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 uses a refrigerant to 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 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 works in reverse. Typical HP require one unit of final energy (electricity) to provide 3-5 units of heat output and 2-4 units of cooling [1]. The ratio between thermal power generated and the electricity consumed is usually referred to as the coefficient of performance (COP) which is the main parameter to express HP efficiency. 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.
Note: BH = borehole.
Source: Bonamente E, Aquino A. Life-Cycle Assessment of an Innovative Ground-Source Heat Pump System with Upstream Thermal Storage. Energies. 2017; 10(11):1854. https://doi.org/10.3390/en10111854
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, the 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 an important technology for the future. However, they can also be installed in existing buildings, even without deep renovation.
New developments of HPs include decarbonisation of industries, electrification of district heating and cooling networks, utilisation 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).
Ground source heat pumps (GSHP) extract the energy from the ground, achieving a higher efficiency compared to air source HP. This makes them highly relevant from a climate mitigation perspective. However, GSHP are more expensive in terms of up-front cost. This makes them particularly relevant in an urban context for apartment buildings, or for shared schemes among groups of houses where the drilling costs can be shared, or for district heating and cooling networks.
In a conventional GSHP, a horizontal or vertical collector is used to extract thermal energy from the ground which is transferred to the HP refrigerant through an external working fluid (brine or water) working in a closed loop. However, direct expansion of the refrigerant of the heat pump can also be done removing the external closed loop. This technology usually refers to direct expansion GSHP (DX-GSHP) and is considered more efficient and cheaper. Nevertheless, DX-GSHP has more complications during the design and needs a high refrigerant charge. Moreover, it may lead to environmental problems such as ground pollution [2].
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Nowadays, GSHP are widely installed in both residential and non-residential buildings. The annual rate of installation is estimated to increase of 10-12% every year [3].
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