Geothermal heat pump

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Ground source heating and cooling

A geothermal heat pump (also called GeoExchange, earth-coupled, ground source or water-source heat pump [1]) system is a heating and/or cooling system that uses the earth´s ability to store heat in the shallow ground or water thermal masses.

Geothermal heat pumps are known also as "GeoExchange" systems, or "ground source heat pumps", to clearly distinguish them from air source heat pumps. It is important to understand that ground source heat pumps draw energy from shallow ground. The energy originates from the sun: none of the energy originates from the centre of the Earth, in spite of the name "geothermal heat pump". Genuine geothermal energy from the centre of Earth is available only in places where volcanic activity comes close to the surface.

These systems operate based on the stability of underground temperatures: the shallow ground, this is the upper 10 feet (3.0 m) of Earth´s surface, has a very stable temperature throughout the year - between 10 and 16 °C (50 and 61 °F), depending upon location's annual climate [2]. Like a cave, the shallow ground temperature is warmer than the air above during the winter and cooler than the air in the summer [3]. A geothermal heat pump uses that available heat in the winter (heating) and puts heat back into the ground in the summer (cooling).

The system cost are returned in energy savings in 5–10 years. System life is estimated at 25 years for the inside components and 50+ years for the ground loop. There are approximately 50,000 geothermal heat pumps installed in the United States each year [4].

Contents

[edit] Other systems

A geothermal system differs from a conventional furnace or boiler by its ability to transfer heat versus the method of producing heat through combustion. 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, 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.

[edit] 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. The source or sink is used to change the state of the refrigeration gas in the refrigeration circuit, which results in the ability of the appliance to remove heat or provide deliverable heat. This is known as a water-source system, and is different from an air source heat pump, that can also be combined with thermal solar cooling, in a geosolar system [5].

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 building 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. DX systems are not gaining acceptance in Canada. Numerous botched installations along with the high cost and liability in Ontario are causing people to shun this technology.

In a double loop system, the refrigerant loop exchanges heat with a secondary loop. This may be an open loop or a closed loop system. In a closed loop system, the loop is made of High-density polyethylene pipe and it contains water and anti-freeze (propylene glycol, denatured alcohol or methanol). After leaving the heat exchanger, the pipe goes out of the building and under the ground below the frost line, and may be submerged in a body of water such as a pond or lake before returning, so the water is exchanging heat with the ground or water. Systems in wet ground or in water are generally more efficient than dryer ground loops since it is less work to move heat in and out of water than solids in sand or soil. In an open loop system the supply piping runs from the machine to a well or body of water (fresh or salt water are ok, but the appliance must be protected from corrosive effect of salt water by using different metals in the heat exchangers and pumps). The return line runs from the machine to a separate rejection well or body of water. The supply and return lines are placed far enough apart to ensure correct thermal transfer and recharge of the source.

[edit] Components

An installed liquid pump pack

Geothermal systems require a ground loop. Some manufacturers have a separate ground loop fluid pump pack, while some integrate the pumping and valving within the unit and a water-source heat pump. Expansion tanks and pressure relief valves can be installed on the heated fluid side.

The tubing can be installed horizontally as a loop field or vertically as a series of long U-shapes in wells(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 soil type and moisture content, the average ground temperature and the heat loss and or gain characteristics of the building being conditioned. Typically, one loop (400 feet (120 m) to 600 feet (180 m)) 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 previous two sentences are extremely subjective and may not be applicable in many cases.

The second component in some cases is a liquid pump pack, which sends the water through the tubing and the water-source heat pump. Some manufacturers have this pumping capacity within the heat pump appliance.

Lastly, the water-source heat pump is the unit that becomes the heating and cooling plant for the building. It can cover space heating, space cooling, (space heating via conditioned air, hydronic systems and / or radiant systems), domestic or pool water preheat (via the desuperheater function, demand hot water all within one appliance with a variety of options with respect to controls, staging and zone control. This is where the heat from the tubing is used to transferred heat in or out of the ground for use in the structure or for water.

Heat pumps have the ability to capture heat at one temperature reservoir and transfer it to another temperature reservoir. An example of an air source heat pump is a refrigerator; heat is removed from the refrigerator's compartments and transferred to the outside. There is no technical barrier to using a water source heat pump system to take the heat out of your leftovers and to put it into the hot water for washing the dishes.

[edit] Common systems

[edit] 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. 3/4 inch (19mm) or 1.25 inch inside diameter are common sizes of 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, 75 to 500 plus feet deep. 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. Grout also protects the ground water from contamination, and prevents artesian wells from flooding the property. Vertical loop fields are typically used when there is a limited area of land available. Bore holes are spaced 5–6 m apart and the length is highly subjective based on ground and building characteristics. Someone thinks they 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.

It aint rocket science folks!

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.
A 3-ton slinky loop prior to being covered with soil. The three slinky loops are running out horizontally with three straight lines returning the end of the slinky coil to the heat pump
Loop field for a 12-ton system (unusually large for most residential applications)
12-ton pond loop system being sunk to the bottom of a pond
Slinky closed loop field
A slinky (also called coiled) closed loop field is a type of horizontal closed loop; 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. Rather than using straight pipe, slinky coils, use overlapped loops of piping laid out horizontally along the bottom of a wide trench. Depending on soil, climate and your heat pumps’ run fraction, slinky coil trenches can be anywhere from one third to two thirds shorter than traditional horizontal loop trenches. Slinky coil ground loops are essentially a more economic and space efficient version of a horizontal ground loop [6].
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.

[edit] 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. In the first type, water can be pumped from a vertical water well and returned to a nearby pond. In the second type of system, water can be pumped from a body of water and returned to the same body of water. In the third type of system, 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.

Some types of open-loop systems are illegal in Ontario, after the Walkerton Tragedy, and other jurisdictions may not allow some of these systems which may drain aquifers or possibly contaminate wells.

[edit] 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 (61 m) 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.

[edit] Common heat pumps

Water-to-air heat pump
Water-to-water heat pump
A heat pump in combination with heat and cold storage

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 or cold. 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 water for heating or cooling the building. Systems such as radiant underfloor heating, baseboard radiators, conventional cast iron radiators and a dual geosolar (solar thermal-geothermal) system would use a water-to-water heat pump. The water-to-water heat pump uses the water from the loop field to heat or cool 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 seasonal thermal storage
Heat pumps, combined with aquifers, can be used for heating of greenhouses.[7] In summer, a greenhouse can be cooled with cold ground water, pumped from an aquifer. This heats the water in the aquifer which can become a warm source for heating in winter. [7][8][9] [10] [11] [12]. The combination of cold and heat storage with heat pumps can be combined with water/humidity regulation. These principles are used more generally in Interseasonal Heat Transfer [13] to provide renewable heat and renewable cooling to all kinds of buildings. Where an aquifer is not present a thermal bank[14] can be used for seasonal thermal storage and an asphalt solar collector[15] can be used to charge a thermal bank with heat in the summer to allow winter heating to be provided very efficiently with a ground source heat pump.

[edit] 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.

[edit] 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.[16] Even with the high initial costs of purchasing a geothermal system the payback period is relatively short, typically between three and five years.[17] 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.[18] Heat pumps also offer significant emission reductions potential, particularly in regions with heating and cooling loads - the GHG emissions savings in a given region can be calculated based on the amount of energy used, efficiency factors, and carbon intensity of the electricity used to power the pump.[1] The life span of the system is longer than conventional heating and cooling systems. Most loop fields are warrantied for 25 to 50 years and are expected to last at least 50 to 200 years.[17][16] 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.[19] 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 and baseboard radiator systems which only require warm temperatures (40°C) to work well (as compared to wall-mounted radiators which normally require 70°C). 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; however, wood or carpet floor coverings dampen this effect because the thermal transfer efficiency of these materials is lower than that of 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. It is usually 7-12°C (45-54°F) at a depth of six meters in locations where heating is needed in winter. Geothermal heat pumps rely on this near constant temperature as a base temperature that is raised or lowered minimally to create a desirable indoor temperature. Because this temperature remains more constant than the air temperature throughout the seasons, geothermal heat pumps perform with far greater efficiency and are stressed less during extreme air temperatures than fueled or electric conventional air conditioners and furnaces. 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, like building orientation, is a natural building technique (bioclimatic building).

The current use of geothermal heat pump technology has resulted in the following emissions reductions:[16]

  • 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:[16]

  • 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:[16]

  • 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.

[edit] Disadvantages

The biggest problem with residential retrofits is the cost of getting heat spread throughout the house. Many existing houses with furnaces use a forced hot air system, which is not ideal for a geothermal heat pump, because the water-to-air units cannot heat the air as much as a furnace can. Therefore the existing duct work must be enlarged to get more air to the required room. For homes with baseboard heat, the cost of the retrofit may make it impractical because water-to-water pumps can only heat the water to 120 degrees Fahrenheit, whereas a boiler can reach 180 degrees or more; this makes it possible to double up on baseboards, but results may vary.

Another problem is that while Geothermal heat pumps can generate enough domestic or process hot water, generally the efficiency of the system declines rapidly after heating water above 120 F, so often another solution is used such as electric, fossil fuels, wood boiler, or solar.

[edit] 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.

The additional costs are returned in energy savings in 5–10 years [20].

The heating efficiency of ground-source and water-source heat pumps is indicated by their coefficient of performance (COP), which is the ratio of heat provided in watts per watt of energy input. Their cooling efficiency is indicated by the Energy Efficiency Ratio (EER), which is the ratio of the heat removed (in btu per hour) to the electricity required (in watts) to run the unit. Efficient pumps must indicate in the ENERGY STAR label a heating COP of 3.3 or greater and an EER of 14.1 or greater [21]

[edit] Installation

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 Geothermal Heat Pump Consortium or the Canadian GeoExchange Coalition for their listing of qualified installers in your area. Installers should be certified and experienced [22].

[edit] References

  1. ^ http://www1.eere.energy.gov/geothermal/heatpumps.html
  2. ^ http://www1.eere.energy.gov/geothermal/geothermal_basics.html
  3. ^ http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12640
  4. ^ http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12640
  5. ^ http://www.lavozdegalicia.es/deza/2008/09/07/0003_7119422.htm (Spanish)
  6. ^ http://www.informedbuilding.com/Geothermal/Main16/Types-of-Geothermal-Ground-Loops/
  7. ^ a b [http://www2.vlaanderen.be/economie/energiesparen/doc/brochure_warmtepomp.pdf Heat pumps in combination with cold/heat storage (see page 28)
  8. ^ Heat and Cold Storage info
  9. ^ diagrams of several types of cold/heat storage system with heatpumps
  10. ^ 2 diagrams of heat/cold storage with heatpumps in summer and winter
  11. ^ Explanation of regular and electrified systems of cold/heat storage with heatpumps
  12. ^ Schematic of similar system of aquifers with fans-regulation
  13. ^ Interseasonal Heat Transfer
  14. ^ thermal bank
  15. ^ Asphalt Solar Collector
  16. ^ a b c d e "Geothermal Heat Pump Consortium, Inc.". Retrieved on 2007-10-19.
  17. ^ a b "Geothermal heat pumps: alternative energy heating and cooling FAQs". Retrieved on 2007-10-19.
  18. ^ Environmental Protection Agency (1993). Space Conditioning: The Next Frontier - Report 430-R-93-004. EPA. 
  19. ^ "Geothermal Heat Pumps". Capital Electric Cooperative. Retrieved on 2008-10-05.
  20. ^ http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12640
  21. ^ http://www.energystar.gov/ia/partners/product_specs/eligibility/geo_heat_pump_elig.pdf
  22. ^ http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12670

[edit] See also

[edit] External links


[edit] Examples

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