heat pump

Dictionary: heat pump

n.
A device that warms or cools a building by transferring heat from a relatively low-temperature reservoir to one at a higher temperature.

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Britannica Concise Encyclopedia: heat pump

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Device for transferring heat from a substance or space at one temperature to another at a higher temperature. It consists of a compressor, a condenser, a throttle or expansion valve, an evaporator, and a working fluid (refrigerant). The compressor delivers vapourized refrigerant to the condenser in the space to be heated. There, cooler air condenses the refrigerant and becomes heated during the process. The liquid refrigerant then enters the throttle valve and expands, coming out as a liquid-vapour mixture at a lower temperature and pressure. It then enters the evaporator, where the liquid is evaporated by contact with the warmer space. The vapour then passes to the compressor and the cycle is repeated. A heat pump is a reversible system and is commonly used both to heat and to cool buildings. It operates on the same thermodynamic principles as refrigeration.

For more information on heat pump, visit Britannica.com.

How Products are Made: How is a heat pump made?

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Background

As a result of society's increasing concern for ecological and environmental issues, the demand for more efficient ways to utilize heat and energy is rising. The heat pump industry uses technological advances such as year-round space heating to displace heat energy to a more useful location and purpose. This concept is accomplished by providing localized or redirected heat, while exchanging cool air with heated air.

The principles of heat pumps are actually the reverse of the technological and thermodynamic principles of an air conditioner unit. The majority of heat pumps give the added benefit of providing both heating in the winter and cooling in the summer. This can be accomplished simply by reversing the flow of the working fluid circulating through the coils. The heat pump is an entire thermodynamic system whereby a liquid and/or gas medium is pumped through an assembly where it changes phases as a result of altering pressure. Although relatively costly to setup, the heat pump system provides a more economical and efficient way to control temperatures and reuse existing heat energy.

Raw Materials

The manufacturing of heat pumps involves the use of large iron castings with stainless steel components and aluminum tubing. The castings, used in the pump and motor, will often have small amounts of nickel, molybdenum, and magnesium to improve the mechanical and corrosion-resisting characteristics of the casting. In smaller heat pumps, some components require the use of alloy steel to reduce weight. Depending on what type of working fluid is used (ammonia, water, or chlorofluorocarbons), the piping in the heat pump system may require corrosion resistant stainless steel or aluminum. In systems where consistency of thermodynamic properties are more critical, copper tubing may improve efficiency. Housing most of the components of the heat pump, the encasements are fabricated out of mild carbon sheet steel. The rest of the piping, fittings, valves, and couplings are stainless steel.

All heat pumps require a working fluid to transfer excess energy from one heat source to another. Traditionally, chlorofluorocarbons (CFCs) have been used as working fluids because of their superior thermodynamic properties. Because of the harmful effects CFCs are now known to have on the environment, they have been gradually phased out of production. Instead, water, hydrocarbons, and ammonia are frequently utilized in heat pump systems despite their lack of efficiency in some heat pump designs.

Design

Heat pumps all have the same basic components. These components consist of a pump, a condenser, an evaporator, and an expansion valve. Despite the relative similarities of these components, heat pump designs vary greatly depending on the specific application of the pump. The two major designs, vapor compression and absorption, utilize different thermodynamic principles, yet both include similar components and provide similar system efficiencies.

Heat pumps demonstrate remarkable versatility in providing both air conditioning and heating in the same system by simply reversing the direction of flow of the working fluid. In this regard, heat pumps eliminate the need for dual systems in order to maintain a desired temperature. However, this will be costly as it requires a system that is able to pump in both directions. In extremely adverse climates, heat pumps lose some of their effectiveness and may require an additional heat source. This supplemental heat can come from geothermally heated water or electric heaters.

The typical heat pump operation uses the working fluid to receive heat from a source positioned close to the evaporator. At the evaporator, the fluid vaporizes into a low pressure vapor. Upon entering the pump, the vapor is compressed to high pressure and enters a condenser which returns the vapor to a liquid and ultimately gives off its stored heat to the desired source. An expansion valve then allows the system to return to its low pressure liquid state, and the cycle begins again.

The Manufacturing
Process

The pump is usually procured as a finished unit and installed into the system by integrating it with coupling and piping components. Designed for the specific size and fluid requirements of the system, the pump may be shipped, depending upon its size, directly to the installation site. This usually occurs with large commercial heat pumps supplying heat and/or refrigeration to office buildings. Smaller residential models may have the pump installed into an assembly that includes the condenser, evaporator, and various piping. These units, encased in a sheet metal box, will be comprised of various subassemblies for the condenser and evaporator in order to bolt every component to the box or to one another. Some of the brackets used will form the base of the unit where the pump will be bolted down to a metal pan and connected to an AC motor.

Encasements

Assembled from several different sheets of metal, encasement units are sheared to size in a shear press. After they are cut to the proper dimensions, small assembly holes are punched in the metal using a Computer Numerically Controlled (CNC) punch press. These punch presses have either a moveable table to move the sheet metal or a moveable die which is able to punch holes in different spots of the metal. Punch presses are often directed where to punch by a computer-aided design (CAD) program. Different shaped punching tools are stored within the machine, providing it with the ability to punch all of the necessary holes by simply changing the computer program.
After punching, the sheet will move to a Numerically Controlled (NC) press brake, where it will be bent in different shapes and configurations. The press brake bends the metal into many different shapes by using dies or tooling. Unlike the CNC punch press, the press brake will require a manual change in tooling to perform a different bend. The sheet is then ready to be welded, riveted, or bolted to the other sheets and brackets. Once assembled, these sheets provide most of the stability of stand-alone units.

Condenser and evaporator

The condenser and evaporator are made of many small, thin copper or aluminum tubes, which are bent around curved dies by tube bending machines. NC tube bending machines will be programmed to provide the same exact bend on each of the tubes, allowing them to be stacked one on top of the other. These tubes will then be attached to plates or fins through which the tubes will pass and be joined through tube expansion or joint welding. This creates a tightly sealed system. The tube and plate assembly will act as a heat exchanger by allowing the working fluid to pass through the system inside the tubes, while giving off the heat in the condenser to another fluid medium passing between the plates and acquiring the heat given off through the tubes.
In order to provide strength or connectivity to the components, small brackets are punched out of mild carbon steel. The brackets are usually punched out of steel coil that is continuously fed first through a decoiler. Once it is decoiled, it is sheared, bent, and formed in one continuous process. This is done with a progressive die configuration, where the bracket remains attached to the coil as it moves from station to station. Each station adds something to the bracket, either a hole or notch, and sends it to the next station, until finally it is sheared from the coil. This process may be outsourced to vendors who specialize in progressive die or transfer press operations and can provide better cost control.

Tubing

More tubing is fabricated and bent to provide the rest of the piping needed to connect the pump with the condenser and evaporator. Various fittings and connection components are utilized. The expansion valve, which is contained within some of the piping lines, is another component purchased as a whole unit. The expansion valve is a designed fitting that provides for the expansion of the working fluid and connection of smaller diameter tubing with larger diameter tubing. In small residential units, the valve is contained within the main box, while in larger commercial units, it may be installed on site in the piping system.

Painting/coating

Components, subassemblies, brackets, and/or plates are painted or powder coated for corrosion resistance. Before painting, however, some parts are treated with a special solvent to remove any grease or oil left from the manufacturing operations. This is usually done by submersing the parts in large tanks filled with solvent and then drying them in a special oven. Some parts, which are specially coated with zinc, nickel, or chrome, will be fed through an acid bath before being dipped into tanks of coating solution. Once cleaned, the parts are manually loaded onto trays or hung on specially designed racks and fed into a paint booth. The paint is applied with a pressurized paint dispenser that will spray paint into each crevice.

Packaging

After passing vigorous inspections, the heat pump is sent to packaging, where the system will be boxed and shipped to the installation site.

Installation

Generally, heat pumps will be installed at the construction site. The compressor and evaporator will be constructed of massive 3 in (7.5 cm) diameter tubing and have larger chambers, where the working fluid will change phases. The pump itself will be bolted to a concrete pad and connected with a large DC motor or natural gas generator. The fittings and valves will be shipped and installed into the piping system, while supported by brackets and braces anchored to existing walls. These installations exhibit significant engineering challenges and often require cooperation between the contractor and heat pump manufacturer.

Quality Control

Each component that is procured from an outside supplier will usually be inspected for dimensional compliance before being assembled. Other components will be checked during their fabrication to ensure quality. The final assembly will then be tested by filling it with the appropriate working fluid and connecting the system to a power source to turn the pump. By measuring, with transducers or switches, the temperature and pressure levels of the fluid in different stages, the final system can be checked against predetermined criteria.

The Future

With the rising energy costs, the demand for the efficient heat pump will increase. The high initial cost will be returned in full as overall energy use decreases. The versatile heat pump will benefit organizations that aim to increase their exposure to new technological developments. As technology improves, the heat pump will ultimately produce more cost effective heating and cooling. Product development will generate competition among industries, causing the high manufacturing costs to decrease. Working fluid technology will continue to expand due to several experimental studies designed to meet future environmental concerns.

Where to Learn More

Other

"HydroHeat Geothermal Systems." October 4,1996. http://www.njhpc.org/njh_uses.html (July 9, 1997).

"Heat Pump Working Fluids." October 1996. http://www.heatpumpcentre.org/hpcwrkf.htm (July 9, 1997).

"Heat Pump Technology." October 1995. http://www.heatpumpcentre.org/hpctek.htm (July 9, 1997).

"Heat Pumps in Industry." October 1996. http://www.heatpumpcentre.org/hpciapp.htm#industrysystems (July 9, 1997).

[Article by: Jason Rude]

Sci-Tech Encyclopedia: Heat pump

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The thermodynamic counterpart of the heat engine. A heat pump raises the temperature level of heat by means of work input. In its usual form a compressor takes refrigerant vapor from a low-pressure, low-temperature evaporator and delivers it at high pressure and temperature to a condenser (see illustration). The pump cycle is identical with the customary vapor-compression refrigeration system. See also Refrigeration cycle.

Basic flow diagram of heat pump with motor-driven compressor. For summer cooling, condenser is outdoors and evaporator indoors; for winter heating, condenser is indoors and evaporator outdoors.

This dual purpose is accomplished, in effect, by placing the low-temperature evaporator in the conditioned space during the summer and the high-temperature condenser in the same space during the winter. Thus, if 70°F (21°C) is to be maintained in the conditioned space regardless of the season, this would be the theoretical temperature of the evaporating coil in summer and of the condensing coil in winter. The actual temperatures on the refrigerant side of these coils would need to be below 70°F in summer and above 70°F in winter to permit the necessary transfer of heat through the coil surfaces. If the average outside temperatures are 100°F (38°C) in summer and 40°F (40°C) in winter, the heat pump serves to raise or lower the temperature 30° (17°C) and to deliver the heat or cold as required.

The heat pump is also used for a wide assortment of industrial and process applications such as low-temperature heating, evaporation, concentration, and distillation.

Architecture: heat pump

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A device that transfers heat from a cooler reservoir to a hotter reservoir by means of a heat exchanger, requiring the expenditure of mechanical energy in the process; used in an air conditioner whose cooling cycle can be reversed so that it can function as a heater.

heat pump

Wikipedia: Heat pump

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A heat pump is a machine or device that moves heat from one location (the 'source') to another location (the 'sink' or 'heat sink') using mechanical work. Most heat pump technology moves heat from a low temperature heat source to a higher temperature heat sink.^[1] Common examples are food refrigerators and freezers, air conditioners, and reversible-cycle heat pumps for providing thermal comfort.

Heat pumps can be thought of as a heat engine which is operating in reverse. One common type of heat pump works by exploiting the physical properties of an evaporating and condensing fluid known as a refrigerant. In heating, ventilation, and air conditioning (HVAC) applications, a heat pump normally refers to a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of heat flow may be reversed. Most commonly, heat pumps draw heat from the air or from the ground. Some air-source heat pumps do not work as well when temperatures fall below around −5°C(23°F).

1 Operation
2 Refrigerants
3 Efficiency
- 3.1 CoP and Lift
4 Types of heat pumps
5 Heat sources
6 History
7 See also
8 References
9 External links

Operation

Main article: Heat pump and refrigeration cycle

According to the second law of thermodynamics heat cannot spontaneously flow from a colder location to a hotter area; work is required to achieve this.^[2] Heat pumps differ in how they apply this work to move heat, but they can essentially be thought of as heat engines operating in reverse. A heat engine allows energy to flow from a hot 'source' to a cold heat 'sink', extracting a fraction of it as work in the process. Conversely, a heat pump requires work to move thermal energy from a cold source to a warmer heat sink.

Since the heat pump uses a certain amount of work to move the heat, the amount of energy deposited at the hot side is greater than the energy taken from the cold side by an amount equal to the work required. Conversely, for a heat engine, the amount of energy taken from the hot side is greater than the amount of energy deposited in the cold heat sink since some of the heat has been converted to work.

One common type of heat pump works by exploiting the physical properties of an evaporating and condensing fluid known as a refrigerant.

A simple stylized diagram of a heat pump's vapor-compression refrigeration cycle: 1) condenser, 2) expansion valve, 3) evaporator, 4) compressor.

The working fluid, in its gaseous state, is pressurized and circulated through the system by a compressor. On the discharge side of the compressor, the now hot and highly pressurized gas is cooled in a heat exchanger, called a condenser, until it condenses into a high pressure, moderate temperature liquid. The condensed refrigerant then passes through a pressure-lowering device like an expansion valve, capillary tube, or possibly a work-extracting device such as a turbine. This device then passes the low pressure, (almost) liquid refrigerant to another heat exchanger, the evaporator where the refrigerant evaporates into a gas via heat absorption. The refrigerant then returns to the compressor and the cycle is repeated.

In such a system it is essential that the refrigerant reach a sufficiently high temperature when compressed, since the second law of thermodynamics prevents heat from flowing from a cold fluid to a hot heat sink. Practically, this means the refrigerant must reach a temperature greater than the ambient around the high-temperature heat exchanger. Similarly, the fluid must reach a sufficiently low temperature when allowed to expand, or heat cannot flow from the cold region into the fluid, i.e. the fluid must be colder than the ambient around the cold-temperature heat exchanger. In particular, the pressure difference must be great enough for the fluid to condense at the hot side and still evaporate in the lower pressure region at the cold side. The greater the temperature difference, the greater the required pressure difference, and consequently the more energy needed to compress the fluid. Thus as with all heat pumps, the energy efficiency (amount of heat moved per unit of input work required) decreases with increasing temperature difference.

Due to the variations required in temperatures and pressures, many different refrigerants are available. Refrigerators, air conditioners, and some heating systems are common applications that use this technology.

A HVAC heat pump system

In HVAC applications, a heat pump normally refers to a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of heat flow may be reversed. The reversing valve switches the direction of refrigerant through the cycle and therefore the heat pump may deliver either heating or cooling to a building. In the cooler climates the default setting of the reversing valve is heating. The default setting in warmer climates is cooling. Because the two heat exchangers, the condenser and evaporator, must swap functions, they are optimized to perform adequately in both modes. As such, the efficiency of a reversible heat pump is typically slightly less than two separately-optimized machines.

In plumbing applications, a heat pump is sometimes used to heat or preheat water for swimming pools or domestic water heaters.

In somewhat rare applications, both the heat extraction and addition capabilities of a single heat pump can be useful, and typically results in very effective use of the input energy. For example, when an air cooling need can be matched to a water heating load, a single heat pump can serve two useful purposes. Unfortunately, these situations are rare because the demand profiles for heating and cooling are often significantly different.

Refrigerants

Until the 1990s, the refrigerants were often chlorofluorocarbons such as R-12 (dichlorodifluoromethane), one in a class of several refrigerants using the brand name Freon, a trademark of DuPont. Its manufacture was discontinued in 1995 because of the damage that CFCs cause to the ozone layer if released into the atmosphere. One widely-adopted replacement refrigerant is the hydrofluorocarbon (HFC) known as R-134a (1,1,1,2-tetrafluoroethane). R-134a is not as efficient as the R-12 it replaced (in automotive applications) and therefore, more energy is required to operate systems utilizing R-134a than those using R-12. Other substances such as liquid R-717 ammonia are widely used in large-scale systems, or occasionally the less corrosive but more flammable propane or butane, can also be used.

Since 2001, carbon dioxide, R-744, has increasingly been used, utilizing the transcritical cycle. In residential and commercial applications, the hydrochlorofluorocarbon (HCFC) R-22 is still widely used, however, HFC R-410A does not deplete the ozone layer and is being used more frequently. Hydrogen, helium, nitrogen, or plain air is used in the Stirling cycle, providing the maximum number of options in environmentally friendly gases. More recent refrigerators are now exploiting the R600A which is isobutane, and does not deplete the ozone and is friendly to the environment.

Efficiency

When comparing the performance of heat pumps, it is best to avoid the word "efficiency" which has a very specific thermodynamic definition. The term coefficient of performance (COP) is used to describe the ratio of useful heat movement to work input. Most vapor-compression heat pumps utilize electrically powered motors for their work input. However, in most vehicle applications, shaft work, via their internal combustion engines, provide the needed work.

When used for heating a building on a mild day, a typical air-source heat pump has a COP of 3 - 4, whereas a typical electric resistance heater has a COP of 1.0. That is, one joule of electrical energy will cause a resistance heater to produce one joule of useful heat, while under ideal conditions, one joule of electrical energy can cause a heat pump to move much more than one joule of heat from a cooler place to a warmer place.

Note that when there is a wide temperature differential, e.g., when an air-source heat pump is used to heat a house on a very cold winter day, it takes more work to move the same amount of heat indoors than on a mild day. Ultimately, due to Carnot efficiency limits, the heat pump's performance will approach 1.0 as the outdoor-to-indoor temperature difference increases. This typically occurs around −18 °C (0 °F) outdoor temperature for air source heat pumps. Also, as the heat pump takes heat out of the air, some moisture in the outdoor air may condense and possibly freeze on the outdoor heat exchanger. The system must periodically melt this ice. In other words, when it is extremely cold outside, it is simpler, and wears the machine less, to heat using an electric-resistance heater than to strain an air-source heat pump. (Geothermal heat pumps are dependent upon the temperature underground, which is "mild" all year round. Their COP is therefore always in the range of 3.5-4.0).

In cooling mode a heat pump's operating performance is described as its energy efficiency ratio (EER) or seasonal energy efficiency ratio (SEER), and both measures have units of BTU/(h·W). A larger EER number indicates better performance. The manufacturer's literature should provide both a COP to describe performance in heating mode and an EER or SEER to describe performance in cooling mode. Actual performance varies, however, and depends on many factors such as installation, temperature differences, site elevation, and maintenance.

Heat pumps are more effective for heating than for cooling if the temperature difference is held equal. This is because the compressor's input energy is largely converted to useful heat when in heating mode, and is discharged along with the moved heat via the condenser. But for cooling, the condenser is normally outdoors, and the compressor's dissipated work is rejected rather than put to a useful purpose.

For the same reason, opening a food refrigerator or freezer heats up the kitchen rather than cooling it because its refrigeration cycle rejects heat to the indoor air. This heat includes the compressor's dissipated work as well as the heat removed from the inside of the appliance.

The COP for a heat pump in a heating or cooling application, with steady-state operation, is:

$COP_{\mathrm{heating}} = \frac{\Delta Q_{\mathrm{hot}}}{\Delta A} \leq \frac{T_{\mathrm{hot}}}{T_{\mathrm{hot}}-T_{\mathrm{cool}}}$

$COP_{\mathrm{cooling}} = \frac{\Delta Q_{\mathrm{cool}}}{\Delta A} \leq \frac{T_{\mathrm{cool}}}{T_{\mathrm{hot}}-T_{\mathrm{cool}}}$

where

$Δ Q c o o l$ is the amount of heat extracted from a cold reservoir at temperature $T c o o l$ ,
$Δ Q h o t$ is the amount of heat delivered to a hot reservoir at temperature $T h o t$ ,
$Δ A$ is the compressor's dissipated work.
All temperatures are absolute temperatures usually measured in kelvin (K).

CoP and Lift

The CoP increases as the temperature difference, or "Lift", decreases between heat source and destination. The CoP can be maximised at design time by choosing a heating system requiring only a low final water temperature (e.g. underfloor heating), and by choosing a heat source with a high average temperature (e.g. the ground). Domestic Hot Water (DHW) and radiators require high water temperatures, affecting the choice of heat pump technology.

Pump type and source	Typical use case	CoP variation with Output Temperature
Pump type and source	Typical use case	35°C (e.g. heated screed floor)	45°C (e.g. heated screed floor)	55°C (e.g. heated timber floor)	65°C (e.g. radiator or DHW)	75°C (e.g. radiator & DHW)	85°C (e.g. radiator & DHW)
High Efficiency air source heat pump (ASHP). Air at −20°C^[3]		2.2	2.0	—	—	—	—
Two Stage ASHP air at −20°C^[4]	Low source temp.	2.4	2.2	1.9	—	—	—
High Efficiency ASHP air at 0°C^[3]	Low output temp.	3.8	2.8	2.2	2.0	—	—
Prototype Transcritical CO₂ (R744) Heat Pump with Tripartite Gas Cooler, source at 0°C^[5]	High output temp.	3.3	—	—	4.2	—	3.0
Ground source heat pump (GSHP). Water at 0°C^[3]		5.0	3.7	2.9	2.4	—	—
GSHP ground at 10°C^[3]	Low output temp.	7.2	5.0	3.7	2.9	2.4	—
Theoretical Carnot cycle limit, source −20°C		5.6	4.9	4.4	4.0	3.7	3.4
Theoretical Carnot cycle limit, source 0°C		8.8	7.1	6.0	5.2	4.6	4.2
Theoretical Lorentz Cycle limit (CO₂ pump), return fluid 25°C, source 0°C^[5]		10.1	8.8	7.9	7.1	6.5	6.1
Theoretical Carnot cycle limit, source 10°C		12.3	9.1	7.3	6.1	5.4	4.8

Types of heat pumps

The two main types of heat pumps are compression heat pumps and absorption heat pumps. Compression heat pumps always operate on mechanical energy (through electricity), while absorption heat pumps may also run on heat as an energy source (through electricity or burnable fuels).^[6]

A number of sources have been used for the heat source for heating private and communal buildings ^[7].

air source heat pump (extracts heat from outside air)
- air–air heat pump (transfers heat to inside air)
- air–water heat pump (transfers heat to a tank of water)
geothermal heat pump (extracts heat from the ground or similar sources)
- geothermal–air heat pump (transfers heat to inside air)
  - ground–air heat pump (ground as a source of heat)
  - rock–air heat pump (rock as a source of heat)
  - water–air heat pump (body of water as a source of heat)
- geothermal–water heat pump (transfers heat to a tank of water)
  - ground–water heat pump (ground as a source of heat)
  - rock–water heat pump (rock as a source of heat)
  - water–water heat pump (body of water as a source of heat)

Heat sources

Most commonly, heat pumps draw heat from the air (outside or inside air) or from the ground (groundwater or soil) ^[8]. The heat drawn from the ground is in most cases stored solar heat, and it should not be confused with geothermal heat, though the latter will contribute in some small measure to all heat in the ground. Other heat sources include water; nearby streams and other natural water bodies have been used, and sometimes domestic waste water which is often warmer than the ambient temperature.

Air-source heat pumps

Air source heat pumps are relatively easy (and inexpensive) to install and have therefore historically been the most widely used heat pump type. However, they suffer limitations due to their use of the outside air as a heat source or sink. The higher temperature differential during periods of extreme cold or heat leads to declining efficiency, as explained above. In mild weather, COP may be around 4.0, while at temperatures below around −8°C (17°F) an air-source heat pump can achieve a COP of 2.5 or better, which is considerably more than the COP that may be achieved by conventional heating systems. The average COP over seasonal variation is typically 2.5-2.8,^[9] and high efficiency model in Japan over 6.0(2.8 kW) written in the IPCC 4th Working Group III report chapter 6 ^[10].

Ground source heat pumps

Ground source heat pumps, which are also confusingly referred to as Geothermal heat pumps, typically have higher efficiencies than air-source heat pumps. This is because they draw heat from the ground or groundwater which is at a relatively constant temperature all year round below a depth of about eight feet (2.5 m). This means that the temperature differential is lower, leading to higher efficiency. Ground-source heat pumps typically have COPs of 3.5-4.0 at the beginning of the heating season, with lower COPs as heat is drawn from the ground. The tradeoff for this improved performance is that a ground-source heat pump is more expensive to install due to the need for the digging of wells or trenches in which to place the pipes that carry the heat exchange fluid. When compared versus each other, groundwater heat pumps are generally more efficient than heat pumps using heat from the soil.

Solid state heat pumps

Main article: Magnetic refrigeration

In 1881, the German physicist Emil Warburg put a block of iron into a strong magnetic field and found that it increased very slightly in temperature. Some commercial ventures to implement this technology are underway, claiming to cut energy consumption by 40% compared to current domestic refrigerators.^[11] The process works as follows: Powdered gadolinium is moved into a magnetic field, heating the material by 2 to 5 °C. The heat is removed by a circulating fluid. The material is then moved out of the magnetic field, reducing its temperature below its starting temperature.

History

This section requires expansion.

Milestones:

1748: William Cullen demonstrates artificial refrigeration.
1834: Jacob Perkins builds a practical refrigerator with diethyl ether.
1852: Lord Kelvin describes the theory underlying heat pump.
1855–1857: Peter Ritter von Rittinger develops and builds the first heat pump.^[12]

References

^ The Systems and Equipment volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, 2004
^ Fundamentals of Engineering Thermodynamics, by Howell and Buckius, McGraw-Hill, New York, 1987
^ ^a ^b ^c ^d The Canadian Renewable Energy Network 'Commercial Earth Energy Systems', Figure 29. (URL accessed March 26, 2008)
^ Technical Institute of Physics and Chemistry, Chinese Academy of Sciences 'State of the Art of Air-source Heat Pump for Cold Region', Figure 5. (URL accessed April 19, 2008)
^ ^a ^b SINTEF Energy Research 'Integrated CO2 Heat Pump Systems for Space Heating and DHW in low-energy and passive houses', J. Steen, Table 3.1, Table 3.3. (URL accessed April 19, 2008)
^ Types of heat pumps (see page 8)
^ Homeowners using heat pump systems
^ Heat pumps sources including groundwater, soil, outside and inside air)
^ Carrier web site: Heat Pumps
^ the IPCC 4th Working Group III report
^ Guardian Unlimited, December 2006 'A cool new idea from British scientists: the magnetic fridge'
^ Banks, David L.. An Introduction to Thermogeology: Ground Source Heating and Cooling. Wiley-Blackwell. ISBN 978-1-4051-7061-1.

External links

Wikimedia Commons has media related to: Heat pumps

Heat Pumps Long Awaited Way out of the Global Warming - Information from Heat Pump & Thermal Storage Technology Center of Japanwhich is a public organization under the Ministry of Economy, Trade and Industry in Japan
Practical information on setting up geothermal heat pump systems at home
Pictures on private/communal heat pump installations
International Energy Agency Heat Pump Programme, Information site for heat pumping technology
Road Energy Systems: Heat from the Street Article on heat pump systems installed in roads and parking lots. Allianz Knowledge, June 2008
Heating and Cooling with a Heat pump - Information from the Canadian Government's Natural Resources Department, Office of Energy Efficiency

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	vapor cycle (thermodynamics)
	coefficient of performance
	thermoelectric cooler (engineering)

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