Heat pump

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For theoretical background, see Heat pump and refrigeration cycle.
Outdoor components of a residential air-source heat pump




In heating, ventilation and air conditioning (HVAC) applications, the term heat pump usually refers to easily reversible vapor-compression refrigeration devices optimized for high efficiency in both directions of thermal energy transfer.

Heat spontaneously flows from warmer places to colder spaces. A heat pump can absorb heat from a cold space and release it to a warmer one. "Heat" is not conserved in this process, which requires some amount of external high grade (low-entropy) energy, such as electricity.

Heat pumps are used to provide heating because less high-grade energy is required for their operation than appears in the released heat. Most of the energy for heating comes from the external environment, and only a fraction comes from electricity (or some other high-grade energy source required to run a compressor). In electrically powered heat pumps, the heat transferred can be three or four times larger than the electrical power consumed, giving the system a coefficient of performance (COP) of 3 or 4, as opposed to a COP of 1 for a conventional electrical resistance heater, in which all heat is produced from input electrical energy.


Reversible heat pumps[edit]

Reversible heat pumps work in either thermal direction to provide heating or cooling to the internal space. They employ a reversing valve to reverse the flow of refrigerant from the compressor through the condenser and evaporation coils.

  • In heating mode, the outdoor coil is an evaporator, while the indoor is a condenser. The refrigerant flowing from the evaporator (outdoor coil) carries the thermal energy from outside air (or soil) indoors, after the vapour temperature has been augmented by compressing it. The indoor coil then transfers thermal energy (including energy from the compression) to the indoor air, which is then moved around the inside of the building by an air handler. Alternatively, thermal energy is transferred to water, which is then used to heat the building via radiators or underfloor heating. The heated water may also be used for domestic hot water consumption. The refrigerant is then allowed to expand, cool, and absorb heat to reheat to the outdoor temperature in the outside evaporator, and the cycle repeats. This is a standard refrigeration cycle, save that the "cold" side of the refrigerator (the evaporator coil) is positioned so it is outdoors where the environment is colder. In cold weather, the outdoor unit is intermittently defrosted by briefly switching to the cooling mode. This will cause the auxiliary or Emergency heating elements (located in the air-handler) to be activated. At the same time, the frost on the outdoor coil will quickly be melted due to the warm refrigerant. The condenser/evaporator fan will not run during defrost mode.

Operating principles[edit]


A fictitious pressure-volume diagram for a typical refrigeration cycle

The working fluid, in its gaseous state, is pressurized and circulated through the system by a compressor. On the discharge side of the compressor, now hot and highly pressurized vapor 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 also called a metering device. This may be an expansion valve, capillary tube, or possibly a work-extracting device such as a turbine. The low pressure liquid refrigerant then enters another heat exchanger, the evaporator, in which the fluid absorbs heat and boils. The refrigerant then returns to the compressor and the cycle is repeated.[citation needed]



To operate in different temperature conditions, different refrigerants are available. Refrigerators, air conditioners, and some heating systems are common applications that use this technology.[citation needed]

Heat transport[edit]

Heat is typically transported through engineered heating or cooling systems by using a flowing gas or liquid. Air is sometimes used, but quickly becomes impractical under many circumstances because it requires large ducts to transfer relatively small amounts of heat. In systems using refrigerant, this working fluid can also be used to transport heat a considerable distance, though this can become impractical because of increased risk of expensive refrigerant leakage. When large amounts of heat are to be transported, water is typically used, often supplemented with antifreeze, corrosion inhibitors, and other additives.

Heat sources/sinks[edit]



Geothermal heat pumps or ground-source heat pumps use shallow underground heat exchangers as a heat source or sink, and water as the heat transport medium. This is possible because below ground level, the temperature is relatively constant across the seasons, and the earth can provide or absorb a large amount of heat. Ground source heat pumps work in the same way as air-source heat pumps, but exchange heat with the ground via water pumped through pipes in the ground. Ground source heat pumps are more simple and therefore more reliable than air source heat pumps as they do not need fan or defrosting systems and can be housed inside. Although a ground heat exchanger requires a higher initial capital cost, the annual running costs are lower, because well-designed ground source heat pump systems operate more efficiently.







District heating[edit]

Main article: Drammen Heat Pump

Commissioned in 2011 this district heating extracts heat from a pond whose temperature is around 8 °C using 3 systems giving a combined capacity of 14 megawatts to town center residences and businesses. A city ordinance mandates this heating system for many new buildings.



One widely adopted replacement refrigerant is the hydrofluorocarbon (HFC) known as R-134a (1,1,1,2-tetrafluoroethane). Heat pumps using R-134a are not as efficient as those using R-12 that they replace (in automotive applications) and therefore, more energy is required to operate systems utilizing R-134a than those using R-12.[citation needed] 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.


More recent refrigerators use R600A which is isobutane, and does not deplete the ozone and is friendly to the environment.[citation needed]

Dimethyl ether (DME) is also gaining popularity as a refrigerant.[4]


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 per work input. Most vapor-compression heat pumps use electrically powered motors for their work input. However, in many vehicle applications, mechanical energy from an internal combustion engine provides the needed work.

According to the US EPA, geothermal heat pumps can reduce energy consumption up to 44% compared with air-source heat pumps and up to 72% compared with electric resistance heating.[5] Heatpumps in general have a COP of 4.2 to 4.6 which places it behind cogeneration with a COP of 9.[6]

When used for heating a building with an outside temperature of, for example, 10 °C, a typical air-source heat pump (ASHP) has a COP of 3 to 4, whereas an electrical resistance heater has a COP of 1.0. That is, one joule of electrical energy will cause a resistance heater to produce only one joule of useful heat, while under ideal conditions, one joule of electrical energy can cause a heat pump to move three or four joules of heat from a cooler place to a warmer place. Note that an air source heat pump is more efficient in hotter climates than cooler ones, so when the weather is much warmer the unit will perform with a higher COP (as it has a smaller temperature gap to bridge). When there is a wide temperature differential between the hot and cold reservoirs, the COP is lower (worse). In extreme cold weather the COP will go down to 1.0.

On the other hand, ground-source heat pumps (GSHP) benefit from the moderate temperature underground, as the ground acts naturally as a store of thermal energy.[7] Their year-round COP is therefore normally in the range of 2.5 to 5.0.










COP_\text{heating} = \frac{\Delta Q_\text{hot}}{\Delta A} \leq \frac{T_\text{hot}}{T_\text{hot}-T_\text{cool}},

COP_\text{cooling} = \frac{\Delta Q_\text{cool}}{\Delta A} \leq \frac{T_\text{cool}}{T_\text{hot}-T_\text{cool}},


  • \Delta Q_\text{cool} is the amount of heat extracted from a cold reservoir at temperature T_\text{cool},
  • \Delta Q_\text{hot} is the amount of heat delivered to a hot reservoir at temperature T_\text{hot},
  • \Delta A is the compressor's dissipated work.

Coefficient of performance (COP) and lift[edit]


COP variation with output temperature
Pump type and source Typical use 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 and DHW)
85 °C
(e.g. radiator and DHW)
High-efficiency air source heat pump (ASHP), air at −20 °C[9] 2.2 2.0
Two-stage ASHP, air at −20 °C[10] Low source temperature 2.4 2.2 1.9
High efficiency ASHP, air at 0 °C[9] Low output temperature 3.8 2.8 2.2 2.0
Prototype transcritical CO
(R744) heat pump with tripartite gas cooler, source at 0 °C[11]
High output temperature 3.3 4.2 3.0
Ground source heat pump (GSHP), water at 0 °C[9] 5.0 3.7 2.9 2.4
GSHP, ground at 10 °C[9] Low output temperature 7.2 5.0 3.7 2.9 2.4
Theoretical Lorentzen cycle limit (CO
pump), return fluid 25 °C, source 0 °C[11]
10.1 8.8 7.9 7.1 6.5 6.1



Compression vs. absorption[edit]


Heat sources and sinks[edit]




A number of sources have been used for the heat source for heating private and communal buildings.[14]

Air (ASHP)[edit]

Main article: Air source heat pumps
  • Air source heat pump (extracts heat from outside air)



In areas where only fossil fuels are available (e.g. heating oil only; no natural gas pipes available) air source heat pumps could be used as an alternative, supplemental heat source to reduce a building's dependence on fossil fuel. Depending on fuel and electricity prices, using the heat pump for heating may be less expensive than using fossil fuel. A backup fossil-fuel, solar hot water or biomass heat source may still be required for the coldest days.[citation needed]


Conventional 47 °F (8.3 °C) 32 °F (0 °C)
Low Temp Optimized 41 °F ( 5 °C) 17 °F (-8.3 °C)

Ground (GSHP)[edit]




Exhaust air (EAHP)[edit]

Main article: Exhaust air heat pump

Water source heat pumps (WSHP)[edit]

  • Uses flowing water as source or sink for heat
  • Single-pass vs. recirculation
    • Single-pass — water source a body of water or a stream
    • Recirculation

Hybrid (HHP)[edit]





Heat distribution[edit]

Heat pumps are only highly efficient when they generate heat at a low temperature differential, ideally around or below 32 °C (90 °F). Normal steel plate radiators are not practical, because they would need to be four to six times their current size. Underfloor heating is one ideal solution. When wooden floors or carpets would spoil efficiency, wall heaters (plastic pipes covered with a thick layer of chalk) and piped ceilings can be used. These systems have the disadvantage that they are slow starters, and that they would require extensive renovation in existing buildings.

The alternative is a warm air system. Such a setup can either complement slower floor heating during warm up, or it can be a quick and economical way to implement a heat pump system into existing buildings. Oversizing the fans and ductwork can reduce the acoustic noise they produce. To efficiently distribute warm water or air from a heat pump, water pipes or air shafts must have significantly larger diameters than in conventional, hotter-source systems, and underfloor heaters should have much more pipes per square meter.[citation needed]

Solid state heat pumps[edit]


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.[19] The process works as follows: Powdered gadolinium is moved into a magnetic field, heating the material by 2 to 5 °C (4 to 9 °F). 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.[citation needed]


Solid state heat pumps using the thermoelectric effect have improved over time to the point where they are useful for certain refrigeration tasks. Thermoelectric (Peltier) heat pumps are generally only around 10-15% as efficient as the ideal refrigerator (Carnot cycle), compared with 40–60% achieved by conventional compression cycle systems (reverse Rankine systems using compression/expansion);[20] however, this area of technology is currently the subject of active research in materials science. A reason why this is popular is because it has a "long lifetime" as there are no moving parts and it does not use potentially hazardous refrigerants.



Historical development[edit]


See also[edit]


  1. ^ Air-source heat pumps National Renewable Energy Laboratory June 2011
  2. ^ "R-410A - wikipedia". Wikipedia. Retrieved 2015-07-13. 
  4. ^ r404a & dme eco-refrigerant blend as a new solution to limit the global warming effect archive 2012.03.14
  6. ^ Lowe, Robert (2011). "Combined heat and power considered as a virtual steam cycle heat pump". Energy Policy 39 (9): 5528–5534. doi:10.1016/j.enpol.2011.05.007. ISSN 0301-4215. 
  7. ^ Thermal Banks store heat between seasons | Seasonal Heat Storage | Rechargeable Heat Battery | Thermogeology | UTES | Solar recharge of heat batteries
  8. ^ BSRIA, "European energy legislation explained", www.bsria.co.uk, May 2010.
  11. ^ 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. . Retrieved April 19, 2008.
  12. ^ Warmtepompen voor woningverwarming brochure 9-10-2013
  13. ^ "Heat pumps sources including groundwater, soil, outside and inside air)" (PDF). Retrieved 2010-06-02. 
  14. ^ "Homeowners using heat pump systems" (PDF). United States Department of Energy. September 1998. Archived from the original (PDF) on -32 January 2008.  Check date values in: |archivedate= (help)
  15. ^ Earth Temperature and Site Geology
  17. ^ Geothermalhelp.com
  18. ^ Asphalt Solar Collector Renewable Heat for IHT | Solar Collectors | Solar Recharge for GSHP | Pavement Solar Collectors | Road Solar Thermal Collector
  19. ^ Guardian Unlimited, December 2006 'A cool new idea from British scientists: the magnetic fridge'
  21. ^ Banks, David L.. An Introduction to Thermogeology: Ground Source Heating and Cooling (PDF). Wiley-Blackwell. ISBN 978-1-4051-7061-1. 
  22. ^ "History of Geothermal Technology" Furnace Repair Edmonton, 2014

External links[edit]