The basic working principle of solar water heating system is thermosyphon process and forced circulation.
A structure of solar tube is installed on a sun exposed area, usually rooftops. Cold water or chemical such as glycol is passed through the tube which is then heated by the sun. Storage tanks in this system consist of an inlet and outlet connected to and from the collector. Tank systems also play a particular role, as in, two-tank systems preheats water before it reaches conventional water heater whereas in (one-tank) backup heater works in combination with the solar storage in one single tank.
Solar Water Heating System use vacuum glass tubes which have a special coating. This coating together with glass tubes called Evacuated Tube Collector system (ETC Systems) is primarily meant to absorb solar energy. Tubes being the main solar energy absorber consist of an assembly of two concentric glass tubes.
Spacing between glass tubes is filled with air which when evacuated creates a vacuum acting as a great insulator, eventually minimizing heat loss from inner tubes. The inner tube’s coating absorbs solar energy transferring it to the water. The water available on the upper side of this vacuum tube becomes hot and consequently lighter causing it to move upward in the tank, whilst, cold water being heavy comes downward from the tank to be stored at the bottom. This particular phenomenon is called natural Thermo-syphon circulation that occurs in every tube.
Differences prevail when it comes to propelling water into the cylinder. Convection systems makes hot water rise and reach the cylinder requiring no mechanics or electricity whereas pump based systems work with an electrical pump for transferring water to the cylinder.
These solar water heating systems always require a backup system for cloudy days or at times of increased demand. Most of these systems require an electric powered backup heater called booster. The booster serves to kick in further heat to warm up the water and kill bacteria as well. Also it goes well for night times and cloudy days when system isn’t getting continuous supply of sun’s energy. A conventional heater mostly come with a storage backup and likely is expected with solar system package. This backup option is also part of solar collector e.g. rooftop tanks with thermosyphon systems. The integral-collector storage system is already storing hot water besides collecting solar heat; the same can come in the package with a tankless or instantaneous water heater as a backup.