creating a vacuum with a flat plate of glass is really hard and would take some really thick glass, that is why they use a tube, but the big problem with the evacuated tube collectors is designing a system for them that can handle the more concentrated heat, you have a pice of copper at the top fo them that can reach 280F+ and is the size of a "C" cell battery.

The high temps (and to keep it from freezing as easily) was why they used the new antifreeze/coolant (propylene glycol). Infact, I would have assumed that this substance is what is used down into some kind of heat exchanger that would be in the attic or basement or what have you.

If you could put a bigger tank, that was well insulated, then you could have a heat 'battery' for hot water use into the night.

I also found another misconception I had, that being that the pipes in the evacuated tubes are not flow through, but dead ends. I thought that cold fluid would be pumped up from the bottom, through the tubes, then hot would come out the top. The pump would be controlled by temp difference of the cold vs hot (and obviously minimums and maximums). But, being dead ends, they need to rely on convection to let heat rise to the top. I still think a loop would be more efficient, even if it was just down the 'back' side in the tube.

I also see that many have little solar panels (assume to run the pumps), but if they work well on cloudy days, then would they draw on other electric sources to run the pump if the system started to overheat?

I have asked a company to come out and evaluate my 'site' (roof/front yard), as solar hot water is not as expensive as PV.

Wonder if anyone ever made a Stirling engine run from excess heat produced during the day, when most people are at work? They supposedly run quietly and if you could stick it just below the panel with maybe a automatic valve that once your storage tank is at temperature and maintaining it, then run a stirling engine to produce electricity?

the evacuated tube collectors use heat pipes, they are much much better at transferring heat then a convection of fluid, almost instant try it sometime, take one of those tubes and holding your hand on the hot end put it in the sun, it gets hot in a matter of seconds, about as much time as it takes for the thin copper fins to get warmed by sun light, and because of how a heat pipe works those fins operate at a cooler temp as well so you have fewer losses from radent heat from the collecting fins.
the reason to run the pump with solar is that in an ideal world the panels gets hot when the sun is out, maybe a small battery and thermostat to keep the pump running would be better, but to have two voltages of pumps or to have the pump run off 120v ac would be more wiring to do and more complexity, as the pump is near the thermostat, near the panel.
the hot water storage tank in most solar hot water systems tends to be 80 gallons or more and should be insulated to keep it warm for 2-3 days... those ideas that are being talked about are not just not new but they have been refined and are common on solar hot water systems, when they are good enough to work.

I have used a sheet of foil faced building paper clamped between 2 thin sheets of polycarbonate mounted behind a solar hot water panel to give an added boost in the winter months when the sun is lower on the horizon. The gain in hot water was approx 15%. In the summer it has little effect as the sun is much higher in the sky.

Cheers
Mike