Geothermal heating and cooling is made possible by a process called heat exchange. Geothermal, also called ground-source heat pumps use the earth’s constant temperature as an exchange medium instead of the outside air. The water circulating in the underground loop piping system exchanges heat between the ground, where absorbed heat from the sun resides (solar energy!), and a heat pump.
Geothermal Heat Pumps
Heat pumps are devices that move thermal (heat) energy opposite to the direction of spontaneous heat flow. The natural direction of heat flow is from warmer places to colder places. In the case of a heat pump, heat absorbed from a cooler space is released to a warmer one. Heat pumps don’t create heat, they pick it up and move it from one place to another. In the case of a geothermal or ground-source heat pump, heat is transferred through the heat pump from the ground and into the home or building. A refrigerator works in much the same way and contains similar components: a compressor and heat exchangers. Essentially, these devices transfer heat energy by moving heat from one place and releasing it into another. A geothermal heat pump has been called a “refrigerator on steroids” because, due to its compressor and earth source, it transfers heat much more efficiently.
During the winter, heat in the ground is carried to the heat pump. Cooled water exits the heat pump and circulates through the underground piping loop. The warmer earth releases heat energy into the water as it passes back to the heat pump. How does it do this? Inside the heat pump is a heat exchanger. A heat exchanger is a device that allows heat from a fluid (an inert liquid or gas) to pass to a second fluid without the two fluids mixing. Heat, however, is exchanged by the fluids with the hot fluid cooling down and the cold fluid warming up. In this heat exchange or transfer process, the cooled incoming water is sent back to the ground loop to pick up more heat energy. Refrigerant in the heat exchanger accepts the heat energy and it becomes a gas. The gaseous refrigerant is then sucked into a compressor in the unit where it is compressed, concentrating the heat energy, and then distributed into the interior as warm, comfortable air. Depending on the heating system in the home or building, the heat is passed through an air coil or a hydronic coil and dispersed via radiators, baseboard or in-floor (radiant) heating tubes.
In warm weather a geothermal system absorbs heat from the home or building and transfers it to the underground loop where it is then absorbed by the cooler earth. The heat pump, which has a reversing valve controlled by a thermostat, now functions in an opposite mode: the hot compressor output is sent to the returning ground loop and the cooler water returning from the earth is used as a heat sink (heat absorbing medium) to help produce cool, dehumidified air distributed via ductwork.