I've seen that putting a black tank inside of an insulated box with a vacuum glass on the side facing the sun.
I've heard placing a hose underneath composting mulch exposed to the sun is very effective as well.
But for the sake of cheapness and less work, given there's already planned to be a water tank above the house for other reasons, I'm going to circulate water through a black hose connected to the tank at each end.
The 500 gallon tank at the top of house is supposed to be for cold water, but who needs cold water anyway? Hell we need to boil the water to drink it anyway.
Except for my cat. The plan for her proximity sensing water faucet would be ruined. What if I want to put a drinking water filter so we don't have to boil water? Well then, I'd need a separate hot water storage tank. But we'll get to that after the proof of concept is established.
A separate tank could be inside an insulated box with a sun facing glass. That may be more elegant and more efficient. But the hose and non-insulated tank is so much easier and less expensive. Also I probably don't need high efficiency. If the result is water warm enough to shower 95% of the time, having more efficiency just means water that's hotter than it needs to be.
I plan to circulate water using the hose's ability to act as a thermosiphon. I think two check valves, one at the inlet and one at the outlet of the tank, allowing flow in only one direction through the black hose with sunlight heated water.
Water expands by 4 percent from very cold to nearly boiling, so even less than that should drive the flow without a pump, and only if the water in the hose is hotter than the water in the tank.
The inlet and outlet should be at about the same height towards the bottom of the tank, so the outlet never gets above the water line in the case municipal water is shut off.
For two columns of water of the same height, the column with colder water will be denser and have a higher hydrostatic pressure at the bottom.
An inlet lower than the outlet will give a higher hydrostatic pressure on the tank side of the inlet if the water in the tank is cooler. If the water in the tank is warmer, the tank side of the inlet will have lower pressure, and the one way valve closes shut.
A more micro-explanation:
If pressure builds in the hose because the temperature increases, the outlet will open and release water. Momentum, and colder water being heavier, will introduce water at the inlet. Continued heating of the hose by the sun will cause siphon flow, water flowing from the inlet to the outlet.
If pressure in the hose reduces because it cools at night, water will flow through the inlet and equalize the pressure to that of the tank. Momentum could push water through the outlet, but because cold water is heavier, the pressure on the hose side of the inlet one way valve will then increase until it closes the valve. No continued flow will take place.
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