A pump circulates a water-antifreeze mix in a closed circulation between the solar collector and the solar heat storage tank. Inside the collectors this mix is heated up by the solar heat radiation. In the solar heat storage tank the heat of the fluid is transfered to fresh water.
The preheated fresh water therefore needs less energy to be heated up to the desired temperature, thus saving a lot of energy.
In the summer, the sun is often so strong that there is no need for heating the preheated water at all.