Originally Posted by Zephod_beeblebrox2
Problem is that while heating water directly from the sun works well during sunny afternoons, the water will be cold in winter and at night.
Well... if you wanted an instant (ie no storage tank) solar water heater then yes you'd be in trouble at night. But just as energy can be stored into a chemical reaction in batteries, it also can be stored as heat in an insulated tank. Certainly in the evening the tank could be piping hot, and with appropriate design it could still be warm enough for use first thing in the morning. And like taskswap says, there's a remarkable amount of solar heat available in the winter. I'd dare say that a solar water heater will behave from summer to winter exactly (or nearly?) the same as a photovoltaic panel does. There's a reduction in the winter certainly, but with PV and with water the system can be designed to allow for that.
You mentioned paupers not being able to experiment in another post... while perhaps that's somewhat true with PV, I would suggest that some basic small-scale water heat experiments can be done "on the cheap" with found or nearly-free stuff. A scrap of aluminum siding/soffit/facia material, some PEX or copper tube, some glass from an old window or even borrow a pane out of the bus.. One thing that might not turn up easily is a water pump, but you might be planning to buy one of those for the bus anyway and could borrow it for solar experiments from time to time. It pays to do small experiments because they breed ideas that help the real thing come together better.