A basic solar water heating system consists of solar collectors (plates) and thermal reservoir (Boiler).
Collector plates are responsible for the absorption of solar radiation. The heat of the sun, captured by the plates of the solar heater, is transferred to the water circulating inside its copper pipes.
The thermal reservoir, also known as Boiler, is a container for storing heated water. They are cylinders of copper, stainless or polypropylene, thermally insulated with CFC-free expanded polyurethane, which does not attack the ozone layer. In this way, the water is kept warm for later consumption. The cold water box feeds the solar tank heat reservoir, keeping it always full.
In conventional systems, water circulates between the collectors and the thermal reservoir through a natural system called a thermosyphon. In this system, the water in the collectors gets warmer and therefore less dense than the water in the reservoir. Thus cold water "pushes" the hot water generating circulation. These systems are called the natural circulation or thermosyphon.
Water circulation can also be done through motor pumps in a process called forced or pumped circulation, and are normally used in swimming pools and large volume systems.