A study on characteristics of thermal storage tank for varying thermal load in multi-use heat pump water heater

A multi-use heat pump water heater (HPWH) is a combined device of heating, air conditioning and hot water supply by using one heat pump. Heat pump is mainly operated for heating or air conditioning the space during the day time and stores the hot water in thermal storage tank during the night time. The stored heated water is used to supply hot water during the day time. When the stored hot water is not enough to supply the required hot water demand, a part of heat pump has to be used for heating the water. To minimize the operation time and power consumption in multi-use HPWH for water supply during the day time, a control method of heat pump operation was studied related to the thermal storage tank. By measuring the temperature within the storage tank, the heat pump was ON/OFF controlled. By varying the water flow rate through the heat pump, the temperature of hot water from the heat pump is maintained to be 60 °C. The effects of size and arrangement of heat exchanger in the thermal storage tank were studied also. The study showed (1) ON/OFF control of heat pump by measuring the temperature within the storage tank reveals an efficient method to reduce the heat pump operation time and power consumption for water supply in multi-use HPWH, (2) varying flow rate through the heat pump shows better performance compared to that of the constant flow rate. The operation time and power consumption are decreased about 12.2% and 11.2%, respectively, and (3) more performance enhancement can be obtained with upper part densely arranged heat exchanger. The operation time and power consumption are decreased about 14.8% and 13.4%, respectively.