A heat pump and a refrigeration unit are the same machine; the difference is in which end is put to use. In refrigeration the heat drawn by the evaporator is what is wanted and the heat rejected by the condenser is waste. In a heat pump, conversely, the heat given by the condenser is the product. Units fitted with a four-way valve reverse the cycle and, with the same heat exchangers, cool in summer and heat in winter. In air-source units the outdoor unit works as the evaporator in winter, frosts up and defrosts. Domestic air conditioners are the best-known example of this arrangement.
Its efficiency is measured by the heating coefficient of performance (COP): the heat delivered for 1 kW of electricity consumed. Typical values are 3–5; that is, the same heat is obtained with a third to a fifth of the electricity of an electric heating element. The COP depends on the difference between the source and delivery temperatures: for every 1 K the difference grows, efficiency falls by 2–3%. That is why underfloor heating at 35 °C works much more efficiently than a radiator circuit at 60 °C.
In food and cold storage plants the most sensible heat pump is the one that uses the waste heat of the refrigeration system as its source. Water arriving from the condenser circuit at 25–35 °C is raised to 60–80 °C by the heat pump and used for pasteurisation, washing or drying. Because the source is warm the COP rises to the 4–6 level. Dairy, meat and beverage plants that need both cooling and heating at the same time benefit most from this solution.
In selection the water temperature required, the distribution of the heat demand through the day and the lowest value of the source temperature are established. The capacity of air-source units falls on exactly the cold days when the demand is highest; at −10 °C it can drop to 60–70% of the rated value. For high temperatures natural refrigerants such as R290, R744 and R717, or R134a, come to the fore. A storage tank prevents the compressor from cycling on and off at short intervals. The payback on the investment is directly related to the annual running hours.