It appears the greatest heat source on Earth is the Earth!
A geothermal heat pump is an extremely efficient heating and cooling system that uses the earth's energy storage capability to heat and cool your home or business. Like all heat pumps, a geothermal heat pump simply moves heat from one place to another. In the summer, the compressor moves Freon through a coil (the evaporator) in the duct system where it absorbs heat from the indoor air. The compressor then moves the Freon to the center tube of a coaxial coil. The outer tube of the coaxial coil contains transfer fluid (a water and methanol mixture) which absorbs the heat from the Freon. This liquid is then circulated through a loop in the ground, where it is cooled. In the winter, the process is reversed. The Freon in the coaxial coil absorbs heat from the transfer liquid and then releases it to the coil in the air distribution system, warming the air passing over it.