The performance of a heat pump is dependent on four main things:
1) Ambient conditions – the temperature conditions will affect performance but are obviously out of anyone’s control!
2) Heating system performance – the technical decisions made by the designer will impact overall performance. It’s vital to ensure the system meets the requirements of the building AND the heat pump itself.
3) Build quality and control of the refrigerant circuits in the heat pump. Things like how optimised the component selections are for the required temperatures, how efficient the compressor operation is for the refrigerant used and how the defrost works.
4) The choice of refrigerant itself.
In this blog we are going to focus on the last point because it is often overlooked. We have written before about why we use CO2 in our heat pumps and why natural refrigerants are a good choice. Below we’re going to explain specifically how refrigerant choice affects heat pump performance.
How To Choose The Right Refrigerant For Your Heat Pump
When designing a heat pump system you need to balance the performance of the heating system (in retro-fit, this is all to do with the cost and practicality of changing components, whereas in new build you can go straight for max efficiency which is CO2) with the performance of the refrigerant.
The top factors that need to be considered are:
1) Flow temperature. Can the right temperature be generated? Not all refrigerants can generate domestic hot water or process heat.
2) Temperature differential, or DT. A high delta temperature means a more efficient heat delivery system and usually no pipework changes in retro fit.
3 Functionality within expected ambient conditions. For example – r32 will not generate temperatures over 45 at zero ambient.
4) GWP – global warming potential. The higher the number the higher the impact. The end user will have to report losses in their carbon footprint assessment. Commercial systems typically lose 10% a year and if you’re using an HFO blend that means a full replacement.
5) Toxicity. HFOs ( Hydrofluoroolefins (HFOs) are a type of refrigerant that are unsaturated organic compounds composed of hydrogen, fluorine and carbon) break down in the environment and are poisoning water, currently the subject of litigation in the US and high on the regulators’ agenda in Europe.
6) Flammability. Some are very flammable, some semi-flammable and some not at all. What are the risks and mitigations for your project? E.g. Propane is flammable but in a rooftop heat pump it is safer.
7) Cost to buy top up/replacement. Single molecule refrigerants can be topped up but blends require full replacement and safe disposal of the old gas. The table below shows relative pricing of different refrigerants:
8) The Coefficient of Performance (COP) and Seasonal Coefficient Of Performance (SCOP). You’ll need to consider the COP and SCOP that can be expected in the specific circumstances – how efficient will the heat pump be in your use case? Here’s a comparison of common refrigerants at -5 Degrees C.
9) Regulatory risk implications. With the EU proposing to ban more refrigerants and increase the phase down, could your heat pump end up being replaced before end of life? Some refrigerants like r410a are being replaced by new blends but these come with toxicity issues.
To add to the complexity different refrigerants can be used in cascade systems to generate different temperatures with a different set of performance characteristics!
Considering all of the factors above, it’s impossible to decide on a refrigerant without full knowledge of the specific circumstances the heat pump will be used for. The above is non an exhaustive list – it merely serves as an illustration. But considering the questions above, it should be clear why at Clade we prefer natural refrigerants – we build both CO2 and propane heat pumps for our clients depending on the use case.