The COP, or Coefficient Of Performance, describes the efficiency of the heat pump and is defined as the ratio between the useful heat transfer for heating or cooling and the required drive energy. The useful energy can either be heating or cooling energy depending on if the heat pump is used to provide heating or cooling. The COP is normally related to a specific operating condition. For cooling and air conditioning applications EER, Energy Efficiency Ratio, is sometimes used instead of COP.
The SCOP, or Seasonal Coefficient of Performance, describes the average COP during a heating season. Depending on definition the SCOP value could also include other parts of the heating system than the heat pump only. For cooling and air conditioning applications SEER, Seasonal Energy Efficiency Ratio, is normally used.
SPF, or Seasonal Performance Factor, is another expression for the average performance of a heat pump during a defined period. It is often used for evaluation or prediction of the performance in a real application and normally italso takes into account the system performance. Which parts of the system that are to be included are different depending on which system boundaries that are used.
For applications where fuel (e.g. gas) is used as drive energy for the heat pumping process, instead or as a complement to electricity, the factor PER, Primary Energy Ratio, SPER, Seasonal Primary Energy Ratio is often used.
There are a number of factors that will affect the efficiency of a heat pump. Major improvements in the efficiency of heat pump systems have been achieved through developments in the fields of heat exchangers, compressors, motors, motor drives and controls. Currently, the best opportunities for improving efficiency are in optimizing the sources and sinks, minimizing auxiliary drive powers and optimizing the temperature and capacity controls. Therefore, when choosing a heat pump and the related sources and sinks, the following things have to be considered:
- Climate – The indoor and outdoor climates and the indoor heat loads will decide the annual heating and cooling demand and maximum peak loads;
- Temperature – the temperatures of the heat source and distribution systems for heating and cooling affect the COP and the capacity of the heat pump;
- Auxiliary equipment – energy consuming auxiliary equipment such as pumps, fans and supplementary heat for bivalent systems could have a much larger impact on system efficiency than is commonly perceived;
- Technology – system design and selection of components such as motors, compressors and heat exchangers have a decisive influence on the operating characteristics of the heat pump;
- Size – sizing of the heat pump and its components in relation to the heat demand will affect energy coverage and part-load operation;
- Control system – the heat pump control system will affect how closely supply and demand for heating and cooling will match and how closely the actual operating temperatures will be to the theoretically optimal temperatures.