I have installed a small (approx. 4000 litres) inflatable pool for my boys in the yard which is used in the Summer months i.e. between June and September.
The pool is exposed to direct sunlight during the morning/early afternoon hours, approx. from 09:00 till 14:00. This would be enough to heat the pool water, however in Summer I cover the yard (and therefore the pool) with shades to reduce the heat inside my house. The problem with shading is that the pool doesn't get heated well anymore and the water can get quite cold, enough for my boys to start feeling cold.
I used to replace some of the pool water on a daily basis by dumping some of the pool water in the grey water tank and replacing it with warm water from the solar water heater, however this year I have a better solution.
I build a small flat plate collector (50cm x 54cm) with a 15 metre 1/2 inch copper pipe inside the collector, all painted matt black. Then I've covered the box with non reflective glass. The theory behind is that I'll be looping the water from the pool through the solar collector, the water gets heated in the process (it will absorb the heat from the collector) and is then dumped back in the pool. The collector is a small one and therefore the pool water can never reach a temperature high enough to scald the boys, however it should be enough to warm the pool water to a more comfortable temperature.
The size of the collector was something which I had little control over simple because it has to be installed in a tight/small space. In theory, at any moment, the sun emits about 3.86 x 1026 watts of energy. Most of that energy goes off into space, but about 1.74 x 1017 watts strikes the earth. (ie: 174,000,000,000,000,000, or 174 quadrillion watts).
If there are no clouds, then 1 square meter of the earth will receive about one kilowatt of that energy.
The image below is showing the underneath of the solar collector box. I have painted it in gloss white and it will be positioned on a 40mm steel pipe.
Below is an image of the inside of the solar box, painted matt black.
Below is an image of the inside of the solar box, with the 15m copper pipe placed and painted/sprayed matt black as well.
And below the ready solar collector with the glass installed.
I'll be installing the finished collector and water pool soon. I'll be posting some performance data once it's up and running.