Solar thermal systems convert incident solar irradiation into heat so that it can be used to heat domestic hot water or for additional heating.

The solar irradiation is converted into heat in the collector whose main component is a selectively-coated absorber. The heat is pumped via a closed piping system, the solar circuit, from the collector to the storage tank. The solar system not only provides hot water in the summer and in transitional periods, but can also even provide some in winter. A frost-resistant mixture of water and glycol is used to prevent the heat-transfer fluid freezing in the solar circuit.

Temperature differential control

A differential temperature controller regulates the delivery of heat from the collector to the storage tank. The controller always starts the circulation pump of the solar circuit when the temperature in the collector is several degrees warmer than the temperature at the bottom of the storage tank. This moves the solar fluid from the collector to the lower heat exchanger, where the heat is transferred to the domestic hot water in the storage tank via the solar circuit heat exchanger. The cooled solar fluid then flows back to the collector in the return pipes. The heated domestic water rises in the storage tank. The water is stratified in the storage tank according to its density or temperature: The hottest water is at the top (where it is drawn off), the coldest water is at the bottom (where cold water is fed in). A standard domestic hot water Solar Thermal System would normally be sized at 1m² of Solar Thermal Collector area and 50 Ltrs of Domestic Hot Water storage volume per occupant.
This could result in a Solar Fraction (percentage of the total energy requirement for heating drinking water provided by solar energy) of up to approx. 60 percent. The remaining 40 percent must be covered by supplementary heating. This is generally performed by a gas or oil boiler in conjunction with the upper coil/heat exchanger of the storage tank.