Comment by EVa5I7bHFq9mnYK 17 hours ago
"Thermodynamics" is singular :) As for the numbers, my AC's manual shows COP of 3.71 for heating and 3.13 for cooling.
So you are spot on, in winter temperature deltas are larger, and efficiency goes up.
You are saying that heat pumps get less efficient when deltas are larger, and the parent post says they get more efficient when deltas are larger. In a sense, you're both correct.
There are multiple relevant temperatures for a heat pump, and the pump is more efficient when some of those are higher and some lower. A heat pump has two heat exchangers, one on the inside of the building and one outside. Each of those heat exchangers has two temperatures: the refrigerant loop temperature at that point, and the ambient temperature (air for air source heat pumps, ground for ground source heat pumps). There's also a fifth relevant temperature that has indirect influence: the setpoint (the desired indoor ambient temperature).
Efficiency increases when the temperature delta between the refrigerant and ambient temperatures is higher (both indoor and outdoor). But those temperature deltas vary inversely with the delta between the indoor and outdoor ambient temperatures.
So, in summary:
- Heat pumps get less efficient when the temperature delta between indoor and outdoor temperature is higher.
- They get more efficient when the temperature delta between refrigerant and ambient temperature is higher.
The net effect of this is that heat pumps become less efficient as the temperature becomes hotter outside in the summer and colder outside in the winter.
Correct!
You can also think about it as far as actually moving heat. Cold is the absence of heat, and so when the air is colder, there is less heat moved for the same effort and you have to work harder -- less efficiently -- for the same amount of head to get moved.
I see, the previous commenter stated the opposite :). Anyway, both numbers are > 1.
> "Thermodynamics" is singular :)
> plural in form but singular or plural in construction
(https://www.merriam-webster.com/dictionary/thermodynamics)
I think American and British English treat words like this differently.
Those high COPs are probably for relatively small temperature deltas. Heat pumps get _less_ efficient when the temperature deltas are larger. See page 18 of the manual linked below for an example. As the temperature gets lower, the heating COP gets lower. The same should be the case with cooling (higher outdoor temperatures lead to lower COPs), but the data is not presented in the same way.
https://backend.daikincomfort.com/docs/default-source/produc...