Ground-coupled heat pumps (GCHPs) have been receiving increasing attention in recent years. In areas where the technology has been properly applied, they are the system of choice because of their reliability, high level of comfort, low demand, and low operating costs. Initially these systems were most popular in rural, residential applications where heating requirements were the primary consideration. However, recent improvements in heat pumps units and installation procedures have expanded the market to urban and commercial applications. This paper discusses some of the current activity in the commercial sector. The basic system and nomenclature are discussed. Several variations for commercial buildings are presented along with examples of systems in operation. Several advantages and disadvantages are listed. Operating and installation costs are briefly discussed. Finally, the GCHP is presented as an alternative that is able to counter much of the criticism leveled by the natural gas industry toward conventional heat pumps.

Direct or non-electric utilization of geothermal energy refers to the immediate use of the heat energy rather than to its conversion to some other form such as electrical energy. The primary forms of direct use include swimming, bathing and balneology (therapeutic use), space heating and cooling including district heating, agriculture (mainly greenhouse heating and some animal husbandry), aquaculture (mainly fish pond and raceway heating), industrial processes, and heat pumps (for both heating and cooling). In general, the geothermal fluid temperatures required for direct heat use are lower than those for economic electric power generation.

Most direct use applications use geothermal fluids in the low-to-moderate temperature range between 50^o and 150^oC, and in general, the reservoir can be exploited by conventional water well drilling equipment.

Low-temperature systems are also more widespread than high-temperature systems (above 150⁰C); so, they are more likely to be located near potential users. In the U.S., for example, of the 1,350 known or identified geothermal systems, 5% are above 150^oC, and 85% are below 90^oC (Muffler, 1979). In fact, almost every country in the world has some low-temperature systems; while, only a few have accessible high temperature systems.

Commercial geothermal power generation is an established industry both here in the U.S. and around the world (Figure 2). Italy was the first country to develop geothermal power commercially in 1914 at Larderello. This was followed by plants at Wairakei, New Zealand in 1958 and at the Geysers in California in 1960 (DiPippo,1999). As indicated in Table1, the U.S. currently leads the world in geothermal power generation with a total of 2850 MW produced from 203 different plants. These plants are located in California, Nevada, Utah, and Hawaii.

Major direct utilization projects exploiting geothermal energy exist in about 38 countries, (Figure 1) and the estimated installed thermal power is almost 9,000 MWt utilizing 37,000 kg/s of fluid. The worldwide thermal energy used is estimated to be at least 112,400 TJ/yr (31,200 GWh/yr). The majority of this energy use is for space heating (33%), and swimming and bathing (19%). In the USA, the installed thermal power is 2,000 MWt, and the annual energy use is 14,800 TJ (4,100 GWh).

The majority of the use (59%) is for the heat pumps (both ground coupled and water source), with space heating, bathing and swimming, and fish and animal farming each supplying about 10%.

At the present time, ground-coupled and groundwater (often called ground-source or geothermal) heat pump systems are being installed in great numbers in the United States, Switzerland and Germany (Kavanaugh and Rafferty 1997; Rybach and Hopkirk 1995). Groundwater aquifers and soil temperatures in the range of 5⁰ to 30 ⁰C are being used in these systems. Ground-source heat pumps (GSHP) utilize groundwater in wells or by direct ground coupling with vertical heat exchangers (Figure 3). Just about every state in the USA, especially in the Midwestern

and eastern states are utilizing these systems in part subsidized by public and private utilities. It is estimated that almost 60,000 groundwater systems, and more than 50,000 closed loop vertical and 100,000 horizontal systems are already in use. Like refrigerators, heat pumps operate on the basic principle that fluid absorbs heat when it evaporates into a gas, and likewise gives off heat when it condenses back into a liquid. A geothermal heat pump system can be used for both heating and cooling. The types of heat pumps that are adaptable to geothermal energy are the water-to-air and the water-to-water. Heat pumps are available with heating capacities of less than 3 kW to over 1,500 kW. Convectors’ heating of individual rooms and buildings is achieved by passing geothermal water (or a heated secondary fluid) through heat convectors (or emitters) located in each room. The method is similar to that used in conventional space heating systems. Three major types of heat convectors are used for space heating: 1) forced air, 2) natural air flow using hot water or finned tube radiators, and 3) radiant panels (see Figure 4).

All these can be adapted directly to geothermal energy or converted by retrofitting existing systems.

Cooling can be accomplished from geothermal energy using lithium bromide and ammonia absorption refrigeration systems (Rafferty, 1983). The lithium bromide system is the most common because it uses water as the refrigerant. However, it is limited to cooling above the freezing point of water. The major application of lithium bromide units is for the supply of chilled water for space and process cooling. They may be either one- or two-stage units. The two-stage units require higher temperatures (about 160 ⁰C); but, they also have high efficiency. The single-stage units can be driven with hot water at temperatures as low as 77 ⁰C (such as at Oregon Institute of Technology ). The lower the temperature of the geothermal water, the higher the flow rate required and the lower the efficiency. Generally, a condensing (cooling) tower is required, which will add to the cost and space requirements.

Figure 5 shows the different systems used for Ground-Coupled, Groundwater, and Lake Water Heat Pump Systems. These systems are essentially used for residential applications in conjunction with the heat pumps.

Geothermal energy has been used extensively in commercial installations. However, until lately very few building owners, engineers, and architects considered GCHPs because in the past the implementation was difficult. There were very few qualified loop installers, design guides were hard to find, and the traditional HVAC network balked at the thought of linking equipment to plastic pipe buried in the ground. However, the experiences of those who tried this "new" concept have led to a sound methodology for the design and installation of highly reliable and efficient systems.

One such firm operates in Pennsylvania. This firm designs, installs, and operates GCHP

systems. The ground coils are typically 200 to 500 ft. deep with 1½ inch (4cm) polyethylene U-bends. Drilling in the area is very difficult compared to the rest of the U.S.A. However, several successful systems have been and are continuing to be installed and operated. A listing is given in Table 2.

Similar firms are operating profitably in areas all over the country. Austin, Texas has several new schools and other commercial buildings that have GCHPs. Activity in Canada is very high compared to the U.S. with utilities promoting the technology with rebates and technical assistance. Oklahoma, a state that derives much of its income from oil and gas, is in the process of installing a GCHP system to heat and cool its state capital complex. The common thread in successful GCHP programs appears to be an individual or set of individuals in a particular location who recognize the advantages of GCHPs. These individuals have the initiative to push forward in spite of the many skeptics who contend that GCHPs will not work.

Heat pump efficiency is primarily dependent upon the temperature difference between the building interior and the environment. If this difference can be minimized, heat pump efficiency (and capacity) will improve. Ground temperatures are almost always closer to room temperature than air temperatures. Therefore, GCHPs are inherently more efficient than units that use outdoor air as a heat source or sink if the ground coil is correctly designed. This principle is one of Mother Nature's rules and is referred to as Carnot's Law.

Secondly, it is important to have large coils for high efficiency. (Compare today's outdoor units with those used 20 years ago). Water is far superior to air with regard to "convecting" heat through coil surfaces. Therefore, the water coils in GCHP are smaller and much more "efficient" in transferring heat.

In addition to being a better heat transfer fluid compared to air, water requires much less energy to be circulated. Although it is heavier than air, a given volume of water contains 3500 times the thermal capacity of atmospheric air. Therefore, the pump motors circulating water through a GCHP system are much smaller than outdoor air or cooling tower fan motors of conventional systems. The indoor fan power is also reduced because the units are in the zone and duct runs are very short and non-existent. Therefore, low pressure (and power) fans can be used.

The conventional GCHP system is extremely simple. The water-to-air unit consists of a compressor, a small water coil, a conventional indoor air coil, one bi-flow expansion device, and a few electrical controls. The flow control can be either a single circulation pump on each unit (that is turned on with the compressor relay) or a normally closed two-way valve for systems with a central circulation pump. If the designer chooses an extended range heat pump as recommended, no water regulating control valves are necessary.

Control is also very simple. A conventional residential thermostat is sufficient. Since units are located in every zone, a single thermostat serves each unit. Zones can be as small as ½ ton (1.8 kW). However, each unit can be linked to a central energy management system if desired. Air volume control is not required. Larger water-to-air heat pumps are available if multi-zone systems are required. This would complicate one of the most attractive benefits of GCHPs, local zone control. The simple system can be installed and serviced by technicians with moderate training and skills. The building owner would no longer be dependent on the controls vendor or outside maintenance personnel. The simple control scheme would interface with any manufacturers

thermostats.

GCHPs eliminate the Achilles’ heel of conventional heat pumps in terms of comfort, "cold blow". Commercial systems can be designed to deliver air temperatures in the 100°to 105°F (38° to 41°C) range without compromising efficiency. Moisture removal capability is also very good in humid climates. The previously discussed advantage of local zone control is also critical to occupant comfort.

One of the most attractive benefits of GCHPs is the low level of maintenance. The heat pumps are closed packaged units that are located indoors. The most critical period for a heat pump compressor is start-up after defrost. GCHPs do not have a defrost cycle. The simple system requires fewer components. Logic dictates that the fewer components, the lower the maintenance.

Because of the limited amount of data for GCHPs, not a great deal of data is available to support the claim of low maintenance in commercial buildings.

GCHPs face the typical barriers of any new technology in the heating and cooling industry. Air source heat pump and natural gas heating technology has been successful. The technical personnel have been trained to install and service this equipment. A great deal of research and development has been successfully devoted to improving this technology. Why go to something new?

The GCHP technology faces an additional barrier in the lack of an infrastructure to bridge two unrelated networks: the HVAC industry and the drilling/trenching industry. There is little motivation on either side to unite. The HVAC industry prefers to continue marketing a proven technology and well drillers continue to profit on existing water well, environmental monitoring well, and core sampling work. It is the task of the two sectors who benefit the most from GCHPs, customers and electric utilities, to force a merger of the two networks.

Some equipment manufacturers are resistance to GCHPs because of the reduced need of their products. Water-to-air heat pumps are relatively simple and potentially inexpensive devices. The control network is especially simple and inexpensive. There will be no need for manufacturer’s technicians to trouble-shoot and service control systems. There will be no need to lock into one manufacturer’s equipment because of incompatibility.

GCHPs are not inexpensive. However, approximately 50% of the system’s first cost must be shared with a driller/trencher. Therefore, some HVAC manufacturers may be reluctant to support the implementation of GCHPs.

Geothermal projects require a relatively large initial capital investment, with small annual operating costs thereafter. Thus, a district heating project, including production wells, pipelines, heat exchangers, and injection wells, may cost several million dollars. By contrast, the initial investment in a fossil fuel system includes only the cost of a central boiler and distribution lines. The annual operation and maintenance costs for the two systems are similar, except that the fossil fuel system may continue to pay for fuel at an ever increasing rate; while, the cost of the geothermal fuel is stable. The two systems, one with a high initial capital cost and the other with high annual costs, must be compared.

The most formidable barrier to GCHP systems is currently high installation costs. While this is especially true in the residential sector, it also applies to commercial applications.

Residential premiums compared to a standard electric cooling/natural gas heating system are typically $600 to $800 per ton for horizontal systems and $800 to $1000 per ton for vertical systems. Simple payback is typically five to eight years. The percent increase is somewhat less for commercial GCHPs.

The cost of vertical ground coil ranges between $2.00 to $5.00 per ft. of bore. Required bore lengths range between 125 ft. per ton for cold climate, high internal load, commercial buildings to 250 ft. per ton for warm climate installations. Pipe cost can be as low as $0.20 per ft.of bore ($.10/ft of pipe) for 3/4 inch (2.0 cm) and as high as $1.00 per ft. of bore ($0.50/ft of pipe) for 1½ inch (4.0 cm) polyethylene pipe. Drilling cost range from less than $1.00 per ft. to as high as $12.00 per ft. However, $5.00 per ft. is typically the upper limit for a drilling rig designed for the small diameter holes required for GCHP bores even in the most difficult conditions. It should be noted that larger diameter pipes result in shorter required bore lengths. Table 3 gives typical costs for low and high drilling cost conditions for 3/4 and 1½ inch U-bends for a 10 ton system.

The table indicates the added cost to be in the $400 to $850 per ton range. If the cost of the boiler, drains, and cooling tower is deducted from the total and the cost of the ground coil and current cost of improved heat pumps ($100/ton) is added, a cost range for GCHPs results. For low cost drilling sites the cost of GCHPs is actually lower than conventional 2-pipe Variable Air Volume systems.

Water- and ground-coupled heat pumps, referred to as geothermal heat pumps (GHP), have several advantages over air-source heat pumps. These are: (1) they consume about 33% less annual energy, (2) they tap the earth or groundwater, a more stable energy source than air, (3) they do not require supplemental heat during extreme high or low outside temperatures, (4) they use less refrigerant (freon), and (5) they have a simpler design and consequently less maintenance. The main disadvantage is the higher initial capital cost, being about 33% more expensive than air source units. This is due to the extra expense and effort to burying heat exchangers in the earth or providing a well for the energy source. However, once installed, the annual cost is less over the life of the system, resulting in a net savings. The savings is due to the coefficient of performance (COP) averaging around 3 for GHP as compared to 2 for air-source heat pumps.

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