November 17, 2008
Geothermal (Ground Source) Heating/Cooling
Geothermal heating and cooling systems, sometimes referred to as ground source systems, are used to provide heating and cooling in buildings. These are highly efficient, renewable energy technology systems that utilize the earth’s relatively constant temperature. The earth’s ground temperature remains constant year-round, ranging from 45º to 70º Fahrenheit. This constant temperature and the depth needed to reach the constant temperature varies depending on geographic location; in Wilson, North Carolina, the depth needed to reach a constant temperature is approximately four to five feet, but in more tropical or arctic regions, the constant temperature could be much deeper. The ground temperature in the Wilson, NC area is approximately 65ºF at a depth of five feet.
Geothermal heating/cooling takes advantage of the constant ground temperature by utilizing the principal of heat transfer. During the winter, a geothermal system absorbs heat from the earth (which is now warmer than the ambient air temperature)and transfersit into the heating system. During the summer, the system absorbs the cooler temperature from the earth (which is now cooler than the ambient air temperature) and transfers it into the cooling system in the building. A properly sized and installed geothermal system can lower heating costs by 40 to 70 percent and reduce cooling costs by 30 to 40 percent compared to conventional heating and cooling systems.
The U. S. Department of Energy estimates that ground-source heat pumps save anywhere from $300 to $1000 per year on typical residential energy bills. The higher initial cost, which runs from $2,000 to $4,000, is paid back in energy savings in three to eight years. (www.advancedenergy.org)
These systems operate by pumping fluid, typically a solution of water and antifreeze that has a freezing point at least 10º below the minimum expected temperature, through a piping system into the ground where the earth acts as a heat exchanger. The heated or cooled fluid is then used to heat or cool a building through its ductwork system.
The most important factor in the success of a ground source heat pump system is the thermal conductivity of the soil where the system will be located. Soil tests and test wells are utilized to measure the thermal conductivity of the soil. In October 2006, Wilson Community College worked with Geothermal Resource Technologies, Inc. (Greenville, TX) to install a geothermal test well (see image) to assess the thermal conductivity of the ground and appropriate sizing of a geothermal heat pump system needed for the new Student Center on campus.
A geothermal test well at WilsonCommunity College was drilled to determine the ground thermal conductivity and to help in sizing the geothermal heating/cooling system for the new Student Center
The image shows three elements of a test well. First, the machine on the right is a generator supplying power for the collection of the sample. Second, the white covered box is a data collector, heater, and system pump. The third element is the well that has been drilled to a depth of 275 feet, and is located by the two white bags on the far left. Underneath the bags are two small pipes that will remain within the well. The data collector is monitoring the thermal conductivity, which is the ability of a material to conduct heat, of the well by pumping fluid down one pipe and reading the temperature change exiting out the other pipe. The data that is collected from the sample will determine how many wells are needed for the proposed system in the Student Center.
For more information on geothermal and ground source heating and cooling visit: www.alliantenergygeothermal.com www1.eere.energy.gov/geothermal
Rob Holsten, Dean of Continuing Education & Sustainability
Tish Scott, Director of Public Relations
Wilson Community College January 2007
