Journal of Solar Energy Engineering, Transactions of the ASME
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Mean and Peak Wind Loads on Heliostats Mean and peak wind loads on flat rectangular or circular heliostats were measured on models in a boundary layer wind tunnel which included an atmospheric surface layer simulation. Horizontal and vertical forces, moments about horizontal axes at the ground level and at the centerline of the heliostat, and the moment about the vertical axis through the heliostat center were measured. Results showed that loads are higher than predicted from results obtained in a uniform, low-turbulence flow due to the presence of turbulence. Reduced wind loads were demonstrated for heliostats within a field of heliostats and upper bound curves were developed to provide preliminary design coefficients.
Journal of Solar Energy Engineering, Transactions of the ASME - Tập 111 Số 2 - Trang 158-164 - 1989
Theoretical Analysis of a Water Desalination System Using Low Grade Solar Heat Theoretical analysis of a solar desalination system utilizing an innovative new concept, which uses low-grade solar heat, is presented. The system utilizes natural means of gravity and atmospheric pressure to create a vacuum, under which liquid can be evaporated at much lower temperatures and with less energy than conventional techniques. The uniqueness of the system is in the way natural forces are used to create vacuum conditions and its incorporation in a single system design where evaporation and condensation take place at appropriate locations without any energy input other than low grade heat. The system consists of solar heating system, an evaporator, a condenser, and injection, withdrawal, and discharge pipes. The effect of various operating conditions, namely, withdrawal rate, depth of water body, temperature of the heat source, and condenser temperature were studied. Numerical simulations show that the proposed system may have distillation efficiencies as high as 90% or more. Vacuum equivalent to 3.7 kPa (abs) or less can be created depending on the ambient temperature at which condensation will take place.
Journal of Solar Energy Engineering, Transactions of the ASME - Tập 126 Số 2 - Trang 774-780 - 2004
High-Efficiency, Thin-Film GaAs Solar Cells The present research is directed toward demonstrating the feasibility of producing high-efficiency GaAs solar cells with high power-to-weight ratio by organo-metallic chemical vapor deposition (OM-CVD) growth of thin epi-layers on suitable substrates. Antireflection-coated, metal-oxide-semiconductor (AMOS), GaAs solar cells grown on bulk polycrystalline Ge substrates were initially studied, with the best efficiency achieved being about 9 percent AM1 (7 percent AM0). Subsequently, a new direct deposition method for fabricating ultra-thin top layer, epitaxial n+ /p shallow homojunction solar cells on (100) GaAs substrates (without anodic thinning) was developed by means of which large area (1 cm2) cells were produced with about 19 percent AM1 (15 percent AM0) conversion efficiency. An AM1 conversion efficiency of about 18 percent (14 percent AM0), or about 17 percent (13 percent AM0) with 5 percent grid coverage, was achieved for a single-crystal, GaAs, n+ /p cell grown by OM-CVD on a Ge wafer. These achievements led to the fabrication, for the first time, of thin GaAs epi-layers OM-CVD grown with good crystallographic quality, using a (100) Si-substrate on which a thin Ge epi-interlayer was first deposited by CVD from GeH4 and processed for improved surface morphology.
Journal of Solar Energy Engineering, Transactions of the ASME - Tập 105 Số 3 - Trang 237-242 - 1983
The Effect of a Thermotropic Material on the Optical Efficiency and Stagnation Temperature of a Polymer Flat Plate Solar Collector Solar hot water and space heating systems constructed of commodity polymers have the potential to reduce the initial cost of solar thermal systems. However, a polymer absorber must be prevented from exceeding its maximum service temperature during stagnation. Here, the addition of a thermotropic material to the surface of the absorber is considered. The thermotropic layer provides passive overheat protection by switching from high transmittance during normal operation to high reflectance if the temperature of the absorber becomes too high. A one dimensional model of a glazed, flat-plate collector with a polymer absorber and thermotropic material is used to determine the effects of the optical properties of the thermotropic material on the optical efficiency and the stagnation temperature of a collector. A key result is identification of the reflectance in the translucent state required to provide overheat protection for potential polymer absorber materials. For example, a thermotropic material in its translucent state should have a solar-weighted reflectance greater than or equal to 52% to protect a polypropylene absorber which has a maximum service temperature of 115 °C.
Journal of Solar Energy Engineering, Transactions of the ASME - Tập 137 Số 2 - 2015
Radiative Heat Transfer Analysis in Plasmonic Nanofluids for Direct Solar Thermal Absorption The present study reports a novel concept of a direct solar thermal collector that harnesses the localized surface plasmon of metallic nanoparticles suspended in water. At the plasmon resonance frequency, the absorption and scattering from the nanoparticle can be greatly enhanced via the coupling of the incident radiation with the collective motion of electrons in metal. However, the surface plasmon induces strong absorption with a sharp peak due to its resonant nature, which is not desirable for broad-band solar absorption. In order to achieve the broad-band absorption, we propose a direct solar thermal collector that has four types of gold-nanoshell particles blended in the aquatic solution. Numerical simulations based on the Monte Carlo algorithm and finite element analysis have shown that the use of blended plasmonic nanofluids can significantly enhance the solar collector efficiency with an extremely low particle concentration (e.g., approximately 70% for a 0.05% particle volume fraction). The low particle concentration ensures that nanoparticles do not significantly alter the flow characteristics of nanofluids inside the solar collector. The results obtained from this study will facilitate the development of highly efficient solar thermal collectors using plasmonic nanofluids.
Journal of Solar Energy Engineering, Transactions of the ASME - Tập 134 Số 2 - 2012
Predicted Efficiency of a Low-Temperature Nanofluid-Based Direct Absorption Solar Collector Due to its renewable and nonpolluting nature, solar energy is often used in applications such as electricity generation, thermal heating, and chemical processing. The most cost-effective solar heaters are of the “flat-plate” type, but these suffer from relatively low efficiency and outlet temperatures. The present study theoretically investigates the feasibility of using a nonconcentrating direct absorption solar collector (DAC) and compares its performance with that of a typical flat-plate collector. Here a nanofluid—a mixture of water and aluminum nanoparticles—is used as the absorbing medium. A two-dimensional heat transfer analysis was developed in which direct sunlight was incident on a thin flowing film of nanofluid. The effects of absorption and scattering within the nanofluid were accounted for. In order to evaluate the temperature profile and intensity distribution within the nanofluid, the energy balance equation and heat transport equation were solved numerically. It was observed that the presence of nanoparticles increases the absorption of incident radiation by more than nine times over that of pure water. According to the results obtained from this study, under similar operating conditions, the efficiency of a DAC using nanofluid as the working fluid is found to be up to 10% higher (on an absolute basis) than that of a flat-plate collector. Generally a DAC using nanofluids as the working fluid performs better than a flat-plate collector, however, much better designed flat-plate collectors might be able to match or outperform a nanofluids based DAC under certain conditions.
Journal of Solar Energy Engineering, Transactions of the ASME - Tập 131 Số 4 - 2009
A Circuit-Based Approach to Simulate the Characteristics of a Silicon Photovoltaic Module With Aging The aging of photovoltaic modules results inevitably in a decrease of their efficiency all through their lifetime utilization. An approach to simulate the evolution of electrical characteristics of a photovoltaic module with aging is presented. The photovoltaic module is modeled by an equivalent electrical circuit whose components have time-dependent characteristics determined under accelerated tests. By entering sun irradiance and temperature, I–V and P–V curves as well as efficiency evolution can be simulated over years assuming equivalent time. The methodology is applied for the case of a monocrystalline photovoltaic module modeled by a one-diode circuit and aging laws are determined with experimental results of damp heat (DH) tests 85 °C/85% RH performed by Hulkoff (2009, “Usage of Highly Accelerated Stress Test (HAST) in Solar Module Aging Procedures,” M.S. thesis, Chalmers University of Technology, Göteborg, Sweden). A power degradation rate of 0.53%/yr is found. A parametric study shows that the rundown of optical transmittance of the upper layers with aging has the most important impact by reducing the initial efficiency by 11.5% over a 25-year exposure contrary to electrical degradations which cause a decrease of 1.85% of the initial efficiency.
Journal of Solar Energy Engineering, Transactions of the ASME - Tập 137 Số 2 - 2015
Exergy Analysis for the Evaluation of the Performance of Closed Thermal Energy Storage Systems The use of exergy analysis, rather than energy analysis, for the evaluation of the performance of thermal energy storage systems is discussed. The energy and exergy relationships for a simple closed tank storage with heat transfers by heat exchanger are obtained. A complete storing cycle, as well as the individual charging, storing, and discharging periods, are considered. A numerical example for a simple case is given. The work reported is preliminary to the task of developing simplified conventions for the evaluation and comparison of the performance of thermal storages using exergy analysis methods. The establishment of such simplified conventions appears to be a necessary prerequisite to general acceptance of these methods by the engineering community.
Journal of Solar Energy Engineering, Transactions of the ASME - Tập 110 Số 4 - Trang 255-261 - 1988
A Fundamental Equation for Exergy Balance on Solar Collectors This paper presents the exergy balance equation on a solar collector which acts as the fundamental and principal expression for the solar thermal design. The equation fully explains the exergy loss processes and can be used to derive the approximate optimum operating condition for solar collectors. Furthermore, using the equation, it can be shown that two different collectors, an evacuated tubular collector and a flat-plate collector, have both nearly equal capabilities in exergy gain despite large differences in technological efforts and expenses to produce them. In addition, ways for improvement for a solar collector are also discussed here briefly.
Journal of Solar Energy Engineering, Transactions of the ASME - Tập 110 Số 2 - Trang 102-106 - 1988
Appropriate Thermodynamic Performance Measures for Closed Systems for Thermal Energy Storage Several definitions of energy and exergy efficiency for closed systems for thermal energy storage (TES) are developed and discussed. A simple model is utilized in which heat quantities are transferred at specified temperatures to and from a TES. Efficiency definitions are considered for the overall storage process and for the three component periods which comprise a complete storage process (charging, storing, and discharging). It is found that (1) appropriate forms for both energy and exergy efficiency definitions depend on which quantities are considered to be products and inputs; (2) different efficiency definitions are appropriate in different applications; (3) comparisons of different TES systems can only yield logical results it they are based on a common definition, regardless of whether energy or exergy quantities are considered; and (4) exergy efficiencies are generally more meaningful and illuminating than energy efficiencies for evaluating and comparing TES systems. A realistic, but simplified, illustrative example is presented. The efficiency definitions should prove useful in the development of valid and generally accepted standards for the evaluation and comparison of different TES systems.
Journal of Solar Energy Engineering, Transactions of the ASME - Tập 114 Số 2 - Trang 100-105 - 1992
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