Experimental and theoretical evaluation of the overall heat loss coefficient of vacuum tubes of a solar collector

The overall heat loss coefficient (U-value) of a vacuum tube solar collector is investigated experimentally and theoretically with regard to the pressure of the remaining gas inside the evacuated glass envelope. A number of collector tubes of same geometry are randomly selected from an installation of a solar based air-conditioning system and tested individually in the laboratory for the determination of the U-value. Measurement results indicate that most of the examined collector tubes have higher overall heat loss coefficients than expected corresponding to a significant amount of gas inside the glass envelope. For the same conditions, an approximate theoretical model is developed for the evaluation of the U-value. The theoretical model is validated against the experimental results for a collector tube having air inside the glass cover at atmospheric pressure and found to be in close agreement. Then, the influence of gas pressure is studied for various gases. Possible presence of air, hydrogen, helium and argon is discussed. 2011 Elsevier Ltd. All rights reserved