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→‎Improving the COP: Made a short list of two options, improved the formula for Delta T (instead of describing the formula in words), added internal link to heat exchanger.
 
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{{short description|Ratio of useful heating or cooling provided to work required}}
{{Original research|date=September 2012}}
The '''coefficient of performance''' or '''COP''' (sometimes '''CP''' or '''CoP''') of a [[Heat pump and refrigeration cycle|heat pump, refrigerator or air conditioning system]] is a ratio of useful heating or cooling provided to work (energy) required.<ref>{{cite web |url=http://www.tetech.com/temodules/graphs/instructions.pdf |title=Archived copy |access-date=2013-10-16 |url-status=dead |archive-url=https://web.archive.org/web/20130124080037/http://www.tetech.com/temodules/graphs/instructions.pdf |archive-date=2013-01-24 }}</ref><ref>{{Cite web|archive-url=https://web.archive.org/web/20140628003757/https://us.grundfos.com/service-support/encyclopedia-search/cop-coefficient-ofperformance.html|archive-date=2014-06-28|url=https://us.grundfos.com/service-support/encyclopedia-search/cop-coefficient-ofperformance.html|title=COP (Coefficient of performance)|website=us.grundfos.com|language=en-US|access-date=2019-04-08}}</ref> Higher COPs equate to higher efficiency, lower energy (power) consumption and thus lower operating costs. The COP is used in [[thermodynamics]].


The '''coefficient of performance''' or '''COP''' (sometimes CP) of a [[heat pump]] is a ratio of heating or cooling provided to work required.<ref>http://www.tetech.com/temodules/graphs/instructions.pdf</ref><ref>http://us.grundfos.com/service-support/encyclopedia-search/cop-coefficient-ofperformance.html</ref> Higher COPs equate to lower operating costs. The COP may exceed 1, because, instead of just converting work to heat (which, if 100% efficient, would be a COP of 1), it pumps additional heat from a heat source to where the heat is required. For complete systems, COP should include energy consumption of all auxiliaries. COP is highly dependent on operating conditions, especially absolute temperature and relative temperature between sink and system, and is often graphed or averaged against expected conditions.<ref>http://www.tetech.com/temodules/graphs/HP-199-1.4-0.8.pdf</ref>
The COP usually exceeds 1, especially in heat pumps, because instead of just converting work to heat (which, if 100% efficient, would be a COP of 1), it pumps additional heat from a heat source to where the heat is required. Most air conditioners have a COP of 2.3 to 3.5{{Citation needed|date=April 2024}}. Less work is required to move heat than for conversion into heat, and because of this, heat pumps, air conditioners and refrigeration systems can have a coefficient of performance greater than one.

The COP is highly dependent on operating conditions, especially absolute temperature and relative temperature between sink and system, and is often graphed or averaged against expected conditions.<ref>{{cite web |url=http://www.tetech.com/temodules/graphs/HP-199-1.4-0.8.pdf |title=Archived copy |access-date=2013-10-16 |url-status=dead |archive-url=https://web.archive.org/web/20090107132318/http://www.tetech.com/temodules/graphs/HP-199-1.4-0.8.pdf |archive-date=2009-01-07 }}</ref>

Performance of [[absorption refrigerator]] chillers is typically much lower, as they are not heat pumps relying on compression, but instead rely on chemical reactions driven by heat.<ref>{{cite web |title=Coefficient of Performance - Measuring Efficiency in HVAC Systems |url=https://www.fargoheatingandcooling.com/coefficient-of-performance-measuring-efficiency-in-hvac-systems/ |website=Fargo Heating and Cooling |access-date=November 6, 2023}}</ref>


== Equation ==
== Equation ==
The equation is:
The equation is:
:<math>COP = \frac{ Q}{ W}</math>
:<math>{\rm COP} = \frac{|Q|}{ W}</math>
where
where
* <math> Q \ </math> is the [[heat]] supplied to or removed from the reservoir.
* <math> Q \ </math> is the useful [[heat]] supplied or removed by the considered system (machine).
* <math>W \ </math> is the [[Mechanical work|work]] consumed by the heat pump.
* <math>W > 0\ </math> is the net [[Mechanical work|work]] put into the considered system in one cycle.


The COP for heating and cooling are thus different, because the heat reservoir of interest is different. When one is interested in how well a machine cools, the COP is the ratio of the heat removed from the cold reservoir to input work. However, for heating, the COP is the ratio of the heat removed from the cold reservoir plus the input work to the input work:
The COP for heating and cooling are different because the heat reservoir of interest is different. When one is interested in how well a machine cools, the COP is the ratio of the heat taken up from the cold reservoir to input work. However, for heating, the COP is the ratio of the magnitude of the heat given off to the hot reservoir (which is the heat taken up from the cold reservoir plus the input work) to the input work:
:<math> COP_{heating}=\frac{| Q_{H}|}{ W}=\frac{| Q_{C}| + W}{ W}</math>
:<math> {\rm COP}_{\rm cooling}=\frac{|Q_{\rm C}|}{ W}=\frac{Q_{\rm C}}{ W}</math>
⚫
:<math> COP_{cooling}=\frac{| Q_{C}|}{ W}</math>
⚫
where
⚫
*<math> Q_{C} \ </math> is the heat removed from the cold reservoir.
*<math> Q_{H} \ </math> is the heat supplied to the hot reservoir.


:<math> {\rm COP}_{\rm heating}=\frac{| Q_{\rm H}|}{ W}=\frac{Q_{\rm C} + W}{ W} = {\rm COP}_{\rm cooling} + 1 </math>
==Derivation==
⚫
where
{{Unreferenced section|date=January 2011}}
⚫
*<math> Q_{\rm C} > 0 \ </math> is the heat removed from the cold reservoir and added to the system;
According to the [[first law of thermodynamics]], in a reversible system we can show that <math>Q_{hot}=Q_{cold}+W </math> and <math>W=Q_{hot}-Q_{cold}</math>, where <math>Q_{hot}</math> is the heat transferred to the hot reservoir and <math>Q_{cold}</math> is the heat collected from the cold heat reservoir.<br>
*<math> Q_{\rm H} < 0 \ </math> is the heat given off to the hot reservoir; it is lost by the system and therefore negative<ref name="PlanckBook">{{cite book |last=Planck |first=M. |title=Treatise on Thermodynamics |page=§90 & §137 |quote=eqs.(39), (40), & (65) |publisher=Dover Publications |year=1945}}.</ref> (see [[heat]]).
Therefore, by substituting for W,<br>
⚫
:<math> COP_{heating}=\frac{Q_{hot}}{Q_{hot}-Q_{cold}}</math>


⚫
Note that the COP of a heat pump depends on its direction. The heat rejected to the hot sink is greater than the heat absorbed from the cold source, so the heating COP is greater by one than the cooling COP.
For a heat pump operating at maximum theoretical efficiency (i.e. Carnot efficiency), it can be shown that
⚫
:<math> \frac{Q_{hot}}{T_{hot}}=\frac{Q_{cold}}{T_{cold}}</math> and <math>Q_{cold}=\frac{Q_{hot}T_{cold}}{T_{hot}}</math>
⚫
where <math>T_{hot} </math> and <math>T_{cold}</math> are the temperatures of the hot and cold heat reservoirs respectively.
'''Note:''' these equations must '''use an absolute temperature scale''', for example, '''K'''elvin or '''R'''ankine.


==Theoretical performance limits==
⚫
At maximum theoretical efficiency, <br>
According to the [[first law of thermodynamics]], after a full cycle of the process <math>Q_{\rm H}+Q_{\rm C}+W = \Delta_{\rm cycle}U = 0 </math> and thus <math>W=-\ Q_{\rm H}-Q_{\rm C}</math>.<br>
⚫
:<math> COP_{heating}=\frac{T_{hot}}{T_{hot}-T_{cold}} </math>
Since <math> |Q_{\rm H}| = -Q_{\rm H} \ </math>, we obtain
⚫
which is equal to the reciprocal of the ideal efficiency for a heat engine, because a heat pump is a heat engine operating in reverse. Similarly, <br>
:<math> COP_{cooling}=\frac{Q_{cold}}{Q_{hot}-Q_{cold}} =\frac{T_{cold}}{T_{hot}-T_{cold}}</math>
:<math> {\rm COP}_{\rm heating}=\frac{Q_{\rm H}}{Q_{\rm H}+Q_{\rm C}}</math>


For a heat pump operating at maximum theoretical efficiency (i.e. [[Carnot efficiency]]), it can be shown<ref name="FermiBook">{{cite book |last=Fermi |first=E. |title=Thermodynamics |page=48 |quote= eq.(64) |publisher=Dover Publications (still in print) |year=1956}}.</ref><ref name="PlanckBook"/> that
⚫
Note that the COP of a heat pump depends on its duty. The heat rejected to the hot sink is greater than the heat absorbed from the cold source, so the heating COP is 1 greater than the cooling COP.
⚫
:<math> \frac{Q_{\rm H}}{T_{\rm H}}+ \frac{Q_{\rm C}}{T_{\rm C}}=0</math> and thus <math>Q_{\rm C}=-\frac{Q_{\rm H}T_{\rm C}}{T_{\rm H}}</math>
⚫
where <math>T_{\rm H} </math> and <math>T_{\rm C}</math> are the [[thermodynamic temperature]]s of the hot and cold heat reservoirs, respectively.


⚫
At maximum theoretical efficiency, therefore
<math>COP_{heating}</math> applies to heat pumps and <math>COP_{cooling}</math> applies to air conditioners or refrigerators. For heat engines, see [[Thermodynamic efficiency|Efficiency]]. Values for actual systems will always be less than these theoretical maximums. In Europe, the standard tests for ground source heat pump units use 35&nbsp;°C (95&nbsp;°F) for <math>{T_{hot}}</math> and 0&nbsp;°C (32&nbsp;°F) for <math>{T_{cold}}</math>. According to the above formula, the maximum achievable COP would be 8.8. Test results of the best systems are around 4.5. When measuring installed units over a whole season and accounting for the energy needed to pump water through the piping systems, seasonal COP's are around 3.5 or less. This indicates room for improvement.<ref>Borgnakke, C., & Sonntag, R. (2013). The Second Law of Thermodynamics. In Fundamentals of Thermodynamics (1st ed., pp. 244-245). Wiley.</ref>
⚫
:<math> {\rm COP}_{\rm heating}=\frac{T_{\rm H}}{T_{\rm H}-T_{\rm C}} </math>
⚫
which is equal to the reciprocal of the [[thermal efficiency]] of an ideal [[heat engine]], because a heat pump is a heat engine operating in reverse.<ref>Borgnakke, C., & Sonntag, R. (2013). The Second Law of Thermodynamics. In Fundamentals of Thermodynamics (8th ed., pp. 244-245). Wiley.</ref>


Similarly, the COP of a refrigerator or air conditioner operating at maximum theoretical efficiency,
⚫
==Improving COP==
:<math> {\rm COP}_{\rm cooling}=\frac{Q_{\rm C}}{\ Q_{\rm H}-Q_{\rm C}} =\frac{T_{\rm C}}{T_{\rm H}-T_{\rm C}}</math>
⚫
As the formula shows, the COP of a heat pump system can be improved by reducing the temperature gap <math>T_{hot} </math> minus <math>T_{cold}</math> at which the system works. For a heating system this would mean two things: 1) reducing the output temperature to around {{convert|30|C|F}} which requires piped floor, wall or ceiling heating, or over sized water to air heaters and 2) increasing the input temperature (e.g. by using an over sized ground source or by access to a solar-assisted thermal bank <ref>Thermal banks and Thermal Energy Storage, http://www.icax.co.uk/ThermalBanks.html</ref> ). For an air cooler, COP could be improved by using ground water as an input instead of air, and by reducing temperature drop on output side through increasing air flow. For both systems, also increasing the size of pipes and air canals would help to reduce noise and the energy consumption of pumps (and ventilators) by decreasing the speed of fluid which in turn lower the Re number and hence the turbulence (and noise) and the head loss (see [[hydraulic head]]). The heat pump itself can be improved by increasing the size of the internal heat exchangers which in turn increase the [[Heat pump#Efficiency|efficiency]] (and the price) relative to the power of the compressor, and also by reducing the system's internal temperature gap over the compressor. Obviously, this latter measure makes such heat pumps unsuitable to produce high temperatures which means that a separate machine is needed for producing hot tap water.


<math>{\rm COP}_{\rm heating}</math> applies to heat pumps and <math>{\rm COP}_{\rm cooling}</math> applies to air conditioners and refrigerators.
==Example==
Measured values for actual systems will always be significantly less than these theoretical maxima.
{{Unreferenced section|date=September 2012|comment=Note that the footnotes aren't refs}}


In Europe, the standard test conditions for ground source heat pump units use 308&nbsp;K (35&nbsp;°C; 95&nbsp;°F) for <math>{T_{\rm H}}</math> and 273&nbsp;K (0&nbsp;°C; 32&nbsp;°F) for <math>{T_{\rm C}}</math>. According to the above formula, the maximum theoretical COPs would be <br>
A [[geothermal heat pump]] operating at <math>COP_{heating}</math> 3.5 provides 3.5 units of heat for each unit of energy consumed (i.e. 1&nbsp;kWh consumed would provide 3.5&nbsp;kWh of output heat). The output heat comes from both the heat source and 1&nbsp;kWh of input energy, so the heat-source is cooled by 2.5&nbsp;kWh, not 3.5&nbsp;kWh.
⚫
:<math> {\rm COP}_{\rm heating}=\frac{308}{308-273} = 8.8</math><br>
⚫
:<math> {\rm COP}_{\rm cooling}=\frac{273}{308-273} = 7.8</math>


Test results of the best systems are around 4.5. When measuring installed units over a whole season and accounting for the energy needed to pump water through the piping systems, seasonal COP's for heating are around 3.5 or less. This indicates room for further improvement.
A heat pump of <math>COP_{heating}</math> 3.5, such as in the example above, could be less expensive to use than even the most efficient gas furnace except in areas where the electricity cost per unit is higher than 3.5 times the cost of natural gas (e.g. [[Connecticut]] or [[New York City]]).


The EU standard test conditions for an air source heat pump is at [[dry-bulb temperature]] of 20&nbsp;°C (68&nbsp;°F) for <math>{T_{\rm H}}</math> and 7&nbsp;°C (44.6&nbsp;°F) for <math>{T_{\rm C}}</math>.<ref>According to European Union COMMISSION DELEGATED REGULATION (EU) No 626/2011 ANNEX VII Table 2</ref> Given sub-zero European winter temperatures, real world heating performance is significantly poorer than such standard COP figures imply.
A heat pump cooler operating at <math>COP_{cooling}</math> 2.0 removes 2 units of heat for each unit of energy consumed (e.g. an [[air conditioner]] consuming 1&nbsp;kWh would remove 2&nbsp;kWh of heat from a building's air).


⚫
==Improving the COP==
Given the same energy source and operating conditions, a higher COP heat pump will consume less purchased energy than one with a lower COP. The overall environmental impact of a heating or air conditioning installation depends on the source of energy used as well as the COP of the equipment. The operating cost to the consumer depends on the cost of energy as well as the COP or efficiency of the unit. Some areas provide two or more sources of energy, for example, natural gas and electricity. A high COP of a heat pump may not entirely overcome a relatively high cost for electricity compared with the same heating value from natural gas.
As the formula shows, the COP of a heat pump system can be improved by reducing the temperature gap <math>(\Delta T = T_\text{hot} - T_\text{cold}) </math> at which the system works. For a heating system this would mean two things:


# Reducing the output temperature to around {{convert|30|C|F}} which requires piped floor, wall or ceiling heating, or oversized water to air heaters.
For example, the 2009 US average price per therm (100,000 BTU) of electricity was $3.38 while the average price per therm of natural gas was $1.16.<ref>Based on average prices of 11.55 cents per kWh for electricity [http://www.eia.doe.gov/cneaf/electricity/epm/table5_3.html] and $13.68 per thousand cubic feet for natural gas [http://tonto.eia.doe.gov/dnav/ng/hist/n3010us3a.htm], and conversion factors of 29.308 kWh per therm and 97.2763 cubic feet per therm [http://www.eia.doe.gov/kids/energyfacts/science/energy_calculator.html].</ref> Using these prices, a heat pump with a COP of 3.5 in moderate climate would cost $0.97<ref>$3.38/3.5~$0.97</ref> to provide one therm of heat, while a high efficiency gas furnace with 95% efficiency would cost $1.22<ref>$1.16/.95~$1.22</ref> to provide one therm of heat. With these average prices, the heat pump costs 20% less<ref>($1.16-$0.95)/$1.16~20%</ref> to provide the same amount of heat. At 0&nbsp;°F (-18&nbsp;°C) COP is much lower. Then, the same system costs as much to operate as an efficient gas heater. The yearly savings will depend on the actual cost of electricity and natural gas, which can both vary widely.
# Increasing the input temperature (e.g. by using an oversized ground source or by access to a solar-assisted thermal bank<ref>{{Cite web|url=http://www.icax.co.uk/ThermalBanks.html|title=Thermal Banks store heat between seasons {{!}} Seasonal Heat Storage {{!}} Rechargeable Heat Battery {{!}} Energy Storage {{!}} Thermogeology {{!}} UTES {{!}} Solar recharge of heat batteries|website=www.icax.co.uk|access-date=2019-04-08}}</ref> ).


⚫
Accurately determining [[thermal conductivity]] will allow for much more precise ground loop<ref>{{Cite web|url=http://www.carbonzeroco.com/field-services/soil-thermal-conductivity-testing/|title=Soil Thermal Conductivity Testing|website=Carbon Zero Consulting|language=en-US|access-date=2019-04-08}}</ref> or borehole sizing,<ref>{{Cite web|url=http://www.carbonzeroco.com/ground-source-heat-pumps/ground-source-heating-cooling/|title=GSHC Viability and Design|website=Carbon Zero Consulting|language=en-US|access-date=2019-04-08}}</ref> resulting in higher return temperatures and a more efficient system. For an air cooler, the COP could be improved by using ground water as an input instead of air, and by reducing the temperature drop on the output side by increasing the air flow. For both systems, also increasing the size of pipes and air canals would help to reduce noise and the energy consumption of pumps (and ventilators) by decreasing the speed of the fluid, which in turn lowers the [[Reynolds number]] and hence the turbulence (and noise) and the head loss (see [[hydraulic head]]). The heat pump itself can be improved by increasing the size of the internal [[Heat exchanger|heat exchangers]], which in turn increases the [[Heat pump#Performance|efficiency]] (and the cost) relative to the power of the compressor, and also by reducing the system's internal temperature gap over the compressor. Obviously, this latter measure makes some heat pumps unsuitable to produce high temperatures, which means that a separate machine is needed for producing, e.g., hot tap water.
However, a COP may help make a determination of system choice based on carbon contribution. Although a heat pump may cost more to operate than a conventional natural gas or electric heater, depending on the source of [[electricity generation]] in one's area, it may contribute less net [[carbon dioxide]] to the atmosphere than burning natural gas or heating fuel. If locally no green electricity is available, then carbon wise the best option would be to drive a heat pump on piped gas or oil, to store excess heat in the ground source for use in winter, while using the same machine also for producing electricity with a built-in [[Stirling engine]]. {{Citation needed|date=February 2011}}


The COP of absorption chillers can be improved by adding a second or third stage. Double and triple effect chillers are significantly more efficient than single effect chillers, and can surpass a COP of 1. They require higher pressure and higher temperature steam, but this is still a relatively small 10 pounds of steam per hour per ton of cooling.<ref>Depart of Energy Advanced Manufacturing office. Paper DOE/GO-102012-3413. January 2012</ref>
Please see Modern Geothermal HVAC Engineering and Control Applications by, Greg Cunniff, and Carl Orio.


==Conditions of use==
{{Unreferenced section|date=September 2012}}
While the COP is partly a measure of the efficiency of a heat pump, it is also a measure of the conditions under which it is operating: the COP of a given heat pump will rise as the input temperature increases or the output temperature decreases because it is linked to a warm temperature distribution system like [[underfloor heating]].


== Seasonal efficiency ==
== Seasonal efficiency ==
A realistic indication of energy efficiency over an entire year can be archived by using Seasonal COP or Seasonal Coefficient of Performance (SCOP) for heat. [[Seasonal energy efficiency ratio|Seasonal energy efficiency ratio (SEER)]] is mostly used for air conditioning. SCOP is a new methodology that gives a better indication of expected real-life performance, using COP can be considered using the "old" scale. Seasonal efficiency gives an indication on how efficient a heat pump operates over an entire cooling or heating season.<ref>{{cite web |url=http://www.daikin.co.uk/binaries/Seer%20fact%20sheet%20stg2_tcm511-261046.pdf?quoteId= |title=A new era of Seasonal Efficiency has begun |publisher=Daikin |date= |website=Daikin.co.uk |access-date=31 March 2015}}</ref>
A realistic indication of [[Thermal efficiency|energy efficiency]] over an entire year can be achieved by using seasonal COP or seasonal coefficient of performance (SCOP) for heat. [[Seasonal energy efficiency ratio|Seasonal energy efficiency ratio (SEER)]] is mostly used for air conditioning. SCOP is a new methodology that gives a better indication of expected real-life performance, using COP can be considered using the "old" scale. Seasonal efficiency gives an indication on how efficiently a heat pump operates over an entire cooling or heating season.<ref>{{cite web |url=http://www.daikin.co.uk/binaries/Seer%20fact%20sheet%20stg2_tcm511-261046.pdf?quoteId= |title=A new era of Seasonal Efficiency has begun |publisher=Daikin |website=Daikin.co.uk |access-date=31 March 2015 |archive-url=https://web.archive.org/web/20140731084805/http://www.daikin.co.uk/binaries/Seer%20fact%20sheet%20stg2_tcm511-261046.pdf?quoteId= |archive-date=31 July 2014 |url-status=dead |df=dmy-all }}</ref>


==See also==
==See also==
Line 68: Line 70:
* [[Seasonal thermal energy storage]] (STES)
* [[Seasonal thermal energy storage]] (STES)
* [[Heating seasonal performance factor]] (HSPF)
* [[Heating seasonal performance factor]] (HSPF)
* [[Power usage effectiveness]] (PUE)
* [[Thermal efficiency]]
* [[Thermal efficiency]]
* [[Vapor-compression refrigeration]]
* [[Vapor-compression refrigeration]]
Line 77: Line 80:


== External links ==
== External links ==
{{Commons category|Coefficient of performance}}
*[http://www.icax.co.uk/gshp.html Discussion on changes to COP of a heat pump depending on input and output temperatures]
*[http://www.icax.co.uk/gshp.html Discussion on changes to COP of a heat pump depending on input and output temperatures]
*[http://www-3.unipv.it/energy/web/Libro%20petrecca/pdf/capitolododicesimo.pdf See COP definition in Cap XII of the book Industrial Energy Management - Principles and Applications]
*[http://www-3.unipv.it/energy/web/Libro%20petrecca/pdf/capitolododicesimo.pdf See COP definition in Cap XII of the book Industrial Energy Management - Principles and Applications]{{dead link|date=August 2017 |bot=InternetArchiveBot |fix-attempted=yes }}


{{DEFAULTSORT:Coefficient Of Performance}}
{{DEFAULTSORT:Coefficient Of Performance}}
[[Category:Heat pumps]]
[[Category:Heat pumps]]
[[Category:Heating, ventilating, and air conditioning]]
[[Category:Heating, ventilation, and air conditioning]]
[[Category:Mechanical engineering]]
[[Category:Dimensionless numbers of thermodynamics]]
[[Category:Dimensionless numbers of thermodynamics]]
[[Category:Engineering ratios]]

Latest revision as of 12:28, 18 June 2024

The coefficient of performance or COP (sometimes CP or CoP) of a heat pump, refrigerator or air conditioning system is a ratio of useful heating or cooling provided to work (energy) required.[1][2] Higher COPs equate to higher efficiency, lower energy (power) consumption and thus lower operating costs. The COP is used in thermodynamics.

The COP usually exceeds 1, especially in heat pumps, because instead of just converting work to heat (which, if 100% efficient, would be a COP of 1), it pumps additional heat from a heat source to where the heat is required. Most air conditioners have a COP of 2.3 to 3.5[citation needed]. Less work is required to move heat than for conversion into heat, and because of this, heat pumps, air conditioners and refrigeration systems can have a coefficient of performance greater than one.

The COP is highly dependent on operating conditions, especially absolute temperature and relative temperature between sink and system, and is often graphed or averaged against expected conditions.[3]

Performance of absorption refrigerator chillers is typically much lower, as they are not heat pumps relying on compression, but instead rely on chemical reactions driven by heat.[4]

Equation[edit]

The equation is:

where

  • is the useful heat supplied or removed by the considered system (machine).
  • is the net work put into the considered system in one cycle.

The COP for heating and cooling are different because the heat reservoir of interest is different. When one is interested in how well a machine cools, the COP is the ratio of the heat taken up from the cold reservoir to input work. However, for heating, the COP is the ratio of the magnitude of the heat given off to the hot reservoir (which is the heat taken up from the cold reservoir plus the input work) to the input work:

where

  • is the heat removed from the cold reservoir and added to the system;
  • is the heat given off to the hot reservoir; it is lost by the system and therefore negative[5] (see heat).

Note that the COP of a heat pump depends on its direction. The heat rejected to the hot sink is greater than the heat absorbed from the cold source, so the heating COP is greater by one than the cooling COP.

Theoretical performance limits[edit]

According to the first law of thermodynamics, after a full cycle of the process and thus .
Since , we obtain

For a heat pump operating at maximum theoretical efficiency (i.e. Carnot efficiency), it can be shown[6][5] that

and thus

where and are the thermodynamic temperatures of the hot and cold heat reservoirs, respectively.

At maximum theoretical efficiency, therefore

which is equal to the reciprocal of the thermal efficiency of an ideal heat engine, because a heat pump is a heat engine operating in reverse.[7]

Similarly, the COP of a refrigerator or air conditioner operating at maximum theoretical efficiency,

applies to heat pumps and applies to air conditioners and refrigerators. Measured values for actual systems will always be significantly less than these theoretical maxima.

In Europe, the standard test conditions for ground source heat pump units use 308 K (35 °C; 95 °F) for and 273 K (0 °C; 32 °F) for . According to the above formula, the maximum theoretical COPs would be


Test results of the best systems are around 4.5. When measuring installed units over a whole season and accounting for the energy needed to pump water through the piping systems, seasonal COP's for heating are around 3.5 or less. This indicates room for further improvement.

The EU standard test conditions for an air source heat pump is at dry-bulb temperature of 20 °C (68 °F) for and 7 °C (44.6 °F) for .[8] Given sub-zero European winter temperatures, real world heating performance is significantly poorer than such standard COP figures imply.

Improving the COP[edit]

As the formula shows, the COP of a heat pump system can be improved by reducing the temperature gap at which the system works. For a heating system this would mean two things:

  1. Reducing the output temperature to around 30 °C (86 °F) which requires piped floor, wall or ceiling heating, or oversized water to air heaters.
  2. Increasing the input temperature (e.g. by using an oversized ground source or by access to a solar-assisted thermal bank[9] ).

Accurately determining thermal conductivity will allow for much more precise ground loop[10] or borehole sizing,[11] resulting in higher return temperatures and a more efficient system. For an air cooler, the COP could be improved by using ground water as an input instead of air, and by reducing the temperature drop on the output side by increasing the air flow. For both systems, also increasing the size of pipes and air canals would help to reduce noise and the energy consumption of pumps (and ventilators) by decreasing the speed of the fluid, which in turn lowers the Reynolds number and hence the turbulence (and noise) and the head loss (see hydraulic head). The heat pump itself can be improved by increasing the size of the internal heat exchangers, which in turn increases the efficiency (and the cost) relative to the power of the compressor, and also by reducing the system's internal temperature gap over the compressor. Obviously, this latter measure makes some heat pumps unsuitable to produce high temperatures, which means that a separate machine is needed for producing, e.g., hot tap water.

The COP of absorption chillers can be improved by adding a second or third stage. Double and triple effect chillers are significantly more efficient than single effect chillers, and can surpass a COP of 1. They require higher pressure and higher temperature steam, but this is still a relatively small 10 pounds of steam per hour per ton of cooling.[12]


Seasonal efficiency[edit]

A realistic indication of energy efficiency over an entire year can be achieved by using seasonal COP or seasonal coefficient of performance (SCOP) for heat. Seasonal energy efficiency ratio (SEER) is mostly used for air conditioning. SCOP is a new methodology that gives a better indication of expected real-life performance, using COP can be considered using the "old" scale. Seasonal efficiency gives an indication on how efficiently a heat pump operates over an entire cooling or heating season.[13]

See also[edit]

Notes[edit]

  1. ^ "Archived copy" (PDF). Archived from the original (PDF) on 2013-01-24. Retrieved 2013-10-16.{{cite web}}: CS1 maint: archived copy as title (link)
  2. ^ "COP (Coefficient of performance)". us.grundfos.com. Archived from the original on 2014-06-28. Retrieved 2019-04-08.
  3. ^ "Archived copy" (PDF). Archived from the original (PDF) on 2009-01-07. Retrieved 2013-10-16.{{cite web}}: CS1 maint: archived copy as title (link)
  4. ^ "Coefficient of Performance - Measuring Efficiency in HVAC Systems". Fargo Heating and Cooling. Retrieved November 6, 2023.
  5. ^ a b Planck, M. (1945). Treatise on Thermodynamics. Dover Publications. p. §90 & §137. eqs.(39), (40), & (65).
  6. ^ Fermi, E. (1956). Thermodynamics. Dover Publications (still in print). p. 48. eq.(64).
  7. ^ Borgnakke, C., & Sonntag, R. (2013). The Second Law of Thermodynamics. In Fundamentals of Thermodynamics (8th ed., pp. 244-245). Wiley.
  8. ^ According to European Union COMMISSION DELEGATED REGULATION (EU) No 626/2011 ANNEX VII Table 2
  9. ^ "Thermal Banks store heat between seasons | Seasonal Heat Storage | Rechargeable Heat Battery | Energy Storage | Thermogeology | UTES | Solar recharge of heat batteries". www.icax.co.uk. Retrieved 2019-04-08.
  10. ^ "Soil Thermal Conductivity Testing". Carbon Zero Consulting. Retrieved 2019-04-08.
  11. ^ "GSHC Viability and Design". Carbon Zero Consulting. Retrieved 2019-04-08.
  12. ^ Depart of Energy Advanced Manufacturing office. Paper DOE/GO-102012-3413. January 2012
  13. ^ "A new era of Seasonal Efficiency has begun" (PDF). Daikin.co.uk. Daikin. Archived from the original (PDF) on 31 July 2014. Retrieved 31 March 2015.

External links[edit]

source: https://en.wikipedia.org/w/index.php?title=Coefficient_of_performance&diff=1229730623&oldid=696165422

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