Experimental investigation of the higher coefficient of thermal performance for water-in-glass evacuated tube solar water heaters in China

•The energy grades system for solar water heater (SWH) in China was introduced.•Heat loss and capacity of heat collection mainly affected SWH thermal performance.•Optimum ratio of tank volume to collector area for solar water heater is 57 to 72 L/m2.•The recommendation polyurethane insulation layer should be around 50 mm thick.•SWH with shorter tube has a better thermal performance.

Solar water heaters (SWHs), now widely used in China, represent an environmentally friendly way to heat water. We tested the performance of more than 1000 water-in-glass evacuated tube SWHs according to Chinese standards and found that the heat loss from the storage tank and capacity of the solar collector affected their thermal performance. The optimum parameters to maximize the performance of water-in-glass evacuated tube SWHs included a ratio of tank volume to collector area of 57–72 L/m2, which should give a system efficiency of 0.49–0.57, meaning that the temperature of water in the tank will exceed 45 °C after one day of heat collection. In addition, the polyurethane insulation layer should be around 50 mm thick with a free foaming density of about 35 kg/m3, and the evacuated tube should be short. The tilt angle did not affect the performance of the SWHs. These results should aid in the design of highly efficient SWHs.