[Freestyler_3]

Yes its heavier. They would have to keep the water moving or for heavy winters have it drained. Also possible to have a liquid in there with much lower freezing point like you said.

The problem I think is that when the panels need cooling the most the system to disperse the heat to would need to be huge, because you just have a large surface heating up water fast and you don't want the house floor to heat up and your water tank for showering is already at max temp. Also gotta be careful not to evaporate the pool.