OT solar water heater

Thought about building one to preheat water to the hot water heater.
Thought about flattening the copper tubes some for more area, but otherwise build it standard, insulated box w/ interior painted black, double glazing (have old sliding glass door)
The questions
If our demand isn't high, won't it need to be built w/ a recirculating pump and a preholding tank for the water to heat up any instead of on minimal demand?
Thought about an insulated door to close over it, manually or auto and a heating coil in case of freezing, what kind of cheapee coil?

Haven't seen the advances in them, now a lot of glass tubing w/ copper tubes in them in a vacuum.

Some I have read about use ananti-freeze solution of some type that doesn't frezee, and use that with a heat exchanger to heat the water.

Thanks, may have to go that route, but was trying to avoid the extra plumbing involved

Depends what your climate is like. The most efficient system is a solar collector hooked to storage tank with NO pump - just a thermo-siphon system. But that only works well in climates warm enough to have a storage tank next to the collector.

In cold areas, a collector on the roof using a closed loop of anti-freeze protected liquid gets hooked to a storage tank in a heated building. For this a circulation pump is needed. Often a small solar panel is used to run the circulation pump. Since no heating occurs unless the sun is shining, it makes a nice system.

There is also a drain-back system for cold areas that does not use a closed loop. It directly heats the domestic water. A bit more complicated, but the solar collector drains itself when it's cold to prevent freeze up.
Even if you just stick an 80 gallon preheat tank in your home with no solar or wood coils, and hook it before the main fuel-fired (or electric fired) heater - it will make a difference, especially if you use deep-well water. My water is 48 degrees F when it comes out of the ground.
It lets the water reach room temps before going into the main heater.
Whatever you do, remember that . . . if you send hot water into a propane tank heater, you can damage it - you must use a tempering valve. Also, if you send preheated water into a tankless, you gain nothing and it heats the already hot water, over again. That is, unless you special order a thermostat controlled tankless - usually marketed as a "solar tankless."

Consider a batch heater (see link) and whether it would work for you.

I picked up the electric heater tank this year, been watching the sun to decide where to hook it up (I'm on the north slope of a hill, so none of my options are "ideal."). For reasons of easiest plumbing, I'll probably put it in the slightly less sunny of the two contender locations.

My penciling says it would be effective for all my hot water for showers / dishes / etc April thru mid-October for me, and would pay for itself in one summer. Probably could tweak a couple more months out of it on either side.
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm Batch

We had a solor water on our home in Western PA for several years and it worked very well. Ours was an air handler system with a circulation pump to the storage tank. We had a four panel unit on the roof and I built the roof with the required degree of slope for best performance and it sloped to the south. We had a 120 gallon storage tank and could heat water to 120 degrees on any sunny day. We later developed a leak in the storage tank and replaced with an 82 gal standard electric heater tank for cost reasons. Our water never got any hotter then 120 - so we just had less volume. I wish I had removed it before I sold the home - it would be handy today.
We used the unit from 1979 to 1993 and only had a leaky tank problem. I plan to build another one sometime next year when I have the time to do it.