Re: Air to air heat pumps efficiency

Romeo Gigli wrote:
""Ivar S. Ertesvåg"" <ivar.s.ertevag@xxxxxxx> ha scritto nel messaggio news:glddf0$fh1$1@xxxxxxxxxxxxxxxxxxxxx

Romeo Gigli wrote:

I' ve noted that many air to air heat pumps today available (Mitsubishi, Daikin, etc...) have coefficient of performance (COP) higher than 5, for a thermal power in the range of 8000-10,000 Btu/h, of course in "standard" condition, i.e. with a temperature of 7 °C outside and 20 °C inside. Just curious, what do these COP figures (and thermal power) become if the outside temperature falls to - 5 °C ? Even we have, in those conditions, 80% of COP/thermal power vs nominal condition an air to air heat pump could be an efficient and low cost strategy for space heating, I suspect at least in hotter climates for example where the coldest temperature typically doesn't fall in winter to less than - 5 degree °C (pratically almost southern and central Europe and Us, I guess)

The Swedish Energy Agency has tested a number of air/air heat pumps; mainly for "mild nordic" climate (down to -15 °C) according to the
European standard EN 14511 (not very different from ISO 5151).
A recent summary is found here
http://www.swedishenergyagency.se/WEB%5CSTEMFe01e.nsf/V_Media00/C12570D10037720FC12574E5003D72B1/$file/luftluft_ny1.pdf
It is in Swedish, though I guess you get the main information.
"Mätpunkter, utomhustemperatur" means measurement points, outdoor temperature. "Avgiven värmeeffekt" means released rate of heat (i.e.indoor) and "COP" means COP. "Lägsta utomhustemperatur" = lowest
outdor temperature (as specified by the manufacturer, not tested)
"buller" = noise.



At a first sight, their figures are quite disappointing to me; even if we have still 2/3 or 3/4 of the nominal power at -7 and -15 °C (of course - 15 °C has little sense for Italy), the COPs at those emperatures are only 2,5 or 2 vs even > 4 or 5 at +7 °C. Frankly, I hoped something more....

This is a matter of thermodynamics:
For 7 C outdor/20 C indor, the Carnot COP is 22.6; that is, the theoretical maximum COP limited by the 2nd law.
Thus a real COP of 5.0, means an exergy (or 2nd law) efficiency of 0.22

For -7/20, the Carnot COP is 10.9
A real COP of 2.5 gives an exergy efficiency of 0.23. That is,
this device is actually thermodynamically more eficient at -7 C than at + 7 C.

With 1-2 exceptions, all the heat pumps in the link are thermodynamically more efficient at lower temperatures. I guess (but do not know) that this results from design choices for the nordic market.


.

Relevant Pages

  • Re: Air to air heat pumps efficiency
    ... heating when the heat pump can't work, because the local climate is such ... The air there is warm ... and then get a lower temperature than outdoor i.e. lower COP. ...
    (sci.energy)
  • Re: Air to air heat pumps efficiency
    ... have coefficient of performance (COP) higher than 5, for a thermal power in the range of 8000-10,000 Btu/h, of course in "standard" ... COP/thermal power vs nominal condition an air to air heat pump could be an efficient and low cost strategy for space heating, I suspect at least in hotter climates for example where the coldest temperature typically doesn't ... European standard EN 14511. ...
    (sci.energy)
  • Re: Air to air heat pumps efficiency
    ... what do these COP figures (and thermal power) ... air heat pump could be an efficient and low cost strategy for space ... coldest temperature typically doesn't fall in winter to less than - 5 ...
    (sci.energy)
  • Re: We Went to the Moon on Feet and Inches
    ... (Rant #1) ... measure heat pump efficiency. ... Most residential air conditioners have a COP ...
    (sci.space.shuttle)
  • Re: We Went to the Moon on Feet and Inches
    ... (Rant #1) ... measure heat pump efficiency. ... Most residential air conditioners have a COP ...
    (sci.space.history)