Warming A Pool

[espookid]

I was wondering if it is possible to warm a pool for free
My idea starts with a lets say radiator painted black and in the water ,hoping the black will attract the warmth but then I get stuck on the fact that it wont circulate and therefore just warm the top ,which of of course wont circulate and leave the bottom cold/colder
I then went on to using a solar panel that powers a small water pump with a set of pipewok below ,which would support the weight and hopefully circulate but that would be quite expensive

[jomifl]

PP, I'm not sure that the inside of the swimming pool needs to be painted black, most of the infra-red is converted to heat in the top metre