8.8: Cooling and Heating (IV)

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Factors Affecting the Type of GHP Loop

Geothermal heat pumps (GHPs) can be used effectively almost anywhere in the country. However, the specific geological, hydrological, and spatial characteristics of a site determine the best type of ground loop for a specific location.

Benefits of a GHP System

Low energy use

The biggest benefit of GHP's is that they use 20 to 50 percent less electricity than conventional heating or cooling systems. This translates into a GHP using one unit of electricity to move three units of heat from the earth.
According to a report by Oak Ridge National Laboratory, statistically valid findings show that the 4,003-unit GHP retrofit project at Fort Polk, LA, will save 25.8 million kilowatt-hours (kWh) in a typical year, or 32.5 percent of pre-retrofit whole-community electrical consumption. This translates to an average annual savings of 6,445 kWh per housing unit. In addition, 100 percent of the whole-community natural gas previously used for space conditioning and water heating (260,000 therms) will be saved.
In housing units that were all-electric in the pre-retrofit period, the GHP's were found to save about 42 percent of the pre-retrofit electrical consumption for heating, cooling, and water heating.

Free or reduced-cost hot water

Unlike any other heating and cooling system, a geothermal heat pump can provide free hot water. A device called a super heater transfers excess heat from the heat pump's compressor to the hot water tank. In the summer, hot water is provided free; in the winter, water heating costs are cut roughly in half.

Year-round comfort

While producing lower heating bills, geothermal heat pumps are quieter than conventional systems, and improve humidity control. These features help explain why customer surveys regularly show high levels of user satisfaction, usually well over 90 percent.

Design features

Geothermal heat pump systems allow for design flexibility, and can be installed in both new and retrofit situations. Because the hardware requires less space than that needed by conventional HVAC systems, the equipment rooms can be greatly scaled down in size, freeing space for productive use. Also, geothermal heat pump systems usually use the existing ductwork in the building and provide simultaneous heating and cooling without the need for a four-pipe system.

Improved aesthetics

Architects and building owners like the design flexibility offered by GHP's. Historic buildings like the Oklahoma State Capital and some Williamsburg, Virginia, structures use GHPs because they are easy to use in retrofit situations and easy to conceal, as they don't require cooling towers.
GHP systems eliminate conventional rooftop equipment, allowing for more aesthetically pleasing architectural designs and roof lines. The lack of rooftop penetrations also means less potential for leaks and ongoing maintenance, and better roof warranties. In addition, the aboveground components of a GHP system are inside the building, sheltering the equipment both from weather-related damage and from potential vandalism.

Low environmental impact

Because a GHP system is so efficient, it uses a lot less energy to maintain comfortable indoor temperatures. This means that less energy is often created from burning fossil fuels than is needed to operate a GHP. According to the EPA, geothermal heat pumps can reduce energy consumption and corresponding emissions up to 44% compared to air-source heat pumps, and up to 72% compared to electric resistance heating with standard air conditioning equipment.

Low maintenance

According to a study completed for the Geothermal Heat Pump Consortium (GHPC), buildings with GHP systems had average total maintenance costs ranging from 6 to 11 cents per square foot, or about one third that of conventional systems. Because the workhorse part of the system, the piping, is underground or underwater, there is little maintenance required. Occasional cleaning of the heat exchanger coils and regularly changing the air filters are about all the work necessary to keep the system in good running order.

Zone heating and cooling

These systems provide excellent zone space conditioning - different areas of the building heated or cooled to different temperatures simultaneously. For example, GHP systems can easily move heat from computer rooms (which need constant cooling) to the perimeter walls for winter heating in commercial buildings. School officials like the flexibility of heating or cooling just auditoriums or gymnasiums for special events rather than the entire school.

Durability

Because GHP systems have relatively few moving parts, and because those parts are sheltered inside a building, they are durable and highly reliable. The underground piping often carries warranties of 25 to 50 years, and the GHP's often last 20 years or more.

Reduced vandalism

GHPs usually have no outdoor compressors or cooling towers, so the potential for vandalism is eliminated.

Typically when heating systems or appliances are compared, all the costs that are incurred—purchase, installation, operation, and maintenance costs—can be combined into a life-cycle cost, the cost of ownership over a period of years. Table 8.8.1 compares the various types of central heating systems.

Table 8.8.1. Comparison of life cycle costs for heat pumps

Compare	Safety	Installation Cost	Operating Cost	Maintenance Cost	Life-Cycle Cost
Combustion-based	A concern	Moderate	Moderate	High	Moderate
Heat pump	Excellent	Moderate	Moderate	Moderate	Moderate
Geothermal heat pump	Excellent	High	Low	Low	Low

Installation and Operating Costs of GHP Systems

On average, a geothermal heat pump (GHP) system costs about $2,500 to $3,500 per ton of capacity, or roughly $7,500 to 10,000 for a 3-ton unit (typical residential size). In comparison, other systems would cost about $4,000 with air conditioning.

When included in the mortgage, the homeowner has a positive cash flow from the beginning. For example, say that the extra $3,500 will add $30 per month to each mortgage payment. A 3,000 square-foot house in Oklahoma City, shown in Figure 8.8.1, has a verified average electric bill of $60 per month, using a geothermal heat pump. This represents significant savings.

A photo of a house using GHP in Oklahoma City

Figure 8.8.1. A 3,000 square-foot house using GHP in Oklahoma City

Credit: Office of Energy Efficiency & Renewable Energy

A system using horizontal ground loops will generally cost less than a system with vertical loops.

Geothermal heat pump installations in both new and existing homes can reduce energy consumption 25 to 75 percent compared to older or conventional replacement systems. Annual operating costs were also lowest with geothermal heat pumps. Add in the benefits of the desuperheater for hot water savings, and it's easy to see how a GHP system is the most efficient available.