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Electric Heat Pumps

Electric Heat Pumps

Electric heat pumps are year-round space-conditioning systems capable of providing heating, cooling, and domestic hot water. Their appeal lies both in that they offer heating and cooling in a single piece of equipment—which usually means a lower capital cost—and in that they provide heat at a lower cost than electric resistance heating (in some cases, lower than gas heating as well). There are two broad categories of electric heat pumps—air source and ground source (also called geothermal). Air-source heat pumps are the most common form found in commercial applications, so we have focused on them. Geothermal heat pumps are discussed in a sidebar.

Air-source heat pumps can be used in most commercial applications and some industrial processes, particularly those that generate waste heat. However, most air-source heat pumps do not perform well in cold climates, because both their capacity and their efficiency decrease significantly at low temperatures.

Figure 1: Schematic diagram of an air-to-air heat pump operation
A heat pump can switch from heating to cooling functions by changing the position of the reversing valve. All common heat pumps contain two heat exchangers (one cold and one hot) plus a compressor charged with refrigerant. The hot heat exchanger delivers heat from condensing refrigerant while the cold heat exchanger absorbs heat as refrigerant evaporates. The refrigerant is forced by the motor-driven compressor to circulate and change phase from liquid to gas in the cold evaporator and back to a liquid in the hot condenser. The heat exchangers typically require a fan or pump to move air or water through them to achieve effective heat transfer from a heat source to a heat sink.

There are three applications where ASHPs are best suited:

  • Where electricity is the only fuel source available. Since ASHPs can provide heat up to almost four times more efficiently than electric resistance heaters, their operating costs will be significantly lower.
  • Where heating loads are small and the capital cost of an ASHP is less than that of a separate air conditioner and furnace. Even if cheap heating fuel produces low energy costs for a furnace, this may not outweigh the lower capital cost of buying just one piece of equipment where heating loads are small.
  • Where heating loads are large and the difference in price between electricity and heating fuel is great enough to produce lower energy costs for the ASHP. ASHPs can be highly cost-effective when they have both a lower capital cost (from buying one piece of equipment instead of two) and a lower energy cost.
Geothermal Heat Pumps

Geothermal heat pump systems (also sometimes called ground-source heat pumps or geoexchange systems) use the relatively constant temperature of the ground to provide a higher efficiency than a conventional air-source heat pump. During the cooling season, an air-source heat pump moves energy from the building to the hotter outdoor air, while a geothermal heat pump transfers energy to the cooler ground—moving energy across a lower temperature difference, thereby gaining efficiency. Similarly, during the heating season, an air-source heat pump moves energy from cold outside air and brings it inside—and when the temperature gets too cold, the air-source heat pump must be supplemented by electric resistance heating. Because the ground stays much warmer than the outside air, even in the heating season, a geothermal heat pump moves energy across a lower temperature difference and can deliver heat on even the coldest days with a high coefficient of performance (COP).

The major elements in a geothermal heat pump include a ground loop (a buried piping system), one or more water-source heat pumps (inside the building), and a distribution system to bring conditioned air where it’s needed. In open-loop systems, a heat exchanger often sits between the ground loop and a water loop that feeds the heat pumps. In facilities with multiple spaces to be conditioned, a distributed heat pump system can be established: Commercial buildings often have simultaneous demands for heating and cooling in different zones, and having a geothermal heat pumps in each space can provide independent heating and cooling. The distributed nature of such a system has an operations and maintenance advantage as well—a problem with a single heat pump will only affect the room it serves, not the performance of the entire system.

The main benefit of geothermal heat pumps, however, is energy savings. The U.S. government has installed thousands of them in its buildings and has found that they typically save 15 to 25 percent of total building energy use in commercial buildings compared to conventional heating and air-conditioning systems. And they can save even more in residential buildings, where savings can be as high as 40 percent.

However, capital costs are high, limiting the current market for geothermal heat pumps to predominantly institutional facilities that make decisions based on life-cycle costs or do not have short payback period requirements, such as federal, state, and local government buildings and K–12 schools. Also, as a general rule of thumb, geothermal heat pumps are most likely to be economical when there are both high heating and high cooling loads—and when those loads are relatively balanced.

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