3 ms·
That's not the reason. Heat engines can reach an efficiency of ~50%, so a COP of 400% still gives you an overall efficiency of 200% which is a net energy gain.
by Turskarama 3y ago
That's not the reason. Heat engines can reach an efficiency of ~50%, so a COP of 400% still gives you an overall efficiency of 200% which is a net energy gain.
The trick here is that the COP of both isn't static, but is usually a peak. Heat engines are more efficient the higher the temperature difference, while heat pumps are more efficient the smaller the difference. For any given heat gradient, you can't have a total efficiency of a heat pump + heat engine that exceeds 100%.
- friend_and_foe 3y agoThe heat gradient here is the big point. If you can use one kilowatt of power to pump 4 kilowatts of heat from one room to another, to turn some of that into electricity you need to use the temperature gradient between the two rooms, and therefore let that heat flow back into the room you're cooling. If 400% is peak, and you get 50% efficiency out of the heat pump, you can do it at peak at 200% efficiency, but that's only at peak, and at full efficiency of the heat pump. Unfortunately, the higher the temperature gradient in the direction you're pumping, the less efficient a heat pump is, so you won't get anything near peak efficiency. Not at peak and/or with too low a temperature gradient and you can't do it, you'll be not cooling the room you're trying to cool at all and spending electricity to accomplish nothing.