Effects of Tube Space on the Thermal Performance of Water-in-Glass Evacuated Tube Solar Water Heaters

Abstract

In the present specification of domestic solar water heating systems (GB/T19141), regardless of the type of collectors used in the systems, Q17, the daily heat gain corresponding to the daily radiation of 17MJ/m2 on the collector surface, is a key indication to rate the thermal performance of a domestic solar water heaters. In this work, an attempt was made to investigate effect of the distance between two adjacent tubes in solar tube collectors of a water-in-glass evacuated tube solar water heater (ETSWH) on by theoretical calculations and experimental measurements. Results obtained by simulations and experimental test showed that the tube space had a significant effect on of the ETSWH, the shorter the tube distance, the higher the Q17, thus better the ETSWH performs. However, the actual situation is the case, the shorter the tube distance, the lower the real daily solar gain of the system, thus worse it performs. This implied that Q17 as the indication of thermal performance of the ETSWH is not reasonable, and specifying the tube distance in solar tube collectors of the ETSWH in the technical specification of GB/T19141 is advisable so as to eliminate effect of the tube distance on Q17.