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. These systems operate based on the stability of underground temperatures: the ground a few feet below surface has a very stable temperature throughout the year, depending upon location's annual climate. A geothermal 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 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.

Geothermal heat pumps are also known as "GeoExchange" systems (a term created by an industry association) [cite web |title=Geothermal Heat Pump Consortium, Inc. |url=http://geoexchange.org/ |accessdate=2008-04-03] and "ground-source heat pumps." The latter term is useful as it clearly distinguishes the technology from air-source heat pumps. Geothermal heat pumps, which can be used in almost any region, should also be distinguished from geothermal heating. Geothermal heating is used in areas where exceptionally high underground temperatures, such as those at hot springs and steam vents, are used to heat indoor spaces without the use of a heat pump.

This article focuses on geothermal heat pumps that use water to exchange heat with the ground, often referred to as "water-source geothermal heat pumps" or "water loop geothermal heat pumps." Another type of geothermal heat pump, the direct exchange geothermal heat pump, is also available and is discussed briefly here and more fully in its own article.

Introduction

A geothermal heat pump is a heat pump that uses the Earth as either a heat source, when operating in heating mode, or a heat sink, when operating in cooling mode.

Geothermal heat pumps can be characterised as having one or two loops. The heat pump itself, explained more fully in the article on heat pumps, consists of a loop containing refrigerant. The refrigerant is pumped through a vapor-compression refrigeration cycle that moves heat from a cooler area to a warmer one.

In a single loop system, the copper tubing refrigerant loop actually leaves the heat pump appliance cabinet and goes out of the house and under the ground and directly exchanges heat with the ground before returning to the appliance. Hence the name "direct exchange" or DX. Copper loop DX systems are gaining acceptance due to their increased efficiency and lower installation costs but the volume of expensive refrigerant remains high.In a double loop system, the refrigerant loop exchanges heat with a secondary loop made of plastic pipe containing water and anti-freeze (propylene glycol, denatured alcohol or methanol). After leaving the heat exchanger, the plastic pipe goes out of the house and under the ground before returning, so the water is exchanging heat with the ground. This is known as a water-source system. In principle this need not be pressurized, so inexpensive plastic tubing could be used, but in practice the heat-exchange coil in the appliance requires pressurization to flush out air and to obtain the necessary flow.

Components

Geothermal systems require a length of buried tubing on the property, a liquid pump pack and a water-source heat pump. Expansion tanks and pressure relief valves can be installed. The tubing can be installed horizontally as a loop field or vertically as a series of long U-shapes (see below). The purpose of the tubing 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 (400 to 600 feet) 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 tubing and the water-source heat pump. Lastly, the water-source heat pump is the unit that replaces the existing furnace or boiler. This is where the heat from the tubing 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. Another example of a heat pump is a refrigerator; heat is removed from the refrigerator's compartments and transferred to the outside.

Common Systems

Closed loop fields

A closed loop system, the most common, circulates the fluid through the loop fields’ pipes and does not pull in water from a water source. In a closed loop system there is no direct interaction between the fluid and the earth; only heat transfer across the pipe. The length of vertical or horizontal loop required is a function of the ground formation thermal conductivity, ground temperature, and heating and cooling power needed, and also depends on the balance between the amount of heat rejected to and absorbed from the ground during the course of the year. A rough approximation of the initial soil temperature is the average daily temperature for the region. Although copper and other metals can be used, polyethylene seems to be the most common tubing material used currently by installers; often 3/4 inch (19mm) inside diameter tubing.

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. The borehole is commonly filled with a bentonite grout surrounding the pipe to provide a good thermal connection to the surrounding soil or rock to maximize the heat transfer. Vertical loop fields are typically used when there is a limited square footage of land available. Bore holes are spaced 5–6 m apart and are generally 15 m (50 ft) deep per kW of cooling. During the cooling season, the local temperature rise in the bore field is influenced most by the moisture travel in the soil. Reliable heat transfer models have been developed through sample bore holes as well as other tests.

;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 pump is used to heat the house.

;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. 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 systems

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 then pumped back into a second well or source body of water. There are three general types of systems: First water can be pumped from a vertical water well and returned to a nearby pond. Second, water can be pumped from a body of water and returned to the same body of water. Third, water can be pumped from a vertical well and then returned to the same well. While thermal contamination (where the ground temperature is affected by the operation of the system) is possible with any geothermal system, with proper design, planning, and installation any loop configuration can work very well for a very long time. Deep lake water cooling uses a similar process with an open loop for air conditioning and cooling. Open loop systems using ground water are usually much more efficient than closed systems because they will be heat exchanging with water always at ground temperature. Closed loop systems, in comparison, have to make do with the inefficient heat-transfer between the water flowing through the tubing and the ground temperature.

One of the benefits of an open loop system is that for most configurations and depending on the local environment you are dealing with ground water at a constant temperature of about 50°F/10°C. In closed loop systems the temperature of the water coming in from the loop is often within 10°F/6°C of the temperature of the water entering the loop showing how little heat was exchanged. The constant ground water temperatures significantly improve heat pump efficiency.

Standing Column Well

A standing column well system is less expensive and more efficient than a comparably sized closed loop system. Water is drawn from the bottom of a deep rock well, passed through a heat pump, and returned to the top of the well, where traveling downwards it exchanges heat with the surrounding bedrock. The choice of a standing column well system is often dictated where there is near-surface bedrock and limited surface area is available. A standing column is typically not suitable in locations where the geology is comprised of mostly clay, silt, or sand. If bedrock is deeper than 200 feet from the surface, the cost of casing to seal off the overburden may become prohibitive.

A multiple standing column well system can support a large structure in an urban or rural application. The standing column well method is also popular in residential and small commercial applications. There are many successful applications of varying sizes and well quantities in the many boroughs of New York City, and is also the most common application in the New England states. This type of Earth-Coupling system has some heat storage benefits, where heat is rejected from the building and the temperature of the well is raised, within reason, during the Summer cooling months which can then be harvested for heating in the Winter months, thereby increasing the efficiency of the heat pump system. As with closed loop systems, sizing of the standing column system is critical in reference to the heat loss and gain of the existing building. As the heat exchange is actually with the bedrock, using water as the transfer medium, a large amount of production capacity (water flow from the well) is not required for a standing column system to work. However, if there is adequate water production, then the thermal capacity of the well system can be enhanced by periodic discharge during the peak Summer and Winter months.

Since this is essentially a water pumping system, standing column well design requires critical considerations to obtain peak operating efficiency. Should a standing column well design be misapplied, leaving out critical shut-off valves for example, the result could be an extreme loss in efficiency and thereby cause operational cost to be higher than anticipated. The development and promotion of Standing Column Well technology is generally credited to Carl Orio CGD from Atkinson, New Hampshire [ [AHSRAE White Papers on Standing Column Wells: 4666 (RP-1119);4794 (RP-1119); QC-06-006] ] .

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 manufacturers 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.

;Geothermal heat pumps in combination with cold/heat storage: is used extensively for applications as the heating of greenhouses. [http://www2.vlaanderen.be/economie/energiesparen/doc/brochure_warmtepomp.pdf Heat pumps combination with cold/heat storage (see page 28)] In summer, the greenhouse is cooled with ground water, pumped from a aquifer, which is the cold source. This heats the water. the water is then stored by the system in a warm source. In winter, the relative warm water is again pumped up, which derives heat. The now cooled water is again stored in the cold source. [ [http://www.iea-eces.org/energy-storage/storage-techniques/underground-thermal-energy-storage.html Heat and Cold Storage info] ] [ [http://www.warmtepompenindeglastuinbouw.nl/bodemkaart/alg_info.html diagrams of several types of cold/heat storage system with heatpumps] ] [ [http://www.geotherm.nl/producten/wko.htm 2 diagrams of heat/cold storage with heatpumps in summer and winter] ] [ [http://www.shpegs.org/ Explanation of regular and electrified systems of cold/heat storage with heatpumps] ] The combination of cold and heat storage with heat pumps can be very interesting for greenhouses as it may be combined with water/humidity application. This obviously is a great advantage for greenhouses. In the (closed circuit) system, the water used as a storage medium for heat is done in a first aquifer, while the cold water is held in a second aquifer. The heat and cold stored in the water mass is when needed spread as hot or cold air through the use of fans. [ [http://www.zonneterp.nl/english/index_uk.html Schematic of similar system of aquifers with fans-regulation] ] In the described system, everything can be automated.

Direct Exchange

While this article focuses on water-source systems in which the refrigerant exchanges its heat with a water loop that is placed in the ground, a direct exchange system (often known as DX geothermal) is one in which the refrigerant circulates through a copper pipe placed directly in the ground. This eliminates the need for a heat exchanger between the refrigerant loop and the water loop, as well as eliminating the water pump. These simpler systems are able to reach higher efficiencies while also requiring a shorter and smaller pipe to be placed in the ground, reducing installation cost. DX systems are a relatively newer technology than water-source. DX systems, like water-source systems, can also be used to heat water in the house for use in radiant heating applications and for domestic hot water, as well as for cooling applications. Though corrosion or cracking of the copper loop has sometimes been a concern, these can be eliminated through proper installation. Since copper is a naturally-occurring metal that survives in the ground for thousands of years in most soil conditions, the copper loops usually have a very long lifetime.

Benefits of Geothermal Heat Pumps

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.cite web |title=Geothermal Heat Pump Consortium, Inc. |url=http://geoexchange.us/ |accessdate=2007-10-19] Even with the high initial costs of purchasing a geothermal system the payback period is relatively short, typically between three and five years.cite web |title=Geothermal heat pumps: alternative energy heating and cooling FAQs |url=http://www.econar.com/faq.htm |accessdate=2007-10-19] Geothermal systems are 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. [cite journal|last1= Environmental Protection Agency|title= Space Conditioning: The Next Frontier - Report 430-R-93-004|publisher= EPA|year= 1993] 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. Geothermal systems use electricity for heating the house. 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 who install geothermal systems for heating/cooling their building.Cite web
url=http://www.capitalelec.com/Energy_Efficiency/ground_source/index.html
title=Geothermal Heat Pumps
publisher=Capital Electric Cooperative
accessdate=2008-10-05] This is due to the fact that 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. It also allows them to reduce peak usage during the summer (due to the increased efficiency of heat pumps), thereby avoiding costly construction of new power plants. For the same reasons, other utility companies have started to pay for the installation of geothermal heat pumps at customer residences. They lease the systems to their customers for a monthly fee, at a net overall savings to the customer.

Geothermal heat pumps are especially well matched to underfloor heating systems which do not require extremely high temperatures (as compared with wall-mounted radiators). Thus they are ideal for open plan offices. Using large surfaces such as floors, as opposed to radiators, distributes the heat more uniformly and allows for a lower temperature heat transfer fluid. Some of the advantages cited for underfloor radiant heat; lower temperature heat transfer fluid, can however, be compromised somewhat when using wood or carpet floor coverings since the thermal transfer efficiency of these materials is lower than masonry floors (tile, concrete).

Undisturbed earth below the frost line remains at a relatively constant temperature year round. This temperature equates roughly to the average annual air-temperature of the chosen location, so is usually 7-21 degrees Celsius (45-70 degrees Fahrenheit) depending on location. Because this temperature remains more constant than the air, geothermal heat pumps perform with far greater efficiency and in a far larger range of extreme temperatures than conventional air conditioners and furnaces, and even air-source heat pumps. A particular advantage is that they can use electricity to heat spaces and water much more efficiently than an electric heater.

Geothermal heat pump technology is a Natural Building technique.

Today there are more than 1,000,000 geothermal heat pump installations in the United States.

The current use of geothermal heat pump technology has resulted in the following emissionsreductions:
* 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 consumptionreductions:
* 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 geothermal heat pumps is equivalent to:
* 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 oilper 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 size 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 US$400-1400/year, reducing the average heating/cooling costs by 35-70% per household.

References

See also

* Direct exchange geothermal heat pump
* Geothermal heating
* Earth cooling tubes
* Microgeneration Certification Scheme
* Geothermal power

External links

* [http://www1.eere.energy.gov/geothermal/heatpumps.html Geothermal Heat Pumps (US Department of Energy)]
* [http://www.geoexchange.org Geothermal Heat Pump Consortium]
* [http://www.cekor.com Geothermal Heat Pump designs in New York and Information]
* [http://www.geo-exchange.ca/en/ Canadian Geoexchange Coalition]
* [http://www.gshp.org.uk/ Ground Source Heat Pump Association - UK]
* [http://www.igshpa.okstate.edu/ International Ground Source Heat Pump Association]
* [http://www.fossilfreedom.com/geothermal.html Fossil Freedom - Crash Course in geothermal heat pumps]
* [http://www.toolbase.org/Technology-Inventory/HVAC/geothermal-heat-pumps PATH Tech Inventory: Geothermal Heat Pumps]
* [http://www.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12640 US Dept. of Energy]
* [http://www.heatpumpsfederation.co.uk Ground Source Heat Pumps - Heat Pump Federation UK]

Examples

* [http://www.channel4.com/4homes/on-tv/grand-designs/episode-guides/maidstone-the-hi-tech-bungalow-08-06-03_p_1.html Maidstone: The Hi Tech Bungalow]