Measured Performance of a High-Efficiency Solar-Assisted Heat Pump Water Heater

This paper describes a novel solar photovoltaic-assisted heat pump water heater (PV-HPWH). The system uses two 310 Wp PV modules, micro-inverters and innovative controls to produce and store daily hot water. The system likely costs half that of solar thermal systems, with greater reliability, no freeze protection and potentially superior performance in cloudy locations. Moreover, no net metering is needed for the PV, which with two modules makes for a simple installation.The HPWH controls have been modified through experimentation such that higher tank temperatures are achieved during the day when solar availability is high while avoiding triggering the compressor during early morning hot water draws. When tank temperatures are satisfied, the remaining PV electricity is stored in the tank using staged electric resistance elements. Typical performance sees hot water storage greater than 65 °C at sunset. A mixing valve provides hot water at the target temperature (52 °C). By altering tank temperature, an equivalent of ~2 kWh of electrical energy is stored for use during evening hours. Typically, there is no grid electricity demand during the utility summer peak demand window.The system has been tested for twelve months in a laboratory at the Florida Solar Energy Center. Realistic hot water draws were imposed with detailed data recorded on system performance. Long term COP, averaging 5.4, has been as high as 8.0 during sunny summer days. Average daily grid electricity consumption has been 1.2 kWh/day' less than many refrigerators. Prospects for further development and refinement are discussed.