ASME Conference Presenter Attendance Policy and Archival Proceedings

2012;():i. doi:10.1115/ES2012-NS.

This online compilation of papers from the ASME 2012 6th International Conference on Energy Sustainability (ES2012) 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to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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

2012;():1-10. doi:10.1115/ES2012-91007.

By employing the fuzzy control theory and Dynamic Matrix Control (DMC) method, the controllers for temperature control of a room cooled by a displacement ventilation system are developed. The fluid flow and heat transfer inside the room is calculated by solving the Reynolds Averaged Navier-Stokes (RANS) equations including the effects of buoyancy in conjunction with a two-equation realizable k-epsilon turbulence model. Thus the physical environment is represented by a nonlinear system of partial differential equations. The system also has a large time delay because of the slowness of the heat exchange. Additionally, the temperature of the exterior wall of the room first increases and then decreases with time during a twenty four hour period, which acts as a strong disturbance in changing the temperature of the room. The goal of this paper is to develop controllers that will maintain the temperature in room within the specified upper and lower bounds by deploying the displacement ventilation system. In order to solve this temperature control problem, we develop a special fuzzy control method. At the same time, we analyze the peak value of the error and employ the DMC method to replace the fuzzy control method with success. Results show that the fuzzy controller is effective in saving energy and the DMC method can contain the error within specified bounds in the worst situation (when the temperature of the exterior wall is highest). This kind of fuzzy control and DMC method can also be employed for other HVAC systems such as Overhead VAV (Variable Air Volume) system and radiant cooling hydronic system.

Commentary by Dr. Valentin Fuster
2012;():11-15. doi:10.1115/ES2012-91010.

This research is aimed to study the system performance for a large solar hot water system constructed by connecting a series of small domestic natural-circulation systems. This type of large solar hot water system has been recently installed in Taiwan without either practical or theoretical support. There are few studies on this type of large solar hot water system available. This paper presents the numerical simulation study for the control and the system operating parameters effects on the system performance to provide important information both for users and system designers.

Commentary by Dr. Valentin Fuster
2012;():17-26. doi:10.1115/ES2012-91032.

Night ventilation is a well known strategy for passive cooling of residences and small commercial buildings. The building’s thermal mass can be cooled at night by ventilating the inside of the space with the relatively lower outdoor air temperatures, thereby lowering indoor temperatures during the warmer daytime period. Numerous experimental and theoretical studies have shown the effectiveness of the method to significantly reduce air conditioning loads or improve comfort levels in those climates where the night time ambient air temperature drops below that of the indoor air. One could develop/adapt computer programs with detailed mathematical component models to simulate and evaluate the performance of night ventilation strategies in a specific location for a particular building. A more basic problem is to develop a methodology whereby potential designers can screen various climatic locations and regions in order to perform a preliminary evaluation of which months of the year are good candidates for implementing such a scheme. Only after completion of such a phase is a detailed evaluation warranted for specific buildings. In this paper, effectiveness of night ventilation is quantified by a parameter called the Discomfort Reduction Factor (DRF) which is the index of reduction of occupant discomfort levels during the day time from night ventilation. Two different thermal network models which provide such insights are evaluated. Daily and monthly DRFs are calculated for two climate zones and three building heat capacities for the whole year. It is verified that night ventilation is effective in seasons and regions when day temperatures are between 30 °C and 36 °C and night temperatures are below 20 °C. The accuracy of these preliminary screening models may be lower than using a detailed simulation program but the loss in accuracy in using such tools is more than compensated by the insights provided, along with better transparency in the analysis approach and results obtained.

Commentary by Dr. Valentin Fuster
2012;():27-32. doi:10.1115/ES2012-91192.

Though the Solar Chimney (SC) concept is one that has been around for nearly a century, this technology has regained momentum in recent decades. Several large-scale projects have been constructed, and others are in the process of being constructed. CFD and numerical analysis have allowed research to progress in characterizing the performance of SCs. Previous research has focused primarily on large-scale, high power production designs. The authors have validated these analyses empirically for small-scale, low power production SCs through the design and fabrication of a small-scale SC with adjustable geometries. The performance of the SC was characterized by the primary variable of chimney height. The effects of secondary variables were also measured and characterized: chimney diameter, collector height, and collector slope.

Results are presented showing that, though the numerical analysis and characteristic equations hold for large scale SCs, in small scale applications the reduction in size creates a greater dependence on the secondary variables in the overall performance of the SC. Optimization of both primary and secondary variables is discussed and demonstrated based on empirically gathered data. Though this study does not consider the turbine generator system, charge controller, or their efficiencies, power output as a function of collector geometry is discussed in detail.

Topics: Solar energy
Commentary by Dr. Valentin Fuster
2012;():33-38. doi:10.1115/ES2012-91194.

The basic solar chimney concept remains largely unchanged since being proposed nearly a century ago. The air inside a collector heats up, and becomes less dense and begins rising toward the center of the collector as the covering is sloped upward towards a centrally located chimney. Heat and air flow increases as it moves toward the chimney. When a turbine is placed in this chimney, electricity can be generated from the air flow.

Using previously published research [1] on small scale solar chimneys, the authors designed and fabricated a small scale solar chimney. Performance variables were optimized to achieve peak performance and theoretical electrical output. The solar chimney was designed to achieve low material cost, and minimal maintenance effort without the use of heavy machinery.

The solar chimney designed by the authors was constructed and tested for implementation in remote regions where land space is practically unrestricted and electrical power is desired for lighting and communication. The project team conducted an assessment of one likely location for this solar chimney in Northern Mongolia during the summer of 2011. There are nomadic people groups who do not have electrical power but have vast amounts of land at their disposal. The solar chimney is a potential source of electricity for these people groups, as it was also designed to be transportable, lightweight, and inexpensive.

Commentary by Dr. Valentin Fuster
2012;():39-47. doi:10.1115/ES2012-91200.

This paper present experimental measurements of velocity distribution at different sections around a reduced scale model of a real domed-roof building with several openings. Measurements are made in an open loop wind tunnel with atmospheric boundary layer. A new modified Counihan scheme which is a combination of the two systems is developed for the purpose of part-depth atmospheric boundary layer (ABL). The measured quantities are wind velocity profile, turbulence intensity, and air flow pattern around the building. For experiments a 1:54 scale model of a domed-roof building with six windows and an aperture on the roof is made. In addition, by using a numerical solution, turbulent air flow around such scale model in the wind tunnel is simulated and flow field inside the model and also the discharge coefficient are computed during the economizer mode.

Commentary by Dr. Valentin Fuster
2012;():49-58. doi:10.1115/ES2012-91246.

The performance of the adsorption cooling system using the zeolite 13X/CaCl2 composite adsorbent was studied using a numerical simulation. The novel zeolite 13X/CaCl2 composite adsorbent with superior adsorption properties was developed in previous studies [11]. It has high equilibrium water uptake of 0.404 g/g between 25°C and 100°C under 870Pa. The system specific cooling power (SCP) and coefficient of performance (COP) were successfully predicted for different operation parameters. The simulated COP with the composite adsorbent is 0.76, which is 81% higher than a system using pure zeolite 13X under desorption temperature of 75°C. The SCP is also increased by 34% to 18.4 W/kg. The actual COP can be up to 0.56 compared to 0.2 for zeolite 13X-water systems, an increase of 180%. It is predicted that an adsorption cooling system using the composite adsorbent could be powered by a low grade thermal energy source, like solar energy or waste heat, using the temperature range of 75°C to 100°C.

The performance of the adsorber with different design parameters was also studied in the present numerical simulation. Adsorbents with smaller porosity can have higher thermal conductivity and may result in better system performance. The zeolite bed thickness should be limited to 10mm to reduce the thermal response time of the adsorber. Addition of high thermal conductivity materials, for example carbon nanotube, can also improve the performance of the adsorber. Multi-adsorber tube connected in parallel can be employed to provide large heat transfer surface and maintain a large SCP and COP. The desorption temperature also showed a large effect on the system performance.

Commentary by Dr. Valentin Fuster
2012;():59-64. doi:10.1115/ES2012-91289.

The heat pipe augmented solar heating system significantly reduces heating loads relative to other conventional passive space heating systems [1–3]. Yet unwanted thermal gains during the cooling season from passive solar systems increase cooling loads and, in extreme cases, may even increase overall space conditioning loads relative to a nonsolar building. The objective of this study was to compare the effectiveness of several design modifications and control strategies for the heat pipe wall to reduce unwanted gains. MATLAB was used to simulate four different unwanted gains reduction mechanisms: 1. shading to block beam radiation from striking the collector, 2. an opaque cover to block all radiation from striking the collector, 3. a mechanical valve in the adiabatic section to eliminate convective heat transfer through the heat pipe into the room, and 4. switching the elevations of the evaporator and condenser sections of the heat pipe to provide heat transfer out of the room during the cooling season. For each mechanism, three different control strategies were evaluated: 1. Seasonal control, for which the prescribed mechanism is deployed at the beginning and removed at the end of the cooling season, 2. ambient temperature-based control, for which the mechanism is deployed if the forecast for the next hour (based on TMY3 weather data) is greater than 65°F, and 3. room temperature-based control, for which the mechanism is deployed if auxiliary cooling was required for the previous hour. For the seasonal strategy, the months for which the unwanted gains reduction mechanism should be deployed to minimize overall space conditioning loads were estimated with a season determination ratio (SD), defined as the monthly ratio of unwanted gains to heating load. Results suggested that SD may be a ‘universal’ parameter that can be applied across a range of climates for quick assessment of its optimal cooling season. With TMY3 data for Louisville, KY, the heat pipe system performed best with ambient temperature-based control. The mechanical valve was the best single mechanism. While in many cases the combination of the valve with a cover or shading produced slightly better performance than the mechanical valve alone, these additional reductions were small. Switching elevations of the evaporator and condenser sections produced little cooling, because of the low thermal emittance of the absorber and low thermal transmittance of the cover, and for the Louisville climate, small diurnal temperature swings during the summer.

Commentary by Dr. Valentin Fuster
2012;():65-68. doi:10.1115/ES2012-91290.

A 12′ by 24′ passive solar test building has been constructed on the campus of the University of Louisville. The building envelope is comprised of structural insulated panels (SIPs), 12″ thick, (R-value of 45 ft2F/Btu) for the floor and walls and 16″ (R-63) for the roof. The building is divided into two symmetrical rooms with a 12″ SIPs wall separating the rooms. All joints between panels are caulked to reduce infiltration. Each room contains one window (R-9) on the north side wall, and two windows (also R-9) facing south for ventilation and daylighting, but which will also provide some direct gain heating. The south wall of each room features an opening that will accommodate a passive solar heating system so that performance of two systems can be compared side-by-side. The overhang above the south openings is purposely left short to accommodate an awning to provide adjustable shading. The calculated loss coefficient (UA) for each room of the building is 6.07 W/K. Each room is also equipped with a data acquisition system consisting on an SCXI 1600 16 bit digitizer and an SCXI 1102B isolation amplifier with an SCXI 1303 thermocouple module. Pyranometers are placed on the south wall and the clerestory wall to measure insolation on the solar apertures.

For initial tests, one room is equipped with an original heat pipe system previously tested in another building, while the other is equipped with a modified heat pipe system. Changes to the modified system include copper absorbers versus aluminum, an adiabatic section constructed of considerably less thermally-conductive DPM rubber than the copper used for the original design, and one of the five condenser sections of the heat pipes is exposed directly to the room air to provide early-morning heating. Experimental results will be compared to simulations with as-built building characteristics and actual weather data. Previous simulations with a load to collector ratio of 10 W/m2K, a defined room comfort temperature range between 65°F to 75°F, and TMY3 weather data for Louisville, KY, showed that the modified heat pipe wall design improves annual solar fraction by 16% relative to the original design.

Commentary by Dr. Valentin Fuster
2012;():69-78. doi:10.1115/ES2012-91295.

This paper presents thermal performance results of an experimental and numerical simulation study of a solar domestic hot water system (SDHW) for Canadian weather conditions. The experimental test setup includes two solar panels, a solar preheat tank, and an auxiliary propane-fired storage water heater, and an air handler unit for space heating. Experiments were performed on the SDHW system during a different season of the year, over the period March through October 2011 to assess the system performance for different solar gain and water draw schedules. Sunny, partly cloudy and cloudy conditions were explored. The test results were analysed in terms of solar fraction, solar efficiency, and the effects of thermosyphoning and stratification in the solar storage tank.

Modelling and simulation of the solar thermal energy system using TRNSYS software was performed. The objective was to optimise key design parameters and to suggest an effective control strategy to maximise the heat extraction from solar collectors. The developed model was based on the experimental test setup. It was first adjusted and verified with the solar gain and water draw schedule experimental data. The results of the numerical simulations were then validated with experimental results obtained with other water draw schedule and weather conditions. Acceptable agreements between the predicted and measured values were obtained at this early stage of development. Further refinements in system and model validation are in progress in order to improve the accuracy of the predictions. Ultimately, as the final product of this investigation, this model will be used to predict the performance of solar domestic hot water and space heating systems in different Canadian locations, different operating conditions and water draw schedules.

Commentary by Dr. Valentin Fuster
2012;():79-87. doi:10.1115/ES2012-91306.

A major challenge for solar water heaters is to provide heat at a cost comparable to or lower than conventional fuels. Since the price of a passive integral-collector-storage (ICS) solar water heater has historically been less than that for active systems with freeze protection, they can potentially heat water at a lower cost. However, ICS panels are subject to freeze damage, as the collector generally has metal tubes carrying pressurized water that can freeze and burst. In order to delineate the geographical areas where ICS panels can be deployed safely, it is necessary to experimentally characterize the conditions causing freeze damage, to develop a model relating the freeze behavior to climatic conditions, to validate that model with experimental data, and to run the model against long-term weather data across the U.S. Two variations of an ICS panel and/or their bare tubes were tested in a walk in freezer and subjected to freezing conditions until freeze damage occurred. The units tested include both a single and double glazed tubular ICS panel. Key data includes the volume expansion of the tube(s) at burst and the collector loss coefficient near 0 degrees C. Under freezing conditions the insulated supply/return lines would freeze solid initiating a pressure-buildup and eventual burst in the collector tubes due to further internal freezing. An additional test on the single glazed unit was also conducted in which heat tape was installed on the inlet and outlet pipes to prevent them from freezing, which increases the freeze tolerance of the panel by forcing small internal interconnection pipes to freeze solid before damage occurs. Existing models for ICS thermal performance were modified to incorporate the freezing process, and have been validated with the experimental data. The validated models were used to predict regions of the country that are safe for installing the ICS panels. Simulations were run using 30 years of weather data available for all TMY2 sites, and maps were created to illustrate regions of safe installation throughout the US for both the with and without heat tape scenarios for the two ICS models. A correlation using record minimum temperature was developed to generalize the maps to any location for which the record minimum is known. The maps show quantitatively the expected conclusions: 1) that double glazing and higher insulation will extend the safe region; and 2) that the use of heat tape on the inlet and outlet pipes significantly increases the region in which ICS panels can be safely installed in the US.

Topics: Storage
Commentary by Dr. Valentin Fuster
2012;():89-95. doi:10.1115/ES2012-91315.

A new concept for long-term solar storage is based on the absorption properties of aqueous calcium chloride. Water, diluted and concentrated calcium chloride solutions are stored in a single tank. An immersed heat exchanger and stratification manifold are used to preserve long-term sorption storage, and to achieve thermal stratification. The feasibility of the concept is demonstrated via measurements of velocity, CaCl2 mass fraction, and temperature in a 1500 liter prototype tank during sensible charging. Experiments are conducted over a practical range of the relevant dimensionless parameters. For Rayleigh numbers from 3.4 × 108 to 5.6 × 1010 and buoyancy ratios from 0.8 to 46.2, measured Sherwood numbers are 11±2 to 62±9 and the tank is thermally stratified. Convective mixing is inhibited by the presence of a steep density gradient at the interface between regions of differing mass fraction. The predicted storage time scales for the reported Sherwood numbers are 160 to 902 days.

Commentary by Dr. Valentin Fuster
2012;():97-104. doi:10.1115/ES2012-91345.

Methodologies have been developed to allow real-time determination of energy production and use as well as sizing of HVAC equipment based on thermal loads at the residential level. The data obtained reflect actual properties of the thermal envelope and appliance efficiencies, as well as actual renewable power production. The use of properly sized HVAC equipment introduces further energy savings. Recovery of energy savings expressed in terms of carbon offsets provides an incentive to improve the occupant’s energy footprint. When monetized, the energy cost savings and carbon offsets have financial benefits. These benefits are evaluated for model homes in various climatic zones within the state of California.

Commentary by Dr. Valentin Fuster
2012;():105-110. doi:10.1115/ES2012-91359.

Published hourly solar irradiance data are used to calculate the hourly clearness indices for three municipalities (Los Angles, Orlando and New York City). The clearness index method is then used to model the hourly total solar irradiance (summation of beam, diffuse and ground reflected) on an arbitrarily oriented surface. The orientations of solar panel for maximum annual solar irradiance for these locations are determined. It is shown that in some cases the southward solar panels do not yield maximum annual solar irradiance. Further, it is shown that using the hourly clearness index results in solar irradiance distribution that is not symmetric with respect to the solar noon.

Commentary by Dr. Valentin Fuster
2012;():111-117. doi:10.1115/ES2012-91361.

A basic principle of well designed greenhouse design emphasizes the utilization of solar energy as much as possible to grow the plants indoors during extreme outdoor climate conditions. Greenhouses can use significant amount of energy due to several factors including poor envelope design, inappropriate maintenance practices, and heavy reliance on fuel-based heating systems. In order to reduce energy consumption in the agricultural industry of Colorado, it is important to design energy efficient greenhouses under Colorado climatic conditions.

Commentary by Dr. Valentin Fuster
2012;():119-127. doi:10.1115/ES2012-91406.

Solar absorption cooling and heating (SACH) systems currently still stay at development and demonstration stage due to the nature of the complex system. It is critical for practitioners and engineers to have a correct and complete performance analyses and evaluation for SACH systems with respects of energy, economics, and environment. Optimization is necessarily involved to find the optimal system design by considering these three aspects. However, many assumptions made in the optimization are sensitive to the energy, economic, and environmental variations, and thus the optimization results will be affected. Therefore, the sensitivity and uncertainty analysis is important and necessary to make optimization robust.

This paper uses a case study to explore the influence of the uncertainties on the SACH system optimization results. The case is a SACH system for a medium size office building in Atlanta. The one parameter at a time (OAT) sensitivity analysis method was applied firstly to determine the most sensitive inputs. Monte Carlo statistical method was utilized to generate the data sets for uncertainty analysis. The optimization problem under uncertainty was then formulated and solved. Due to the uncertainty associated with system inputs, the optimization solutions were found with certain types of the distributions. In addition, the scenario analysis on electricity price does not show large sensitivity to the objectives.

Commentary by Dr. Valentin Fuster
2012;():129-136. doi:10.1115/ES2012-91495.

Research on phase change materials (PCM) as a potential technology to reduce peak loads and HVAC energy use in buildings has been conducted for several decades, resulting in a great deal of literature on PCM properties, temperature, and peak reduction potential. However, there are few building energy simulation programs that include PCM modeling features, and very few of these have been validated. Additionally, there is no previous research that indicates the level of accuracy when modeling PCMs from a building energy simulation perspective. This study analyzes the effects a nonlinear enthalpy profile has on thermal performance and expected energy benefits for PCM-enhanced insulation. The impact of accurately modeling realistic, nonlinear enthalpy profiles for PCMs versus simpler profiles is analyzed based on peak load reduction and energy savings using the Conduction Finite Difference (CondFD) algorithm in EnergyPlus. The PCM and CondFD models used in this study have been previously validated after intensive verification and validation done at the National Renewable Energy Laboratory. Overall, the results of this study show annual energy savings are not very sensitive to the linearization of enthalpy curve. However, hourly analysis shows that if simpler linear profiles are used, users should try to specify a melting range covering roughly 80% of the latent heat, otherwise, hourly results can differ by up to 20%.

Commentary by Dr. Valentin Fuster
2012;():137-142. doi:10.1115/ES2012-91523.

The dynamic performance of a PCM thermal storage system is investigated. The most affecting parameters on the system performance are the type and properties of PCM material, and the heat transfer fluid (HTF) inlet temperature. The time variant solar collector discharge temperature and that from the heating/cooling coils due to space load variation results in time dependent HTF inlet temperature. This paper is to study the behavior of the PCM thermal storage system under such time dependent HTF inlet temperature operating condition. A simple one dimensional model were used and solved numerically using the finite difference technique. To assure stability of solution the right time step and element size were applied. A MATLAB Program is formulated and used to solve the result system of equations. Results are presented in terms of the storage tank fluid temperature profiles, effectiveness of PCM usage and capacity of the storage system. The above were calculated for the cases of constant and a time-dependent HTF inlet temperature conditions for comparison. The performance of the PCM storage system is found to be substantially different for the above two mode of operation.

Commentary by Dr. Valentin Fuster

Combined Energy Cycles, CHP and CCHP

2012;():143-150. doi:10.1115/ES2012-91023.

Combined Cooling, Heating, and Power (CCHP) systems have been widely recognized as a key alternative for heat and electricity generation because of their ability to consume fuel more efficiently, which translates into a reduction in carbon dioxide emission, considered the main factor contributing to global warming. However, economic analyses do not always favor the implementation of this technology. Even though CCHP systems offer other benefits such as power reliability, power quality, and fuel source flexibility, they are often negated as a feasible alternative because of these poor economic indicators. Therefore, a more comprehensive evaluation of the system should be considered. This is particularly true in an environment where economic, environmental, political, and logistical problems associated with increasing centralized electrical power production are becoming more difficult to overcome. In addition, as consumers continue to be more involved and to develop a better understanding of energy choices, the demand for technology that better meets their energy needs is increasing.

To promote the development of CCHP projects, it is important to facilitate, without any cost, a first order analysis of this technology to determine if a more cost intensive, in-depth analysis should be performed. This analysis can be done by using screening tools such as the CCHP Screening Tool for Existing Office Buildings (CCHP-ST-EOB) proposed in this study. Screening tools should be as accurate as possible while maintaining the simplicity of their data input in order to make it easy to use by a broad audience that may include building owners and managers without engineering background. In this sense, the CCHP-ST-EOB uses a methodology that translates energy consumption from utility bills as input into hourly energy consumption for a more accurate analysis of the matching between the demand and supply sides. This tool takes into consideration partial load efficiencies for the power generation unit and absorption chiller for a more realistic simulation of the system performance. Results are presented in terms of cost, primary energy consumption, and CO2 emission. The tool is available to be downloaded free of charge at http://microchp.msstate.edu/thankyou.html.

Commentary by Dr. Valentin Fuster
2012;():151-159. doi:10.1115/ES2012-91045.

Pending or recently enacted greenhouse gas regulations and mandates are leading to the need for current and feasible GHG reduction solutions including combined heat and power (CHP). Distributed generation using advanced reciprocating engines, gas turbines, microturbines and fuel cells has been shown to reduce greenhouse gases (GHG) compared to the U.S. electrical generation mix due to the use of natural gas and high electrical generation efficiencies of these prime movers. Many of these prime movers are also well suited for use in CHP systems which recover heat generated during combustion or energy conversion. CHP increases the total efficiency of the prime mover by recovering waste heat for generating electricity, replacing process steam, hot water for buildings or even cooling via absorption chilling. The increased efficiency of CHP systems further reduces GHG emissions compared to systems which do not recover waste thermal energy. Current GHG mandates within the U.S Federal sector and looming GHG legislation for states puts an emphasis on understanding the GHG reduction potential of such systems. This study compares the GHG savings from various state-of-the-art prime movers. GHG reductions from commercially available prime movers in the 1–5 MW class including, various industrial fuel cells, large and small gas turbines, micro turbines and reciprocating gas engines with and without CHP are compared to centralized electricity generation including the U.S. mix and the best available technology with natural gas combined cycle power plants. The findings show significant GHG saving potential with the use of CHP. Also provided is an exploration of the accounting methodology for GHG reductions with CHP and the sensitivity of such analyses to electrical generation efficiency, emissions factors and most importantly recoverable heat and thermal recovery efficiency from the CHP system.

Commentary by Dr. Valentin Fuster
2012;():161-166. doi:10.1115/ES2012-91046.

This paper evaluates the economic, energetic, and environmental feasibility of using two power generation units (PGUs) to operate a combined heat and power (CHP) system. A benchmark building developed by the Department of Energy for a full-service restaurant in Chicago, IL is used to analyze the proposed configuration. This location is selected since it usually provides favorable CHP system conditions in terms of cost and emissions reduction. In this investigation, one PGU is operated at base load to satisfy part of the electricity building requirements (PGU1), while the other is used to satisfy the remaining electricity requirement operating following the electric load (PGU2). The dual-PGU configuration (D-CHP) is modeled for several different scenarios in order to determine the optimum operating range for the selected benchmark building. The dual-PGU scenario is compared with the reference building using conventional technology to determine the economical, energetic, and environmental benefits of this proposed system. This condition is also compared to a CHP system operating following the electric load (FEL) and to a base-loaded CHP system, and it provides greater savings in operating cost, primary energy consumption, and carbon dioxide emissions than the optimized conditions for base loading and FEL.

Commentary by Dr. Valentin Fuster
2012;():167-174. doi:10.1115/ES2012-91148.

Gas turbine-based power plants generate a significant portion of world’s electricity. This paper presents the modeling of a gas turbine-based cogeneration cycle. One of the reasons for the relatively low efficiency of a single gas turbine cycle is the waste of high-grade energy at its exhaust stream. In order to recover this wasted energy, steam and/or hot water can be cogenerated to improve the cycle efficiency. In this work, a cogeneration power plant is introduced to use this wasted energy to produce superheated steam for industrial processes. The cogeneration system model was developed based on the data from the Whitby cogeneration power plant in ASPEN PLUS®. The model was validated against the operational data of the existing power plant. The electrical and total (both electrical and thermal) efficiencies were around 40% and 70% (LHV), respectively. It is shown that cogenerating electricity and steam not only significantly improve the general efficiency of the cycle but it can also recover the output and efficiency losses of the gas turbine as a result of high ambient temperature by generating more superheated steam. Furthermore, this work shows that the model could capture the operation of the systems with an acceptable accuracy.

Commentary by Dr. Valentin Fuster
2012;():175-184. doi:10.1115/ES2012-91175.

Combined heat and power (CHP) or cogeneration systems provide both electricity and useful heat to a building. CHP systems can result in lower operational cost, primary energy consumption (PEC), and carbon dioxide emissions when compared to the standard alternative of purchasing electricity from the grid and supplying heat from a boiler. However, the potential for these benefits is closely linked to the relationship between the ratio of power to heat supplied by the CHP system and the ratio of power to heat demanded by the building. Therefore, the benefits of the CHP system also vary with the size of the prime mover.

In the model presented in this paper, the CHP system is base-loaded, providing a constant power-to-heat ratio. The power-to-heat ratio demanded by the building depends on the location and the needs of the building, which vary throughout the day and throughout the year. At times when the CHP system does not provide the electricity needed by the building, electricity is purchased from the grid, and when the CHP system does not provide the heat needed by the building, heat is generated with a supplemental boiler. Thermal storage is an option to address the building’s load variation by storing excess heat when the building needs less heat than the heat produced by the CHP system, which can then be used later when the building needs more heat than the heat produced by the CHP system.

The potential for a CHP system with thermal storage to reduce cost, PEC, and emissions is investigated, and compared with both a CHP system without thermal storage and with the standard reference case. This proposed model is evaluated for three different commercial building types in three different U.S. climate zones. The size of the power generation unit (PGU) is varied and the effect of the correspondingly smaller or larger base load on the cost, PEC, and emissions savings is analyzed. The most beneficial PGU size for a CHP system with the thermal storage option is compared with the most beneficial PGU size without the thermal storage option. The need for a supplemental boiler to provide additional heat is also examined in each case with the thermal storage option.

Commentary by Dr. Valentin Fuster
2012;():185-194. doi:10.1115/ES2012-91218.

A recently patented hybrid technology may prove to be an energy game-changer. This innovative integrated combined cycle uses two fuels and a large gas (combustion) turbine in tandem with a small, efficient helium nuclear reactor to cleanly produce electrical power.

The hybrid approach to energy sustainability combines the strengths of individual energy assets to yield an optimal solution to meet the planet’s needs. This integration is more effective than the sum of the individual technologies by themselves. The hybrid is able to efficiently use all of fuel resources available in the US in a single power plant.

The hybrid-nuclear family of technologies is a fail-safe, environmentally friendly and evolutionary new direction for nuclear power and energy production.

Commentary by Dr. Valentin Fuster
2012;():195-202. doi:10.1115/ES2012-91390.

This paper presents a solar Organic Rankine Cycle (ORC) for electricity generation; where a regression based approach is used for the working fluid. Models of the unit’s sub-components (pump, evaporator, expander and condenser) are also presented. Heat supplied by the solar field can heat the water up to 80–95 °C at mass flow rates of 2–12 kg/s and deliver energy to the ORC’s heat exchanger unit. Simulation results of steady state operation using the developed model shows a maximum power output of around 40 kWe. Both refrigerant and hot water mass flow rates in the system are identified as critical parameters to optimize the power production and the cycle efficiency.

Commentary by Dr. Valentin Fuster
2012;():203-208. doi:10.1115/ES2012-91427.

Data centers play an important role in modern business. They require a large amount of electricity and cooling energy simultaneously. In 2007, the percentage of total energy consumed by data centers in total US energy doubled over seven years and it is estimated to be double again by 2012. Currently data centers typically employ separate cooling, heating and power (SCHP) systems. The Combined Cooling Heating and Power (CCHP) system is an efficient, clean, and reliable approach to generating power and thermal energy from a single fuel source simultaneously on site. They could be suitable energy supply systems for data centers since demands from data centers match with the energy generation of the CCHP systems. This paper assesses the energy performance of a CCHP system for the Qualcomm data center in San Diego, California, by means of modeling and operational data analysis. The CCHP system mainly consists of four gas turbines, one exhaust fired absorption chiller, three hot water fired absorption chillers, three electrical chillers and seven cooling towers. System performance models have been developed and validated by experimental data in TRNSYS. The modeling result shows that the CCHP system is capable of meeting the electricity and cooling demands with an overall system efficiency of 46%. As a result, the CCHP system could approximately save 12.9GWh of energy per year compared with SCHP systems. Therefore, the CCHP system is a sustainable and green option for data centers.

Commentary by Dr. Valentin Fuster
2012;():209-215. doi:10.1115/ES2012-91452.

Combined heat and power (CHP) has the potential to decrease greenhouse gas emissions by utilizing waste heat that is typically rejected to the environment. CHP systems have been used to satisfy loads on university and corporate campuses but there may be other clusters of mixed used buildings that are viable for a CHP system. In an urban environment, such as New York City, high electricity loads and space heating loads are located in close proximity to each other, whether in a single building or in a neighborhood. This indicates a potential for clusters of buildings demand that could be satisfied by CHP. The analysis presented attempts to determine the potential for CHP systems for the 28,840 blocks of New York City many of which incorporate buildings of mix use. The systems are sized to meet the electrical base load and are considered viable if the CHP efficiency (useful electrical and thermal energy divided by the fuel input) is greater than 60% and the system size is larger than 30kW. The analysis determined that of the 28,840 blocks in New York City, 3,205 could be considered for a CHP system.

Commentary by Dr. Valentin Fuster
2012;():217-226. doi:10.1115/ES2012-91471.

Pacific Northwest National Laboratory (PNNL) is working with industry to independently monitor up to fifteen distinct 5 kilowatt-electric (kWe) combined heat and power (CHP) high temperature (HT) proton exchange membrane (PEM) fuel cell systems (FCSs) installed in light commercial buildings. This research paper discusses an evaluation of the first six months of measured performance data acquired at a one-second sampling rate from real-time monitoring equipment attached to the FCSs at building sites. Engineering performance parameters are independently evaluated.

Based on an analysis of the first few months of measured operating data, FCS performance is consistent with manufacturer-stated performance. Initial data indicate that the FCSs have relatively stable performance and a long term average production of about 4.57 kWe of power. This value is consistent with, but slightly below, the manufacturer’s stated rated electric power output of 5 kWe. The measured system net electric efficiency has averaged 33.7%, based on the higher heating value (HHV) of natural gas fuel. This value, also, is consistent with, but slightly below, the manufacturer’s stated rated electric efficiency of 36%. The FCSs provide low-grade hot water to the building at a measured average temperature of about 48.4°C, lower than the manufacturer’s stated maximum hot water delivery temperature of 65°C.

The uptime of the systems is also evaluated. System availability can be defined as the quotient of total operating time compared to time since commissioning. The average values for system availability vary between 96.1 and 97.3%, depending on the FCS evaluated in the field. Performance at Rated Value for electrical efficiency (PRVeff) can be defined as the quotient of the system time operating at or above the rated electric efficiency and the time since commissioning. The PRVeff varies between 5.6% and 31.6%, depending on the FCS field unit evaluated. Performance at Rated Value for electrical power (PRVp) can be defined as the quotient of the system time operating at or above the rated electric power and the time since commissioning. PRVp varies between 6.5% and 16.2%. Performance at Rated Value for electrical efficiency and power (PRVt) can be defined as the quotient of the system time operating at or above both the rated electric efficiency and the electric power output compared to the time since commissioning. PRVt varies between 0.2% and 1.4%. Optimization to determine the manufacturer rating required to achieve PRVt greater than 80% has been performed based on the collected data. For example, for FCS unit 130 to achieve a PRVt of 95%, it would have to be down-rated to an electrical power output of 3.2 kWe and an electrical efficiency of 29%.The use of PRV as an assessment metric for FCSs has been developed and reported for the first time in this paper.

For FCS Unit 130, a 20% decline in electric power output was observed from approximately 5 kWe to 4 kWe over a 1,500 hour period between Dec. 14th 2011 and Feb. 14th 2012.

Commentary by Dr. Valentin Fuster

Concentrating Solar Power

2012;():227-235. doi:10.1115/ES2012-91030.

Solar Two was a demonstration of the viability of molten salt power towers. The power tower was designed to produce enough thermal power to run a 10-MWe conventional Rankine cycle turbine. A critical component of this process was the solar tower receiver. The receiver was designed for an applied average heat flux of 430 kW/m2 with an outlet temperature of 565°C (838.15 K). The mass flow rate could be varied in the system to control the outlet temperature of the heat transfer fluid, which was high temperature molten salt. The heat loss in the actual system was calculated by using the power-on method which compares how much power is absorbed by the molten salt when using half of the heliostat field and then the full heliostat field. However, the total heat loss in the system was lumped into a single value comprised of radiation, convection, and conduction heat transfer losses. In this study, ANSYS FLUENT was used to evaluate and characterize the radiative and convective heat losses from this receiver system assuming two boundary conditions: (1) a uniform heat flux on the receiver and (2) a distributed heat flux generated from the code DELSOL. The results show that the distributed-flux models resulted in radiative heat losses that were ∼14% higher than the uniform-flux models, and convective losses that were ∼5–10% higher due to the resulting non-uniform temperature distributions. Comparing the simulations to known convective heat loss correlations demonstrated that surface roughness should be accounted for in the simulations. This study provides a model which can be used for further receiver design and demonstrates whether current convective correlations are appropriate for analytical evaluation of external solar tower receivers.

Commentary by Dr. Valentin Fuster
2012;():237-246. doi:10.1115/ES2012-91039.

Central receiver power towers are regarded as a proven concentrating solar power (CSP) technology for generating utility-scale electricity. In central receiver systems, improper alignment (canting and focusing) of heliostat facets results in beam spillage at the receiver and leads to significant degradation in performance. As a result, proper alignment of heliostats is critical for increasing plant efficiency. Past tools used for analyzing and correcting heliostat alignment at the National Solar Thermal Test Facility (NSTTF) have proven to be laborious and inaccurate, sometimes taking up to six hours per heliostat. In light of these drawbacks, Sandia National Labs (SNL) and New Mexico Tech (NMT) have created the Heliostat Focusing and Canting Enhancement Technique (H-FACET). H-FACET uses a high-resolution digital camera to observe the image of a stationary target reflected by a heliostat facet. By comparing this image to a theoretical image generated via a custom software package, technicians can efficiently identify and correct undesirable deviations in facet orientation and shape. Previous tests have only proven the viability of H-FACET for canting heliostats. As a result, SNL and NMT have expanded H-FACET’s capabilities and analyzed the system’s ability to simultaneously cant and focus heliostats. Initial H-FACET focusing test results have shown improved beam sizes and shapes for single facets. Furthermore, simulations of these tests revealed an approximated system accuracy of better than 1.80 milliradians. This accuracy accounted for technician, position, and additional error sources, suggesting that H-FACET was capable of focusing facets to an even greater accuracy than those seen in the initial tests. When implemented for simultaneous canting and focusing of heliostats, H-FACET has demonstrated its capability to increase peak flux and decrease beam size. These full alignment test results demonstrated an average total system accuracy of 1.17 milliradians on five heliostats. As before, this accuracy included multiple error sources which cannot be corrected by H-FACET. Additionally, these tests revealed that H-FACET can align heliostats in about 1 hour and 30 minutes. Finally, two heliostats aligned with H-FACET maintained average accuracies 1.46 and 1.24 milliradians over a four hour window centered about solar noon. This implies that H-FACET is capable of aligning heliostats to a true off-axis alignment over NSTTF’s operating window. In light of these results, SNL has implemented both the focusing and canting portions of H-FACET at the NSTTF.

Commentary by Dr. Valentin Fuster
2012;():247-253. doi:10.1115/ES2012-91050.

The Southern Nevada Water Authority (SNWA) obtained six of the Amonix, Inc. multi-junction Concentrating Photovoltaic (CPV) systems in 2009. A description of the CPV systems, plant layout, and operating description is given. Data showing the power and that the systems have generated over 1,200 MWh of grid energy during the last 33 months are presented in the paper. An estimate is given of the net annual energy generated based upon the last 33 months of daily energy performance. Data is also presented showing the daily peak power divided by peak DNI and the energy performance during this period. The effect of shading is discussed and data are presented showing its effects on the annual field performance. Data are also given on the operating reliability and the long-term performance of the plant. A discussion and pictures are given of how the terrain is returning to its natural state and how the habitat has accepted the CPV systems.

Topics: Water
Commentary by Dr. Valentin Fuster
2012;():255-261. doi:10.1115/ES2012-91053.

The Steam Injection Gas turbine (STIG) cycle offers a way to use solar steam at a low temperature and pressure, generated by low-cost concentrators, in order to augment the power output of the turbine. In conventional STIG, the steam is generated from the gas turbine exhaust and injected into the combustion chamber. In previous work we proposed the solar augmentation of a STIG cycle, using solar concentrators to generate steam in much larger amounts compared to the natural limit of heat recovery in the conventional STIG. In the current work, an annual analysis of the Solar STIG cycle is presented for two sites with moderate and high annual DNI, under two scenarios: constant power with a varying Solar Fraction (SF), and variable power with a nearly constant SF. Results show typical annual SF in the range of 20–30%, and solar to electricity efficiency of around 15%, similar to the annual efficiency of current parabolic trough plants that operate at much higher pressure and temperature. The variable power scenario improves the SF with only a minor decrease in efficiency.

Topics: Solar energy , Cycles
Commentary by Dr. Valentin Fuster
2012;():263-271. doi:10.1115/ES2012-91055.

Technical and economical evaluation of solar thermal power plants constantly gains more importance for industry and research. The reliability of the results highly depends on the assumptions made for the applied parameters. Reducing a power plant system to one single, deterministic number for evaluation, like the levelized cost of electricity (LCOE), might end in misleading results. Probabilistic approaches can help to better evaluate systems [1] and scenarios [2]. While industry looks for safety in investment and profitability, research is predominantly interested in the evaluation of concepts and the identification of promising new approaches. Especially for research, dealing with higher and hardly quantifiable uncertainties, it is desirable to get a detailed view of the system and its main influences. However, to get there, also a good knowledge on the stochastic interrelations and its interpretation is required.

Therefore, this paper mainly assesses the influences of basic stochastic assumptions and suggests a methodology to consider suitable stochastic input, especially for parameters of systems still under research. As examples, the comparison between a parabolic trough plant with synthetic oil and direct steam generation is used.

Commentary by Dr. Valentin Fuster
2012;():273-280. doi:10.1115/ES2012-91056.

For future parabolic trough plants direct steam generation in the receiver pipes is a promising option for reducing the costs of solar thermal energy [1]. These new solar thermal power plants require innovative storage concepts, where the two phase heat transfer fluid poses a major challenge [2]. For the regions where the heat transfer fluid is in a single phase (water or steam), sensible heat storage using molten salt [3] or concrete [4] as storage material can be applied. However, efficient energy storage in the two-phase evaporation/condensation region requires heat storage operation within a narrow temperature range.

For this two phase region, a high performance PCM storage technology was developed and demonstrated by DLR. A test module using 14 tons of PCM with 700 kWh capacity was built in 2009 and commissioned in 2010 in a direct steam test loop, set up at the power plant Litoral of Endesa in Carboneras, Spain [5]. The PCM-storage uses Sodium nitrate as phase change material with a melting temperature of 305 °C. Cycle testing has started end of 2010. Cycling tests have proven the expected discharge capacity of approx. 700 kWh for the PCM-storage module. System operation in constant pressure mode and sliding pressure mode has been conducted for the PCM-storage. While in the constant pressure mode a peak performance of the storage of more than 700 kW could be demonstrated, in the sliding pressure mode a constant power output over almost the whole charge and discharge period could be provided.

The paper discusses the test results and evaluation for different operation modes for the phase change storage for discharge operation. Charging of the phase change storage is always in a once-through mode. However, for discharge, the steam can be generated either in forced or natural circulation mode or in once-through mode, leading to very different effects for the two-phase flow and filling level inside the heat exchanger pipes in the storage. The effects for forced and natural circulation discharge will be analysed and described in the paper.

Commentary by Dr. Valentin Fuster
2012;():281-290. doi:10.1115/ES2012-91064.

Cavity receivers used in solar power towers and dish concentrators may lose considerable energy by natural convection, which reduces the overall system efficiency. A validated numerical receiver model is desired to better understand convection processes and aid in heat loss minimization efforts. The purpose of this investigation was to evaluate heat loss predictions using the commercial computational fluid dynamics software packages FLUENT 13.0 and SolidWorks Flow Simulation 2011 against experimentally measured heat losses for a heated cubical cavity model [1] and a cylindrical dish receiver model [2]. Agreement within 10% was found between software packages across most simulations. However, simulated convective heat loss was under predicted by 45% for the cubical cavity when experimental wall temperatures were implemented on cavity walls, and 32% when implementing the experimental heat flux from the cavity walls. Convective heat loss from the cylindrical dish receiver model was accurately predicted within experimental uncertainties by both simulation codes using both isothermal and constant heat flux wall boundary conditions except at inclination angles below 15° and above 75°, where losses were under- and over-predicted by FLUENT and SolidWorks, respectively. Comparison with empirical correlations for convective heat loss from heated cavities showed that correlations by Siebers and Kraabel [1] and for an assembly of heated flat plates oriented to the cavity geometry [3] predicted heat losses from the cubical cavity within experimental uncertainties, while correlations by Clausing [4] and Paitoonsurikarn et al. [8] were able to do the same for the cylindrical dish receiver. No single correlation was valid for both receiver models. Different turbulence and air-property models within FLUENT were also investigated and compared in this study.

Commentary by Dr. Valentin Fuster
2012;():291-297. doi:10.1115/ES2012-91067.

A newly developed analytical optical approach — First-principle OPTical Intercept Calculation (FirstOPTIC) — is employed to study the optical impact of receiver position error on parabolic trough collectors. The FirstOPTIC program performs first-principle treatment system optical error sources. By analyzing a large number of cases with varying system parameters such as the overall system optical error and the collector geometrical parameters, the paper quantitatively examines the difference between the first-principle treatment and probability approximation to receiver position error. In addition, a practical correlation between actual measurement data and its probability approximation for receiver position errors is established from parametric study; the correlation can be used to evaluate the relative importance of receiver position error to the collector’s optical performance. The effective coefficients defining the correlation of receiver position errors are also summarized for some existing trough collectors and make it convenient to conduct error-convolution-based optical analysis, which was not straightforward before. It is also shown that FirstOPTIC is a suitable tool for in-depth optical analysis and fast collector design optimization, which otherwise requires computationally intensive ray-tracing simulations.

Commentary by Dr. Valentin Fuster
2012;():299-305. doi:10.1115/ES2012-91069.

The overall efficiency of a Concentrating Solar Power (CSP) plant depends on the effectiveness of Thermal Energy Storage (TES) system [1]. A single tank TES system consists of a thermocline region which produces the temperature gradient between hot and cold storage fluid by density difference [2]. Preservation of this thermocline region in the tank during charging and discharging cycles depends on the uniformity of the velocity profile at any horizontal plane. Our objective is to maximize the uniformity of the velocity distribution using a pipe-network distributor by varying the number of holes, distance between the holes, position of the holes and number of distributor pipes. For simplicity, we consider that the diameter of the inlet, main pipe, the distributor pipes and the height and the width of the tank are constant. We use Hitec® molten salt as the storage medium and the commercial software Gambit 2.4.6 and Fluent 6.3 for the computational analysis. We analyze the standard deviation in the velocity field and compare the deviations at different positions of the tank height for different configurations. Since, the distance of the holes from the inlet and their respective arrangements affects the flow distribution throughout the tank; we investigate the impacts of rearranging the holes position on flow distribution. Impact of the number of holes and distributor pipes are also analyzed. We analyze our findings to determine a configuration for the best case scenario.

Commentary by Dr. Valentin Fuster
2012;():307-316. doi:10.1115/ES2012-91077.

In this work, a novel concentrating solar power system consisting of a small heliostat field utilizing simplified two-axis tracking is proposed for distributed-scale solar thermal power generation. Monte Carlo ray tracing is used to characterize the optical performance of the system and to parametrically evaluate its design. Radiative flux distributions are obtained in the receiver plane for solar irradiation at an example location of Albuquerque, NM, and date of June 21. The system delivers an 8-hour daily average optical efficiency of 64.4%, flux concentration ratio of 122.8 suns, and daily average thermal power of 47.3 kWt for a receiver of 0.35 m radius. The peak optical efficiency at solar noon was found to be 97.9% with a concentration ratio of 201.3 and thermal power of 77.5 kWt for the base simulation parameters.

Commentary by Dr. Valentin Fuster
2012;():317-325. doi:10.1115/ES2012-91095.

Increased receiver temperatures of solar tower power plants are proposed to decrease the plants levelized electricity costs (LEC) due to the utilization of supercritical steam power plants and thus higher overall plant efficiency. Related to elevated receiver temperatures preliminary concept studies show a distinct LEC reduction potential of the internal direct absorption receiver (IDAR), if it is compared to liquid in tube (LIT) or beam down (BD) receiver types. The IDAR is characterized by a downwards oriented aperture of a cylindrical cavity, whose internal lateral area is illuminated from the concentrator field and cooled by a liquid molten salt film. The objective is the further efficiency enhancement, as well as the identification and assessment of the technical critical aspects. For this a detailed fluid mechanic and thermodynamic receiver model of the novel receiver concept is developed to be able to analyze the IDAR’s operating performance at full size receiver geometries. The model is used to analyze the open parameters concerning the feasibility, functionality and performance of the concept. Hence, different system management strategies are examined and assessed, which lead to the proposal of a cost optimized lead-concept. This concept involves a rotating receiver system with inclined absorber walls. The spatial arrangements of the absorber walls minimize thermal losses of the receiver and enhance film stability. The centrifugal forces acting on the liquid salt film are essential to realize the required system criteria, which are related to the maximal molten salt temperature, film stability and droplet ejection. Compared to the state of the art at a 200 MWel power level the IDAR concept can lead to a LEC reduction of up to 8%. The cost assumptions made for the assessment are quantified with sensitivity analysis.

Topics: Absorption
Commentary by Dr. Valentin Fuster
2012;():327-332. doi:10.1115/ES2012-91097.

A solar stove using a giant Fresnel lens has been developed in the Energy and Fuel Cell Laboratory at the University of Arizona. Solar tracking is required to control the Fresnel lens to maintain a stationary focal point on the heat transfer surface of the solar stove. A two-axis passive control system for solar tracking is adopted. Characteristics of the optical system are analyzed in order to find a reasonable tracking and adjustment frequency and overall system control accuracy. Defocus of the lens due to the angular offset (related to tracking resolution) of the lens’ axis versus the sunray and the change of the shape of the focal point on a static plate is calculated. The results of the analysis are used in the design of the control algorithm which has been implemented in the control system of the prototype solar stove. The proposed tracking scheme is expected to improve heat collection, thermal protection and thereby reduction of heat loss in the solar stove.

Commentary by Dr. Valentin Fuster
2012;():333-344. doi:10.1115/ES2012-91109.

Within the last years, Linear Fresnel (LF) collector systems have been developed as a technical alternative to parabolic trough collector (PT) systems. In the past, LF systems focused on low- and medium temperature applications. Nowadays, LF systems equipped with vacuum receivers can be operated at the same temperatures as PT systems. Papers about the technical and economical comparison of specific PT and LF systems have already been published, [1–3]. However, the present paper focuses on the systematic differences in optical and thermodynamic performance and the impact on the economic figures.

In a first step the optical performance of typical PT and LF solar fields has been examined, showing the differences during the course of the day and annually. Furthermore, the thermodynamic performance, depending on the operating temperature, has been compared.

In a second step, the annual electricity yield of typical PT and LF plants are examined. Solar Salt has been chosen as heat transfer fluid. Both systems utilize the same power block and storage type. Solar field size, storage capacity, and power block electrical power are variable, while all examined configurations achieve the same annual electricity yield. As expected for molten salt systems, both systems are the most cost-effective with large storage capacities. The lower thermodynamic performance of the LF system requires a larger solar field and lower specific costs in order to be competitive. Assuming specific PT field costs of 300 €/m2 aperture, the break-even costs of the LF system with Solar Salt range between 202 and 235 €/m2, depending on the site and storage capacity.

Commentary by Dr. Valentin Fuster
2012;():345-351. doi:10.1115/ES2012-91131.

The solution proposed in this paper presents a new modeling approach that integrates a generalized thermal storage performance model into a concentrating solar power (CSP) plant. The overall performance, including round trip efficiency, for a thermal energy storage system is highly dependent on the operating parameters and operation strategy of the complete power plant. Previous methods used for analysis of thermal storage have followed one of two approaches: The first requires time-intensive customized detailed performance models of the thermal storage system and the power cycle to account for the effects of charging and discharging storage on conversion efficiency and heat transfer fluid (HTF) return temperature to the solar field. The second method uses a simple energy balance with “derate” factors that do not accurately predict the effects of storage on other plant components. In this paper, we develop a generalized method based on efficiency metrics and discuss the application in TES sizing and performance evaluation for an early concept study. The method is an integral approach and complements the detailed models that simulate yearly operation of a CSP plant.

Commentary by Dr. Valentin Fuster
2012;():353-362. doi:10.1115/ES2012-91143.

In the framework of the National Laboratory of Solar Concentrating and Solar Chemistry Systems (LACYQS, for its Spanish acronym), a Heliostat Test Field (HTF) was built in México. This research facility is located 10 km away from the city of Hermosillo, in the state of Sonora. The main purpose of the HTF, at the present stage, is to serve as platform for the development and testing of heliostat technology.

In order to evaluate the performance of heliostats, various optical tests have been implemented. In the sun tracking test, the heliostat is operated as a solar tracker. A camera is attached to the heliostat, which is pointed directly to the sun. Images are captured throughout the day to quantify the wandering of the solar disc in the image. In the reflected spot test, the image produced on the Lambertian target by the concentrating heliostat, due to the reflection of the sun, is recorded by a CCD camera throughout the day. Image processing algorithms calculate the centroid of energy of the image and evaluate the position and wandering across the white screen at all times. After this information is gathered, and the influence of wind and external factors eliminated, data are interpreted to characterize the behavior of solar projection algorithms and mechanical components.

In the fringe projection analysis, also known as deflectometry, fringe patters are projected at night on a Lambertian target. The image of the pattern reflected by the heliostat is recorded with a camera. Distortions in the fringes, due to mirror stress and canting, allows the characterization of the surface error of the facets.

Commentary by Dr. Valentin Fuster
2012;():363-373. doi:10.1115/ES2012-91154.

Monte Carlo ray tracing, coupled to a finite volume solver, is used to model 3D heat transfer in a parabolic trough solar concentrator system. The non-uniform distribution of the incident solar radiation, the radiative exchange between the various receiver surfaces, and the heat gain/loss around the receiver’s circumference and along the system’s axis are determined for spectral radiative properties of the receiver and concentrator surfaces. The computed heat losses and thermal efficiencies agree well with experimental data. Besides the beneficial information on peak temperatures and heat flux, the 3D model also has a potential to predict glass temperatures more accurately than previous gray models and temperature correlations.

Commentary by Dr. Valentin Fuster
2012;():375-383. doi:10.1115/ES2012-91179.

In 2011, the U.S. Department of Energy (DOE) initiated a “SunShot Concentrating Solar Power R&D” program to develop technologies that have the potential for much higher efficiency, lower cost, and/or more reliable performance than existing CSP systems. The DOE seeks to develop highly disruptive Concentrating Solar Power (CSP) technologies that will meet 6¢/kWh cost targets by the end of the decade, and a high-efficiency, low-cost thermal power cycle is one of the important components to achieve the goal. Supercritical CO2 (s-CO2) operated in a closed-loop Brayton cycle offers the potential of equivalent or higher cycle efficiency versus superheated or supercritical steam cycles at temperatures relevant for CSP applications. Brayton-cycle systems using s-CO2 have a smaller weight and volume, lower thermal mass, and less complex power blocks versus Rankine cycles due to the higher density of the fluid and simpler cycle design. The simpler machinery and compact size of the s-CO2 process may also reduce the installation, maintenance and operation cost of the system.

Commentary by Dr. Valentin Fuster
2012;():385-392. doi:10.1115/ES2012-91187.

Heliostat image drift is defined as the wandering of the irradiance spot produced by a heliostat on a receiver or observation screen. Two factors producing heliostat image drift have been analyzed theoretically in the present paper: errors in reference position, and time delay. Some regular behavior is found in the drift curves. Maximum deviations from target tend to be linearly dependent with either angular error, or time delay. In particular, the curves produced by this later effect are circular or elliptical. Heliostats at different distances from a tower have been analyzed. For heliostats far from the tower the drift curves due to errors in reference position also have a more or less elliptical shape. Results are presented as dimensionless quantities. Some practical implications of these results are discussed.

Topics: Delays , Errors , Shapes
Commentary by Dr. Valentin Fuster
2012;():393-398. doi:10.1115/ES2012-91204.

HiTek Services, Inc. has designed, fabricated, and tested a solar monitoring system that can measure the sun’s position with high accuracy. Sun position data, measured by the University of Nevada, Las Vegas Center for Energy Research, is presented showing that the instrument’s accuracy is better than 0.004 degrees (one σ). A histogram is presented showing that nearly 100 % of the time the measurement error is less than 0.008 degrees. The capability of using multiple sun monitors with synchronized data collection to measure the structural bending of one part of a tracking structure relative to another part of the structure is discussed and data are presented. Other operating features, such as being self-contained, no external cables required, and the ability to record data for a week without recharging the batteries, are discussed.

Topics: Solar energy , Testing
Commentary by Dr. Valentin Fuster
2012;():399-403. doi:10.1115/ES2012-91229.

Atmospheric attenuation loss between a heliostat field and receiver has been recognized as a significant source of loss in Central Receiver System. Methods that can improve estimation of attenuation loss using available measurements will be useful in reducing uncertainty in estimation of CSP plant production, particularly in locations and climates that differ in atmospheric composition from typical arid desert locations. In clear sky situations, Direct Normal Irradiance (DNI) is primarily impacted by aerosols in the atmosphere. Aerosols extinct direct radiation with the photons either being absorbed or scattered based on the aerosols optical characteristics. As aerosol loading is high close to the surface, the attenuation loss between heliostat and receivers is significantly influenced by amount of aerosols present on a particular day. The purpose of the study is to understand the impact of aerosols on attenuation loss and model this loss as a function of the ratio of measured DNI to a calculated DNI for an “aerosol-free” atmosphere. The assumption here is that the reduction in clear sky DNI due to aerosols when compared to a theoretical “clean environment” value can provide valuable information about aerosol loading at the surface and therefore attenuation loss between heliostat and receiver. Preliminary analysis shows that such an approach is viable.

Historically, human observers have measured visibility on a daily basis. While these observations are subject to varying levels of uncertainty they may be a good indicator of atmospheric attenuation between heliostat and receiver. In this paper we will review historical and recent publications to show how visibility observations contain useful information for estimating attenuation loss in central receiver systems. We will also present a simple relationship that uses visibility observations to estimate heliostat to receiver attenuation for varying separation distances.

Commentary by Dr. Valentin Fuster
2012;():405-412. doi:10.1115/ES2012-91235.

This research expands on previous work by coupling the in-house Monte Carlo Ray Trace (MCRT) radiation model with the more sophisticated fluid dynamics modeling capabilities of ANSYS FLUENT. This allows for the inclusion of more realistic inlet and outlet geometries in the receiver, as well as a turbulence model and much finer grid sizing. Taken together, these features give a more complete picture of the heat transfer, mixing, and temperature profiles within the receiver than previous models. This flow solution is coupled to the MCRT code, by using the in-house MCRT radiation solver to provide the source term of the energy equation. The temperature data output from FLUENT is then fed back into the FORTRAN MCRT code, via a User Defined Function written in C#, and the two models iterate until convergence. The solar input has been modified from the previous model to provide a Gaussian fit to a calculated flux distribution, which is more realistic than a uniform flux. Initial results for a 5 MW solar input agree with the trend identified in Ruther’s work regarding the influence of particle mass loading on heating in the receiver. The maximum outlet temperature reached is 1430K, which is on target for driving a Brayton cycle gas turbine. Cylinder wall temperatures are consistently below those of the gas boundary layer, and significantly below the maximum gas temperature in the receiver cavity.

Commentary by Dr. Valentin Fuster
2012;():413-422. doi:10.1115/ES2012-91237.

Concentrating solar power (CSP) plants with thermal energy storage offer several advantages to plants without storage. Thermal energy storage (TES) allows CSP plants to produce power for longer periods of time each day, making them produce energy more like traditional, fossil fuel power plants. TES also gives the ability to time shift production of energy to times of peak demand, allowing the plant to sell the energy when prices are highest. A CSP plant with storage can increase turbine performance and reach higher levels of efficiency by load leveling production and can remain productive through cloud transients.

Power tower CSP plants are capable of producing extremely high temperatures, as they have the ability to oversize their solar field and achieve a greater concentration ratio. Studies have been conducted on variable working fluids, leading to higher working temperatures. This theoretically allows power towers to use more efficient, higher temperature cycles including the recuperated air Brayton cycle, although none currently exist on a commercial scale. This research focuses on developing a model of a high temperature TES system for use with an air Brayton cycle for a power tower CSP plant.

In this research we model one module of a latent heat TES system designed to meet the thermal needs of a recuperated Brayton engine of 4.6 MWe capacity for six hours. A metal alloy, aluminum-silicide (AlSi), is considered as the phase change medium. The storage tank is approximately 161 m3, or a cylinder with a 5 m diameter that is 8 m tall filled with AlSi with several air pipes throughout the volume. We model the volume around a single pipe in a 2-D cylindrical coordinate system, for a module size of 0.2 m in diameter and 8 m long. The advantages of using AlSi alloys is that they have variable melting temperatures depending on the relative concentration of the two metals, from 577 C for the eutectic composition of 12.6% silicon to 1411 C for 100% silicon. This attribute is taken advantage of by the TES model as the composition of the AlSi alloy will vary axially. This will allow a cascaded type storage system within one tank and with one material. The use of FLUENT to model this problem is first validated by several analytical solutions including Neumann’s exact solution for a one-dimensional Cartesian geometry and the Quasi-Steady Approximation in a 1-D cylindrical geometry. The model developed will establish charge/discharge times for the storage system, round-trip efficiency of the system, ability of the system to meet the demand of the Brayton cycle, and the validity of using off-eutectic metal alloys in a cascade as a latent heat TES medium.

Commentary by Dr. Valentin Fuster
2012;():423-428. doi:10.1115/ES2012-91256.

At present, the utilization of thermal energy from sunlight has been widely adopted as the working principle of concentrated solar power (CSP) generation systems. In this research, we suggest a CSP technology based on the properties of transparent conductive oxide (TCO) thin films on metal substrates which is compatible with mass production of solar selective absorbers that can be utilized at high temperatures. Since the plasma wavelength of TCO materials is in the infrared region, electromagnetic waves with wavelengths longer than the plasma wavelength are reflected at the surface, whereas electromagnetic waves with shorter wavelengths pass through the surface layer and reach the substrate. In other words, a TCO thin film behaves as an antireflection film only in the transparency range of TCO coating. This phenomenon is demonstrated through numerical simulations based on rigorous coupled-wave analysis (RCWA). The prepared samples also show favorable spectral selectivity and satisfactory performance as solar selective absorbers, with a solar absorptance of 0.76, a thermal emittance of 0.12 at 800°C and a spectral selectivity of 6.5 at 800°C.

Commentary by Dr. Valentin Fuster
2012;():429-435. doi:10.1115/ES2012-91264.

Deviations from the ideal shape of reflector panels for parabolic trough solar power plants have relevant impact on field efficiency and thus on the performance of the whole power plant. Analyzing the gravity-induced deformation of mirror shape for different mirror angles is relevant for performance calculation of solar parabolic trough collectors and identifying optimization potential of the mirror panels.

Two mirror model versions (stiff and elastic supports) are evaluated in four angles: in horizontal laboratory angle (mirrors facing upward with mounting points horizontally aligned), and in 0°, 45° and 90° collector angle. The resulting slope maps are calculated in a separate post-processing.

In order to evaluate the effect of gravity load on mirror shape, the deformed mirror in each evaluated angle is compared to the non-deformed mirror shape, and to the shapes in 0° (zenith) collector angle, respectively. The resulting slope deviation maps show the mirror deformation in different mirror angles. Stiffness of the mounting to the support structure has a relevant impact. Mirror deformation on elastic brackets (SDx up to 1.6 mrad) is much more pronounced than on an ideal stiff support structure (SDx up to 1.0 mrad).

Commentary by Dr. Valentin Fuster
2012;():437-447. doi:10.1115/ES2012-91283.

As we pursue efforts to lower the capital and installation costs of parabolic trough solar collectors, it is essential to maintain high optical performance. While there are many optical tools available to measure the reflector slope errors of parabolic trough solar collectors, there are few tools to measure the absorber alignment. A new method is presented here to measure the absorber alignment in two dimensions to within 0.5 cm. The absorber alignment is measured using a digital camera and four photogrammetric targets. Physical contact with the receiver absorber or glass is not necessary. The alignment of the absorber is measured along its full length so that sagging of the absorber can be quantified with this technique. The resulting absorber alignment measurement provides critical information required to accurately determine the intercept factor of a collector.

Commentary by Dr. Valentin Fuster
2012;():449-457. doi:10.1115/ES2012-91305.

This paper investigates the possibility of using a post-industrial ceramic commercially called Cofalit as a promising, sustainable, and inexpensive ($10/ton) thermal energy storage material. This ceramic presents relevant properties to store thermal energy by means of sensible heat in the temperature range of concentrated solar power (CSP) plants from ambient temperature up to 1100 °C. In the present study, the compatibility of this ceramic was studied with two conventional heat transfer fluids: nitrate molten salts for medium-temperature applications (200 to 500 °C) and air for high-temperature applications (500 to 900 °C). The use of this ceramic in direct contact with the heat transfer fluid should significantly reduce the cost of thermal energy storage systems in CSP applications and help to achieve the U.S. Department of Energy’s SunShot Initiative cost targets.

Commentary by Dr. Valentin Fuster
2012;():459-468. doi:10.1115/ES2012-91317.

This paper presents the technical formulation and demonstrated model performance results of a new direct-steam-generation (DSG) model in NREL’s System Advisor Model (SAM). The model predicts the annual electricity production of a wide range of system configurations within the DSG Linear Fresnel technology by modeling hourly performance of the plant in detail. The quasi-steady-state formulation allows users to investigate energy and mass flows, operating temperatures, and pressure drops for geometries and solar field configurations of interest.

The model includes tools for heat loss calculation using either empirical polynomial heat loss curves as a function of steam temperature, ambient temperature, and wind velocity, or a detailed evacuated tube receiver heat loss model. Thermal losses are evaluated using a computationally efficient nodal approach, where the solar field and headers are discretized into multiple nodes where heat losses, thermal inertia, steam conditions (including pressure, temperature, enthalpy, etc.) are individually evaluated during each time step of the simulation.

This paper discusses the mathematical formulation for the solar field model and describes how the solar field is integrated with the other subsystem models, including the power cycle and optional auxiliary fossil system. Model results are also presented to demonstrate plant behavior in the various operating modes.

Topics: Steam
Commentary by Dr. Valentin Fuster
2012;():469-477. doi:10.1115/ES2012-91337.

The concept of absorbing concentrated solar radiation volumetrically, rather than on a surface, is being researched by several groups with differing designs for high temperature solar receivers. The Small Particle Heat Exchange Receiver (SPHER), one such design, is a gas-cooled central receiver capable of producing pressurized air in excess of 1100 C designed to be directly integrated into a Brayton-cycle power block to generate electricity from solar thermal power. The unique heat transfer fluid used in the SPHER is a low-density suspension of carbon nano-particles (diameter ∼ 200 nm) to absorb highly concentrated solar radiation directly in a gas stream, rather than on a fixed absorber like a tube or ceramic foam. The nano-particles are created on-demand by pyrolyzing a small flow of natural gas in an inert carrier gas just upstream of the receiver, and the particle stream is mixed with air prior to injection into the receiver. The receiver features a window (or multiple windows, depending on scale) on one end to allow concentrated sunlight into the receiver where it is absorbed by the gas-particle suspension prior to reaching the receiver walls. As they pass through the receiver the carbon nano-particles oxidize to CO2 resulting a clear gas stream ready to enter a downstream combustor or directly into the turbine. The amount of natural gas consumed or CO2 produced is miniscule (1–2%) compared to what would be produced if the natural gas were burned directly to power a gas turbine.

The idea of a SPHER, first proposed many years ago, has been tested on a kW scale by two different groups. In the new work, the engineering for a multi-MW SPHER is reported. An in-house Monte Carlo model of the radiation heat transfer in the gas-particle mixture has been developed and is coupled to FLUENT to perform the fluid dynamic calculations in the receiver. Particle properties (size distribution and complex index of refraction) are obtained experimentally from angular scattering and extinction measurements of natural gas pyrolysis in a lab-scale generator, and these are corroborated using image analysis of Scanning Electron Microscope (SEM) pictures of particles captured on a filter. A numerical model of the particle generator has been created to allow for scale-up for a large receiver. We have also designed a new window for the receiver that will allow pressurized operation up to 10 bar with a 2 m diameter window. Recent progress on overcoming the engineering challenges in developing this receiver for a prototype test is reported.

Commentary by Dr. Valentin Fuster
2012;():479-489. doi:10.1115/ES2012-91358.

This paper introduces a new heat engine using a gas, such as air or nitrogen, as the working fluid that extracts thermal energy from a heat source as the energy input. The heat engine is to mimic the performance of an air-standard Otto cycle. This is achieved by drastically increasing the time duration of heat acquisition from the heat source in conjunction with the timing of the heat acquisition and a large heat transfer surface area. Performance simulations show that the new heat engine can potentially attain a thermal efficiency above 50% and a power output above 100 kW under open-cycle operation. Additionally, it could drastically reduce engine costs and operate in open cycles, effectively removing the difficulties of dry cooling requirement. The new heat engine may find extensive applications in renewable energy industries, such as concentrating solar power and geothermal energy power. Furthermore, the heat engine may be employed to recover energy from exhaust streams of internal combustion engines, gas turbine engines, and various industrial processes. It may also work as a thermal-to-mechanical conversion system in a nuclear power plant, and function as an external combustion engine in which the heat source is the combustion gas from an external combustion chamber.

Commentary by Dr. Valentin Fuster
2012;():491-498. doi:10.1115/ES2012-91363.

Concentrated solar power (CSP) plants are one of several renewable energy technologies with significant potential to meet a part of our future energy demand. By now, CSP systems are used to supply photovoltaic or thermal power plant, but results on nanorectennas suggest the possibility to use this technology for direct energy conversion of solar radiation into electricity. A rectenna is a rectifying antenna that can be used to directly convert wave energy into DC electricity. Experiences in microwave applications have shown energy conversion efficiency in the order of 85%, and recently empirical tests have demonstrated that this technology can be used up to the infrared wavelength. The present paper, together with first preliminary results on the fabrication of the rectifier (the key element of a rectenna) and its electrical behavior, proposes the numerical simulation of a new CSP system where a receiver, heated by concentrated solar radiation, reemits infrared energy on the nanorectenna, which converts the incoming energy into electricity. In this way the receiver plays the role of a sunlight radiation converter to infrared energy.

The numerical simulation of the system consists of two steps. The first is a ray-tracing model to calculate the concentrator optical efficiency and the energy distribution on the focusing area of the parabolic mirror. The second step consists in the receiver temperature calculation as function of the incident solar radiation. The numerical procedure allows the calculation of the concentrator/receiver assembly performance which returns the energy incident on the nanorectenna as a function of external environmental conditions.

Commentary by Dr. Valentin Fuster
2012;():499-507. doi:10.1115/ES2012-91364.

Power tower concentrated solar plants have the potential to reach temperatures higher than those achievable by a parabolic trough plant. These higher temperatures allow for greater power cycle efficiencies and therefore make power towers an attractive option and a growing topic of research. One common design is to pump water through the tower such that it boils and returns to the power cycle as saturated or superheated vapor. One option to increase power cycle efficiency for a direct steam system is to send the steam exiting the high pressure turbine through a committed reheat receiver section and then through a low pressure turbine.

This paper details a new semi-empirical, first-principles thermal model of a direct steam receiver consisting of dedicated boiler, superheater, and reheater sections. This thermal model — integrated with a regression power cycle model and a heliostat field model in SAM — is used to simulate the performance of a direct steam power tower concentrated solar plant and the analysis results are presented.

Commentary by Dr. Valentin Fuster
2012;():509-518. doi:10.1115/ES2012-91374.

Pyromark 2500 is a silicone-based high-temperature paint that has been used on central receivers to increase solar absorptance. The cost, application, curing methods, radiative properties, and absorber efficiency of Pyromark 2500 are presented in this paper for use as a baseline for comparison to high-temperature solar selective absorber coatings currently being developed. The directional solar absorptance was calculated from directional spectral absorptance data, and values for pristine samples of Pyromark 2500 were as high as 0.96–0.97 at near normal incidence angles. At higher irradiance angles (>40°–60°), the solar absorptance decreased. The total hemispherical emittance of Pyromark 2500 was calculated from spectral directional emittance data measured at room temperature and 600°C. The total hemispherical emittance values ranged from ∼0.80–0.89 at surface temperatures ranging from 100°C – 1,000°C. The aging and degradation of Pyromark 2500 with exposure at elevated temperatures were also examined. Previous tests showed that solar receiver panels had to be repainted after three years due to a decrease in solar absorptance to 0.88 at the Solar One central receiver pilot plant. Laboratory studies also showed that exposure of Pyromark 2500 at high temperatures (750°C and higher) resulted in significant decreases in solar absorptance within a few days. However, at 650°C and below, the solar absorptance did not decrease appreciably after several thousand hours of testing. Finally, the absorber efficiency of Pyromark 2500 was determined as a function of temperature and irradiance using the calculated solar absorptance and emittance values presented in this paper.

Commentary by Dr. Valentin Fuster
2012;():519-528. doi:10.1115/ES2012-91389.

The primary purpose of a thermal energy storage system in a concentrating solar power (CSP) plant is to extend the operation of plant at times when energy from the sun is not adequate by dispatching its stored energy. Storing sun’s energy in the form of latent thermal energy of a phase change material (PCM) is desirable due to its high energy storage density which translates to less amount of salt required for a given storage capacity. The objective of this paper is to analyze the dynamic behavior of a packed bed encapsulated PCM energy storage subjected to partial charging and discharging cycles, and constraints on charge and discharge temperatures as encountered in a CSP plant operation. A transient, numerical analysis of a molten salt, single tank latent thermocline energy storage system (LTES) is performed for repeated charging and discharging cycles to investigate its dynamic response. The influence of the design configuration and operating parameters on the dynamic storage and delivery performance of the system is analyzed to identify configurations that lead to higher utilization. This study provides important guidelines for designing a storage tank with encapsulated PCM for a CSP plant operation.

Commentary by Dr. Valentin Fuster
2012;():529-536. doi:10.1115/ES2012-91392.

A detailed numerical and empirical systems analysis tool was developed which incorporated component scaling cost equations. It was benchmarked against the known data from the Andasol-1 plant in Spain, and then used to evaluate the effect of changes in the size of the solar field, the thermal energy storage system, and the power block on the levelized cost of electricity (LCOE) for the plant. The simulation result indicates that when the power plant capacity increases from 50 MW to 400 MW, the LCOE decreases by 32%. Also, the model’s results indicate that an oversized field and thermal energy storage tanks help to lower the LCOE.

Commentary by Dr. Valentin Fuster
2012;():537-543. doi:10.1115/ES2012-91396.

Advancement of supercritical carbon dioxide Brayton cycle technology in concentrated solar power plants requires an improved understanding of duct-flow convection in the supercritical region. Numerical simulation, based on a modified carbon dioxide hot gas bypass load stand with an external heat source, is conducted to determine carbon dioxide convective heat transfer coefficients at supercritical pressures and temperatures beyond the range for which results are available in the literature.

The simulation geometry is derived from the heated test section included in the physical load stand. Inlet pressure, temperature, and mass flux are varied to assess the influence on Nusselt number. Cases that achieve fully developed flow and temperature conditions inside the tube geometry agree with predictions from a Nusselt number correlation in the literature with a mean absolute error of 6.4 percent, less than the 6.8% average error reported for the correlation. This agreement includes pressure and temperature conditions outside the defined range of the correlation. Future experiments will provide additional validation of the model and correlation, enabling analysis farther into the supercritical region necessary for Brayton cycle operation.

Commentary by Dr. Valentin Fuster
2012;():545-550. doi:10.1115/ES2012-91401.

In order to meet the demands of future high-temperature solar thermal power generation, 37 kinds of mixed carbonate molten salts were prepared by mixing potassium carbonate, lithium carbonate, sodium carbonate in accordance with different proportions in this paper. Melting point of molten salt, as the most important basic character, is the primary parameter to select molten salt. Melting point and decomposition temperature are measured by Simultaneous Thermal Analyzer. The results show that melting points of major ternary carbonates are close at around 400°C and decomposition temperatures of most ternary carbonate are between 800 and 850°C. In accordance with energy variation, when the system is cooled from the molten state, precipitates of crystalline phases is orderly. Crystallization temperatures of some samples are much higher than their melting points. Therefore, through comparative experimental study of heating and cooling, 10 kinds of mixed carbonates with low melting point and crystallization temperature were selected primarily. Then, latent heat, density and thermal stability of these mixed salts were studied.

Topics: Temperature
Commentary by Dr. Valentin Fuster
2012;():551-560. doi:10.1115/ES2012-91416.

Latent thermal energy storage (LTES) system offers high energy storage density and nearly isothermal operation for concentrating solar power generation. However, the low thermal conductivity possessed by the phase change material (PCM) used in LTES system limits the heat transfer rates. Utilizing thermosyphons to charge or discharge a LTES system offers a promising engineering solution to compensate for the low thermal conductivity of the PCM. The present work numerically investigates the enhancement in the thermal performance of charging and discharging process of LTES system by embedding thermosyphons. A transient, computational analysis of the LTES system with embedded thermosyphons is performed for both charging and discharging cycles. The influence of the design configuration of the system and the arrangement of the thermosyphons on the charge and discharge performance of the LTES installed in a concentrating solar power plant (CSP) is analyzed to identify configurations that lead to improved effectiveness.

Commentary by Dr. Valentin Fuster
2012;():561-566. doi:10.1115/ES2012-91417.

The purpose of this study is two-fold: 1) To examine the performance of the Global Solar Insolation Project (GSIP) physics-based model in characterizing global horizontal solar radiation across the United States by comparing to the ground measured data, and 2) to examine improvements of the GSIP data to address temporal and spatial variations. The study enumerates and examines the spatial and temporal limitations of the GSIP model. Most comparisons demonstrate relatively good statistical agreement. However, the methodology used in the satellite model to distinguish microclimate conditions presents significant challenges, and the model requires refinement in addressing aerosol estimates, water vapor estimates, and clear sky optical properties. Satellite derived datasets are only available at half-hour intervals. Surface measurement can easily be made at temporal resolution in the order of seconds. Therefore intra-hour variability, an important quantity for understanding how power production in power plants will vary, cannot be directly derived from satellites. This paper illustrates how intra-hour variability in ground measurements cannot be captured by the satellite based datasets. We also discuss the potential for improved next-generation geostationary satellite data to improve the accuracy of surface radiation estimates.

Topics: Satellites
Commentary by Dr. Valentin Fuster
2012;():567-571. doi:10.1115/ES2012-91425.

Heliostat canting is the alignment of facets on a common frame which provides focusing of sunlight on a prescribed target. Traditionally, this alignment has been parabolic, in which the focal point of the heliostat lies on its optical axis. Two alternative off-axis canting methods are compared in this article, (i) fixed facet (static) canting in which the facet alignment is optimized for a single design day and time and then rigidly mounted to the frame and (ii) dynamic canting in which the facets are actively controlled such that the center of each facet is always perfectly focusing. For both methods, two case studies are considered: (i) a power tower with planar heliostat field (heliostat dimensions and tower height modeled after the 11 MWe plant PS10) and (ii) a hillside heliostat field which directs light down to a ground-level salt pond. In both case studies, static heliostat canting provides a small improvement in focusing by reducing the average annual insolation-weighted spot size by roughly 1–2%. Dynamic canting, in contrast, provides a 20–25% reduction in spot size.

Topics: Dimensions , Design , Sunlight
Commentary by Dr. Valentin Fuster
2012;():573-584. doi:10.1115/ES2012-91435.

A comprehensive parametric investigation on the fluid flow and heat transfer in a Latent-heat based energy storage/release system is explored for the axisymmetric melting and solidification process inside an encapsulated spherical container of 10, 20, 30 and 40mm in diameter. A numerical solution is developed using the finite-volume method and the enthalpy-porosity technique to solve Navier–Stokes and energy equations for natural convection coupled to a solid-liquid phase change. The study focused on Phase Change Materials (PCMs) with a melting temperature lying in the practical range of operation of concentrated solar thermal power generation (573.15K to 673.15K). Numerical calculations are performed in order to compute the evolution of the melting front and the velocity and temperature fields for different Grashof, Stefan and Fourier numbers. Also the effect of different metal coating materials subjected to a uniform wall temperature from 5K to 11K above the mean melting temperature of the PCM is presented. Simulation results show that a recirculating vortex is formed between the top region of the solid phase and the inner wall of the capsule that causes a more intense melting process in the upper part of the solid phase compared to the bottom region. The location of the eye of the recirculation pattern is observed to be dependent on the Grashof number and moves toward the unmelted portion of the PCM as natural convection is intensified.

Commentary by Dr. Valentin Fuster
2012;():585-593. doi:10.1115/ES2012-91437.

Thermally-stratified air layers overlying solar-heated ground are exploited for power generation by the deliberate formation of intense buoyancy-induced vertical columnar vortices, similar to naturally-occurring desert “dust devils.” In hot-climate regions, such buoyancy-driven columnar vortices occur spontaneously with core diameters of 1–50 m at the surface and heights up to one km, leading to flows with considerable angular and axial momentum. Unlike “dust devil” vortices, which are typically free to wander laterally and are therefore susceptible to crosswind, anchored columnar vortices are deliberately triggered and confined within a domain laterally bounded by an azimuthal array of stationary ground-mounted vertical vanes, and are sustained by continuous entrainment of heated air passing between these vanes. Hot air near the solar-heated ground plane sustains the anchored vortex, and electric power is generated by using the resulting rotational and vertical flow induced by an “anchored” vortex to drive a vertical-axis wind turbine coupled to an electric generator. This novel approach to the collection of solar energy provides a low-cost, scalable, and sustainable method for generation of electric power from vast amounts of solar-heated air in arid regions. Meter-scale laboratory experiments have demonstrated the nucleation and sustainment of strong, buoyancy-driven vortices above a thermally controlled ground plane. The present investigation focuses on fundamental mechanisms of columnar vortex formation, evolution, and dynamics using stereo particle image velocimetry (PIV), with particular emphasis on scaling and assessment of the available mechanical power. The strength and scaling of theses vortices can be significantly altered by adjustment of the vanes and the rate of sensible heat uptake by the air flow, related to the “buoyancy flux”. Recent outdoor tests of a meter-scale prototype coupled with a simple vertical-axis turbine placed on a surface directly heated by solar radiation, have demonstrated continuous rotation of the turbine with significant extraction of kinetic energy from the column vortex, in both the absence and presence of crosswind.

Commentary by Dr. Valentin Fuster
2012;():595-599. doi:10.1115/ES2012-91447.

Storage systems based on latent heat storage have high-energy storage density, which reduces the footprint of the system and the cost. However, phase change materials (PCMs) have very low thermal conductivities making them unsuitable for large-scale use without enhancing the effective thermal conductivity. In order to address the low thermal conductivity of the PCMs, macroencapsulation of PCMs is adopted as an effective technique. The macro encapsulation not only provides a self-supporting structure but also enhances the heat transfer rate.

In this research, Sodium nitrate (NaNO3), a low cost PCM, was selected for thermal storage in a temperature range of 300–500°C. The PCM was encapsulated in a metal oxide cell using self-assembly reactions, hydrolysis, and simultaneous chemical oxidation at various temperatures. The metal oxide encapsulated PCM capsule was characterized using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The cyclic stability and thermal performance of the capsules were also studied.

Commentary by Dr. Valentin Fuster
2012;():601-610. doi:10.1115/ES2012-91453.

The collector accuracy requirements for parabolic trough systems are a function of the concentrator and receiver geometry. As the current trend is to use larger trough designs the need for higher accuracy is generally more important. Concentrating solar power (CSP) companies developing and deploying collectors need to meet stringent optical performance requirements and thus require accurate surface characterization instruments to validate that performance. All reflective characterization processes are sensitive to the instrument resolution, experimental setup, data fitting process, and analysis. Small changes in any of these factors can impact the estimated optical performance. It is desirable to have total local surface measurement uncertainties less than 0.5 milliradians (mrad) for a parabolic trough reflector and many instruments are capable of achieving this. Most surface characterization instruments perform a fitting process on measured data that yields a best fit description of the collector surface using some sort of polynomial. Because of this relationship it is desirable to have a robust fitting process. The type and order of the fitted polynomial and fitting process are the two major contributors to describing a facet’s surface based on the measured data. The order of the polynomial can increase or decrease the accuracy of surface description relative to the true surface. This is a function of the existing aberrations in the facet and the surface naturally described by the polynomial. An accurate description of a surface is typically obtained by performing a least squares fit on measured surface data relative to the polynomial. The best analytical description of the surface is achieved when residual errors relative to the polynomial are minimized. The difference between measured data and the best fit description is completed using an iterative process. However, not all surface imperfections on a single reflector can be accurately described with a polynomial as an exact mathematical description of the surface can never be truly achieved. Local positional errors exist in isolated areas of a facet cannot always be fit accurately. The sensitivity in the best fit description of the surface and the surface resulting from the fitting process using higher order polynomials will be discussed in this paper. The change in calculated facet location and surface slope are compared to determine the sensitivity of the process. The results are then used to calculate the intercept factor using ray tracing and estimate the sensitivity in this calculated performance metric.

Commentary by Dr. Valentin Fuster
2012;():611-614. doi:10.1115/ES2012-91458.

The SkyTrough is an advanced integrated parabolic trough concentrator designed for high performance and low cost to achieve economic objectives in the market for high grade heat for industrial processes and electrical generation. To achieve low cost, a comprehensive optimization process was carried out for every component based on the choice of low cost silvered polymer film as the reflector. To verify high performance, the optical efficiency of a single module was measured at the National Renewable Energy Lab (NREL), and a demonstration loop was constructed in December, 2009 at the SEGS-II solar power plant in Daggett, CA, USA. This paper compares operating data recorded over eighteen months for the commercial demonstration at the SEGS-II plant with model predictions based on the NREL efficiency measurement. The comparison demonstrates that the SkyTrough system will perform predictably over time. Additional data illustrating the good performance of the collector in wind, and the sustained reflectance of the mirror film, are presented.

Commentary by Dr. Valentin Fuster
2012;():615-622. doi:10.1115/ES2012-91459.

The American Society of Mechanical Engineers (ASME) Performance Test Codes (PTCs) have provided the power industry with the premier source of guidance for conducting and reporting performance tests of their evolving base technologies of power producing plants and supporting components. With an overwhelming push for renewable energy in recent years, ASME PTCs are in the development of similar standards for the testing of concentrating solar thermal technologies based power plants by the formation of a committee to develop “PTC 52, Performance Test Code on Concentrated Solar Plants”, on July 2009.

The U.S. Department of Energy’s (DOE) SunShot Initiative goal is to reduce costs and eliminate market barriers to make large-scale solar energy systems cost-competitive with other forms of energy by the end of the decade. The ASME PTC-52 similarly removes critical barriers hindering deployment and speeds the implementation of concentrating solar power technologies by reducing commercial risk by facilitating performance testing procedures with quantified uncertainty. As with any commercialization of power producing technologies, clearly defining risk and providing methods to mitigate those risks are essential in providing the confidence necessary to secure investment funding. The traditional power market accomplishes this by citation of codes and standards in contracts; specifically ASME PTCs which supply commercially accepted guidelines and technical standards for performance testing to validate the guarantees of the project (Power Output, Heat Rate, Efficiency, etc.). Thus providing the parties to a power project with the tools they need to ensure that the planned project performance was met and the proper transfer of funds are accomplished. To enable solar energy systems to be fully embraced by the power industry, they must have similar codes and standards to mitigate commercial risks associated with contractual acceptance testing. The ASME PTC 52 will provide these standard testing methods to validate Concentrating Solar Power (CSP) systems performance guarantees with confidence.

This paper will present the affect that solar resource variability and measurement accuracies have on concentrating solar field performance uncertainty based on calculation methods like those used for conventional fossil power plants. Measurement practices and methods will be discussed to mitigate that uncertainty. These uncertainty values will be correlated to the levelized cost of electricity (LCOE), and LCOE sensitivities will be derived. The results quantify the impact of resource variability during testing, test duration and sampling rate to annual performance calculation. These uncertainties will be further associated with costs and risks based on typical technology performance guarantees. The paper will also discuss how the development of standard measurements and calculation methods help to produce lower uncertainty associated with the overall plant result, which is already being accomplished by ASME PTCs in conventional power genreation.

Commentary by Dr. Valentin Fuster
2012;():623-630. doi:10.1115/ES2012-91475.

As the importance of latent heat thermal energy storage increases for utility scale concentrating solar power (CSP) plants, there lies a need to characterize the thermal properties and melting behavior of phase change materials (PCMs) that are low in cost and high in energy density. In this paper, the results of an investigation of the melting temperature and latent heat of two binary high temperature salt eutectics are presented. Melting point and latent heat are analyzed for a chloride eutectic and carbonate eutectic using simultaneous Differential Scanning Calorimetry (DSC) and Thermogravimetric Analsysis (TGA). High purity materials were used and the handling procedure was carefully controlled to accommodate the hygroscopic nature of the chloride eutectic. The DSC analysis gives the values of thermal properties of the eutectics, which are compared with the calculated (expected/published) values. The thermal stability of the eutectics is also examined by repeated thermal cycling in a DSC and is reported in the paper along with a cost analysis of the salt materials.

Commentary by Dr. Valentin Fuster
2012;():631-635. doi:10.1115/ES2012-91511.

In order for Concentrating Solar Power (CSP) to become a significant contributor to utility scale baseload power, dramatic reductions in cost and increases in performance must be achieved. 3M Company and Gossamer Space Frames have developed advanced collectors that are centered on a step-change in solar technology aimed at transforming the economics and industrialization of CSP. In particular, we focus on mirror film based reflective materials, stiff and shape accurate panel constructions, and lightweight and accurate space frames. These technology elements have been combined into a new parabolic collector design with an aperture of 7.3 m and length of 12 m. The geometric concentration ratio of the design is 103, far exceeding current designs. The National Renewable Energy Laboratory (NREL) has measured an intercept factor exceeding 99% on the subject collector fielded at SEGS II (Daggett, CA). The successful implementation of this technology platform has implications for new solar collector designs for both point and line focus systems.

Commentary by Dr. Valentin Fuster

Economic, Environmental, Policy, Education, Markets and Legal Aspects of Alternate Energy

2012;():637-643. doi:10.1115/ES2012-91020.

A study of the state of solar energy development in Taiwan has been performed. In this work, general energy use, solar research issues, and solar manufacturing status and applications were surveyed in late 2011. It was found that there are active research efforts underway in a variety of solar technologies, primarily in photovoltaics, and to a limited extent in solar domestic water heating. Significant manufacturing capabilities in photovoltaic cells have developed within the last decade, and this has grown rapidly, such that Taiwan has edged out Germany for the number two spot in the list of top manufacturers. Very little in the line of photovoltaic installations are found on the island, however. Another characteristic in terms of solar water heating manufacturing and application is that not much is found in Taiwan in contrast to what has taken place on the mainland of China. Some government efforts to stimulate the Taiwanese consumer market both in photovoltaics as well as water heating are outlined, but goals for the PV installations seem overly optimistic based upon recent history.

Topics: Solar energy
Commentary by Dr. Valentin Fuster
2012;():645-650. doi:10.1115/ES2012-91201.

Renewable energy continues to grow its share of the energy market during the past few years. This is evident by the fact that in 2010, renewable energy supplied an estimated 16% of global final energy consumption, and that the trend still moves upward despite the overall downturn in global energy consumption in 2009. In Iran, a total of 7797.3 Megawatts renewable energy has been utilized for electricity production during the period between 3/2009 and 3/2010 (i.e. the Iranian official year). This sum yields from four renewables, namely hydro (7704.7 MW), wind (90.6 MW), biogas (1.9 MW) and solar (0.1 MW). During the four year period between 2006 and 2010, the capacities of hydro and wind plants have experienced increases of 27.5% and 90.2%, respectively. In this paper, the current status of the renewable energy in the world in general and in Iran in specific is reviewed and the growth trends are presented with particular emphasis on solar energy. A brief assessment of solar radiation in Iran is then presented and accordingly, the potential locations for each application are identified. These potential locations generally absorb a daily solar irradiation of more than 4.5 kWh/m2, a value that most probably ensures the financial feasibility for a variety of solar thermal applications. A solar radiation map for Iran, indicating appropriate application for each region (based on the average calculated daily radiation for that region), is also presented. Moreover, the relevant researches and investigations conducted in Iran are introduced (including pilot projects and those under development and/or construction).

Commentary by Dr. Valentin Fuster
2012;():651-659. doi:10.1115/ES2012-91217.

This analysis will examine the relationship between increased levels of wind energy generation and emissions per unit of electricity produced using historical data for electricity output and CO2, SO2 and NOx emissions in the Electric Reliability Council of Texas (ERCOT). Renewable Portfolio Standards (RPS) are generally seen in part as a policy tool for reducing overall system CO2 emissions, although renewable energy goals do not directly regulate such emissions. Limiting this analysis to ERCOT provides two important advantages: transmission of wind energy output is constrained by the physical boundaries of the ERCOT grid, simplifying the analysis and avoiding associated ‘leakage issues’; and ERCOT has the highest level of wind generation as a percentage of total system demand of any grid in the continental U.S.

The intermittent nature of wind generation has resulted in the need to ramp conventional thermal generation up and down to compensate for variability in wind output. Such ramping leads to inefficiencies in many fossil-fueled power plants that increase emissions of CO2, SO2, and NOx relative to a respective unit’s peak efficiency emissions rate.

Using EPA’s Clean Air Markets hourly emissions data, we calculate the total combustion emissions of CO2, SO2 and NOx per MWh of electricity output for the ERCOT system from 2003–2011. The EPA database includes CO2, SO2 and NOx emissions reported by facility owner and operators on an hourly basis in a manner that incorporates facility inefficiencies during ramping periods, allowing us to fully evaluate the CO2 emissions reductions achieved in ERCOT as a result of increased wind generation. The study is ongoing as we wait for emissions statistics from the final quarter of 2011 to be released by the EPA in early 2012.

Commentary by Dr. Valentin Fuster
2012;():661-672. doi:10.1115/ES2012-91324.

It is broadly accepted that current energy systems should become more sustainable in both a global and local context. However, setting common goals and shared objectives and determining the appropriate means by which to get there is the subject of heavy debate. Therefore, the American Society of Mechanical Engineers (ASME) and the German Association of Engineers (VDI) initiated a joint project aimed at providing a young engineers’ perspective to the global energy conversation. The young engineer project teams set a common goal of assembling a completely sustainable energy system for the U.S. and Germany by 2050. This includes not only the electricity market, but the overall energy system. Based on the current global energy paradigm, a completely sustainable energy system seems very ambitious. However, multiple analyses show that this path is possible and would in the medium to long run not only be desirable, but also competitive in the market. This future ‘energy puzzle’ consists of many important pieces, and the overall picture must be shaped by an overarching strategy of sustainability. Besides the many detailed pieces, four main critical issues must be addressed by engineers, politicians and everybody else alike. These challenges are:

i) Rational use of energy:

This uncomfortable topic is rather unappealing to communicate, but is a key issue to reduce energy demand and to meet the potentials of renewable energy carriers.

ii) Balancing of electricity demand and generation:

This is a challenge to the electricity markets and infrastructures that are currently designed for base-load, mainly fossil power plants. The overall mix of renewable energy generation, storage technologies, grid infrastructure, and power electronics will decide how efficient and reliable a future energy system will be.

iii) Cost efficiency and competitiveness:

It is a prerequisite for industrialized countries to stay competitive and to establish RE in the market. Developing economic technologies while at the same time establishing a strong RE market is the secret of success.

iv) Acceptance of the system and its consequences:

The best energy strategy cannot be realized without broad public acceptance for it. Therefore, the understanding of the energy technologies and an objective discussion must be promoted — without old fashioned emotionalizing of certain risks.

The paper will present details on the four mentioned aspects, compare the situations between the U.S. and Germany, and propose solutions for appropriate political frame conditions to achieve a sustainable energy system.

Commentary by Dr. Valentin Fuster
2012;():673-680. doi:10.1115/ES2012-91419.

It is estimated that within the next 40 years, solar thermal power plants would be capable of supplying more than half of the electricity needs of EUMENA. While solar irradiance differs widely in Europe due to seasonal variations, in the MENA region, there is abundant and continuous solar irradiance. This make the MENA region suitable for establishing CSP plants and exporting the electricity generated to Europe. This has driven many institutions and agencies, including the World Bank and the Desertec Foundation, to propose various schemes to promote the use of CSP systems in the MENA region.

The objective of this paper is to examine the existing policy and regulatory frameworks in the MENA countries, identify any barriers and make recommendations on how to surmount these barriers, to increase the scale and scope of utilizing CSPs and other renewable energy technologies (RETs) in the region. The paper concludes by making a number of policy and regulatory recommendations to support utilization of solar thermal energy resource within the MENA region.

Commentary by Dr. Valentin Fuster
2012;():681-689. doi:10.1115/ES2012-91465.

Petroleum-based conventional fuels dominate the transportation sector due to simple economics. Per unit of energy, few fuels can rival gasoline and diesel in terms of total cost of ownership to the consumer. While some fuels, such as natural gas and electricity, offer lower fuel costs and/or higher vehicle efficiencies than conventional fuels, the fuel price differentials may not be sufficient to offset the higher initial costs of the vehicles, especially if petroleum prices are low. Even when total costs of ownership are similar or slightly lower for alternative fuels than conventional fuels, differences in attributes, such as vehicle range, fueling time, cargo space, vehicle availability, and fuel availability, and consumer loss aversion suggest that more substantial differences in costs are required before consumers are willing to adopt the alternatives.

In order for the transportation sector to achieve greater energy sustainability, the traditional economic paradigm for the vehicle purchase decision must expand to incorporate the true benefits of alternatives to conventional fuels, namely the societal benefits of increased energy security, lower criteria pollutant emissions, and lower greenhouse gas emissions. These benefits are not purely economic and yet are crucial to the future of transportation. To capture these benefits in the economic scheme, the societal costs of transportation fuels to the U.S. have been monetized according to measurable impacts. For energy security, the costs are tied to decreased economic output, loss of national gross product, economic strain and volatility, oil supply shocks and price spikes, supply disruption, and import costs. For criteria pollutant and greenhouse gas emissions, the costs are tied to human health impacts, property damage, loss of agricultural productivity, and destruction of terrestrial and aquatic ecosystems. These societal costs then applied to the use of specific fuels in two representative market segments, representing distinct applications, duty cycles, fuel consumption, and vehicle lifetime. Incorporating the monetized societal costs of transportation fuels in the total costs of ownership enables a fair comparison that reflects the benefits of alternatives to conventional fuels. As a result, these societal costs provide a justifiable framework for a real discussion on incentives and the direction of energy policy, beyond the mere objective of low fuel prices that has pervaded policy discussions to date.

Commentary by Dr. Valentin Fuster

Energy Systems Design and Thermoeconomic Analysis

2012;():691-698. doi:10.1115/ES2012-91001.

This paper describes a thermodynamic model that simulates the discharge cycle of a single-tank thermal energy storage (TES) system using supercritical fluid in a concentrating solar power plant.

Current state-of-the-art TES design utilizes a two-tank system with molten nitrate salts; one major problem is the high cost of the fluid. The alternate design explored here involves the use of less expensive fluids at supercritical temperatures and pressures. By cycling the storage fluid between a relatively low temperature two-phase state and a high temperature supercritical state, a large excursion in internal energy can be accessed which includes both sensible heat and latent heat of vaporization.

Supercritical storage allows for the consideration of fluids that are significantly cheaper than molten salts; however, a supercritical TES system requires high pressures and temperatures that necessitate a relatively high cost containment vessel that represents a large fraction of the system capital cost. To mitigate this cost, the proposed design utilizes a single-tank TES system, effectively halving the required wall material. A single-tank approach also significantly reduces the complexity of the system in comparison to the two-tank systems, which require expensive pumps and external heat exchangers. However, a single-tank approach also results in a loss of turbine power output as the storage fluid temperature declines over time during the discharge cycle.

The thermodynamic model is used to evaluate system performance; in particular it predicts the reduction in energy output of the single-tank system relative to a conventional two-tank storage system. Tank wall material volume is also presented and it is shown that there is an optimum average fluid density that generates a given turbine energy output while minimizing the required tank wall material and associated capital cost.

Overall, this study illustrates opportunities to further improve current solar thermal technologies. The single-tank supercritical fluid system shows great promise for decreasing the cost of thermal energy storage, and ensuring that renewable energy can become a significant part of the national and global energy portfolio.

Commentary by Dr. Valentin Fuster
2012;():699-707. doi:10.1115/ES2012-91008.

A novel approach to storing thermal energy with supercritical fluids is being investigated, which if successful, promises to transform the way thermal energy is captured and utilized. The use of supercritical fluids allows cost-affordable high-density storage with a combination of latent heat and sensible heat in the two-phase as well as the supercritical state. This technology will enhance penetration of several thermal power generation applications and high temperature water for commercial use if the overall cost of the technology can be demonstrated to be lower than the current state-of-the-art molten salt using sodium nitrate and potassium nitrate eutectic mixtures. An additional attraction is that the volumetric storage density of a supercritical fluid can be higher than a two-tank molten salt system due to the high compressibilities in the supercritical state.

This paper looks at different elements for determining the feasibility of this storage concept — thermodynamics of supercritical state with a specific example, naphthalene, fluid and system cost and a representative storage design. A modular storage vessel design based on a shell and heat exchanger concept allows the cost to be minimized as there is no need for a separate pump for transferring fluid from one tank to another as in the molten salt system. Since the heat exchangers are internal to the tank, other advantages such as lower parasitic heat loss, easy fabrication can be achieved.

Results from the study indicate that the fluid cost can be reduced by a factor of ten or even twenty depending on the fluid and thermodynamic optimization of loading factor. Results for naphthalene operating between 290 °C and 475 °C, indicate that the fluid cost is approximately $3/kWh compared with $25-$50/kWh for molten salt. When the storage container costs are factored in, the overall system cost is still very attractive. Studies for a 12-hr storage indicate that for operating at temperatures between 290–450 °C, the cost for a molten salt system can vary between $66/kWh to $184/kWh depending on molten salt cost of $2/kg or a more recent quote of $8/kg. In contrast, the cost for a 12-hr supercritical storage system can be as low as $40/kWh. By using less expensive materials than SS 316L, it is possible to reduce the costs even further.

Commentary by Dr. Valentin Fuster
2012;():709-717. doi:10.1115/ES2012-91300.

This paper presents an artificial neural network (ANN) for forecasting the short-term electrical load of a university campus using real historical data from Colorado State University. A spatio-temporal ANN model with multiple weather variables as well as time identifiers, such as day of week and time of day, are used as inputs to the network presented. The choice of the number of hidden neurons in the network is made using statistical information and taking into account the point of diminishing returns. The performance of this ANN is quantified using three error metrics: the mean average percent error (MAPE); the error in the ability to predict the occurrence of the daily peak hour; and the difference in electrical energy consumption between the predicted and the actual values in a 24-hour period. These error measures provide a good indication of the constraints and applicability of these predictions. In the presence of some enabling technologies such as energy storage, rescheduling of non-critical loads, and availability of time of use (ToU) pricing, the possible DSM options that could stem from an accurate prediction of energy consumption of a campus include the identification of anomalous events as well the management of usage.

Commentary by Dr. Valentin Fuster
2012;():719-729. doi:10.1115/ES2012-91310.

Simulation results from a hybrid ground source heat pump model are presented for a residential home that integrates a compact cooling tower into an existing ground source heat pump model. The tower is introduced to assess its impact on the operational and economic performance over that of a GSHP alone. Metrics include initial and lifetime operational costs, ground heating effects, heat pump efficiency, and ability to control the temperature of the conditioned space. A single story, 195 m2 house located in Austin, Texas is used as a cooling-dominated test case. Simulations spanning 10-years of operation show that adding the cooling tower is cost effective, but more importantly, it extends the lifetime of the borehole system and maintains the heat pump efficiencies at high levels.

Commentary by Dr. Valentin Fuster
2012;():731-750. doi:10.1115/ES2012-91481.

The United States (U.S.) Department of Energy (DOE)’s Pacific Northwest National Laboratory (PNNL) is spearheading a program with industry to deploy and independently monitor five kilowatt-electric (kWe) combined heat and power (CHP) fuel cell systems (FCSs) in light commercial buildings. This publication discusses results from PNNL’s research efforts to independently evaluate manufacturer-stated engineering, economic, and environmental performance of these CHP FCSs at installation sites. The analysis was done by developing parameters for economic comparison of CHP installations. Key thermodynamic terms are first defined, followed by an economic analysis using both a standard accounting approach and a management accounting approach. Key economic and environmental performance parameters are evaluated, including (1) the average per unit cost of the CHP FCSs per unit of power, (2) the average per unit cost of the CHP FCSs per unit of energy, (3) the change in greenhouse gas (GHG) and air pollution emissions with a switch from conventional power plants and furnaces to CHP FCSs; (4) the change in GHG mitigation costs from the switch; and (5) the change in human health costs related to air pollution.

CHP FCS heat utilization is expected to be less than 100% at several installation sites. Specifically at six of the installation sites, during periods of minimum building heat demand (i.e. summer season), the average in-use CHP FCS heat recovery efficiency based on the higher heating value of natural gas is expected to be only 24.4%.

From the power perspective, the average per unit cost of electrical power is estimated to span a range from $15–19,000/kilowatt-electric (kWe) (depending on site-specific changes in installation, fuel, and other costs), while the average per unit cost of electrical and heat recovery power varies between $7,000 and $9,000/kW. From the energy perspective, the average per unit cost of electrical energy ranges from $0.38 to $0.46/kilowatt-hour-electric (kWhe), while the average per unit cost per unit of electrical and heat recovery energy varies from $0.18 to $0.23/kWh. These values are calculated from engineering and economic performance data provided by the manufacturer (not independently measured data). The GHG emissions were estimated to decrease by one-third by shifting from a conventional energy system to a CHP FCS system. The GHG mitigation costs were also proportional to the changes in the GHG gas emissions. Human health costs were estimated to decrease significantly with a switch from a conventional system to a CHP FCS system.

A unique contribution of this paper, reported for the first time here, is the derivation of the per unit cost of power and energy for a CHP device from both standard and management accounting perspectives. These expressions are shown in Eq. (21) and Eq. (31) for power, and in Eq. (24) and Eq. (34) for energy. This derivation shows that the average per unit cost of power is equal to the average per unit cost of electric power applying a management accounting approach to this latter calculation. This term is also equal to the average per unit cost of heat recovery power applying a management accounting approach. A similar set of relations hold for the average per unit cost of energy. These derivations underscore the value of using Eq. (21) for economic analyses to represent the average per unit cost of electrical power, heat recovery power, or both, and using and Eq. (24) for energy.

Commentary by Dr. Valentin Fuster

Fuels and Infrastructure, Including Biofuels

2012;():751-758. doi:10.1115/ES2012-91061.

Global cultivation of canola increased by approximately 22% between 2000 and 2009, due to increased demand for canola oil for biodiesel production and as an edible oil. In 2009 over 290,000 km2 of canola was cultivated globally. In contrast to oilseed, the commercial market for canola straw is minimal and it is generally ploughed back into the field. The high carbohydrate content (greater than 50 % by dry weight) of canola straw suggests it would be a good feedstock for second-generation bioethanol production. There are four major steps involved in bioethanol production from lignocellulosic materials: (i) pretreatment, (ii) hydrolysis, (iii) fermentation, and (iv) further purification to fuel grade bioethanol through distillation and dehydration. Previous research demonstrated a glucose yield of (440.6 ± 14.9) g kg−1 when canola straw was treated using alkaline pretreatment followed by enzymatic hydrolysis. Whilst bioethanol can be produced using cells free in solution, cell immobilization provides the opportunity to reduce bioethanol production costs by minimizing the extent to which down-stream processing is required, and increasing cellular stability against shear forces. Furthermore, the immobilization process can reduce substrate and product inhibition, which enhances the yield and volumetric productivity of bioethanol production during fermentation, improves operational stability and increases cell viability ensuring cells can be used for several cycles of operation. Previous research used cells of Saccharomyces cerevisiae immobilized in Lentikat® discs to convert glucose extracted from canola straw to bioethanol. In batch mode a yield of (165.1 ± 0.1) g bioethanol kg−1 canola straw was achieved.

Continuous fermentation is advantageous in comparison to batch fermentation. The amount of unproductive time (e.g. due to filling, emptying and cleaning) is reduced leading to increased volumetric productivity. The higher volumetric productivity of continuous fermentation means that smaller reactor vessels can be used to produce the same amount of product. This reduces the capital costs associated with a fermentation plant. Research demonstrated a higher bioethanol yield was attained (224.7 g bioethanol kg−1 canola straw) when glucose was converted to bioethanol using immobilized cells in packed-bed continuous flow columns. On an energy generation basis, conversion of 1 kg of canola straw to bioethanol resulted in an energy generation of 6 MJ, representing approximately 35% energy recovery from canola straw. The amount of energy recovered from canola straw could be improved by increasing the amount of energy recovered as bioethanol and by utilising the process by-products in a biorefinery concept.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2012;():759-767. doi:10.1115/ES2012-91091.

Anaerobic digestion is a waste-to-energy conversion process that offers potential economic and environmental benefits of organic waste diversion and renewable energy generation. However, these systems are often not feasible for small-to-medium size food processors, due to the significant capital investment involved. The key objective of this study is to identify the volume and composition of dairy manure and liquid-phase food manufacturing waste streams available in New York State (NYS) to make co-digestion of multiple feedstocks in centralized anaerobic digester facilities an economically attractive alternative. Organic waste volume and property data were obtained via Freedom of Information Law (FOIL) requests at the county and municipal levels for each of the 62 counties in NYS. Spatial analyses of dairy confined animal feeding operations (CAFO) locations relative to food manufacturing facility locations were analyzed using Microsoft MapPoint imaging software, which identified concentrations of high strength liquid-phase waste in the upstate corridor extending between Buffalo and Albany. The results show that if anaerobically digested, dairy CAFO manure and food manufacturing waste can contribute significantly to the State’s renewable energy portfolio. A laboratory scale two-phase anaerobic digester (bioDrillTS-AD200©) can help establish the correlation between waste properties (e.g. total solids, etc.) and quantity and quality of biogas produced.

Commentary by Dr. Valentin Fuster
2012;():769-775. doi:10.1115/ES2012-91096.

Making fuel from algae is one of the promising approaches of producing biofuels. Open channel raceway is a typical facility of growing algae in medium and large scales. The algae growth rate in water raceway is affected by conditions of water temperature, nutrients, and sunlight. These conditions are essentially associated with the fluid mixing in the flow field. A good mixing of fluid allows better diffusion of nutrients and equal opportunities of exposure to the water surface and therefore sunlight, as well as a uniform temperature everywhere in water raceway so that all algae cells grow in the same rate. While a better fluid mixing is benefit to the growth of algae, it is also desirable that the energy needed to drive the flow and mixing being the minimum. In this work, a novel flow field has been proposed and the flow field was studied through flow visualization and CFD analysis. Optimization of the flow field for better flow mixing and low energy cost for the flow has been considered.

Commentary by Dr. Valentin Fuster
2012;():777-783. doi:10.1115/ES2012-91105.

The province of Cordoba, Colombia, has a 250.000 tons production of corn, which generate about 45.000 tons of cobs per year, which do not represent any value for farmers. The disposal of this agricultural waste has become a source of contamination because is burned in open areas. On the other hand, this province has a considerable production of raw coal; nevertheless, it is characterized by its high sulfur content (1.65%) and low heat value (5111 cal/gr), as a consequence, it has a limited market, but is still used as a fuel. This study proposes the use of biobriquettes that are a composite fuel prepared from a mixture of biomass (corn cob) and coal in a low die press briquetting machine. They consist of different proportions of milled cob (up to 80%) and coal (up to 40%) mixed with a cassava starch based binder. For the mixtures proximal analysis, including sulfur content and heat values of the material was determined according to their composition. The experimental results showed that the biobriquettes compared with the raw coal have between a 92 to 32% lower sulfur content, while the heat value was reduced by 14 to 29%. Therefore, its use will reduce the amount of burned residue in open spaces and consequently the pollution.

Commentary by Dr. Valentin Fuster
2012;():785-794. doi:10.1115/ES2012-91139.

This project investigates the effects of sparger geometry and placement on bubble and fluid flow patterns and convective heat transfer within a column photobioreactor (PBR) using Computational Fluid Dynamics (CFD). Experimental and computational studies have been completed that focused on the hydrodynamics and heat transfer within a rectangular column photobioreactor (34.29 cm long × 15.25 cm wide × 34.29 cm tall) with a single sparger located at the center of its base (33.02 cm × 1.27 cm) running lengthwise. This study extends previous work by investigating the flow patterns and heat transfer effects due to full bottom sparger or porous sparger. The gas bubbles and the water-based media within the photobioreactor are modeled using the Lagrangian-Eulerian approach. A low Reynolds k-Epsilon turbulence model is used to predict near-wall flow patterns. A flat surface photobioreactor is used to achieve sufficient light penetration into the system. The main interaction forces between the bubbles and the media, including drag forces, added mass forces, and lift forces are considered. The overarching goal of this research is to produce biofuels and bioproducts through the improved design of column PBRs used for microalgae production. An important factor in designing photobioreactors is the appropriate selection of sparger geometry and placement. The sparger governs the bubble size distribution and gas holdup. These factors in turn influence flow pattern, effective interfacial area, rates of mass transfer, heat transfer, and mixing. It is hypothesized that increasing the sparger width will improve uniformity of bubble distribution as well as mixing. Despite its importance, optimizing the sparger geometry and placement in PBRs for microalgae production is still largely not understood. In this study, the simulation’s results are presented for various spargers, which can be helpful in designing appropriate sparger geometry and proper placement for increased microalgae production.

Commentary by Dr. Valentin Fuster

Geothermal Energy, Harvesting, Ocean Energy and Other Emerging Technologies

2012;():795-803. doi:10.1115/ES2012-91006.

With recent concerns on CO2 emissions from coal fired electricity generation plants; there has been major emphasis on the development of safe and economical Carbon Dioxide Capture and Sequestration (CCS) technology worldwide. Saline reservoirs are attractive geological sites for CO2 sequestration because of their huge capacity for sequestration. Over the last decade, numerical simulation codes have been developed in U.S, Europe and Japan to determine a priori the CO2 storage capacity of a saline aquifer and provide risk assessment with reasonable confidence before the actual deployment of CO2 sequestration can proceed with enormous investment. In U.S, TOUGH2 numerical simulator has been widely used for this purpose. However at present it does not have the capability to determine optimal parameters such as injection rate, injection pressure, injection depth for vertical and horizontal wells etc. for optimization of the CO2 storage capacity and for minimizing the leakage potential by confining the plume migration. This paper describes the development of a “Genetic Algorithm (GA)” based optimizer for TOUGH2 that can be used by the industry with good confidence to optimize the CO2 storage capacity in a saline aquifer of interest. This new code including the TOUGH2 and the GA optimizer is designated as “GATOUGH2”. It has been validated by conducting simulations of three widely used benchmark problems by the CCS researchers worldwide: (a) Study of CO2 plume evolution and leakage through an abandoned well, (b) Study of enhanced CH4 recovery in combination with CO2 storage in depleted gas reservoirs, and (c) Study of CO2 injection into a heterogeneous geological formation. Our results of these simulations are in excellent agreement with those of other researchers obtained with different codes. The validated code has been employed to optimize the proposed water-alternating-gas (WAG) injection scheme for (a) a vertical CO2 injection well and (b) a horizontal CO2 injection well, for optimizing the CO2 sequestration capacity of an aquifer. These optimized calculations are compared with the brute force nearly optimized results obtained by performing a large number of calculations. These comparisons demonstrate the significant efficiency and accuracy of GATOUGH2 as an optimizer for TOUGH2. This capability holds a great promise in studying a host of other problems in CO2 sequestration such as how to optimally accelerate the capillary trapping, accelerate the dissolution of CO2 in water or brine, and immobilize the CO2 plume.

Commentary by Dr. Valentin Fuster
2012;():805-813. doi:10.1115/ES2012-91271.

As the reduction of carbon emissions becomes an increasingly pressing issue, a larger emphasis is being placed on the need for the development of renewable energy. One such option is geothermal energy which utilizes the heat from the earth’s crust; it presents a vast potential for the production of commercial scale base-load power generation. However, the conventional techniques used in the stimulation of hot dry rocks (HDR) geothermal wells are not very effective in producing a permeable reservoir for heat exchange between the rock mass and the working fluid. To increase the permeability of geothermal reservoirs, a new stimulation technique (developed by CSIRO - Commonwealth Scientific and Industrial Research Organisation) which involves isolating sections of the well for controlled planar fracture growth can be used. However, if these notches/fractures are placed too closely together they will interact with one another, resulting in a deviated fracture path. A two dimensional numerical model has thus been developed to study conditions under which adjacent fractures will interact with one another. This study aims to verify the numerical model through stimulating a number of granite blocks, and drawing comparisons between the observed fracture pattern and that predicted by the model. To achieve this goal, the stimulated and fractured granite blocks were sectioned and their fracture patterns were extracted using a MATLAB code, before being reconstructed in their respective positions. Stimulation was carried out firstly using conventional techniques, and then by trialling the method proposed by CSIRO. Observation of the reconstructed images showed good agreement between the model predictions and the observed fracturing patterns in two-dimensions. However, the three-dimensional pattern in the notched, perpendicular well-bore was observed as a ‘half cylinder’. This was counter intuitive as it was expected that radial symmetry of the fractures would be observed resulting in a ‘bowl’ shape. It was therefore concluded that while the model was unable to accurately predict the three-dimensional geometry of an array of fractures, stimulation through a notched perpendicular wellbore was very effective in the production of a controlled system of fractures with an improved fluid flow and heat exchanging surface area of the reservoir in comparison to the conventional techniques.

Commentary by Dr. Valentin Fuster
2012;():815-825. doi:10.1115/ES2012-91309.

An integrated building load-ground source heat pump model is developed in this paper to serve as a test bed for assessing the short- and long-term performance of GSHP and HGSHP systems with vertical boreholes. The model uses the Simulink/Matlab environment to take advantage of their built-in functionality, allowing for full coupling of the component building load, heat pump, ground loop, and supplemental heat rejection models at every time step The building load model uses the HAMBASE thermal program which can model residential and commercial buildings. The heat pump model uses available data provided by GSHP manufacturers and sensible heat corrections to accurately model operation across a wide range of input conditions. The vertical borehole ground loop model is based on Eskillsons g-function model, but includes a one-dimensional numerical model by Xu to calculate the short-term thermal response of the borehole and ground. The supplemental heat rejection section allows various techniques to be tested. The integrated model captures system performance over a wide range of time scales from seconds to 10–20 years. Results of a 15-year simulation are presented to illustrate the different time scale information that reveal the slow degradation in system performance due to the effects of ground heating.

Topics: Stress , Heat pumps
Commentary by Dr. Valentin Fuster
2012;():827-836. doi:10.1115/ES2012-91311.

Deployment of ground source heat pumps in Texas and the Southwest has been limited by high initial cost and potential ground heating. To address these limitations for a residential application a sensitivity study of the ground loop design parameters was completed. The study uses an integrated building load-ground source heat pump model that is designed to be a test bed for assessing the short- and long-term performance of GSHPs. This study examines a 195 m2 residential house located in Austin, Texas with a 14.6 kW heat pump and 4 vertical boreholes each with a length of 68.5 m. The performance effects and the long-term economics of the total ground loop length, spacing of the boreholes, placement of the boreholes, grout thermal conductivity, and supplemental heat rejector sizes are compared. The study shows the importance of proper sizing, design, and placement of the borehole in locations with severely unbalanced heating and cooling loads.

Commentary by Dr. Valentin Fuster
2012;():837-846. doi:10.1115/ES2012-91347.

Geothermal power generation is achieved by feeding the harnessed geothermal heat into the boiler of an ORC (Organic Rankine Cycle) based engine which uses an organic working fluid characterised by its low boiling temperature. For the purpose of this study, such a system was designed, built and operated to verify the concept of small-scale power generation using heat from a low temperature source. This experimental facility used hot water as the source of heat, brazed plate heat exchangers as the boiler and the condenser, an automotive inline fuel pump for circulation and a Scroll compressor operated in reverse to act as the expander. The working fluid was R245fa with a boiling point of 80°C at a pressure of 790kPa (or 40°C at 250kPa). A Prony Brake was fitted on the shaft of the expander to determine its power output. Experimental investigations found that ∼600W of power could be produced under optimum conditions at a rotational speed of ∼2000RPM. The details of the experimental facility as well as the results of the experiments are provided in this paper.

Commentary by Dr. Valentin Fuster
2012;():847-854. doi:10.1115/ES2012-91485.

A fluidized bed reactor has been developed which uses a two-step thermochemical water splitting process with a peak hydrogen production rate of 47 Ncm3/min.gFe at an oxidation temperature of 850°C. Of particular interest, is that a mixture of iron and zirconia powder is fluidized during the oxidation reaction using a steam mass flux of 0.58 g/min-cm2, and the zirconia powder serves to virtually eliminate iron powder sintering while maintaining a high reaction rate. The iron/zirconia powder is mixed with a ratio of 1:2 by apparent volume, equivalent mass ratio, and both iron and zirconia particles are sieved to sizes ranging from 125–355 μm. Fluidized bed reactors are advantageous because they have high reactivity, strong thermal and chemical transport, and tend to be compact. There has been significant interest in developing fluidized bed reactors for solar thermochemical reactors, but sintering of the reactive powder has inhibited their development. The current powder mixture and reactor configuration shows great potential for achieving high hydrogen production rates for operation at high temperature.

The experimental investigations for utilizing zirconia as a sintering inhibitor was found to be dependent on the iron and zirconia particle size, particle size distribution and iron/zirconia apparent volume ratio.

For example at 650 °C the oxidation of iron powder with a mean particle size of 100 μm and a wide particle size distribution (40–250 μm) mixed with 44 μm zirconia powder with an iron/zirconia apparent volume ratio of 1:1 results in 75–90 % sintering. In all cases when iron is mixed with zirconia, the hydrogen production rate is not affected when compared with the pure iron case. When iron powder is mixed with zirconia, both with a narrow particle size distribution (125–355 μm) the first oxidation step results in 3–7% sintering when the reactions are carried out at temperatures ranging between 840–895 °C. The hydrogen fractional yield is high (94–97%). For subsequent redox reactions, the sintering is totally eliminated at 867 and 895 °C although the hydrogen fractional yield decreases to 27 and 33%, respectively. This study demonstrates that mixing iron with zirconia in an equivalent mass ratio and similar particle size can eliminate sintering in a fluidized bed reactor at elevated temperatures up to 895°C.

Commentary by Dr. Valentin Fuster

Materials; Micro and Nano Technologies Applications to Energy Systems

2012;():855-862. doi:10.1115/ES2012-91031.

Trapping of carbon in deep underground brine-filled reservoirs is a promising approach for the reduction of atmospheric greenhouse gas emissions. However, estimation of the amount of carbon dioxide (CO2) that can be captured in a reservoir remains a challenge. One difficulty lies in the estimation of local capillary pressure effects that arise from mineral surface heterogeneity inherent in underground geological formations. As a preliminary step to address this issue, we present a series of pore network modeling (PNM) simulations of two-phase immiscible flow in 3D structured porous media with contact angle heterogeneity. We present saturation patterns for networks with homogeneous and heterogeneous wettability under typical reservoir conditions, taking into account varying contact angles for CO2 on mica and quartz at supercritical conditions. At lower flow rates, our preliminary results showed higher saturations for the heterogeneous networks than for the homogeneous ones. To characterize the fingering patterns, we have introduced R as the ratio of filled throats to the total network saturation. Based on this measure, the heterogeneous networks demonstrated thicker fingering patterns than the homogeneous networks. These preliminary results highlight the importance of micro-scale surface heterogeneity for the modeling of carbon storage processes.

Commentary by Dr. Valentin Fuster
2012;():863-866. doi:10.1115/ES2012-91116.

In this paper, we characterized the microstructure of Indiana Limestone rock samples using X-ray micro-computed tomography (microCT) measurements. Our preliminary results include the porosity, and three-dimensional pore reconstructions for each sample. The resulting porosity values are consistent with experimental permeability tests.

Commentary by Dr. Valentin Fuster
2012;():867-874. doi:10.1115/ES2012-91223.

Gas and air-side heat transfer is ubiquitous throughout many technological sectors, including HVAC (heating, ventilating, and air conditioning) systems, thermo-electric power generators and coolers, renewable energy, electronics and vehicle cooling, and forced-draft cooling in the petrochemical and power industries. The poor thermal conductivity and low heat capacity of air causes air-side heat transfer to typically dominate heat transfer resistance even with the use of extended area structures. In this paper, we report design, analysis, cost modeling, fabrication, and performance characterization of micro-honeycombs for gas-side heat transfer augmentation in thermoelectric (TE) cooling and power systems. Semi-empirical model aided by experimental validation was undertaken to characterize fluid flow and heat transfer parameters. We explored a variety of polygonal shapes to optimize the duct shape for air-side heat transfer enhancement. Predictions using rectangular micro-honeycomb heat exchangers, among other polygonal shapes, suggest that these classes of geometries are able to provide augmented heat transfer performance in high-temperature energy recovery streams and low-temperature cooling streams. Based on insight gained from theoretical models, rectangular micro-honeycomb heat exchangers that can deliver high performance were fabricated and tested. High- and low-cost manufacturing prototype designs with different thermal performance expectations were fabricated to explore the cost-performance design domain. Simple metrics were developed to correlate heat transfer performance with heat exchanger cost and weight and define optimum design points. The merits of the proposed air-side heat transfer augmentation approach are also discussed within the context of relevant thermoelectric power and cooling systems.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster
2012;():875-883. doi:10.1115/ES2012-91316.

Flow boiling experiments with sub-cooled Isopentane and n-Pentane at 3.0bar pressure assess the utility of compressed copper- and steel-filament screen laminate surface coatings as high performance boiling surfaces. High-speed video show that at high heat flux ebullition is unsteady. Isopentane and n-Pentane are found to produce nearly identical boiling characteristic curves. At the same applied heat flux, the superheat of copper filament coatings are much smaller than the steel filament coating superheats.