With the last few days, we have seen a drop in our pool temperature to the 70's. At this rate we will be in the 60's in no time, and that is just a little to cold for us. So I might have to get a pool heater, if we are planing on trying to make it longer, and not a gas one. I will probably make a solar one, or we will do some sort of copper coil one around a fire pit.
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https://www.google.com/search?q=solar+pool+heater&sxsrf=ALeKk03QruNiWTQCRvKfF1aM03zJQlDd3g:1599742517903&source=lnms&tbm=isch&sa=X&ved=2ahUKEwj62Yuk0d7rAhVIrZ4KHUqtDpcQ_AUoA3oECA8QBQ&biw=1600&bih=708
I know a couple of fellow engineers that made their own.
I'm not sure where you are, but they are almost in the North Atlantic. They don't get a lot of sun (lots of fog) and it still performed pretty well.
Yeah I think that you can make use of a coiled copper and a fire pit that can be used to make the water heated to your pool @bigram13 :D it is more economical than using electricity.