Most pool heating systems include a:
Solar collector :The device through which pool water is circulated to be heated by the sun
Filter :Removes debris before water is pumped through the collector
Pump :Circulates water through the filter and collector and back to the pool
Flow control valve :Automatic or manual device that diverts pool water through the solar collector.

How pool heating systems work.
Pool water is pumped through the filter and then through the solar collector(s), 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.

Some systems include sensors and an automatic or manual valve to divert water through the collector when the collector temperature is sufficiently greater than the pool temperature. When the collector temperature is similar to the pool temperature, filtered water simply bypasses the collectors and is returned to the pool.

Solar pool collectors.
Solar pool collectors are made out of different materials; the type that you buy depends on your climate and how you intend to use the collector. Collectors designed to operate only when temperatures are above freezing are inexpensive, made from specially formulated plastic materials, and generally do not include glazing (a glass covering). Collectors appropriate for colder climates, or that are designed to operate year-round, generally have metal components and a low iron-tempered glass glazing (glazing greatly increases the cost of the collector).

Solar Pool Covers
The largest heat loss from a swimming pool is from evaporation, by adding a simple barrier such as a pool cover can cut almost all of these losses. Covering the pool with a pool cover when it is not in use is the single most effective means of reducing pool heating costs. Savings of 50-70% are possible. It is also worthwhile investing in a pool cover reel, to make covering the pool easier.