Heat pump COP vs. capacity
虽然回到Marc Rosenbaum提到调整他的热泵，以更加紧密地跟踪加热模式的所需容量，并报告了显着的效率提高。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
这个系统的实例并不完全跟上负载 -
overnight the house didn’t get above 64F, with a 68F setpoint.
它与除霜的不可避免的暂停连续，
在那些外壳上，有一个轻微的降额曲线，所以我是
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
下降的BTU / HR / DEGF图[这是一个降低的COP 3]
configuration, which might be indicative but there may be a couple
of other factors in play from minor insulation improvements hereabouts.
真的很难告诉这些事情,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）减少与泵送流体周围相关的摩擦损失 - 通常是一个主要因素，但在分裂系统中可能更重要，因为管较长。
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.nrech.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.
是的，我读了52175.pdf - 和马尔科的经验......并实现了
我的系统在加热模式下没有动态自我调整，
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可以用一般可配置性做出一个*多*更好的工作
与这些天空系统的可视性，但至少有
service and engineering manuals freely online helps quite a bit in
了解如何工作以及如何调整它们。
_H*