Image: Willemferguson via Wikimedia CommonsThe solar collectors are the main solar water heating components in a heating system. They collect the sun´s energy and transfer it into usable heat energy. This energy is then transferred to hot water storage.
Relevant solar water heating components for this task are the recirculation pump and the solar controller. The solar controller recognizes when the temperature in the water storage is lower than in the collectors. It then activates the recirculation pump, and the solar fluid inside the piping is circulated. A solar heat exchanger then transfers the heat indirectly into the hot water storage so fluid and water don´t mix - a lot of systems feature solar antifreeze which should not mix with the water in the storage.
Next among the solar water heating components is the expansion vessel. Pressure within a solar water heating system is not constant, it changes with rising or falling temperatures. For example, the fluid expands when temperatures rise. The solar expansion vessel prevents damage by giving the solar fluid space to expand safely.
Solar water heating systems often need a backup heater to provide additional heat on cloudy days. A tankless water heater is a device used fairly often for this task.
Solar Water Heating Components in Passive & Indirect Systems
The components described above are mostly for active indirect solar water heating systems which feature the most components. There are simpler systems that do not need all of these such as thermosyphon systems or batch heaters. For batch heaters, the potable water is heated and used directly - there is no need for a dedicated extra storage or a solar heat exchanger. Additionally, thermosyphons do not need a recirculation pump as they use natural convection to achieve fluid circulation.