Ground-source heat pumps (GSHPs), also called geothermal heat pumps or ground-coupled heat pumps, are gaining popularity as efficient space-conditioning systems. A 1993 report by the U.S. Environmental Protection Agency, "Space Conditioning: The Next Frontier," rated advanced and emerging GSHP technologies among the best alternative residential space-conditioning systems, providing performance, cost savings for consumers, along with low CO2 emissions and overall environmental costs. The trend is evidenced by recent and planned installations nationwide, and the formations of both a national consortium and a California committee to further study and promote the use of ground-source heat pumps. The U.S. Department of Energy (DOE) also plans to initiate a program with the goal of increasing total annual sales and installations ten-fold, from the estimated 40,000 in 1993 to 400,000 by 2000, a total that would be roughly half the current annual sales of conventional air-source heat pumps.

A ground-source heat pump heats and cools just like a conventional heat pump, but instead of an outdoor unit with heat exchanger coils and a fan in a box, the outside coils are buried in the ground. Ground-source heat pumps take advantage of the more-constant ground temperature, which in the lower 48 states varies only from 42deg.F-77deg.F, while air temperatures range from -40deg.F to more than 100deg.F.

The most common ground-source heat pump is a closed-loop system in which water or an anti-freeze solution is circulated through plastic pipes buried underground (see "Ground-Source Heat Pumps: Earth as Heat Source and Heat Sink," HE Nov/Dec '90 p.32). The loop may be vertical, installed in a shaft drilled much the same as a water well, or horizontal, installed in underground trenches. In trench systems, the piping loops commonly have a serpentine or "slinky" configuration. Available land, and the types of soil and rock determine the most economical choice. In winter, the circulating fluid collects heat from the earth and carries it through the system and into the building. In summer, the system flow is reversed to remove heat from the building and place it in the ground. Open loop systems that employ groundwater, or surface water such as a pond or lake, are used in some parts of the country, but water supply and water quality issues impose limitations on such applications.

Ground-source heat pumps are available with an optional de-superheater that siphons off some hot refrigerant to a secondary heat exchanger for domestic water heating. In the cooling mode, this surplus heat provides free hot water and in the winter heating mode, it reduces water-heating costs, providing average overall savings of 60% on annual water-heating bills.

Equipment efficiencies generally exceed high-efficiency air-source heat pumps. Energy-efficiency ratios (EER) for standard ground-source heat pumps begin at about 13.2 EER and 3.1 coefficient of performance (COP), and the most-efficient equipment available exceeds 20 EER and 6 COP. Although they have successfully saved energy and costs in many installations (based on consumer and utility information), actual measured data from field tests is not abundant, and the savings vary based on location, climate, and utility rates. In a Louisiana test, a four-ton air conditioner and a gas furnace were replaced with a ground-source heat pump, electricity costs were reduced by 21% and gas service was eliminated. Annual energy cost savings for the home averaged $580 over a five-year period. The system cost $2,200 over the replacement of the air conditioner and gas furnace, so payback on the system was less than four years.

Citing advantages, Jim Bose, executive director of the ten-year-old International Ground-Source Heat Pump Association (IGSHPA), said, "Based on user surveys, it has the highest percentage of customer satisfaction (97%) of any heating and cooling technology, and on average it reduces customer utility bills 30-40%." IGSHPA claims that systems can produce cost savings of 30%-60% in the heating mode, and savings of 10%-25% in the cooling mode, when compared to other temperature control systems. Utilities also benefit from a reduction in peak load demand. A 1989 comparison of ground-source heat pumps with air-source heat pumps in PSI Energy's Indiana service area showed a peak heating-load reduction of 50% and peak cooling-load reductions of 20-25%.

Equipment and system longevity of this new technology has not been fully tested; however, ground-loop heat exchangers made of high-quality polyethylene or polybutylene pipe are warrantied up to 50 years by many manufacturers. Based on Ontario Hydro's experience with the installation of 7,000 units from 1990-1993, the utility's Randall Lee says the integrity and efficiency of the systems are dependent on proper installation. Unlike conventional heat pumps, there is no maintenance for outside fan coil units which are exposed to weather, environmental hazards, and vandalism, and there is no outside unit noise.

IGSHPA is based at Oklahoma State University in Stillwater where Bose is an engineering professor. The 480-member association carries out training, education, and research, publishes guides for installation and design, and has accredited 1,300 installers.

Bose believes the main obstacle to expanding the market is lack of awareness of the technology, but initial costs also affect market growth. Equipment prices run high compared to conventional heat pumps, and the ground-loop drilling or trenching adds to the installation costs. Generally, applications have been for new construction, with ground loops installed during lot or subdivision preparation, or for owners of rural or larger lots with adequate ground space.

Residential sales and installations, particularly in the central states from Canada to the Gulf Coast, provided the first major surge in ground-source heat pump use beginning in the 1980s, with promotion and assistance from rural electric cooperatives, and later from investor-owned utilities. Ground-source heat pumps comprise an estimated 2.5%-3% of the total annual residential heat-pump market. The light-commercial market and schools are currently the fastest growing segments.

An example of a successful new-construction project is the 126-home Walden Pond subdivision in Indiana in which ground loops for the entire development were installed prior to home construction, as a joint effort of PSI Energy and the homebuilder. In a 1990 report, the Electric Power Research Institute (EPRI) estimated that the ground-source heat pumps produce $400 per year energy savings per customer for all-electric homes, with pre-installation producing a 34% cost reduction for home buyers, when compared to one-at-a-time loop installations.¹ PSI Energy has since entered agreements to pre-install ground loops in 40 more subdivisions totaling 1,400 homes, and about 10% of new homes in the utility's service area are being built with ground-source heat pumps.

DOE Consortium

DOE senior program manager Lew Pratsch said that the agency had identified the potential of ground-source heat pumps as part of the Bush Administration's National Energy Strategy, but President Clinton's 1993 Climate Change Action Plan has enabled a formal program to get underway (see "Carrots and Sticks from Washington," p.2). In 1993, the national Geothermal Heat Pump Consortium was formed. It includes DOE, EPA, EPRI, IGSHPA, Edison Electric Institute, National Rural Electric Co-op Association, and manufacturer representatives.

Pratsch anticipates that DOE's major efforts will begin in 1995 and said that the key program goals are "to get utilities more involved, create market incentives, and find ways to reduce the cost and time of installation, which will involve getting more work for installation crews, finding ways to reduce the size of the ground loop to expand the market for urban lots, and obtaining more data for utilities to justify incentive programs." The proposed DOE consortium program calls for spending $100 million over six years, with the private sector contributing two-thirds of program costs.

Pratsch also believes that ground-source heat pumps can be tied into building envelope improvements in retrofit applications. He cited a recent Department of Housing and Urban Renewal project for a 348-unit apartment complex in Tulsa, retrofitted with a ground-source heat pump system and envelope improvements which enabled a reduction in the building's Heating, Ventilating and Cooling (HVAC) tonnage from 724 to 516 tons. The largest single replacement program in the nation is underway at Fort Polk in Louisiana where the Department of Defense is replacing 4,000 conventional heat pumps with ground-source heat pumps, for military housing.

The western states have been slower to use and promote ground source heat pumps, but the California Geothermal Heat Pump Committee, formed in 1993, has completed an initial economic assessment of the technology in the state, with funding assistance from DOE. Committee participants include the California Energy Commission (CEC), local electric utilities, the Geothermal Resources Council, the Western Area Power Administration, DOE, and equipment manufacturers.

The California study evaluated six locations statewide, comparing ground-source heat pumps with conventional HVAC appliances.² Results favored use of ground-source heat pumps in four of the six areas, for both retrofit and new construction. Adding a water-heating option would make all six locations cost-effective in new construction, and five of the locations favorable for retrofits. Utility rebates and incentives would further enhance the economics for consumers.

Committee member David Maul of the CEC said that ground-source heat pumps have the potential to significantly reduce total energy consumption for home heating and cooling in California. However, overall economic feasibility and cost effectiveness will be closely tied to actual installed costs. Maul said, "Market demand and competition is critical in order to bring manufacturers here with competitive unit prices, and to bring down the average cost currently charged by well-drilling companies for drilling."

Several California projects are planned or underway. The Truckee-Donner Public Utility District near Lake Tahoe will install six residential ground-source heat pumps this year as a pilot project, with an eventual goal of a full-scale marketing and incentive program for the utility's 8,000 customers. In this cold-winter mountain area, load factor is a concern, as is air quality, with the units providing a clean alternative to wood burning. The Plumas-Sierra Rural Electric Co-op is also involved in the installation of 100 GSHPs in a resort condominium complex.

Another project will be a cooperative effort between the Sacramento Municipal Utility District and the Sacramento Housing and Redevelopment Agency (SHRA). The two plan to install 556 residential ground-source heat pumps within three years. A primary motivation for SHRA's interest is the need to reduce maintenance and vandalism costs now resulting from the use of outdoor units for conventional heat pumps.

-- Ted Rieger

Ted Rieger is a Sacramento, California-based freelance writer who specializes in energy issues.

Notes

1. "Geothermal, Ground-Loop Pre-installation Project at Walden Pond," Electric Power Research Institute, Palo Alto, California, August 1990, Report#CU-6969. Tel: (415)855-2411.

2. "Economic Assessment of Geothermal Heat Pumps in the State of California (Phase 1 Activity)," December 1993, presented by The Fleming Group, Inc., East Syracuse, New York. Tel: (315)437-1869.

Figure 1. Three configurations of ground-source heat exchanger installation.