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ASME Conference Presenter Attendance Policy and Archival Proceedings

2014;():V001T00A001. doi:10.1115/ES2014-NS1.
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This online compilation of papers from the ASME 2014 8th International Conference on Energy Sustainability (ES2014) 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to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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

Combined Energy Cycles, CHP, CCHP, and Smart Grids

2014;():V001T01A001. doi:10.1115/ES2014-6311.

Estimation of the mean lifetime and reliability of sophisticated systems such as cogeneration power plant (CHP) is a challenging problem in energy-engineering applications. A simulation model that predicts failure and repair times is described and used to calculate operating time availability and costs to operate the system. The new concept of estimated repair time with a Weibull probability density function is the key to the study of the down time. This probability distribution was defined for each component to estimate the time necessary to repair and the associated costs in economic and sustainability terms. The described simulation model provides a tool that demonstrates the competitive advantage or disadvantage of the CHP system for an application and enables more informed decisions as to their designs.

Commentary by Dr. Valentin Fuster
2014;():V001T01A002. doi:10.1115/ES2014-6365.

The operation strategies are key points that can influence the annual total energy efficiency, energy saving efficiency and economic benefits of BCHP systems. This paper mainly aims at establishing the model about how to realize several typical operation strategies of BCHP systems based on gas engines, such as operation following the power load and operation following the heating load. Also, in a specific project, operation strategies always depend on the load feature and the government policy and the price of power and etc. This paper includes methods about how to choose the operation strategies reasonably.

Topics: Gas engines
Commentary by Dr. Valentin Fuster
2014;():V001T01A003. doi:10.1115/ES2014-6453.

The carbon reduction and sustainability goals of the University of California, Irvine require increased penetrations of intermittent renewables on the campus microgrid. These increased intermittent renewables create operational challenges related to conventional energy resources. To study these operational challenges, a holistic campus resource dispatch model was developed. The campus energy resources consist of a microgrid with ten 12 kV circuits emanating from one substation, 4 MW of solar photovoltaic, a central combined heat and power plant (19 MW), a district heating and cooling system, and an electric chiller-thermal energy storage system that provide electricity, heat, and cooling. The holistic model includes dynamic models of the combined heat and power (CHP) plant, the electric chiller-thermal energy storage system, and various renewable resources. In addition, models for complimentary technologies were also created to investigate their potential to increase renewable penetration on the campus microgrid. These include battery energy storage, demand response, and energy efficiency. Simulations with the holistic campus resource model revealed several important conclusions: (1) Regardless of renewable resource type, impacts on the CHP plant remains the same, i.e., increased renewable penetrations create reduced CHP plant capacity factors; (2) Local two axis CPV provides lower costs of electricity than local fixed PV at renewable penetrations below 23% after which local fixed PV provides a lower cost of electricity (3) Introduction of a battery into the campus microgrid achieves higher renewable penetrations and improves the operation of CHP plant; and (4) Electric energy storage does not always prove cost effective (i.e., At low renewable penetrations, electric energy storage is not cost effective; At 17% renewable penetration, electric energy storage begins to become cost effective).

Topics: Sustainability
Commentary by Dr. Valentin Fuster
2014;():V001T01A004. doi:10.1115/ES2014-6487.

In this paper, a combined heat and power (CHP) system utilizing two power generation units operating simultaneously with differing operational strategies (D-CHP) is analyzed on the basis of operational cost savings. An operating cost optimization metric, based on the facility monthly power-to-heat-ratio (PHR), is presented. The PHR is defined as the ratio between the electric load and the thermal load required by the facility. Previous work in this field has suggested that D-CHP system performance may be improved by limiting operation of the system to months in which the PHR is relatively low. The focus of this paper is to illustrate how the facility PHR can be used to determine the potential of a D-CHP system to reduce operational cost. This paper analyzed the relationship between the PHR and the operational cost savings of six different benchmark buildings, including buildings that are traditionally poor candidates for CHP or D-CHP systems, due to high cost of operation as compared with conventional systems with separate heating and power (SHP). Achieving operational costs savings through optimal operation based on monthly PHR for these building types can enhance the practical implementation potential of D-CHP and CHP systems.

Commentary by Dr. Valentin Fuster
2014;():V001T01A005. doi:10.1115/ES2014-6527.

The main goal of gas turbine design is the effective use of energy. Usually, the efficient high temperature first and second stage turbine blade surface is cooled by jet of coolant flow from extended exit holes (EEH). Against the prevailing hot gas flow, the flow through EEH must be designed to form a film of cool air over the blade. Computational analyses are performed to examine the cooling effectiveness of flow from EEH over the suction side of a blade by solving conservation equations (mass, momentum and energy) and the ideal gas equation of state for the three-dimensional, turbulent, compressible flow. A diverging flow through EEH is typically choked at its throat, resulting in a supersonic flow, a shock and then a subsonic flow downstream. The location of the shock relative to the high-temperature gas flow over the blade determines the temperature distribution along the blade surface; which is analyzed in detail when the coolant flow rate is varied.

Commentary by Dr. Valentin Fuster
2014;():V001T01A006. doi:10.1115/ES2014-6530.

Good room temperature control is beneficial for user thermal comfort and health. It also helps reduce energy consumption and carbon dioxide emissions inside the buildings. However, there are certain limitations with the current thermostatic valves commonly used in central heating systems, especially the ones used in China, since they cannot satisfactorily adjust room temperatures. In response to such problems, an intelligent on-off regulation method is presented in this paper. The room temperature can be maintained by controlling the valves according to the on-time ratio predicted by this method. The on-time ratio is predicted for each cycle, and the current ratio is decided by checking a 6-order fuzzy-control table with room temperature deviations in the last step, as well as room temperature set points. The design of the fuzzy-control table as a major factor that may affect the controlling effect is also discussed. A proper controlling cycle and optimal controlling table have been worked out. Finally, this paper discusses the application effectiveness of this strategy. Results show that the room temperature can be maintained in the range of set point ±0.5°C using this method in different conditions. It is fair to say that this method is a stable, reliable and advanced controlling method with accurate control.

Commentary by Dr. Valentin Fuster
2014;():V001T01A007. doi:10.1115/ES2014-6672.

Integrating high penetration variable renewables in economically and operationally plausible ways is a current clean energy challenge facing many countries and regions, including California. Renewable energy deployment is a relevant pathway to decarbonize the electricity sector and reduce greenhouse gas emissions (GHGs) and mitigate the harmful effects of climate change. This paper highlights the key findings from a recently completed study, funded by the California Solar Initiative, to develop and investigate strategies to integrate high penetration renewable energy and solar photovoltaic (PV) systems using distributed energy resources (DER). We develop hypothetical operating strategies that utilize the DER present in campus microgrids, such as combined heat and power (CHP) systems and thermal energy storage, and evaluate these based on economic criteria.

Our host site is the University of California, San Diego (UCSD) microgrid, which has a rich DER base that includes a 2.8 MW fuel cell powered by directed biogas, 30 MW of onsite generation, steam and electric chillers, thermal storage and roughly 1.5 MW of onsite solar PV. We develop and evaluate three types of strategies for integrating renewable generation: peak load shifting, on-site PV firming, and grid support. We analyze these strategies with an hourly dispatch optimization model and one year of data. We define a successful renewable integration strategy as one that is operationally plausible and economically viable.

We find all three classes of strategies are technically feasible and can be cost-effective under certain conditions. However, we find that the value proposition to customers such as the UCSD campus, under current tariff structures and market prices, will need to be higher to motivate such customers to offer these services, given the risks associated with changing microgrid operations from regular practice. Our findings suggest alternative incentive mechanisms and engagement strategies beyond those pathways currently available are needed to leverage the potential of DER at campuses for renewables integration purposes. Such efforts are relevant not only to campus resources but to similar commercial and industrial loads across California, including the vast combined heat and power resources.

Topics: Microgrids
Commentary by Dr. Valentin Fuster
2014;():V001T01A008. doi:10.1115/ES2014-6680.

Electrical power generation in austere settings, such as combat zones, places a heavy burden on the US Army; high costs in both dollars and lives lost require that every drop of fuel be used effectively and efficiently. In remote locations such as combat outposts (COPs) and small forward operating bases (FOBs) in Afghanistan, electrical power derived from the Army’s standard Advanced Medium Mobile Power Sources (AMMPS) generator is even used to heat water for showers and heat living spaces. This heating requires conversion of thermal energy to mechanical energy, which is then converted to electrical energy and back to heat. Thus, a significant fuel savings could be realized through the more efficient production of heat. A combined heat and power system is proposed; efficiency is increased by routing the generator exhaust through simple ducting to a standard gas hot water heater to produce hot water with waste heat. With funding from the U.S. Army Rapid Equipping Force, cadets and faculty at the United States Military Academy designed, built and tested a system for under $1,000 in parts which was readily coupled to a 5 kW AMMPS generator to produce hot shower water. Results indicate a possible fuel savings of 1500–2000 gallons per year, 20–35% increased fuel utility, and the ability to provide 10–20 five gallon showers during every 5 hours of operation of each 5 kW generator. At a fuel cost of $20–50 per gallon in the deployed environment, and considering the large inventory of deployed generators, the payback for the Army could be tremendous.

Commentary by Dr. Valentin Fuster

Concentrating Solar Power, Solar Thermochemistry and Thermal Energy Storage

2014;():V001T02A001. doi:10.1115/ES2014-6321.

With a large capacity thermal storage system using phase change material (PCM), Concentrated Solar Power (CSP) is a promising technology for high efficiency of solar energy utilization. In a thermal storage system, a dual-media thermal storage tank is typically adopted in industry for the purpose of reducing the use of the heat transfer fluid (HTF). While the dual-media sensible heat storage system has been well studied, a dual-media latent heat storage system (LHSS) still needs more attention and study; particularly, the sizing of volumes of storage tanks considering actual operation conditions is of significance. In this paper, a strategy for LHSS volume sizing is proposed, which is based on computations using an enthalpy-based 1D model. One example of 60MW solar thermal power plant with 35% thermal efficiency is presented. In the study, potassium hydroxide (KOH) is adopted as PCM and Therminol VP-1 is used as HTF. The operational temperatures of the storage system are 390°C and 310°C, respectively for the high and low temperatures. The system is assumed to operate for 100 days with 6 hours charge and 6 hours discharge every day. From the study, the needed height of the thermal storage tank is calculated from using the strategy of tank sizing. The method for tank volume sizing is of significance to engineering application.

Commentary by Dr. Valentin Fuster
2014;():V001T02A002. doi:10.1115/ES2014-6324.

Experiments are performed to analyze melting and solidification of a phase change material (PCM) enclosed in a vertical cylinder by a concentrically-located heat pipe (HP) surrounded by either aluminum foam or foils. The liquid fraction, temperature distribution, melting (solidification) rates and effectiveness are reported to quantify the improvement in performance relative to a base case, a Rod-PCM configuration. Parameters of interest include the porosity of the PCM-metal composite, the foil thickness, the number of foils and the foam pore density. The main contributor to enhanced performance is shown to be the porosity for both the HP-Foil-PCM and HP-Foam-PCM configurations. Both of these configurations improve heat transfer rates relative to either the HP-PCM or the Rod-PCM configuration. However, the HP-Foil-PCM configuration is shown to have approximately the same performance as the HP-Foam-PCM configuration with one third of the metal mass, for the range of porosities studied here (0.870 to 0.987). The HP-Foil-PCM configuration, with a porosity of 0.957 using 162 foils of thickness 0.024 mm, attained an overall rate of phase change that is about 15 times greater than that of the Rod-PCM configuration and about 10 times greater than that of the HP-PCM configuration. The greatest degree of enhancement was achieved with the HP-Foil-PCM configuration (with porosity 0.957) yielding an average effectiveness during melting (solidification) of 14.7 (8.4), which is an extraordinary improvement over the base case.

Commentary by Dr. Valentin Fuster
2014;():V001T02A003. doi:10.1115/ES2014-6328.

High-temperature receiver designs for solar powered supercritical CO2 Brayton cycles that can produce ∼1 MW of electricity are being investigated. Advantages of a supercritical CO2 closed-loop Brayton cycle with recuperation include high efficiency (∼50%) and a small footprint relative to equivalent systems employing steam Rankine power cycles. Heating for the supercritical CO2 system occurs in a high-temperature solar receiver that can produce temperatures of at least 700 °C. Depending on whether the CO2 is heated directly or indirectly, the receiver may need to withstand pressures up to 20 MPa (200 bar). This paper reviews several high-temperature receiver designs that have been investigated as part of the SERIIUS program. Designs for direct heating of CO2 include volumetric receivers and tubular receivers, while designs for indirect heating include volumetric air receivers, molten-salt and liquid-metal tubular receivers, and falling particle receivers. Indirect receiver designs also allow storage of thermal energy for dispatchable electricity generation. Advantages and disadvantages of alternative designs are presented. Current results show that the most viable options include tubular receiver designs for direct and indirect heating of CO2 and falling particle receiver designs for indirect heating and storage.

Commentary by Dr. Valentin Fuster
2014;():V001T02A004. doi:10.1115/ES2014-6332.

The nonstoichiometric cerium oxide (ceria) redox cycle is an attractive pathway for storing energy from concentrated sunlight in chemical bonds by splitting water and carbon dioxide. The endothermic reduction reaction Display Formula

(R1)
CeO2-δoxCeO2-δed+Δδ2O2
is favored thermodynamically at high temperatures and low oxygen partial pressures, while the CO2 and H2O splitting reactions (R2, R3) are exothermic and favored at lower temperatures and higher oxygen partial pressures. The produced hydrogen and carbon monoxide, referred to collectively as syngas, are important feedstocks used in the synthesis of ammonia and liquid fuels.Display Formula
(R2)
CeO2-δed+ΔδCO2CeO2-δox+ΔδCO
  Display Formula
(R3)
CeO2-δed+ΔδH2OCeO2-δox+ΔδH2

Commentary by Dr. Valentin Fuster
2014;():V001T02A005. doi:10.1115/ES2014-6336.

Based on the characteristics of the oxide redox pair system Co3O4/CoO as a thermochemical heat storage medium and the advantages of porous ceramic structures like honeycombs and foams in heat exchange applications, the idea of employing such ceramic structures coated with or manufactured entirely from a redox material like Co3O4, has been implemented.

Thermo-Gravimetric Analysis (TGA) experiments have demonstrated that laboratory-scale Co3O4-coated, redox-inert ceramic foams and honeycombs exhibited repeatable, cyclic reduction-oxidation operation within the temperature range 800–1000°C, employing all the redox material incorporated, even at loading levels exceeding 100 wt% loading percentages.

To further improve the volumetric heat storage capacity, monolithic porous ceramic foams made entirely of Co3O4 were manufactured, together with analogous pellets. Such porous structures were also capable of cyclic reduction–oxidation, exploiting the entire amount of Co3O4 used in their manufacture. In this perspective, “open” porous structures like the ones of ceramic foams seem to have significant advantages in addressing problems associated with cyclic expansion-contraction that could be detrimental to structural integrity.

Commentary by Dr. Valentin Fuster
2014;():V001T02A006. doi:10.1115/ES2014-6338.

The present experimental investigation covers the construction of a latent heat thermal energy storage system (LHTES), which uses sodium nitrate (NaNO3) as phase change material (PCM). The storage unit is filled with 300 kg of the PCM. For the heat transfer a vertically arranged bimetallic mono tube with longitudinal fins is used. The fins increase the heat flux into/from the PCM. Thermal oil is used as a heat transfer medium, as it allows working temperatures up to 400°C. This thermal energy storage is able to store 60 kWh of thermal energy and can be loaded with a power up to 200 kW. One part of the investigation results presented in this paper was the determination of the storable energy and the comparison with data from literature and calculations. Additionally, the melting behavior of the PCM was measured with temperature sensors located at different positions over the height of the storage unit. Finally, the entrance of the heat transfer medium was changed from the top to the bottom of the thermal energy storage unit and a different melting behavior could be detected.

Commentary by Dr. Valentin Fuster
2014;():V001T02A007. doi:10.1115/ES2014-6343.

While the levelized cost of energy (LCOE) is generally emphasized when assessing the economic viability of renewable energy systems, the other benefits such as the operational and capacity value are often ignored. Concentrating solar power with thermal energy storage (CSP-TES) can be dispatched similarly to conventional thermal generation, which may lead to incorrect economic comparisons between CSP-TES and variable renewable generation technologies such as wind and photovoltaics (PV). However, unlike most conventional thermal power plants, CSP-TES plants are energy limited, meaning that their response might be restricted by solar availability. Therefore, the use of this limited solar energy must be optimally scheduled to provide the greatest value to the power system. The timing of CSP-TES dispatch depends on a variety of factors, including electricity demand patterns, the penetration of variable generation sources, and the configuration of the CSP-TES plant itself. We use an established CSP-TES modeling framework in a commercially available production cost model to study the dispatch and value of a molten salt tower CSP-TES plant in a test system simulating the Colorado grid. We examine a range of configuration parameters by varying the relative sizes of three separate but interrelated parts of the CSP-TES plant: the solar collection field, the power block, and the thermal storage tank. Firstly, we observe that increasing the thermal energy storage capacity of a plant reduces spilled energy and increases the ability to deploy thermal energy to displace the most costly forms of conventional generation. Additional storage, however, shows only marginal benefit beyond 6–9 hours of rated plant capacity. Secondly, we observe that reducing the ratio of solar collection field size to power block capacity (a design parameter often called the solar multiple [SM]) provides the most benefit to this test system per unit of energy produced. A large solar collection field size, relative to the rated capacity of the power block, forces some solar energy to be stored during the periods when conventional generation is most costly. The stored solar energy must then be used during off-peak hours to displace less costly fossil generation. The relative value of CSP-TES presented here is dependent on many system variables, including existing solar energy penetration. Future work must examine the impact of capital cost on the overall value proposition of these configurations of CSP-TES, since the upfront cost is not considered here.

Commentary by Dr. Valentin Fuster
2014;():V001T02A008. doi:10.1115/ES2014-6347.

Energy storage is key to expanding the capacity factor for electric power from solar energy. To accommodate variable weather patterns and electric demand, storage may be needed not just for diurnal cycles, but for variations as long as seasonal. Five solar electric systems with energy storage were simulated and compared, including an ammonia thermochemical energy storage cycle, compressed air energy storage (CAES), pumped hydroelectric energy storage (PHES), vanadium flow battery, and thermal energy storage (TES). To isolate the influence of the storage system, all systems used the same parabolic concentrator and Stirling engine. For CAES, PHES and battery, the engine directly produced electricity, which was then converted and stored. For TES, heat transfer fluid was heated by the dish and stored, and later used to drive the engine to produce electricity. For ammonia, the dish heated an ammonia dissociation reactor to produce nitrogen and hydrogen, which was stored. Heat was recovered to drive the engine by reforming ammonia from the stored gases. Each system was simulated in TRNSYS with weather data for Louisville, KY and Phoenix, AZ with subsystem efficiencies and storage losses estimated from previous experimental results. All systems including the ammonia cycle involved time dependent storage losses. Losses from the receiver included convection and emitted radiation, both of which depend on receiver temperature.

Overall (solar-storage-electric) efficiency of the ammonia cycle depended strongly on synthesis reactor temperature, ranging from less than 1% to ∼18% for both Louisville, KY and Phoenix, AZ, at 500 K to 800 K, respectively. In contrast, the effect of dissociation reactor temperature was less. Overall (solar-electric-storage-electric) efficiencies of the CAES, systems in the limit of zero storage time ranged from ∼10% to ∼18% for solar receiver temperature of 500 K to 800 K. The vanadium flow battery and PHES efficiencies ranged from ∼9% to ∼17% for the same conditions. TES initially provided 12 to 23% efficiency over the same range of temperature. When time-dependent storage losses were included, however, efficiencies for all systems declined rapidly except the ammonia cycle in both locations and PHES in Louisville. The ammonia system had the highest efficiency after one month of storage, an advantage that increased with time of storage.

The simulations showed that TES was most efficient for diurnal-scale storage and the ammonia cycle for longer storage. Full capacity factor for solar electric production may be most efficiently accomplished with a combination of direct solar-electric production and systems with both diurnal and long-term storage, the proportions of which depending on weather conditions and electric demand profiles.

Commentary by Dr. Valentin Fuster
2014;():V001T02A009. doi:10.1115/ES2014-6353.

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) films on metal substrates which is compatible with mass production of solar selective absorbers that can be utilized at high temperatures. TCO material has plasma wavelength in infrared region. Therefore the electromagnetic wave with shorter wavelength than plasma wavelength goes through the material, while the electromagnetic wave with longer wavelength is reflected on the surface. By coating metal surface with a TCO film, interference is occurred in transparent wavelength range of TCO. Therefore, solar energy is highly absorbed, though thermal radiation from the absorber is suppressed. The optical property of fabricated TCO coated metal is well consistent with the simulated property. It is revealed that the performance of the absorber is improved by fabricating microstructures on the metal substrate. Thermal stability is confirmed at 700°C in vacuum for 3 hours. Solar absorptance and hemispherical emittance of the fabricated absorber are 0.82 and 0.17, respectively, which is comparable to that of commercialized absorbers.

Commentary by Dr. Valentin Fuster
2014;():V001T02A010. doi:10.1115/ES2014-6369.

Slag is one of the main waste materials of the iron and steel manufacturing. Every year about 20 million tons of slag are generated in the United States and 43.5 million tons in Europe. The revalorization of this by-product as heat storage material in thermal energy storage systems would have numerous advantages which include: the possibility to extend the working temperature range up to 1000 °C, the reduction of the system cost and, at the same time, the decrease of the quantity of waste in the iron and steel industry.

In this paper, two different electric arc furnace slags from two companies located in the Basque Country (Spain) are studied. Their thermal stability and compatibility in direct contact with the most common heat transfer fluids used in the concentrated solar power plants are analyzed. The experiments have been designed in order to cover a wide temperature range up to the maximum operation temperature of the future generation of concentrated solar power plants (1000 °C). In particular, three different fluids have been studied: synthetic oil (Syltherm 800®) at 400 °C, molten salt (Solar Salt) at 500 °C and air at 1000 °C. In addition, a complete characterization of the studied slags and fluids used in the experiments is presented showing the behavior of these materials after 500 hour laboratory-tests.

Commentary by Dr. Valentin Fuster
2014;():V001T02A011. doi:10.1115/ES2014-6376.

This paper describes the modelling and design of an external receiver using supercritical CO2 as the heat transfer fluid that can reach up to 700 °C outlet temperature with ∼85% thermal efficiency. The internal pressure of the tubes is 20 MPa. The receiver tubes are arranged in a serpentine fashion and are coated with Pyromark 2500. Analyses were performed to evaluate the thermal efficiency of the receiver as a function of incidence angle of the incident radiation. Two different radiation models, discrete ordinates and surface-to-surface ray tracing, were used in the computational fluid dynamics model (ANSYS FLUENT). The receiver thermal efficiency ranged from 75% for incidence angles of 80 degrees to 88% for near-normal incidence angles of 10 degrees.

Commentary by Dr. Valentin Fuster
2014;():V001T02A012. doi:10.1115/ES2014-6393.

Concentrating solar power (CSP) is an effective means of converting solar energy into electricity with an energy-storage capability for continuous, dispatchable, renewable power generation. However, challenges with current CSP systems include high initial capital cost and electricity price. The U.S. Department of Energy’s (DOE) SunShot program aims to reduce cost and improve performance of CSP technology. To this end, NREL is developing a solid-particle based CSP system projected to have significant cost and performance advantages over current nitrate-based molten salt systems. The design uses gas/solid, two-phase flow as the heat transfer fluid and separated solid particles as the storage medium. A critical component in the system is a novel near-blackbody (NBB) enclosed particle receiver with high-temperature capability developed with the goal of meeting DOE’s SunShot targets for receiver cost and performance. Development of the NBB enclosed particle receiver necessitates detailed study of the dimensions of the receiver, particle flow conditions, and heat transfer coefficients. The receiver utilizes an array of absorber tubes with a granular medium flowing downward through channels between tubes. The current study focuses on simulation and analysis of granular flow patterns and the resulting convective and conductive heat transfer to the particulate phase. This paper introduces modeling methods for the granular flow through the receiver module and compares the results with an in-situ particle flow test.

Commentary by Dr. Valentin Fuster
2014;():V001T02A013. doi:10.1115/ES2014-6412.

A new type of high temperature solar receiver for Brayton Cycle power towers is being designed and built in the Combustion and Solar Energy Laboratory at San Diego State University under a DOE Sunshot Award. The Small Particle Solar Receiver is a pressurized vessel with a window to admit concentrated solar radiation that utilizes a gas-particle suspension for absorption and heat transfer. As the particles absorb the radiation that enters the receiver through the window, the carrier fluid (air in this case) heats which oxidizes the particles and the flow leaves the receiver as a clear gas stream. After passing through an in-line combustor if needed, this hot gas is used to power a turbine to generate electricity.

The numerical modelling of the receiver is broken into three main pieces: Monte Carlo Ray Trace (MCRT) method (written in FORTRAN), ANSYS Fluent (CFD), and the User Defined function (written in C code) for oxidation. Each piece has its advantages, disadvantages, and limitations and the three pieces are coupled to finalize the calculation. While we have successfully demonstrated this approach to obtaining the velocity and temperature fields, one big challenge to this method is that the definition of the geometry is a time consuming programming task when using MCRT. On the other hand, arbitrary geometries can be easily modelled by Computational Fluid Dynamics (CFD) codes such as FLUENT.

The goal of this study is to limit the use of MCRT method to determining the appropriate input boundary condition on the outside of the window of the receiver and to use the built-in Discrete Ordinates (DO) method for all the radiation internal to the receiver and leaving the receiver due to emission. To reach the goal, this paper focuses on the DO method implemented within FLUENT. An earlier study on this subject is based and advanced. Appropriate radiation input for the DO method is extensively discussed. MIRVAL is used to simulate the heliostat field and VEGAS is used to simulate a lab-scale solar simulator; both of these codes utilize the MCRT method and provide intensity information on a surface. Output from these codes is discretized into DO parameters allowing the solution to proceed in FLUENT. Suitable benchmarks in FLUENT are used in a cylindrical geometry representing the receiver for the comparison and validation. This method will allow FLUENT to be used for a variety of problems involving concentrated solar energy.

Commentary by Dr. Valentin Fuster
2014;():V001T02A014. doi:10.1115/ES2014-6413.

The efficiency of solar thermochemical cycles to split water and carbon dioxide depends in large part on highly effective gas phase heat recovery. Heat recovery is imperative for approaches that rely on an inert sweep gas to reach low partial pressures of oxygen during thermal reduction and/or use excess oxidizer to provide a higher thermodynamic driving potential for fuel production. In this paper, we analyze heat transfer and pressure drop of a tube-in-tube ceramic heat exchanger for the operating conditions expected in a prototype solar reactor for isothermal cycling of ceria. The ceramic tubes are filled with reticulated porous ceramic (RPC). The impacts of the selection of the composition and morphology of the RPC on heat transfer and pressure drop are explored via computational analysis. Results indicate a 10 pore per inch (ppi), 80–85% porous alumina RPC yields effectiveness from 85 to 90 percent.

Commentary by Dr. Valentin Fuster
2014;():V001T02A015. doi:10.1115/ES2014-6417.

Pyrolysis of biomass (wood) was studied at the focus of a 1.5 kW vertical axis solar furnace. The sweeping gas feeding system was designed based on the CFD simulation results. The effects of temperature on the product yields and compositions of gas were investigated by experiments performed at heating rate of 50°C/s up to 600, 800, 1000, and 1200°C. The role of heating rates influencing the product yields and gas composition was studied by experiments carried out at different heating rates of 5, 100, 25 °C/s to 1200°C. The results show that the increase of gas product from 27.7% to 47.8% with increasing temperature and heating rate comes from the tar decomposition. In the solar reactor, heating rate plays a more important role for the product yields than temperature does, which is different with respect to conventional reactors.

Commentary by Dr. Valentin Fuster
2014;():V001T02A016. doi:10.1115/ES2014-6421.

A system approach was used for the development of a new process concept for solar driven thermochemical redox cycles. Two aspects of this concept will be presented here. Since a high heat recovery rate for cycles using non-stoichiometric reduction has been identified as elementary in order to reach meaningful overall process efficiencies, a special focus was directed on this aspect. A quasi-countercurrent heat recovery system, which makes use of a particulate heat transfer medium, was outlined and numerically analyzed. The analysis shows that recovery rates of more than 70% seem realistic. Even though the heat recovery system is based on an arrangement of stages including relative complex flow pattern the basic principle seems promising and opens up new pathways for system design and optimization. The second aspect highlighted of the developed process concept is the use of a multi chamber system with optimized reaction conditions for the reduction of the redox material. By optimizing the pressure in a multi chamber system energy savings related to the pumping work of more than 20% are predicted. Also the execution of pre-reduction in the heat recovery system is discussed.

Topics: Heat recovery , Cycles
Commentary by Dr. Valentin Fuster
2014;():V001T02A017. doi:10.1115/ES2014-6429.

Spherical tanks have the potential for cost reduction in sensible thermal energy storage (TES) systems, by using less tank building material and insulation.

The current CFD study compares the Thermal Efficiency (TE) of a thermocline storage system in a spherical tank to a cylindrical tank of the same volume. A parametric study is then performed on a spherical tank during the discharge process to determine the flow parameters that govern the thermocline formation and entrainment. The following parameters are used: tank diameter to inlet diameter ratio D/d = 10, inlet velocity (0.02–0.1 m/s), and ΔT (10–70° C), leading to an inlet Froude number (0.4–3), inlet Reynolds number (500–7500), and tank Richardson number (2–100).

The results show a significant correlation between the inlet Reynolds and inlet Froude numbers, and the tank TE, in addition to a weak correlation between the tank Richardson number, based on the tank diameter, and the tank TE. The parametric study also shows a maximum tank TE at a Froude number equal to 0.5, and a proportional decrease of TE as the Reynolds number increases.

Commentary by Dr. Valentin Fuster
2014;():V001T02A018. doi:10.1115/ES2014-6437.

The optical design and engineering features of a 10 kW solar furnace now operational at Valparaiso University are described. The solar furnace is anticipated to achieve a mean concentration ratio of 3000 suns over a 6 cm diameter focus. It will support high-temperature solar chemistry research and undergraduate engineering pedagogy. Many of the components of the solar furnace were designed and constructed by undergraduate engineering students. Some of these students cite their participation in the solar furnace project as the motivating factor for continuing to work in the area of energy science in industry or graduate school.

Commentary by Dr. Valentin Fuster
2014;():V001T02A019. doi:10.1115/ES2014-6444.

Because of the capability of large capacity thermal storage and extended operation during night and cloudy days, concentrated solar thermal power generation is getting more and more attention in the recent years. Dual-media thermal energy storage system is typically adopted in industry for reducing the use of the heat transfer fluid, which is usually expensive. In such a dual-media system, the solid filler material can be a phase change material relying on latent heat or a regular solid material using sensible heat for energy storage. Two strategies of starting-up fluid charge and discharge are considered for the operation of a concentrated solar thermal power plant incorporated with a dual-media thermal storage system. These two strategies include: 1) starting daily cyclic charge and discharge operation with an initially cold tank; 2) to fully charge the thermal storage system before operation of the cyclic discharge/charge for the power plant. The energy storage efficiency and the effects to the power plant operation due to the application of these two strategies are studied in the current work based on an enthalpy-based 1-D model, and significant difference is found in starting-up process of the daily cyclic operations, which will help us decide the best strategy of operating a thermal energy storage system with more electrical energy output.

Commentary by Dr. Valentin Fuster
2014;():V001T02A020. doi:10.1115/ES2014-6446.

Cavity receivers have been an integral part of Concentrated Solar Power (CSP) plants for many years. However, falling solid particle receivers (SPR) which employ a cavity design are only in the beginning stages of on-sun testing and evaluation. A prototype SPR has been developed which will be fully integrated into a complete system to demonstrate the effectiveness of this technology in the CSP sector. The receiver is a rectangular cavity with an aperture on the north side, open bottom (for particle collection), and a slot in the top (particle curtain injection). The solid particles fall from the top of the cavity through the solar flux and are collected after leaving the receiver. There are inherent design challenges with this type of receiver including particle curtain opacity, high wall fluxes, high wall temperatures, and high heat losses. CFD calculations using ANSYS FLUENT were performed to evaluate the effectiveness of the current receiver design. The particle curtain mass flow rate needed to be carefully regulated such that the curtain opacity is high (to intercept as much solar radiation as possible), but also low enough to increase the average particle temperature by 200°C. Wall temperatures were shown to be less than 1200°C when the particle curtain mass flow rate is 2.7 kg/s/m which is critical for the receiver insulation. The size of the cavity was shown to decrease the incident flux on the cavity walls and also reduced the wall temperatures. A thermal efficiency of 92% was achieved, but was obtained with a higher particle mass flow rate resulting in a lower average particle temperature rise. A final prototype receiver design has been completed utilizing the computational evaluation and past CSP project experiences.

Commentary by Dr. Valentin Fuster
2014;():V001T02A021. doi:10.1115/ES2014-6451.

Thermal energy storage (TES), when combined with a concentrating solar power (CSP) plant has potential to produce electricity at a cost-competitive rate to traditional sources of electricity production. In single tank TES system both the hot fluid and cold fluid settle in the same tank. The region of contact of these two fluids is called thermocline. Preservation of this thermocline region in the cylindrical tank during charging and discharging cycles depends on the uniformity of the velocity profile at any horizontal plane. So to maintain this thermocline region, a pipe flow distributor was placed near the inlet and outlet of the cylindrical tank. To optimize the efficiency of this single tank TES system is to increase the thermo-physical properties of heat transfer fluid. This addition will result in harnessing solar energy by increasing thermal efficiency of the thermodynamic cycles. Adding of nanoparticles, in the heat transfer fluid give rise of this thermo-physical properties i.e. thermal conductivity (k) and specific heat capacity (Cp). Hitec® molten salt is used as the base-fluid and synthesized with five different types of nanoparticles (SiO2, Al2O3, Fe3O4, ZnO and Ag) with different concentrations. The values of effective k and Cp are calculated for the new Hitec® nanofluid. The doping of nano-particles results in higher k and Cp when compared to the base fluid. Higher Cp is expected to improve the thermal storage capacity but higher value of k is expected to increase the thermal diffusivity, thereby affecting the performance of the thermocline. The diffusivity depends on the ratio of k to Cp and density of the effective properties. So there is a need to balance the effective properties to improve thermal storage performance. The total energy storage capacity is then checked by finite volume based computational fluid dynamics software. The simulation shows how the performance of the nanofluid changes at different concentrations in a single tank TES system during its charging-discharging cycle.

Commentary by Dr. Valentin Fuster
2014;():V001T02A022. doi:10.1115/ES2014-6471.

The hybrid air Brayton concentrated solar power plant (CSP) combines a natural gas fired combustor in series with a traditional CSP system. The combination boosts turbine inlet temperature above the receiver temperature and reduces output variability. However, a combustor operating in this mode must tolerate an inlet air temperature equal to the solar receiver outlet temperature, which is expected to be as much as 1,000°C for next generation designs. High inlet temperature hybrid combustors must achieve low NOx emissions in spite of the increased risk for autoignition and flashback. In addition, the hybrid injector must be able to adjust to the variability inherent to the solar source. The design of a multibank micromix injector that meets these challenges is described with emphasis on its NOx and CO emissions characteristics.

Commentary by Dr. Valentin Fuster
2014;():V001T02A023. doi:10.1115/ES2014-6475.

A low-cost rigid foam-based concentrator technology development program was funded by the DOE SunShot Initiative to meet installed cost goals of $75/m2 vs. current costs of $200–250/m2. The cost reduction in this approach focuses primarily on designing a mirror module with a rigid foam center with stainless steel facesheets and reflective film. The low mechanical strength of the foam is compensated by optimizing the densities and dimensions to meet pointing accuracy requirements of 4 milliradians (mrad) in 27mph winds. Two alpha concentrators were built to validate the mirror module manufacturing process and one of them was accurate to 0.15 mrad RMS vs. the design requirement of 1 mrad RMS. To understand the lifetime reliability of the panels, fifteen 4-inch square samples were exposed to various environmental conditions including acid rain, bird droppings, thermal cycling, and the final results indicated no loss in reflectivity of 95%. UV testing will be performed in the next phase. Three mechanical structure options covering the range of large multi-faceted heliostats with diagonal load carrying elements, small single facet heliostats low to the ground and optimized truss-based deep structure designs were analyzed with FEA and analytically; results indicated a significant cost benefit (>2×) for the truss-based design over the other options. Other elements such as the controls, actuators were also considered in th analysis with vendor data. Cost trades were performed for heliostats ranging from 10m2 to 250m2. The results indicated a broad installed cost minimum around $113/m2 for heliostat sizes ranging from 80 m2 to 130 m2. Additional cost saving approaches will be considered in Phase 2 of the project.

Commentary by Dr. Valentin Fuster
2014;():V001T02A024. doi:10.1115/ES2014-6482.

Recently, the supercritical CO2 (s-CO2) Brayton cycle has been identified as a promising candidate for solar-thermal energy conversion due to its potentially high thermal efficiency (50%, for turbine inlet temperatures of ∼ 1000K). Realization of such a system requires development of solar receivers which can raise the temperature of s-CO2 by over 200K, to a receiver outlet temperature of 1000K. Volumetric receivers are an attractive alternative to tubular receivers due to their geometry, functionality and reduced thermal losses. A concept of a ceramic pressurized volumetric receiver for s-CO2 has been developed in this work. Computational Fluid Dynamics (CFD) analysis along with a Discrete Ordinate Method (DOM) radiation heat transfer model has been carried out, and the results for temperature distribution in the receiver and the resulting thermal efficiency are presented. We address issues regarding material selection for the absorber structure, window, coating, receiver body and insulation. A modular small scale prototype with 0.5 kWth solar heat input has been designed. The design of a s-CO2 loop for testing this receiver module is also presented in this work.

Commentary by Dr. Valentin Fuster
2014;():V001T02A025. doi:10.1115/ES2014-6493.

Solar thermal power plants are a promising option for future solar electricity generation. Their main advantage is the possibility to utilize integrated thermal storage capacities, allowing electricity generation on demand. In state of the art solar thermal power plants, two-tank molten-salt thermal energy storages are used. Significant cost reductions are expected by using thermocline thermal energy storage by storing the liquid storage material inside a single tank when compared to a two tank storage system. By embedding a low cost solid filler material inside the storage tank further cost reductions can be achieved.

In earlier studies [1, 2] several potential filler materials have been investigated. In these study quartzite turned out to be a promising candidate due to its satisfying thermal stability and availability. At a temperature of approx. 573°C the crystal structure of quartzite changes from trigonal α-quartz phase to the hexagonal β-quartz phase [3]. This quartz conversion results in a volume change [4] that may cause cracking of the quartzite crystals due to weight loads in a packed bed. Since these thermal tests of the study mentioned were limited to 500°C this dunting was not considered. Thus, despite of the published studies there is a need for further, more detailed analysis.

One trend in today’s development of solar thermal power plants is to use molten salt as storage material and heat transfer fluid at operating temperatures of 560°C and above. Accordingly, the quartz inversion might limit the applicability of quartzite as a filler material at elevated operating temperatures. Due to this concern, an investigation has been started to investigate the utilizability of natural rocks as low cost filler materials.

In the first phase of this investigation a comprehensive literature survey was conducted. Based on this study, magmatic and sedimentary rocks turned out to the most promising rock classes for this application. For the further investigation, basalt was chosen as a suited representative for magmatic and quartzite for sedimentary rocks. In lab-scale tests, these candidate materials were investigated with respect to their:

• Calcite content

• Thermal stability up to 900°C in air

• Thermal stability up to 560°C in molten salt

• Cyclic stability between 290°C and 560°C in molten salt

• Specific heat capacity up to 600°C

In this paper the results of these investigations are presented and future activities are outlined.

Commentary by Dr. Valentin Fuster
2014;():V001T02A026. doi:10.1115/ES2014-6496.

Integration of electro-dynamic screens (EDS) on mirrors in CSP power plants is an emergent and environmentally conservative technology. It can remove the deposited dust from these mirrors and thus maintain high reflectivity continuously through the plant life. We propose a levelized cost of mirror cleaning (LCOMC) metric to link the EDS-enhanced reflectivity gains with the relevant product and installation costs, as well as with the direct and indirect costs associated with plant operation and maintenance. The LCOMC metric accounts for the fact that enhanced reflectivity owing to EDS technology allows the plant operators to specify a suitably smaller optical capacity plant in order to deliver a fixed power production target. We illustrate our proposal with a dataset on deluge cleaning of a scaled solar power plant configuration. For the configuration studied, it is shown that, if the EDS technology production and installation cost is $10/m2, then its LCOMC is 7.9% below the LCOMC for a comparable deluge cleaning alternative. Thus, the proposed LCOMC metric provides a methodology for systemic assessment of the economic impact of the EDS technology (and other mirror cleaning technologies), early in its technology development cycle.

Commentary by Dr. Valentin Fuster
2014;():V001T02A027. doi:10.1115/ES2014-6500.

The performance of parabolic trough (PT) receiver tubes (RT) has a direct impact on concentrated solar power (CSP) plant production. As a result, one major need of operation and maintenance (O&M) in operating plants is to monitor the state of the receiver tube as a key element in the solar field. In order to fulfill this necessity, Abengoa Solar has developed the first existing portable device for measuring transmittance and reflectance of parabolic trough receiver tubes directly in the field. This paper offers a description of the technical features of the instrument and reviews the issues related to its usability as a workable portable device in operating solar fields. To evaluate its performance, laboratory studies have been carried out using two patterns to determine the accuracy and standard deviation of the measurements, obtaining excellent results.

This information is complemented with data collected by O&M using this instrument in solar power plants. Studies have been carried out to determine the effect of both rainfall and artificial cleaning on the increase of transmittance. These values are then compared to those obtained from hand-cleaning and show important differences. The results are discussed in this paper.

Commentary by Dr. Valentin Fuster
2014;():V001T02A028. doi:10.1115/ES2014-6503.

The Riyadh Techno Valley Solar Tower, an innovative type of concentrator solar power plant, is being developed by King Saud University (KSU) and Georgia Tech (GT). The facility is being constructed at the Riyadh Techno Valley development near the KSU campus and will store thermal energy collected from the sun in solid particles, which can be heated to higher temperatures than is currently possible using molten salts. The particles must be well insulated to stop energy loss to the environment. Hence, GT and KSU have incorporated an insulated storage bin into the plant design.

The bin will be constructed in several layers: an inner layer of firebrick, which can endure direct exposure to the heated particles; a specially prepared refractory insulating concrete, which maintains good insulating value at high temperatures; and a conventional structural concrete shell surrounding the entire bin. This paper presents a thermal analysis of this storage device and discusses structural analyses. Simplified analytical solutions are compared with the finite element results from a 3D ANSYS model of the entire bin. A temperature distribution is obtained, and heat loss through the bin is also evaluated.

Modeling of rebar and concrete cracking are described, and methods of reducing stress on the outer concrete shell are considered. Structural support for an access tunnel into the bin is also explored. The current tunnel design involves a material with a relatively high thermal conductivity, necessitating modifications to the bin. Finally, material selection is considered, particularly with regard to the insulating concrete layer. Limitations on the use of Portland cement based insulating concretes are discussed, and alternative base materials are evaluated.

Commentary by Dr. Valentin Fuster
2014;():V001T02A029. doi:10.1115/ES2014-6504.

One increasingly viable option for high temperature concentrator solar power (CSP) is a central receiver system with a particle heating receiver (PHR). A PHR system uses suitable particulates to capture and store energy. It is expected that the particles will be sustained at high temperatures (in the range of 300°C or 400°C to 700°C or 800°C or even 1000°C) on most typical days of plant operation, so there is interest in how the particle optical properties might change after prolonged high-temperature exposure.

This paper presents the results from experiments conducted over a 5-month period in which samples of various types of particulates including silica sands and alumina proppants were exposed to high temperatures for extended periods of time. The reflectance of a bed of particles was measured at room temperature in 8 wavelength bands using the 410-Solar reflectometer device developed by Surface Optics Corporation. The infrared emittance was determined using the ETS-100 emissometer instrument, also developed by Surface Optics Corporation [1,2].

Particles were heated to 950°C and 350°C, and measurements were recorded at intervals during the exposure so that trends in the optical properties over time could be observed. From the measured data, the total solar absorptance and total hemispherical emittance at high temperature were computed; these results are also presented.

Commentary by Dr. Valentin Fuster
2014;():V001T02A030. doi:10.1115/ES2014-6505.

Optically transparent superhydrophobic (SH) coatings based on multifunctional silica nanoparticles and polymeric binders were developed and their optical and abrasion resistance properties were studied. Three key factors are emphasized: i) The optical clarity of the coatings. Particle deagglomeration and surface functionalization techniques were developed to obtain particle size distributions with an average size smaller than 200 nm. The particles were uniformly dispersed in organic binders and resulted in coatings with an average roughness value smaller than 30 nm. The nano-sized particles do not scatter light at wavelengths > 250 nm because of their small dimensions. ii) Enhanced particle-binder interfaces. We have introduced a double functionality on the particle’s surface in order to partially crosslink them with the polymeric binder. This multifunctional configuration significantly improves the abrasion resistance of the coatings without degrading their SH properties. iii) Accelerated weathering durability. Coatings were subjected to simulated solar UV exposure. Our ongoing studies indicate that the coatings are environmentally durable over several years of simulated UVA exposure. The nanostructure-property interdependencies underlined in the above three key factors are utilized in the development of transparent SH coatings with enhanced durability.

Commentary by Dr. Valentin Fuster
2014;():V001T02A031. doi:10.1115/ES2014-6506.

For large scale CSP power plants, vast areas of land are needed in deserts and semi-arid climates where uninterrupted solar irradiance is most abundant. These power facilities use large arrays of mirrors to reflect and concentrate sunlight onto collectors, however, dust deposition on the optical surfaces causes obscuration of sunlight, resulting in large energy-yield losses in solar plants. This problem is compounded by the lack of natural clean water resources for conventional cleaning of solar mirrors, often with reflective surface areas of large installations exceeding a million square meters. To investigate the application of transparent electrodynamic screens (EDS) for efficient and cost effective dust removal from solar mirrors, both optical modeling and experimental verifications were performed. Prototype EDS-integrated mirrors were constructed by depositing a set of parallel transparent electrodes into the sun-facing surface of solar mirrors and coating electrodes with thin transparent dielectric film. Activation of the electrodes with a three-phase voltage creates an electrodynamic field that charges and repels dust electrostatically by Coulomb force and sweeps away particles by a traveling electrodynamic wave. We report here brief discussions on (1) rate of deposition and the properties of dust with respect to their size distribution and chemical composition in semi-arid areas of the southwest US and Mojave Desert and their adhesion to solar mirrors, (2) optical models of: (a) specular reflection losses caused by scattering and absorption by dust particles deposited on the surface based on Mie scattering theory, and (b) reflection loss by the integration of EDS on the mirror surface, computed by FRED ray-tracing model. The objective is to maintain specular reflectivity of 90% or higher by frequent removal of dust by EDS. Our studies show that the incorporation of transparent EDS would cause an initial loss of 3% but would be able to maintain specular reflectivity more than 90% to meet the industrial requirement for CSP plants. Specular reflection measurements taken inside a climate controlled environmental chamber show that EDS integration can restore specular reflectivity and would be able to prevent major degradation of the optical surface caused by the deposition of dust.

Commentary by Dr. Valentin Fuster
2014;():V001T02A032. doi:10.1115/ES2014-6528.

In order to increase the thermal efficiency and produce process heat for hydrogen production, the operating temperature of the heat-transfer fluid in thermal solar plants needs to increase, but to increase the operating temperature, new heat-transport liquids need to be evaluated. Liquid metals have been proposed as heat-transport fluids because of the large temperature ranges over which they remain liquid. One of the most studied liquid metals for non-solar applications has been lead-bismuth eutectic alloy (LBE), for the nuclear industry. The main challenge with using LBE as a coolant is that the major constituents of structural steels have high solubility in LBE. In this work, the challenges of using LBE as a high temperature heat-transport fluid are discussed, as well as initial results of high-temperature static corrosion tests of structural steels to evaluate their potential use in a thermal solar power plant.

Commentary by Dr. Valentin Fuster
2014;():V001T02A033. doi:10.1115/ES2014-6545.

Turbine inlet pressures of ∼ 300 bar in case of CO2 based cycles call for redesigning the cycle in such a way that the optimum high side pressures are restricted to the discharge pressure limits imposed by currently available commercial compressors (∼150 bar) for distributed power generation. This leads to a cycle which is a combination of a transcritical condensing and a subcritical cycle with an intercooler and a bifurcation system in it. Using a realistic thermodynamic model, it is predicted that the cycle with the working fluid as a non-flammable mixture of 48.5 % propane and rest CO2 delivers ∼37.2 % efficiency at 873 K with a high and a low side pressure of 150 and 26 bar respectively. This is in contrast to the best efficiency of ∼36.1 % offered by a transcritical condensing cycle with the same working fluid at a high side pressure of ∼ 300 bar.

Commentary by Dr. Valentin Fuster
2014;():V001T02A034. doi:10.1115/ES2014-6546.

Photocatalytic water splitting is a potential route for future carbon-free production of hydrogen. However catalysts still need to be enhanced in order to reach acceptable solar-to-fuel efficiency. In the context of the project HyCats funded by the Federal Ministry of Education and Research of Germany a high performance test facility for the evaluation of the activity of photocatalysts under practical conditions was established. It mainly consists of a solar concentrator and a planar receiver reactor. A modified linear Fresnel concentrator configuration was chosen based on ray tracing simulation results and improved concerning the number of different facets and the tolerance of tracking errors. It meets the major demand of a homogeneous irradiance distribution on the surface of the reactor. The SoCRatus (Solar Concentrator with a Rectangular Flat Focus) is a 2-axis solar concentrator with a geometrical concentration ratio of 20.2 and an aperture area of 8.8 m2. The tracking accuracy is better than 0.1° respecting both the solar azimuth and altitude angle. Its 22 highly UV/Vis-reflective flat aluminum mirror facets reflect the sunlight resulting in a rectangular focus with a nominal width of 100 mm and a nominal length of 2500 mm. The reactor is placed in the focal plane at a distance of 2500 mm from the mounting plane of the facets and allows concentrated solar radiation to penetrate suspensions of water, electrolytes and photocatalyst particles flowing through it. Corresponding to a maximum angle of incidence of 36.6° the Quartz window reflects not more than 5% of the incoming radiation and assures only marginal absorption, particularly in the UV-part of the sun’s spectrum. The material of the receiver body is PTFE (polytetrafluoroethylene) providing reflection coefficients above 90% concerning wavelengths of UV-A and UV-B. The design of the reactor features two parallel reaction chambers, offering the possibility to test two separate suspensions at the same irradiation conditions. A pump transports the tempered suspension to the reactor. The geometry of the reactor inlet and outlet minimizes critical regions with inadequate flow caused by vortices. Any evolved gases are separated from the suspension in a tank together with nitrogen introduced in the piping upstream and are analyzed by micro chromatographs. Numerous devices are installed in order to control and monitor the reaction conditions. First experiments have been carried out using methanol as a sacrificial reagent.

Topics: Design , Water
Commentary by Dr. Valentin Fuster
2014;():V001T02A035. doi:10.1115/ES2014-6562.

BrightSource solar power plants consist of fields of tens of thousands of mirrors, spread across kilometers of open areas. These huge mirrors are in constant motion, reflecting the sun’s rays on to the solar thermal collector. Maintaining high reflectivity of the mirrors is essential for the solar field’s performance, a task that becomes complex when expanded to encompass the solar field’s features. The solution for mirror cleaning must be efficient, cost-effective, time-saving, and easy to maintain for dozens of years.

BrightSource has designed and constructed a system of GPS-based mirror washing machines (MWMs) that are controlled and managed by end-to-end software. The system generates optimized cleaning tasks, positions the mirrors, and efficiently controls the navigation and state of the MWMs with their 25-meter-long extendable cranes. All of these actions together provide an optimal mirror cleaning solution.

This article describes the BrightSource cleaning control technology, for example, in the Ivanpah project, the world’s largest solar thermal facility. The Ivanpah solar field includes 173,500 heliostats divided among three solar fields. Each heliostat holds two mirrors of approximately 2.5 × 3.5 meters, all of which require periodic cleaning.

Specifically, this article addresses issues such as the following:

• The mirror washing machine (MWM) types: truck and tractor-based, and their differing usage in the solar field

• Designation and choice of the cleaning area

• Estimation of the stopping points in the designated area, and association of the mirrors to clean from each stopping point

• Cleaning time optimization: stopping point density, order in which to clean heliostats, and heliostat position during cleaning

• Heliostat positioning: opening clear corridors through which the MWM can travel, and setting heliostats in cleaning orientations

• Receiving and responding to callback messages from the MWMs, such as cleaning progress and machine faults

• Working in the real world: resources shared with the power plant, and recovery from system faults

Commentary by Dr. Valentin Fuster
2014;():V001T02A036. doi:10.1115/ES2014-6566.

Solar central receiver systems are viewed as one of the most promising concentrated solar power (CSP) technologies for power production, in which solar radiation is concentrated through large mirrors (heliostats) onto a central receiver. This is due to the fact that very high temperatures can be reached at the receiver and thus higher thermal efficiency can be achieved compared with other CSP technologies. Heliostat field layout optimization is an essential task for any solar central receiver system.

In this paper, a heuristic algorithm, i.e. the differential evolution (DE), was employed to perform efficient optimization of the conventional radial staggered heliostat field layout using MATLAB. The model calculates all the required optical performance parameters at every step of the optimization process for each heliostat and consequently more reliable results are obtained as compared with many other optimization methods. Two cases were considered: one with single variable optimization and the other with multi-variable optimization. For the first case, the azimuthal spacing between the adjacent heliostats or the radial distance between the rows of heliostats were optimized independently and for the second case both of these variables were optimized simultaneously. Both cases were examined for high sun altitude angle and low altitude angle and a comparison study was performed between them to check their effect on the heliostat field efficiency. Finally, it was noted that varying the radial distance between the rows of the heliostats yields slightly better efficiency as compared with when optimizing the azimuthal spacing.

Commentary by Dr. Valentin Fuster
2014;():V001T02A037. doi:10.1115/ES2014-6568.

The use of solid particles as a heat transfer and thermal energy storage (TES) medium in central receiver systems has received renewed attention in recent years due to the ability of achieving high temperatures and the potential reduction in receiver and TES costs. Performance of TES systems is primarily characterized by the percentage of heat loss they allow over a prescribed period of time. Accurate estimation of this parameter requires special attention to the transient nature of the process of charging the TES bin during solar field operation and discharging during nighttime or at periods where solar field operation is interrupted. In this study, a numerical model is built to simulate the charge-discharge cycle of a small cylindrical-shaped TES bin that is currently under construction. This bin is integrated into the tower of an experimental 300-kW (thermal) central receiver field being built in Riyadh, Saudi Arabia, for solid particle receiver research, most notably on-sun testing of the falling particle receiver concept within the context of a SunShot project. The model utilizes a type of wall construction that had been previously identified as showing favorable structural characteristics and being able to withstand high temperatures. The model takes into account the anticipated charge-discharge particle flow rates, and includes an insulating layer at the ceiling of the bin to minimize heat loss by convection and radiation to the receiver cavity located immediately over the TES bin. Results show that energy loss during the full charge-discharge cycle is 4.9% and 5.9% for a 5-hour and 17-hour discharge period, respectively. While large, these energy loss values are primarily due to the high surface-to-volume ratio of the small TES bin being investigated. Preliminary analysis shows that a utility-scale TES bin using the same concept will have an energy loss of less than 1%.

Commentary by Dr. Valentin Fuster
2014;():V001T02A038. doi:10.1115/ES2014-6569.

Solar power is a sustainable resource which can reduce the power generated by fossil fuels, lowering greenhouse gas emissions and increasing energy independence. The U.S. Department of Energy’s SunShot Initiative has set goals to increase the efficiency of concentrating solar power (CSP) systems. One SunShot effort to help CSP systems exceed 50% efficiency is to make use of high-temperature heat transfer fluids (HTFs) and thermal energy storage (TES) fluids that can increase the temperature of the power cycle up to 1300°C.

Sporian has successfully developed high-temperature operable pressure, temperature, thermal flux, strain, and flow sensors for gas path measurements in high-temperature turbine engines. These sensors are based on a combination of polymer derived ceramic (PDC) sensors, advanced high-temperature packaging, and integrated electronics. The overall objective is the beneficial application of these sensors to CSP systems.

Through collaboration with CSP industry stakeholders, Sporian has established a full picture of operational, interface, and usage requirements for trough, tower, and dish CSP architectures. In general, sensors should have accurate measurement, good reliability, reasonable cost, and ease of replacement or repair. Sensors in contact with hot salt HTF and TES fluids will experience temperature cycling on a daily basis, and parts of the system may be drained routinely. Some of the major challenges to high-temperature CSP implementation include molten salt corrosion and flow erosion of the sensors. Potential high-temperature sensor types that have been identified as of interest for CSP HTF/TES applications include temperature, pressure, flow, and level sensors.

Candidate solar salts include nitrate, carbonate, and chloride, with different application temperatures ranging from 550°C-900°C. Functional ceramics were soaked for 500 hours in molten nitrate, carbonate, and chloride salts, showing excellent corrosion resistance in chloride salts and good resistance in nitrate salts. The demonstration of functional ceramics in relevant HTFs laid the foundation for full prototype sensor and packaging demonstration.

Sporian has developed a packaging approach for ceramic-based sensors in various harsh gaseous environments at temperatures up to 1400°C, but several aspects of that packaging are not compatible with corrosive and electrically conductive HTFs. In addition to consulting published literature, a 300 hour soak test in molten chloride salt allowed the authors to identify suitable structural metals and ceramics.

Based on discussions with stakeholders, molten salt corrosion testing and room-temperature water flow testing, suitable for CSP sensor/packaging concepts were identified for future development, and initial prototypes have been built and tested.

Commentary by Dr. Valentin Fuster
2014;():V001T02A039. doi:10.1115/ES2014-6573.

A lab-scale solar thermochemical reactor is designed and fabricated to study the thermal reduction of non-volatile metal oxides, which operates simultaneously as solar collector and as chemical reactor. The main purpose of this reactor is to achieve the first step in two-step thermochemical cycles. The chemical conversion rate strongly depends on the temperature and fluid flow distribution around the reactant, which are determined by the reactor geometry. The optimal design depends on the constraints of the problem and on the operating parameters. Hence, the objective of this investigation is to analyze the heat and mass transfer in the vertically-oriented chemical reactor by a CFD model and to optimize the reactor design. The developed numerical model is validated by comparing the simulation results with reported model. The influence of different technical and operating parameters on the temperature distribution and the fluid flow of the reactor are studied.

Commentary by Dr. Valentin Fuster
2014;():V001T02A040. doi:10.1115/ES2014-6578.

A group of eutectic ternary halide salts were surveyed and studied for the objective of developing a high temperature heat transfer fluid with a freezing point below 250°C and a low vapor pressure, below 1.0 atm, at temperatures up to 800°C. The studied salts include: 1) NaCl-KCl-ZnCl2 with a mole fractions of 18.6%-21.9%-59.5% and a melting point of tm=213°C; 2) NaCl-KCl-ZnCl2 with a mole fraction of 13.4%-33.7%-52.9% and a melting point of tm=204°C; and 3) NaCl-KCl-ZnCl2 with mole fraction of 13.8%-41.9%-44.3% and a melting point of tm=229 °C. Vapor pressures of these salts at different temperatures were experimentally obtained using an in-house developed test facility. The results show that vapor pressures of all the three eutectic molten salts are below 1.0 atm at a temperature of 800 °C. The salt of ZnCl2-KCl-NaCl in mole faction of 44.3%-41.9%-13.8% has lowest vapor pressure which is only about 1.0 atm even at a temperature of 900 °C. Viscosities of these salts were measured in the temperature range from after melting to 850°C. At low temperatures near their melting points of the salts, the viscosities are about 16 × 10−3Pa s, while at high temperatures above 700°C the viscosities are around 4 × 10−3Pa s, which is satisfactorily low to serve as heat transfer fluid for circulation in a CSP system. Both the vapor pressure and the viscosities of the studied three eutectic salts demonstrated satisfaction to serve as high temperature heat transfer fluids. Other thermal and transport properties of these salts are expected to be reported in the future for screening out a satisfactory high temperature heat transfer fluid.

Commentary by Dr. Valentin Fuster
2014;():V001T02A041. doi:10.1115/ES2014-6586.

Using solid particulates as a heat absorption and transfer medium in solar concentrated systems is a solution for collecting and storing thermal energy. Solid particulates, such as sand, are relatively inexpensive and are much less corrosive and expensive to maintain than molten salts. Small particles may be stored easily, and can be used as a heat transfer medium for use with a suitable heat exchanger. Despite their anticipated low cost, excessive degradation of the particulates requiring replenishment or disrupting operation could impair the overall economics. Consequently, the durability of the particulates should be verified. Responding to this need, this study examines the durability of solid particulates as a heat transfer medium in a closed cycle for concentrated solar power central receiver systems. Specifically, this study analyzes the combination of attrition and sintering of sand with varying temperatures. Attrition is the reduction of a particle’s mass and sintering is a process of fusing two or more particles together to form a larger agglomerate. In a closed cycle, particularly for a concentrated solar power tower, a particle will experience typical temperatures from 600°C to 1000°C. The increase in temperature may change the physical characteristics of the particles and along with any impurities may promote lower softening point bonding. Thus, it is important to investigate particle durability at high temperatures.

The experimental procedure used in this investigation involves heating and abrading particulates of a known mass and size distribution to temperatures between 600°C and 1000°C, and also at 25°C to observe attrition only. The testing is conducted using a specially designed experimental apparatus described below. The heated particulates are contained in a metal cylinder. Inside the cylinder is another cylinder made of a porous silicon carbide foam. As the temperature is held constant, the particulate sample is rotated 180 degrees around a horizontal axis every 15 seconds from a low position to a higher position so that the particulates fall and abrade against each other. This process is repeated for a known number of cycles (many thousands). Then the resulting particulate size distribution is measured to determine the amount of attrition and sintering occurred during the experiment. The particulates tested are various types of sand with varying mean diameters and composition, along with a ceramic particulate similar to hydraulic fracturing proppants. Sample composition, sample size distribution, and temperature will be used to establish parameters for rates of attrition and sintering. These rates will be used to predict the behavior of particulates in a concentrated solar power tower closed cycle.

Commentary by Dr. Valentin Fuster
2014;():V001T02A042. doi:10.1115/ES2014-6588.

This research is a part of the DOE-funded SunShot project on “High Temperature Falling Particle Receiver.” Storing thermal energy using solid particulates is a way to mitigate the time of day dependency of concentrated solar power. Small particles may be stored easily, and can be used as a heat transfer medium to transfer heat to the power cycle working fluid through a heat exchanger. This study examines the physical characteristics of solid particulates of different materials kept inside large storage containers. Particle behavior at the expected high temperatures of the concentrated solar power cycle combined with the elevated pressure experienced within the storage container must be evaluated to assess the impact on their physical properties and ensure that the particles would not sinter thereby impacting flow through the system components particularly the receiver and heat exchanger. Sintering is a process of fusing two or more particles together to form a larger agglomerate. In the proposed concentrated solar power tower design, particles will experience temperatures from 600°C to 1000°C. The increase in temperature changes the physical characteristics of the particle, along with any impurities that could form particle to particle bonds. In addition, the hydrostatic pressure exerted on particles stored inside a storage unit increases the probability of sintering. Thus, it is important to examine the characteristics of particles under elevated temperatures and pressures.

The experimental procedure involves heating particulates of a known mass and size distribution to temperatures between 600°C and 1000°C inside a crucible. As the temperature is held constant, the particulate sample is pressed upon by a piston pushing into the crucible with a known constant pressure. This process is repeated for different temperatures and pressures for varying lengths of time. The resulting particulates are cooled, and their size distribution is measured to determine the extent of sintering, if any, during the experiment. The particulates tested include various types of sand, along with alumina particles. The data from this experiment will allow designers of storage bins for the solid particulates to determine when significant sintering is expected to occur.

Commentary by Dr. Valentin Fuster
2014;():V001T02A043. doi:10.1115/ES2014-6592.

DSG plants in a once-through configuration convert water into superheated steam suitable for a steam turbine, with a single pass of the fluid through the receiver. The control problem is to manipulate the feed-water mass flow to maintain a desired steam condition (e.g. temperature) under variable solar radiation. This paper presents a full state linear feedback controller for the steam temperature for a once-through direct steam generation system, featuring a 500 m2 paraboloidal dish concentrator and a mono-tube cavity receiver at the Australian National University. The controller manipulates the feed-water mass flow at the receiver inlet to maintain a predetermined specific enthalpy at the receiver outlet, compensating for variations in direct normal irradiation (DNI) and other ambient conditions. The linear controller features three separate regulation mechanisms: a feedforward law to anticipate changes in DNI; a full state feedback loop that uses a state observer for the receiver and an additional output feedback integrator loop for robustness. Experimental results show that the linear controller can successfully control the temperature of the SG4 receiver, provided that it is adequately tuned.

Commentary by Dr. Valentin Fuster
2014;():V001T02A044. doi:10.1115/ES2014-6597.

The integration of transparent electro-dynamic screen (EDS) on the front surface of solar mirrors and glass cover plates of photovoltaic panels has a strong potential to significantly reduce the frequency of water-based cleaning needed to mitigate losses from dust depositions present in arid regions. The objective of our research was to develop and evaluate prototype transparent EDS-integrated mirrors and solar panels for their self-cleaning functions, with an aim to keep the collectors clean at a low cost without water or manual labor. This paper focuses on the design, fabrication, and laboratory evaluation of a prototype EDS integrated second surface mirrors and solar panels. The EDS consists of a set of parallel transparent electrodes screen-printed on the optical surface and embedded in a thin transparent dielectric film. By applying three-phase, low current, low frequency high voltage-pulses to the electrodes, electro-dynamic repulsion forces and a traveling wave are created for removing dust particles from the surface of the collectors. Design and construction of an environmental test chamber to simulate different atmospheric conditions of semi-arid and arid areas with respect to temperature, RH, and dust deposition conditions are briefly described. A non-contact specular reflectometer was designed, constructed and calibrated for measuring specular reflection efficiency of the mirrors. Laboratory evaluation of the performance of the EDS-integrated collectors was completed using humidity controlled environment test chamber where the prototype mirrors and panels were examined for their self-cleaning action. In each experiment, the solar collectors were loaded with dust until the specular reflectance of the test mirror or the short circuit current of the panel showed a significant decrease. The EDS was then operated for one minute and the relative output was recorded. The results show that the specular reflectivity of EDS mirrors and the short circuit current of the EDS panels can be restored by more than 90% of the values measured under the clean conditions.

Commentary by Dr. Valentin Fuster
2014;():V001T02A045. doi:10.1115/ES2014-6603.

Research of advanced power cycles has shown supercritical carbon dioxide power cycles may have thermal efficiency benefits relative to steam cycles at temperatures around 500–700°C. To realize these benefits for CSP, it is necessary to increase the maximum outlet temperature of current tower designs. Research at NREL is investigating a concept that uses high-pressure supercritical carbon dioxide as the heat transfer fluid to achieve a 650°C receiver outlet temperature. At these operating conditions, creep becomes an important factor in the design of a tubular receiver and contemporary design assumptions for both solar and traditional boiler applications must be revisited and revised. This paper discusses lessons learned for high-pressure, high-temperature tubular receiver design. An analysis of a simplified receiver tube is discussed, and the results show the limiting stress mechanisms in the tube and the impact on the maximum allowable flux as design parameters vary. Results of this preliminary analysis indicate an underlying trade-off between tube thickness and the maximum allowable flux on the tube. Future work will expand the scope of design variables considered and attempt to optimize the design based on cost and performance metrics.

Commentary by Dr. Valentin Fuster
2014;():V001T02A046. doi:10.1115/ES2014-6611.

In response to the DOE Sunshot Initiative to develop low-cost, high efficiency CSP systems, UCLA is leading a multi-university research effort to develop new high temperature heat transfer fluids capable of stable operation at 800°C and above. Due to their operating temperature range, desirable heat transfer properties and very low vapor pressure, liquid metals were chosen as the heat transfer fluid. An overview of the ongoing research effort is presented.

Development of new liquid metal coolants begins with identification of suitable candidate metals and their alloys. Initial selection of candidate metals was based on such parameters as melting temperature, cost, toxicity, stability/reactivity Combinatorial sputtering of the down selected candidate metals is used to fabricate large compositional spaces (∼ 800), which are then characterized using high-throughput techniques (e.g., X-ray diffraction). Massively parallel optical methods are used to determine melting temperatures. Thermochemical modeling is also performed concurrently to compliment the experimental efforts and identify candidate multicomponent alloy systems that best match the targeted properties. The modeling effort makes use of available thermodynamic databases, the computational thermodynamic CALPHAD framework and molecular-dynamics simulations of molten alloys. Refinement of available thermodynamics models are performed by comparison with available experimental data. Characterizing corrosion in structural materials such as steels, when using liquid metals, and strategies to mitigate them are an integral part of this study. The corrosion mitigation strategy we have adopted is based on the formation of stable oxide layers on the structural metal surface which prevents further corrosion. As such oxygen control is crucial in such liquid metal systems. Liquid metal enhanced creep and embrittlement in commonly used structural materials are also being investigated. Experiments with oxygen control are ongoing to evaluate what structural materials can be used with liquid metals. Characterization of the heat transfer during forced flow is another key component of the study. Both experiments and modeling efforts have been initiated. Key results from experiments and modeling performed over the last year are highlighted and discussed.

Commentary by Dr. Valentin Fuster
2014;():V001T02A047. doi:10.1115/ES2014-6618.

This paper presents an analysis of an organic Rankine cycle (ORC) with dry cooling system aided by an earth-coupled passive cooling system. Several organic fluids were considered as working fluids in the ORC in the temperature range of 125–200°C. An earth-air-heat-exchanger (EAHE) is studied for a location in the United States (Las Vegas) and another in India (New Delhi), to pre cool the ambient air before entering an air-cooled condenser (ACC). It was observed that the efficiency of the system improved by 1–3% for the system located in Las Vegas and fluctuations associated with temperature variations of the ambient air were also reduced when the EAHE system was used. A ground-coupled heat pump (GCHP) is also studied for these locations where cooling water is pre cooled in an underground buried pipe before entering a condenser heat exchanger in a closed loop. The area of the buried pipe and the condenser size are calculated per kW of power generation for various working fluids.

Commentary by Dr. Valentin Fuster
2014;():V001T02A048. doi:10.1115/ES2014-6620.

Solar optical modeling tools are valuable for modeling and predicting the performance of solar technology systems. Four optical modeling tools were evaluated using the National Solar Thermal Test Facility heliostat field combined with flat plate receiver geometry as a benchmark. The four optical modeling tools evaluated were DELSOL, HELIOS, SolTrace, and Tonatiuh. All are available for free from their respective developers. DELSOL and HELIOS both use a convolution of the sunshape and optical errors for rapid calculation of flux profiles on the receiver surfaces. SolTrace and Tonatiuh use ray-tracing methods to determine reflected solar rays on the receiver surfaces and construct flux profiles. We found the ray-tracing tools, although slower in computation speed, to be more flexible for modeling complex receiver geometries, whereas DELSOL and HELIOS were limited to standard receiver geometries. We provide an example of using SolTrace for modeling non-conventional receiver geometries. We also list the strengths and deficiencies of the tools to show tool preference depending on the modeling and design needs.

Commentary by Dr. Valentin Fuster
2014;():V001T02A049. doi:10.1115/ES2014-6632.

The use of an air curtain blowing across the aperture of a falling-particle receiver has been proposed to mitigate convective heat losses and to protect the flow of particles from external winds. This paper presents experimental and numerical studies that evaluate the impact of an air curtain on the performance of a falling particle receiver. Unheated experimental studies were performed to evaluate the impact of various factors (particle size, particle mass flow rate, particle release location, air-curtain flow rate, and external wind) on particle flow, stability, and loss through the aperture. Numerical simulations were performed to evaluate the impact of an air curtain on the thermal efficiency of a falling particle receiver at different operating temperatures. Results showed that the air curtain reduced particle loss when particles were released near the aperture in the presence of external wind, but the presence of the air curtain did not generally improve the flow characteristics and loss of the particles for other scenarios. Numerical results showed that the presence of an air curtain could reduce the convective heat losses, but only at higher temperatures (>600°C) when buoyant hot air leaving the aperture was significant.

Commentary by Dr. Valentin Fuster
2014;():V001T02A050. doi:10.1115/ES2014-6637.

This paper discuses the design of several micro-channel solar receiver devices. Due to enhanced heat transfer in micro-channels, these devices can achieve a higher surface efficiency than current receiver technology, leading to an increase in overall plant efficiency. The goal is to design an efficient solar receiver based on use of super-critical carbon-dioxide and molten salt as heat-transfer fluids. The super-critical Brayton cycle has shown potential for a higher efficiency than current power cycles used in CSP. Molten salt has been used in CSP applications in the past. The required inlet and outlet temperatures of the fluid are 773.15 K and 923.15 K for carbon-dioxide and 573.15 K and 873.15 K for molten salt. These temperature values are determined by the power cycles the devices are designed to operate in. The required maximum pressure drop is 0.35 bar for carbon-dioxide and 1 bar for molten salt. These pressure values are intended to be a practical goal for maximum pressure drop. The super-critical carbon-dioxide power cycle requires an operating pressure of is 120 bar. Finally, each device must withstand any mechanical and thermal stresses that may exist. Devices presented range in size from 1 cm2 to 4 cm2 and in heat transfer rates from 200 W to 400 W. The size of the device is based on the output capacity of the solar simulator which will be used for testing. For carbon-dioxide, three designs were developed with varying manufacturability. The low risk design features machined and welded parts and straight parallel channels. The medium risk design features machined and diffusion bonded parts and straight parallel channels. The high risk design features a circular micro-pin-fin array created using EDM and is constructed using diffusion bonding. The absence of high operating pressure for molten salt made structural design much easier than for carbon-dioxide. Conjugate heat-transfer simulations of each design were used to evaluate pressure drop, receiver efficiency, and flow distribution. Two and three dimensional structural analyses were used to ensure that the devices would withstand the mechanical and thermal stresses. Based on the numerical analyses, a receiver efficiency of 89.7% with a pressure drop of 0.2 bar were achieved for carbon-dioxide. The design was found to have a structural safety factor of 1.3 based maximum mechanical stress occurring in the headers. For molten salt, an efficiency of 92.1% was achieved with a pressure drop of 0.5 bar.

Commentary by Dr. Valentin Fuster
2014;():V001T02A051. doi:10.1115/ES2014-6640.

Central receivers being installed in recent commercial CSP plants are liquid-cooled and power a steam turbine in a Rankine cycle. San Diego State University’s (SDSU) Combustion and Solar Energy Laboratory has built and is testing a lab-scale Small Particle Heat Exchange Receiver (SPHER). The SPHER is an air-cooled central receiver that is designed to power a gas turbine in a Brayton cycle. The SPHER uses carbon nanoparticles suspended in air as an absorption medium. The carbon nanoparticles should oxidize by the outlet of the SPHER, which is currently designed to operate at 5 bar absolute with an exit gas temperature above 1000°C.

Carbon particles are generated from hydrocarbon pyrolysis in the carbon particle generator (CPG). The particles are mixed at the outlet of the CPG with dilution air and the mixture is sent to the SPHER. As the gas-particle mixture flows through the SPHER, radiation entering the SPHER from the solar simulator is absorbed by the carbon particles, which transfer heat to the gas suspension and eventually oxidize, resulting in a clear gas stream at the outlet.

Particle mass loading is measured using a laser opacity measurement combined with a Mie calculation, while particle size distribution is determined by scanning electron microscopy and a diesel particulate scatterometer prior to entering the SPHER. In predicting the performance of the system, computer models have been set up in CHEMKIN-PRO for the CPG and in ANSYS Fluent for the SPHER, which is coupled with VeGaS ray trace code for the solar simulator.

Initial experimentation has resulted in temperatures above 850°C with around a 50K temperature difference when particles are present in the air flow. The CPG computer model has been used to estimate performance trends while the SPHER computer model has been run for conditions to match those expected from future experimentation.

Commentary by Dr. Valentin Fuster
2014;():V001T02A052. doi:10.1115/ES2014-6648.

A new experimental set-up has been introduced at San Diego State University’s Combustion and Solar Energy Lab to study the thermal oxidation characteristics of in-situ generated carbon particles in air at high pressure. The study is part of a project developing a Small Particle Heat Exchange Receiver (SPHER) utilizing concentrated solar power to run a Brayton cycle. The oxidation data obtained will further be used in different existing and planned computer models in order to accurately predict reactor temperatures and flow behavior in the SPHER.

The carbon black particles were produced by thermal decomposition of natural gas at 1250 °C and a pressure of 5.65 bar (82 psi). Particles were analyzed using a Diesel Particle Scatterometer (DPS) and scanning electron microscopy (SEM) and found to have a 310 nm average diameter. The size distribution and the complex index of refraction were measured and the data were used to calculate the specific extinction cross section γ of the spherical particles. The oxidation rate was determined using 2 extinction tubes and a tube furnace and the values were compared to literature. The activation energy of the carbon particles was determined to be 295.02 kJ/mole which is higher than in comparable studies. However, the oxidation of carbon particles bigger than 100 nm is hardly studied and almost no previous data is available at these conditions.

Commentary by Dr. Valentin Fuster
2014;():V001T02A053. doi:10.1115/ES2014-6663.

Electrodynamic Screens (EDS) is a promising alternative for removing dust particles from solar collectors and advantageous compared to the current water-based cleaning methods used widely in solar industries. To operate the EDS for efficient removal of dust layer having different size distributions and compositions of particles, it is necessary to optimize the design of the EDS and the materials used for construction. Since the electric field is the main component in removal of the dust particles, this paper reports optimization of the electric field as the function of geometric parameters of the EDS. For the optimization of the EDS, two distinct objective functions have been defined and the optimal values for the electrode width and inter-electrode spacing have been provided. The EDS model has been implemented in the COMSOL Multiphysics finite element analysis (FEA) software and analytical results have been verified. Based on the optimized parameters, different designs have been gone under fabrication process and then testing. This is a work in progress paper and the experimental results will be provided later to corroborate the higher clearance rates for the optimal designs.

Commentary by Dr. Valentin Fuster
2014;():V001T02A054. doi:10.1115/ES2014-6679.

The advantages of high temperature central receiver particle heating solar heat supply systems in concentrator solar power (CSP) have been recognized in recent years. The use of particulate as the collection medium provides two critical advantages: (1) Ordinary particulate minerals and products will allow higher collection temperatures approaching 1000°C compared with conventional molten salts, which are limited to around 650°C, and (2) the low cost high temperature particulate material can also be used as the storage medium in a highly cost effective thermal energy storage (TES) system. The high operating temperature allows use of high efficiency power conversion systems such as supercritical steam in a vapor power cycle or supercritical carbon dioxide in a Brayton cycle. Alternatively, a lower cost gas turbine can be used for the power conversion system. High conversion efficiency combined with inexpensive TES will yield a highly cost effective CSP system. The 300 kW-th prototype is being constructed as a solar heat supply system only, deferring the power conversion system for later demonstration in a larger integrated CSP system.

This paper describes the general design and development efforts leading to construction of the 300 kW prototype system located in the Riyadh Techno Valley development near King Saud University in Riyadh, Saudi Arabia, which is the first sizeable solar heat supply system purposely designed, and constructed as a particle heating system.

An important component in a particle heating system is the particle heating receiver (PHR), which should be durable and efficient while remaining cost-effective. A critical enabling technology of the PHR being implemented for this project was invented by researchers on our team. In our version of the PHR, the particulate flows downwards through a porous or mesh structure where the concentrated solar energy is absorbed. The porous structure will reduce the speed of the falling particulate material allowing a large temperature rise on a single pass. The new design will also increase the absorption of solar energy and mitigate convective heat loss and particle loss. Other innovative aspects of this design include low cost thermal energy storage bins and a cost effective particle to working fluid heat exchanger. Certain features of these design elements are subjects of ongoing patent applications. Nevertheless, the overall design and the development process of the prototype system is presented in this paper.

Commentary by Dr. Valentin Fuster
2014;():V001T02A055. doi:10.1115/ES2014-6696.

Parabolic trough and power tower technologies provide inherent advantage of thermal energy storage and high efficiency of the Concentrating Solar Power (CSP) systems for utility scale solar plants. High efficiency CSP power generation with minimal water use is one of the SunShot goals of the US Department of Energy. The specular reflectance efficiency of the solar mirrors plays a critical role in the efficiency of power generation. The optical surface of the mirrors and the receiver must be kept clean for efficient operation of the plant. Some environmental challenges in operating the large-scale CSP plants at high reflectance efficiency arise from high concentration of atmospheric dust, wind speed and variation of relative humidity (RH) over a wide range. Deposited dust and other contaminant particles, such as soot, salt, and organic particulate matters attenuate solar radiation by scattering and absorption. Adhesion of these particles on the mirror surface depends strongly by their composition and the moisture content in the atmosphere. Presence of soluble inorganic and organic salts cause corrosion of the mirror unless the contaminants are cleaned frequently.

In this paper, we briefly review (1) source of atmospheric dust and mechanisms involved in degradation of mirrors caused by salt particles, (2) loss of specular reflection efficiency as a function of particle size distribution and composition, and (3) an emerging technology for removing dust layer by using thin transparent electrodynamic screen (EDS). Feasibility of integration of EDS on the front surface of the solar collectors has been established to provide active self-cleaning properties for parabolic trough and heliostat reflectors.

Prototype EDS-integrated solar collectors including second-surface glass mirrors, metallized acrylic film mirrors, and dielectric mirrors, were produced and tested in an environmental test chambers simulating desert atmospheres. The test results show that frequent removal of dust layer can maintain the specular reflectivity of the mirrors above 90% under dust deposition at a rate ranging from 0 to 10 g/m2, with particle size varying from 1 to 50 μm in diameter. The energy required for removing the dust layer from the solar was less than 10 Wh/m2 per cleaning cycle. EDS based cleaning could therefore be automated and performed as frequently as needed to maintain reflection efficiency above 90% and thus reducing water usage for cleaning mirrors in the solar field. A comparative cost analysis was performed between EDS and deluge water based cleaning that shows the EDS method is commercially viable and would meet water conservation needs.

Topics: Dust , Solar energy , Mirrors
Commentary by Dr. Valentin Fuster
2014;():V001T02A056. doi:10.1115/ES2014-6699.

Commercial-scale implementation of concentrating solar thermal (CST) technologies for electricity generation has been increasing worldwide, but technology assessments produced by engineering consultancies typically indicate that electricity production using CST is more expensive than most other renewable energy technologies. A review of a selection of costing studies that have been prepared in recent years for Australian government and industry bodies suggests that electricity cost estimates for CST technologies are exaggerated by a combination of high capital cost estimates and the financial analysis methods used. The results of these assessments are often used in investment decision-making processes of industry and government bodies, so this may have a negative impact on further development of CST technologies. While it is apparent that revision of the methods used in these analyses could improve the apparent cost effectiveness of CST, it is also apparent that the competitiveness of CST technologies needs to be improved through cost reduction and generation improvement. One major driver for this is that some CST technologies have the capability to efficiently store energy in thermal form for electricity production on demand and this could have significant benefits to both specific users and to the general electricity network stability. As a stage in identifying potential targets for new research that will improve competitiveness of CST technologies, a sensitivity analysis was performed to examine the influence of a broad range of factors on the cost of electricity using combined performance modeling and financial analysis. This largely reconfirms the commonly held view that reduction of solar collector costs is a critical target, but also identifies the importance of improving the performance of the overall power generation cycle and general cost reduction throughout the plants.

Commentary by Dr. Valentin Fuster
2014;():V001T02A057. doi:10.1115/ES2014-6739.

Next-generation solar power conversion systems in concentrating solar power (CSP) applications require high-temperature advanced fluids in the range of 600° to 900°C. Molten salts are good candidates for CSP applications, but they are generally very corrosive to common alloys used in vessels, heat exchangers, and piping at these elevated temperatures. The majority of the molten-salt corrosion evaluations for sulfates with chlorides and some vanadium compounds have been performed for waste incinerators, gas turbine engines, and electric power generation (steam-generating equipment) applications for different materials and molten-salt systems. The majority of the molten-salt corrosion kinetic models under isothermal and thermal cyclic conditions have been established using the weight-loss method and metallographic cross-section analyses. Electrochemical techniques for molten salts have not been employed for CSP applications in the past. Recently, these techniques have been used for a better understanding of the fundamentals behind the hot corrosion mechanisms for thin-film molten salts in gas turbine engines and electric power generation. The chemical (or electrochemical) reactions and transport modes are complex for hot corrosion in systems involving multi-component alloys and salts; but some insight can be gained through thermochemical models to identify major reactions. Electrochemical evaluations were performed on 310SS and In800H in the molten eutectic NaCl-LiCl at 650°C using an open current potential followed by a potentiodynamic polarization sweep. Corrosion rates were determined using Tafel slopes and the Faraday law. The corrosion current density and the corrosion potentials using Pt wire as the reference electrode are reported.

Topics: Alloys , Corrosion
Commentary by Dr. Valentin Fuster

Geothermal, Ocean, and Emerging Energy Technologies

2014;():V001T05A001. doi:10.1115/ES2014-6310.

Energy production from salinity gradients is one of the developing renewable energy sources, and has significant potential for satisfying electrical demands. Urmia Lake is the second hyper-saline lake in the world and there is a significant salinity gradient between the lake’s water and the waters of those rivers that flow into the lake. A methodology for determining the feasibility for electrical production using Salinity Gradient Power (SGP) is developed for two different types of systems using this location as an example. Reverse Electrodialysis (RED) and Pressure Retarded Osmosis (PRO), The Gadar Chay River is one of thirteen rivers that run into Urmia Lake; it supports about 10% of the lake’s water. In this study, important parameters such as river discharge and the salinity content of river and lake’s waters for several years were investigated. The theoretical and technical potential of salinity gradient energy was also determined. These calculations indicate that 206.08 MW of electrical power could be produced at this location when the river flow is approximately 29.82 m3/s and they illustrates the potential for salinity gradient energy extraction between Urmia Lake and The Gadar Chay River.

Commentary by Dr. Valentin Fuster
2014;():V001T05A002. doi:10.1115/ES2014-6342.

Groundwater source heat pumps exploit the difference between the ground surface temperature and the nearly constant temperature of shallow groundwater. This project characterizes two areas for geothermal heating and cooling potential, Mason County in central Illinois and the American Bottoms area in southwestern Illinois. Both areas are underlain by thick sand and gravel aquifers and groundwater is readily available. Weather data, including monthly high and low temperatures and heating and cooling degree days, were compiled for both study areas. The heating and cooling requirements for a single-family house were estimated using two independent models that use weather data as input. The groundwater flow rates needed to meet these heating and cooling requirements were calculated using typical heat pump coefficient of performance values. The groundwater in both study areas has fairly high hardness and iron concentrations and is close to saturation with calcium and iron carbonates. Using the groundwater for cooling may induce the deposition of scale containing one or both of these minerals.

Commentary by Dr. Valentin Fuster
2014;():V001T05A003. doi:10.1115/ES2014-6422.

In this paper, we present the results of the design, fabrication, installation, and operation of a 20-kW OTEC (ocean thermal energy conversion) pilot plant. The results can be used as basic data for the design of commercial plants with capacities in excess of 40 MW. To perform an experiment on the 20 kW OTEC, a closed OTEC cycle was designed and fabricated at the Ocean Water Plant Research Center. The cycle utilizes surface ocean water as its heat source and deep ocean water as its heat sink. R32 (Difluoromethane, CH2F2) was used as the working fluid, and the temperature of the heat source and heat sink were 26°C and 5°C, respectively. A semi-welded-type heat exchanger was used as the evaporator and condenser, and the OTEC cycle was designed for a gross power of 20 kW. The advantages of the semi-welded-type heat exchanger include easy maintenance of the gasket-type heat exchanger and the rare leakage of the welded-type heat exchanger. The plate arrangement of the semi-welded-type heat exchanger comprised one welded channel for the working fluid and another gasket-type channel. The gross power of the turbine was determined to be 20.49 kW. The evaporating capacity was calculated as 1,020 kW, and the cycle efficiency was determined to be about 2.00%.

Commentary by Dr. Valentin Fuster
2014;():V001T05A004. doi:10.1115/ES2014-6438.

Today’s rate of fossil fuel consumption rapidly depletes fuel reserves and leads to a number of adverse environmental effects. Although the scope of these effects has yet to be fully realized, it is clear that the development of alternative energy sources is very important. A relatively new form of alternative energy known as reverse electrodialysis (RED) appears to be one of the promising energy sources of the future. This technology harvests the energy stored in the salinity gradient between two different liquids, and converts it directly into electric power. This power is generated by pumping water through an array of alternating pairs of cation and anion exchange membranes called cells. Various academic sources calculate the available energy to be 1.5 MJ for every cubic meter of sea and river water mixed, making all river basins a potential location for power production.

Small prototype systems using 50 cells with areas of 100 cm2 were assembled by a group in the Netherlands, but larger stacks remain to be tested. An understanding of the feasibility of RED as a possible energy source relies on testing of cells with larger membrane area and different numbers of membrane pairs. An experimental system was designed with cells 61 cm × 16.5 cm, which will increase the output by nearly a factor of 10. Along with having much larger dimensions than previous systems, the design has an adjustable number of cells in the stack, allowing users obtain test results at a variety of settings. Comparing the output of systems with few cells to systems with many cells will help us to optimize the stack size in terms of hydrodynamic losses.

Initial testing of the system resulted in a positive result. The tests showed that the system produced power, and the 1.98 volts measured was 83% of the predicted value. Leakage of the electrode rinse solution contaminated the membranes, and prevented more testing. Once the electrode rinse system is redesigned, more testing will be done.

Commentary by Dr. Valentin Fuster
2014;():V001T05A005. doi:10.1115/ES2014-6549.

A supercritical CO2 test facility is currently being developed at Indian Institute of Science, Bangalore, India to analyze the performance of a closed loop Brayton cycle for concentrated solar power (CSP) generation. The loop has been designed for an external heat input of 20 kW, a pressure range of 75–135 bar, flow rate of 11 kg/min, and a maximum cycle temperature of 525 °C. The operation of the loop and the various parametric tests planned to be performed are discussed in this paper. The paper addresses various aspects of the loop design with emphasis on design of various components such as regenerator and expansion device. The regenerator design is critical due to sharp property variations in CO2 occurring during the heat exchange process between the hot and cold streams. Two types of heat exchanger configurations 1) tube-in-tube (TITHE) and 2) printed circuit heat exchanger (PCHE) are analyzed and compared. A PCHE is found to be ∼5 times compact compared to a TITHE for identical heat transfer and pressure drops. The expansion device is being custom designed to achieve the desired pressure drop for a range of operating temperatures. It is found that capillary of 5.5 mm inner diameter and ∼2 meter length is sufficient to achieve a pressure drop from 130 to 75 bar at a maximum cycle temperature of 525 °C.

Commentary by Dr. Valentin Fuster
2014;():V001T05A006. doi:10.1115/ES2014-6593.

There are many possibilities for the direct carbon fuel cell approach including hydroxide and molten carbonate electrolytes, solid oxides capable of consuming dry carbon, and hybrids of solid oxide and molten carbonate technologies. The challenges in fabricating this type of fuel cell are many including how to transport the dry solids into the reactant chamber and how to transport the spent fuel (ash) out of the chamber for continuous operation[1]. We accomplish ash removal by utilizing a hydrodynamic approach, where inert gas or steam is injected into the anode chamber causing the carbon particles to circulate. This provides a means of moving the particles to a location where they can be separated or removed from the system. The graphic below illustrates how we segregate the spent fuel from the fresh fuel by creating multiple chambers. Each sequential chamber will have a reduced performance until the fuel is fully spent. At that point, the electrolyte/ash mixture can be removed from the cell area and cleaned for recycling or discarded.

Commentary by Dr. Valentin Fuster

Hydrogen Energy Technologies

2014;():V001T07A001. doi:10.1115/ES2014-6345.

Hydrogen has many properties that make it an attractive energy storage medium for a sustainable future. But hydrogen is also difficult to store safely and cheaply. By storing hydrogen in an oil-based slurry with powdered magnesium hydride, cheap and safe hydrogen storage can be realized. This paper describes the characteristics and benefits of cycling hydrogen in and out of magnesium hydride slurry.

Based on our experience with magnesium hydride slurry, we have performed a study to evaluate the cost effectiveness of applying bulk hydrogen storage, using magnesium hydride slurry, in a baseload wind power system that we will also discuss. This study concludes that a 150 MW baseload wind power system would produce an Internal Rate of Return (IRR) of 10% with an electric price of $0.088/kWh. The costs and performance characteristics of this power plant are described.

Commentary by Dr. Valentin Fuster

Low/Zero Emission Power Plants and Carbon Sequestration

2014;():V001T08A001. doi:10.1115/ES2014-6414.

TRNSYS simulation software was used to modify a validated Air Source Heat Pump (ASHP) model in an Archetype Sustainable House (ASH) in Toronto. In this model, a Building Integrated Photovoltaic-Thermal Collector (BIPV/T) was coupled with ASHP. The PV/T system arrangement was considered as a part of the south-oriented roof of the house. The warm air generated in the BIPV/T collector was considered the source of the heat pump for heat production. The coupling of BIPV/T and ASHP enables a highly efficient heating system in harsh winter conditions. The developed TRNSYS model of the house along with integrated PV/T system with ASHP was simulated for the whole year to predict the hourly outlet air temperature, thermal energy and electricity obtained from the PV/T array. The results from the simulation were used to estimate the saving in energy and cost as well as to predict the electricity related GHG emission reduction potential from the PV panels. Monthly greenhouse gas (GHG) emission credit from PV production based on hourly GHG emission factor was obtained; the results showed that annual GHG emission due to electricity demand by the ASHP was reduced by 225 kg CO2 (19.3%) when the heat pump was integrated with the PV/T array. Also, in this study, the annual electricity cost credit from PV production based on Time-of-Use (TOU) and the reduction in electricity cost of the heat pump when connected with PV/T systems was calculated and compared with the cost of working the heat pump alone. The results show that there is a saving of $500 in annual electricity bills and GHG emission credit of 862.6 kg CO2 from renewable electricity generation.

Topics: Heat pumps , Emissions
Commentary by Dr. Valentin Fuster
2014;():V001T08A002. doi:10.1115/ES2014-6514.

With the ever rising concern of global warming, carbon capture is gaining the reputation of one of the most challenging fields of research. A very promising technology to capture CO2 is oxy-combustion. Oxy-combustion offers several advantages over conventional combustion technologies, such as flue gas volume reduction, high combustion efficiency, low fuel consumption and significant reduction in NOx emissions. Liquid fuel is available and it is the most widely used source of energy in the world. Easy handling and transportation, less storage volume and higher flame temperature are some of the features of liquid fuel which give it an upper hand over other sources. In this study, an experimental work on oxygen enriched combustion of ethanol in a vertical reactor by Lacas F. et. al. has been modeled numerically. Non-premixed model using Probability Density Function has been incorporated to simulate the combustion process of ethanol droplets. Predicted combustion characteristics are found to be in good compliance with the experimental data. In addition to this, effects of dilution of carbon-dioxide in oxygen on the flame properties have also been presented. Combustion of ethanol in oxygen-carbon dioxide environment has been compared with that of the conventional air environment.

Topics: Combustion , Ethanol
Commentary by Dr. Valentin Fuster

Photovoltaics

2014;():V001T09A001. doi:10.1115/ES2014-6743.

Efficiency of solar-thermophotovoltaic (TPV) systems can be improved by controlling spectral property of thermal radiation to match the photovoltaic (PV) cells spectral response. We developed a spectrally selective emitter which can emit quasi-monochromatic thermal radiation to improve the efficiency of TPV systems. From the evaluation by the detailed-balance model for gallium antimonite (GaSb) TPV cells, the photovoltaic conversion efficiency over 50 % is achievable in the case of the emitter temperature over 1800K and emitter Q-value over 15. The numerical simulation based on Rigorous-Coupled Wave Analysis revealed that the quasi-monochromatic thermal radiation can be obtained by closed-end microcavity structure. A solar-TPV system equipped with a solar-simulator is fabricated to evaluate the effect of spectrally selective emitter. By using GaSb TPV cells and spectrally selective emitter based on dielectric layer coating, photovoltaic conversion efficiency of 26% and total efficiency of 7.7% are obtained at emitter temperature of 1654 K under 384 suns.

Commentary by Dr. Valentin Fuster

Wind Energy Systems and Technologies

2014;():V001T13A001. doi:10.1115/ES2014-6664.

This paper presents the work done by the authors to analyze the method of performance characterization of a 100W scale vertical axis wind turbines using a controlled-velocity test apparatus. The design of the power transfer system containing a gearbox and generator requires test data to determine the peak and operating range of wind speed, corresponding to RPM and torque. Multiple methods of turbine testing were considered, including in situ, wind tunnel, and control-velocity. Controlled-velocity, a method where the turbine is moved through a fluid, was selected based on lack of test location wind speeds or access to a wind tunnel of sufficient size. The test apparatus is designed to be effective for VAWT turbines of a diameter range from 1.45 to 4.2 meters in a wind velocity range of 1 to 17 m/s. This covers a Reynolds number range between (2.5 × 10^5 < Re < 4.2 × 10^6). A change from previous control-velocity test apparatus is the use of a separate truck and trailer compared to a flatbed truck, which allows greater distance between the truck cab and the turbine, to decrease any flow interference of the cab. This previous work and testing has shown to be a valid test method in that the turbine is in similar turbulent conditions as near the ground and buildings which the turbine is designed for. The main advantage of this test apparatus is the ability to test turbines in a region with low average wind speeds and minimum infrastructure.

Commentary by Dr. Valentin Fuster

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