Air-Source, Ground-Source, and
Absorption Heat Pumps
As we discussed in the previous section, air-source heat pumps use a fan in an outdoor unit to bring air over refrigerant-filled coils. Two sets of these coils transfer heat indoors, where the heat is blown away from the coils by another fan and distributed through a home. Some air-source heat pump systems consist of a single "packaged" unit containing both sets of coils in one box. This box is then installed on the roof of a building with the ductwork extending through the wall. Larger systems for commercial buildings often are installed in this way. Home heat pumps are usually "split" systems with an outdoor and indoor component installed through the wall. Depending on the type of system, there may be one or more indoor components to distribute heat.
Ground-source heat pumps absorb heat from the ground or an underground body of water and transfer it indoors, or vice versa. The most common type of ground-source heat pump transfers heat directly from the ground by absorbing it through buried pipes filled with water or a refrigerant. Ground-source heat pumps that pump liquid through pipes can be either closed-loop or open-loop systems. In a closed-loop system, the same refrigerant or water circulates through the pipes repeatedly. In an open-loop system, water is pumped out of the water source, such as a well or a man-made lake, and when the heat has been extracted from the water, that water returns to the well or surface lake. More water is then pumped from the well to extract more heat.
Absorption heat pumps are air-source heat pumps that are powered by natural gas, solar power, propane, or geothermally heated water rather than by electricity. Absorption pumps can be used for large-scale applications, but are now available for large homes as well. The main difference between a standard air-source heat pump and an absorption pump is that rather than compressing a refrigerant, an absorption pump absorbs ammonia into water, and then a low-power pump pressurizes it. The heat source then boils the ammonia out of the water, and the process starts over. Manufacturers rate absorption heat pumps differently from standard heat pumps, using a measurement called a coefficient of performance (COP). Consumers should look for a COP above 1.2 for heating and above 0.7 for cooling. (We'll discuss ratings for standard heat pumps a little later.) Air-source, ground-source, and absorption heat pumps are the most common kinds of heat pumps, but they won't work in every situation. Read on to learn about special kinds of heat pumps.