You can significantly reduce swimming pool heating costs by installing a solar pool heater. They're cost competitive with both gas and heat pump pool heaters, and they have very low annual operating costs. Actually, solar pool heating is the most cost-effective use of solar energy in many climates.
Pool water is pumped through a filter and then through the solar collector, where it is heated before it is returned to the pool. In hot climates, the collector can also be used to cool the pool during peak summer months by circulating the water through the collector at night.