Heat Pumps (HP) are conversion devices able to transfer heat from a lower temperature heat source into a higher temperature heat sink. HP can be used to 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 of 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. 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 (NEZB), district heating and cooling networks (distributed in every building or central heat pumps), positive energy districts, energy communities and by 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 as a technology for the future. However, they can also be installed in existing buildings, even without deep renovation.
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New developments of HPs include decarbonisation of industries, electrification of district heating and cooling networks, utilization of waste heat (to upgrade it and inject it in networks) and for utilizing both sinks (condenser and evaporator) at the same time (using dual source heat exchanger, see example below).
A water source heat pump (WSHP) works on a similar principle to that of ground source heat pumps (GSHP) by rejecting or extracting heat to a water pipe system (open loop) or a water loop (closed loop). In the first case water is pumped from the source and the heat is extracted from the heat pump before being discharged again to the water source. In the second case sealed pipes with anti-freeze are submerged into the water source. The fluid is heated/cooled by the water and returns to the heat pump. WSHP can be connected to aquifers, rivers, lakes or the sea, and to wastewater, cooling water from industrial systems, or a district heating system [1]. Compared to air source HP, the efficiency of WSHP is increased by the higher heat transfer coefficient of water compared to air, making them more efficient. Moreover, the temperature of the water is more stable during the year compared to air. In the United States, ASHRAE sets the minimum efficiency requirements for WSHPs to be higher than air source HP being able provide 4 to 6 units of heating for every unit of energy consumed. In Europe the seasonal efficiency requirements set by regulation 813/2013 for water to water heat pump is set to 3.33 and 2.95 for low and medium temperature heat pump respectively [2]. With a water-source heat pump in heating mode, the heat of compression in the refrigerant circuit can be recovered for heating purpose.
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WSHP is a mature technology installed in both residential and non-residential buildings. Nevertheless, The WSHP industry is putting effort to further developments in in improving efficiency implementing technologies such as microchannel heat exchangers, variable-speed compressors, building management software integration, wireless thermostats, and occupancy sensors [3].
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