The first usage for solar energy was solar water heaters
The collector absorb the sun rays then transfer it to heat water
Kinds of solar water heaters
Flat plate type
The flat collector absorb the sun rays then transfer it to heat the copper network which is placed behind it, inside the copper network we found water mixed with propylene glycol to prevent freezing water, by thermosiphone theory the heated water become low density than cold water then it goes up to heat the water inside the tank.
Vacuum tube type