Not very hot and highly variable sun today but I did some experimenting anyway.
84F with occasional 90F during sunny stretches.
I managed to get 11.1 degrees of water cooling at 2 GPM, so my performance goal is now met, though the high pressure is a bit higher than I'd like when a get a sunny stretch.
I started with low pressures, both high and low side, and crappy performance. As I kept adding in refrigerant in sub ounce increments, pressures slowly rose and so did water cooling performance. It never stopped doing so but it took a long time to see the water temperature drop further, but it did, after some minutes.
Pressure on the high side is now above where I'd like to be when the sun comes out and it gets hotter. I think this indicates that I should get a TXV valve, or shorten the capillary tube (which I'm less interested in), so pressure could be lower.
Here's the data:
High Side:415 psi cloudy to 440 sunny (122 Saturation temp)
Low Side: 128-132 psi (45 Saturation temp)
Suction temp near evaporator 68F
Liquid line temp a foot before capillary tube: 109F
Superheat: 23F
Subcool: 13F
Running amps: 5.8 (full specified running amps for compressor and fan) with sunny at max pressure 440psi. I forgot to measure fan separately. It's less than 0.3A I need to recheck this with my RMS amp meter; my clamp on meter was showing steady low amps until I moved the wire deeper in the meter hook, and it gained an amp. So my amps checking was crap due to clamp on meter unreliability.
Water flow rate 2 GPM
Water temp in: 67.6F
Water temp out: 56.6F
Delta temperature: 11.1F
One thing I noticed is no more frost on the capillary tube. A little sweating but that's it. At the higher pressure it's no longer flashing in the tube. A good thing; the flashing is happening inside the BPHE where it should be.
I can see the value of a TXV for lower pressures and better efficiency and will look into sourcing one, but I'm in no hurry now as I have met the goal and can afford to remove a little R410a to get the high pressure down 10 lbs without screwing up performance much. I stopped adding tiny burps of liquid when the time to see improvement took minutes, and improvements were diminishing. I need to do some more measurements on cooling the house. I ran out of sun before I could start that process; I want to monitor incoming and outgoing water temps plus slab temp over about 4-5 hrs of running (my intended use, to use my sunny day PV surplus).
I'm very happy with performance as it is, but would be glad for advice regarding my high side pressure. When the temperature rises, it's a bit much by my references. But it really does work best like this.
Suggested online sources for TXV's would also be appreciated.
Best Wishes,
Bruce