Solar water heating systems have both a tank as well as solar collectors. They can be a very effective way to heat the water in your home, since they use the sun to heat the water, which is essentially free. In a two tank solar water heater system the water is preheated before it enters into the conventional storage thank. In a one tank system there is a backup heater and it's combined with solar energy storage in one tank.
There are three types of collectors in a solar water heater.
Flat-plate collector- Insulated box with a dark observer plate under one or more glass covers.
Integral collector-storage systems- One or more blank tanks or tubes in a well insulated box. Cold water is sent through the collector and pre-heated before it is sent to the conventional backup water heater. These should be installed in mild climates as the outside pipes may freeze in colder temperatures.
Evacuated-tube solar collectors- Parallel rows of transparent glass tubes. Each tube has a metal absorber attached to a fin. The coating on the fin absorbs solar heat but limits heat loss. Commercial installers in the US use these the most.