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How do heat pumps work?

Last week, I started writing an article about whether a heat pump is cheaper to run than a conventional heating system, but I realised that the concept of heat pump efficiency might not make sense – how can a piece of equipment have an efficiency of more than 100%? Well, it can’t, but the term efficiency is commonly used when describing heat pumps and I feel it’s misleading. I ran it past Fran, my long-suffering wife. She glazed over a bit and then said that she would summarise heat pumps like this:

"Let’s say you put 1kW of energy into running a heat pump. Once that 1kW of electricity is in the heat pump, then the heat pump fairies apply their magic et voila, lots more energy comes out. For reference, heat pump fairies are closely related to T'internet fairies and Wi-Fi-Pixies."

At this point I had a decision to make – I could divorce Fran or I could write something that made more sense. I have decided to go with the latter, tempting as the former may have been!

So how does a heat pump work? A heat pump uses a refrigerant to extract heat from either the air, ground or water. Everything is in the name – it pumps heat. So, when someone states that a heat pump has an efficiency of 350%, what they really mean is that for every kilowatt (kW) of electricity put into the heat pump, the heat pump delivers 3.5 kW of usable heat.

The following schematic is a simplified diagram of an air source heat pump. The system uses a refrigerant which has its temperature and pressure adjusted in the cycle, enabling it to absorb heat from a low temperature source (the air in this case) and transfer the absorbed heat at a usable temperature to the heating system. The process is continuous while the heat pump is running:

  1. The compressor raises the pressure and temperature of the refrigerant. The temperature is high enough to be of use to a heating system (more than 70°C).

  2. The condenser heat exchanger transfers heat to the heating system. Most heat pumps can deliver a flow temperature of 50°C to 55°C, and some specialist units can deliver up to 80°C.

  3. The expansion valve drops the pressure and temperature of the refrigerant; the temperature is dropped to a very low value. One common refrigerant (R32) boils at minus 52°C, so even during the coldest winters in the UK, it can still extract heat from the outside air.

  4. The very cold refrigerant flows through the external heat exchanger where it picks up heat from the outside air. Heat energy wants to flow from a higher temperature to a lower temperature and so, even in the middle of winter, the outside air at minus 10°C is still 42°C warmer than the refrigerant at minus 52°C, so heat energy is transferred.

You might see on websites about heat pumps that they have an efficiency of 350%. What this really means is, for every kW of electricity put into the heat pump to run it, the heat pump delivers 3.5 kW of usable heat. It’s more useful to talk about either CoP (Coefficient of performance) or SCoP (Sessional Coefficient of Performance). The CoP is a measure of how much heat is delivered for one kW of energy input at a specific time, and the SCoP is the averaged-out CoP for a typical installation over a typical year. The most important figure is the SCoP as this will aid in determining how much the heat pump will cost to run.

Hopefully this explains what heat pump fairies do and how a heat pump delivers more heat than the energy put into it. For my next article, I will consider explaining the secret behind how magicians cut people in half….or I might look at whether heat pumps are cost efficient. We’ll see...

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