I recently read an article comparing costs and efficiencies of flat plate solar (FP) collectors with evacuated tube (ET) solar collectors. In summary, it said that FP solar collectors cost less than ET collectors, and if the water inlet temperature equals the air temperature, FP collectors are more
efficient too. However, as the water inlet temperature rises, all solar collectors become less efficient, and rising inlet temperatures make FP collectors become less efficient more quickly than ETs do.
Under most circumstances, the water entering the solar collector is indeed hot, and therefore evacuated tube collectors are generally more efficient than flat plate collectors.
What if... a heat pump was used to chill the water cycling through the solar collector, causing the water inlet temperature to always equal the air temperature?
By not chilling the water below air temperature, condensation (or in cold weather, frost) is avoided, but solar gain is otherwise maximized.
Obviously, the destination for the heat removed by the heat pump would be wherever the solar system would normally have sent that heat... a hot water tank or other thermal mass.
Now, I'll freely admit that this uses much more energy than a more passive solar heating system, which would typically be only pumping water, but on the other hand, in most conditions, it will use less energy to heat a home (or to heat a home's hot water) than either a ground-source or air-source heat pump.
As an added bonus, if the heat pump can be switched to "reverse", then on winter mornings, when the solar collectors might be snowed over, some heat and electricity can be expended to very quickly melt off the snow.