Stagnation temperature test methods for determining solar collector thermal performance degradation

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Virginia Polytechnic Institute and State University

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.