A fast response heat pump water heater using thermostat made from shape memory alloy

The heat pump water heater produces hot water so slow at low ambient temperature that it frequently could not meet the hot water load demand in winter. The present study develops a fast response heat pump water heater (FRHP) designed with two separate tanks (supply and holding tank) which are connected by a thermostat made from shape memory alloy (SMAV). The SMAV is a mechanical heat-sensitive device made from shape memory alloy which keeps the valve closed when the water temperature is not high enough. This will isolate the tanks and let the vapor compression cycle heat up the supply tank only. The speed of temperature rise thus is increased. The SMAV will open and induce a natural circulation between two tanks to transfer the heat from the supply tank to the holding tank, when water is heated to a designated temperature. A 100 l FRHP was built and tested in the present study. The experimental results showed that the temperature response speed of the supply tank, before SMAV is opened, reaches 1.056 °C/min and the holding tank, after SMAV is opened, reaches 0.828 °C/min at ambient temperature 20 °C. The FRHP will heat up 50 l water in the supply tank with 30 °C temperature rise within 40 min in winter which is acceptable in domestic application. The energy consumption is in the range 0.008-0.016 kWh/l of hot water at about 55 °C.