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large magnifying glasses to heat swimming pool? Answered

why cant you use the large magnifying glasses if seen used to melt metal etc. to heat a pool by pointing them at the water instead of all the solar panels and black pipe?

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Privacy issues -  Anyone flying above would be able to look down and see you in great detail.

 
 The other thing not mentioned previously is that the focussed sunlight would pass through the water and not release it's energy. It needs to be absorbed, ideally by a black surface to have a heating effect.

I would only use them when no one in pool to get the water to swimable temps

You get 1000W/sq metre, roughly, at noon, and with that area perpendicular to the sun. Without lenses in the sq. metre class, you don't gather that power. If you need 10,000W to keep your pool heated, you need 10 sq m of lenses, all pointing at the sun, all focussed into the water.

For magnifying glasses to have an effect, they must be much larger than the thing they are heating.

If you want to warm a whole pool...

You would be FAR better off running the pool water through some old radiators painted black and put behind double glazing.

1. bigger area for heat transfer
2. Less likelihood of frying the swimmers
3.Much less invasive
4. Cheaper

Like kiteman says you need the AREA, not the power density