Geothermal heat pump

A geothermal heat pump system is a heating and/or an air conditioning system that uses the Earth's ability to store heat in the ground and water thermal masses. This system will take advantage of a land mass as a heat exchanger to either heat or cool a building structure. These systems operate on a very simple premise; the ground few feets below surface stays around annual average temperature throughout the year, typically somewhere in range of 50-85 °F (10-30 °C) depending upon location's annual climate. A water-source heat pump uses that available heat in the winter and puts heat back into the ground in the summer. A geothermal system differs from a conventional furnace or boiler by its ability to transfer heat versus the standard method of producing the heat. As energy costs continue to rise and pollution concerns continue to be a hot topic, geothermal systems may hold a solution to both of these concerns. A particular advantage is that they can use electricity produced from renewable sources, like solar and wind power, to heat spaces and water much more efficiently than an electric heater. This allows buildings to be heated with renewable energy without transporting and burning biomass on site, producing biogas for use in gas furnaces or relying solely upon solar heating. Geothermal heat pump systems are straightforward and do not require high tech components.

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Components

Geothermal systems require three primary components; a loop field on the property, a liquid pump pack and a water-source heat pump. A loop field can be installed horizontally or vertically on the property; we will focus on the different types of loop fields later in this article. The purpose of the loop field is to transfer heat to and from the ground. The size of the loop field depends on the size of the building being conditioned. Typically, one loop has the capacity of one ton or 12,000 British thermal units per hour (BTU/h) or 3.5 kilowatts. An average house will range from 3 to 5 tons (10 to 18 kW) of capacity. The second component is a liquid pump pack, which sends the water through the loop field and the water-source heat pump. An example of an installed liquid pump pack can be seen in the image to the right. Lastly, the water-source heat pump is the unit that replaces the existing furnace or boiler. This is where the heat from the loop field is transferred for heating the structure. Heat pumps have the ability to capture heat at one temperature reservoir and transfer it to another temperature reservoir. An example of a heat pump is a refrigerator; heat is removed from the refrigerator's compartments and transferred to the outside. (See the article on heat pumps for more information regarding heat pumps.)

Common loop fields

Closed loop fields

A closed loop system, the most common, circulates the fluid through the loop fields’ pipes. In a closed loop system there is no direct interaction between the fluid and the earth; only heat transfer across the pipe. There are four common types of closed loop systems; vertical, horizontal, slinky, and pond. (Slinky and pond loops depicted below.)

Vertical closed loop field

A vertical closed loop field is composed of pipes that run vertically in the ground. A hole is bored in the ground, typically, 150 to 250 feet deep (45–75 m). Pipe pairs in the hole are joined with a U-shaped cross connector at the bottom of the hole. Vertical loop fields are typically used when there is a limited square footage of land available.

Horizontal closed loop field

A horizontal closed loop field is composed of pipes that run horizontally in the ground. A long horizontal trench, deeper than the frost line, is dug and U-shaped coils are placed horizontally inside the same trench. A trench for a horizontal loop field will be similar to one seen under the slinky loop field; however, the width strictly depends on how many loops are installed. Horizontal loop fields are very common and economical if there is adequate land available.

Slinky closed loop field

A slinky closed loop field is also installed in the horizontal orientation; however, the pipes overlay each other. The easiest way of picturing a slinky field is to imagine holding a slinky on the top and bottom with your hands and then move your hands in opposite directions. A slinky loop field is used if there is not adequate room for a true horizontal system, but it still allows for an easy installation. The image above shows a 3-ton slinky loop prior to being covered with soil. In the picture you can see the three slinky loops running out horizontally and three straight lines returning the end of the slinky coil to the heat pump.

Closed pond loop

A closed pond loop is not as common, but is becoming increasingly popular. A pond loop is achieved by placing coils of pipe at the bottom of an appropriately sized pond or water source. This system has been promoted by the DNR (Department of Natural Resources), who support geothermal systems and the use of ponds for geothermal systems. The two images below show a pond loop close up and the pond loop as it about to be sunk to the bottom of a pond. This loop field is for a 12-ton system, which is unusually large for most residential applications. As you can tell by the pictures; a pond loop is extremely similar to a slinky loop, except that it is attached to a frame and located in a body of water versus soil.

Open loop field

In contrast to the closed loop systems, an open loop system pulls water directly from a well, lake, or pond. Water is pumped from one of these sources into the heat pump, where heat is either extracted or added. The water is usually dumped back into some other location where it will be returned to the Earth. Examples: 1) Water is pumped from a vertical water well and returned to a nearby pond. 2) Water is pumped from a pond/lake and returned (although usually in a different area) to the pond/lake. The only downfall to the 2nd example is the potential for dirt, fish, algae, etc. to contaminate the piping, pumps, and heat pump, thus possibly leading to a failure. Also, if the water source is fairly small and/or the heating/cooling load is very large, the potential for thermal contamination exists. This means that the water may become either too hot or cold for the proper operation of the heat pump. Though proper design, planning, and installation will allow any one of the possible loop configurations to work very well for a very long time.

Note that the heat pump does not pollute or harm the water it uses in any way with an open loop setup.

Common heat pumps

There are also different types of water-source heat pumps. A variety of products are available, for both residential and commercial applications; there are water-to-air heat pumps, water-to-water heat pumps and hybrids between the two. Some manufactures are now producing a reversible heat pump for chillers also.

Water-to-air

The water-to-air heat pumps are designed to replace a forced air furnace and possibly the central air conditioning system. The term water-to-air signifies that the heat pump is designed for forced air applications and indicates that water is the source of heat. The water-to-air system is a single central unit that is capable of producing heat during the winter and air conditioning during the summer months. There are variations of the water-to-air heat pumps that allow for split systems, high-velocity systems, and ductless systems.

Water-to-water

A water-to-water heat pump is designed for a heating-system that utilizes hot water for heating the building. Systems such as radiant underfloor heating, baseboard radiators and conventional cast iron radiators would use a water-to-water heat pump. The water-to-water heat pump uses the warm water from the loop field to heat the water that is used for conditioning the structure. Just like a boiler, this heat pump is unable to provide air conditioning during the summer months.

Hybrid

A hybrid heat pump is capable of producing forced air heat and hot water simultaneously and individually. These systems are largely being used for houses that have a combination of under-floor and forced air heating. Both the water-to-water and hybrid heat pumps are capable of heating domestic water also. Almost all types of heat pumps are produced commercially and residentially for indoor and outdoor applications.

Characteristics

Geothermal systems are able to transfer heat to and from the ground with minimal use of electricity. When comparing a geothermal system to an ordinary system a homeowner can save anywhere from 30% to 70% annually on utilities^[1]. Even with the high initial costs of purchasing a geothermal system the payback period is relatively short, typically between three and five years^[2]. Geothermal systems are environmentally friendly; they are a renewable energy source, non-polluting, and recognized as one of the most efficient heating and cooling systems on the market. The U.S. Environmental Protection Agency (EPA) has called geothermal the most energy-efficient, environmentally clean, and cost-effective space conditioning systems available.^[3] The life span of the system is longer than conventional heating and cooling systems. Most loop fields are warranted for 25 to 50 years and are expected to last at least 50 to 200 years^[2][1]. Geothermal systems do not use fossil fuels for heating the house and eliminate threats cause by combustion, like carbon monoxide poisoning. The fluids used in loop fields are designed to be biodegradable, non-toxic, non-corrosive and have properties that will minimize pumping power needed. Some electric companies will offer special rates to customers that install geothermal systems for heating/cooling their building. This is due to the fact that heat pumps only use electricity for heating and no fossil fuels are being purchased. However, if the electricity comes from fossil fuel burning power plants this is no longer so. Renewable electricity such as a solar photo-voltaic system could be used to power the heat pumps. Electrical plants have the largest loads during summer months and much of their capacity sits idle during winter months. This allows the electric company to use more of their facility during the winter months and sell more electricity.

Today there are more than 1,000,000 geoexchange installations in the United States.

The current use of geothermal heat pump technology has resulted in the following emissions reductions^[1]:

• Elimination of more than 5.8 million metric tons of CO₂ annually
• Elimination of more than 1.6 million metric tons of carbon equivalent annually

These 1,000,000 installations have also resulted in the following energy consumption reductions^[1]:

• Annual savings of nearly 8,000 GWh
• Annual savings of nearly 40 trillion Btus of fossil fuels
• Reduced electricity demand by more than 2.6 GW

The impact of the current use of geoexchange is equivalent to^[1]:

• Taking close to 1,295,000 cars off the road
• Planting more than 385 million trees
• Reducing U.S. reliance on imported fuels by 21.5 million barrels (3,420,000 m³) of crude oil

per year.

Costs and savings

The initial cost of installing a Geothermal Heat Pump system can be two to three times that of a conventional heating system in most residential applications, new construction or existing. In retrofits, the cost of installation is affected by the square footage of living area, the home's age, insulation characteristics, the geology of the area, and location of the home/property. For new construction, proper duct system design and mechanical air exchange should be considered in initial system cost. These systems can save the average family from 400-1400$/year, reducing the average heating/cooling costs by 35-70% per household. The cost of installation may be reduced by many governmental programs which all the home owners use to reduce their taxes at the end of the year.^[4]

References

See also

• Geothermal exchange heat pump
• Geothermal heating
• Geo-exchange