Re: Using waste heat from PV panels to generate residential hot water

You can't do that unless you're okay running your photovoltaic panels at upwards of 70 C. Second law of thermodynamics, and all that.

Concentrator panels run much hotter than that, of course.

'Evacuated-tube solar collectors They feature parallel rows of

Sure, we were using selective coatings too, though not vacuum. But you aren't using those to improve photovoltaic efficiency.

Cheers

Phil Hobbs

It's not, someone was just reinventing the wheel. The funniest part is that nowadays pv panels have gotten cheap enough that the most common "good enough" way to make solar hot water is to put up pv panels and use them to drive the bottom heating element in a two-element electric water heater (with a controller), just to avoid all the pumps and plumbing and freezing hassles .

That sounds about right - in the aftermath of the first OPEC induced oil crisis. I worked on something similar about then too. I still have a couple of good books from that era on alternative energy...

ISTR there were slightly more sophisticated ones that had a modest sized water tank at the top of the panel which thermal syphoning would allow to get to quite a respectable temperature (at least on a sunny day) and then a second heat exchange circuit to move that heat to where it was needed in the domestic HW tank.

Or preheat the water to one of the fancier ones. ISTR simple FPC were popular in California back then for heating outdoor swimming pool water.

Pool collectors made much more sense, then and now. They don’t need an insulated box, for one thing.

These were all-copper and self-draining, with a manifold top and bottom, and parallel tubes soldered to flashing-copper absorbers.

We used a neat little tool to make the manifolds—it was sort of like making spun-aluminum bowls. It had a screw+nut arrangement with a small ball on a hook. You drilled a small hole, stuck the hook in, and twisted the screw part so that the hook pulled the metal out into a tee fitting.

Each manifold had eight or ten of them. They weren’t long or smooth enough to use like a normal sweat fitting, so the parallel tubes were brazed on. That annealed the copper too, which helped prevent leaks.

Fun job, for three months at least.

Cheers

Phil Hobbs

An easier use case would be to use extra energy from the PV when you can't sell back to grid, power the house or charge the batteries.

boB