How Geothermal Works
At the heart of a geothermal heating & cooling system is the heat pump. As the name implies, a heat pump pumps or moves heat from one place to another; unlike a furnace or boiler, it does not create any heat. The heat pump is connected to the building (e.g. your house) by either an air distribution system – a blower with duct work – or a series of distributed pipes recirculating water or other liquid. On the opposite side of the heat pump, it is connected to the earth by a series of buried pipes (known as the ground or earth loop) through which a water solution is recirculated or by a pair of wells – one to supply and a second to receive water. The heat pump moves heat between the two – from the earth to the building in heating mode and from the building to the earth in cooling mode.
As the laws of thermodynamics tell us, heat will only move from an area of higher temperature to one of lower temperature, so the heat pump must concentrate the heat from the earth which is at a fairly constant 45F at a depth of 6 feet or so (where the ground loop is) in order to raise its temperature so that its heat can be moved to a house maintained at approximately 70F. To accomplish this, the heat pump utilizes a refrigeration cycle, just like your refrigerator or freezer. The important difference is that the cycle is reversible, i.e. it can move heat to or from your house.
How the Refrigeration Cycle Works
Key to a refrigeration cycle is the use a special fluid (called refrigerant) that has a low boiling point which can be significantly altered by changing its pressure. The heat pump takes heat from a source (e.g. the earth in heating mode) and concentrates it to raise its temperature high enough so that it can be moved to the targeted area (e.g. your house). The schematic below shows a heat pump (in heating mode) between the earth and your house.
Water from the ground loop at approximately 40F comes in close thermal (but not physical) contact with the refrigerant on the low-pressure side of the heat pump where the boiling point is approximately 20F. The higher temperature water causes the refrigerant to boil or vaporize.
Once vaporized, the refrigerant can be compressed or pressurized which raises it temperature significantly.
Now the heat from the refrigerant can be transferred to your house by blowing air across (or bring water in close thermal contact with) the coiled metal tubing containing the hot refrigerant gas. This causes the refrigerant to condense back to a liquid.
Finally, the hot refrigerant liquid is passed through an expansion valve to the low-pressure side of the heat pump which reduces it temperature so that it can absorb more heat from the ground loop.
And the cycle repeats.
In cooling mode, a reversing valve switches sides of the heat pump: the cold evaporator is in contact with the building’s distribution system and the hot condenser with the ground loop.