Geothermal Heat Pumps

Geothermal Heat Pumps

A natural exchange of heat with the earth

Geothermal, or ground source, heat pumps have been in use since the 1970s. Although the initial cost is usually higher than a traditional heating and cooling system, in the right application the investment can be recovered in a fairly short time through the savings in energy costs. System life is estimated at 25 years for the inside components and 50+ years for the ground loop. There are approximately 50,000 geothermal heat pumps installed in the United States each year.

While Vermont and New Hampshire can be extremely cold in the winter—a few feet below the earth’s surface the ground remains at a relatively constant temperature. Depending on latitude, ground temperatures range from 45°F (7°C) to 75°F (21°C). This ground temperature is warmer than the air above it during the winter and cooler than the air in the summer. The GHP takes advantage of this by exchanging heat with the earth through a ground heat exchanger.

Types of Geothermal Heat Pump Systems

Open-Loop System
These systems pump well water to the heat pump. Once the water has circulated through the heat pump, the water goes to an injection well or to a pond or brook. This option is practical only where there is an adequate supply of relatively clean water, and all local codes and regulations regarding groundwater discharge are met.

Water quality is a significant factor when evaluating the use of a well. Very hard water (low pH) or water with hydrogen sulphide (rotten egg smell) can cause problems that need to be addressed when considering an open loop system.

Closed-Loop Systems
These types of systems have been in use in both residential and commercial markets since the late 1980s. Closed loop systems can be horizontal or vertical and depend on site conditions.

Vertical systems can be used for small parcels of land, while horizontal systems require more space. Installing a vertical system requires between 150 to 220 feet of borehole per ton, so if a 2-ton BTU heating system is needed, you’ll need to drill between 300 and 440 feet.

These types of systems are the least common for residential uses since not many homes are situated on a suitable body of water. A lake or pond needs to be at least 12 feet deep and have sufficient surface area.

For a ground source heat pump system to work, it needs a way to increase the temperature of the heat that resides naturally in the ground to a sufficent level for it to be useful heat for the home. Since energy is needed to do this, the term heat ‘pump’ is used to describe the equipment. A heat pump is basically a refrigeration unit. Refrigeration equipment (window AC, freezer, refrigerator, etc.) moves heat from a space to cool it, and discharges the heat at higher temperature to the room or the outside air. The only difference between a heat pump and a refrigeration unit is the refrigeration unit cools and the heat pump heats. But heat pumps are reversible and can heat and/or cool the space.

The energy efficiency of a heat pump and the power needed to operate it are directly related to the temperature range that the heat pump works. Geothermal heat pump systems are more efficient than air-source heat pump systems simply because the ground is warmer (or cooler in summer) than the outside air temperature. An air-source heat pump removes heat from cold outdoor air in the winter and delivers heat to hot outside air in the summer. But a ground source heat pump pulls heat from relatively warm soil or groundwater in the winter and returns heat to the same relatively cool soil or groundwater in summer.

The heat pump unit is the main player in any ground source heat pump system. The heat pump includes a refrigerant-to-water heat exchanger, refrigerant piping and control valve, air coil (to heat air in winter, and to cool and dehumidy in summer), fan and controls.

Most GHP units have an option to be equipped with a desuperheater to partially heat domestic hot water. In the summer, the desuperheater uses some of the excess heat from the air conditioning to heat water. So during the summer, its essentially free to heat water. In the winter, some of the heat pump’s capacity is used to heat water. Although energy is still used to heat the water, the hot water is produced for less than a convetionaly water heater.

For more information
DOE’s Energy Efficiency and Renewable Energy’s Geothermal Technologies Program’s website.
International Ground Source Heat Pump Association’s website.