Why are air-to-water heat pumps more efficient at lower water temperatures?

[SteamyTea]

40 minutes ago, Garald said:

Right, mathematically the two things are the same

And that was where old Claude was observant.

 

[Garald]

12 minutes ago, Marvin said:

Hi @Garald

 

Why are air-to-water heat pumps more efficient at lower water temperatures?

 

My simple explenation to your question is that air passing through the ASHP changes the temperature of the water(or air) output and the bigger the difference between the air temperature and the required water temp (hotter or colder) the harder the fan and compressor have to work.

 

Of course they have to work harder - on less water, so that doesn't really answer the question. At any rate, it seems we are satisfied now.

 

12 minutes ago, Marvin said:

After that, any further detail is unnecessary unless your an academic and couldn't be bothered to google it.

 

I was Googling badly :). The two wikipedia pages cited above do a reasonable job (and of course Feynman is very readable).

[DanDee]

7 hours ago, Garald said:

 

Well, obviously heating a *given volume* of water to a higher temperature takes more energy than heating it to a lower temperature. But why is it more efficient, overall? (Operating at a lower temperature, you presumably have to heat a larger volume of water.)

You've asked about the difference in energy used to deliver a given power rate of 5kW at 45C(water temp) vs 5kW at 60C(water temp), why do you include the energy used to get to 60C?

That is accumulated energy of the heated medium.

 

Getting from 0 to 45 or from 0 to 60, or from 45 to 60, takes an amount of energy that has nothing to do with the energy used to compress the gas in order to sustain the pressure required for the condenser to be at 45(50 )vs 60(65).

 

A crane has to lift a weight from 0 to 50m.

B crane has to lift the same weight from 0 to 100m.

Both cranes need to deliver the weight at the same time, for the B crane to achieve this, it needs to accelerate(use more energy) to deliver at the same time with crane A.

 

[Garald]

3 minutes ago, DanDee said:

You've asked about the difference in energy used to deliver a given power rate of 5kW at 45C(water temp) vs 5kW at 60C(water temp), why do you include the energy used to get to 60C?

That is accumulated energy of the heated medium.

 

 

Exactly. I think we already zeroed into the right question, and got an answer; see the discussion above.