Heat Pumps (HP) are conversion devices able to transfer heat from a lower temperature heat source into a higher temperature heat sink. HP can provide heating, cooling, or domestic hot water both for residential and non-residential applications. There are several types of HP (electric compression heat pumps, gas driven HP, heat driven HP, etc.), with electric HP being by far the most common. An electric HP is made of five main components: an evaporator, a compressor, a condenser, an expansion valve and a refrigerant. The working principle is based on the use of a refrigerant that release heat to the working fluid (air or water). The refrigerant is compressed using electrical energy and the heat of the refrigerant is released during the process of condensation (heat sink), passing from gas to liquid state. The refrigerant then returns then to its gas state passing through an expansion valve and using a cold source (energy source) to reject the heat. When the HP is working in heating mode, the energy source is cooled down and heat is provided to the energy sink of the process, while in cooling mode the process work in reverse. An air source heat pump uses ambient air as heat sink. The parameter that has to be considered in the efficiency of a heat pump is the coefficient of performance (COP). Typical air source HP require one unit of final energy (electricity) to provide 3-5 units of heat output and 2-4 units of cooling . This parameter expresses the ratio between thermal power generated and the electricity consumed. Due to its versatility and high energy efficiency, the development of HP technology will be crucial in this transition towards electrification and decarbonisation of cities.
Heat pumps have traditionally used refrigerants with high global warming potential (GWP) releasing greenhouse gases to the atmosphere (through leakages). The use of low-impact refrigerants is a need already covered by regulations, and expected to increase in the future. In fact, F-Gas regulation imposes a series of restrictions on the use of refrigerants until 2030, phasing out some higher GWP refrigerants soon, which paves the way for the use of natural refrigerants (e.g. CO2, propane and ammonia).
Heat pumps have characteristics that make them very interesting for use in near-zero energy buildings (NZEB), district heating and cooling networks (distributed in every building or central heat pumps), positive energy districts, energy communities and in combination with other sources (geothermal, solar, etc.). NZEB are designed to have a very low energy demand, which is largely covered by energy from renewable sources, including self-production of renewable energy. In this context and considering residential energy consumption, the HP is imposed as a technology for the future. However, they can also be installed in existing buildings, even without deep renovation.
New developments of HPs include for decarbonisation of industries, electrification of district heating and cooling networks, utilization of waste heat (to upgrade it and inject it in networks) and use of both condenser and evaporator at the same time (using a dual source heat exchanger, see example below).