Ground Source Heat Pumps - Residential - 9/19/2003 - House Systems HVAC Heating Air Conditioning

Ground Source Heat Pumps - Residential

A Technology Profile from the Federal Energy Management Program (FEMP)

Technology Group:

HVAC

Description:

Ground source heat pumps (GHPs) are similar to ordinary air conditioners and air-source heat pumps, but they dump building heat into or extract heat from the ground rather than the outside air. The tempering effect of the earth provides for either heat extraction or heat rejection, allowing GHPs to operate more efficiently than conventional equipment in particularly cold or hot climates. GHP systems are known by many different names, such as geothermal, ground-coupled, or GeoExchange systems.

All of these systems consist of a "ground loop" for exchanging heat with soil, groundwater, surface water, or wastewater streams. In most systems, the heat exchanger consists mainly of long pipes either placed in vertical bores or, if sufficient land is available, buried in horizontal trenches. One or more heat pumps are then connected to this ground loop to provide indoor space heating and cooling. Some systems provide water heating as well. The ground loop is most commonly a closed loop system, but some open loop systems are used whereby water is pumped through the GHP and then re-injected into the ground or into surface water.

Applications:

GHPs can be applied to virtually any size residential building as well as specialized applications such as swimming pools. The vertical ground loops average 400 feet in length per ton of capacity, so limited site area is usually not a problem. Horizontal loops may be 600 to 1000 feet per ton, depending on soil type. GHP's tend to be the most cost effective in the following situations:

• New construction
• Climates characterized by high daily temperature swings, or where winters are quite cold or summers quite hot
• Areas where electricity costs are over $.07/kWh
• Areas where natural gas is unavailable or where cost is high compared with electricity.

When considering an open-loop system, check with local environmental authorities to ensure compliance with regulations. There are one-to-two-million GHPs are operating in homes, schools, and commercial buildings in the United States. They are adaptable to virtually any kind of building; the Federal government has installed many thousands of GHPs, mostly in residential applications.

Performance/Costs:

GHPs energy cost savings vary with the electric rates, climate loads, soil conditions, and other factors. In residential building applications, typical annual energy savings are in the range of 30 to 60 percent compared to conventional HVAC equipment.

Unlike air source units, GHP systems do not need defrost cycles nor expensive backup electric resistance heat at low outdoor air temperatures. The stable temperature of a ground source is a tremendous benefit to the longevity and efficiency of the compressor.

A residential GHP system is more expensive to install than a conventional heating system. It is most cost-effective when operated year-round for both heating and cooling. In such cases, the incremental payback period can be as short as 3–5 years. A ground source heat pump for a new residence will cost around 9-12% of the home construction costs. A typical forced air furnace with flex ducting system will cost 5-6% of the home construction costs.

Stated in an alternative form, the complete cost of a residential ground source heat pump system is $3,500-$5,500 per ton. Horizontal loop installations will generally cost less than vertical bores. For a heating dominated residence, figure around 550 square feet/ton to size the unit. A cooling dominated residence would be estimated around 450 square feet/ton. The table below compares three types of systems.

Availability:

GHP's are available from a number of manufacturers. Contact the International Ground Source Heat Pump Association or the Geothermal Heat Pump Consortium (see website addresses below) for a listing of member manufacturers and qualified installers.

For Additional Information:

• International Ground Source Heat Pump Association: This is a non-profit group supporting the ground source heat pump industry with publications, information and research. Training programs are offered for engineers, installers and trainers.
• Information Survival Kit for the Prospective Geothermal Heat Pump Owner: Geo-Heat Center Oregon Institute of Technology, February 1997. This is a compilation of information focused on a residential installation.
• Geo-Heat Center: This site provides the transfer of technological information to consultants, developers, potential users, and the general public. This site includes meeting and workshop announcements, publications, technical assistance, quarterly bulletins, newsletters and directories of manufacturers and consultants.
• Geothermal Heat Pump Consortium, Inc.: This is a non-profit organization whose goal is to advance the use of GeoExchange heating and cooling systems. This content-rich site includes Federal case studies, Federal performance contracting information, Federal building data, Federal geothermal slide presentations, and a list of members. The Consortium's meetings and conferences are also listed on this site.
• FEMP Product Efficiency Recommendations: This website provides recommended efficiency ratings for commercial heat pump equipment as well as other energy consuming equipment used in buildings.
• FEMP's Technology-Specific Super ESPC Program: This article describes the program and an example of a GHP project.
• The Fort Polk GHP Project: IFederal facility managers who need to see hard data before committing to GHPs will be interested in a project at a U.S. Army base, the Fort Polk Joint Readiness Training Center in Louisiana, where 4003 GHPs were installed in 1995-96. This project has demonstrated that the technology can yield substantial energy and cost savings for Federal facilities.
• U.S. Department of Energy's (DOE) geothermal home page: This site, hosted by the DOE Office of Geothermal Technologies, provides a variety of information on geothermal energy, from the history of geothermal applications to the latest technological developments.
• Geothermal Heat Pumps Deliver Big Savings for Federal Facilities: This DOE Technology Focus Report is well written and focused on Federal applications of ground source heat pumps, super energy savings performance contracts specifically for Federal facilities, and basic information about geothermal technologies.
• PrimeNet - The Air-Conditioning and Refrigeration Institute's On-Line Directories of Certified Equipment and Certified Unitary Products (for air-source heat pumps, ASHPs)
• Do You Dig Ground Source Heat Pumps?: This article from the Nov/Dec 1996 issue of Home Energy gives a good overview of the technology and the pros and cons.

Definitions

Heat pumps: A device that transfers heat from a lower temperature reservoir to a higher temperature reservoir by doing work through a compressor.

Open loop: A heating or cooling system, such as a solar water heater or ground source heat pump, in which the working fluid is heated or cooled and used directly, not returning to the heating device.

Ton: A 'ton' unit of air cooling capacity equals 12,000 Btu per hour.

Payback: The amount of time required (usually in years) for positive cash flows to equal the total investment costs. This is often used to describe how long it will take for energy savings resulting from using more energy-efficient equipment to equal the premium paid to purchase the more energy-efficient equipment.

View PDF Format

Credits:
Diagram and performance data from Energy Efficiency and Renewable Energy Network factsheet “Renewable Energy Geothermal Heat Pump”