Solar thermal: the function briefly explained

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Viessmann Vitosol 100-FM on the roof of a house

Every day, the sun emits free energy that homeowners can benefit from with a solar system. Solar thermal is very straightforward: collectors capture the radiant heat and convert it into thermal energy before a storage unit absorbs the heat. Depending on the size of the system, that heat can then be used for domestic hot water heating or as a central heating backup.


Flat-plate collectors such as the Vitosol  100-FM or the Vitosol  200-FM consist of a thermally insulated aluminium frame covered with highly transparent special-purpose glass that is directed towards the sun. This allows a large proportion of the radiant heat into the interior of the collectors, where it hits the absorber. The component heats up and transfers thermal energy to the solar medium, which flows through meandering pipes inside the collector.

Vacuum tube collectors such as the Vitosol 200-TM or Vitosol 300-TM consist of individual tubes that are interconnected via a collector. The tubes contain the absorber and a heat transfer medium. The latter evaporates during heating, condenses on the collector and transfers solar energy to the solar fluid (solar medium). A vacuum in the collector tubes reduces heat losses and ensures that the solar thermal system functions efficiently.

Viessmann solar collectors feature the patented ThermProtect temperature-dependent shutdown. In the case of flat-plate collectors, this consists of a special absorber coating that changes its properties as the temperature changes. If the collector temperature rises above the shutdown point, a change in the crystal structure causes more heat to be radiated away. The collector temperature then falls, and the solar collectors are no longer at risk of overheating. In vacuum tube collectors such as the Vitosol  300-TM or  200-TM, the temperature-dependent shutdown functions via a self-regulating heat pipe. This prevents condensation of the medium in the heat pipes whenever their temperature rises above 150  degrees Celsius. The ThermProtect temperature-dependent shutdown function works without additional components. It simplifies the design of solar thermal systems and ensures safe operation with all new Viessmann solar collectors.

A thermal store buffers the solar energy in the house

If the supply of solar energy is greater than the heat demand in the home, a solar cylinder absorbs the energy from the solar system before releasing it to consumers later on. This helps to ensure that as much solar heat as possible can be used in the home. The type of energy store used depends on the function of the solar thermal system. If the system is used for domestic hot water heating, our trade partners will install a DHW cylinder. If the solar system is used as a central heating backup, they will fit a buffer cylinder instead.

DHW cylinder for domestic hot water solar thermal systems

DHW cylinders such as the Vitocell  100-B/-W are filled with DHW. They are equipped with two coil heat exchangers that sit in the tank much like immersion heaters. While the lower heat exchanger guides the hot solar medium through the cylinder, the upper heat exchanger is connected to a boiler, such as the Vitodens  300-W gas condensing boiler. If the output from the solar system is not sufficient to bring the DHW to the required temperatures, a control unit activates the second heat generator. This ensures that DHW always flows out of the pipes warm, even on cloudy days or when demand is particularly high. This process is particularly efficient when all system components are perfectly matched, as in the Vitosol  141-FM DHW solar package from Viessmann.

Heating water buffer cylinder for solar systems for central heating backup

If the solar thermal system is designed to work as a central heating backup, a heating water buffer cylinder will be installed. This is filled with heating water that is heated with solar heat via a heat exchanger. If this heat is not sufficient to achieve the required temperatures, a Viessmann gas heating system, a wood heating system or a heat pump will provide the additional energy required.

Solar thermal: functionality relies on a solar circuit

The solar circuit ensures that the heat from the collectors reaches the home. It connects the system components and is filled with a solar medium (a mixture of water and antifreeze). The medium is heated in the solar collectors and is pumped to the storage unit by a pump. Here, the medium releases thermal energy, heating up the storage unit and cooling itself down in the process. It then flows back to the solar collectors on the roof and the cycle can continue. A number of safety fittings are required to ensure that the solar thermal system can achieve its full potential.