Coefficient of Performance

The efficiency of refrigeration systems and heat pumps is denoted by its Coefficient Of Performance (COP). The COP is determined by the ratio between energy usage of the compressor and the amount of useful cooling at the evaporator (for a refrigeration instalation) or useful heat extracted from the condensor (for a heat pump). A high COP value represents a high efficiency.

Most of the electric energy needed to drive the compressor is released to the refrigerant as heat. Therefore more heat is available at the condensor than is extracted at the evaporator of the heat pump.

For a heat pump a COP value of 4 means that the addition of 1 kW of electric energy is needed to have a release of 4 kW of heat at the condensor. At the evaporator side 3,0-3,5 kW of heat is extracted. The additional heat is generated by the compressor. On the other hand: For a refrigeration system a COP of 4 indicates that 1 kW of electricity is needed for a evaporator to extract 4 kW of heat. Due to this important difference in COP definition, for a heat pump one often speaks of COPh. In this abbreviation 'h' means heating.

The efficiency of a heat pump, COPh, depends on several factors. Especially the temperature difference between waste heat source and potential user is an important factor. The temperature difference between condensation and evaporation temperature mainly determines the efficiency: the smaller the difference, the higher the COPh. The figure on the left shows the influence of this temperature difference on the COPh value. These values are based on figures from a Grasso 65HP compressor with the refrigerant Ammonia. The figure shows an increase in COPh with an increasing evaporation temperature. Futhermore it shows a decrease in COPh with a decreasing condensation temperature. In general the COPh decreases with an increase in temperature difference between condensation and evaporation. The figure below gives an indication of the dependence of the COPh of an Ammonia heat pump as a function of this temperature difference.

Another important factor that influences efficiency is the applied refrigerant. Ammonia, for example, is a very efficient refrigerant with a COPh of 6 for a evaportion temperature of 30 °C and condensation temperature of 70 °C. These same conditions only give a COPh of 4,5 for refrigerant R134A. Other factors that will effect the efficiency of a heat pump are system controls, efficiency of pheripheral equipement like fans, pumps, etc.

Carnot efficiency

The theoretical maximum efficiency of a heat pump is described by the Carnot-efficiency:

The equation shows that the Carnot-efficiency depends on the condensation and evaporation temperature. With an ideal compression cycle without losses it is possible to achieve the Carnot efficiency. However, in practice there are a lot of parameters that have a negative influence on the efficiency. Therefore the real COPh is given by the product of the Carnot efficiency and the system efficiency:

The system efficiency is usually 50% to 70%.

Lorentz efficiency

With a transcritical heat pump the Carnot-efficiency can not be used, because there is no condensation temperature, but a temperature range in the gas cooler. The theoretic maximum efficiency of a transcritical heat pump is described by the Lorentz efficiency.

Tm is the mean temperature in the gas cooler. This temperature is calculated from the temperature at the inlet and the outlet of the gas cooler:

Similar to Carnot, the Lorentz efficiency will not be reached in practice due to all kind of losses. To determine the real COP, a system efficiency must be taken into account: