One thing that made me happy about using flat plate Solar Panels was that the pump would work when the sun is shining, so there would always be PV power to meet the demand of the pump.
Conversely, one thing that made me unhappy about using the plastic radiator to make smaller but continuous gains, even at night time or during rainy days was that a pump would be working longer hours.
Talked again to David at Ice Energy, and:
1. He comes up with the possibility that it needs only a 40watt pump (I was worried that it would be in the region of 200watt).
2. We can tweak the 'delta-T' to trigger the pump, so that it isnt running all night, unless temperature is favourable - becomes mostly a daytime device.
3. We can have a small PV panel on the south facade next to the plastic radiator, that is charging up a DC battery (car or motorcycle) and have a DC pump. So it is definitely able to work in daytime, and works some hours into the evening, while temperatures are still warm, and never requires power from the grid.