Investigation on system efficiency of a flat plate solar water heater

A design method of a flat plate solar water heating system is presented. By analyzing the thermal efficiency of the whole system, it is obtained that daily radiant exposure is inversely proportional to the system efficiency. The condition that the heat gain in the system is proportional to the daily radiant exposure is achieved. The influencing factors of the appearance time of the maximum temperature in the water tank are obtained. The applicability of the double loop solar water heating system in Beijing is demonstrated in the experiments, and the system efficiency is 59.5%.