Geothermal heat pumps have become more popular in the past decade, even though they have been around since the 1940’s. Geothermal heat pumps (GHP), which are also referred to as earth-coupled, ground-source, or water-source heat pumps, use the constant temperature of the Earth as the exchange medium instead of the outside air temperature as seen in the typical installation of residential air conditioning and heat pump systems. This allows the system to reach fairly high efficiencies (300%-600%) on the coldest of winter nights, compared to 175%-250% for air-source heat pumps on cool days. Savings can be high as $600 to $1,200 annually and there are also tax credit and utility rebates available to offset the installation costs.
The reason GHP are more efficient that air-source heat pumps is that a few feet below the Earth's surface the ground remains at a relatively constant temperature. Depending on the latitude, ground temperatures range from 45°F to 75°F. The GHP takes advantage of this by exchanging heat with the Earth through a ground heat exchanger, which is a loop of high-density polyethylene pipe.
The heat pump’s basic components include a heat exchanger, compressor, reversing valve, air condenser and an expansion valve. In the heating cycle, an anti-freeze solution from the ground loop enters the heat pump and passes through a heat exchanger where latent heat energy is transferred to a refrigerant, which is piped to the compressor via the reversing valve. The compressor raises the temperature of the refrigerant to approximately 140 degrees Fahrenheit. Basically, the water pumped through the loop adds or removes heat from the refrigerant depending on the mode of operation. The heat exchanger is where this takes place and is usually a “Coaxial” heat exchanger. The water or anti-freeze flows in the center rifled tube, and refrigerant flows around the outside of the copper tube. This is where the exchange takes place between water or loop fluid and refrigerant. In heating, heat from the water, in the center of the tube, exchanges to the refrigerant in the outer area inside this coaxial coil.
Coiled Coaxial Heat Exchanger - The refrigerant line runs through the center of the coiled water jacket tubing.
Tubular Coaxial Heat Exchanger - Water flows down the center and refrigerant is in the copper coil lining the outside
After the compressor the refrigerant passes through the domestic hot water heater to warm the building's water supply. Next, the system pushes the refrigerant to the next stage where it is used to heat the building as it gives up its latent energy and re-condenses in a refrigerant-to-air or a refrigerant-to-water heat exchanger.
Geothermal heat pumps can be of an open- or closed-loop configuration. Open-loop systems draw well water for use as the heat source or heat sink, and after use, return the well water to a drainage field or another well. Closed-loop or earth-coupled systems use a water and antifreeze solution, circulated in a ground loop of pipe to extract heat from the earth. Open-loop systems are less common due to the permitting requirements. Well water is pumped out of the ground, goes through the heat exchanger and then is pumped back into the ground by way of a recharge well. Some jurisdictions will not allow this for fear of ground water contamination. Water testing must be conducted to determine the hardness of the water which can also have an adverse effect on system efficiency.
Ground loops can be installed in a vertical well or a horizontal loop. Vertical wells are usually more expensive and used where space is limited. The length of loop pipe required will vary with soil type, loop configuration, and system capacity. Loop length can range from 250 to 1,000 feet per ton of capacity.
A desuperheater makes it possible for GHP systems to provide almost free water heating throughout the summer. Instead of sending all the waste heat into the ground, some can be captured and channeled to a water heater. Once the desuperheater has been installed, for about $400, the only cost of using it is the energy needed to circulate the water.
Evaluating Your Site for a Geothermal Heat Pump
Because shallow ground temperatures are relatively constant throughout the United States, geothermal heat pumps can be effectively used almost anywhere. However, the specific geological, hydrological, and spatial characteristics of your land will help your local system supplier/installer determine the best type of ground loop for your site:
Factors such as the composition and properties of your soil and rock (which can affect heat transfer rates) require consideration when designing a ground loop. For example, soil with good heat transfer properties requires less piping to gather a certain amount of heat than soil with poor heat transfer properties. The amount of soil available contributes to system design as well — system suppliers in areas with extensive hard rock or soil too shallow to trench may install vertical ground loops instead of horizontal loops.
Hydrology Ground or surface water availability also plays a part in deciding what type of ground loop to use. Depending on factors such as depth, volume, and water quality, bodies of surface water can be used as a source of water for an open-loop system, or as a repository for coils of piping in a closed-loop system. Ground water can also be used as a source for open-loop systems, provided the water quality is suitable and all ground water discharge regulations are met.
Before you purchase an open-loop system, you will want to be sure your system supplier/installer has fully investigated your site's hydrology, so you can avoid potential problems such as aquifer depletion and groundwater contamination. Antifreeze fluids circulated through closed-loop systems generally pose little to no environmental hazard.
The amount and layout of your land, landscaping, and the location of underground utilities or sprinkler systems also contribute to your system design. Horizontal ground loops (generally the most economical) are typically used for newly constructed buildings with sufficient land. Vertical installations or more compact horizontal, coiled installations are often used for existing buildings because they minimize the disturbance to the landscape.
Direct Exchange Geothermal Heat Pumps
Recently, one manufacturer has developed a heat pump design that utilizes the refrigerant piping for the ground source heat exchange. They call this system a ground-coupled system. It uses the phase change from liquid evaporation to vapor condensation and the heat pump compressor to transport heat. Copper tubing which is run from the compressor is buried in the ground where the copper tubing last almost indefinitely in most soils. Copper buried in the Earth is naturally corrosion resistant because it generally requires an oxidizing environment to corrode, and most soils are reducing, thus they contribute electrons to the copper and protect it against corrosion. In areas where corrosive conditions may exist, corrosion can be prevented with a small amount of "impressed electrical current" applied to the copper loop via an insulated cable. The system installation is usually less costly since it requires about 100 feet of tubing per ton, 1/3 to 1/10 the requirement for standard geothermal systems.
Because of the technical knowledge and equipment needed to properly install the piping, a GHP system installation is not a do-it-yourself project. To find a qualified installer, call your local utility company, the International Ground Source Heat Pump Association or the Geothermal Heat Pump Consortium for their listing of qualified installers in your area. Installers should be certified and experienced. Ask for references, especially for owners of systems that are several years old, and check them.
U.S. Department of Energy
NAHB Research Center
Geothermal Heat Pump Consortium