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
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.
Here the collector is vacuum tube, its inside temperature reaches to 240 degrees
And because of vacuum the heat gained can’t transfer to the air it must transfer inside to either water in case of open loop system or to copper heat pipe in case of closed loop system, then the heat transfer to the water inside the tank