The term geothermal when applied to heat pump technology means that the ground is used as a heat exchange medium, rather than the air. Heat pumps are nothing more than reversible devices to heat and cool a home.
The technology is the same as refrigeration. When a gas expands, it absorbs energy from the air, thus cooling the surroundings. Refrigeration works by using a pump to compress a gas, called the working fluid. The compressed gas, now a fluid, is moved to the area to be cooled and then allowed to expand. The heat being moved by a heat pump is expelled away from the area to be cooled. For most systems, “away” is the air outside the house.
The hotter it is outside in the summer, the harder a heat pump has to work to cool your home. This is where the ground comes into play. Geothermal heat pumps use the ground as “away”. The heat exchanger portion of a geothermal heat pump is connected via wells drilled or lateral lines on the property to water or some other liquid which transfers the heat to the much cooler ground, rather than the much warmer air. This process is more efficient at moving heat, and therefore summer cooling costs are lower.
The process is reversed in the winter. Compressing a gas inside a home produces heat, then the compressed gas is moved out of doors and allowed to expand and cool out of doors. Heat pumps are quite efficient at heating in the winter as long as the temperature is not too low. The colder it gets the less efficient the system. For a geothermal heat pump, it doesn’t matter what the air temperature is because the heat exchange is with the ground which is about fifty to sixty degrees Fahrenheit year round. In the winter the ground is warmer than the air so geothermal heat pumps are more effective than traditional air source systems.
Overall geothermal heat pumps are more efficient than normal air source heat pumps, particularly during the temperature extremes of summer and winter. In a study at Fort Polk Louisiana, heating costs during winter days below freezing were forty per cent lower. During the summer days with the temperature over ninety degrees, the costs of cooling were also about forty percent lower with the ground source heat pumps compared to air source heat pumps.
Heat pumps work best when the difference between the outside and inside temperatures are not great. Geothermal heat pumps take advantage of the more stable ground temperature, keeping the difference lower than for air source heat pumps.
Geothermal systems require the drilling of wells or laying lateral lines to create the ground contact so the systems are inherently more expensive that simple air source systems, but because of their greater efficiency, usually have payback periods on the order of five to ten years.