A study of the effects of water capacity on the thermal performance of water-in-glass evacuated tube solar collectors applied to space heating

Abstract

Water-in-glass evacuated tube solar collectors are widely used in China for their low costs. The water-in-glass collector is characterized in its large heat capacity due to the large capacity of water filled in the inner tubes. This article studies the thermal performance of water-in-glass evacuated tube collectors for space heating and mainly focuses on the effects of water capacity. A dynamic model of the water-in-glass evacuated tube collector applied to space heating systems is developed and experiments are conducted. The simulated results by the dynamic model agree well with the experimental results, while the simulated results acquired by the zero-capacity model produce significant deviations. The amounts of heat collection at different water capacities are nearly the same; the heat output increases with the reduction of water capacity. The reduction of heat output during daytime is close to the heat loss during nights. A simplified equation is proposed to calculate the heat loss during nights and can be used to evaluate the effects of water capacities on reduction of the daily heat output under various circumstances directly.