Conference Year



Solar Thermal, Heat Pump, Energy Factor


According to the U.S. Department of Energy, 40% of the energy consumed in the U.S. is used in buildings and almost half of that amount is used for heating or cooling. Current technologies allow for efficient thermal management, but most utilize energy harvested from fossil fuels or convert electricity directly into thermal energy. Alternative heating technologies such as heat pumps and solar thermal collectors can greatly reduce the energy used for heating while producing reliable heating performance. This paper documents how an off-the-shelf residential heat pump water heater was integrated with a solar thermal system to improve overall heating performance. Solar thermal is the primary source of heat and the heat pump is used as a back-up when the sun is not shining with enough intensity. A primary/secondary pumping system is used to allow multiple modes of operation, including both hot water and hydronic heating. Since the heat pump water heater is located entirely inside the conditioned space, a secondary benefit/detriment for space cooling is noted based on the weather conditions. An energy dashboard that evaluates the heating performance of this hybrid solar heat pump system in real time was developed.  The primary efficiency metric is the Energy Factor (EF), which is the ratio of thermal energy delivered to electrical energy consumed.  As a point of reference, heat pump water heaters earn an Energy Star rating for an Energy Factor of 2.2 under test conditions.  During two-weeks of rigorous 24 hours a day testing in March of 2016, the instantaneous EF for the solar heat pump system varied from 1 to greater than 40 depending on the weather conditions.  The overall average EF for the two weeks of continuous testing was 2.29, indicating an efficiency significantly higher than traditional direct electrical heating systems.