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.
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