Consultant Energy Information Services Parker, Colorado, United States
The decarbonization of residential buildings requires eliminating or replacing combustion appliances for space and water heating. Hence, there is an obvious need to have an alternative source of energy for heating houses in both temperate and cold climates. While the use of electric heat pumps instead of gas-fired furnaces and boilers is one possible path under grid electrification, the most common air-source heat pumps (ASHPs) quickly lose efficiency as outside temperatures decrease. Below freezing temperatures, even variable-speed inverter-driven “cold-climate” ASHPs struggle to operate with a coefficient of performance (COP) above 2.0, which means they are barely able to pull enough heat out of the outside air in order to supply heated indoor air. As outdoor temperatures approach 0 degrees F, performance gets considerably worse, to the point that most ASHP systems have to use electric resistance heat strips to provide supplemental indoor space heating.
While it is possible to use PV systems to supply electricity to a typical home’s ASHP, winter operation at freezing temperatures and below would require a much larger PV system (in the range of 5-10 kW larger) than makes current economic sense, especially if the heat pump transitions to supplying electric resistance heating. However, it is possible to provide a solar assist to a home’s heat pump by using solar thermal collectors in combination with the outdoor evaporator coil. In one configuration, refrigerant flowing through the collectors during the day is heated above the outside air evaporator temperature, which in turn increases the efficiency of the heat pump when that refrigerant is pumped to the indoor condenser fan coil.
In addition to solar-assisted ASHPs, water-source heat pumps (WSHPs) can be used with solar thermal collectors to increase the efficiency of the heat pump at cold outdoor temperatures. Solar-assisted WSHPs also provide the ability to store water heated during the day for use at night by using a storage tank and immersing the heat pump’s indoor condenser inside the storage tank. This is the most common solar-assisted heat pump configuration when it is replacing a natural gas-fired boiler in a baseboard or radiant heating system.
This poster shows these and other configurations of solar-assisted heat pumps that have been demonstrated to work in cold and temperate climates. Electric and gas utilities, regulators, and policy-makers all need to understand how heat pumps and solar thermal systems can work together to achieve decarbonization goals.
