The concept of achieving low cost (≈ $20/m²) and ultra low weight (5 kg/m²) for heliostats is explored theoretically and experimentally. The objective of this work is to significantly improve the cost and performance of the structure under concern, without sacrificing strength and efficiency. The focus is on an innovative design of stretched-membrane heliostats. A reflective membrane of thin film is supported by a taut fishnet structural membrane consisting of graphite fiber-polymer matrix composite. The reflective and structural membranes are attached to a ring frame made of wood. The nonlinear problem of stress-strain analysis is formulated and solved using the finite-element code NASTRAN. The analysis is done for loads which include the initial stretching of the film and structural membrane and the pressure load due to wind. The scope of the present work is limited to analyzing the structural deformation behavior of flat-plate heliostats and partial extension to parabolic and semi-hemispherical dish reflectors.

An analytical and experimental investigation was undertaken to evaluate a proposed method for determining the thermal degradation of materials used in flat-plate solar collectors. The proposed method is based on measuring stagnation (no-flow) temperatures of the absorber plate. A comparison of the advantages and limitations of the proposed method is made with the conventionally used existing method which is based on measuring the energy output from the collector. Previous investigations have shown that the existing test method may not be sufficiently sensitive to detect expected changes in material properties, is influenced by the test environment, and is relatively expensive to perform. The material properties of interest are primarily the cover transmittance, the solar absorptance of the absorber, the infrared emittance of the absorber, and the thermal conductivity of insulation. Experimental results were obtained from both on and off-campus test sites. The data includes those from solar simulator tests and indoor tests using a highly instrumented solar collector. This indoor collector was equipped with electrical strip heaters mounted on the backside of the absorber plate to simulate the absorbed solar radiation in a controlled environment. The experiments included an investigation of four techniques for measuring the absorber stagnation temperature. Steady-state and transient analytical models are developed to evaluate stagnation temperature test methods. These models are validated using extensive experimental data. The proposed method is based on measuring stagnation temperatures before and after prolonged exposure to prevailing environmental conditions. While these measurements are simpler than those required in the energy output method, other.factors, such as transient effects, are important for outdoor tests. Test methods using instantaneous and allday integrated stagnation temperatures are considered. Both of these test methods were shown to be highly sensitive to environmental conditions. Wind speed was shown to potentially have the most serious influence on results. The measured temperature distribution of the absorber plate was shown to be highly nonisothermal as a result of collector edge heat losses and thermal stratification of the air underneath collector covers. Instantaneous measurements were observed to be very sensitive to transients as a result of intermittent cloud cover. All-day integrated measurements were not affected by such transients. The investigation revealed that proposed stagnation temperature test methods have potential in determining collector property changes after environmental exposure. Results indicate that the proposed method is more sensitive to small property changes than the current energy output method. However, variations in environmental conditions should be limited or taken into account when using stagnation temperature test methods. / Ph. D.

An analysis of outdoor collector performance data collected as part of the NBS Durability/Reliability Test Program is presented. The data is analyzed for consistent patterns of performance degradation resulting from prolonged outdoor exposure at low flow rate and stagnation conditions. The analysis includes examination of the effects of pyranometer sensitivity changes with time and sensitivity dependence on temperature. Performance results are compensated for differences in test environments by use of an analytic correction routine. A statistical examination of the implementation of the testing procedure is made to identify interlaboratory differences. / Master of Science

The National Bureau of Standards conducted outdoor durability tests on eight different types of commercially available flat plate solar collectors. The test results for five types of collectors are analyzed. The purpose of the test program was to investigate the feasibility of determining the durability of materials by measuring collector thermal efficiency at specified intervals. The important material properties include the solar absorptance and long wave emittance of the absorber surface, solar and long wave transmittance of the cover material and the thermal conductivity of the insulation. Tests were conducted by three independent testing laboratories located in Phoenix, Arizona, Cape Canaveral, Florida, and Palo Alto, California. The test sites were chosen to investigate the effects of the various environmental conditions found in the United States on collector degradation. Three test series were considered to study the effect of various operating conditions on collector degradation. The collectors were exposed to the environment for up to 240 days. The thermal performance test results did not reveal significant degradation in the performance parameters, F_r(T_α) and F_rU_L, of the collectors considered. Any degradation that may have occurred was overshadowed by experimental uncertainty. The test results did not depend on test series, geographic location or season of the year. Referring the test results to a common set of weather conditions did not reduce the scatter in the results. / Master of Science

All-day experimental tests were performed to determine the thermal performance of two commercial integral storage collectors for solar domestic hot water systems. These tests were performed under a variety of ambient conditions and irradiance levels, both with and without forced circulation and noontime hot water draws. An analytical model was developed to predict the thermal performance of one of the two systems tested and predicted performance was compared with experimental results. Experimental and analytical results indicate that thermal stratification has a mininal effect on the daily collection efficiency of integral storage collector, so that a standard test similar to ASHRAE Standard 93-77 might reasonably be used to obtain the performance characteristics of the collection element of these systems. The results of an ASHRAE 93-77 type test might then be used to obtain performance ratings under ASHRAE Standard 95 procedures using an in-line heat source. The results of the present investigation may be used to validate such an ASHRAE 95 test method. / Master of Science

This study presents a comparison of field measurements of energy delivered by five integral collector storage (ICS) passive solar water heater systems installed at various geographic locations in Virginia to predicted values calculated using Annual Performance Methodology (APM). APM is a prediction method developed by Alan Zollner that offers quick and easy comparisons of design option for ICS systems. Several different methods exist today that are meant to predict the performance of this type of system and that might be used as a design tool to help a designer make appropriate design decisions. Some of these methods are quite complicated and do not lend themselves to quick and easy comparisons of various design options. This study measured the amount of energy delivered by five ICS systems over a six month period. The amount of water drawn out of these systems daily was also recorded. This data was compared to predicted values calculated using APM to determine if APM could predict the performance of these systems within plus or minus 15 percent of the measured values. This study demonstrates that APM was able to predict long term performance of ICS systems within plus or minus 15 percent eighty percent of the time. Short term performance however demonstrated variation that in some cases were quite large and could not be considered reliable predictions. / M. Arch.