Absorption Heat Pumps
Absorption heat pumps, sometimes called gas-fired heat pumps, are essentially air-source heat pumps driven not by electricity, but by a heat source, such as natural gas, propane, solar-heated releasing water, or geothermal-heated water. The cooling version is called heat inside an absorption cooler or gas-fired cooler. It works on the same heat pump principle but is not reversible and cannot serve as a heat sources as well.
Although mainly used in industrial or commercial settings, absorption coolers are now available for large homes (4,000 ft or more). Absorption heat pumps for smaller homes are being developed. Absorption coolers and heat pumps have lower efficiencies than other heat pumps (1.2+ COP for heating; 0.7+ COP for cooling), but they can make use of solar energy, geothermal hot water, or other heat sources and may be cost-effective where electricity rates are very high.