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Basics of Geothermal

Comfort

GeoExchange systems produce an abundance of warm or cooled air and deliver it gently to the space. There are no blasts of hot or cold air that are associated with other types of systems. Since these air temperature fluctuations are substantially reduced, GeoExchange systems provide superior room comfort control.

Economical

How about lower energy bills? Heat from the ground is free and the only electricity needed is for moving that heat between your home and the ground.

A GeoExchange system can result in up to:

GeoExchange systems are recognized by the Environmental Protection Agency (EPA) as the most environmentally friendly heating and cooling technology available. It is a system you can feel good about using. CO2 emissions are significantly reduced.

Clean

No flames, no flue, no odors – just safe, reliable operation.

Reliable

A GeoExchange system has very few moving parts to maintain. A geothermal heat pump is located indoors, where it is protected from weather extremes, vandalism and abuse.

Low Maintenance

GeoExchange systems require little maintenance. Homeowners only need to change air filters. Businesses could eliminate expensive maintenance contracts or on-staff operators.

Efficient

GeoExchange systems are more than three times as efficient as the most efficient fossil fuel furnace. Instead of burning a combustible fuel to make heat, they simply move heat that already exists.

How A GeoExchange System Works

The process is simple. It is based on the basic concept that heat will naturally move from something warm to something cool. A geothermal heat pump provides a means for this natural heat transfer to occur so the earth’s heat can be moved into the heat pump and used to heat the building.

GeoExchange technology moves heat rather than creates heat.

The heat source for a geothermal heat pump in Wisconsin is found at a depth of six to eight feet below the surface. At that depth, the ground temperature is around 50 degrees. The earth’s heat is extracted by means of a ground loop heat exchanger consisting of polyethylene pipes containing a water solution. The water solution inside the pipes captures heat from the ground and with a circulating pump, moves the water into the heat pump. Within the heat pump, a heat exchanger removes the heat from the water solution, concentrates the heat and then distributes the heat to a forced air or hydronic system in the building. For cooling, the process is just reversed. Excess heat from the building is removed by the heat pump and transferred into the earth loop heat exchanger. About 65% of a geothermal heat pump’s heating capacity is actually heat that was removed from the ground and transferred into the heat pump. It is little wonder why these systems are so economical – they are using free heat from the ground. GeoExchange provides 100% of your heating and cooling needs.

Geothermal Heat Pumps

A geothermal heat pump is no more complicated than a refrigerator. The heat pump uses a basic refrigeration cycle to extract heat from a water source and move that heat into the building.

By remembering that heat will naturally move from something warm to something cool and that a heat pump provides the necessary temperature link between the heat in the earth and the need to heat a building–the entire process is very simple.

Important components within a geothermal heat pump are:

Free Hot Water

In the cooling season, the GeoExchange system removes heat from the space. Before this energy is moved to the ground loop it can be used to provide domestic hot water. This means no cost water heating in the summer and at substantially lower costs in the winter.

Do you have Dirt?

The notable flexibility of GeoExchange systems means there are few application barriers. Whether installed in a residential or commercial setting, the GeoExchange system captures and moves the renewable energy from the earth into the geothermal Heat Pump through a geothermal Heat Exchanger – often referred to as the ground loop. The question “Do you have Dirt?” simply means is there sufficient area, at the project site, to install the ground loop.

GeoExchange systems rely on the naturally stored energy in the earth to provide 70% of the energy needed to heat and cool the building. The size of the ground loop must be adequate to accomplish this important benefit. The GeoExchange system designer will determine the heating and cooling requirements of the building, specify the needed capacity of the geothermal heat pump, and design the correct corresponding ground loop length. With some very large ground loop designs, a test is performed to determine the thermal conductivity of the area where the ground loop will be installed.

The least expensive way to install a geothermal ground loop is a horizontal system put into a trench. The ground loops consisting of ?” plastic piping are installed in a trench which is usually 8 feet deep. Horizontal systems do not prohibit the use of the property above them. To the contrary, Heat Exchangers are installed under lawns, farm fields and recreational areas, because of the space requires and the equipment needed for installation, horizontal Heat Exchangers are difficult to install on heavily wooded lots.

Vertical Heat Exchangers minimize the footprint of the loop field installation. Vertical bore holes are made and two pipes, connected at the bottom with an elbow or u-bend, are placed down the bore hole. The bore hole is sealed completely, from bottom to top, with a special compound called grout. The grout insures good heat transfer between the pipes and the earth and protects underground aquifers from contamination. Vertical loop fields are the choice for larger GeoExchange installations because considerable energy can be moved from the earth to the building, while occupying a fraction of the space needed for a comparably sized horizontal loop field. The Evansville high school, in southern Wisconsin, is heated and cooled with renewable energy from the earth, through their GeoExchange system, by virtue of its 460 bore hole vertical Heat Exchanger, installed under a small athletic field. The greatest barrier to vertical Heat Exchangers is their high cost, relative to horizontal systems. As more water well drillers consider expansion of their business to include vertical Heat Exchanger installation, the cost of these systems will become more competitive.

Properties of Refrigerants

All Heat Pumps use a vapor compression cycle (refrigeration cycle) to utilize the properties of refrigerants to move heat from one location to another. Refrigerants are compounds, that have special thermal properties, which circulate through the Heat Pump (See Bulletin: How does a Geothermal Heat Pump Work?)

The refrigeration cycle is difficult to understand without knowing a little about phase changes of a substance, like water. Let us say we are going to make ice cubes. We place a tray of water in the freezer. Energy (heat) is released by the water as it cools, one unit of energy for every one-degree drop in the ice cube tray. If the temperature of the water is 33 degrees, one unit of energy is released by the water and it becomes 32 degrees water. To change 32 degree water into 32 degree ice, however, about 120 units of energy must be released. Changing liquid water to ice is a phase change.

Water can illustrate another common phase change, liquid to steam (vapor). Water must absorb about 1,000 units of energy to go from 212 degrees water to 212 degrees steam or vapor.

Refrigerants “boil” at much lower temperatures than water, depending on the pressure maintained by the refrigeration device (such as a Heat Pump) and the type of refrigerant. The refrigerant in most kitchen refrigerators vaporizes at 20 degrees. This allows the refrigerant to adsorb heat and cool the refrigerator compartment.

Key Point: It is not the change in temperature of the refrigerant that is important, it is the phase change of the refrigerant. 30 degrees to 50 degrees water circulating from the earth can not heat your home or business, but it can enable the refrigerant to phase change, absorbing a great deal of energy, allowing the Heat Pump to heat the space.

The unique characteristicof a Geothermal Heat Pump, compared to conventional refrigeration appliances, I s the refrigeration process can be reversed. This is discussed in more detail (in How does a Heat Pump work?). As it relates to the refrigerant circulation through the Heat Pump, many units of heat are released by the vapor, useable heat for making he home or business comfortable or providing for the domestic hot water needs.

212 degree Steam (vapor) releases 1,000 units of energy to change back to 212 degree Water. Remember only one unit of energy is released if the temperature of the water is lowered to 211 degrees.

The phase change properties of water are an appropriate way to show the important concept of absorption and release of energy. We are familiar with freezing and boiling points of water. A refrigerant is able to accomplish this phenomenon at more useful temperatures and do so much more efficiently. The phase changes of the refrigerant are the “magic” of Geothermal Heat Pumps. The refrigerants give them the ability to move the stored energy from the earth in the winter and remove heat from the space and transfer it to the ground in the summer.

New refrigerants are being developed to improve efficiency and eliminate the impact on the ozone layer. Geothermal Heat Pump manufacturers’ are already re-engineering ad supplying equipment free of ozone depleting compounds. How can something operate at 400% efficiency? Or How can 50 degree dirt keep me warm? Far as we know there are no perpetual motion machines, if you want something, you have to pay the price, and there is no free lunch. Yet when we talk about the heating efficiency of a Geothermal Heat Pump (or GeoExchange) system it sure sounds unbelievable.

As described in How does a Geothermal Heat Pump work?, the refrigeration cycle and refrigerants, the substance circulating in a Heat Pump, were discussed. As the liquid refrigerant moves through the Evaporator, the refrigerant absorbs energy from the water circulating through the Evaporator from the loop field. The water temperature is warm enough to cause the refrigerant to boil (change to a vapor). Changing to a vapor allows the refrigerant to move a tremendous amount of energy from the loop field. At this point, most of the work is done; 70% of the energy needed to heat the space has been moved from the 40 degree circulating water of the lop field. The job of the compressor is to simply pressurize the vapor. Remember gases readily compress, so the compressor does not have to work very hard. As the refrigerant vapor is compressed the temperature increases, providing the required temperature (120 degrees to 140 degrees) for effective space heating. The hot vapor moves to the condenser, where it changes to a liquid, giving up its heat to the ductwork, or other heating distribution system, serving the space.

The key point is, if the compressor uses one unit of electricity to operate and the heat pup produces four units of heat, the heat pump is said to have a Coefficient of Performance (C.O.P.) or 4. Saying it another way, the system is 400% efficient un using an electrical energy input to produce a heating energy output.

GeoExchange systems are not always 400% efficient, sometimes the C.O.P. is higher or lower, depending on a number of factors. The Air-Conditioning & Refrigeration Institute (ARI) provides performance standards for Geothermal Heat Pumps.

ARI Standard 13256-1 is the name of the rating for all Geothermal Heat Pumps, which the ARI tests for heating and cooling efficiency. ARI charts listing heat pumps from various manufacturers show the C.O.P. the Institute has assigned to each model. This independent testing is one of the ways consumers know that the GeoExchange system will provide the expected performance.

Loop field sizing, making sure there is sufficient ability to move heat from the earth to the Geothermal Heat Pump, is critical for maintaining peak operating efficiency. If the loop field is too small, it will not have the ability to maintain temperature as the refrigerant removes heat from the circulating water. If the loop field temperature drops, GeoExchange system efficiency drops. Consumers installing a GeoExchange system for their home or business will benefit from years of research to perfect loop field sizing methods. Scientific-based calculations help make certain of optimum GeoExchange system performance.

While 400% efficiency deserves some explanation, cooling efficiencies for GeoExchange systems are remarkable as well. ARI standards list cooling efficiencies as E.E.R., the same rating given for conventional cooling equipment. Removing heat from the space, during the heating season, and transferring that energy to the relatively cool earth, via the loop field, is considerably more efficient than conventional cooling. A conventional cooling unit must work hard to reject heat into the hot, humid air surrounding the exterior condenser. As outside air temperatures increase, more and more cooling is required by the space, the conventional system becomes less and less efficient. How does a Geothermal Heat Pump work?

All Heat Pumps use a vapor compression cycle (refrigeration cycle) to move heat from one location to another. It shares the same components as other refrigeration appliances, the refrigerator in your kitchen, the walk-in cooler, standard air conditioners, with one special difference; heat pumps can reverse the cycle to deliver both heating and cooling.

The substance circulating in the heat pump is a refrigerant. It goes through phase changes (liquid to gas) during the refrigeration cycle. Refrigerants adsorb and release energy as they change from the liquid phase to the gas phase and back again. Refrigerants are very efficient phase change substances and accomplish the phase changes at very low temperatures (See: Properties of Refrigerants).

To follow the refrigeration cycle for a Geothermal Heat Pump in the cooling mode. Liquid refrigerant enters the Evaporator, an indoor coil (looks a little like a car radiator), where it adsorbs heat from the room air (makes Cool Air) by changing from a liquid to a gas. The warm vapor moves to the compressor and leaves the compressor as a hot (about 120 to 140 degrees) high pressure vapor as it moves to a refrigerant-to-water heat exchanger. The heat exchanger serves as the Condenser when the system is delivering Cool Air to the space. Inside the heat exchanger (Condenser) the refrigerant moves through tubing that are installed close to separate tubing where water is circulating from the loop field. The heat from the hot vapor moves readily to the cool water, where it will be rejected to the cooler soil around the loop field piping. It is moving energy to and from the earth that adds Geothermal to the system’s name and provides for the unmatched efficiency of the Geothermal Heat Pump systems. The warm vapor moves to the compressor and leaves the compressor as a hot (about 120 to 140 degrees) high pressure vapor as it moves to a refrigerant-to-water heat exchanger. The heat exchanger serves as the Condenser when the system is delivering Cool Air to the space. Inside the heat exchanger (Condenser) the refrigerant moves through tubing that are installed close to separate tubing where water is circulating from the loop field. The heat from the hot vapor moves readily to the cool water, where it will be ejected to the cooler soil around the loop field piping. It is moving energy to and from the earth that adds Geothermal to the system’s name and provides for the unmatched efficiency of the Geothermal Heat Pump systems.

Since the hot vapor gives up its heat to the loop field, it condenses to a high pressure liquid (a phase change again). This is very similar to water vapor condensing on the outside of a glass of ice water. The high pressure liquid refrigerant moves to the expansion valve, which reduces the pressure of the liquid. When the pressure is reduced the liquid cools, like how an aerosol can feels when it is sprayed, the cycle is complete as the cool liquid refrigerant re-enters the Evaporator to pick up more room heat.

In the heating season, the Reversing Valve allows the Heat Pump to circulate refrigerant through the cycle in the opposite direction from the cooling mode. The refrigerant-to-water heat exchanger now becomes the Evaporator. Now cool liquid refrigerant is circulated through the tubing and heat is readily transferred from the circulating water and the liquid refrigerant changes to a vapor. The vapor moves to the compressor, which increases the refrigerant’s pressure and raises its temperature. The hot gas circulates through the Condenser where the heat is removed and sent into the space for heating. When the refrigerant loses its heat, it changes back to a liquid. The liquid refrigerant cools further as it passes through the expansion valve, and the process begins again.