I took a few classes on solar water heating, but still learning. A big issue was freezing at night in the north, that seemed to be handled by either a drain back system that drained the collectors if it was too cold, or using a two stage system where a non-toxic anti-freeze liquid was used in the lines like ethel-glycol. The other problem they addressed was if it the collectors produced too much heat they had to be shut down or the heat dumped some way.
I put up a 10kw grid tie generating system, and since I bought a pallet lot of PV panels, I put another 1.1kw I intend to use as an off grid backup for well pump and refrigeration during power outages. I ran out of cash for the large battery bank and inverter, but decided to go ahead and mount the panels on the roof of the house, and wired them to the lower heating element of my electric hot water heater. (after disconnection the ac of course)
The heating element is a resistive load which works for ac or dc, so the energy from my PV panels is just dumped into the hot water heater and acts as a pre-heater for the water. The top element in a water heater usually does most of the work so I never have a problem of no hot water. I put some arc protection of the lower thermostat and have it set pretty high, but it hardly ever cuts off.
I wired a separate kwh meter to the input of the ac and it seems to use about 25% of the electricity it would use without the solar power dump. I have only had the water get too high once when I had about a week of sunny days last summer, which almost never happens in Ohio. PV panels are not the most efficient use for heating, but they are nearly zero maintenance, and the price has really come down, so I think we may see more PV used for heating in the future.
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