PS another useful observation from the last chart I posted is the effect of low OATs and defrost cycles on hourly COPs: around 4 on the left hand side, when the OAT was around 10 degrees, to around 2 on the right hand side, when the OAT was just below zero degrees.
Midea 14kW (for now...) ASHP heating both building and DHW
@tasos - never one to knowingly miss an opportunity to say weather compensation isn't any good!
Ha, ha true, but I am not saying it isn't any good, I am saying it is not that good. A simple argument is that it is open loop. And to answer your "anticipation" argument, let me say that if a reliable OAT measurement is available, it can be used in closed loop control by a technique, called "disturbance feed-forward".
I think you are mixing up the chicken and the egg here.
I don't think I am. The order is:
1. The system orders a defrost cycle.
2. The OAT sensor sends erroneous measurement (warmer).
3. The set LWT (T1S) falls (because of 2).
4. The actual LWT falls much more.
The problem here is twofold:
1. The pump should have enough (reserve) power to maintain T1S, despite the OAT sensor spike. Perhaps, this should must be taken into consideration at design stage.
2. The drop in actual LWT should not be that much (it is undershooting by 50%).
Midea MHCV10WD2N7 R290, 4.8kW peak energy community solar power.
PS another useful observation from the last chart I posted is the effect of low OATs and defrost cycles on hourly COPs: around 4 on the left hand side, when the OAT was around 10 degrees, to around 2 on the right hand side, when the OAT was just below zero degrees.
This is to be expected, isn't it?
Midea MHCV10WD2N7 R290, 4.8kW peak energy community solar power.
The flow rate also shows another Midea characteristic, much of the time the flow rate is fixed, at around 1.3 to 1.4 cubic metres per hour (note it is charted as actual value x 10, to improve visibility):
Perhaps also worth pointing out that when using defrost cycles (right hand side), the actual LWT mostly only reaches the set LWT, ie the mean actual LWT is below the set LWT, whereas in 'normal' cycling (left hand side), the actual LWT varies either side of the set LWT, to give a mean close to the set LWT.
I think the flow rate is meant to be constant. In my machine is set at 0.36 m3/h. But I am not sure of this. On the other hand, why shouldn't it be ?
As for the LWT, chaotic, fluctuation, I am sure it is just bad control strategy of the compressor speed. I would very much like to know what it is.
Midea MHCV10WD2N7 R290, 4.8kW peak energy community solar power.
I think you are mixing up the chicken and the egg here.
I don't think I am. The order is:
1. The system orders a defrost cycle.
2. The OAT sensor sends erroneous measurement (warmer).
3. The set LWT (T1S) falls (because of 2).
4. The actual LWT falls much more.
I've zoomed in on a section of the earliest chart, and added drop down arrows at the start of each defrost cycle:
It seems me the order is:
1. The system orders a defrost cycle, which starts where the down arrows are
2. Heat is extracted from the circulating fluid as it passes through the heat pump which causes the LWT to drop sharply and, a little later, with a slight lag, the warmer exhaust air warms the OAT
(1) happens first, (2) follows.
The set LWT does not change ie event (3) in your series of events does not happens
Because (3) doesn't happen, (4) can't happen because of (3). It happens because it is defrosting, which overrides normal behaviour.
Midea 14kW (for now...) ASHP heating both building and DHW
I think the flow rate is meant to be constant. In my machine is set at 0.36 m3/h. But I am not sure of this. On the other hand, why shouldn't it be ?
As for the LWT, chaotic, fluctuation, I am sure it is just bad control strategy of the compressor speed. I would very much like to know what it is.
A flow rate of 0.36 cubic metres per hour seems quite low. But the real point here it seems to me is Midea are missing a trick here. By using a steady(ish) flow rate, they lose one of the variable that affects heat transfer, and so have less ability to modulate output.
The LWT fluctuations aren't chaotic, they do clearly follow a pattern, both in normal cycling and defrost cycling. The compressor speed (red dotted line on the earlier chart) on the other hand does appear rather more chaotic in the middle and right hand parts of the chart. I am not sure we can work out what the control strategy is just by looking at that red dotted line, eg what's going on in the middle of the chart, reasonably steady OAT, reasonably steady set LWT, reasonably steady LWT/RWT delta t, reasonably steady flow rate, but compressor speed all over the place
Midea 14kW (for now...) ASHP heating both building and DHW
