The Science of Geothermal Energy: Ground-Source Heat Transfer
Geothermal HVAC systems (also known as ground-source heat pumps) leverage the stable temperature of the earth to provide heating, cooling, and hot water. While ambient air temperatures fluctuate dramatically between winter and summer, the ground temperature 6 feet below the surface remains constant year-round, typically between 50°F and 60°F. A geothermal system uses a closed loop of high-density polyethylene pipes filled with water and antifreeze to exchange heat with the earth.
The efficiency of heating systems is measured by the Coefficient of Performance (COP), which represents the ratio of heating output to electrical energy input: $$\text{COP} = \frac{Q_{\text{heating}}}{W_{\text{electrical}}}$$. Standard electric resistance heaters have a COP of 1.0. Air-source heat pumps achieve average winter COPs of 2.50. Geothermal heat pumps operate at COPs between 4.0 and 5.0 because they draw heat from a warm ground source rather than freezing outdoor air.
To compare geothermal systems with other high-efficiency technologies, explore our heat pump savings calculator or look at window energy losses using the double glazing payback calculator.