[teragram (OP)]

I understand that heat pumps these days claim a co-efficient of performance in excess of 4. Can anyone confirm that these devices can actually achieve such performance in real life (as opposed to laboratory testing)?

[chris]

What sorts of devices, operating under what sorts of conditions are you referring to? Are these domestic systems, for instance?

[alancalverd]

Theoretical max COP = T_H/(T_H - T_C) so you can work out the theoretical efficiency of an ideal pump for any temperature differential.

Wikipedia states

In Europe, the standard tests for ground source heat pump units use 35 °C (95 °F) for T _H and 0 °C (32 °F) for T _C. According to the above formula, the maximum achievable COP would be 8.8. Test results of the best systems are around 4.5. When measuring installed units over a whole season and accounting for the energy needed to pump water through the piping systems, seasonal COP's are around 3.5 or less. This indicates room for improvement.

Maintaining an indoor temperature of 293K against an average outdoor temperature of 283K, the theoretical limit or a UK domestic aircon or underfloor system is about 29, but most systems maintain the underfloor water temperature at 313 and a hot water tank at 333, plus losses in the water circulating pump and occasional weeks at outdoor temperatures below freezing, so however good the primary heat pump may be, an overall annual CoP above 3 is pretty good.

[teragram (OP)]

What sorts of devices, operating under what sorts of conditions are you referring to? Are these domestic systems, for instance?

I was referring to, mainly domestic, space heating systems, with air source heat pumps in mind. The rear of my house has a tall fence on each side, with no space between the building and the fences. The garden slope is downhill toward the building. My concern is that an air source device would create a reservoir of very cold air against the wall of the building, diminishing the device's COP, and perhaps causing other problems as well.

[teragram (OP)]

Maintaining an indoor temperature of 293K against an average outdoor temperature of 283K, the theoretical limit or a UK domestic aircon or underfloor system is about 29, but most systems maintain the underfloor water temperature at 313 and a hot water tank at 333, plus losses in the water circulating pump and occasional weeks at outdoor temperatures below freezing, so however good the primary heat pump may be, an overall annual CoP above 3 is pretty good.

Thanks for your informed reply. You confirm my suspicions. As I said in my reply to Chris, my main interest was in air source heat pumps, and my concern is with, not only performance, but impact on the local environment of "pooling" of extremely cold air. Regarding ground source devices, I suppose one limiting factor on performance must be the conductivity of the ground in the heat collecting area. Also, as with air source, is there any risk involved in substantially reducing the temperature of the ground in the heat collection area (tree roots, unknown water pipes, whatever)? 

[alancalverd]

If there's no planning restriction, you can position a domestic air source pump at any height. Window or first floor level is usually convenient so no problem with it getting choked with leaves or recirculating chilled air.

Ground source must become less efficient with time as the immediate ground cools, so you need an oversize collecting coil at these latitudes.

[teragram (OP)]

If there's no planning restriction, you can position a domestic air source pump at any height. Window or first floor level is usually convenient so no problem with it getting choked with leaves or recirculating chilled air.Ground source must become less efficient with time as the immediate ground cools, so you need an oversize collecting coil at these latitudes.

Thanks again. From your previous comments however, even a COP of only 3 represents a good saving in energy!

[alancalverd]

Yes. Strongly recommended for a new build or major refurbishment!

[syhprum]

Apart from environmental benefits a COP of 3 would not be of any benefit to me as a Joule from electricity costs me more than three times that of one from burning natural gas.
If there is cheap electricity available from Hydro or Plutonium producing plants it would be a help to me but not in the UK

[Bored chemist]

Apart from environmental benefits a COP of 3 would not be of any benefit to me as a Joule from electricity costs me more than three times that of one from burning natural gas.
If there is cheap electricity available from Hydro or Plutonium producing plants it would be a help to me but not in the UK

Build your own turbine etc and then use the heat pump to pull waste heat from the generator.

[syhprum]

Where can I borrow the money at a reasonable rate of interest to finance these projects ?, I think that I will just stick with my gas fired boiler.

[Galaser]

Yes. Strongly recommended for a new build or major refurbishment!

I agree with you.