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Geothermal Systems

Ground source heating and cooling

Geothermal shilsholepointe

Ground source heating and cooling

A geothermal heat pump or ground source heat pump (GSHP) is a central heating and/or cooling system that pumps heat to or from the ground. It uses the earth as a heat source (in the winter) or a heat sink (in the summer). This design takes advantage of the moderate temperatures in the ground to boost efficiency and reduce the operational costs of heating and cooling systems, and may be combined with solar heating to form a geosolar system with even greater efficiency. Geothermal heat pumps are also known by a variety of other names, including geoexchange, earth-coupled, earth energy or water-source heat pumps. The engineering and scientific communities prefer the terms "geoexchange" or "ground source heat pumps" to avoid confusion with traditional geothermal power, which uses a high temperature heat source to generate electricity.[1] Ground source heat pumps harvest a combination of geothermal power and heat from the sun when heating, but work against these heat sources when used for air conditioning.[2]

Almost everywhere, the upper 10 feet (3.0 m) of Earth's surface maintains a nearly constant temperature between 50 and 60°F (10 and 16°C), depending on latitude.[3] Like a refrigerator or air conditioner, these systems use a heat pump to force the transfer of heat from there. Heat pumps can transfer heat from a cool space to a warm space, against the natural direction of flow, or they can enhance the natural flow of heat from a warm area to a cool one. The core of the heat pump is a loop of refrigerant pumped through a vapor-compression refrigeration cycle that moves heat. Heat pumps are always more efficient at heating than pure electric heaters, even when extracting heat from cold winter air. But unlike an air-source heat pump, which transfers heat to or from the outside air, a ground source heat pump exchanges heat with the ground. This is much more energy-efficient because underground temperatures are more stable than air temperatures through the year. Seasonal variations drop off with depth and disappear below seven meters due to thermal inertia.[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. A ground source heat pump extracts ground heat in the winter (for heating) and transfers heat back into the ground in the summer (for cooling). Some systems are designed to operate in one mode only, heating or cooling, depending on climate.

The geothermal pump systems reach fairly high efficiencies (300%-600%) on the coldest of winter nights, compared to 175%-250% for air-source heat pumps on cool days.[4] Ground source heat pumps (GSHPs) are among the most energy efficient technologies for providing HVAC and water heating.[5][6]

The setup costs are higher than for conventional systems, but the difference is usually returned in energy savings in 3 to 10 years. System life is estimated at 25 years for inside components and 50+ years for the ground loop.[7] As of 2004, there are over a million units installed worldwide providing 12 GW of thermal capacity, with an annual growth rate of 10%.[8]

Differing terms and definitionsEdit

There is a great deal of controversy and confusion with regard to exactly what geothermal heat pumps do. There are several concepts commonly attached to the idea of geothermal:

  • Utilizing geologically hot rocks, which have little relationship to the surface climate and derive their heat from deep in the earth, to run a heat engine which produces electricity. Such a system can be operated only until the rock around the bore cools, then it gradually loses its generating ability. All of these systems are in tectonically or volcanically active areas. Most people are pretty clear that this should be called "geothermal power".
  • Utilizing geologically hot rocks, which have little relationship to the surface climate and derive their heat from deep in the earth, to heat some type of liquid or gas which is pumped up to be used to heat a building. Such a system can be operated only until the rock around the bore cools, then it gradually loses its heating ability. Most of the these systems are in tectonically or volcanically active areas. This is not a perfectly distinct concept, but when this is the primary mechanism of heating it is often called "geothermal heating".
  • Utilizing a heat exchanger with a finite amount of external material to incorporate additional thermal mass to a building. This makes the building change temperature slowly, and allows the inhabitants to go through a time period with less overall temperature variation. This is the main focus of this article, and many terms have been applied. The most common ones appear to be "geothermal heat pump" by laymen and "ground-source heat pump" by experts, but even these are broad, barely understood terms about which there is no consensus.

Builders may try to smooth out the indoor climate over surface temperature variations resulting from the day-night cycle, variations due to short-term weather patterns, or variations due to entire seasons. The amount of thermal mass incorporated is on a spectrum, so one cannot say their system addresses any of these cycles specifically – a system sized for day-night cycling will still help somewhat in a week-long blizzard. Such a system requires power to pump the coolant, but can be operated indefinitely.

To further complicate things, even though most home-sized systems termed "geothermal" operate primarily on the former principle, the thermal mass in such systems is rarely perfectly finite and closed. Groundwater flows through the area, and heat leaks out and warms/cools the surrounding area. True geothermal heat may play a small or large role in such systems.

When trying to explain this subject, experts may go through a series of explanations and divisions.

First, people separate out terms for geothermal electricity generation:

Then, they split out geothermal heating, which is commonly used in tectonically or volcanically active regions:

Then, they explain the traditional concept of a heat pump which uses only inside and outside air:

After that, they try to identify simple systems in which the coolant is air which is pumped directly out of and back into the building, going through a simple hole in the ground:

After that, they remove systems which depend on large quantities of water or wet ground, primarily for cooling:

At this point they may explain the concept of a seasonal thermal store or a thermal mass climate control strategy:

Then, they may try to figure out the size of the system. Is it targeted at a home? A building? Is it a full-scale district heating system?

Then they go into the specifics of the system. First, is the coolant water, and if so is it "open loop" – exposed to groundwater – or "closed loop" – not exposed.

Are other energy sources helping? Is solar absorbed from the house or from a dedicated thermal collector?

After this they concentrate on the specific form factor of the system. Is it a grid of pipes buried 3 feet (0.91 m) underneath the owner's garden? Does it consist of a hundred-foot borehole? A thousand-foot borehole? Dozens of 8-foot boreholes?

Finally they may try to decide what the locals call the system, as identical systems are often called different things in different countries, and in some countries generic terms may be trademarked in others:

  • Geoexchange is a trademarked product in the US, but is a standards coalition in Canada.
  • Earth tubes, Air-earth heat exchangers and "heat exchanger" in general, appear to be primarily used in the UK.

HistoryEdit

The heat pump was described by Lord Kelvin in 1853 and developed by Peter Ritter von Rittinger in 1855. After experimenting with a freezer, Robert C. Webber built the first direct exchange ground-source heat pump in the late 1940s.[9] The first successful commercial project was installed in the Commonwealth Building (Portland, Oregon) in 1946, and has been designated a National Historic Mechanical Engineering Landmark by ASME.[10] The technology became popular in Sweden in the 1970s, and has been growing slowly in worldwide acceptance since then. Open loop systems dominated the market until the development of polybutylene pipe in 1979 made closed loop systems economically viable.[10] As of 2004, there are over a million units installed worldwide providing 12 GW of thermal capacity.[8] Each year, about 80,000 units are installed in the USA (geothermal energy is used in all 50 U.S. states today, with great potential for near-term market growth and savings)[11] and 27,000 in Sweden.[8]

Ground heat exchangerEdit

Pond Loop Close Up

Loop field for a 12-ton system (unusually large for most residential applications)

Heat pumps provide wintertime heating by extracting heat from a source and transferring it to the building. In theory, heat can be extracted from any source, no matter how cold, but a warmer source allows higher efficiency. A ground source heat pump uses the shallow ground as a source of heat, thus taking advantage of its seasonally moderate temperatures.

In the summer, the process can be reversed so the heat pump extracts heat from the building and transfers it to the ground. Transferring heat to a cooler space takes less energy, so the cooling efficiency of the heat pump gain benefits from the lower ground temperatures.

Shallow horizontal heat exchangers experience seasonal temperature cycles due to solar gains and transmission losses to ambient air at ground level. These temperature cycles lag behind the seasons because of thermal inertia, so the heat exchanger can harvest heat deposited by the sun several months earlier. Deep vertical systems rely heavily on migration of heat from surrounding geology, unless they are recharged annually by exhaust heat from air conditioning.

Ground source heat pumps must have a heat exchanger in contact with the ground or groundwater to extract or dissipate heat. This component accounts for a third to a half of the total system cost. Several major design options are available for these, which are classified by fluid and layout. Direct exchange systems circulate refrigerant underground, closed loop systems use a mixture of anti-freeze and water, and open loop systems use natural groundwater.

Direct exchangeEdit

Main article: Direct exchange geothermal heat pump

The Direct exchange geothermal heat pump is the oldest type of geothermal heat pump technology. It is also the simplest and easiest to understand. The ground-coupling is achieved through a single loop circulating refrigerant in direct thermal contact with the ground (as opposed to a combination of a refrigerant loop and a water loop). The refrigerant leaves the heat pump appliance cabinet, circulates through a loop of copper tube buried underground, and exchanges heat with the ground before returning to the pump. The name "direct exchange" refers to heat transfer between the refrigerant and the ground without the use of an intermediate fluid. There is no direct interaction between the fluid and the earth; only heat transfer through the pipe wall. Direct exchange heat pumps are not to be confused with "water-source heat pumps" or "water loop heat pumps" since there is no water in the ground loop. ASHRAE defines the term ground-coupled heat pump to encompass closed loop and direct exchange systems, while excluding open loops.

Direct exchange systems are slightly more efficient and have potentially lower installation costs than closed loop water systems. Copper's high thermal conductivity contributes to the higher efficiency of the system, but heat flow is predominantly limited by the thermal conductivity of the ground, not the pipe. The main reasons for the higher efficiency are the elimination of the water pump (which uses electricity), the elimination of the water heat exchanger (which is a source of heat losses), and the phase change of the refrigerant in the ground itself, allowing a higher temperature gradient between loop and ground resulting in a higher rate of heat transfer.

While they require much more refrigerant and their tubing is more expensive per foot, a direct exchange loop is shorter than a closed water loop for a given capacity. A direct exchange system requires 1/2 to 3/4 the length of tubing and half the diameter of drilled holes, and the drilling or excavation costs are therefore lower. Refrigerant loops are less tolerant of leaks than water loops because gas can leak out through smaller imperfections. This dictates the use of brazed copper tubing, even though the pressures are similar to water loops. The copper loop must be protected from corrosion in acidic soil through the use of a sacrificial anode or cathodic protection.

Closed loopEdit

Most installed systems have two loops on the ground side: the primary refrigerant loop is contained in the appliance cabinet where it exchanges heat with a secondary water loop that is buried underground. The secondary loop is typically made of High-density polyethylene pipe and contains a mixture of water and anti-freeze (propylene glycol, denatured alcohol or methanol). After leaving the internal heat exchanger, the water flows through the secondary loop outside the building to exchange heat with the ground before returning. The secondary loop is placed below the frost line where the temperature is more stable, or preferably submerged in a body of water if available. Systems in wet ground or in water are generally more efficient than drier ground loops since it is less work to move heat in and out of water than solids in sand or soil. If the ground is naturally dry, soaker hoses may be buried with the ground loop to keep it wet.

Pump Pack

An installed liquid pump pack

Closed loop systems need a heat exchanger between the refrigerant loop and the water loop, and pumps in both loops. Some manufacturers have a separate ground loop fluid pump pack, while some integrate the pumping and valving within the heat pump. Expansion tanks and pressure relief valves may be installed on the heated fluid side. Closed loop systems have lower efficiency than direct exchange systems, so they require longer and larger pipe to be placed in the ground, increasing excavation costs.

Closed loop tubing can be installed horizontally as a loop field in trenches or vertically as a series of long U-shapes in wells(see below). 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. A rough approximation of the initial soil temperature is the average daily temperature for the region.

VerticalEdit

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 feet (23–150 m) 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 thermal connection to the surrounding soil or rock to improve the heat transfer. Thermally enhanced grouts are available to improve this 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 depth depends on ground and building characteristics. For illustration, a detached house needing 10 kW (3 ton) of heating capacity might need three boreholes 80 to 110 m (260 to 360 ft) deep.[12] (A ton of heat is 12,000 British thermal units per hour (BTU/h) or 3.5 kilowatts.) 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.

HorizontalEdit

3-ton Slinky Loop

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

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 or slinky coils are placed horizontally inside the same trench. Excavation for horizontal loop fields is about half the cost of vertical drilling, so this is the most common layout used wherever there is adequate land available. For illustration, a detached house needing 10 kW (3 ton) of heating capacity might need 3 loops 120 to 180 m (390 to 590 ft) long of NPS 3/4 (DN 20) or NPS 1.25 (DN 32) polyethylene tubing at a depth of 1 to 2 m (3.3 to 6.6 ft).[13]

As an alternative to trenching, the horizontal loop field may be laid by mini horizontal directional drilling. (mini-HDD) This technique can lay piping under yards, driveways or other structures without disturbing them, with a cost between those of trenching and vertical drilling.

A slinky (also called coiled) closed loop field is a type of horizontal closed loop where the pipes overlay each other (not a recommended method). 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 pump's 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.[14]

PondEdit

Pond Loop Being Sunk

12-ton pond loop system being sunk to the bottom of a pond

A closed pond loop is not common because it depends on proximity to a body of water, where an open loop system is usually preferable. A pond loop may be advantageous where poor water quality precludes an open loop, or where the system heat load is small. A pond loop consists of coils of pipe - similar to a slinky loop - attached to a frame and located at the bottom of an appropriately sized pond or water source.

Open loopEdit

In an open loop system (also called a groundwater heat pump), the secondary loop pumps natural water from a well or body of water into a heat exchanger inside the heat pump. ASHRAE calls open loop systems groundwater heat pumps or surface water heat pumps, depending on the source. Heat is either extracted or added by the primary refrigerant loop, and the water is returned to a separate injection well, irrigation trench, tile field or body of water. The supply and return lines must be placed far enough apart to ensure thermal recharge of the source. Since the water chemistry is not controlled, the appliance may need to be protected from corrosion by using different metals in the heat exchanger and pump. Limescale may foul the system over time and require periodic acid cleaning. Also, as fouling decreases the flow of natural water, it becomes difficult for the heat pump to exchange building heat with the groundwater. If the water contains high levels of salt, minerals or hydrogen sulfide, a closed loop system is usually preferable.

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 more efficient than closed systems because they are better coupled with ground temperatures. Closed loop systems, in comparison, have to transfer heat across extra layers of pipe wall and dirt.

A growing number of jurisdictions have outlawed open-loop systems that drain to the surface because these may drain aquifers or contaminate wells. This forces the use of more environmentally sound injection wells.

Standing column wellEdit

A standing column well system is a specialized type of open 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.[15] 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 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 ground source 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 discharging a small percentage of system flow 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.

Building distributionEdit

Water to Air Heat Pump

Liquid-to-air heat pump

The heat pump is the central unit that becomes the heating and cooling plant for the building. Some models may cover space heating, space cooling, (space heating via conditioned air, hydronic systems and / or radiant heating systems), domestic or pool water preheat (via the desuperheater function, demand hot water, and driveway ice melting all within one appliance with a variety of options with respect to controls, staging and zone control. The heat may be carried to its end use by circulating water or forced air. Almost all types of heat pumps are produced for commercial and residential applications.

Liquid-to-air heat pumps (also called water-to-air) output forced air, and are most commonly used to replace legacy forced air furnaces and central air conditioning systems. There are variations that allow for split systems, high-velocity systems, and ductless systems. Heat pumps cannot achieve as high of a fluid temperature as a conventional furnace, so they require a higher volume flow rate of air to compensate. When retrofitting a residence, the existing duct work may have to be enlarged to reduce the noise from the higher air flow.

Water to Water Heat Pump

Liquid-to-water heat pump

Liquid-to-water heat pumps (also called water-to-water) are hydronic systems that use water to carry heating or cooling through the building. Systems such as radiant underfloor heating, baseboard radiators, conventional cast iron radiators would use a liquid-to-water heat pump. These heat pumps are preferred for pool heating or domestic hot water pre-heat. Heat pumps can only heat water to about 50 °C (122 °F) efficiently, whereas a boiler normally reaches 65–95 °C (149–203 °F). Legacy radiators designed for these higher temperatures may have to be doubled in numbers when retrofitting a home. A hot water tank will still be needed to raise water temperatures above the heat pump's maximum, but pre-heating will save 25-50% of hot water costs.

Ground source heat pumps are especially well matched to underfloor heating and baseboard radiator systems which only require warm temperatures (40°C) to work well. 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 water temperature. 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). Underfloor piping, ceiling or wall radiators can also be used for cooling in dry climates, although the temperature of the circulating water must be above the dew point to ensure that atmospheric humidity does not condense on the radiator.

Combination heat pumps are available that can produce forced air and circulating water simultaneously and individually. These systems are largely being used for houses that have a combination of air and liquid conditioning needs, for example central air conditioning and pool heating.

Seasonal thermal storageEdit

HeatAndColdStorageWithHeatPump

A heat pump in combination with heat and cold storage

Main article: seasonal thermal storage

The efficiency of ground source heat pumps can be improved by using seasonal thermal storage. If heat loss from the ground source is sufficiently low, the heat pumped out of the building in the summer can be retrieved in the winter. Heat storage efficiency increases with scale, so this advantage is most significant in commercial or district heating systems. Geosolar combisystems further augment this efficiency by collecting extra solar energy during the summer (more than is needed for air conditioning) and concentrating it in the store.

Such a system has been used to heat and cool a greenhouse using an aquifer for thermal storage.[16] In summer, the greenhouse is cooled with cold ground water. This heats the water in the aquifer which can become a warm source for heating in winter.[16][17] The combination of cold and heat storage with heat pumps can be combined with water/humidity regulation. These principles are used to provide renewable heat and renewable cooling[18] to all kinds of buildings.

Thermal efficiencyEdit

Main article: thermal efficiency

The net thermal efficiency of a heat pump should take into account the efficiency of electricity generation and transmission, typically about 40%.[8] Since a heat pump moves 3 to 5 times more heat energy than the electric energy it consumes, the total energy output is much greater than the input. This results in net thermal efficiencies greater than 100% for most electricity sources. Traditional combustion furnaces and electric heaters can never exceed 100% efficiency, but heat pumps provide extra energy by extracting it from the ground.

Geothermal heat pumps can reduce energy consumption— and corresponding air pollution emissions—upto 44% compared to air source heat pumps and up to 72% compared to electric resistance heating with standard air-conditioning equipment.[19]

The dependence of net thermal efficiency on the electricity infrastructure tends to be an unnecessary complication for consumers and is not applicable to hydroelectric power, so performance of heat pumps is usually expressed as the ratio of heating output or heat removal to electricity input. Cooling performance is typically expressed in units of BTU/hr/watt as the Energy Efficiency Ratio, (EER) while heating performance is typically reduced to dimensionless units as the Coefficient of Performance. (COP) The conversion factor is 3.41 BTU/hr/watt. Performance is influenced by all components of the installed system, including the soil conditions, the ground-coupled heat exchanger, the heat pump appliance, and the building distribution, but is largely determined by the "lift" between the input temperature and the output temperature.

For the sake of comparing heat pump appliances to each other, independently from other system components, a few standard test conditions have been established by the American Refrigerant Institute (ARI) and more recently by the International Organization for Standardization. Standard ARI 330 ratings were intended for closed loop ground-source heat pumps, and assumes secondary loop water temperatures of 77°F for air conditioning and 32°F for heating. These temperatures are typical of installations in the northern USA. Standard ARI 325 ratings were intended for open loop ground-source heat pumps, and include two sets of ratings for groundwater temperatures of 50°F and 70°F. ARI 325 budgets more electricity for water pumping than ARI 330. Neither of these standards attempt to account for seasonal variations. Standard ARI 870 ratings are intended for direct exchange ground-source heat pumps. ASHRAE transitioned to ISO 13256-1 in 2001, which replaces ARI 320, 325 and 330. The new ISO standard produces slightly higher ratings because it no longer budgets any electricity for water pumps.[1]

Efficient compressors, variable speed compressors and larger heat exchangers all contribute to heat pump efficiency. Residential ground source heat pumps on the market today have standard COPs ranging from 2.4 to 5.0 and EERs ranging from 10.6 to 30.[1][20] To qualify for an Energy Star label, heat pumps must meet certain minimum COP and EER ratings which depend on the ground heat exchanger type. For closed loop systems, the ISO 13256-1 heating COP must be 3.3 or greater and the cooling EER must be 14.1 or greater.[21]

Actual installation conditions may produce better or worse efficiency than the standard test conditions. COP improves with a lower temperature difference between the input and output of the heat pump, so the stability of ground temperatures is important. If the loop field or water pump is undersized, the addition or removal of heat may push the ground temperature beyond standard test conditions, and performance will be degraded. Similarly, an undersized blower may allow the plenum coil to overheat and degrade performance.

Soil without artificial heat addition or subtraction and at depths of several meters or more remains at a relatively constant temperature year round. This temperature equates roughly to the average annual air-temperature of the chosen location, usually 7–12 °C (45–54 °F) at a depth of six meters in the northern USA. Because this temperature remains more constant than the air temperature throughout the seasons, geothermal heat pumps perform with far greater efficiency during extreme air temperatures than air conditioners and air-source heat pumps.

Standards ARI 210 and 240 define Seasonal Energy Efficiency Ratio (SEER) and Heating Seasonal Performance Factors (HSPF) to account for the impact of seasonal variations on air source heat pumps. These numbers are normally not applicable and should not be compared to ground source heat pump ratings. However, Natural Resources Canada has adapted this approach to calculate typical seasonally adjusted HSPFs for ground-source heat pumps in Canada.[12] The NRC HSPFs ranged from 8.7 to 12.8 BTU/hr/watt (2.6 to 3.8 in nondimensional factors, or 255% to 375% seasonal average electricity utilization efficiency) for the most populated regions of Canada. When combined with the thermal efficiency of electricity, this corresponds to net average thermal efficiencies of 100% to 150%.

Environmental impactEdit

The U.S. Environmental Protection Agency (EPA) has called ground source heat pumps the most energy-efficient, environmentally clean, and cost-effective space conditioning systems available.[22] Heat pumps offer significant emission reductions potential, particularly where they are used for both heating and cooling and where the electricity is produced from renewable resources.

Ground-source heat pumps have unsurpassed thermal efficiencies and produce zero emissions locally, but their electricity supply almost always includes components with high greenhouse gas emissions. Their environmental impact therefore depends on the characteristics of the electricity supply. The GHG emissions savings from a heat pump over a conventional furnace can be calculated based on the following formula:[2]

Annual greenhouse gas savings from using a ground source heat pump instead of a high-efficiency furnace in a detached residence
Country Electricity CO2
Emissions Intensity
GHG savings relative to
natural gas heating oil electric heating
Canada 223 ton/GWh[23][24][25] 2.7 ton/yr 5.3 ton/yr 3.4 ton/yr
Russia 351 ton/GWh[23][24] 1.8 ton/yr 4.4 ton/yr 5.4 ton/yr
USA 676 ton/GWh[24] -0.5 ton/yr 2.2 ton/yr 10.3 ton/yr
China 839 ton/GWh[23][24] -1.6 ton/yr 1.0 ton/yr 12.8 ton/yr

GHG\ Savings=HL \left( \frac{FI}{AFUE \times 1000\frac{kg}{ton}}-\frac{EI}{COP \times 3600\frac{sec}{hr}}\right)

  • HL = seasonal heat load ≈ 80 GJ/yr for a modern detached house in the northern USA
  • FI = emissions intensity of fuel = 50 kg(CO2)/GJ for natural gas, 73 for heating oil
  • AFUE = furnace efficiency ≈ 95% for a modern condensing furnace
  • COP = heat pump coefficient of performance ≈ 3.2 seasonally adjusted for northern USA heat pump
  • EI = emissions intensity of electricity ≈ 200-800 ton(CO2)/GWh, depending on region

Ground-source heat pumps always produce less greenhouse gases than air conditioners, oil furnaces, and electric heating, but natural gas furnaces may be competitive depending on the greenhouse gas intensity of the local electricity supply. In countries like Canada and Russia with low emitting electricity infrastructure, a residential heat pump may save 5 tons of carbon dioxide per year relative to an oil furnace, or about as much as taking an average passenger car off the road. But in countries like China or USA that are highly reliant on coal for electricity production, a heat pump may result in 1 or 2 tons more carbon dioxide emissions than a natural gas furnace.

The fluids used in closed loops may be designed to be biodegradable and non-toxic, but the refrigerant used in the heat pump cabinet and in direct exchange loops was, until recently, chlorodifluoromethane, which is an ozone depleting substance.[1] Although harmless while contained, leaks and improper end-of-life disposal contribute to enlarging the ozone hole. This refrigerant is being phased out in favor of ozone-friendly R410A for new construction.

Open loop systems that draw water from a well and drain to the surface may contribute to aquifer depletion, water shortages, groundwater contamination, and subsidence of the soil. A geothermal heating project in Staufen im Breisgau, Germany, is suspected to have caused considerable damage to buildings in the city center. The ground has subsided by up to eight millimeters under the city hall while other areas have been uplifted by a few millimeters.[26]

Ground-source heat pump technology, like building orientation, is a natural building technique (bioclimatic building).

EconomicsEdit

Ground source heat pumps are characterized by high capital costs and low operational costs compared to other HVAC systems. Their overall economic benefit depends primarily on the relative costs of electricity and fuels, which are highly variable over time and across the world. Based on recent prices, ground-source heat pumps currently have lower operational costs than any other conventional heating source almost everywhere in the world. Natural gas is the only fuel with competitive operational costs, and only in a handful of countries where it is exceptionally cheap, or where electricity is exceptionally expensive.[2] In general, a homeowner may save anywhere from 20% to 60% annually on utilities by switching from an ordinary system to a ground-source system.[27][28]

Capital costs and system lifespan have received much less study, and the return on investment is highly variable. One study found the total installed cost for a system with 10 kW (3 ton) thermal capacity for a detached rural residence in the USA averaged $8000–$9000 in 1995 US dollars.[29] More recent studies found an average cost of $14,000 in 2008 US dollars for the same size system.[30][31] The US Department of Energy estimates a price of $7500 on its website, last updated in 2008.[32] Prices over $20,000 are quoted in Canada,[33] with one source placing them in the range of $30,000-$34,000 Canadian dollars.[34] The rapid escalation in system price has been accompanied by rapid improvements in efficiency and reliability. Capital costs are known to benefit from economies of scale, particularly for open loop systems, so they are more cost-effective for larger commercial buildings and harsher climates. The initial cost can be two to five 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. Proper duct system design and mechanical air exchange should be considered in the initial system cost.

Payback period for installing a ground source heat pump in a detached residence
Country Payback period for replacing
natural gas heating oil electric heating
Canada 13 years 3 years 6 years
USA 12 years 5 years 4 years
Germany net loss 8 years 2 years
Notes:


  • Highly variable with energy prices.
  • Government subsidies not included.
  • Climate differences not evaluated.

Capital costs may be offset by substantial subsidies from many governments, for example totaling over $7000 in Ontario for residential systems installed in the 2009 fiscal year. Some electric companies offer special rates to customers who install a ground-source heat pump for heating/cooling their building.[35] 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 ground-source heat pumps at customer residences. They lease the systems to their customers for a monthly fee, at a net overall savings to the customer.

The lifespan of the system is longer than conventional heating and cooling systems. Good data on system lifespan is not yet available because the technology is too recent, but many early systems are still operational today after 25–30 years with routine maintenance. Most loop fields have warranties for 25 to 50 years and are expected to last at least 50 to 200 years.[27][36] Ground-source heat pumps use electricity for heating the house. The higher investment above conventional oil, propane or electric systems may be returned in energy savings in 2–10 years for residential systems in the USA.[7][28][36] If compared to natural gas systems, the payback period can be much longer or non-existent. The payback period for larger commercial systems in the USA is 1–5 years, even when compared to natural gas.[28]

Ground source heat pumps are recognized as one of the most efficient heating and cooling systems on the market. They are often the second-most cost effective solution in extreme climates, (after co-generation), despite reductions in thermal efficiency due to ground temperature. (The ground source is warmer in climates that need strong air conditioning, and cooler in climates that need strong heating.)

Commercial systems maintenance costs in the USA have historically been between $0.11 to $0.22 per m2 per year in 1996 dollars, much less than the average $0.54 per m2 per year for conventional HVAC systems.[10]

Governments that promote renewable energy will likely offer incentives for the consumer (residential), or industrial markets. For example, in the United States, incentives are offered both on the state and federal levels of government.[37]

InstallationEdit

Because of the technical knowledge and equipment needed to properly install the piping, a GSHP system installation requires a professional's services. The International Ground Source Heat Pump Association (IGSHPA), Geothermal Heat Pump Consortium and the Canadian GeoExchange Coalition maintain listings of qualified installers in the USA and Canada.[38]

See alsoEdit

ReferencesEdit

  1. 1.0 1.1 1.2 1.3 Rafferty, Kevin (April 1997). "An Information Survival Kit for the Prospective Residential Geothermal Heat Pump Owner". Geo-Heat Centre Quarterly Bulletin (Klmath Falls, Oregon: Oregon Institute of Technology) 18 (2): pp. 1–11. ISSN 0276-1084. http://geoheat.oit.edu/bulletin/bull18-2/art1.pdf. Retrieved 2009-03-21  The author issued an updated version of this article in February 2001.
  2. 2.0 2.1 2.2 2.3 Hanova, J; Dowlatabadi, H (9 November 2007). "Strategic GHG reduction through the use of ground source heat pump technology". Environmental Research Letters (UK: IOP Publishing) 2: pp. 044001 8pp. doi:10.1088/1748-9326/2/4/044001. ISSN 1748-9326. http://www.iop.org/EJ/article/1748-9326/2/4/044001/erl7_4_044001.pdf?request-id=dd247af1-1eb0-4c8d-a20b-426d37d8ee5f. Retrieved 2009-03-22 
  3. http://www1.eere.energy.gov/geothermal/geothermal_basics.html
  4. http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12640
  5. http://apps1.eere.energy.gov/geothermal/projects/projects.cfm/ProjectID=108
  6. http://www.cres-energy.org/techbasics/geothermal_div1.html
  7. 7.0 7.1 "Energy Savers: Geothermal Heat Pumps". Apps1.eere.energy.gov. 2009-02-24. http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12640. Retrieved 2009-06-08. 
  8. 8.0 8.1 8.2 8.3 Lund, J.; Sanner, B.; Rybach, L.; Curtis, R.; Hellström, G. (September 2004). "Geothermal (Ground Source) Heat Pumps, A World Overview". Geo-Heat Centre Quarterly Bulletin (Klmath Falls, Oregon: Oregon Institute of Technology) 25 (3): pp. 1–10. ISSN 0276-1084. http://geoheat.oit.edu/bulletin/bull25-3/art1.pdf. Retrieved 2009-03-21 
  9. "History". About Us. International Ground Source Heat Pump Association. http://www.igshpa.okstate.edu/about/about_us.htm. Retrieved 2009-03-24. 
  10. 10.0 10.1 10.2 Bloomquist, R. Gordon (December 1999). "Geothermal Heat Pumps, Four Plus Decades of Experience". Geo-Heat Centre Quarterly Bulletin (Klmath Falls, Oregon: Oregon Institute of Technology) 20 (4): pp. 13–18. ISSN 0276-1084. http://geoheat.oit.edu/bulletin/bull20-4/art3.pdf. Retrieved 2009-03-21 
  11. http://www1.eere.energy.gov/geothermal/pdfs/40665.pdf
  12. 12.0 12.1 "Ground Source Heat Pumps (Earth Energy Systems)". Heating and Cooling with a Heat Pump. Natural Resources Canada, Office of Energy Efficiency. http://oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/gsheatpumps.cfm. Retrieved 2009-03-24.  Note: contrary to air-source conventions, the NRC's HSPF numbers are in units of BTU/hr/watt. Divide these numbers by 3.41 BTU/hr/watt to arrive at non-dimensional units comparable to ground-source COPs and air-source HSPF.
  13. Chiasson, A.D. (1999). Advances in modeling of ground source heat pump systems. Oklahoma State University. http://www.solis.pl/index.php/content/download/375/1291/file/GROUND%20SOURCE%20HEAT.pdf. Retrieved 2009-04-23 
  14. "Geothermal Ground Loops". Informed Building. http://www.informedbuilding.com/Geothermal/Main16/Types-of-Geothermal-Ground-Loops/. Retrieved 2009-06-08. 
  15. Orio, Carl D.; Johnson, Carl N.; Rees, Simon J.; Chiasson, A.; Deng, Zheng; Spitler, Jeffrey D. (2004). "A Survey of Standing Column Well Installations in North America". ASHRAE Transactions (ASHRAE) 11 (4): pp. 637–655. http://www.hvac.okstate.edu/research/Documents/Orio_Johnson_Rees_Chiasson_Deng_Spitler_05.pdf. Retrieved 2009-03-25 
  16. 16.0 16.1 Van Passel, Willy; Sourbron, Maarten; Verplaetsen, Filip; Leroy, Luc; Somers, Yvan; Verheyden, Johan; Coupé, Koen. Organisatie voor Duurzame Energie Vlaanderen. ed (PDF). Warmtepompen voor woningverwarming. p. 28. http://www2.vlaanderen.be/economie/energiesparen/doc/brochure_warmtepomp.pdf. Retrieved 2009-03-23. 
  17. Schematic of similar system of aquifers with fans-regulation
  18. Capture, storage and release of Renewable Cooling.
  19. http://www.nrel.gov/docs/legosti/fy98/24782.pdf
  20. "AHRI Directory of water-to-air geothermal heat pumps". http://www.ahridirectory.org/ahriDirectory/pages/wbahp/defaultSearch.aspx. 
  21. "Energy Star Program Requirements for Geothermal Heat PUmps". Partner Commitments. Energy Star. http://www.energystar.gov/ia/partners/product_specs/eligibility/geo_heat_pump_elig.pdf. Retrieved 2009-03-24. 
  22. Environmental Protection Agency (1993). Space Conditioning: The Next Frontier - Report 430-R-93-004. EPA. 
  23. 23.0 23.1 23.2 European Environment Agency (2008). Energy and environment report 2008. EEA Report. No 6/2008. Luxemburg: Office for Official Publications of the European Communities. p. p83. doi:10.2800/10548. ISBN 978-92-9167-980-5. http://www.eea.europa.eu/publications/eea_report_2008_6. Retrieved 2009-03-22 
  24. 24.0 24.1 24.2 24.3 Energy Information Administration, US Department of Energy (2007). "Voluntary Reporting of Greenhouse Gases, Electricity Emission Factors". http://www.eia.doe.gov/oiaf/1605/pdf/Appendix%20F_r071023.pdf. Retrieved 2009-03-22 
  25. "annex 9". National Inventory Report 1990–2006:Greenhouse Gas Sources and Sinks in Canada. Government of Canada. May 2008. ISBN 978-1-100-11176-6. http://www.ec.gc.ca/pdb/ghg/inventory_report/2004_report/ta9_7_e.cfm. 
  26. Waffel, Mark (March 19, 2008). "Buildings Crack Up as Black Forest Town Subsides". Spiegel Online International (Der Spiegel). http://www.spiegel.de/international/zeitgeist/0,1518,541296,00.html. Retrieved 2009-02-24. 
  27. 27.0 27.1 "Geothermal Heat Pump Consortium, Inc.". http://geoexchange.us/. Retrieved 2007-10-19. 
  28. 28.0 28.1 28.2 Lienau, Paul J.; Boyd, Tonya L.; Rogers, Robert L. (April 1995). Ground-Source Heat Pump Case Studies and Utility Programs. Klamath Falls, OR: Geo-Heat Center, Oregon Institute of Technology. http://geoheat.oit.edu/pdf/hp1.pdf. Retrieved 2009-03-26 
  29. Kavanaugh, Steve; Gilbreath, Christopher (December 1995). Joseph Kilpatrick. ed (PDF). Cost Containment for Ground-Source Heat Pumps (final report ed.). http://geoheat.oit.edu/pdf/tp72.pdf. Retrieved 2009-03-24 
  30. Cummings, Paul (June 2008) (PDF). Indiana Residential Geothermal Heat Pump Rebate, Program Review. Indiana Office of Energy and Defense Development. http://www.in.gov/oed/files/GHPProgramreport.pdf. Retrieved 2009-03-24 
  31. Template:Cite doi
  32. "Energy Savers: Selecting and Installing a Geothermal Heat Pump System". Energysavers.gov. 2008-12-30. http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12670. Retrieved 2009-06-08. 
  33. Example costs in Canada
  34. [|RETscreen International], ed (2005). "Ground-Source Heat Pump Project Analysis". Clean Energy Project Analysis: RETscreen Engineering & Cases Textbook. Natural Resources Canada. Catalogue no.: M39-110/2005E-PDF. ISBN 0-662-39150-0. http://www.retscreen.net/ang/textbook_gshp.html. Retrieved 2009-04-20 
  35. "Geothermal Heat Pumps". Capital Electric Cooperative. http://www.capitalelec.com/Energy_Efficiency/ground_source/index.html. Retrieved 2008-10-05. 
  36. 36.0 36.1 "Geothermal heat pumps: alternative energy heating and cooling FAQs". http://www.econar.com/faq.htm. Retrieved 2007-10-19. 
  37. http://www.dsireusa.org/searchby/searchtechnology.cfm?&CurrentPageID=2&EE=1&RE=1
  38. "Energy Savers: Selecting and Installing a Geothermal Heat Pump System". Apps1.eere.energy.gov. 2008-12-30. http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12670. Retrieved 2009-06-08. 

External linksEdit

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