NET ZERO & ZERO CARBON BUILDINGS
What to consider when choosing a heat pump?
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Jason Allen, commercial product manager, Groupe Atlantic UK, ROI & NA looks at synthetic and natural refrigerant options and discusses the factors to consider when choosing the heat pump with the right refrigerant
n line with the Government’s drive to decarbonise the UK , heat pumps make a valuable contribution to the delivery of a low-carbon future, but at the heart of an efficient heat pump system lies an important element: the refrigerant.
The role of refrigerants in air source heat pumps
Essentially, refrigerants are the working liquids used in the process of the refrigeration cycle but first, let’s look at how this works in terms of a typical heat pump system. Heat pumps gather heat energy from the surrounding air, which, via a heat exchanger, is transferred into the heat pump refrigerant and turned into vapour. This vapour is then passed through a compressor and tuned into a high-pressure, high-temperature refrigerant, raising the temperature to a point that can deliver heat throughout a building’s heating system, via a Plate Heat Exchanger. Essentially, refrigerants are the lifeblood of heat pumps.
With traditional refrigerants such as CFCs (halocarbons) and HCFCs (hydro halocarbons) now phased out due to their high Ozone Depletion and Global Warming Potential (GWP), refrigerant types are now broadly split into two categories: HFCs such as R32 and natural refrigerants, which include R290 and CO2. Factors to consider when choosing the right refrigerant for your heat pump:
Flow temperature of the heat pump
When it comes to heat pump performance, one of the hot topics of discussion is temperature. It is widely believed that a heat pump needs to generate the same temperatures as a gas boiler. This is a myth.
Another common misconception is that heat pumps using synthetic refrigerants such as R32 cannot reach high enough temperatures. The
reality is, many conventional R32 air source heat pumps on the market can reach up to 60°C and when controlled correctly for the type and construction of a building, they can efficiently keep a building warm all year round. In comparison, natural refrigerants such as R290 and CO2. can produce very high temperatures of up to 75°C. However, assuming the heat pump is calibrated correctly, this temperature will not need to be reached in most cases but there are some instances where it might be appropriate. For installation projects such as process heating (where specific temperatures are required for manufacturing or cleaning), large heating systems, such as district heating and shopping centres, and settings where a large volume of domestic hot water is needed, such as healthcare and leisure settings, where there are increased controls for risks such as Legionella, it may be more appropriate to use a high-temperature heat pump with a natural refrigerant. In terms of generating domestic hot water (DHW), a heat pump using R32 would be perfectly adequate, but it would need to be supplemented by another means of heating, to raise the temperature to 62C and prevent harmful Legionella bacteria growth from forming. This could be achieved with another heat source, such as a boiler, or by the use of immersion heaters but this can come with higher installation, running and capital costs. In this type of setting, a heat pump using natural refrigerant could reach the desired heat without additional backup and have a high COP at useable temperatures from 60C upwards.
The temperature differential (Delta T) of a heat pump system is another factor that should be considered as this can help to achieve an efficient heat delivery system. An efficient system should return water that is significantly colder than when it left the heat pump. The benefits of which are: smaller pipes needed for the same kWh of heat delivered, lower pumping cost because the volume of water is lower and decreased heat loss from the
return pipe. Although possible to achieve heat pump designs based on 5 or 10C Delta Ts with refrigerants, such as R32 and R290, heat pumps using CO2 can perform equally as well with a much higher Delta Ts for instance, CO2 is actually at its most efficient at a temperature differential of 30 or 40°C.
Concerns over Global Warming
When we talk about the Global Warming Potential (GWP) of a refrigerant, we’re referring to the measure of how much heat a greenhouse gas traps in the atmosphere over a specific period (usually 100 years), compared to carbon dioxide (CO2). Simply put, the higher the GWP value, the higher the global warming potential, so the ultimate goal is to use refrigerants with lower GWP to mitigate climate change. This is where natural refrigerants take the spotlight.
Based on substances that already exist in the natural environment, natural refrigerants generally have a low impact on the environment with close to zero levels of ozone depletion and global warming potential. That being said, the quantities of HFC refrigerant used in today’s heat pumps is typically small, with modern heat pump design offering low risk when it comes to leakage. Regardless, it is an important consideration for end users who need to report losses in their carbon footprint assessments.
Understanding COP and SCOP are two important factors that also need to be considered. Providing a measure of a system’s performance, they enable environmental factors to be balanced against any practical considerations that need to be accounted for. When we refer to COP, we’re referring to the ratio of heating capacity and total power input for example, a COP of 3 would mean, you generate 3 kW of heating capacity by using 1kW of electric power (assuming the
18 BUILDING SERVICES & ENVIRONMENTAL ENGINEER MAY 2024
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