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ASME Conference Presenter Attendance Policy and Archival Proceedings

2013;():V001T00A001. doi:10.1115/ES2013-NS.
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This online compilation of papers from the ASME 2013 7th International Conference on Energy Sustainability (ES2013) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Advances in Solar Buildings and Conservation; Climate Control and the Environment; Solar Heating and Cooling

2013;():V001T01A001. doi:10.1115/ES2013-18011.

The technical performance of a non-tracking hybrid PV/T concept that uses a wavelength selective film is modeled. The wavelength selective film is coupled with a compound parabolic concentrator to reflect and concentrate the infrared portion of the solar spectrum onto a tubular absorber while transmitting the visible portion of the spectrum to an underlying thin-film photovoltaic module. The optical performance of the CPC/selective film is obtained through Monte Carlo Ray-Tracing. The CPC geometry is optimized for maximum total energy generation for a roof-top application. Applied to a rooftop in Phoenix, Arizona USA, the hybrid PV/T provides 20% more energy compared to a system of the same area with independent solar thermal and PV modules, but the increase is achieved at the expense of a decrease in the electrical efficiency from 8.8% to 5.8%.

Topics: Wavelength
Commentary by Dr. Valentin Fuster
2013;():V001T01A002. doi:10.1115/ES2013-18015.

Thermal stratification of solar water storage tanks improves collector efficiency and provides higher quality energy to the user. A crucial aspect of maintaining stratification is preventing mixing in the tank, particularly during solar charging and hot water draws. An effective and simple approach to flow control is an internal stratification manifold. In this paper, the performance of the rigid porous manifold, which consists of a series of vertical hydraulic resistance elements placed within a perforated tube, is considered for charging operation. A 1-D model of the governing mass, momentum, and energy conservation equations is used to illustrate the procedure for designing a manifold and to explore its performance over a broad range of operating conditions expected in solar water storage tanks. A manifold performance indicator (MPI) is used to evaluate the effectiveness of the manifold relative to an inlet pipe positioned at the top of the tank. The rigid porous manifold improves the stratification in the tank over a wide range of operating conditions unless the inlet flow rate is significantly reduced from the design point.

Topics: Design , Manifolds
Commentary by Dr. Valentin Fuster
2013;():V001T01A003. doi:10.1115/ES2013-18016.

Chilled water systems constitute a major portion of energy consumption in air conditioning systems of the large buildings and process cooling of the manufacturing plants. These systems do not operate optimally in most of the cases because of the operating parameters set and/or the components used. The Chilled water system analysis tool software (CWSAT) is developed as a primary screening tool for energy evaluation. This tool quantifies the energy usage of the various chilled water systems and typical measures that can be applied to these systems to conserve energy. The tool requires minimum number of inputs to analyze the component-wise energy consumption and incurred overall cost. Both air-cooled and water-cooled systems can be analyzed with this tool; however, this paper focuses on water-cooled systems. The tool uses weather data of the chilled water system location and loading schedules to calculate the chilled water system energy consumption. The Air-Conditioning and Refrigeration Institute (ARI) standard 550/590 typical loading schedule is also incorporated for the chiller(s) loading. The tool is capable of comparing economics by analyzing the energy consumption and relevant cost of the existing system and the new system with cost reduction opportunities considered like: (1) increase chilled water temperature set point, (2) lowering the condenser cooing water supply temperature set point, (3) replace chiller(s), (4) Apply variable speed control to chilled and/or condenser water pumps, (5) upgrade cooling tower fan speed control, (6) Use free cooling when possible for water-cooled systems. The savings can be calculated separately for each cost reduction opportunity or can be combined. The economics comparison can be a primary decision criterion for further detailed engineering and cost analysis related with system changes. The comparison between actual system energy consumption and CWSAT results are also shown.

Commentary by Dr. Valentin Fuster
2013;():V001T01A004. doi:10.1115/ES2013-18026.

In recent years Computational Fluid Dynamics (CFD) simulations are increasingly used to model the air circulation and temperature environment inside the rooms of residential and office buildings to gain insight into the relative energy consumptions of various HVAC systems for cooling/heating for climate control and thermal comfort. This requires accurate simulation of turbulent flow and heat transfer for various types of ventilation systems using the Reynolds-Averaged Navier-Stokes (RANS) equations of fluid dynamics. Large Eddy Simulation (LES) or Direct Numerical Simulation (DNS) of Navier-Stokes equations is computationally intensive and expensive for simulations of this kind. As a result, vast majority of CFD simulations employ RANS equations in conjunction with a turbulence model. In order to assess the modeling requirements (mesh, numerical algorithm, turbulence model etc.) for accurate simulations, it is critical to validate the calculations against the experimental data. For this purpose, we use three well known benchmark validation cases, one for natural convection in 2D closed vertical cavity, second for forced convection in a 2D rectangular cavity and the third for mixed convection in a 2D square cavity. The simulations are performed on a number of meshes of different density using a number of turbulence models. It is found that k-epsilon two-equation turbulence model with a second-order algorithm on a reasonable mesh gives the best results. This information is then used to determine the modeling requirements (mesh, numerical algorithm, turbulence model etc.) for flows in 3D enclosures with different ventilation systems. In particular two cases are considered for which the experimental data is available. These cases are (1) air flow and heat transfer in a naturally ventilated room and (2) airflow and temperature distribution in an atrium. Good agreement with the experimental data and computations of other investigators is obtained.

Commentary by Dr. Valentin Fuster
2013;():V001T01A005. doi:10.1115/ES2013-18037.

This paper aims to study the performance of a solar-powered adsorption chiller with a novel composite adsorbent material (silica activated carbon/CaCl2) operating during some typical months in Hong Kong. Modeling is established to investigate the cooling performance of this adsorption chiller driven by flat-type solar collectors with three different configurations of glaze: 1) single glazed cover; 2) double glazed cover and 3) transparent insulation material (TIM) cover. The simulation results show that the higher the solar collector temperature is, the better the coefficient of performance (COP) and the specific cooling power (SCP) of the adsorption chiller are. It is suggested to select a double glazed collector with a small value of the lumped capacitance for this adsorption chiller. Seasonal effects are discussed in which the solar COP achieves its highest value during autumn. However, the cooling capacities in spring, summer and autumn are similar. All in all, this newly developed composite material as adsorbent used in the adsorption chiller could achieve a mean solar COP of 0.36 and SCP of 94W/kg on a typical summer day of operation.

Commentary by Dr. Valentin Fuster
2013;():V001T01A006. doi:10.1115/ES2013-18089.

In this paper, a building energy simulation code, EnergyPlus, was used to study selected important conditions, i.e. wall boundary conditions and locations, which potentially affect the energy consumption and thermal management of a popular data center model. The data center model having 1120 servers distributed in four rows of rack was investigated under two major climate conditions — hot and humid (Miami, FL), and cool and humid (Chicago, IL), and under five different wall boundary conditions. The data center model was first simulated under a well-mixed single-zone condition as a baseline. Then, a multi-zone approach was proposed to resolve the hot and cold aisles and used to investigate the data center performance. Both monthly and annual overall energy consumption as well as cooling load reports were analyzed and compared among various boundary conditions. In addition, monthly thermal behavior of hot and cold aisle zones within the data center was analyzed. The simulation results show that thermal performance of the data center is significantly affected by locations or climate conditions. The effects of location and wall boundary conditions are particularly appreciable during the summer and winter seasons.

Commentary by Dr. Valentin Fuster
2013;():V001T01A007. doi:10.1115/ES2013-18093.

Limiting solar power is the inability to cost effectively store energy. The most cost effective means to store solar energy is thermally in the ground, which can then be used for direct conversion to electricity. However, doing so is limited by a historically poor thermal efficiency of such engines. A novel Stirling engine is posed which more closely mimics a Carnot heat engine. It does this through the use of a new passive thermal ‘switch’ which permits heat flow into the expansion chamber of the Stirling engine only when the temperature of the chamber is above a desired value. Ideally heat would be added only at the end of the compression stroke and the beginning of the expansion stroke. Central to this thermal switch is the use of a vanadium dioxide (VO2) low mass heat exchanger internal to the expansion chamber. This low mass heat exchanger allows the film material to track and react to the temperature changes within the expansion chamber, permitting it to transfer heat only when needed. An adiabatic model of this enhanced solar Stirling engine is developed. Results indicate that the thermal efficiency can be nearly doubled, delivering a second law efficiency of over 0.6. Further, a year round overall efficiency accounting for losses in the Stirling engine and solar thermal collectors of 7% appears to be feasible when this engine is integrated with ground solar storage, providing the necessary power to meet loads in a low energy residence. Such results demonstrate promise for future application of this technology.

Commentary by Dr. Valentin Fuster
2013;():V001T01A008. doi:10.1115/ES2013-18130.

In order for the solar air conditioners (A/Cs) to become a real alternative to the conventional systems, their performance and total cost has to be optimized. In this study, an innovative hybrid solar A/C was simulated using the transient systems simulation (TRNSYS) program, which was coupled with MATLAB in order to carry out the optimization study. Two optimization problems were formulated with the following design variables: collector area, collector mass flow rate, storage tank volume, and number of batteries. The Genetic Algorithm (GA) was selected to find the global optimum design for the lowest electrical consumption. To optimize the two objective functions simultaneously, a Multi-Objective Genetic Algorithm (MOGA) was used to find the Pareto front within the design variables’ bounds while satisfying the constraints. The optimized design was also compared to a standard vapor compression cycle. The results show that coupling TRNSYS and MATLAB expands TRNSYS optimization capability in solving more complicated optimization problems.

Commentary by Dr. Valentin Fuster
2013;():V001T01A009. doi:10.1115/ES2013-18177.

Heat pumps are commonly used for space-heating and cooling requirements. The combination of solar thermal and heat pump systems as a single solar-assisted heat pump (SAHP) system is a promising technology for offsetting domestic hot water, space-heating and cooling loads more efficiently. Task 44 of the Solar Heating and Cooling Programme of the International Energy Agency is currently investigating ways to optimize SAHP systems for residential use. This paper presents a review of past and current work conducted on SAHP systems. Specifically, the key performance data from many studies are highlighted and different system configurations are compared in order to establish insight towards which system configurations are suitable for the Canadian residential sector.

It was found that the most suitable configuration for Canadian residential buildings depend on a combination of factors which may include occupant behavior, building characteristics, operation parameters, system components, the performance criteria of interest and climate. A large variety of configurations and parameters exist for SAHP systems and this made analyzing a specific system, comparing differing systems and establishing an optimal design fairly difficult. It was found that different authors used various different performance criterions and this inconsistency also added to the difficulty of comparing the studies of different systems. Overall, a standard performance criterion needs to be established for SAHP systems in order to meaningfully compare different configurations and determine optimal configurations for certain requirements.

Commentary by Dr. Valentin Fuster
2013;():V001T01A010. doi:10.1115/ES2013-18207.

Team Ontario is one of twenty collegiate teams selected to design and build a solar powered, net positive home for the U.S. Department of Energy Solar Decathlon 2013. One aspect of Team Ontario’s competition design entry is a high R-value wall using vacuum insulation panels. This paper details the method used for theoretical evaluation of the high R-value wall, stating all simplifying assumptions made. Theoretical simulations were performed in THERM, a two dimensional finite element heat transfer modelling program. Following a weighted average method used by industry experts, the whole-wall thermal resistance value was calculated. To verify the modelling results, an in-situ experimental validation was conducted. An 8′ × 8′ wall test specimen was built to the specifications of Team Ontario’s wall design. Experimental heat flux and temperature readings were collected from the test specimen in Carleton University’s Vacuum Insulation Test Facility located in Ottawa, Ontario, Canada, with the test specimen exposed to exterior weather elements. The experimental and theoretical results are compared and conclusions drawn to determine the effective thermal resistance of the vacuum insulation panels installed in the wall assembly. Finally the theoretical model is refined based on the previous study and a more accurate whole-wall thermal resistance of Team Ontario’s wall design is determined.

Commentary by Dr. Valentin Fuster
2013;():V001T01A011. doi:10.1115/ES2013-18222.

Heat pumps are commonly used for residential space-heating and cooling. The combination of solar thermal and heat pump systems as a single solar-assisted heat pump (SAHP) system can significantly reduce residential energy consumption in Canada. As a part of Team Ontario’s efforts to develop a high performance house for the 2013 DOE Solar Decathlon Competition, an integrated mechanical system (IMS) consisting of a SAHP was investigated. The system is designed to provide domestic hot water, space-heating, space-cooling and dehumidification. The system included a cold and a hot thermal storage tank and a heat pump to move energy from the low temperature reservoir, to the hot. The solar thermal collectors supplies heat to the cold storage and operate at a higher efficiency due to the heat pump reducing the temperature of the collector working fluid. The combination of the heat pump and solar thermal collectors allows more heat to be harvested at a lower temperature, and then boosted to a suitable temperature for domestic use via the heat pump. The IMS and the building’s energy loads were modeled using the TRNSYS simulation software. A parametric study was conducted to optimize the control, sizing and configuration of the system. This paper provides an overview of the model and summarizes the results of the study. The simulation results suggested that the investigated system can achieve a free energy ratio of about 0.583 for a high performance house designed for the Ottawa climate.

Commentary by Dr. Valentin Fuster
2013;():V001T01A012. doi:10.1115/ES2013-18312.

Sustainable development could be seen as indispensable condition for survival of civilization. The development of timber products is a new paradigm in material and building science research in China, requiring the accounting for carbon emissions, carbon sequestration, material embodied energy, and material thermal properties for this renewable resource. This paper studies the application of the timber lattice wall in Chinese solar buildings. Firstly, it is analyzed timber structural form and mechanical property of the multi-ribbed composite wall, proving its high anti-seismic property and firmness by finite element modeling. Secondly, the timber structure filled with wheat straw brick contributes to low U-value of the wall, which is only 0.3 Watts per square meter per degree Celsius and far better than the code of Chinese building, greatly promoting building energy efficiency. Besides, the timber structure and straw brick are both “Cradle to Cradle” materials and reduce carbon emission compared to common building envelope. In the end, the paper is analyzed the promising market of the multi-ribbed composite wall for its competitive price and outstanding environmental performance.

Commentary by Dr. Valentin Fuster
2013;():V001T01A013. doi:10.1115/ES2013-18409.

Among all types of concentrators, compound parabolic concentrators (CPCs) have been designed as stationary solar collectors for relative high temperature operations with high cost effectiveness. The CPCs are potentially the favorable option for solar power systems and high temperature solar thermal system. This paper provided a review on studies of CPCs in solar thermal applications. It covered basic concepts, principles, and design of CPCs. It also reviewed optical models and thermal models of CPCs, as well as the thermal applications of CPCs. The challenges were also summarized.

Topics: Solar energy
Commentary by Dr. Valentin Fuster

Combined Energy Cycles, CHP and CCHP

2013;():V001T02A001. doi:10.1115/ES2013-18008.

The round trip efficiency of compressed air for energy storage is greatly limited by the significant increase in the temperature of the compressed air (and the resulting heat loss) in high-ratio adiabatic compression. This paper introduces a multi-stage compression scheme with low-compression-ratio compressors and inter-compressor natural convection cooling resulting in a quasi-isothermal compression process that can be useful for large-scale energy storage. When many low pressure ratio compressors work inline, a high overall compression ratio can be achieved with high efficiency. The quasi-isothermally compressed air can then be expanded adiabatically in turbines to generate power with the addition of thermal energy, from either fuel or a solar thermal source. This paper presents mathematical models of such an energy storage system and discusses its round-trip performance with different operating schemes.

Commentary by Dr. Valentin Fuster
2013;():V001T02A002. doi:10.1115/ES2013-18256.

This paper presents results of the combined cycle power plant (CCPP) modeling when the ambient temperature is varying. The model of the CCPP was developed using a gas turbine and a heat recovery steam generator (HRSG) models that had been already developed and validated. The model of the components was developed based on an actual existing power plant and then the operational data of the power plant was used to validate the model. The results of running the model for various ambient temperatures demonstrated that the performance of the gas turbine part of the cycle was heavily affected by the changes in the ambient temperature, particularly the output power of the gas turbines. However, the performance of the steam cycle was almost untouched by the changes of ambient temperature. This suggests that operation of the CCPP is more stable than stand-alone gas turbine in hot summer days especially if the cycle is not equipped with an inlet air cooling system.

Commentary by Dr. Valentin Fuster
2013;():V001T02A003. doi:10.1115/ES2013-18286.

Because of the performance of the power generation equipment is almost perfect, how to integrate the thermally-activated technologies and use the waste heat deeply are a critical issue for CCHP (Combined cooling heating and power) system. According to the characteristics of a typical end user’s demands, a CCHP system with the flue gas and geothermal energy is proposed. The system is composed of an internal combustion engine, a soil source absorption heat pump driven by the flue gas, and other assistant facilities, such as pumps, fans, and end user devices. In the winter, the flue gas is used to drive absorption heat pump to recover the waste heat of the soil source and the condensation heat of the flue gas simultaneously, and in the summer, the waste heat of the flue gas is used to drive absorption heat pump to cooling, and the heat sink is the soil. In the paper, the energy analysis of the system is done. Compared with the conventional CCHP system, the operation cost is lowered greatly and the increased investment could be returned within one year. It is show that the system is the efficient integration of clean energy, renewable energy, the discharge of the flue gas could be reduced to below 30°C, and the water steam could be catch to avoid the white smoke of the stack.

Commentary by Dr. Valentin Fuster
2013;():V001T02A004. doi:10.1115/ES2013-18293.

As an energy-saving and environmentally friendly technology, the combined cooling heating and power system (CCHP) had been applied in the field of heating and air conditioning. Chinese researchers recently designed a CCHP system with the condensation heat recovery of the flue gas, which composed of a gas-powered internal combustion engine (ICE), an exhaust-gas-driven absorption heat pump (AHP), a flue gas condensation heat exchanger (CHE), and other assistant facilities, such as pumps, fans, and end user devices. The system was built and operated in 2011. We tested the parameters of the system on the heating and cooling status from the ICE to the CHE, including the temperature and flux of water, the inlet and outlet parameters of different facilities, and the performance of different facilities for a typical operation status. Based on the test results, the overall COP of the system in the heating and cooling mode was computed, and the energy efficiency level was analyzed. The results indicated that the energy utilization efficiency is about 94% on the heating status, and the energy utilization efficiency is about 84% on the cooling status. These results could serve as a reference for designing or evaluating the CCHP systems.

Commentary by Dr. Valentin Fuster
2013;():V001T02A005. doi:10.1115/ES2013-18296.

With the process acceleration of China’s energy conservation and the full development of the market economy, the environmental protection is to coexist with the power plants’ benefits for thermal power plants. Relative to the traditional mode named “determining power by heat”, it is not adequate that the heating demand is only to be met, the maximizations of economy benefits and social benefits are also demanded. At present, several large-scale central heating modes are proposed by domestic and foreign scholars, such as the parallel arrangement and series arrangement of heating system for the traditional heating units and NCB heating units (NCB heating unit is a new condensing-extraction-backpressure steam turbine and used to generate the power and heat, it has the function of extraction heating turbine at constant power, back pressure turbine or extraction and back pressure heating turbine and extraction condensing heating turbine.), and running mode with heating units and absorbed heat pumps, and so on. Compare and analyze their heating efficiency, heating load, heating area, power generation, and the impact on the environment. The best heating mode can be found under the different boundary conditions, it can be used to instruct the further work. The energy utilization efficiency will be further improved.

Commentary by Dr. Valentin Fuster
2013;():V001T02A006. doi:10.1115/ES2013-18327.

Combined cooling heating and power (CCHP) systems based on natural gas is widely applied abroad, which have resulted in increasing domestic attention to distributed energy systems (DES). In order to reduce heat loss of exhaust-gas and recover condensation heat of it, an innovative system is advanced by Tsinghua, which can recover exhaust-gas condensation heat utilizing an exhaust-gas-driven absorption heat pump (AHP). In 2007, Tsinghua bears a research task from National Ministry of Science and Technology, that is, ‘Integration and Demonstration study of high-efficiency natural gas CCHP technique’. As a part, a natural gas CCHP system based on AHP has been set up at the Tsinghua University energy-saving building, in Beijing, China, and a lot of research has been made. As another part, a combined cooling heating and power (CCHP) project based on the gas-powered internal combustion engine (ICE) (electricity volume MW) is built in Beijing Southern Station. This paper mainly analyzes and introduces the running condition of this demonstration project.

Topics: Natural gas
Commentary by Dr. Valentin Fuster

Concentrating Solar Power

2013;():V001T03A001. doi:10.1115/ES2013-18055.

Thermal efficiencies of the solar field of two different parabolic trough concentrator (PTC) systems are evaluated for a variety of operating conditions and geographical locations, using a detailed 3D heat transfer model. Results calculated at specific design points are compared to yearly average efficiencies determined using measured direct normal solar irradiance (DNI) data as well as an empirical correlation for DNI. It is shown that the most common choices of operating conditions at which solar field performance is evaluated, such as the equinox or the summer solstice, are inadequate for predicting the yearly average efficiency of the solar field. For a specific system and location, the different design point efficiencies vary significantly and differ by as much as 11.5% from the actual yearly average values. An alternative simple method is presented of determining a representative operating condition for solar fields through weighted averages of the incident solar radiation. For all tested PTC systems and locations, the efficiency of the solar field at the representative operating condition lies within 0.3% of the yearly average efficiency. Thus, with this procedure, it is possible to accurately predict year-round performance of PTC systems using a single design point, while saving computational effort. The importance of the design point is illustrated by an optimization study of the absorber tube diameter, where different choices of operating conditions result in different predicted optimum absorber diameters.

Commentary by Dr. Valentin Fuster
2013;():V001T03A002. doi:10.1115/ES2013-18057.

Industrial power utilities are using molten binary nitrate salt as a heat transfer fluid and thermal storage media for solar energy generation. Currently, the maximum bulk temperature is 565°C, due to concerns of salt degradation and materials compatibility with containment vessels.

To increase overall cycle efficiency, one must increase the upper temperature of the nitrate salt, thereby lowering the levelized cost of electricity (LCoE) through higher power cycle efficiency. The corrosion performance of 316 stainless steel and Inconel 625 is currently characterized at 600°C. However, the 316SS has exhibited stress corrosion cracking (thought due to aqueous flush in Solar Two [1]), and while In625 performs well, its cost is prohibitive. Therefore, current research seeks to evaluate heat-resistant austenitic alloys for use with nitrate salts, ascertaining if they have superior performance characteristics, as well as assessing their mechanisms of corrosion.

Sandia National Laboratory is researching four alloys (S35140, ATI332Mo, RA330, and HA556) for corrosion performance at 600°C for 3000 hours, under a cover gas of air. Air is used to simulate the chemistry conditions expected in a power plant.

This work details the corrosion rate and the oxide structure for each alloy. Research indicates all alloys are very corrosion-resistant, with metal loss rates projected to be less than 21μm/year after 3000 hours. Though all alloys performed well, corrosion rate data for RA330 (Fe-19Cr-35Ni + minor elements) currently appears to exhibit a linear loss mechanism. In conclusion, this paper will explore the differences in oxide formation between these similar alloys.

Topics: Alloys , Corrosion
Commentary by Dr. Valentin Fuster
2013;():V001T03A003. doi:10.1115/ES2013-18078.

Parabolic trough collectors are economically and technically attractive options for process heat applications that require temperatures in excess of 200 °C. One of the reasons is that low-cost non-evacuated receivers are used in this type of application. However, at higher temperatures, the performance of non-evacuated receivers deteriorates considerably due to excessive radiation and natural convection losses. A new idea had been preliminarily investigated by the authors both numerically and experimentally. The idea was to introduce a thermally insulating layer to the part of the receiver’s annular gap that does not receive concentrated sunlight from the parabolic mirrors, and the results had been quite promising. This paper presents additional, more extensive experiments on this concept. In these experiments, a cartridge heater is inserted along the axis of the receiver tube of a non-evacuated receiver. The heater is surrounded by a conductive material to ensure uniform heating of the receiver tube. A number of thermocouples are affixed near the inner surface of the receiver as well as on the outer surface of the glass envelope to monitor temperature uniformity. Two sets of experiments are then conducted, one with the insulating layer, and the other without. In each set, the power input is set to a certain level and the receiver temperature is measured once steady state conditions are attained. The power level is then increased, and the measurements are repeated. The heat loss values from each set are compared to determine whether adding the insulating layer enhances receiver performance. Results show that a reduction in heat loss of as much as 15% can be achieved using this design, and collector efficiency can increase by up to about 6%. However, it was also found that the extent of improvement in collector efficiency depends on the operating temperature and direct normal irradiance, with the improvement being more significant at higher temperature applications and at low direct normal irradiance.

Commentary by Dr. Valentin Fuster
2013;():V001T03A004. doi:10.1115/ES2013-18136.

With the success of CSP technology in the last years more players are active in the market, inducing the need for harmonization of technical terms and methodologies. The mission of the SolarPACES “guiSmo” project which was started in 2010 is to develop a guideline for CSP yield analysis [1]. Activities carried out so far have shown that people have different understandings of many terms used in daily CSP practice. Especially for the development of guidelines, the essential terms need to be clearly defined in order to avoid inconsistencies within the same project. A first version of a nomenclature has been compiled by the “guiSmo” team and will undergo final discussion. The aim is to come to a harmonized version by Summer 2013 which will then be presented at the ASME Energy Sustainability conference. The compilation so far includes essential definitions of terms like direct normal irradiance, incident angles, heat flows, and efficiencies on a system level. The definitions presented will be discussed together with existing standards like the ISO 80000 (physical quantities and units of measurement), the ISO 9488 (Solar energy-vocabulary) and other relevant sources. Although the list of terms is primarily put together for the work in the “guiSmo” project, it might serve as a basis for standardization in the official councils. An international group of solar experts is involved in the preparation of the document in order to ensure high quality and international support for the results.

Commentary by Dr. Valentin Fuster
2013;():V001T03A005. doi:10.1115/ES2013-18143.

Currently, increasing world population demands a higher cement production. Therefore atmospheric emissions and energy consumption become two of the most important environmental and economic issues. Fuel and electricity consumption for the production of cement represent 40% of the total production cost [1]. It is known that cement production is an energy-intensive process which contributes with approximately 5% of the worldwide carbon dioxide (CO2) emissions [2] [3].

By using Concentrated Solar Thermal (CST) at the calcination process in the cement production line, CO2 emissions can be reduced by 40% and savings of up to 60% through fuel substitution can be obtained if all the fuel used at the calcination step is substituted.

The aim of the study is not to propose a detailed design of the solar process but to examine and quantify the various options in order to define the favorable economic conditions and the technical issues to face in a conventional cement plant aiming: substituting energy sources and achieving continuous operation of the cement plant employing a hybrid mode.

Three options related with how to apply the CST technology were evaluated. The best solution is a Central Tower with Solar Reactor at the Top of the Tower since it allows energy substitution with high thermal energy efficiency. This implies, compared with the other options, the minimum changes in the process.

Several energy substitution scenarios are investigated considering different energy losses and amount of energy to be replaced. It was found that the solar energy availability is not a constraint, meaning that from the technical point of view it is possible to replace up to 100% of the energy requirements for the calcination process.

Economic results are promissory since the application of the proposed approach (Go Process) became attractive. The Payback Time (PBT) obtained (from 6 to 10 years) is lower when it is compared with the PBT for applications of CST for electricity production. Besides, the IRR values obtained (from 8% to 11%) are adequate in accordance with the typical values expected by most of the equity investors in renewable energy projects (between 8% and 12%) [4].

It is expected that CST technology will become more attractive and profitable due to economic aspects like increments in fossil fuels and alternative fuels cost and the current deployment of the CST technology to produce electricity. Other aspects such as more strict legislation related with CO2 emissions combined with encouraging legislation to use of renewable energy also play an important role in the economic attractiveness of the proposed application.

Commentary by Dr. Valentin Fuster
2013;():V001T03A006. doi:10.1115/ES2013-18159.

The thermal performance of parabolic trough concentrating solar collectors depends on both the structural and optical characteristics of the design. In order to reduce the cost of energy, advanced concentrating structures must significantly reduce the cost of collectors while maintaining good optical performance. This paper discusses a Finite Element Ray Tracer (FERT) that has been developed specifically to support the commercial design process. This is achieved by tying the whole of the support structure directly to its optical effects. Consequently, the optical performance metrics go beyond the typical reflector slope error RMS or average intercept factor to present the designer with spatially resolved analysis of localized performance. By incorporating this analytical method into the structural design process, collector cost and performance can be balanced efficiently and rapidly, allowing for an accelerated design period. At times, this insight has driven better, albeit unexpected, design decisions.

The paper presents an overview of the development process that Abengoa R&D uses to take advantage of its analytical optical analysis capability throughout all phases of a project, as well as a review of its implementation. A selection of case studies is also presented to illustrate how FERT enables the designer to identify local areas of concern, diagnose the cause, and quickly develop possible redesign strategies. Finally, the significance of various parameters within the ray tracer are discussed.

Commentary by Dr. Valentin Fuster
2013;():V001T03A007. doi:10.1115/ES2013-18195.

Concentrating solar power (CSP) continues to advance as worldwide interest in renewable energy continues to grow. CSP technologies, including parabolic troughs, power towers, and dish/engines, provide the unique potential for low-cost thermal energy storage that will ensure that renewable energy can become cost-competitive with traditional fossil fuel sources on a large scale and comprise a significant portion of the global energy portfolio.

The challenge is to develop cost-effective thermal energy storage to ensure that renewable energy can become a major part of the national and global energy supply. Storage fluid selection is a critical decision that must fulfill a number of criteria to not only provide long-term reliability, but also to remain cost-competitive in the power generation arena. The state-of-the-art thermal storage design uses a 2-tank molten salt configuration. However, most molten salt mixtures have a relatively high freezing temperature, which poses some system design issues. Additionally, the price of molten salt mixtures is steadily increasing. Current laboratory and industry research efforts have shifted focus to exploration of alternative storage fluids to significantly reduce costs.

In this study, several storage fluid candidates have been selected based on an attractive combination of thermodynamic properties, cost, and availability. In this paper, rapid screening of fluid candidates is reported, and an expanded series of thermal cycling and supercritical characterization experiments have been planned and are being implemented to determine the long-term durability of the fluid candidates over a range of operating temperatures for extended periods of time. Commercial-grade materials were used, and in the case of naphthalene and biphenyl, the testing procedure was carefully controlled to prevent sublimation of the sample. This paper presents the results of a study investigating the thermal stability of several organic fluids. Samples were extracted and chemical analyses such as nuclear magnetic resonance (NMR) and gas chromatography (GC) were conducted to observe degradation behavior and decomposition pathways. The rapid screening phase provided a timely and effective filter of the best-performing fluid candidates for supercritical thermal energy storage.

Commentary by Dr. Valentin Fuster
2013;():V001T03A008. doi:10.1115/ES2013-18238.

Current heliostats cost ∼$200/m2 of reflective area and are estimated to contribute up to 50% of the total solar power tower plant costs. A drastic overall cost reduction is required in order for concentrated solar thermal power to be economically viable. The Department of Energy has set forth the SunShot initiative targeting a levelized cost of energy (LCOE) of $0.06/kWh by the year 2020. The cost of each heliostat must be brought down to an estimated $75/m2 to achieve this rigorous goal. One of the driving aspects of heliostat design and cost are the heliostat optical errors. At the moment, it is relatively unclear about the amount of error that can be present in the system while still maintaining low cost and high optical accuracy. The optical errors present on heliostat mirror surfaces directly influence the plant LCOE by causing beam spillage. This can result in an increase in the number of heliostats, an increased receiver size, and decreased thermal efficiency. Assuming a fixed heliostat cost of $75/m2, the effects of optical errors on LCOE are evaluated within the software DELSOL. From a probabilistic analysis, beam quality errors (i.e. slope error, alignment errors, etc.) are shown to have more importance on the LCOE than tracking errors. This determination results in a realization that the tracking errors and beam quality errors can be combined into a “bundled” root-sum-square (RSS) error value. A “bundled” error value of 2 mrad results in an LCOE of $0.06/kWh. This “bundled” value is the basis for a new optical error budget and is decomposed into five individual errors. These five errors can be used as design specifications for new generation heliostats.

Topics: Errors
Commentary by Dr. Valentin Fuster
2013;():V001T03A009. doi:10.1115/ES2013-18245.

Heliostat optical performance can be affected by both wind- and gravity-induced deflections in the mirror support structure. These effects can result in decreased energy collection efficiency, depending on the magnitude of structural deflections, heliostat orientation and field position, and sun position. This paper presents a coupled modeling approach to evaluate the effects of gravity loading on heliostat optical performance, considering two heliostat designs: The National Solar Thermal Test Facility (NSTTF) heliostat and the Advanced Thermal Systems (ATS) heliostat. Deflections under gravitational loading were determined using finite element analysis (FEA) in ANSYS Mechanical, and the resulting deformed heliostat geometries were analyzed using Breault APEX optical engineering software to evaluate changes in beam size and shape. Optical results were compared against images of actual beams produced by each respective heliostat, measured using the Beam Characterization System (BCS) at Sandia National Laboratories. Simulated structural deflections in both heliostats were found to have visible impacts on beam shape, with small but quantifiable changes in beam power distribution. In this paper, the combined FEA and optical analysis method is described and validated, as well as a method for modeling heliostats subjected to gravitational deflection but canted in-field, for which mirror positions may not be known rigorously. Furthermore, a modified, generalized construction method is proposed and analyzed for the ATS heliostat, which was found to give consistent improvements in beam shape and up to a 4.1% increase in Annual Incident Power Weighted Intercept (AIPWI).

Commentary by Dr. Valentin Fuster
2013;():V001T03A010. doi:10.1115/ES2013-18292.

The ability of thermal energy storage (TES) to avoid the major intermittency issues associated with solar photovoltaic power generation is a key differentiator for concentrating solar power (CSP) systems. Infinia Corporation is a pioneer in phase change salt TES systems, with one DOE contract based on heat pipes [1], and a second DOE contract that uses an inherently simpler, patented sodium pool boiler that is integral with the TES salt. This paper describes the Phase 1 results for that second contract, which is targeted for cost effective extended duration storage for CSP systems up to a level that can provide baseload power, with a particular focus on dish Stirling systems.

Commentary by Dr. Valentin Fuster
2013;():V001T03A011. doi:10.1115/ES2013-18297.

A finite-volume-based model of a molten-salt thermocline tank is developed to achieve simulation at a sufficient level of detail but at low computational cost. Combination of this storage model with a system-level power tower plant model enables yearlong thermocline tank simulation in response to historical weather data and corresponding plant control. The current study simulates a 100 MWe molten-salt power tower plant to optimize annual plant performance as a function of the thermocline tank size and the plant solar multiple.

Thermocline storage performance is characterized by the effectiveness of the tank in storing and delivering utilizable heat for steam generation and power production. Additional system-level metrics include thermal energy discard due to saturation of storage capacity and annual plant capacity factor. Economic assessment of the power output is characterized with a simple levelized cost of electricity. Minimum cost is observed with a solar multiple of 3 and a thermocline tank storage capacity of 16 hours.

Commentary by Dr. Valentin Fuster
2013;():V001T03A012. doi:10.1115/ES2013-18333.

The ambitious DOE SunShot cost target ($0.06/kWh) for concentrated solar power (CSP) requires innovative concepts in the collector, receiver, and power cycle subsystems, as well as in thermal energy storage (TES).

For the TES, one innovative approach is to recycle waste from metallurgic industry, called slags, as low-cost high-temperature thermal energy storage material. The slags are all the non-metallic parts of cast iron which naturally rises up by lower density at the surface of the fusion in the furnace. Once cooled down some ceramic can be obtained mainly composed of oxides of calcium, silicon, iron, and aluminum. These ceramics are widely available in USA, about 120 sites in 32 States and are sold at a very low average price of $5.37/ton. The US production of iron and steel slag was estimated at 19.7 million tons in 2003 which guarantees a huge availability of material.

In this paper, electric arc furnace (EAF) slags from steelmaking industry, also called “black slags”, were characterized in the range of temperatures of concentrated solar power. The raw material is thermo-chemically stable up to 1100 °C and presents a low cost per unit thermal energy stored ($0.21/kWht for ΔT = 100 °C) and a suitable heat capacity per unit volume of material (63 kWht/m3for ΔT = 100°C). These properties should enable the development of new TES systems that could achieve the TES targets of the SunShot (temperature above 600 °C, installed cost below $15/kWht, and heat capacity ≥25 kWht/m3). The detailed experimental results are presented in the paper.

After its characterization, the material has been shaped in form of plates and thermally cycled in a TES system using hot-air as heat transfer fluid. Several cycles of charge and discharged were performed successfully and the concept was validated at laboratory scale. Apart from availability, low-cost, and promising thermal properties, the use of slag promotes the conservation of natural resources and is a noble solution to decrease the cost and to develop sustainable TES systems.

Commentary by Dr. Valentin Fuster
2013;():V001T03A013. doi:10.1115/ES2013-18342.

Conversion of direct solar energy, in particular the Concentrated Solar Power (CSP) technologies, has a significant role on conventional energy cost and efficiency. A single tank thermocline Thermal Energy Storage (TES) system is accountable for the overall efficiency of this conversion system. A single tank TES system has a thermocline region that produces the temperature gradient between hot and cold storage fluid by density difference. The overall energy storage capacity depends on sustaining of this region at uniform manner. This paper analyzes how the difference in the percentage of porous medium influences the effectiveness of the flow-distribution and hence, the overall performance of the TES system. The effectiveness is assessed by the optimal flow distribution. The optimal distribution is obtained by examining the velocity profile at any horizontal plane. This plane should be uniform for sustaining the thermocline region during the operation period. To achieve a uniform velocity distribution, two symmetric perforated plate flow distributors were placed in the tank. The distributors were positioned near the inlet and outlet, and checked the performance by varying the percentage of porous medium since the distribution is influenced by the porosity. Porous distributors with hexagonal shape pore were considered and Hitec® molten salt was used as a heat transfer fluid. These respective percentages of porosity affect the flow distribution throughout the tank during the flow distribution. The standard deviations of the velocity field at different positions along z-plane and thermal diffusivity were analyzed. The analyses of our cases were done to distinguish a configuration for the minimum thermal diffusivity and velocity deviation from the mean flow.

A finite volume based computational fluid dynamics software was used to execute the computational analysis.

Commentary by Dr. Valentin Fuster
2013;():V001T03A014. doi:10.1115/ES2013-18348.

Heliostat reflective facets have traditionally been constructed with glass/silver and a metal back support. During the past year, Sandia National Laboratories evaluated low-cost materials and alternative manufacturing methods to construct facets with the goal of reducing current facet cost by at least 25% while maintaining surface slope errors at 1 milli-radians rms or below. Several companies developed prototype facet samples, which were optically evaluated at Sandia and compared to baseline facet samples using a proposed cost-to-performance metric.

A cost-performance metric for comparing facets was developed by modeling and optimizing a 200 MWe power tower plant scenario in DELSOL, a computer code for system-level modeling of power tower systems. We varied the slope error on the facets and adjusted the cost on the facets to maintain the constant plant levelized cost of energy. The result of these models provides a chart of the facet optical performance and the allowable facet cost for a constant plant LCOE.

The size of the prototype facet samples ranged from 1.4 to 3 m2. The measured optical slope errors were between 1 and 2 milli-radians rms when compared to a flat mirror design shape. Despite slope errors greater than 1 mrad rms, some of the prototype samples met the cost goals for this project using the cost-performance metric. Next steps are to work with the companies to improve the manufacturing processes and further reduce the cost and improve on the optical performance to reach DOE SunShot goal of $75/m2 for heliostats.

Commentary by Dr. Valentin Fuster
2013;():V001T03A015. doi:10.1115/ES2013-18352.

Thermally-stratified air layers over solar-heated ground are exploited for a scalable, low-cost method of power generation by the deliberate formation of intense buoyancy-induced vertical columnar vortices, similar to naturally-occurring desert “dust devils.” Such vortices collect low-grade thermal energy in solar-heated air in regions with high surface heating rates, and convert the (gravitational) potential energy in the concentrated buoyant air into “solar-induced wind” with significant kinetic energy. Unlike dust devil vortices that are typically free to wander laterally and are therefore susceptible to cross winds, the anchored columnar vortex is deliberately triggered and anchored within a cylindrical domain bounded by an azimuthal array of stationary ground-mounted vertical vanes sustained by continuous entrainment of the heated air through these vanes. Solar energy collected by the ground plane sustains the anchored vortex and electric power is generated by using the air motion to drive a vertical-axis turbine to form a simple, low-cost electric power generating unit. In terms of cost-of-energy, intermittency, mechanical simplicity, and environmental impact, the approach has significant advantages compared to solar PV, CSP, and conventional wind turbines. Meter-scale laboratory experiments have demonstrated the nucleation, anchoring, and sustainment of strong, buoyancy-driven vortices centered within an azimuthal array of stationary vertical vanes above a controlled thermal ground plane. Outdoor tests of a meter-scale prototype have demonstrated autonomous start-up, formation, anchoring, and sustainment of a buoyancy-induced vortex generated solely by absorbed solar energy.

Commentary by Dr. Valentin Fuster
2013;():V001T03A016. doi:10.1115/ES2013-18363.

Valves used in molten salt thermal energy storage systems often utilize conventional packing methodology and materials. These packing materials often exhibit relatively short lifetimes because of the reactive interaction of the salt and packing. Past research has indicated that valve packing lifetime is affected by both stress and temperature in the packing. Because of this interaction, it is important to understand the stress in the packing and to find ways to reduce the packing stress. A finite element model of a valve stem/packing system was created and material properties of the packing were determined and validated against previous work. The model was then used to evaluate the stress induced in the packing system through linear axial motion and then extended to include rotational stem motion as well. The analysis confirmed previous results that axial translation created a significant amount of stress in the valve packing. The newly included rotational motion of the valve stem was found to affect the packing stress only minimally. This result suggests that development of better rotary valves would be very useful for utilization in molten salt service, especially as the temperature of the salts are increased in an effort to achieve higher power cycle efficiencies.

Commentary by Dr. Valentin Fuster

Energy Systems Design and Thermoeconomic Analysis

2013;():V001T04A001. doi:10.1115/ES2013-18058.

All the previous theoretical models of the dynamic thermal behavior of the periodic–flow Air Pre-Heater (APH) have ignored the effects of the baskets walls. Therefore, the purpose of this work is to develop experimental and a numerical model simulating transient energy transport in a periodic-flow regenerative APH to involve the important thermal effect of the basket walls surrounding the heating element matrices.

Three main different types of matrices that are intended to be used in periodic-flow APH have been investigated and comparison was made between them. These materials are corrugated plates matrices (CPMs), wire mesh matrices (WMMs) and stone media matrices (SMMs). Total of (13) different configurations of matrices have been tested. The investigations covered a range of geometrical parameters of corrugated plate angle, wire diameter of mesh, pitch and screen to screen position and size of stones. The investigation has been carried out for hydraulic – diameter base Reynolds number, Re, in the range of 680<Re < 10100.

The basket wall was determined to have a significant influence on the thermal behavior of the packing matrices especially in the near–wall region, and had been seen to introduce different thermal response for each type of the tested materials.

New (13) correlation equations for the overall J-factor have been developed for each matrix type to provide the best matching between the gathered experimental data with the numerical model.

Commentary by Dr. Valentin Fuster
2013;():V001T04A002. doi:10.1115/ES2013-18121.

Approximately half of the water heaters sold in the U.S. and Canada for residential and small commercial applications are natural gas fired storage water heaters, with a maximum theoretical thermal efficiency of 96%. A packaged water heater heated by a 2.9 kW absorption heat pump was designed and demonstrated in this study to achieve performance exceeding these limitations. The modeling and validation of the absorption cycle and of the natural gas-fired combustion system are discussed here. Heat transfer characteristics of the absorption components at expected operating conditions were used to model cycle performance. A single-effect system based on these models was fabricated and yielded a cyclic COP of 1.63, within 3% of predictions. A corresponding GAX cycle-based system yielded performance 20% lower than predicted values, indicating the need for larger heat and mass exchangers to achieve the expected system level performance. The gas-fired burner configuration required for this heat pump is governed by the water heater envelope, desorber geometry and process requirements, coupled with emissions requirements. Parametric CFD analyses were conducted to estimate the impact of chamber design on burner performance, and revealed a beneficial recirculation pattern within the combustion chamber that was strongly influenced by chamber height. Emission reductions depended on chamber diameter, and prototype burners with smaller outer diameter fabricated based on these designs met emission targets.

Commentary by Dr. Valentin Fuster
2013;():V001T04A003. doi:10.1115/ES2013-18147.

Portable energy storage will be a key challenge if electric vehicles become a large part of our future transportation system. A big limiting factor is vehicle range. Range can be further limited if heating and air conditioning systems are powered by the electric vehicle’s batteries. The use of electricity for HVAC can be minimized if a thermal battery can be substituted as the energy source to provide sufficient cabin heating and cooling. The aim of this project was to model, design, and fabricate a thermal storage battery for electric vehicles. Since cost and weight are the main considerations for a vehicular application — every attempt was made to minimize them in this design. Thus, the final thermal battery consists of a phase change material Erythritol (a sugar alcohol commonly used as artificial sweetener) as the storage medium sealed in an insulated, stainless steel cooking pot. Heat exchange to the thermal battery is accomplished via water (or low viscosity engine oil) which is pushed through a copper coil winding. A CFD model was used to determine the geometry (winding radius and number of coils) and flow conditions necessary to create adequate heat transfer. Testing of the fabricated design indicates that the prototype thermal battery module losses less than 5% per day and can provide enough heat to meet the demand of cruising passenger vehicle for up to 1 hour of full heating on a cold day. Other metrics, such as $/kJ and kJ/kg, are competitive with Lithium ion batteries for our prototype.

Topics: Modeling , Testing , Batteries
Commentary by Dr. Valentin Fuster
2013;():V001T04A004. doi:10.1115/ES2013-18243.

This paper describes the design and construction of a solar thermal adsorption refrigerator in Patna, Bihar, India. After a brief description of the local situation and planning prerequisites the reasons for choosing an ethanol based adsorption system are explained. The following sections are focused on the description of the design and the theory behind the system. Lastly, practical aspects that arose during the construction of the first prototype are mentioned.

Commentary by Dr. Valentin Fuster
2013;():V001T04A005. doi:10.1115/ES2013-18308.

Thermal energy storage (TES) is a vital component of concentrated solar power (CSP). TES makes up for intermittent solar radiation, bad weather, and peak power demand. Currently, a sensible heat storage system using two tanks containing molten salt is considered the most practical and economical TES. Yet further system development is needed in order to improve its performance and economics.

In this study of molten salt storage tanks, spherical tanks were investigated as an alternative to cylindrical tanks. Structural and thermal aspects of cylindrical tanks with varying H/D ratios (0.25–5) and spherical tanks of the same volume were compared.

Comparison showed that utilization of spherical instead of cylindrical tanks resulted in significant savings in shell building material (28–47%). Heat transfer from the spherical tank’s shell is at least 35% less than cylindrical tanks. Reduction in building material, foundation, and insulation cost can lead to significant cost savings.

Commentary by Dr. Valentin Fuster
2013;():V001T04A006. doi:10.1115/ES2013-18316.

Condensing boiler for flue gas waste heat recovery is widely used in industries. In order to gain a portion of the sensible heat and latent heat of the vapor in the flue gas, the flue gas is cooled by return water of district heating through a condensation heat exchanger which is located at the end of flue. At low ambient air temperature, some boilers utilize the air pre-heater, which makes air be heated before entering the boiler, and also recovers part of the waste heat of flue gas. However, there are some disadvantages for these technologies. For the former one, the low temperature of the return water is required while the utilization of flue gas heat for the latter one is very limited. A new flue gas condensing heat recovery system is developed, in which direct contact heat exchanger and absorption heat pump are integrated with the gas boiler to recover condensing heat, even the temperature of the return water is so low that the latent heat of vapor in the flue gas could not be recovered directly by the general condensing technologies. Direct contact condensation occurs when vapor in the flue gas contacts and condenses on cold liquid directly. Due to the absence of a solid boundary between the phases, transport processes at the phase interface are much more efficient and quite different from condensation phenomena on a solid surface. Additionally, the surface heat exchanger tends to be more bulky and expensive. In this study, an experimental platform of the new system is built, and a variety of experimental conditions are carried out. Through the analysis of the experimental data and operational state, the total thermal efficiency of the platform will be increased 3.9%, and the system is reliable enough to be popularized.

Commentary by Dr. Valentin Fuster
2013;():V001T04A007. doi:10.1115/ES2013-18350.

Parabolic trough technology is currently one of the most extended solar thermal systems for the production of electricity.

This paper describes a thermo-economic study of an integrated, combined-cycle parabolic trough power plant. The parabolic trough plant is considered an economizer or a superheater of the HRSG (heat recovery steam generator). The main objective is to obtain the optimum design of the different sections of the boiler and the size of the parabolic field. The configurations analyzed are two pressure levels with and without a reheater. A Euro Trough (ET) concentrator was used in this study, the working fluid being water with direct steam generation. There will be no problem with the evaporation in the absorber, since the solar plant will be the economizer of the HRSG and an approach point greater than 3°C is considered.

The methodology applied for the optimization is Genetic Algorithms. This methodology was employed in previous works developed by the authors and yielded good results. So that method is applied to the configurations analyzed but including the parabolic trough plant. As a result, a thermoeconomic optimum design of a parabolic trough plant used as the section of the HRSG is obtained. The results show that the solar field increases the power and efficiency of the combined-cycle plant during the operation and makes it less susceptible to load variations.

Commentary by Dr. Valentin Fuster
2013;():V001T04A008. doi:10.1115/ES2013-18399.

The current heating system in Yinchuan city, the capital of the Ningxia Autonomous Region in northwest China, is investigated and analyzed. Lacking an integrated planning, the heating systems have developed with low energy efficiency, high environment emission and economic cost. The choice of heating energy structure vary between coal and gas, the heating modes including gas-fired CHP, coal-fired CHP, gas-fired boiler and coal-fired boiler are facing challenges. In this paper, several innovative planning scenarios are proposed to achieve high energy efficiency, low environment emission and reasonable economic cost. In the heating schemes, three innovative technologies are designed. The first technology is waste heat recovery based on the Co-generation-based absorption heat-exchange (Co-ah) cycle. The waste heat can be both from circulating water or flue gas in CHP heating system and the industrial waste heat recovery. The second technology is the heating network with large temperature difference. The third technology is the gas distributed peak-shaving, gas-driven absorption heat-exchange in the substation.

Topics: Design , Heating
Commentary by Dr. Valentin Fuster
2013;():V001T04A009. doi:10.1115/ES2013-18402.

In order to use real-time energy measurements to identify system operation faults and inefficiencies, a cooling coil energy baseline is studied in an air-handling unit (AHU) through an integration of physical models and a data driven approach in this paper. A physical model for an AHU cooling coil energy consumption is first built to understand equipment mechanism and to determine the variables impacting cooling coil energy performance, and then the physical model is simplified into a lumped model by reducing the number of independent variables needed. Regression coefficients in the lumped model are determined statistically through searching optimal fit using the least square method with short periods of measured data. Experimental results on an operational AHU (8 ton) are presented to validate the effectiveness of this approach with statistical analysis. As a result of this experiment, the proposed cooling energy baselines at the cooling coil have ±20% errors at 99.7% confidence. Six-day data for obtaining baseline is preferred since it shows similar results as 12-day.

Topics: Cooling , Ducts
Commentary by Dr. Valentin Fuster
2013;():V001T04A010. doi:10.1115/ES2013-18410.

Subcooling of the refrigerant at the exit of the condenser in a vapor compression refrigeration system could be an effective method to improve the coefficient of performance (COP). This method allows the refrigerant entering the evaporator with a lower mass fraction of vapor and absorbs more heat in the evaporator. The effort in this paper investigates a solar based integrated system of an electricity-driven vapor-compression chiller (VC) and an absorption heat pump (ABS) to provide both heating and cooling to space. Thermodynamic and heat transfer models of the integrated system were developed to estimate the system performance. The modeling results showed the integrated system can achieve 40% of the reduction on electricity consumption and 66% of the improvement in cooling COP. Furthermore, the hot water supply temperature can reach 50.25°C.

The models have also been used to conduct parametric sensitivity analysis. The key parameters which affect the performance of the system were the heat source temperature, the hot water return temperature, and the outdoor air flow rate. Hot water flow rate only has significant influence on the hot water supply temperature. Increasing the temperature and flow rate of the heat source can have benefits on both heating and cooling performance. However, increasing the outdoor air flow rate can only benefit on energy saving and cooling performance.

Commentary by Dr. Valentin Fuster

Materials and Micro/Nano Technologies for Energy Applications

2013;():V001T05A001. doi:10.1115/ES2013-18079.

Heat transfer to the storage fluid is a critical subject in thermal energy storage systems. The storage fluids that are proposed for supercritical thermal storage system are organic fluids that have poor thermal conductivity; therefore, pure conduction will not be an efficient heat transfer mechanism for the system. The current study concerns a supercritical thermal energy storage system consisting of horizontal tubes filled with a supercritical fluid. The results of this study show that the heat transfer to the supercritical fluid is highly dominated by natural convection. The buoyancy-driven flow inside the storage tubes dominates the flow field and enhances the heat transfer dramatically. Depending on the diameter of the storage tube, the buoyancy-driven flow may be laminar or turbulent. The natural convection has a significant effect on reducing the charge time compared to pure conduction. It was concluded that although the thermal conductivity of the organic supercritical fluids are relatively low, the effective laminar or turbulent natural convection compensates for this deficiency and enables the supercritical thermal storage to charge effectively.

Commentary by Dr. Valentin Fuster
2013;():V001T05A002. doi:10.1115/ES2013-18145.

Next generation Concentrating Solar Power (CSP) system requires high operating temperature and high heat storage capacity heat transfer fluid (HTF), which can significantly increase the overall system efficiency for power generation. In the last decade several research going on the efficacy of ionic liquids (ILs) as a HTF in CSP system. ILs possesses superior thermophysical properties compare to currently using HTF such as Therminol VP-1 (mixture of biphenyl and diphenyl oxide) and thermal oil. However, advanced thermophysical properties of ILs can be achieved by dispersing small volume percentage of nanoparticles forming nanofluids, which is called Nanoparticle Enhanced Ionic Liquids (NEILs). In the present study NEILs were prepared by dispersing 0.5% Al2O3 nanoparticles (spherical and whiskers) in N-butyl-N, N, N-trimetylammonium bis(trifluormethylsulfonyl)imide ([N4111][NTf2]) IL. Viscosity, heat capacity and thermal conductivity of NEILs were measured experimentally and compared with the existing theoretical models for liquid–solid suspensions. Additional, the convective heat transfer experiment was performed to investigate thermal performance. The thermal conductivity of NEILs enhanced by ∼5%, heat capacity enhanced by ∼20% compared to the base IL, which also gives 15% enhancement in heat transfer performance.

Commentary by Dr. Valentin Fuster
2013;():V001T05A003. doi:10.1115/ES2013-18164.

The purpose of this study is to analyze structural properties of biomass materials, namely corn stover. The structural properties of the biomass corn stover are examined at macro and fiber levels by performing a series of tests including three-point bending and tensile strength. Results of the stated tests are statistically analyzed. The goal of this analysis is to test the strength under loading from various directions to gather a full understanding of the structural properties of corn stalk fibers. Tests are performed using universal testing machines (UTMs). The results of these studies will be used to compile a database of the structural properties of biomass. These properties have the potential to be used in finite element computer simulations for structural analysis and bulk solid flows. The bulk fluid motion of the pulverized/chopped biomass can be simulated in storage and transportation equipment, including auguring screws and pneumatic conveyance systems, as well as devices for feeding biomass feedstocks in biorefineries. Traditional biochemical and thermochemical reactors operate as batch systems because of the difficulty of feeding the biomass feedstock in a continuous manner. Having a clearer background about the structural and rheological properties of biomass feedstock will help simulate and design the bulk-solid flows within storage bins and conveyance systems.

Commentary by Dr. Valentin Fuster
2013;():V001T05A004. doi:10.1115/ES2013-18337.

In this work, we present a low cost method for the fabrication of a heat exchanger utilizing metal-based microchannels using the LiGA technique. Lithography is used to pattern dry film negative photoresist (Ordyl P-50100) on the substrate. The resist is laminated over the substrate and exposed with a UV source. The use of dry film resist allows for simple and inexpensive microchannel patterns without requiring advanced cleanroom equipment. Following the lithography process, electrodepostion of metals is used to fill the recesses patterned in the resist. In this work, nickel has been electroplated into the bounding resist structure. After electroplating, the remaining resist is dissolved leaving free standing metal structures. The fabricated exchanger is then evaluated based on thermal absorption of simulated waste heat sources and capillary action of the metal channels themselves.

Channels are fabricated to heights of 60, 70 and 90 μm respectively using these methods. Working fluid mass transfer rate from the heated MHE is utilized as a basic metric of operation. The mass transfer rate recorded from the nickel-based MHE is 3.35, 3.37 and 3.41 mg/s respectively for the different channel heights. This implies an effective thermal power consumption rate of 2.54, 2.56 and 2.59 kW/m2 respectively.

Commentary by Dr. Valentin Fuster
2013;():V001T05A005. doi:10.1115/ES2013-18386.

We report improved performance of Li-ion polymer batteries through advanced gel polymer electrolytes (GPEs). Compared to solid and liquid electrolytes, GPEs are advantageous as they can be fabricated in different shapes and geometries; also ionic properties are significantly superior to that of solid and liquid electrolytes. We have synthetized GPE in form of membranes by trapping ethylene carbonate and propylene carbonate in a composite of polyvinylidene fluoride and N-methylpyrrolidinore. By applying phase-transfer method, we synthetized membranes with micro-pores, which led to higher ionic conductivity. The proposed membrane is to be modified further to have higher capacity, stronger mechanical properties, and lower internal resistance. In order to meet those requirements, we have doped the samples with gold nanoparticles (AuNPs) to form nanoparticle-polymer composites with tunable porosity and conductivity. Membranes doped with nanoparticles are expected to have higher porosity, which leads to higher ion mobility; and improved electrical conductivity. Four-point-probe measurement technique was used to measure the sheet resistance of the membranes. Morphology of the membranes was studied using electron and optical microscopies. Cyclic voltammetry and potentiostatic impedance spectroscopy were performed to characterize electrochemical behavior of the samples as a function of weight percentage of embedded AuNPs.

Commentary by Dr. Valentin Fuster
2013;():V001T05A006. doi:10.1115/ES2013-18387.

Au/α-Fe2O3 catalyst was synthesized using a modified co-precipitation method to generate an inverse catalyst model. The effects of introducing CO2 and H2O during preferential oxidation (PROX) of CO were investigated. The goal of this work was ≥99.8% CO conversion at 80°C. There was an increase in the conversion at all temperatures with the introduction of CO2 and 100% of the CO was converted at the target temperature of 80°C for any amount of CO2. Furthermore, there was an increase in conversion to 100% for water fractions ranging from 3% to 10%.

Finally, for realistic conditions of (bio-)fuel reforming, 24% CO2 and 10% water, 99.85% conversion was achieved. A long-term test of 200 hours showed no significant deactivation of the catalyst at a temperature of 80°C in presence of 24% CO2 and 3% water. The mechanism for PROX is not known definitively, however, current literature believes the gold particle size is the key. In contrast, we emphasize the tremendous role of the support particle size.

Commentary by Dr. Valentin Fuster
2013;():V001T05A007. doi:10.1115/ES2013-18388.

In the present study, a catalyst produced by flame spray pyrolysis (FSP) was evaluated for its ability to produce hydrogen-rich gas mixtures. Catalyst particles fabricated by a novel flame spray pyrolysis method resulting in a highly active catalyst with high surface-to-volume ratio were compared to a commercially produced catalyst (BASF F3-01). Both catalysts consisted of CuO/ZnO/Al2O3 of identical composition (CuO 40wt%, ZnO 40wt%, Al2O3 20wt%). Reaction temperatures between 220 and 295 °C, methanol-water inlet flow rates between 2 and 50 μl/min, and reactor masses between 25 and 100 mg were tested for their effect on methanol conversion and the production of undesired carbon monoxide. 100% methanol conversion can be easily achieved within the operational conditions mentioned for this flame-made catalyst — at reactor temperatures of 255 °C (achievable with non-concentrating solar collectors) more than 80% methanol conversion can be reached for methanol-water inlet flow rates as high as 10 μl/min. The FSP catalyst demonstrates similar catalytic abilities as the BASF, produces a consistent gas composition and produces lower overall CO production. Furthermore, the FSP catalyst demonstrates a better suitability to fuel cell use through its higher resistance to degradation and smaller production of carbon monoxide over long-term use.

In the present study, the merits of using flame spray pyrolysis to produce CuO/ZnO/Al2O3 methanol steam reforming catalysts are examined, and directly compared to catalysts that are commercially produced in bulk pellet form, and then ground and sieved. The comparison is performed from several different perspectives: catalytic activity and CO production at various temperatures and fuel inlet flow rates; surface and structure characteristics are determined via scanning electron and transmission electron microscopy; surface area characteristics are determined via BET tests.

Commentary by Dr. Valentin Fuster
2013;():V001T05A008. doi:10.1115/ES2013-18394.

There is an active need to develop compact mass transfer systems for high efficiency gas-liquid absorption applications, such as solvent-based carbon capture and natural gas sweetening processes. The present paper focuses on the absorption of carbon dioxide in aqueous diethanolamine using microreactors having hydraulic diameters of 762, 508 and 254 μm. The mass transfer phenomenon was studied and characterized with respect to absorption efficiency and mass transfer coefficient. Parametric studies were conducted varying the liquid and gas phase concentrations. Liquid-side volumetric mass transfer coefficients as high as 620 s−1 were achieved, which is between 2–3 orders of magnitude higher than that reported for most conventional gas-liquid absorption systems. High levels of absorption efficiency, close to 100%, were observed under certain operating conditions. The presently observed process intensification was attributed to an increase in the specific interfacial area with reduction in the channel diameter.

Commentary by Dr. Valentin Fuster

Photovoltaics

2013;():V001T06A001. doi:10.1115/ES2013-18002.

This article presents significant experimental data about the dye-sensitized nano solar cells (DSSCs) using new photoelectrode fabrication technique to achieve low energy consumption process purposes. The typical nano TiO2 solution preparation usually spends some hours for nano TiO2 particle dispersion with extremely high input ultrasonic energy. Moreover, a sintering process for adhesion enhancement often requires temperature over 500°C, which also results in very large energy consumption. This work develops a composite apparatus for nano TiO2 particle dispersion using ultrasonic dispersion with help of agitation to reduce the dispersion time and uses the conductive binder to replace the sintering process. The new developed fabrication process of the photoelectrode spends 30 mins with input power ca. 300 W for dispersion and 10 mins with 150°C for drying, which saves ca. 70% energy. The result shows that short-circuit density, open-circuit voltage, fill factor, and photoelectric conversion efficiency of the best DSSCs in this work are 3.04 mA/cm2, 0.80 V, 49%, and 1.18% respectively.

Commentary by Dr. Valentin Fuster
2013;():V001T06A002. doi:10.1115/ES2013-18039.

Concentrated photovoltaic (CPV) is an alternative solution to reduce the cost of solar PV systems by using less semiconductor materials. One key component in CPV systems is a solar tracker that enables to keep them in an optimal position to maximize solar concentration. However, CPV solar trackers typically are expensive, often unreliable and require lots of power, because they are composed of bulky, complex and heavy mechanical moving parts such as motors and supporting frames. These bulky and heavy tracking components make CPV systems difficult to be installed on building or residential rooftop.

We present a microfluidic tunable liquid prim panel that enables to track the daily and seasonal sun’s motion and concentrate steered sunlight onto a solar cell for solar power generation. The panel consists of arrayed tunable liquid prisms. An apex angle in each prism is tuned by electrowetting, which allows incident light to be adaptively steered and focused onto a solar cell. Our systems consume very little power in the range of ∼mW as well as require no heavy and expensive supporting hardware or moving parts for solar tracking. We discuss concept, design and analytical estimation of the system performances. It is able to steer incoming light beam with incident angle up to α = ± 70°, while only causing additional optical reflection loss about 5 ∼ 10%. We have fabricated the liquid prism with a 1cm × 1cm aperture area and demonstrated apex angle modulation up to φ = ± 30°, and beam steering up to Δα = 14.6°. By eliminating expensive and inefficient motor-driven mechanical solar trackers, our optofluidic solar tracking system can offer a cost-effective CPV system with low power consumption for residential or building rooftop installation.

Commentary by Dr. Valentin Fuster
2013;():V001T06A003. doi:10.1115/ES2013-18248.

Rising electricity prices, falling photovoltaic (PV) system costs and the availability of net metering are encouraging consumers to consider PV systems. However, the variety and complexity of utility rate structures can be a formidable barrier to consumers in making economically informed decisions. This paper describes a methodology to integrate Green Button energy use data from electric utilities, with solar and temperature data to analyze the economics of PV systems, with and without battery storage, under different rate structures. Case study results indicate that the economics of PV systems are nearly identical under PG&E’s time-of-use and inverted-block rate structures, and are more favorable than under flat rate structures with the same average annual cost per kWh. However, simple paybacks remain well short of the typical life of PV systems. The simple payback for the addition of batteries is initially competitive with PV systems, but rises rapidly as battery size is increased.

Commentary by Dr. Valentin Fuster
2013;():V001T06A004. doi:10.1115/ES2013-18349.

This paper presents preliminary results of a field study focused in the study of the heat patterns of a PV array in tropical conditions. The research system is comprised by four sub arrays of four mono-crystalline Silicon PV Modules. The system was installed facing to the South direction in a static configuration according to the geographical location of the study site. A set of temperature sensors were installed on the back of the PV module in order to monitor their thermal patterns on daily basics. Ambient temperature, solar radiation on the PV surface and on the horizontal surface as well as the wind speed and wind direction have been also monitored concurrently with the thermal patterns of the whole PV array under study.

Commentary by Dr. Valentin Fuster

Policy, Education, and Economic Challenges of Alternative Energy Sources

2013;():V001T07A001. doi:10.1115/ES2013-18257.

In this paper, research will be discussed on how to scientifically, systematically, and economically reduce greenhouse gas emissions within the state of West Virginia, USA. While fossil fuels such as coal and natural gas remain the top resources within this particular state, there are new technologies, different approaches and modifications to current power generation cycles, and different fuels that can be presented to gain further reduction of these harmful emissions.

To achieve this objective, eight different scenarios were introduced. In the first scenario, existing power stations’ fuel was switched to natural gas. Existing power plants were replaced by natural gas combined cycle (NGCC), integrated gasification combined cycle (IGCC), solid oxide fuel cell (SOFC), hybrid SOFC, and SOFC-IGCC hybrid power stations in scenarios number 2 to 6, respectively. The last two scenarios involved carbon capture systems. It has been found that the CO2 emissions can be significantly reduced by introducing changes and alternatives to the current cycles and methods that are in place today.

Commentary by Dr. Valentin Fuster
2013;():V001T07A002. doi:10.1115/ES2013-18264.

This paper introduces the newly established renewable energy engineering program at Alfred University (AU) and a solar house project used as an educational tool for this program. Topics include solar energy harvesting and adaptation of solar systems, engineering calculations/simulation, and global collaboration and marketing.

Commentary by Dr. Valentin Fuster
2013;():V001T07A003. doi:10.1115/ES2013-18284.

Solar houses are important for Chinese farmers to improve quality of life. What are the farmers’ perceived values (FPVs)? In this paper, we report a focus group study in Chongqing, China. Four constructs are considered in the FPV model: economic, quality, social and emotional values. The most important construct is the economic value and the second one is the quality value. Between social and emotional values, this study cannot yield the proper ranking for the third and fourth places.

Topics: Solar energy , China
Commentary by Dr. Valentin Fuster

Posters in Energy Topics

2013;():V001T08A001. doi:10.1115/ES2013-18265.

The degradation of silicon nanostructure / poly(3,4-ethylenedioxylthiophene : poly(styrenesulphonic acid) (SiNS/PEDOT:PSS) hybrid solar cell due to the moisture is investigated with an environmental chamber. The unencapsulated devices were tested under different relative humidity (RH) varied from (15% to 100%). Under different RH, the devices show various degradation trends. After 3hrs of storage under 100% RH, the average device power conversion efficiency (PCE) drops from 6.52% to 1.27%. While the device is stored under 15% RH, the averaged PCE just drop from 6.40% to 5.49% and the device at 60% RH degrades from 5.97% to 3.12%. To understand the cause of the device degradation, we compare the ITO conductivity and apply tunneling electron microscopy (TEM) to study the growth of the silicon dioxide layer on the silicon nanostructures. We confirmed that the major cause of the PCE drop in the current devices are due to the decrease of the PEDOT:PSS conductivity and the increase of the interface resistances. By re-depositing the PEDOT:PSS layer onto the degraded device and recycling the Si (and fresh ITO), we demonstrated that the efficiency of the device can be partially recovered (to fully recovered). The current work not only highlighted the importance of the humidity control in these SiNS/PEDOT:PSS hybrid solar cells, but also identified the major causes of the device degradation. The observation has been re-confirmed by recovering the PCE of the degraded device with a fresh PEDOT:PSS layer and a fresh ITO.

Commentary by Dr. Valentin Fuster

Smart Grid, Micro-Grid Concepts; Energy Storage

2013;():V001T09A001. doi:10.1115/ES2013-18103.

Phase-change materials (PCMs) can store large amounts of heat without significant change of their temperature during the phase-change process. This effect may be utilized in thermal energy storage, especially for solar-thermal power plants. In order to enhance the rate of heat transfer into PCMs, one of the most common methods is the use of fins which increase the heat transfer area that is in contact with the PCM.

The present work deals with a latent heat thermal storage device that uses a finned tube with an array of radial fins. A heat transfer fluid (HTF) flows through the tube and heat is conducted from the tube to the radial fins that are in contact with the bulk of the PCM inside a cylindrical shell. The thermal storage charging/discharging process is driven by a hot/cold HTF inside the tube that causes the PCM to melt/solidify.

The main objective of the present work is to demonstrate that close-contact melting (CCM) can affect the storage unit performance. Accordingly, two different types of experiments are conducted: with the shell exposed to ambient air and with the shell submerged into a heated water bath. The latter is done to separate the PCM from the shell by a thin molten layer, thus enabling the solid bulk to sink. The effect of the solid sinking and close-contact melting on the fins is explored. It is found that close-contact melting shortens the melting time drastically.

Accordingly, two types of models are used to predict the melting rate: numerical CFD model and analytical/numerical close-contact melting model. The CFD model takes into account convection in the melt and the PCM property dependence on temperature and phase. The analytical/numerical CCM model is developed under several simplifying assumptions. Good agreement is found between the predictions and corresponding experimental results.

Topics: Fins , Latent heat , Storage
Commentary by Dr. Valentin Fuster
2013;():V001T09A002. doi:10.1115/ES2013-18118.

The availability of cost effective storage capacity is considered essential for increasing the share of renewables in electricity generation.

With the development of solar thermal power plants large thermal storage systems have become commercial in recent years. Various storage concepts are applied, systems using solid storage media are operated at a maximum temperature of 680 °C, other systems using molten salt as storage medium show thermal capacities in the GWh range. Heating these storage systems directly by surplus electricity and using the heat later during the discharge process to operate turbines is not very attractive, since the process is limited by the Carnot efficiency.

Alternatively, surplus electricity can be used to transform low temperature heat into high temperature heat which is stored in a thermal storage system during the charging process. During discharge, this heat is used to drive a turbine generating electric energy. Theoretically, this concept allows a roundtrip efficiency of 100%.

Various options for the implementation of this storage concept have been suggested, using air or CO2 as working fluids. Recently, DLR has demonstrated the operability of a latent heat storage system connected to a steam circuit at 100 bar. The availability of this latent heat storage technology allows new implementations of the storage concept based on heat transformation. Using a left-running Rankine cycle during the charging process, heat from the environment is used to evaporate steam, which is compressed using the surplus electricity. Superheated steam exiting the compressor flows through the thermal storage system composed of latent heat storage sections and sensible heat storage sections. After throttling, the water enters the evaporator again. During discharging, heat from the storage system is used to evaporate and superheat steam, which drives the turbine. A cascaded implementation of this concept, using ammonia for the low temperature part of the process, while water is used for the high temperature part, reaches a storage efficiency of 70%. The integration of low temperature waste heat sources allows the compensation of losses.

Commentary by Dr. Valentin Fuster
2013;():V001T09A003. doi:10.1115/ES2013-18142.

A latent thermal storage system LTSS of spherical PCM capsules packed bed is considered in this investigation. Such a LTSS is considered to be one of the most practical systems for the purpose. It is compact, easy to construct and maintain, with very good storage characteristics. Physiochemical properties of PCM and the capsules size are the two most effective properties of the storage system. Number of researchers investigated the performance of such systems under different designs and operating conditions. In this work a LTSS system is constructed with PCM capsules of different sizes and of different PCM properties. The capsules are packed with different capsules sizes and varying PCM properties along the bed to match the temperature profile best. The performance of the LTSS as described is investigated numerically and presented in terms of the bed temperature profiles function of location and charging time. The effectiveness of the storage system is presented in terms of time evolution of the liquid fraction of the PCM and in terms of the percentage of the energy storage in the PCM. The first represent the degree of utilization of the PCM and the later shows the effectiveness of the heat transfer and storage process of the LTSS for different system designs. The results as above are compared with the reference cases of uniform arrangements with uniform and constant PCM properties along the bed.

Commentary by Dr. Valentin Fuster
2013;():V001T09A004. doi:10.1115/ES2013-18204.

A passive induced cooling system using phase change material (PCM) energy storage is presented in this analysis for providing indoor cooling and energy saving. Also, the latent heat performance of the PCM is analyzed. The supplied cooling capacity was evaluated using an indoor cooling temperature performance while the PCM characteristic performance was achieved by relating the applications sensible heat ratio efficiency to the charging and discharging effectiveness of the PCM. This is carried out for an office building in a warm humid climate. Obtained result delivered 24.54 % of the required indoor cooling load for 24°C indoor cooling temperature. Moreover, delivered indoor cooling capacity increased at constant increasing mean indoor temperature and PCM melting temperatures. Application sensible heat ratio efficiency was 77.66 % and average energy saving of 37.77 % in total energy operation cost was obtained. A CO2 emission reduction of 0.071 tons can also be achieved by the system.

Commentary by Dr. Valentin Fuster
2013;():V001T09A005. doi:10.1115/ES2013-18390.

It is widely understood that energy storage is the key to integrating variable generators into the grid. It has been proposed that the thermal mass of buildings could be used as a distributed energy storage solution and several researchers are making headway in this problem. However, the inability to easily determine the magnitude of the building’s effective thermal mass, and how the heating ventilation and air conditioning (HVAC) system exchanges thermal energy with it, is a significant challenge to designing systems which utilize this storage mechanism. In this paper we adapt modal analysis methods used in mechanical structures to identify the primary modes of energy transfer among thermal masses in a building. The paper describes the technique using data from an idealized building model. The approach is successfully applied to actual temperature data from a commercial building in downtown Boise, Idaho.

Commentary by Dr. Valentin Fuster

Solar Chemistry

2013;():V001T10A001. doi:10.1115/ES2013-18040.

A transient heat transfer model is developed for a solar reactor prototype for H2O and CO2 splitting via two-step non-stoichiometric ceria cycling. Counter-rotating cylinders of reactive and inert materials cycling between high and low temperature zones permit continuous operation and heat recovery. To guide the reactor design a transient three-dimensional heat transfer model is developed based on transient energy conservation, accounting for conduction, convection, radiation, and chemical reactions. The model domain includes the rotating cylinders, a solar receiver cavity, and insulated reactor body. Radiative heat transfer is analyzed using a combination of the Monte Carlo method, Rosseland diffusion approximation, and the net radiation method. Quasi-steady state distributions of temperatures, heat fluxes, and the non-stoichiometric coefficient are reported. Ceria cycles between temperatures of 1708 K and 1376 K. A heat recovery effectiveness of 28% and solar-to-fuel efficiency of 5.2% are predicted for an unoptimized reactor design.

Commentary by Dr. Valentin Fuster
2013;():V001T10A002. doi:10.1115/ES2013-18042.

A dynamic numerical model of a solar cavity-type reactor for the thermal dissociation of ZnO is formulated based on a detailed radiative heat transfer analysis combining the Monte Carlo ray-tracing technique and the radiosity enclosure theory. The quartz window is treated as a semi-transparent spectrally-selective glass layer with directionally dependent optical properties. Model validation is accomplished by comparison with experimental results obtained with a 10-kW solar reactor prototype in terms of cavity temperatures, reaction extents, and quartz window temperature distribution measured by IR thermography. The solar-to-fuel energy conversion efficiencies obtained experimentally are reported and the various energy flows are quantified.

Commentary by Dr. Valentin Fuster
2013;():V001T10A003. doi:10.1115/ES2013-18050.

We present a kinetic study performed in a solar-driven vacuum thermogravimeter (solar-TG), in which solid reactants are directly exposed to high-flux irradiation while their weight change is continuously monitored. The system allows testing under a total vacuum pressure as low as 10 mbar. With this arrangement, the rate of thermochemical reactions can be examined under the same radiative heat transfer characteristics and heating rates typical of solar reactors. The solar-TG system is used to investigate metal oxides redox cycles for splitting H2O and CO2 and for high-temperature heat storage. Operation of the metal oxide reduction under vacuum pressures is of special interest because it eliminates the need for purge gas, thus simplifying the process and avoiding energy penalties associated with inert gas recycling.

Topics: Metals , Vacuum , Solar energy
Commentary by Dr. Valentin Fuster
2013;():V001T10A004. doi:10.1115/ES2013-18156.

Kinetic analysis is essential for chemical reactor modeling. This study proposes a methodology to use available kinetic analysis methodologies, including conventional (modelistic) graphical representation, isoconversional (model free), models based on first principles and reduced time scale analysis (Sharp and Hancock procedure) to predict the kinetics of an investigated reaction. Even though these methods have some limitations, a methodology comprised of combining their results can help in determining the kinetic parameters for reaction. The isoconversional approach can be used to determine the activation energy without the need of using a reaction model. The modelistic graphical representation can aid is determining the group (i.e. diffusion, first order, phase boundary or nucleation) to which the reaction generally belongs. The reduced time scale analysis can guide in isolating the reaction kinetics in the early stages of the reaction when the conversion ranges between 0.15 and 0.5. This proposed methodology uses the various methods and applies them to experimental data for high temperature reactions in fluidized bed reactors. Particular attention is given to steam driven iron oxidation kinetics for hydrogen production. When only the modelistic approach is used, the activation energy computed using the selected models varies from 59–183 kJ/mol, depending on the model used. However, by combining the predictive capabilities of various approaches discussed in this study, the activation energy range narrows to 80–147 kJ/mol. It is also shown that the iron oxidation with steam under the studied conditions can be described by a combination of two models. The early stage of the reaction is represented by either a contracting volume or first order model. Later stages of reaction can be described by either a contracting volume, first order or 3-D diffusion model. In addition, when analyzing reaction kinetics using a fundamental approach, it is observed that the fluidized bed oxidation reaction of iron with pure steam can be best represented by a combination of two mechanisms, namely shrinking sphere surface area and diffusion controlled mechanisms and the estimated activation energy is 103 kJ/mol.

Commentary by Dr. Valentin Fuster
2013;():V001T10A005. doi:10.1115/ES2013-18254.

This paper reports the synthesis, characterization and evaluation of different weight loadings of cobalt ferrite (CoFe2O4) in 8 mol% yttria-stabilized zirconia (8YSZ) via the co-precipitation method. Prepared powders were calcined at 1350 °C for 36 hours and 1450 °C for 4 hours in air. These powders were then formed into a porous structure using sacrificial pore formation via oxidation of co-mixed graphite powder. These formed structures obtained were then characterized using thermogravimetric analysis (TGA), X-ray diffraction (XRD), high temperature X-ray diffraction (HT-XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Brunauer-Emmett-Teller (BET) surface area analysis was performed on the most promising of the structures before being subjected to 50 thermal reduction-CO2 oxidation (redox) cycles using TGA. Together, these results indicate that CoFe2O4-8YSZ can provide a lower reduction temperature, maintain syngas production performance from cycle to cycle, and enhance utilization of the reactive material within the inert support in comparison to iron oxide only structures.

Commentary by Dr. Valentin Fuster
2013;():V001T10A006. doi:10.1115/ES2013-18403.

In this study we have developed a unique method for synthesizing very reactive water splitting materials that will remain stable at temperatures as high as 1450 °C to efficiently produce clean hydrogen from concentrated solar energy. The hydrogen production for a laboratory scale reactor using a “Thermo-mechanical Stabilized Porous Structure” (TSPS) is experimentally investigated for oxidation and thermal reduction temperatures of 1200 and 1450 °C, respectively. The stability and reactivity of a 10 g TSPS over many consecutive oxidation and thermal reduction cycles for different particle size ranges has been investigated. The novel thermo-mechanical stabilization exploits sintering and controls the geometry of the matrix of particles inside the structure in a favorable manner so that the chemical reactivity of the structure remains intact. The experimental results demonstrate that this structure yields peak hydrogen production rates of 1–2 cm3/(min.gFe3O4) during the oxidation step at 1200 °C and the 30 minute thermal reduction step at 1450 ° C without noticeable degradation over many consecutive cycles. The hydrogen production rate is one of the highest yet reported in the open literature for thermochemical looping processes using thermal reduction. This novel process has strong potential for developing an enabling technology for efficient and commercially viable solar fuel production.

Commentary by Dr. Valentin Fuster

SunShot CSP Symposium

2013;():V001T11A001. doi:10.1115/ES2013-18069.

As one of the viable concentrating solar power (CSP) technologies, linear Fresnel collectors differ from parabolic troughs by virtue of their low-profile mirror arrays and fixed receiver assemblies. This technology is capable of achieving high concentration ratios and so is applicable to high-temperature solar power plant designs. In addition, its low wind profile and linear nature lead to low system and operation and maintenance (O&M) costs.

In this report two linear Fresnel solar plant configurations, namely a direct steam generation (DSG) system and a direct high-temperature molten-salt plant, are examined via a levelized cost of electricity (LCOE) analysis. By treating LCOE as a function of the annual investment energy return (IER, or the ratio of annual net electricity to the total direct system cost) under various assumptions of O&M cost, a few plant scenarios employing high-temperature linear Fresnel technology are carefully configured to meet the aggressive LCOE goals of 8 cents/kWh and 6 cents/kWh. The latter is the Department of Energy (DOE) SunShot Initiative goal aimed at making CSP cost competitive in the current energy market. In particular, a linear Fresnel scenario with the potential to meet the SunShot goal is featured with a collector cost of $100/m2, an annual system energy efficiency of 18%, a storage system cost of $15/kWh-th, and an O&M cost of $7.5/MWh. One of the most aggressive assumptions is an advanced power block with about 52% cycle efficiency and a turbine inlet temperature of 700°C.

This work addresses unanswered questions regarding linear Fresnel cost and performance and identifies future research and development directions for linear Fresnel technology, including economic optimization of collectors and receivers, development of physical plant performance models, development of automated O&M mechanisms and sophisticated plant control software.

Commentary by Dr. Valentin Fuster
2013;():V001T11A002. doi:10.1115/ES2013-18149.

We present an investigation of the effects of the solar irradiation and mass flow conditions on the behavior of a Small Particle Solar Receiver employing our new, three-dimensional coupled fluid flow and radiative heat transfer model. This research expands on previous work conducted by our group and utilizes improved software with a set of new features that allows performing more flexible simulations and obtaining more accurate results. For the first time, it is possible not only to accurately predict the overall efficiency and the wall temperature distribution of the solar receiver, but also to determine the effect on the receiver of the window, the outlet tube, real solar irradiation from a heliostat field, non-cylindrical geometries and 3-D effects. This way, a much better understanding of the receiver’s capabilities is obtained. While the previous models were useful to observe simple trends, this new software is flexible and accurate enough to eventually act as a design and optimization tool for the actual receiver.

The solution procedure relies on the coupling of the CFD package ANSYS Fluent to our in-house Monte Carlo Ray Trace (MCRT) software. On the one hand, ANSYS Fluent is utilized as the mass-, momentum- and energy-equation solver and requires the divergence of the radiative heat flux, which constitutes a source term of the energy equation. On the other hand, the MCRT software calculates the radiation heat transfer in the solar receiver and needs the temperature field to do so. By virtue of the coupled nature of the problem, both codes should provide feed-back to each other and iterate until convergence. The coupling between ANSYS Fluent and our in-house MCRT code is done via User-Defined Functions.

After developing the mathematical model, setting up and validating the software, and optimizing the coupled solution procedure, the receiver has been simulated under fifteen different solar irradiation and mass flow rate cross combinations. Among other results, the behavior of the receiver at different times of the day and the optimum mass flow rate as a function of the solar thermal input are presented. On an average day, the thermal efficiency of the receiver is found to be over 89% and the outlet temperature over 1250 K at all times from 7:30 AM to 4:00 PM (Albuquerque, NM) by properly adapting the mass flow rate. The origin of the losses and how to improve the efficiency of the Small Particle Solar Receiver are discussed as well.

Commentary by Dr. Valentin Fuster
2013;():V001T11A003. doi:10.1115/ES2013-18186.

Concentrated solar power (CSP) systems use heliostats to concentrate solar radiation in order to produce heat, which drives a turbine to generate electricity. We, the Combustion and Solar Energy Laboratory at San Diego State University, are developing a new type of receiver for power tower CSP plants based on volumetric absorption by a gas-particle suspension. The radiation enters the pressurized receiver through a window, which must sustain the thermal loads from the concentrated solar flux and infrared reradiation from inside the receiver. The window is curved in a dome shape to withstand the pressure within the receiver and help minimize the stresses caused by thermal loading. It is highly important to estimate how much radiation goes through the window into the receiver and the spatial and directional distribution of the radiation. These factors play an important role in the efficiency of the receiver as well as window survivability.

Concentrated solar flux was calculated with a computer code called MIRVAL from Sandia National Laboratory which uses the Monte Carlo Ray Trace (MCRT) method. The computer code is capable of taking the day of the year and time of day into account, which causes a variation in the flux. Knowing the concentrated solar flux, it is possible to calculate the solar radiation through the window and the thermal loading on the window from the short wavelength solar radiation. The MIRVAL code as originally written did not account for spectral variations, but we have added that capability.

Optical properties of the window such as the transmissivity, absorptivity, and reflectivity need to be known in order to trace the rays at the window. A separate computer code was developed to calculate the optical properties depending on the incident angle and the wavelength of the incident radiation by using data for the absorptive index and index of refraction for the window (quartz) from other studies and vendor information. This method accounts for regions where the window is partially transparent and internal absorption can occur.

A third code was developed using the MCRT method and coupled with both codes mentioned above to calculate the thermal load on the window and the solar radiation that enters the receiver. Thermal load was calculated from energy absorbed at various points throughout the window. In our study, window shapes from flat to concave hemispherical, as well as a novel concave ellipsoidal window are considered, including the effect of day of the year and time of the day.

Topics: Solar energy
Commentary by Dr. Valentin Fuster
2013;():V001T11A004. doi:10.1115/ES2013-18211.

Thermal energy Storage is a critical component of Concentrating Solar Power (CSP) plant, enabling uninterrupted operation of plant during periods of cloudy or intermittent solar weather. Investigations of Latent Thermal Energy Storage (LTES) which utilizes Phase Change Material (PCM) as a heat storage medium is considered due to its high energy storage density and low capital cost. However, the low thermal conductivity of the PCM restricts the solidification rate of the PCM leading to inefficient heat transfer between the PCM and the HTF which carries thermal energy to the power block. To address this, LTES embedded with heat pipes and PCM’s stored within the framework of porous metal foams possessing one to two orders of magnitude higher thermal conductivity than the PCM are considered in the present study. A transient, computational analysis of the metal foam (MF) enhanced LTES system with embedded heat pipes is performed to investigate the enhancement in the thermal performance of the system for different arrangement of heat pipes and design parameter of metal foams, during both charging and discharging operation.

Commentary by Dr. Valentin Fuster
2013;():V001T11A005. doi:10.1115/ES2013-18213.

Integrating a thermal energy storage (TES) in a concentrating solar power (CSP) plant allows for continuous operation even during times when solar radiation is not available, thus providing a reliable output to the grid. In the present study, the cost and performance models of an encapsulated phase change material thermocline storage system are integrated with a CSP power tower system model to investigate its dynamic performance. The influence of design parameters of the storage system is studied for different solar multiples of the plant to establish design envelopes that satisfy the U.S. Department of Energy SunShot Initiative requirements, which include a round-trip exergetic efficiency greater than 95% and storage cost less than $15/kWht for a minimum discharge period of 6 hours. From the design windows, optimum designs of the storage system based on minimum LCOE, maximum exergetic efficiency, and maximum capacity factor are reported and compared with the results of two-tank molten salt storage system. Overall, this study presents the first effort to construct a latent thermal energy storage (LTES)-integrated CSP plant model, that can help decision makers in assessing the impact, cost and performance of a latent thermocline energy storage system on power generation from molten salt power tower CSP plant.

Commentary by Dr. Valentin Fuster
2013;():V001T11A006. doi:10.1115/ES2013-18215.

High temperature central receivers are on the forefront of concentrating solar power research. Current receivers use liquid cooling and power steam cycles, but new receivers are being designed to power gas turbine engines within a power cycle while operating at a high efficiency. To address this, a lab-scale Small Particle Heat Exchange Receiver (SPHER), a high temperature solar receiver, was built and is currently undergoing testing at the San Diego State University’s (SDSU) Combustion and Solar Energy Laboratory. The final goal is to design, build, and test a full-scale SPHER that can absorb 5 MWth and eventually be used within a Brayton cycle.

The SPHER utilizes air mixed with carbon particles generated in the Carbon Particle Generator (CPG) as an absorption medium for the concentrated solar flux. Natural gas and nitrogen are sent to the CPG where the natural gas undergoes pyrolysis to carbon particles and nitrogen is used as the carrier gas. The resulting particle-gas mixture flows out of the vessel and is met with dilution air, which flows to the SPHER.

The lab-scale SPHER is an insulated steel vessel with a spherical cap quartz window. For simulating on-sun testing, a solar flux is produced by a solar simulator, which consists of a 15kWe xenon arc lamp, situated vertically, and an ellipsoidal reflector to obtain a focus at the plane of the receiver window. The solar simulator has been shown to produce an output of about 3.25 kWth within a 10 cm diameter aperture. Inside of the SPHER, the carbon particles in the inlet particle-gas mixture absorb radiation from the solar flux. The carbon particles heat the air and eventually oxidize to carbon dioxide, resulting in a clear outlet fluid stream.

Since testing was initiated, there have been several changes to the system as we have learned more about the operation. A new extinction tube was designed and built to obtain more accurate mass loading data. Piping and insulation for the CPG and SPHER were improved based on observations between testing periods. The window flange and seal have been redesigned to incorporate window film cooling. These improvements have been made in order to achieve the lab scale SPHER design objective gas outlet flow of 650°C at 5 bar.

Commentary by Dr. Valentin Fuster
2013;():V001T11A007. doi:10.1115/ES2013-18217.

Windows are being evaluated for use in some high temperature solar receivers to reduce radiative and convective losses. The design process of a 1.7 meter diameter quartz dome window is explained to arrive at a window geometry able to maintain acceptable stresses when exposed to pressure differentials. The dome must be able to withstand the operational differential pressures of 0.5 MPa where the efficiency of the solar receiver/power cycle is maximized, and maximum temperatures upwards to 800°C may be observed. Brittle materials like glass need the tensile stresses to be reduced or eliminated to maximize the reliability of the dome window. However, glass does not possess a consistent characteristic strength and it is dependent on the flaw size. The dome mount is critical to maintaining an environmental seal, but careful attention must be taken in the glass-metal interface to minimize tensile bending stresses that can cause a catastrophic or rapid failure.

A method to characterize the strength of the quartz dome is discussed and aides in determining the maximum design stresses allowable during operation of the solar receiver. To determine an accurate model of the dome stress, a statistical analysis based on strength data has been carried out using the Weibull failure probability method. Finite Element Analysis (FEA) analysis of the dome is discussed, and a design trade study of the dome window geometry (ranging from a shallow angle to a full hemisphere) is presented. The combination of these perspectives will give insight on the process to design a glass dome window for this challenging environment and to predict the reliability.

Commentary by Dr. Valentin Fuster
2013;():V001T11A008. doi:10.1115/ES2013-18236.

This paper presents simulations and designs of a prototype falling particle solar receiver with air recirculation as a means of mitigating heat loss and impacts of external wind on particle flow. The flow and dispersion of different sizes of ceramic proppant (CARBO HSP) particles (1 mm, 100 μm, and 10 μm) were simulated in recirculating air flows without heating effects. Particles on the order of 0.1–1 mm yielded desirable simulated flow patterns when falling through the cavity receiver with an air-injection velocity of 3 m/s. Simulations of smaller particles on the order of 10 microns yielded unstable flow patterns that may lead to large losses of particles through the aperture. A prototype cavity receiver with air recirculation was designed and fabricated to validate the unheated simulations. The blower nozzles and suction plenum were engineered to yield the most uniform flow pattern along the entire width of the aperture. Observed and simulated air velocity distributions around the aperture and particle flow patterns using CARBO HSP particles with an average diameter of ∼0.7 mm were found to be qualitatively similar.

Commentary by Dr. Valentin Fuster
2013;():V001T11A009. doi:10.1115/ES2013-18283.

A sulfur based thermochemical energy storage cycle for baseload power generation is being developed under the support of US DOE Sunshot program. Solar heat is stored in elemental sulfur via thermal decomposition of sulfuric acid and disproportionation of sulfur dioxide into elemental sulfur and sulfuric acid. Heat energy is recovered upon sulfur combustion. On-sun decomposition of sulfuric acid in a solar furnace has been demonstrated between 650 and 850°C. Near equilibrium conversion was obtained at high temperature but conversion was reduced due to catalyst poisoning at the lower temperatures. Sulfur dioxide disproportionation modeling showed the reaction driving force is maximized at the high system pressure and low system temperature. The effect of system pressure was validated experimentally. However, the disproportionation rate was found to increase with system temperature as a result of increased reaction kinetics. Homogenous iodide catalysts were used to further enhance the degree of disproportionation and the reaction rate. The process steps required to recover the catalyst for reuse have been verified.

Commentary by Dr. Valentin Fuster
2013;():V001T11A010. doi:10.1115/ES2013-18299.

This paper introduces separate-type heat pipe (STHP) based solar receiver systems that enable more efficient operation of concentrated solar power plants without relying on a heat transfer fluid. The solar receiver system may consist of a number of STHP modules that receive concentrated solar flux from a solar collector system, spread the high concentrated solar flux to a low heat flux level, and effectively transfer the received heat to the working fluid of a heat engine to enable a higher working temperature and higher plant efficiency. In general, the introduced STHP solar receiver has characteristics of high heat transfer capacity, high heat transfer coefficient in the evaporator to handle a high concentrated solar flux, non-condensable gas release mechanism, and lower costs. The STHP receiver in a solar plant may also integrate the hot/cold tank based thermal energy storage system without using a heat transfer fluid.

Commentary by Dr. Valentin Fuster
2013;():V001T11A011. doi:10.1115/ES2013-18335.

Thermal energy storage (TES) system integrated with concentrated solar power provides the benefits of extending power production, eliminating intermittency issues, and reducing system LOCE. Infinia Corporation is under the contract with DOE in developing TES systems. The goal for one of the DOE sponsored TES projects is to design and build a TES system and integrate it with a 3 KWe free-piston Stirling power generator. The Phase Change Material (PCM) employed for the designed TES system is a eutectic blend of NaF and NaCl which has a melt temperature of 680° C and energy storage capacity of 12 KWh. This PCM was selected due to its low cost and desired melting temperature. This melt temperature ensures the Stirling being operated at designed operating hot end temperature. The latent heat of this eutectic PCM offers 5 to 10 times the energy density of a typical molten salt. The technical challenges associated with low cost molten salt TES systems are the low thermal conductivity of the salt and large thermal expansion. To address these challenges, an array of sodium filled Heat Pipes (HP) is embedded in the PCM to enhance the heat transfer from solar receiver to PCM and from PCM to Stirling engine. The oversized dish provides sufficient thermal energy to operate a 3KWe Stirling engine at full power and to charge up the TES. The HP arrays are optimally distributed so that the solar energy is transferred directly from receiver to Stirling engine heat receiver. During the charge phase, the Stirling engine absorbs and converts the transferred solar energy to electricity and the excess thermal energy is re-directed and stored to PCM. The stored energy is transferred via distributed HP from PCM to Stirling engine heat receiver during discharge phase.

The HP based PCM thermal energy storage system was designed, built, and performance tested in laboratory. The TES/engine assembly was tested in two different orientations representing the extremes of system operation when mounted on sun-tracking dish, horizontal and vertical. Horizontal represents the zero elevation at sun rise and the vertical represents the extreme of solar noon. The testing allows the examination of orientation effect on the heat pipe performance and the maximum charge and discharge rates. The total energy stored and extracted was also examined. The areas for further system refinements were identified and discussed.

Commentary by Dr. Valentin Fuster
2013;():V001T11A012. doi:10.1115/ES2013-18353.

This computational study investigates design of microchannel based solar receiver for use in concentrated solar power. A design consisting of a planar array of channels with solar flux incident on one side and using supercritical carbon dioxide as the working fluid is sought. Use of microchannels is investigated as they offer enhanced heat transfer in solar receivers and have the potential to dramatically reduce the size and increase the performance. Designs are investigated for an incident heat flux of 1 MW/m2, up to 3.3 times that of current solar receivers [1], resulting in significant reduction of size and cost. The goal is to design a microchannel receiver with inlet and outlet temperatures of the working fluid of 500°C and 650°C, operating pressure of 100 bar, pressure drop less than 0.35 bar and surface efficiency greater than 90% defined by radiation and convection losses to the environment. Three micro-channel designs are considered: rectangular cross section with high and low aspect ratio (designs A and B) and rectangular cross section with an array of micro pin-fins of various shape spanning the height of the channel (design C). Numerical simulations are performed on individual channels and on a unit cell of the pin-fin design. Structural analysis is performed to ensure that the design can withstand the operating pressure and thermal stresses. The effects of flow maldistribution and header system in an array of channels are also investigated. Preliminary results show that all three designs are capable of meeting the requirements, with the pin-fin design having the lowest pressure drop and highest efficiency.

Commentary by Dr. Valentin Fuster
2013;():V001T11A013. doi:10.1115/ES2013-18365.

Concentrated Solar Power (CSP) systems used for photothermal conversion of solar energy to electricity are capable of meeting a large fraction of the global energy requirements. CSP plants are inherently robust with respect to the availability of materials, technology, and energy storage. However, dust depositions on solar collectors cause energy yield loss annually, ranging from 10 to 50% depending upon their location in the semi-arid and desert lands. Mitigation of energy loss requires manual cleaning of solar mirrors with water. A brief review of the soiling related losses in energy yield of the CSP plants is presented, which shows that cleaning of the CSP mirrors and receivers using water and detergent is an expensive and time-consuming process at best and is often impractical for large-scale installations where water is scarce. We report here our research effort in developing an electrodynamic dust removal technology that can be used for keeping the solar collectors clean continuously without requiring water and manual labor. Transparent electrodynamic screens (EDS), consisting of rows of transparent parallel electrodes embedded within a transparent dielectric film can be integrated on the front surface of the mirrors and on the receivers for dust removal for their application as self-cleaning solar collectors. When the electrodes are activated, over 90% of the deposited dust is removed. A summary of the current state of prototype development and evaluation of EDS integrated solar mirrors and experimental data on the removal of desert dust samples are presented. A brief analysis of cost-to-benefit ratio of EDS implementation for automated dust removal from large-scale solar collectors is included.

Commentary by Dr. Valentin Fuster
2013;():V001T11A014. doi:10.1115/ES2013-18395.

Inorganic materials and organic salts are usually used as phase change materials (PCMs) for thermal energy storage. Some of these materials have high latent heat of fusion; however one major drawback of these materials is the low thermal conductivity, which limits the rate of charging and discharging process. In this paper, we studied metallic alloys (eutectic alloys or alloys with a narrow melting temperature range) as phase-change materials, which have both high thermal conductivity and high latent heat of fusion. A formula was presented from entropy change to predict the latent heat of fusion of metallic alloys. We found that the latent heat of fusion of alloys can be expressed from three different contributions: the latent heat from each element, the sensible heat, and the mixing entropy. From the theory we also showed that latent heat of fusion could be greatly increased by maximizing the entropy of mixing, which can be realized by introduce more elements in the alloys, i.e., form ternary alloys by adding elements to binary alloys. This idea is demonstrated by the synthesis and measurement of the binary alloy 87.8Al-12.2Si (at%) and ternary alloy 45Al-40Si-15Fe (at%). The metallic alloy is synthesized by hot pressing method. The latent heat of fusion of 45Al-40Si-15Fe (at%) is about 865 kJ/kg with melting temperature from 830 °C to 890 °C from the differential scanning calorimetry (DSC) measurement, comparing with 554.9 kJ/kg and 578.3 °C for the binary alloy 87.8Al-12.2Si (at%). From the binary to the ternary alloy, the contribution to the latent heat from mixing entropy increases by 17%.

Commentary by Dr. Valentin Fuster

Sustainable Cities and Communities, Transportation (JOINT)

2013;():V001T12A001. doi:10.1115/ES2013-18166.

Costs of Helium are increasing more rapidly than provided by economic analysts because of the high demanded if compared with actual production possibilities. These economic disadvantages are slowing many airship projects, because of the necessary costs that need to be faced for initial inflation. They are forcing the definition of novel airship architectures which could use hydrogen as buoyant gas with a higher level of safety than in the past.

This paper starts from those economic issues to define a general model based on the variable volume airship defined in the MAAT cruiser-feeder airship project. It defines a future hydrogen airships concept which could avoid the disasters of the past and maximize the safety of people and goods.

The model has been compared with the results of an optimization of the very general airship conceptual design based on Constructal theory. The goal of the optimization has been defined in terms of maximization of solar energy caption possibility, minimization of frontal area, volume, and energy consumption at a predefined operative altitude. A surprising accordance of the proposed model with the results of the Constructal optimization results has been obtained.

A comparison with a traditional model of airship has been performed and results have been validated by CFD simulation at different altitudes.

Commentary by Dr. Valentin Fuster
2013;():V001T12A002. doi:10.1115/ES2013-18167.

This paper presents the EVITA electric car. EVITA is the acronym of Electric Vehicle Improved by Three-phase Asynchronous cooled motor. It is a research project developed jointly by RGEngineering and University of Modena and Reggio Emilia. It aims to produce a novel electric power train with the capability of solving three fundamental problems of today commercial electric vehicles:

1. direct torque dependency of the rotation speed, and its reduction at high speed regimes;

2. electric motors performances reduction due to the overheating effects under heavy load conditions;

3. acclimatization of the car cabin interior in winter times.

Commentary by Dr. Valentin Fuster
2013;():V001T12A003. doi:10.1115/ES2013-18347.

Nowadays there is a trend that more people prefer to live in cities than in rural areas, so there is the necessity of different urbanization strategies with various transportation schemes. While in most cities in the US the population density is very low with an interval of 20 to 50 inhabitants per hectare, in cities like Hong Kong more than 250 inhabitants live on an area of one hectare. As a consequence transportation needs are different, for example gasoline driven cars as preferred transportation systems for US citizens, giving rise to an average consumption of 2000 liters per year for every inhabitant; in comparison to Hong Kong, where average consumption is only 300 Liters per year. So there is a great challenge to reduce these emissions. Apparently solutions are not planned in relation to studies by the International Energy Agency that places the United States as biggest crude oil producer in the year 2017, due the increments of tight oil exploitation. In the shown scenery electro mobility can be a solution for sustainable transportation, when it is combined with PV systems.

This study starts with the feasibility of such a combination, and shows the coverage of population where this system can be implemented. From the analysis of economy results: It can be observed that the return of investment of a PV-system can be reduced when it is combined with electric mobility. Today cars driven by an electronic motor with cost of 0.035±0.015 USD/km afforded for transportation energy are more efficient than gasoline motors (based on fuel costs of 0.5±0.1 USD/km for economic cars with a coverage of 14 km/L). So it’s demonstrated that the return of investment is minor than 5 years for the PV system. It’s worth mentioning that charging batteries of electric cars could help to smooth energy peaks in the grid caused by renewables like sun and wind energy (a challenge for further grid systems). Besides the economic study the ecological impact has to be evaluated.

Commentary by Dr. Valentin Fuster

Sustainable Fuels and Infrastructure

2013;():V001T13A001. doi:10.1115/ES2013-18003.

This paper addresses issues of flow field optimization for a water raceway which is used to grow algae for biofuels. An open channel raceway is the typical facility to grow algae in medium to large scales. The algae growth rate in a raceway is affected by conditions of temperature, nutrients, and sunlight intensity etc. These conditions are essentially associated with the fluid mixing in the flow field. Good flow mixing at low consumption of pumping power for the water flow is desirable for an economic algal growth facility. A novel design of an open channel raceway for medium- and large-scale algae growth field has been proposed by the authors previously, which is called High Velocity Algae Raceway Integrated Design (ARID-HV). Optimization analysis using CFD and experimental visualization has been applied to a table-sized ARID-HV test model with various geometries of dams and their spacing in the system. CFD results and flow visualization allow us to understand the flow mixing in the entire raceway. Data is also processed to show the statistics of the locations of ‘fluid particles’ at different height and time period during one flow path. Different flow field designs were thus compared quantitatively based on this statistics according to the understanding that the “tumbling times” of fluid particles at bottom/top of the water is tightly related to the growth rate of algae.

Commentary by Dr. Valentin Fuster
2013;():V001T13A002. doi:10.1115/ES2013-18023.

In this paper, a thermodynamic model of a spark ignition internal combustion engine fueled with natural gas is developed in order to estimate the air-fuel-unburned gas temperature at before top dead center (BTDC). This temperature is used as controlled variable in a control loop in order to avoid the autoignition phenomena when the engine operates with a fuel with different methane number from the methane number requirement of the engine. The model formulation is based on a polytropic compression proccess whose coefficient was determined experimentally in a turbocharged internal combustion engine fueled with natural gas. To make feasible the use of differents gaseous fuels from natural gas, it was necessary to design two control strategies to avoid the knocking phenomenon and choose the best one. The ambient temperature is the disturbance considered, whose changes are significant in different places in the world. The first control strategy that was implemented is called “Robust”, which employs a conventional feedback control loop with a robust controller which is designed. The response of this control loop is compared to the response of the second control strategy called “Feedforward control”. The results obtained reveals that Feedforward control strategy has better performance than robust control strategy for this application. The control strategy and the model proposed will allow increase the range of application of gaseous fuels with low methane number (MN) leading to guarantee a safe running in internal combustion engines that currently are fueled with natural gas.

Commentary by Dr. Valentin Fuster
2013;():V001T13A003. doi:10.1115/ES2013-18028.

Currently there are nearly 750 million ground vehicles in service worldwide. They are responsible for 50% of petroleum (oil) consumption and 60% of all greenhouse gas (GHG) emissions worldwide. The number of vehicles is forecasted to double by 2050. Therefore the environmental issues such as noise, emissions and fuel burn have become important for energy and environmental sustainability. This paper provides an overview of specific energy and environmental issues related to ground transportation. The technologies related to reduction in energy requirements such as reducing the vehicle mass by using the high strength low weight materials and reducing the viscous drag by active flow control and smoothing the operational profile, and reducing the contact friction by special tire materials are discussed along with the portable energy sources for reducing the GHG emissions such as low carbon fuels (biofuels), Lithium-ion batteries with high energy density and stability, and fuel cells. The technological challenges and opportunities for innovations are discussed.

Commentary by Dr. Valentin Fuster
2013;():V001T13A004. doi:10.1115/ES2013-18140.

This study investigates the effects of torrefaction temperature and time on the energy content and storage stability of bio-char derived from corn stover feedstock. The batch torrefaction system in this study uses 91.4 cm × 25.4 cm diameter reactors with electrical heating elements to torrefy large samples of corn stover. The reactor is typically loaded with 0.5 kg samples of chopped corn stover (stalks, leaves, and cobs) with particle surface areas ranging from 0.5–2.4 cm2. The operating temperatures range from 230–340 degrees C, while the operating times range from 30–60 minutes. After each reaction trial, the energy content of the bio-char is quantified using the heat of combustion value obtained from bomb calorimeter tests on each of the samples. Values from these tests are compared to previous research to investigate the feasibility of larger-scale torrefaction reactions with mixed component stover. Temperature and time profiles are obtained from an Arduino output to investigate temperature behaviors during the reaction. The temperature and energy content can provide the basis for defining the phases of torrefaction.

The long-term goal of this research is to assess the viability of producing a high energy, storable bio-char as well as a usable biogas from the torrefaction process of corn stover feedstocks. Since compounds within the corn stover have different reaction rates, a composition analysis of samples at various stages in torrefaction will indirectly provide information on the usability of the biogas and behaviors of organic compounds in the reaction. Therefore, the torrefaction conditions must be specified before moving forward.

Topics: Temperature
Commentary by Dr. Valentin Fuster
2013;():V001T13A005. doi:10.1115/ES2013-18187.

Anaerobic codigestion of dairy manure and food-based feedstocks reflects a cradle-to-cradle approach to organic waste management. Given both of their abundance throughout New York State, waste-to-energy processes represent promising waste management strategies. The existing waste-to-energy literature has not yet fully realized the environmental impacts associated with displaced grid electricity generation and feedstock-hauling emissions on the net environmental impact of centralized codigestion facilities. The key objective of this study is to provide a comprehensive environmental impact assessment with the purpose of understanding the existing environmental status of centralized codigestion facilities. Real-time data from an operational codigestion facility located in Western New York State was used to conduct this environmental impact statement. A comprehensive inventory of greenhouse gas emissions associated with renewable electricity production at the codigestion facility was developed using the Emissions & Generation Resource Integrated Database (eGRID) (U.S. EPA), while emissions associated with feedstock hauling were quantified using the fuel life-cycle approach developed by the Greenhouse gases, Regulated Emissions, and Energy use in Transportation model (GREET) (U.S. DOE). With each of the emissions models used for this analysis, it was determined that the net environmental impact associated with hauling food-related feedstocks from the many locations throughout the Northeast U.S. region would be acceptably low, and thus could be part of future sustainable development of centralized codigestion facilities.

Commentary by Dr. Valentin Fuster
2013;():V001T13A006. doi:10.1115/ES2013-18189.

Prior research conducted by our Institute has revealed the large quantities of food waste available in New York State, particularly in the Upstate corridor extending from Buffalo to Syracuse. The Finger Lakes region is heavily populated with agricultural operations, dairy farms and food processing plants, including those producing milk, yogurt, wine, and canned fruits and vegetables. The diverse supply of organic waste generated by these facilities offers the opportunity for sustainable energy production through one of three primary pathways:

• Anaerobic digestion to produce methane

• Fermentation to produce alcohols

• Transesterification to produce biodiesel.

Generally speaking, food wastes are better suited for biochemical conversion instead of thermo-chemical conversion (combustion, gasification, pyrolysis) due to their relatively high moisture content. The current paper provides an initial assessment of food wastes within the 9-County Finger Lakes region around Rochester, New York. Available databases were utilized to first identify all the relevant companies operating in one of four broad industry sectors: agriculture, food processing, food distribution and food services (including restaurants). Our analysis has demonstrated that anaerobic digestion can be a viable method for sustainable energy production from food waste in the Finger Lakes region, due to the dual economic benefits of effective disposal cost reduction and production of methane-rich biogas.

Commentary by Dr. Valentin Fuster

Symposium on Integrated/Sustainable Building Equipment and Systems (JOINT)

2013;():V001T14A001. doi:10.1115/ES2013-18043.

Trigeneration systems are closely associated with sorption cooling technology because prime mover waste heat can be recovered to produce cooling. The working pair and cycle type of the sorption cooling system needs to be matched to the waste heat temperature of the prime mover, as well as with the capacity and application of the trigeneration system. A residential trigeneration system with a 4 kWelec internal combustion engine, a 220 gallon (830 L) hot water tank and a 3 kW adsorption chiller powered by 70°C waste heat with separate sensible and latent cooling control strategy is presented in this study. Transient experiments were conducted under 5 day long hot water and space cooling load profiles from a simulated house to evaluate the performance from a practical perspective. The fuel consumption was measured and compared with that of two baseline systems. An analytical criterion was derived and discussed to further evaluate the trigeneration system with different loads under different climates. It was found that the presented residential trigeneration system could save about 30% of fuel consumption compared with conventional off-grid operation mode, but is not more fuel efficient than the conventional on-grid and vapor compression cooling combination.

Commentary by Dr. Valentin Fuster
2013;():V001T14A002. doi:10.1115/ES2013-18146.

An educational building located in the Caribbean area of Colombia that uses a VAV system has been monitored in order to quantify the energy consumption. The energy values attained has been compared with a simulation of the building running with a constant volumetric flow system. The results show that a 43% and 18% energy reduction can be achieved for the ventilation system and for the entire air conditioning system that includes a chiller with variable primary pump, a variable air volume system and a variable cooling tower fan.

Commentary by Dr. Valentin Fuster
2013;():V001T14A003. doi:10.1115/ES2013-18251.

Energy Crisis and Environment Balance have been occupying a leading position internationally in both, the sociopolitical arena and technological developments. A developing country like India is under dual pressure to provide economic prosperity to its burgeoning population while maintaining the energy-environment balance. Current challenging situation is a good opportunity to develop products and systems which not only provide customer satisfaction at competitive prices but also does it in a sustainable, resource-friendly way. This paper outlines a proposal to set up a Research Park in an academic institute with the theme of Sustainability in HVAC field. The idea is to collaborate with the industry and association partners so that the students and faculty together can work on joint R & D projects which ultimately will result into innovative energy-saving and environment-friendly technological products and systems. The metric on which the Research Park will base its output target is the reduction in greenhouse gas emissions through a multi-pronged approach. In this paper, a baseline of annual energy consumption by the HVAC sector in India has been drawn which works out to 137,026 GWh translating into 123.32 Million Metric Tonnes of CO2 equivalent emissions.

Commentary by Dr. Valentin Fuster
2013;():V001T14A004. doi:10.1115/ES2013-18274.

This paper presents a calibration process and its tool of the vertical ground heat exchanger model used in a building energy simulation program, Energyplus. To adequately analyze the performance of the system, calibration of the system model is crucial. The calibration procedure is to estimate input data of the simulation that match the results of the simulation with measured data by an inverse method. The vertical ground loop heat exchanger consists of ground and borehole systems. The thermal properties of the borehole system usually can be found from manufacturer’s data. However, the thermal property of the ground is hard to evaluate. In this paper, an evaluation tool of the thermal properties of the ground around the borehole is developed using Matlab. This tool consists of three submodels. The first one is a G-function curve fit model which represents the relationship between variation of thermal conductivity and g-function values. The second model is the vertical ground loop heat exchanger model which predicts the return water temperature from a ground loop heat exchanger using the short time response factor method. The vertical ground loop heat exchanger model in Energyplus is converted to Matlab code and integrated into the calibration model for this research. The last sub-model is the optimization model that uses the Nelder and Mead simplex optimization scheme to find parameters which minimize the difference between the simulation results and the field measurement data. This tool estimates the ground thermal propertiesusing an optimization scheme based on data collected from field measurement. Far field ground temperature and the ground thermal conductivity are estimated to be used as input data of the vertical ground loop heat exchanger model in Energyplus. This program is validated using a case study which is performed for an actual building, ZOE which is located in the University of North Texas and its system. 2 weeks’ measurement data were compared with the simulation result. The average deviation between the simulation result and measurement data for 2 weeks is 0.27 °C.

Commentary by Dr. Valentin Fuster

Thermofluids and Thermodynamics of Energy Systems; Exergy Analysis; Systems Integration

2013;():V001T15A001. doi:10.1115/ES2013-18025.

Geological carbon sequestration (GCS) is one of the most promising technologies to address the issue of excessive anthropogenic CO2 emissions in the atmosphere due to fossil fuel combustion for electricity generation. For GCS, the saline aquifer geological carbon sequestration is considered very attractive compared to other options because of their huge sequestration capacity in U.S. and other parts of the world. However, in order to fully exploit their potential, the injection strategies need to be investigated that can address the issues of both the CO2 storage efficiency and safety along with their economic feasibility. Numerical simulations can be used to determine these strategies before the deployment of full scale sequestration in saline aquifers. This paper presents the numerical simulations of CO2 sequestration in three large identified saline aquifers (Mt. Simon, Frio, Utsira) where the sequestration is currently underway or has recently been completed (in case of Frio). The numerical simulations are in acceptable agreement with the seismic data available for plume migration. The results of large scale history-matching simulation in Mt. Simon, Frio, and Utsira formations provide important insights in the uncertainties associated with the numerical modeling of saline aquifer GCS.

Commentary by Dr. Valentin Fuster
2013;():V001T15A002. doi:10.1115/ES2013-18036.

India has always been victim of power failures or blackouts and the recent July 2012 countrywide blackout is a perfect example for it. It is expected that due to the widening gap between supply and demand, such instances of power failure would occur more regularly in future. Such blackouts are also be foreseen in other parts of the world. The electricity grids in many countries are highly centralized and are mostly dependent on fossil-fuel based energy sources (coal, oil, natural gas etc). Due to the rapid rise in the living standards of developing countries such as India and China, there is an increase in demand for electricity for running various appliances, as well as for heating and air-conditioning equipment. Such an increased in demand places tremendous strain on ailing centralized grid burning fossil fuel. The use of renewable energy sources (such as solar and wind) could potentially allow large amount of demand to be met though alternative means and offset the demand on the grid.

The advancement in technology has encouraged the implementation of renewable resources especially solar and wind. Hybrid power systems (HPS) that consist of these resources can significantly lower storage requirements. Furthermore, besides being cost-efficient, it is coherent to the weather conditions since solar and wind complement each other well. For highly efficient hybrid power systems to be developed, a significant degree of research must be applied to further their development. This includes tasks such as modeling these systems and applying proficient control algorithms to maximize efficiency. This paper focuses on simulation of an HPS consisting of a photovoltaic (PV) module, wind turbine (WT), and a lead acid battery through MATLAB/SIMULINK software. Moreover, a control algorithm is proposed, which leads to an efficient and autonomous operation of the HPS, along with maximizing power output from PV module and WT. The model and control system were tested using sample hourly solar radiation, temperature, and wind speed data to generate the power output from the PV module and WT, which was then processed through the proposed algorithm, to power a sample hourly load profile. The results indicate that a simple HPS can meet the type of load demand provided in an efficient and effective manner.

Commentary by Dr. Valentin Fuster
2013;():V001T15A003. doi:10.1115/ES2013-18067.

Archimedes screw generators (ASGs) are beginning to be widely adopted at low head hydro sites in Europe, due to high efficiency (greater than 80% in some installations), competitive costs and low environmental impact. Compared to other microhydro generation technologies, ASGs have greatest potential at low head sites (less than about 5 m). The performance of an Archimedes screw used as a generator depends on parameters including screw inner and outer diameter, slope, screw pitch and number of flights, and inlet and outlet conditions, as well as site head and flow. Despite the long history of the Archimedes screw, there is very little on the dynamics of these devices when used for power generation in the English literature. Laboratory tests of small Archimedes screws (approximately 1 W mechanical power) have been conducted to support the design and validation of ASG design tools. This paper reports experimental results examining the relationship between torque, rotation speed and power. The laboratory screw maintained reasonable efficiency over wide ranges of operating conditions, although distinct efficiency peaks were found to occur. The cause of changes in power output caused by varying the water level at the outlet of the screw were attributed primarily to the corresponding variation in head, and dynamic limiting of screw rotation speed causing corresponding limits in volume flow through the screw. Test results were qualitatively consistent with data from a prototype ASG installed by Greenbug Energy in southern Ontario, Canada, and recent data reported from European laboratory tests and commercial installations.

Commentary by Dr. Valentin Fuster
2013;():V001T15A004. doi:10.1115/ES2013-18152.

Proper light penetration is an essential design consideration for effective algae growth in column photobioreactors. This research focuses on the placement of light guides within a photobioreactor (PBR), and the effect they have on heat transfer, mass transfer, bubble and fluid flow patterns, and mixing. Studies have been done on a rectangular column photobioreactor (34.29 cm long × 15.25 cm wide × 34.29 cm tall) with two light panels along the front and back of the PBR. A bubble sparger is placed along the center of the bottom length of the PBR with both height and width of 1.27 cm and a length of 33.02 cm. Different configurations and numbers of light guides (1.27 cm diameter) running horizontally from the front to the back of the PBR are modeled using the Computational Fluid Dynamics (CFD) software Star-CCM+. It is hypothesized that the addition of light guides will change the flow pattern but not adversely affect the heat or mass transfer of the carbon dioxide bubbles within the PBR. Potential concerns of light guide placement include inhibiting the flow of the carbon dioxide bubbles or creating regions of high temperature, which could potentially kill the algae. Benefits of light guides include increased light penetration and photosynthesis within the PBR. Five different light guide setups are tested with the carbon dioxide bubbles and water modeled as a turbulent multiphase gas-liquid mixture. The near wall standard k-epsilon two layer turbulence model was used, as it takes into account the viscosity influences between the liquid and gaseous phases. Eight different bubble volumetric flow rates are simulated. The bubble flow patterns, temperature distribution, Nusselt number, Reynolds number, and velocity are all analyzed. The results indicate square arrays of light guides give the most desirable velocity distribution, with less area of zero velocity compared to the staggered light guide setup. Temperature distribution is generally even for all configurations of light guides.

Commentary by Dr. Valentin Fuster
2013;():V001T15A005. doi:10.1115/ES2013-18182.

An alternate to the two-tank molten salt thermal energy storage system using supercritical fluids is presented. This technology can enhance the production of electrical power generation and high temperature technologies for commercial use by lowering the cost of energy storage in comparison to current state-of-the-art molten salt energy storage systems. The volumetric energy density of a single-tank supercritical fluid energy storage system is significantly higher than a two-tank molten salt energy storage system due to the high compressibilities in the supercritical state. As a result, the single-tank energy storage system design can lead to almost a factor of ten decrease in fluid costs. This paper presents results from a test performed on a 5 kWht storage tank with a naphthalene energy storage fluid as part of a small preliminary demonstration of the concept of supercritical thermal energy storage. Thermal energy is stored within naphthalene filled tubes designed to handle the temperature (500 °C) and pressure (6.9 MPa or 1000 psia) of the supercritical fluid state. The tubes are enclosed within an insulated shell heat exchanger which serves as the thermal energy storage tank. The storage tank is thermally charged by flowing air at >500 °C over the storage tube bank. Discharging the tank can provide energy to a Rankine cycle (or any other thermodynamic process) over a temperature range from 480 °C to 290 °C. Tests were performed over three stages, starting with a low temperature (200 °C) shake-out test and progressing to a high temperature single cycle test cycling between room temperature and 480 °C and concluding a two-cycle test cycling between 290 °C and 480 °C. The test results indicate a successful demonstration of high energy storage using supercritical fluids.

Commentary by Dr. Valentin Fuster
2013;():V001T15A006. doi:10.1115/ES2013-18201.

Near-critical CO2 flow has been studied because of its potential application in carbon dioxide capture and sequestration, which is one of the proposed solutions for reducing greenhouse gas emission. Near the critical point the thermophysical properties of the fluid undergo abrupt changes that affect the flow structure and characteristics. Pressure drop across a stainless steel tube, 2 ft long with 0.084 in ID, at different inlet conditions and mass flow rates have been measured. The effects of variations of inlet conditions have been studied. The results show extreme sensitivity of pressure drop to inlet conditions especially inlet temperature in the vicinity of the critical point. Also, shadowgraphs have been acquired to study the flow structure qualitatively.

Commentary by Dr. Valentin Fuster
2013;():V001T15A007. doi:10.1115/ES2013-18285.

In recent years, with the continuous urban expansion, the central heating sources are commonly insufficient in the areas of Northern China. Besides, the increasing heat transfer temperature difference results in more and more exergy loss between the primary heat network and the secondary heat network. This paper introduces a new central heating system which combines the urban heat network with geothermal energy (CHSCHNGE). In this system, the absorption heat exchange unit, which is composed of an absorption heat pump and a water to water heat exchanger, is as alternative to the conventional water to water heat exchanger at the heat exchange station, and the doing work ability of the primary heat network is utilized to drive the absorption heat pump to extract the shallow geothermal energy. In this way, the heat supply ability of the system will be increased with fewer additional energy consumptions. Since the water after driving the absorption heat pump has high temperature, it can continue to heat the supply water coming from the absorption heat pump. As a result, the water of the primary heat network will be stepped cooled and the exergy loss will be reduced. In this study, the performance of the system is simulated based on the mathematical models of the heat source, the absorption heat exchange unit, the ground heat exchanger and the room. The thermodynamic analyses are performed for three systems and the energy efficiency and exergy efficiency are compared. The results show that (a) the COP of the absorption heat exchange unit is 1.25 and the heating capacity of the system increases by 25%, which can effectively reduce the requirements of central heating sources; (b) the PER of the system increases 14.4% more than that of the conventional co-generation central heating system and 54.1% more than that of the ground source heat pump system; (c) the exergy efficiency of the CHSCHNGE is 17.6% higher than that of the conventional co-generation central heating system and 45.6% higher than that of the ground source heat pump system.

Commentary by Dr. Valentin Fuster
2013;():V001T15A008. doi:10.1115/ES2013-18336.

A numerical analysis of the characterization of the water flow through a flat solar collector is presented. The manifold area change for minimizing the water flow variation in the solar collector is analyzed. The area ratio in the inlet and outlet of the manifolds were modified in a range of Am/Ao = 1 to 4, where Am and Ao are the cross-sectional area modified and original of the manifolds, respectively. The solar collector investigated is equipped with six riser tubes, which are attached to the manifolds pipe. The numerical study was developed in a commercial Computational Fluid Dynamics (CFD) using FLUENT®. This code allows to solve the Reynolds averaged Navier-Stokes equations and the transport equations of the turbulence quantities. The results shown that increasing the inlet and outlet area of the manifolds allow a more uniform flow distribution compared to the original configuration of the solar collector. It also shows that the overall pressure drop in the solar collector is reduced.

Commentary by Dr. Valentin Fuster
2013;():V001T15A009. doi:10.1115/ES2013-18382.

The purpose of this study is the proof that non-concentrating solar-thermal collectors can supply the thermal energy needed to power endothermic chemical reactions such as steam reforming of alcoholic (bio-) fuels. Traditional steam reformers require the combustion of up to 50% of the primary fuel to enable the endothermic reforming reaction. Our goal is to use a selective solar absorber coating on top of a collector-reactor surrounded by vacuum insulation. For methanol reforming, a reaction temperature of 220–250°C is required for effective methanol-to-hydrogen conversion. A multilayer absorber coating (TiNOX) is used, as well as a turbomolecular pump to reach ultra-high. The collector-reactor is made of copper tubes and plates and a Cu/ZnO/Al2O3 catalyst is integrated in a porous ceramic structure towards the end of the reactor tube. The device is tested under 1000 W/m2 solar irradiation (using an ABB class solar simulator, air mass 1.5).

Numerical and experimental results show that convective and conductive heat losses are eliminated at vacuum pressures of <10−4 Torr. By reducing radiative losses through chemical polishing of the non-absorbing surfaces, the methanol-water mixture can be effectively heated to 240–250°C and converted to hydrogen-rich gas mixture. For liquid methanol-water inlet flow rates up to 1 ml/min per m2 of solar collector area can be converted to hydrogen with a methanol conversion rate above 90%.

This study will present the design and fabrication of the solar collector-reactor, its testing and optimization, and its integration into an entire hydrogen-fed Polymer Electrolyte Membrane fuel cell system.

Commentary by Dr. Valentin Fuster
2013;():V001T15A010. doi:10.1115/ES2013-18398.

Recovery of industrial waste heat is significant to both energy saving and emission reduction. In the present paper, two types of supercritical CO2 Rankine cycles with and without internal heat exchanger (IHX) are integrated for analyzing the performance of low and medium temperature industrial heat recovery. Cycles were simulated with Aspen software, by which the influences of the initial temperature, initial pressure and temperature of cooling water were observed. The results indicate that cycle efficiency and net output work increase with growth of initial temperature, yet they fell as temperature of cooling water ramps down for the two types of cycle. For a given initial temperature, the cycle efficiency and net output work have maximum values under various initial pressures. This can be attributed to the power consumption of CO2 pump, which goes up significantly with increase of initial pressure. The performances of supercritical CO2 Rankine cycles with and without IHX utilizing four typical industrial heat sources at low and medium temperature were analyzed, which were the heat of non-concentrating solar collector, the exhaust gas heat of a 600MW coal fired plant, the exhaust gas heat of a CFB boiler and the exhaust gas heat of industrial furnace of a cement plant. The optimal cycle efficiencies range from 7% to 12% without IHX and 9% to 15% with IHX, respectively, under the temperature of heat source varying from 130°C to 200°C.

Commentary by Dr. Valentin Fuster

Wind Energy Systems and Technologies

2013;():V001T16A001. doi:10.1115/ES2013-18024.

It is well established that the power generated by a Horizontal-Axis Wind Turbine (HAWT) is a function of the number of blades B, the tip speed ratio λ (blade tip speed/wind free stream velocity) and the lift to drag ratio (CL/CD) of the airfoil sections of the blade. The airfoil sections used in HAWT are generally thick airfoils such as the S, DU, FX, Flat-back and NACA 6-series of airfoils. These airfoils vary in (CL/CD) for a given B and λ, and therefore the power generated by HAWT for different blade airfoil sections will vary. The goal of this paper is to evaluate the effect of different airfoil sections on HAWT performance using the Blade Element Momentum (BEM) theory. In this study, we employ DU 91-W2-250, FX 66-S196-V1, NACA 64421, and Flat-back series of airfoils (FB-3500-0050, FB-3500-0875, and FB-3500-1750) and compare their performance with S809 airfoil used in NREL Phase II and III wind turbines; the lift and drag coefficient data for these airfoils sections are available. The output power of the turbine is calculated using these airfoil section blades for a given B and λ and is compared with the original NREL Phase II and Phase III turbines using S809 airfoil section. It is shown that by a suitable choice of airfoil section of HAWT blade, the power generated by the turbine can be significantly increased. Parametric studies are also conducted by varying the turbine rotor diameter.

Commentary by Dr. Valentin Fuster
2013;():V001T16A002. doi:10.1115/ES2013-18220.

Previous work at the institution has successfully shown that a novel VAWT design can be employed to provide electrical power to remote rural villages in a cost effective manner. The VAWT’s design can effectively utilize the non-laminar, low level winds and survive the increased turbulence present at remote and non-optimal installation locations. Previous efforts have improved the overall aerodynamic characteristics of the turbine and scaled these designs from a 100W to a 1kW scaled turbine. In order to remain a viable and affordable solution for use worldwide by truly rural users, these turbines need to have low manufacturing cost and low maintenance costs. This paper presents the work done by the authors to analyze the main cost contributors, manufacturing methods, techniques, and tooling used to improve productivity in the manufacturing process. Design improvements and construction materials were analyzed to reduce overall weight which leads to cost reduction and overall improvements in manufacturability. The specific improvements explored by the authors include redesigning the arms of the turbine to improve aerodynamic efficiency of the turbine, reducing construction materials to minimum allowable values, and designing manufacturing tooling which will allow for rapid production of large quantities of the turbine. Results are presented from over 4000 hours of in-situ testing of the turbine showing that the manufacturing improvements reduced construction time to 25% of the original design and reduced weight by 25% while maintaining full functionality and high-wind survivability.

Commentary by Dr. Valentin Fuster
2013;():V001T16A003. doi:10.1115/ES2013-18423.

Wind and solar power generation differ from conventional energy generation because of the variable and uncertain nature of their power output. This variability and uncertainty can have significant impacts on grid operations. Thus, short-term forecasting of wind and solar power generation is uniquely helpful for balancing supply and demand in an electric power system. This paper investigates the correlation between wind and solar power forecast errors. The forecast and the actual data were obtained from the Western Wind and Solar Integration Study. Both the day-ahead and 4-hour-ahead forecast errors for the Western Interconnection of the United States were analyzed. A joint distribution of wind and solar power forecast errors was estimated using a kernel density estimation method; the Pearson’s correlation coefficient between wind and solar forecast errors was also evaluated. The results showed that wind and solar power forecast errors were weakly correlated. The absolute Pearson’s correlation coefficient between wind and solar power forecast errors increased with the size of the analyzed region. The study is also useful for assessing the ability of balancing areas to integrate wind and solar power generation.

Topics: Errors , Solar power , Wind
Commentary by Dr. Valentin Fuster

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