REFRIGERANTS (the working fluid inside the heat pump)
Every normal heat pump and fridge uses a fluid to enable the heat transfer. This pressurised fluid evaporates and condenses in different heat-exchangers in the unit.
(NB this is different to any glycol antifreeze used in the system)
Gone are the days of refrigerants that affected the ozone layer. They are now all ‘ozone-friendly’. However, most refrigerants do have a high global warming potential, meaning that they are far more potent than the gas – carbon dioxide. That said, the quantities used of these fluids is relatively very small.
The refrigerant within the heat pump is very unlikely to leak (when did you last hear of a refrigerator leaking its refrigerant?) but should it do so, this would detract from the net CO2 savings.
The following information attempts to give a balanced view of the issue and concludes that the savings made during the heat pump’s working life outweigh considerably any negative effects caused by loss of refrigerant.
That said, it is far from ideal having millions of tons of refrigerant out there sealed into equipment. The Montreal Protocol does at least help to contain these refrigerants by making it illegal to release them into the atmosphere. When equipment is scrapped, the refrigerant should be recovered and re-cycled.
The following table lists the Global Warming Potential (GWP) of various refrigerants. This gives a figure of how damaging the gas is, as compared to CO2 which is given a value of 1.
Inside a typical 8kW heat pump there is approximately 2kg of HFC refrigerant. A refrigerant will not wear out and it should stay contained within the unit for its whole life. It is a requirement to recover the refrigerant charge at the end of the heat pumps life. This can then be used in another heat pump, or destroyed.
As an example, I have taken a typical house with the figures below.
Given 2017 UK grid carbon intensity, and typical heat pump might produce around 1.8Tonnes of CO2 per year. Below it, I have estimated the equivalent quantity of CO2 in terms of global warming that would be produced if all the refrigerant were released to the atmosphere.
I have taken a 10-year period as an average worst case figure. (on average 1 loss of charge per 10 years). A few units will have failures and leaks, however most units should never leak over the equipment’s life, and say 80% of the refrigerant will be recovered when it is scrapped.
The following graph shows the total expected CO2 saving when using a heat pump (as numbers above), compared to various heating methods over 10 years. The dark columns show the reduction in saving if the refrigerant is lost in 10 years.
Whilst this might show that the commonly-used HFC refrigerants are acceptable, it is obviously preferable to use a much more benign alternative e.g. Hydrocarbon (HC) refrigerant if possible. Heat pumps using Carbon Dioxide as the refrigerant is a recent development. However, care should be taken when selecting such units to ensure that they will be energy-efficient for the application.
Refrigerant R290 has been used for many applications, but flammability concerns have limited its use Here is an example of a general trend back to use of hydrocarbons.
Notes and assumptions
Gas condensing boiler efficiency 88%
Oil boiler efficiency 87%
Electricity 100% efficient at point of use
2017 grid carbon 0.4kg CO2 per kWh of electricity
Heat Pump COP 3.4 (i.e. 2.35 kW input for 8kW output).