Theoretical And Experimental Study Of The Flat Plate Solar Water Heater Temperature Drop At Night

Compared with all-glass evacuated solar heaters, a flat-plate collector has the advantages of high thermal efficiency, low price and facilitating to integrate with buildings. It is estimated that flat-plate collectors take account about 95% share of solar water heaters' market in the world with an exception of China. However, due to its poor performance in anti-freezing, the application of flat-plate collectors is restricted by climatic condition in China where all-glass evacuated solar heaters take about 88% share of the solar market. To expand the use of flat-plate collector heaters in China, especially in Yunnan, it is extremely important to develop a simple anti-freezing technique of the collectors so as to accelerate the development of solar industry in Yunnan.The air temperature at night is the lowest in the whole day, and the freezing-damage of a flat-plate collector is usually taken place at night. In the daytimes, a collector acts a solar absorber, but functions as a thermal emitter at night due the thermal radiation to the sky dome, and thus the thermal radiation to the sky dome is a key cause to lead the freezing damage of a collector at night. The objective of this study was aimed at experimentally investigating the temperature depression of the absorber (the temperature difference between ambient air and absorber) of a flat-plate collector under different climatic conditions and different operating modes (namely with and without water flowing through the collector at night). To perform this work, two solar water heaters with each consisting of two flat-plate collectors (one is glazed and other is unglazed) was examined at the center of Kunming (the roof of our institute) and a fishing garden in the suburb respectively from 9 to 22 of January, 2006. The experimental results showed that an averaged temperature depression of about 6°C～8°C was found at clear nights and that of 1°C was observed at overcast nights for collectors without water flowing through, and the maximum value was appeared in between 5:00～7:00, indicating that the collectors were easy to be damaged by freezing during this period. In addition, a theoretical analysis was done in this study to simulate the effects of emissivity of the absorber and those of convective coefficient between collector and the ambient air on the temperature depression of collectors. A simple comparative way to measure emissivity of a absorber was suggested in this study.