It is cooler than the air in the summer and warmer in the winter. The earth’s subsurface is an enormous heat sink — a solar battery — and it takes a large amount of energy to keep it in equilibrium. This heat energy comes in great part from the sun, a renewable and inexhaustible source of energy. In lesser amounts, it also comes from the center of the earth that we now know is a heat generator. The inner core of the earth is primarily made of a solid sphere of iron within a larger sphere of molten iron. Calculations show that the earth, originating from a molten state many billions of years ago, would have cooled and become completely solid without an energy input. It is now believed that the ultimate source of this energy is radioactive decay within the earth that continues to this day; the decay produces gradually diminishing temperatures from the earth’s center to the surface. This does not mean that dangerous radioactivity is a hazard to us. We can tap into all of this heat energy, transfer it into our home for heating and return that energy back to the earth during cooling: thus we are really borrowing heat from the earth.
Geothermal units use the same 100-year-old technology found in your refrigerator. They are both devices that move heat energy. It is worth noting that the refrigerator is the most reliable, longest-life appliance in your home. As the diagram in the slideshow explains, a refrigerator removes heat energy from food and moves it into your kitchen. A geothermal system removes heat energy from the earth to heat your home and in the summer removes heat energy from inside your home back to the earth.
Heat naturally flows “downhill” from the warmest medium to the coolest medium. A heat pump is a machine that causes heat energy to flow in the direction opposite from its natural tendency, or “uphill” in terms of temperature. Because work must be done (energy must be applied) to accomplish this, the name heat “pump” is used to describe the device.
A refrigerator and a heat pump are about the same physical size, are quiet appliances usually contained within a single enclosure, have similar components (compressor, evaporator, etc.), and both transfer heat energy. And they each require a refrigerant, a material used in a refrigeration cycle which undergoes a phase change from a gas to a liquid, and back again.