Heat pump 68316 226149201 2008-07-17T01:17:33Z User A1 2062655 Reverted edits by [[Special:Contributions/193.68.47.132|193.68.47.132]] ([[User talk:193.68.47.132|talk]]) to last version by User A1 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|work]]. Most heat pump technology moves heat from a low temperature 'heat source'' to a higher temperature ''heat sink''.<ref>The Systems and Equipment volume of the ''[[ASHRAE Handbook]]'', ASHRAE, Inc., Atlanta, GA, 2004</ref> Common examples are food [[refrigerator]]s and [[freezer]]s, [[air conditioner]]s, 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 [[Condensation|condensing]] fluid known as a [[refrigerant]]. In heating, ventilation, and cooling ([[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. Air-source heat pumps with a [[coefficient of performance]] (COP) 3 are developed in Japan at −20 °C. ==Operation== {{Main|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.<ref>''Fundamentals of Engineering Thermodynamics'', by Howell and Buckius, McGraw-Hill, New York, 1987</ref> Heat pumps differ in how they apply this work to move heat, but they can essentially be thought of as [[heat engine]]s 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 [[Condensation|condensing]] fluid known as a [[refrigerant]]. [[Image:Heatpump.svg|thumb|300px|A simple stylized diagram of a heat pump's [[vapor-compression refrigeration]] cycle: 1)&nbsp;condenser, 2)&nbsp;expansion valve, 3)&nbsp;evaporator, 4)&nbsp;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 (heat transfer)|condenser]] until it condenses into a high pressure, moderate temperature liquid. The condensed refrigerant then passes through a pressure-lowering device like an [[Thermal expansion valve|expansion valve]], [[capillary]] tube, or possibly a work-extracting device such as a [[turbine]]. This device then passes the low pressure, barely liquid (saturated vapor) 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 reaches 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. Similarly, the fluid must reach a sufficiently low temperature when allowed to expand, or heat cannot flow from the cold region into the fluid. 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 more energy is needed to compress the fluid. Thus as with all heat pumps, the [[SEER|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. [[Image:Heat pump system.jpg|thumb|right|150px|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 [[SEER|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 [[Water heating|domestic water heater]]s. 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 [[refrigerant]]s were often [[chlorofluorocarbon]]s 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 [[Earth's atmosphere|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 [[ammonia]], or occasionally the less corrosive but 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, but it is a powerful global warming gas and is nevertheless increasingly being used. Hydrogen, helium, nitrogen, or plain air is used in the [[Stirling engine#Stirling cryocoolers|Stirling cycle]], providing the maximum number of options in environmentally friendly gases. ==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 engine]]s, 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. Sometimes this is inappropriately expressed as an efficiency value greater than 100%, as in the statement, "XYZ brand heat pumps operate at up to 400% efficiency!" This is inaccurate, since the work does not ''make'' heat, but instead ''moves'' existing heat "upstream"; otherwise, this would be a [[perpetual-motion machine]]. The effective heating per [[watt]] of electric energy used can be up to 450% as much as resistance heating however, making this more an issue of semantics than science. 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 cycle|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 pump]]s 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: :<math> COP_{\mathrm{heating}} = \frac{\Delta Q_{\mathrm{hot}}}{\Delta A} \leq \frac{T_{\mathrm{hot}}}{T_{\mathrm{hot}}-T_{\mathrm{cool}}} = \frac{1}{\eta_{\mathrm{carnotcycle}}} </math> :<math> COP_{\mathrm{cooling}} = \frac{\Delta Q_{\mathrm{cool}}}{\Delta A} \leq \frac{T_{\mathrm{cool}}}{T_{\mathrm{hot}}-T_{\mathrm{cool}}} </math> <br> where *<math>\Delta Q_{cool}</math> is the amount of heat extracted from a cold reservoir at temperature <math>T_{cool}</math>, *<math>\Delta Q_{hot}</math> is the amount of heat delivered to a hot reservoir at temperature <math>T_{hot}</math>, *<math>\Delta A</math> is the compressor's dissipated work. ====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 chosing 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. {| class="wikitable" |- style="font-weight:bold" align="center" ! width="160" rowspan="2" | Pump type and source ! width="110" rowspan="2" | Typical use case ! colspan="6" | CoP variation with Output Temperature |- style="font-weight:bold" align="center" ! 35°C<BR>(e.g. heated screed floor) ! 45°C<BR>(e.g. heated screed floor) ! 55°C<BR>(e.g. heated timber floor) ! 65°C <BR>(e.g. radiator or DHW) ! 75°C <BR>(e.g. radiator & DHW) ! 85°C <BR>(e.g. radiator & DHW) |- | height="15" valign="top" | High Efficiency ASHP air at -20°C<ref name="CREN">The Canadian Renewable Energy Network [http://www.canren.gc.ca/app/filerepository/EARTH-BuyersGuide-CommercialEarthEnergySystems.pdf 'Commercial Earth Energy Systems', Figure 29]. (URL accessed [[Mar 26]], [[2008]])</ref> | valign="top" | &nbsp; | align="center" | 2.2 | align="center" | 2.0 | align="center" | - | align="center" | - | align="center" | - | align="center" | - |- | height="30" | Two Stage ASHP air at -20°C<ref name ="TIPC">Technical Institute of Physics and Chemistry, Chinese Academy of Sciences [http://repositories.tamu.edu/bitstream/handle/1969.1/5474/ESL-IC-06-11-312.pdf 'State of the Art of Air-source Heat Pump for Cold Region', Figure 5]. (URL accessed [[Apr 19]], [[2008]])</ref> | | Low source temp. | align="center" style="text-decoration:underline;color:#FF0000" | 2.4 | align="center" | 2.2 | align="center" | 1.9 | align="center" | - | align="center" | - | align="center" | - |- | height="15" | High Efficiency ASHP air at 0°C<ref name="CREN"/> | | Low output temp. | align="center" style="text-decoration:underline;color:#FF0000" | 3.8 | align="center" | 2.8 | align="center" | 2.2 | align="center" | 2.0 | align="center" | - | align="center" | - |- | height="45" | Prototype Transcritical {{chem|CO|2}} (R744) Heat Pump with Tripartite Gas Cooler, source at 0°C<ref name ="STEEN">SINTEF Energy Research [http://www.r744.com/knowledge/papers/files/pdf/pdf_379.pdf '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 [[Apr 19]], [[2008]])</ref> | | High output temp. | align="center" | 3.3 | align="center" | - | align="center" | - | align="center" style="text-decoration:underline;color:#FF0000" | 4.2 | align="center" | - | align="center" | 3.0 |- | height="15" | GSHP water at 0°C<ref name="CREN"/> | | &nbsp; | align="center" | 5.0 | align="center" | 3.7 | align="center" | 2.9 | align="center" | 2.4 | align="center" | - | align="center" | - |- | height="30" | GSHP ground at 10°C<ref name="CREN"/> | | Low output temp. | align="center" style="text-decoration:underline;color:#FF0000" | 7.2 | align="center" | 5.0 | align="center" | 3.7 | align="center" | 2.9 | align="center" | 2.4 | align="center" | - |- | height="15" | Theoretical Carnot cycle limit, source -20°C | | &nbsp; | align="center" | 5.6 | align="center" | 4.9 | align="center" | 4.4 | align="center" | 4.0 | align="center" | 3.7 | align="center" | 3.4 |- | height="15" | Theoretical Carnot cycle limit, source 0°C | | &nbsp; | align="center" | 8.8 | align="center" | 7.1 | align="center" | 6.0 | align="center" | 5.2 | align="center" | 4.6 | align="center" | 4.2 |- | height="30" | Theoretical Lorentz Cycle limit ({{chem|CO|2}} pump), return fluid 25°C, source 0°C<ref name ="STEEN"/> || &nbsp; |align="center" | 10.1 |align="center" | 8.8 |align="center" | 7.9 |align="center" | 7.1 |align="center" | 6.5 |align="center" | 6.1 |- | height="15" | Theoretical Carnot cycle limit, source 10°C | | &nbsp; | align="center" | 12.3 | align="center" | 9.1 | align="center" | 7.3 | align="center" | 6.1 | align="center" | 5.4 | align="center" | 4.8 |} ==Heat sources== Most commonly, heat pumps draw heat from the air (outside or inside air) or from the ground ([[groundwater]] or [[soil]]) <ref>[http://www2.vlaanderen.be/economie/energiesparen/doc/folder_warmtepomp.pdf Heat pumps sources including groundwater, soil, outside and inside air)]</ref>. 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. ==Types of heat pumps== A number of sources have been used for the heat source for heating private and communal buildings <ref>[http://www1.eere.energy.gov/geothermal/pdfs/26161b.pdf Homeowners using heat pump systems]</ref>. 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). <ref>[http://www2.vlaanderen.be/economie/energiesparen/doc/brochure_warmtepomp.pdf Types of heat pumps (see page 8)]</ref> ===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 a lower efficiency, as explained above. In mild weather, COP may be around 3.5, while at temperatures below around −5°C (23°F) an air-source heat pump's COP will drop below 2. But Air-source heat pumps with a COP 3 are developed in Japan at −20 °C. The average COP over seasonal variation is typically 2.5-2.8,high efficiency model in Japan over 6.0(2.8kW).<ref>[http://www.residential.carrier.com/products/acheatpumps/heatpumps/index.shtml Carrier web site: Heat Pumps]</ref><ref>[http://www.hptcj.or.jp/about_e/contribution/pdf/hpe-all. pumps Long Awaited Way out of the Global Warming]</ref> Domestic air-source heatpump water heater called Eco-cute was developed in 2001. ===Geothermal heat pumps=== [[Geothermal heat pump]]s 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 with little seasonal variation. 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|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<!-- The article referenced is unclear what the reference "standard fridge" is -->.<ref>Guardian Unlimited, December 2006 [http://environment.guardian.co.uk/energy/story/0,,1971818,00.html 'A cool new idea from British scientists: the magnetic fridge']</ref> The process works as follows: Powdered [[gadolinium]] is moved into a magnetic field, heating the material by 2 to 5&nbsp;°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. ==Heat transfer== After the heat has been absorbed from the source (air or ground), the heat is transferred and used in the home or building (for [[space heating]]. This is generally done by pipes in the [[floor]], [[wall]] or [[ceiling]] <ref>[http://www2.vlaanderen.be/economie/energiesparen/doc/brochure_warmtepomp.pdf Heat pumps used with floor, ceiling and wall heating for space heating]</ref>. The heat pump can also be used to heat water (thus in [[solar hot water|solar hot water systems]]), yet this is generally not done as it is much less efficient. Finally, the heat pumps may also be used to cool the house (eg in summer) <ref>[http://oee.nrcan.gc.ca/publications/infosource/pub/home/Heating_and_Cooling_with_a_Heat_Pump_Section4.cfm Explanation of heating and cooling with heat pumps by NRCAN]</ref> . However, the latter is also less efficient, and therefore less commonly practiced.<ref>[http://www2.vlaanderen.be/economie/energiesparen/doc/brochure_warmtepomp.pdf Other uses with heat pumps]</ref> ==History== {{Expand-section|date=June 2008}} Milestones: * 1834 – [[Jacob Perkins]] built [[refrigerator]] with [[diethyl ether]]; * 1852 – [[William Thomson, 1st Baron Kelvin|Lord Kelvin]] built first heat pump. ==References== {{reflist}} ==See also== * [[Flash evaporation]] * [[Geothermal heat pump]] * [[Geothermal exchange heat pump]] * [[Geothermal Systems]] * [[Renewable heat]] * [[Thermoelectric]] heat pumps that use the [[Peltier effect]] * [[Vapor-compression refrigeration]] * [[Vortex tube]] * [[IEA-ECBCS Annex 48 : Heat Pumping and Reversible Air Conditioning]] == External links == {{Commonscat|Heat pumps}} *[http://www.hptcj.or.jp/about_e/contribution/pdf/hpe-all.pdf Heat pumps Long Awaited Way out of the Global Warming] - Information from Heat Pump & Thermal Storage Technology Center of Japan *[http://www1.eere.energy.gov/geothermal/heatpumps.html Practical information on setting up heat pump systems at home] *[http://geoexchange.us/illustrations/graphics.htm Pictures on private/communal heat pump installations] *[http://www.heatpumpcentre.org/ International Energy Agency Heat Pump Programme, Information site for heat pumping technology] * ('''Dutch''') [http://www.energiesparen.be/algemeen/klik.php?link=http://www2.vlaanderen.be/ned/sites/economie Useful publication called ""Warmtepompen voor woningverwarming"]. Info on heat pump systems with drawings, schematics and explications of practically set-up systems). * ('''Dutch''') [http://www2.vlaanderen.be/economie/energiesparen/doc/folder_warmtepomp.pdf Useful publication called "Warmtepompen - De natuur als bron van verwarming"]. Info on heat pump systems with drawings, schematics and explications. * [http://knowledge.allianz.com/en/globalissues/climate_change/climate_solutions/heat_pump_street.html Road Energy Systems: Heat from the Street] Article on heat pump systems installed in roads and parking lots. Allianz Knowledge, June 2008 *[http://oee.nrcan.gc.ca/publications/infosource/home/index.cfm?act=online&id=4427&format=PDF&lang=01&PrintView=N&Text=N Heating and Cooling with a Heat pump] - Information from the Canadian Government's Natural Resources Department, Office of Energy Efficiency [[Category:Heat pumps]] [[Category:Building engineering]] [[Category:Geothermal power and heating plants]] <!--Interwiki--> [[bg:Топлинна помпа]] [[cs:Tepelné čerpadlo]] [[da:Varmepumpe]] [[de:Wärmepumpe]] [[es:Bomba de calor]] [[eo:Varmopumpilo]] [[fr:Pompe à chaleur]] [[it:Pompa di calore]] [[nl:Warmtepomp]] [[ja:ヒートポンプ]] [[no:Varmepumpe]] [[nn:Varmepumpe]] [[pl:Pompa ciepła]] [[ru:Тепловой насос]] [[simple:Heat pump]] [[fi:Lämpöpumppu]] [[sv:Värmepump]] [[tr:Isı pompası]] [[yi:היץ פאמפ]] [[zh:熱泵]]