286 Technology Performance

In 1999, an estimated 400,000 ground-source heat pumps were operating in residential and commercial applications, up from 100,000 in 1990. With a projected annual growth rate of 10%, 120,000 new units would be installed in 2010, for a total of 1.5 million units in 2010 (Lund and Boyd 2000). The majority of new ground-source heat pump installations in the United States are for residential applications in the southern and mid-western states. Environmental concerns and a general lack of understanding of the technology by HVAC companies and installers have limited installations in the west.

28.6.1 Field Experience

Observations about field performance of ground-source heat pumps obtained from federal and private-sector users are summarized in this section.

The large numbers of reported installations testify to the stability of this technology. Most sites contacted report satisfaction with the overall performance (energy efficiency, maintenance, and comfort) of the technology. One of the world's largest installations of ground-source heat pumps is at Fort Polk, Louisiana, where over 4000 units were installed in family housing as part of a major retrofit project (Hughes and Shonder 1998). They also have systems installed in several larger facilities, including some cooling-tower hybrid systems (discussed earlier in this chapter). Personnel at Fort Polk are obviously pleased with the performance of their ground-source heat pumps, and the energy manager reports that the systems have performed better than expected and they plan to use the technology for all heating and cooling requirements on the base.

The maintenance engineer at the hospital in Love County, Oklahoma, reported that no problems have occurred since nine units were installed in their facility in the early 1990s. They perform routine maintenance on the system. They particularly like the technology because it takes up less space and gives better heating and cooling control than former systems.

An interview with the energy manager primarily responsible for the ground-source heat pump installation at the Oklahoma State Capitol Building revealed no problems with the heat pumps or the ground loop; however, the building was poorly zoned, which impeded effective temperature control. Approval from the local Water Resource Board for drilling the wells proved difficult but was primarily a matter of educating the board about the technology and the closed-loop system. The Capitol building system, which is a cooling-tower-supplemented design, contains 855-cooling tons (3,009.6 kW) connected to a common ground-loop system consisting of 372 250-foot (76.2 m) vertical wells.

One facility manager did report problems with the ground-source heat pump system installed in a facility at the Dugway Proving Grounds, Utah. The system uses

22.5-ton (8.8-kW) units. Although the actual problem had not been identified, the system is not performing up to expectations. The belief was that the ground loop was undersized. Sizing of the ground loop is one of the most important design factors and is a function of system load, ground temperature, and local soil conditions. As noted earlier in this chapter, the type of soil and rock in contact with the pipe loop has serious implications for the length of ground loop required for adequate heat transfer. Inexperienced designers or installers are more likely to undersize the systems, resulting in inadequate system performance, or to oversize the systems, resulting in increased installation costs. The importance of good design, documentation, installation, and commissioning cannot be overemphasized.

In early 1995, at the National Training Center in Fort Irwin, California, 220 new single-family houses were built with an innovative ground-source heat pump system. The thermal sink/source for the heat pumps is geothermally heated 75°F (23.9°C) groundwater. The groundwater is pumped into a storage reservoir adjacent to the housing project. The reservoir water is then circulated through a double-walled heat exchanger, transferring thermal energy to/from a secondary closed circulation loop connected to individual heat pumps in each residence. Operating conditions and problems with maintaining correct water flow through the central facility heat exchanger contributed to lower than expected energy savings during the first year of operation. There was no indication that the heat pumps themselves were not performing as designed. Limited anecdotal information from the occupants indicates that overall satisfaction with the level of thermal comfort from the system was high.9

Vertical-bore, ground-coupled heat pump systems were installed in four new elementary schools in Lincoln, Nebraska, in 1995. Each school required 54 heat pumps ranging in size from 1.4 to 15 tons (204-tons total cooling capacity). With the heat pump systems, utility costs for these four schools are nearly half of that of other schools in the district, and the systems are providing a comfortable, complaint-free environment (Shonder et al. 1999).

Additional case studies are identified on the Geo-thermal Heat Pump Consortium web page (http://www. geoexchange.org/).

28.6.2 Energy Savings

The most important reason to consider the application of ground-source heat pumps to commercial build-

"D.L. Hadley and L. Lkevgard. August 1997. Family Housing Energy Savings Verification, National Training Center, Fort Irwin, CA. Preliminary Findings: Cooling Season Energy Savings. Letter Report, Pacific Northwest National Laboratory, Richland, Washington.

ing is the potential energy savings and its impact on overall life-cycle cost of the heating and cooling system. Ground-source heat pumps save energy and money because the equipment operates more efficiently than conventional systems, the maintenance costs are lower (see next section for maintenance benefits), and the equipment has a longer life expectancy than conventional unitary equipment. In addition, a ground-source heat pump does not require a defrost cycle, or, in most situations, backup electric resistance heat, as do air-source heat pumps.

By comparison, the average cooling efficiency at commercial-type facilities is estimated to be an EER of 8.0 (2.33 COP) for existing facilities and an EER of 10.0 (2.93 COP) for new facilities. Ground-source heat pump systems have the potential to reduce consumption of cooling energy by 30% to 50% and to reduce heating energy by 20% to 40% compared with typical air-source heat pumps.

A review of manufacture's literature on commercially available systems indicates that cooling efficiencies (EERs) of 13.4 to 20 Btu/W-h and heating efficiencies (COPs) of 3.1 to 4.3 are readily available.10 A study prepared by DOE estimates energy-saving ranges of 17% to 42% comparing ground-source heat pumps to air-source heat pumps, depending on region (Calm 1987). Figure 28.7 illustrates the range of efficiencies typical of various heating and cooling equipment.

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