Here I am responding to Gideon who wants to understand heat pumps. We are only discussing the science and engineering of them, and whether domestic use is a good and economic idea.
Here I am responding to Gideon who wants to understand heat pumps. We are only discussing the science and engineering of them, and whether domestic use is a good and economic idea.
Let's look at the COP of heat pumps, first from a theoretical perspective.
All thermal machines operate with the Carnot cycle, a thermodynamic description of an ideal “heat engine”, typically your vehicle engine and your refrigerator, or the vehicle air conditioning. The ideal efficiency of all of these depends on two temperatures, the input temperature and the output temperature. The maximum COP of any of these is given only by these two temperatures, and for a heat pump is
Input temperature / (output temperature - input temperature)
measured in absolute temperatures (Kelvin). Zero centigrade is about 273K.
These theoretical numbers sound quite reasonable, for example:
Input 273K (0C), output 293K (20C) gives 273/20 = 13.65
Input 263K (-10C), output 333K (60C) = 4.38
These numbers assume adiabatic conditions (no heat loss) in the machine, In practise this is not possible, and the loss may be as much as 50%, making the numbers much less impressive. All this ignores the mechanical input required and thus the electricity that will be used. This depends on the working fluid (gas), and the best were “designer” fluorocarbons, which are now outlawed. This leaves hydrocarbons and ammonia, propane being a common choice. Ammonia is used in large plants because it is very good, but it has unfortunate properties, it is both very poisonous and explosive under the conditions found when used as a refrigerant, very unsuitable for domestic use.
The effect of increasing temperature differentials across the pump is that the quantity of heat transferred falls and the driving power increases, and neither is desirable. The best solution to this is to transfer the input heat (low temperature) to air, as in conventional air conditioning. However, this is difficult to install in existing properties as ducts are needed to each room, for which there is no obvious space. The air also needs to be isolated acoustically from the fan used to transfer it from the heat exchanger, another inconveniently big box.
Therefore the rather unthinking government suggests heating the water used to transfer the heat (very efficiently) from a gas boiler, but then finds that the COP becomes low! OK, reduce the water temperature they say, but this is foolish. The output heat from radiators (actually convectors mainly) responds to another thermodynamic number, and as the temperature is lowered the heat transfer reduces by somewhere between a square and quadratic law! Therefore MUCH larger radiators will be needed, even if the temperature is only lowered by 10 degrees to improve the COP.
The overall effect is therefore that installation is very expensive, and the achieved COP is quite low, and unless electricity is very cheap the system is much more expensive than gas. All of these things work in exactly the wrong direction, as does the next problem and that is that the installed overall supply capacity to each house is between 1 and 2 kW, and if many change to a 24/7 heat pump the supply will fail. National Grid has an estimate that upgrading all the supplies to 6kW would cost £4.2 TRILLION pounds, and cannot be achieved even at this cost in the suggested timescale. It would need another 30 nuclear power stations like Hinkley C to be built starting now, or 1000 of the “mini modular” type not yet designed, and the electrical infrastructure in the entire country to be replaced. Clearly, this is impossible. You will also realise that wind and solar are very unsuitable to provide this energy, during long cold periods it is either dark or there is very little wind!
Let's look at the COP of heat pumps, first from a theoretical perspective.
All thermal machines operate with the Carnot cycle, a thermodynamic description of an ideal “heat engine”, typically your vehicle engine and your refrigerator, or the vehicle air conditioning. The ideal efficiency of all of these depends on two temperatures, the input temperature and the output temperature. The maximum COP of any of these is given only by these two temperatures, and for a heat pump is
Input temperature / (output temperature - input temperature)
measured in absolute temperatures (Kelvin). Zero centigrade is about 273K.
These theoretical numbers sound quite reasonable, for example:
Input 273K (0C), output 293K (20C) gives 273/20 = 13.65
Input 263K (-10C), output 333K (60C) = 4.38
These numbers assume adiabatic conditions (no heat loss) in the machine, In practise this is not possible, and the loss may be as much as 50%, making the numbers much less impressive. All this ignores the mechanical input required and thus the electricity that will be used. This depends on the working fluid (gas), and the best were “designer” fluorocarbons, which are now outlawed. This leaves hydrocarbons and ammonia, propane being a common choice. Ammonia is used in large plants because it is very good, but it has unfortunate properties, it is both very poisonous and explosive under the conditions found when used as a refrigerant, very unsuitable for domestic use.
The effect of increasing temperature differentials across the pump is that the quantity of heat transferred falls and the driving power increases, and neither is desirable. The best solution to this is to transfer the input heat (low temperature) to air, as in conventional air conditioning. However, this is difficult to install in existing properties as ducts are needed to each room, for which there is no obvious space. The air also needs to be isolated acoustically from the fan used to transfer it from the heat exchanger, another inconveniently big box.
Therefore the rather unthinking government suggests heating the water used to transfer the heat (very efficiently) from a gas boiler, but then finds that the COP becomes low! OK, reduce the water temperature they say, but this is foolish. The output heat from radiators (actually convectors mainly) responds to another thermodynamic number, and as the temperature is lowered the heat transfer reduces by somewhere between a square and quadratic law! Therefore MUCH larger radiators will be needed, even if the temperature is only lowered by 10 degrees to improve the COP.
The overall effect is therefore that installation is very expensive, and the achieved COP is quite low, and unless electricity is very cheap the system is much more expensive than gas. All of these things work in exactly the wrong direction, as does the next problem and that is that the installed overall supply capacity to each house is between 1 and 2 kW, and if many change to a 24/7 heat pump the supply will fail. National Grid has an estimate that upgrading all the supplies to 6kW would cost £4.2 TRILLION pounds, and cannot be achieved even at this cost in the suggested timescale. It would need another 30 nuclear power stations like Hinkley C to be built starting now, or 1000 of the “mini modular” type not yet designed, and the electrical infrastructure in the entire country to be replaced. Clearly, this is impossible. You will also realise that wind and solar are very unsuitable to provide this energy, during long cold periods it is either dark or there is very little wind!
We're about to take you to the IET registration website. Don't worry though, you'll be sent straight back to the community after completing the registration.
Continue to the IET registration site