Is there an energy efficient way to heat an outdoor pool?
There are multiple systems for sale that incorporate a solar tube and pump that heats the water at an average rate of around 470 gpm (gallons per minute). These systems are bulky and a bit of an eyesore. If you have warm, sunny weather, just buy a black pool cover. Black is the most absorbent color of energy there is. The heat from the sun will literally be stored in the cover and distribute at least 50% of that heat into the first 6 to 8 inches of water. The pool pump can be used to circulate the water and create an even temperature.
This method is inexpensive, effective, and is definitely not an eyesore. Please realize that this is a natural convective heating method and will not be able to be controlled like a pool water heater.