Heat pumps: a terrible idea

Did you know that you can make your own heat pump using some everyday household items? First, empty your fridge and remove its door. Next, take your front door off its hinges. Now carefully manoeuvre your fridge into position so that its interior cavity faces outwards into the open air. Carefully seal the gaps around the edges using duct tape. Now plug the fridge in, and turn it up to high. Enjoy the warmth given off by the vanes at the back of the fridge that were formerly tucked away against the wall of your kitchen.

Does this sound like an efficient way to heat your home?

Of course it doesn’t, because of course it wouldn’t be an efficient way to heat your home. You would in effect be paying money to your electricity supplier for slightly cooling the outside air, with the side effect of slightly warming the air inside your home.

Would you be doing something good for the environment? — If cooling the outside air down a bit means doing the environment a favour, maybe so for a moment or two, but in accordance with the Second Law of Thermodynamics, it takes more energy overall to heat the inside and cool the outside than it would take to just heat the inside on its own. Furthermore, in accordance with the law of Conservation of Energy, that extra heat energy wouldn’t just go away. No matter how well insulated your house may be, the extra energy must eventually dissipate into the outside world, reversing the effects of the cooling whose generation it was a by-product of in the first place.

That is all a heat pump amounts to: a sort of fridge doing the opposite of what fridges usually do. No matter how super-efficient a heat pump may be, no matter how stringently your house may be insulated, your heat pump cannot contravene two of the most basic laws of physics. It unavoidably creates a warmer place and a cooler place, and it uses energy to do so.

Why do otherwise intelligent people think that a heat pump can do something magical? Speculating, I think many of them are nowadays so worshipful of “science” and “the science” — without knowing much about it — that they think it’s capable of miracles such as contravening its most basic laws. They like the sound of a machine that “works like a fridge in reverse”, and assume that since “fridges cool things down, heat pumps warm them up”, and since “fridges use energy, heat pumps produce energy”. This sort of thinking led earlier generations to believe in perpetual motion machines.

From The Daily Telegraph, 20 October 2021:

It works rather like a fridge in reverse, says Will Rivers, senior manager at the Carbon Trust consultancy. “A heat pump is taking a very large quantity of low-temperature heat [from outside the house], and then compressing it into a smaller volume of high-temperature heat. It might only be two degrees outside, but there is still energy in that air if you capture enough of it.”

I’m afraid this is very misleading. There is indeed energy in two-degree air, but it’s not usable energy unless you have access to something whose temperature is lower than two degrees. In the absence of such access, it takes extra energy from somewhere else to “capture” the heat in two-degree air, and again in accordance with the Second Law of Thermodynamics, it would take more energy to “capture” that heat than the energy of the heat that gets “captured”.

If you are lucky enough to live in a place like Iceland, where there is an abundance of geothermal energy and no shortage of cold air, then there is plenty of usable energy available. That usable energy could be harnessed to provide energy to drive a heat pump which in effect “moves heat around”, from a hot spring (say) to your home. But if you live in the UK, your only hope of avoiding an energy deficit with a heat pump is by digging a very deep hole.

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