Not content with merely having a pool, almost as soon as it was obvious that it was going to fill up with water, the pool boy set about inventing a solar heater for it. He has wanted to play with passive solar for a while, but I won't let him do crazy things like this inside the house with the hot water heater. I know, I know, I'm such a kill joy.
His theory has always been, that a black hose filled with water will heat up in the sun, and if you could use a circulator pump to take cool water out of the pool and put the warm hose water into the pool, it would help heat up the pool. His first version was 200 feet of 1/2 inch poly pipe coiled onto a piece of plywood pointing at the sun. When the circulator pump was turned on, it added warm water to the pool for about 90 seconds and then the water coming out of the hose was pool temperature again. The pump pumped too fast. So about every 1/2 hour, you could add a few gallons of warm water for a few seconds. Promising but not perfect.
Then he remembered that we had lots of copper and aluminum finned base board radiators kicking around from when we renovated the house and replaced the baseboard with in-floor radiant. So he soldered them all together, spray painted them black and mounted them on another piece of plywood. The extra pipe added more water to the heating loop which adds extra heat. The aluminum fins collect more solar and transfer it to the copper pipe, and the copper pipe heats up more than the poly pipe. A lot more. The water coming out of the return hose is too hot to hold your hand under, and the water is warm for a lot longer, about ten minutes in fact. The cool water from the pool continues to steal heat from the hot copper pipe until it is the same temperature as the water, so it is not just the extra volume of pipe that adds heated water, but several minutes of heat transfer after the initial water has gone through the loop.
The next design stage saw the two pieces of plywood attached together and mounted on a pivoting bolt so the solar collector can be swivelled to always face the sun. It all works really well on sunny days. We have had the pool water up to 26 degrees Celsius on sunny warm days. On overcast days though the temperature is only 22 degrees Celsius, which is what is would probably be without a solar heater. On breezy days the aluminum fins actually act as a cooling device, and the wind will steal heat away from the pipe as the water goes through.
He has talked about enclosing the whole system under a clear sheet of plastic or glass to protect it from wind. I don't dare suggest that the pump operation could be put on a timer or that the swivel bolt be hooked up to a motorized cart. Perhaps that will be next year's pool adventure.
I'm told that some people use their pools to swim in.