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SERIES 23 / Module 03 Heat Pumps


Heat pumps Eur Ing John Pooley, CEng. CMC, FEI, FIC H


eat pumps are a well- established technology that are currently seen as a key element in meeting our


future net zero commitments as a replacement for fossil fuel (oil and gas) heating systems. However, they are not a simple drop-in replacement. The current focus for heat pumps is on the replacement of domestic/residential heating systems. The government has an ambition of 600,000 heat pump installations a year by 2028 – this is from a base of 72,000 per year in 2022. However, heat pumps also have a role to play in both commercial and industrial applications.


How do heat pumps work? Heat pumps work to transfer energy from a low temperature source (for example, ambient air, water, ground or waste heat) and raise it to a higher, useful temperature. The renewable nature of the heat source makes a heat pump a low carbon solution – the level of which is dependent on the energy used to drive the heat pump. To be a true low carbon system the ‘drive’ energy needs to be renewable energy. The most usual form of the heat pump is based on the vapour compression system driven by an electric motor. There are also gas-fired absorption systems. One way of looking at a heat pump


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is to see it as a refrigeration system working in reverse. The objective of a refrigeration system is to make the controlled space cooler and reject the heat. With the heat pump we are


making the controlled space warmer. The key components of a basic


heat pump are: an evaporator to collect the heat from the source (e.g. outside air); a compressor to raise the pressure and temperature of the refrigerant; a condenser to deliver the thermal energy into the building/ process; an expansion valve to lower the pressure and temperature of the refrigerant. Input energy is required for the compressor (the largest energy use of the system) but also for fans and pumps that are part of the system. The more efficient these components the better the overall performance of the system. The performance improvements in heat pumps are closely related to those for other refrigeration systems. For example, more efficient compressors, variable speed drives, etc.


Types of heat pump system Heat pumps are normally described by the heat source they utilise


● Air source heat pumps (ASHP) – typically ambient air


● Ground source heat pumps (GSHP) – soil and aquifers


● Water source heat pumps (WSHP) – lakes, ponds, rivers, etc


● Waste heat recovery heat pumps (WHRHP) – could be air or water


Whatever type of heat pump is used,


the choice of refrigerant is important. Ideally the chosen refrigerant should have the lowest possible Global Warming Potential (GWP).


Air source heat pumps (ASHPs) This is the most common type of heat pump. In commercial applications it is not uncommon to combine summer cooling and winter heating with a ‘reversible’ heat pump unit. We may also see this happening residentially if the frequency of heat waves continues. A key aspect is the location of the outdoor unit. As with the outdoor unit for an AC unit there should be free air flow around the unit. Poor air flow and coil fouling can lead to a significant decline in performance. An installation aspect of increasing importance, particularly with domestic installations, is the noise level of the external unit – not only for the householder, but also neighbours. Current regulations require that heat pumps installed from April 2021 onwards are allowed to produce a maximum noise of 40 decibels in the evening and at night between 7pm and 7am. This is calculated at the property boundary with neighbours or at an open window on the same plot. During the day, the noise may be slightly higher at 45 decibels.


Ground source heat pumps (GSHPs) The temperature at around 2m below ground remains between 8-12ºC


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23


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