How Heat Pumps Work.

HOW HEAT PUMPS WORK: PART 2: WATER IS NOT PUMPED IN A WATER-SOURCE HEAT PUMP Last night I was contacted by a Brussels lobbyist for heat pumps (of 17 years) to ask me how heat pumps work, but then told me I was misleading; our parting was 'tense'. It is apparent to me that one cannot begin to understand a thermodynamic cycle unless one physically has an understanding for what things look like. With respect to a recent piece in the Guardian (see attached figure taken from there) about a water-source heat pump the journalist has drawn a situation where ‘hot water’ is extracted and sent to the surface, whence ‘5 degC of heat’ is taken, whence the water is returned downhole so as to heat up again. It is as plain as day to me that this is what the majority of people commenting on this scheme think is happening, but it is not. There is no chance of anybody even beginning to understand a water source heat pump if this is the picture one has in one's mind’s eye. Water is not being pumped at all, neither from the lower seam nor back into the upper seam. Water is not the working fluid. I know it looks that way in the Guardian graphic, but they don’t know what they are on about. The working fluid will be a refrigerant, which could be butane, and heat is transferred (at low temperature) from the water in the flooded mine seam, to a coil containing the working fluid, typically causing this fluid to evaporate from liquid to gas. The refrigerant is then compressed and sent to the surface, whence it condenses and gives up its heat to the high temperature sink. The refrigerant is then throttled back downhole and the cycle begins again. Note that: 1.      The compressor requires energy in the form of work (i.e. electricity) to run which is typically taken from the grid and, unless the grid is supplied by completely renewable energy, it is not a renewable process. The ratio of heat supplied to the high temperature sink and the work consumed by the pump is called the coefficient of performance, and we hope that is greater than 1 (which it almost always is, of course) otherwise we’d have been better off just using the work supplied to the compressor and converting it directly to heat. 2.      Water is not being pump anywhere at all. Nowhere. It is the working fluid (possible butane) that is making the round trip from downhole to the surface and back again, in this case. Other water-source heat pumps may have a coil in a nearby lake or river, for instance. Ground-source heat pumps have a coil dug underground. Air-source heat pumps use as their low temperature source the air, and so their coils are outside in the air. It really isn’t that difficult to picture. But picture it you must, if you are going to be commenting on water-source heat pumps.