The use of a heat exchanger in solar water heating systems reduces the overall system efficiency. In this paper, we derive a simple “penalty factor” that gives the fractional reduction in thermal efficiency imposed by a heat exchanger immersed in an isothermal integral collector storage (ICS) system. The penalty factor depends only on the effectiveness of the heat exchanger, the capacitance of the hot water delivered, and the product of the loss coefficient and aperture area of the ICS. This factor allows the designer to select an appropriate heat exchanger without extensive simulation or experiments. Application of the penalty factor for a load-side (or on-demand) heat exchanger, which operates intermittently at varying flow rates, is discussed and illustrated through example.

Since incident solar radiation and cooling demand are in phase for most of the day at Caribbean sites, and between 4 and 7 kWh/m 2 of solar energy is available virtually every day, solar cooling makes sense. A solar ice maker with a capacity of 50 lbs (22.7 kg) of ice per day was operated at CERMES, UWI, Barbados, for six months using 1.1kW p of BP Solar photovoltaic (PV) panels. This was a pilot study for a full scale, one-tonne-per-day solar ice maker at the fishing village of Skeete’s Bay on the southeast coast of Barbados at a new fishing complex. The Skeete’s Bay solar PV ice maker system uses 148 BP Solar 275F solar PV modules with a maximum output of 4.75A and 17.0V. These are wired in series-parallel to form an array with a nominal output of 11,100W (48V at 231A). This power is directed into a battery bank of twenty four12V, 200Ah (100hour rate) lead acid batteries. Two Trace SW4048 inverters power two Scotsman model 1200AE-32A flake ice machines, each producing 1170 lbs (531kg) per day and feeding into a common storage bin of 4.1m 3 .

Feasibility of ionic liquids as liquid thermal storage media and heat transfer fluids in a solar thermal power plant was investigated. Many ionic liquids such as [C 4 min][PF 6 ], [C 8 mim][PF 6 ], [C 4 min][bistrifluromethane sulflonimide], [C 4 min][BF 4 ], [C 8 mim][BF 4 ], and [C 4 min][bistrifluromethane sulflonimide] were synthesized and characterized using thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), nuclear magnetic resonance (NMR), viscometry, and some other methods. Properties such as decomposition temperature, melting point, viscosity, density, heat capacity, and thermal expansion coefficient were measured. The calculated storage density for [C 8 mim][PF 6 ] is 378 MJ/m 3 when the inlet and outlet field temperatures are 210°C and 390°C. For a single ionic liquid, [C 4 mim][BF 4 ], the liquid temperature range is from −75°C to 459°C. It is found that ionic liquids have advantages of high density, wide liquid temperature range, low viscosity, high chemical stability, non-volatility, high heat capacity, and high storage density. Based on our experimental results, it is concluded that ionic liquids could be excellent liquid thermal storage media and heat transfer fluids in solar thermal power plant.

With the increasing concerns with the quality and the safety of foods, several standards and guidelines have been developed to improve the design, construction, and operation of storage warehouses. Several cool storage buildings have been constructed in Korea during the last decade. However, there are no specific standards or guidelines for energy use reduction in refrigerated structures. The main objective of this study to determine the impact of various insulation systems on the total cooling load of the cool storage structures with particular consideration given to the product thermal mass effect to find optimal insulation thickness for each envelope component for various climatic locations in Korea. An energy analysis model was developed using the DOE-2.1E program and over 3500 energy simulations were performed. To determine the optimal configuration for die storage building insulation system, life-cycle cost analysis was conducted. The selection of optimal insulation configuration for each climatic location is based on various criteria including those of crop storage owners and of national energy conservation policy. The results presented in this paper provide easy-to-use design guidelines to select optimal insulation thickness for crop storage facility in Korea.

A detailed performance model of the 30 MWe SEGS VI parabolic trough plant was created in the TRNSYS simulation environment using the Solar Thermal Electric Component model library. Both solar and power cycle performance were modeled, but natural gas-fired hybrid operation was not. Good agreement between model predictions and plant measurements was found, with errors usually less than 10%, and transient effects such as startup, shutdown, and cloud response were adequately modeled. While the model could be improved, it demonstrates the capability to perform detailed analysis and is useful for such things as evaluating proposed trough storage systems.

This paper presents a novel process comprising solar upgrading of hydrocarbons by steam reforming in solar specific receiver reactors and utilizing the upgraded, hydrogen-rich fuel, in high efficiency conversion systems, such as gas turbines or fuel cells. In comparison to conventionally heated processes, about 30 % of fuel can be saved with respect to the same specific output. Such processes can be used in small scale as a stand-alone system for off-grid markets, as well as in large scale to be operated in connection with conventional combined-cycle plants. The solar reforming process has an intrinsic potential for solar/fossil hybrid operation, as well as a capability of solar energy storage to increase the capacity factor. The complete reforming process will be demonstrated in the SOLASYS project, supported by the European Commission in the JOULE/THERMIE framework. The project has been started in June 1998. The SOLASYS plant is designed for 300 kWel output, it consists of the solar field, the solar reformer and a gas turbine, modified to enable operation both on fossil fuel as well as on the product gas from the solar reformer. The SOLASYS plant will be operated at the experimental solar test facility of the Weizmann Institute of Science, Israel. Start-up of the pilot plant is scheduled for the end of the year 2000. The midterm goal is to replace fossil fuel feedstock by renewable or non conventional feedstocks in order to increase the share of renewable energy and to establish processes with only minor or no CO2 emissions. Examples are given for solar upgrading of bio-gas from municipal solid waste as well as for upgrading of weak gas resources. With some feedstock pretreatment (removal of sulfur components, adjustment of composition) the product gases after solar reforming can be used for further processing to methanol or other chemical compounds.

This paper investigates adaptive optimal control of a grid-independent photovoltaic system consisting of a collector, storage, and a load. The algorithm is based on Q -Learning, a model-free reinforcement learning algorithm, which optimizes control performance through exploration. Q -Learning is used in a simulation study to find a policy which performs better than a conventional control strategy with respect to a cost function which places more weight on meeting a critical base load than on those non-critical loads exceeding the base load.

It has been established that the development of a storage option and increasing the operating temperature for parabolic trough electric systems can significantly reduce the levelized electricity cost (LEC) compared to the current state of the art. Both improvements require a new heat transfer fluid that must have a very low vapor pressure at the hot operating temperature and combined with a high thermal stability, higher than 450°C. Further, the piping layout of trough plants dictates that the fluid not be allowed to freeze, which dictates the use of extensive insulation and heat tracing unless the fluid has a freezing point near 0°C. At present, it seems likely that this “ideal” fluid will have to be found among organic rather than inorganic salts. We are therefore investigating the chemical and thermal properties of ‘room temperature ionic liquids’ (RTILs) that hold much promise as a new class of heat transfer or storage fluids.

A simplified ice storage system model was developed in which the icemaking mode is reflected by a higher power consumption per unit cooling than in chilled-water mode. The performance of four control strategies for ice storage systems is evaluated. The four control strategies investigated are chiller-priority and constant-proportion as conventional, instantaneous controls, while storage-priority and optimal control represent sophisticated controls employing load forecasting. Six parameters were investigated with respect to their influence on the ice storage system performance: Storage losses, utility rate structures, rate periods, penalty for icemaking, storage capacity, and the impact of load forecasting. Optimal control was determined to provide maximal operating cost savings. The storage-priority control yields operating costs only slightly higher than those of optimal control. Chiller-priority control realized savings that were typically on the order of 50% of what is theoretically possible (optimal control). Constant-proportion control proved to be a simple control strategy yielding higher savings than chiller-priority, yet lower than storage-priority control.

This paper investigates the technical feasibility of using a compact, air-cooled, solar absorption air conditioning system when coupled to an innovative array of solar collectors. The particular absorption system of study is a single effect that uses lithium bromide and water as the absorbent and refrigerant fluid pair. The geographical location of interest is Puerto Rico and similar subtropical regions. The heat input to the absorption system generator is provided by an array of novels flat plate solar collectors that integrate the thermal storage component into them. The proposed collectors have a phase change material (PCM) integrated into them as a storage mechanism. The PCM-integrated solar collector eliminates the need of conventional storage tanks reducing cost and space. The present work uses a paraffin-graphite composite as the PCM to increase the conductivity of the PC matrix. The paraffin’s melting point is around 89°C that is appropriate for use in absorption systems. The mathematical model that describes the thermal process in the PCM is presented and differs from the analysis of conventional flat plate solar collectors. The proposed model for the PCM considers the temporal changes but not the spatial variations. The resulting set of equations for the fluid flow, the PCM, and the collector’s surface are solved simultaneously. Results for the collectors’ thermal performance are presented along with the effects of the composition of the PCM material. The thermal performance of an absorption machine coupled to an array of the proposed PCM’s solar collectors was investigated for nominal cooling capacities of 10.5, 14, and 17.5 kW. These cooling loads are suitable for a typical house or a small business building in Caribbean Islands. Computer simulations were conducted to evaluate the overall system’s performance when subjected to dynamic cooling loads. Within the computer model, heat and mass balances are conducted on each component of the system, including the solar collectors, the air-cooled condenser, and the air-cooled absorber. Comparisons are made with an absorption air conditioning system that uses a cooling tower with conventional flat plate collectors instead of air-cooled and PCM components. Useful information about physical dimensions of collectors, number of collectors needed, and efficiency of the overall system is presented.

Heat transfer rates of a single horizontal tube immersed in a water-filled enclosure tilted at 30 degrees are measured. The results serve as a baseline case for a solar water heating system with a heat exchanger immersed in an integral collector storage. Experiments are conducted for isothermal and stratified enclosures with both adiabatic and uniform heat flux boundary conditions. Natural convection flow in the enclosure is interpreted from measured water temperature distributions. Formation of an appropriate temperature difference that drives natural convection is determined. Correlations for the overall heat transfer coefficient in terms of the Nusselt and Rayleigh numbers are reduced to the form Nu D = 0.675Ra D 0.25 for 10 6 ≤ Ra D ≤ 10 8 .

A 2 kW photovoltaic (PV) system with battery storage for emergency power supply was designed and installed as a test demonstration in a 112 square meter apartment. The system was installed to demonstrate the potential of a photovoltaic system (using thin-film solar modules) as an environmentally friendly and renewable source of energy compared to conventional electricity generation. Barriers to installation and preliminary estimations of system performance and costs are presented.

A method of test is defined and instantiated for integral-collector-storage solar systems with a load-side heat exchanger immersed in the storage. A “system test/model calibration” approach is used. The test protocol consists of short-term tests and an extended validation test. Short-term tests include warmup tests, a decay test, and heat exchanger tests. The model parameters to be determined are the normal incidence transmission-absorption product, an additive correction to the total heat loss, and heat exchanger parameters correlating the ratio of modeled to measured heat exchanger fluxes. The calibrated system model resulting from the short-term tests is then validated against typical operation data. The calibrated model presented here under-predicts delivered energy over a 5-day period of normal operation by 11%, perhaps because of the present lack of modeling for the tank stratification induced during charging by solar radiation.

A finite difference model was developed that predicts the transient temperature profile of the water in an integral collector-storage unit during charge from solar radiation incident on the absorber. The model uses a forward difference technique. A Nu(Gr*) correlation for a flat plate in an infinite medium under prescribed uniform heat flux was used on the inside surface of the absorber to relate imposed flux to boundary layer mass flow and temperature. Nodal mass-energy balances were imposed to derive the overall mass flows and temperatures. Model results were compared to experimental and numerical data with good agreement.

The availability of storage capacity plays an important role for the economic success of solar thermal power plants. For today’s parabolic trough power plants, sensible heat storage systems with operation temperatures between 300°C and 390°C can be used. A solid media sensible heat storage system is developed and will be tested in a parabolic trough test loop at PSA, Spain. A simulation tool for the analysis of the transient performance of solid media sensible heat storage systems has been implemented. The computed results show the influence of various parameters describing the storage system. While the effects of the storage material properties are limited, the selected geometry of the storage system is important. The evaluation of a storage system demands the analysis of the complete power plant and not only of the storage unit. Then the capacity of the system is defined by the electric work produced by the power plant, during a discharge process of the storage unit. The choice of the operation strategy for the storage system proves to be essential for the economic optimization.

A zero net energy building (ZNEB) is grid-tied, net metered and produces as much energy on-site as it uses in a typical year. In this paper, we describe methods to cost optimize designs for ZNEB’s. The goal is to find a constrained optimum, i.e., the minimum-cost design that achieves ZNE. Increased first costs for improved efficiency are balanced against the size of the PV system required to achieve ZNE (where the net-metered PV system is the energy source of last resort, because it is expensive but can meet the remaining load “using the grid for storage”). Discrete building component options are considered within various categories (e.g., walls, ceilings, foundations, glass types, air conditioners, furnaces, etc.). Each category is optimized by balancing the marginal cost of saved energy for efficiency options against the cost of energy from the PV system. Categories are optimized sequentially with iteration to adjust for possible interactions between categories. The process is automated with software that manipulates input files, launches DOE2 simulations, reads the results, and executes the optimization methodology.

Parabolic trough power systems that utilize concentrated solar energy to generate electricity are a proven technology. Industry and laboratory research efforts are now focusing on integration of thermal energy storage as a viable means to enhance dispatchability of concentrated solar energy. One option to significantly reduce costs is to use thermocline storage systems, low-cost filler materials as the primary thermal storage medium, and molten nitrate salts as the direct heat transfer fluid. Prior thermocline evaluations and thermal cycling tests at the Sandia National Laboratories’ National Solar Thermal Test Facility identified quartzite rock and silica sand as potential filler materials. An expanded series of isothermal and thermal cycling experiments were planned and implemented to extend those studies in order to demonstrate the durability of these filler materials in molten nitrate salts over a range of operating temperatures for extended timeframes. Upon test completion, careful analyses of filler material samples, as well as the molten salt, were conducted to assess long-term durability and degradation mechanisms in these test conditions. Analysis results demonstrate that the quartzite rock and silica sand appear able to withstand the molten salt environment quite well. No significant deterioration that would impact the performance or operability of a thermocline thermal energy storage system was evident. Therefore, additional studies of the thermocline concept can continue armed with confidence that appropriate filler materials have been identified for the intended application.

Natural convection is measured in an enclosure that represents an integral collector storage solar system with an immersed heat exchanger. The enclosure has an aspect ratio of 9.3:1 and is inclined 30 deg to the horizontal. Heat transfer coefficients for bundles of 240 tubes are obtained for a range of transient operating modes and pitch-to-diameter ratios of 1.5, 2.4, and 3.3. Results for isothermal and stratified enclosures yield a correlation for the overall Nusselt number, Nu D = ( 2.45 ± 0.03 ) Ra D 0.188 , 230 ≤ Ra D ≤ 9800 . Nusselt numbers are three times larger than those for a similarly configured single-tube and an eight-tube bundle. This increase is attributed to stronger fluid motion within the bundle and greater overall circulation rates in the enclosure.

In contrast to building energy conversion equipment, less improvement has been achieved in thermal energy distribution, storage and control systems in terms of energy efficiency and peak load reduction potential. Cooling of commercial buildings contributes significantly to the peak demand placed on an electrical utility grid. Time-of-use electricity rates are designed to encourage shifting of electrical loads to off-peak periods at night and weekends. Buildings can respond to these pricing signals by shifting cooling-related thermal loads either by precooling the building’s massive structure (passive storage) or by using active thermal energy storage systems such as ice storage. Recent theoretical and experimental work showed that the simultaneous utilization of active and passive building thermal storage inventory can save significant amounts of utility costs to the building operator, yet in many cases at the expense of increased electrical energy consumption. This article investigates an approach to ensure that a commercial building utilizing both thermal batteries does not incur excessive energy consumption. The model-based predictive building controller is modified to trade off energy cost against energy consumption. This work shows that buildings can be operated in a demand-responsive fashion to substantially reduce utility costs, however, at the expense of increased energy consumption. Placing a greater emphasis on energy consumption led to a reduction in the savings potential. In the limiting case of energy-optimal control, the reference control was replicated, i.e., if only energy consumption is of concern, neither active nor passive building thermal storage should be utilized. On the other hand, cost-optimal control suggests strongly utilizing both thermal storage inventories.

Parabolic trough power plant technology is one of the most demonstrated solar power options commercially available. While trough power plants are the least expensive solar option, cost of electricity still exceeds that needed to directly compete with conventional fossil-fired large-scale central power technologies. Several evaluations have been done that identify a series of mechanisms for significant cost reduction over the next decade. One of the opportunities for improving the economics of parabolic trough plants is the development of lower cost and more efficient thermal energy storage (TES) technologies. This paper focuses on several of the TES technologies currently under development, namely: the use of an indirect molten-salt storage system, the use of molten-salt as a heat transfer fluid in the solar field and thermal energy storage system, and the development of new types of storage fluids. The assessment compares the cost and performance of these candidate thermal energy storage technologies by evaluating their impact on the levelized cost of electricity from the plant. This analysis is updated based on work conducted on these technologies during the last year.