My fumbling attempt at solar heating a new swimming pool in Oman is hereby documented to help others who might wish to try something similar.

A few salient realities: We are basically located on the Tropic of Cancer, so, in winter, when you most need the heating, the Sun is quite far from zenith, say, at about 45deg during December around noon. People in Oman, even kids, like their pools to be warm, 28deg C (82deg F) or warmer! From what I have seen at other residential pools, most of them go unused after the first three years of novelty, and even the kids do not bother to use unheated pools during the 3 or 4 "winter" months when ambient temperature drops below 25deg C (77deg F). This led me to design our pool towards ornamental longevity, i.e. include a fountain of some kind. During the summer the ambient temperature gets to over 40deg C (105deg F) and some people actually install chillers. A heat exchanger can serve as both a summmer chiller and a heater during winter. Heat exchangers, though a lot more expensive than simple electrical resistance (heating element) or even gas heaters, can have an energy efficiency around 500%; i.e. for each kWh you input as electricity to run the compressor (basically they work like room air conditioners in reverse, by taking heat from the atmosphere and putting it into the water) you get 5kWh of heat transferred into the water. Solar heaters cost very little electricity, since you have to run your filtration pump anyway. Run it during sunlit hours and you collect the solar heat on the way, essentially for free.