How efficient is a heat pump?

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Answer by Dale Blundell (Atkins)

An Air Source Heat Pump (ASHP) will typically produce around 3kW thermal energy for every 1kW of electrical energy consumed, giving an effective “efficiency” of 300%. It is thermodynamically impossible to have an efficiency of more than 100%, as this implies that more energy is being produced than is being put in.  For this reason the performance is expressed as a Coefficient of Performance (COP) rather than an efficiency. The above example would be expressed as having a COP of 3.  The reason that it appears that more energy is being produced than is consumed, is because the only “valuable” energy input is electricity used to drive the compressor and circulating pumps.  The remainder of the energy simply transferred from a heat source that would otherwise not be used (such as the ambient air, ground or a river) so is not considered as an energy input.

While the term Coefficient of Performance (CoP) is commonly used, it is important to understand the differences in order that the heat pump will work successfully, and to be able to compare different models that might be using different measures.

• Seasonal Coefficient of Performance (SCoP). Used mainly with ASHPs to give a measure of heat pump performance over the year, taking account of varying air temperatures.  There are three climatic zones defined across Europe, defined in BSEN14825.
• Seasonal Energy Efficiency Ratio (SEER. Used mainly with ASHPs to give a measure of heat pump cooling performance over the year, taking account of varying air temperatures.  There are three climatic zones defined across Europe, defined in BSEN14825.
• Energy Efficiency Ratio (EER). This is a measure of the ratio of delivered thermal power to total electrical power.  This includes ancillaries such as any fans, pumps and controls.
• Seasonal Performance Factor (SPF). This is the most useful definition, as I gives a measure of the ratio of delivered thermal energy over the year, to total electrical consumption.

But, as with other forms of “renewable” energy, where the source of fuel is virtually limitless and free, it is the total cost of generation rather than the efficiency that really matters. 

For comparison, other forms of heat generation have the following efficiencies:

• Condensing gas/oil boiler: 90-96% efficiency
• Conventional gas/oil boiler: 70-80% efficiency
• Direct electric heating: 35-45% efficiency (including losses in generation and distribution).

As the temperature difference between input and output increases, so does the compressor pressure needed to power it, and hence the COP decreases.  It is therefore important to understand the temperature at which it is rated, and the range over which it will be used.

Future Developments

There is a general trend towards the development of more compact, higher efficiency equipment, which will reduce the capital and operating costs. 

Absorption heat pumps (GAHP) using either natural gas or waste heat avoid the need for refrigerant gases.  These operate in a similar way to absorption chillers, however their COP may be only 1.4 to 1.7 compared to higher COP’s for electric heat pumps .  They are capable of delivering higher temperatures and since they can be connected to a gas source offer a good retro fitting option in particular to older buildings with an existing hydronic gas fired central heating system that operates at higher temperatures than other heat pump technology can deliver.

The GAHP’s in the market are currently geared up for small to medium size commercial end-users (schools, offices, hotels) with unit capacity ranging in the order of 35-40kW heat output with larger demands being met using multiple units.  Whilst absorption technology is mature, the market for GAHP’s in Europe is not fully developed with perhaps only 45,000 units installed to date by a limited number of suppliers.  However, GAHP and heat networks are predicted to be key applications in the challenge of meeting emissions reduction targets and energy demand as far as 2050.  GAHP’s growing role as part of bivalent systems shall play a significant part in the supply of heat to buildings.

Other developments that will help the uptake and effectiveness of heat pumps include:

• Use of high efficient compressors – centrifugal compressors are best when the load is steady and at or near the maximum rating, whereas scroll compressors are best when the load is variable.
• Compression systems for natural working fluids – avoiding the use of Fluorinated gases which may have Ozone depletion or global warming potential.
• Use of thermal stores, to permit operation during cheaper night time tariff periods and storing the heat for daytime use. This also helps to minimise heat pump cycling.

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