Running at the higher temperature of the thermal panel reduces the efficiency of the solar cells. Current should be the same but voltage drops. We should be able to look at the voltage vs. temperature curves for other panels to see what a reasonable drop would be for running at hot-water (plus thermal resistance of the mounting material) temperature vs. air-cooled. Dropping from 18% to 12% seems reasonable. (I presume the panel has more extra cells than an air-cooled panel to keep the voltage up when the coolant is hot.)
Meanwhile, the power pulled out by the solar cells as electricity reduces the thermal heating of the output fluid. With 12% going out as electricity you'd see a 12% drop in thermal efficiency. So the 40% number says the panel's design as a heat collector is good.
I'll be interested to see how long these panels last, given that they combine water with solar cells and have a greater daily cycle of thermal stress than an air-cooled panel, which could lead to moisture leaks.[ Parent ]
The panels designed to run at a maximum 50C water ("PV-panels" rather than "PV-collectors" in PVTwins terminology), eg for underfloor heating, do claim a higher PV efficiency though not 18% which is the highest in the market and thus atypical and too high to expect.
However, much of the time when I especially care about PV performance, eg in the winter, I don't expect the water to get to anything like the specified 90C and the solar heated water will be the pre-heated feed to my tankless natural-gas combi at that point and maybe closer to 30C or less. Thus, I'd expect the PV efficiency to often potentially be several points above the 11--12% figure quoted outside of the summer months when I have far more energy than I need!
Rgds
Damon[ Parent ]
Unless you have a voltage-converting max-power-point controller, of course. That can turn the extra voltage into more amps at the desired output voltage, salvaging far more of the extra power than it consumes for itself.[ Parent ]
At the sort of premium I'd be paying I think I have to strain every sinew to use it effectively.
