Absorption heating and cooling systems are not driven by electricity, but by solar power, geothermal power or natural gas, with natural gas being the most common fuel. Absorption heat pumps work just like any other heat pump. There are, however, two main differences. First, the absorption heat pump is driven by a natural gas burner instead of electricity. The second difference is that absorption pumps use a water-ammonia solution instead of a refrigerant. During the winter, that solution absorbs Earth's heat and the pump moves the heated liquid into the house to warm the air. In the summertime, the heat exchange works in reverse [source: U.S. Department of Energy].