Heat Pumps

Heat pumps (HPs) can transfer heat from a lower temperature heat source to a higher temperature heat sink. This process does not occur spontaneously and needs a certain energy input to happen (i.e. electricity, fuel or high-temperature heat). This energy input is also transferred to the heat sink as in form of thermal energy. Therefore, HPs can be used for either transferring heat to a heat sink or removing heat from a heat source. They are indeed widely used to supply heat and cold for residential, commercial and industrial uses, e.g. space heating and cooling, and water heating. In the so-called compressor HPs the energy input is usually electricity or gas while in the sorption HPs (either absorption or adsorption HPs) the energy input is provided by different fuels or energy sources (i.e., gas, pressurized hot water, oil or waste heat). A heat pump can use natural heat sources (e.g. ambient air, river-, lake- and sea-water, as well as ground sources). As a consequence, HPs are also referred to as the “heat sink-to-heat source”, e.g. air-to-air HP, air-to-water HP, etc., In most national and international regulations the HP technology is regarded as a renewable energy technology.

The HPs energy performance is given by the coefficient of performance (COP), which is the ratio of the amount of energy injected into the heat sink to the amount of energy input (electricity, fuel or primary energy) to make the process happen, under well-defined boundary conditions. For the reversible HPs (which can provide either heat and cold) the energy performance is also given by the energy efficiency ratio (EER), which is the ratio of the amount of energy taken to the heat source to the energy input. The seasonal performance factor, e.g. Seasonal COP or SCOP, is used to define the HP efficiency over an entire cooling or heating season. The SCOP values depend on the heat source and range from 2.3 to 4.5 for air-to-water HPs, from 2.2 and 4.5 for air-to-air HPs, and 3.5 and 6 for ground-source HPs (GSHPs).

In 2015, the total number of HPs in operation in Europe was approximately 29.5 million, with approximately 18.4 million in Italy, corresponding to an installed thermal capacity of approximately 121.7 GW. The Italian market is dominated by HPs for seasonal air conditioning. In 2014, the HPs produced approximately 110,000 TJ of renewable energy.