Heat pump COP vs. capacity
A while back Marc Rosenbaum mentioned tuning his heat pump to more closely track the needed capacity in heating mode, and reported significant efficiency improvement. I dug into the documentation for the Daikin here and while it doesn’t really have truly *variable* capacity in heating mode — it tends to just run full-blast because there’s no issue about latent load — it does have a “demand” setting which simply reduces the output to a configurable percentage. This is either a fixed setting or can be externally switched in and out via wired control from, say, a building-management system. The default reduction is to 70% of nominal capacity, so I chose that to play around with. In this case that brings an 18,000 btu/hr down to around a ton, which is approximately the design load for this house anyways.
Last night in our [rather rare!] subzero temps I saw the first
instance of this system not quite keeping up with the load —
overnight the house didn’t get above 64F, with a 68F setpoint.
It ran continuously with the inevitable pauses for defrost,
and at those ambients there’s a slight derating curve so I’m
guessing that it was doing about 10,000 btu/hr on average.
[I was undoubtedly also getting bit by the “polyiso problem”
*and* heavy night-sky radiation at the same time.]
The problem is that there’s no real way to measure the present
running COP of the system. So here’s the question: how likely
is it that deliberate capacity reduction in a heat pump, in
heating mode, will raise [or lower?] the effective COP? The
major observable change seems to be that the delta between
evaporation and condensation temperatures shrinks a little bit.
Doing my standard meter-read calculations seems to so far return
a lower BTU/hr/degF figure [that assumes a COP of 3] in this reduced
configuration, which might be indicative but there may be a couple
of other factors in play from minor insulation improvements hereabouts.
It’s really hard to tell with these things, but if someone can offer
a little theory on whether reduced compressor RPM might affect
performance that’s sort of what I’m fishing for.
_H*
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Replies
Reduced compressor RPM can, in theory, help COP because of at least the following:
1) It reduces the condensor temperature and increases the evaporator temperature, as you mention, thus decreasing the temperature "lift" that it has to work against.
2) It reduces the frictional losses associated with pumping fluid around--not normally a major factor but maybe more important in a split system because the tubes are longer.
3) If the motor and its control are a modern high performance type, the motor efficiency might go up, particularly if the torque it has to provide is reduced.
Most of the popular mini-split heat pumps out in the market (Mitsubishi, Fujitsu, Daikin, Panasonic, LG, etc.) have fully variable output within their low and high range. For example, the Mitsubishi MUZ/MSZ-FH09 has a published capacity range of 1,600Btu/hr-18,000Btu/hr at 47F outdoors...and it can provide everything in between in very small increments.
Higher COP at lower compressor speed/frequency is not theory. This is how these units achieve the high performance ratings they get. NREL did some lab testing of a couple of units at many different temperatures, compressor speeds and air handler fan speeds. http://www.nrel.gov/docs/fy11osti/52175.pdf
Side note - the COP's for these machines when running at maximum compressor speed are often mediocre at best. It's when they're running at <50% that the "gravy" is made.
Yeah, I'd read 52175.pdf -- and Marc's experiences ... and realized
that my system isn't doing dynamic self-adjusting in heating mode,
so the only way to approximate that was to set a deliberately
limited-capacity configuration and see if that could hold up against
whatever ambient conditions came along. That's caused it to hit a
limit on those really cold nights, but not in a bad way, and early
data seems to indicate that it's overall being more efficient so
I'm probably going to run with this for the rest of the season.
Daikin could have done a *much* better job with general configurability
of and visibility into these Skyair systems, but at least having the
service and engineering manuals freely online helps quite a bit in
understanding how they work and how to tune them.
_H*