What is a heat pump and how does it work?

  • On April 15, 2019

A heat pump is an electrical device that transfers heat from a low-temperature medium to a high temperature one. Refrigerators and heat pumps operate on the same cycle but they differ in their objective. The objective of a refrigerator is to maintain the refrigerated space at a low temperature by removing heat from it. Discharging this heat to a higher temperature medium is merely a necessary part of the operation, not the purpose. The objective of the heat pump, however, is to maintain a heated space at a high temperature. This is accomplished by absorbing heat from a low temperature source, such as well water or cold outside air in winter, and supplying this heat to the high-temperature medium such as residential house.

Heat pumps transfer heat by circulating a substance called refrigerant through a cycle of evaporation and condensation. A compressor pumps the refrigerant between two heat exchanger coils. In one coil, the refrigerant is evaporated at low pressure and absorbs heat from its surroundings. The refrigerant is then compressed and supplied to the other coil, where it condenses at high pressure and temperature. At this point, it releases the heat it absorbed earlier in the cycle. The necessary energy to transfer the low potential heat to high potential heat is the input power of the compressor.

As already mentioned, an air source heat pump absorbs heat from the outdoor air in winter and transfers it to the building. In summer mode the heat pump works as a refrigerator, e.g. it absorbs heat from the building and rejects that heat into outdoor air. The heat pump cycle is fully reversible, and heat pumps can provide year-round climate control for your home – heating in winter and cooling and dehumidifying in summer. Since the ground and air outside always contain some heat, a heat pump can supply heat to a house even on cold winter days. In fact, air at –15°C contains about 80 % of the heat it contained at 20°C.

Air source heat pumps are the most common type of heat pumps found on the market. However, ground source (also called geothermal or water-water) heat pumps, which draw heat from the ground or ground water, are becoming more widely used. The difference is that they do not heat directly the air in the building but prepare intermediate hot water. That water is then distributed to the indoor heating units such as fan coils, radiators or underfloor heating.

The efficiency of the heat pumps and refrigerators is described by the Coefficient of Performance (COP). The coefficient is defined as the ratio of the output heat to the input work. By definition this coefficient could not be lower than 1. The COP values are indicative of the efficiency of the heat pump systems, but only in fixed conditions (outside air and room air temperature). In order to be able to describe the performance of the systems throughout the year (i.e. under different conditions), two more concepts are introduced – SCOP (Seasonal Coefficient of Performance) and SEER (Seasonal Energy Efficiency Ratio). The typical values for the SEER are around 5÷8, while the SCOP values are about 4÷5.