Heat pump performance
The heat delivered by a heat pump is theoretically the sum of the heat extracted from the heat source and the energy needed to drive the cycle. The steady-state performance of an electric compression heat pump at a given set of temperature conditions is referred to as the coefficient of performance (COP). It is defined as the ratio of heat delivered by the heat pump and the electricity supplied to the compressor.
For engine and thermally driven heat pumps the performance is indicated by the primary energy ratio (PER). The energy supplied is then the higher heating value (HHV) of the fuel supplied. For electrically driven heat pumps a PER can also be defined, by multiplying the COP with the power generation efficiency.
The COP or PER of a heat pump is closely related to the temperature lift, i.e. the difference between the temperature of the heat source and the output temperature of the heat pump. The COP of an ideal heat pump is determined solely by the condensation temperature and the temperature lift (condensation - evaporation temperature).
Figure 1 shows the COP for an ideal heat pump as a function of temperature lift, where the temperature of the heat source is 0°C. Also shown is the range of actual COPs for various types and sizes of real heat pumps at different temperature lifts.
The ratio of the actual COP of a heat pump and the ideal COP is defined as the Carnot-efficiency. The Carnot-efficiency varies from 0.30 to 0.5 for small electric heat pumps and 0.5 to 0.7 for large, very efficient electric heat pump systems.
An indication of achievable COP/PERs for different heat pump types at evaporation 0°C and condensing temperature 50°C is shown in Table 1.
The operating performance of an electric heat pump over the season is called the seasonal performance factor (SPF). It is defined as the ratio of the heat delivered and the total energy supplied over the season. It takes into account the variable heating and/or cooling demands, the variable heat source and sink temperatures over the year, and includes the energy demand, for example, for defrosting.
|Heat pump type
||2.5 - 5.0
||0.8 - 2.0 |
||1.0 - 1.8 |
The SPF can be used for comparing heat pumps with conventional heating systems (e.g. boilers), with regards to primary energy saving and reduced CO2 emissions. For evaluating electric heat pumps the power generation mix and the efficiencies of the power stations must be considered.
Factors affecting heat pump performance
The performance of heat pumps is affected by a large number of factors. For heat pumps in buildings these include:
- the climate - annual heating and cooling demand and maximum peak loads;
- the temperatures of the heat source and heat distribution system;
- the auxiliary energy consumption (pumps, fans, supplementary heat for bivalent system etc.);
- the technical standard of the heat pump;
- the sizing of the heat pump in relation to the heat demand and the operating characteristics of the heat pump;
- the heat pump control system.
|Heat pump type 
||10 - 30
|Closed cycle, electric
||3.0 - 8.0
|Closed cycle, enigne
||1.0 - 2.0 |
|Absorption (Type I)
||1.0 - 1.8 |
||0.45 - 0.48 |
Industrial heat pumps often have a higher COP/PER than heat pumps for buildings. This is mainly due to small temperature lifts and stable operating conditions. Typical COP/PER ranges for industrial heat pumps are given in Table 2.
 For a discussion of the various heat pump types see the section on Heat Pump Technology and Heat pumps in industry.