In my musings and putative debunking of the heat pump, I have been blindsided up to now by focusing on the heat pump itself. I don’t feel too bad about this as everyone else who writes about heat pumps has been similarly led astray. Thinking of the heat pump itself leads to agonizing about its internal mechanics, the Carnot cycle and a mysterious factor, the COP, the coefficient of performance which is relates the amount of energy coming out of the system to the electrical energy required to drive the pump.
Forget all that stuff for the moment and consider the heat pump as a
black box with three connectors. There is electricity going in to power
the system. There is heat coming into your home. And there is heat
going into the system from some external source. If we imagine a
perfect heat pump which runs on zero electricity, then the energy from
the outside going in is the same as the energy produced in your home.
So what is this perfect heat pump actually doing? It is not multiplying
energy, it is transforming it. A heat pump transforms a high heat flow
at a lower temperature into a lower heat flow at a higher temperature
(that’s why the fan on the outside is a lot bigger than the one on the
inside). But the energy on both sides is the same.
So where are the calories or kilowatts actually coming from? This is the big question that nobody seems to want to address. The rationale for the heat pump is that it chills some fluid outside the house, sucking out the calories and restituting them on the inside. let’s consider some different configurations. An air source heat pump has a lot of source material to play with, but air is not a great carrier of calories, with a very low specific heat. The fan has to be big and work hard (wasting energy), especially when the outside temperature is low (just when you need most heat). This prompts many to seek more other sources of lukewarm fluids, by drilling shallow boreholes or placing serpentines under the lawn, circulating around the subsurface. For closed loop systems, the problem is different to the air-air system. The subsurface may be capable of heating the fluid some, but as it is chilled and recycled, the issue is how quickly can the ground resupply calories into the chilled exhaust fluid. I recently visited a ground-source system that had both the outgoing chilled fluid and the incoming ‘heated’ fluid traveling in a closed loop down and up the same shallow borehole. I found this to be extremely optimistic, a sentiment that was borne out by the paltry 1°C temperature rise that I noted in the piping.
Many other configurations are in operation. All are predicated on a mystical COP that ‘produces 4 kw for every 1kw of electricity’. let’s say your house needs 16 kw to stay warm. If you believe in the COP, that means that you are looking to extract 12 kw from the cold air outside or from the lukewarm subsurface. That’s a lot of watts! Is it conceivable that there is anything like this energy available from these systems.
The earth scientists and reservoir engineers who are seeking rebirth in green energy are rather well placed to answer this question. Fluid flow in the subsurface? Check! Heat flow? Check! Heat exchange across pipe boundary? Check. All the engineering know-how is there in the reservoir/digital twin toolkit. Why is it not applied routinely? I suspect that this is because such calculations would show the extreme unlikelihood of achieving anything like the required amount of energy from air or the subsurface. Much easier to just blindside folk with the Carnot cycle and the ‘COP’.
Post script. France is currently heat pump crazy. The President of the République is installing a ground-source heat pump in the gardens of the Elysée Palace hoping to heat what is generally recognized to be a ‘passoire thermique’ (i.e. impossible to insulate). Good luck with that Monsieur le Président!
Post post script. While thinking
about this editorial I was surprised to see a report on France’s main
TV channel on a heat pump that is heating Marseilles with energy
derived from the Mediterranean sea! A map of Dalkia’s ‘thalassothermie’
systems popped up showing future installations located all around the
French coastline. I was shocked. It was like the TV was talking to me!
Perhaps I am crazy, although there may be an explanation for this too.
Dalkia is a subsidiary of France’s EDF, the main supplier of grid
electricity. Now ask yourself, who is likely to benefit most if heat
pumps are actually getting most of their energy from the grid? Who will
benefit when all those heat pumps switch over to cooling in the summer
making electricity consumption rocket?
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