Flat-plate collectors: construction, function & advantages

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Collectors are the most visible components of solar thermal. In addition to vacuum tube collectors, Viessmann also offers flat-plate collectors for residential, commercial and local authority buildings. The two collector versions are similar in that they use free and almost universally available solar energy for heating and domestic hot water.
In addition, they are made of corrosion and UV-resistant materials. However, the design and functional principle of solar collectors differ. Please read on to find out what distinguishes Viessmann's Vitosol flat-plate collectors.

Viessmann flat-plate collectors

Design and function of Viessmann flat-plate collectors

Powerful flat-plate collectors are a good addition to any heating system. These collectors consist of an absorber and a casing with cover panel. The difference here lies in the detail: the collector housing of a Viessmann flat-plate collector consists of a circumferentially curved aluminium frame. With the Vitosol 200-FM model, this can be customised and painted in a RAL colour. The frame is painted dark blue as standard. The seamless pane enclosure ensures a permanently tight fit and maximum stability of the collector.

The cover, made of special-purpose glass, is highly transparent, allowing ample solar energy to penetrate through. In addition, Vitosol flat-plate collectors have a puncture-proof and corrosion-resistant back panel and highly effective thermal insulation. The heart of the collectors is the absorber –– a meandering pipe through which a solar fluid (solar medium) flows. Above this is the absorber panel with the ThermProtect coating.

How does sunlight become heat?

The function of the flat-plate collectors is based on a circuit. When the sunlight hits the surface of the flat-plate collectors, it heats the solar medium circulating inside the tubes. The solar medium then transfers the heat to a DHW cylinder or heating buffer cylinder. The cycle can then start again.

On days with abundant sunshine, flat-plate collectors can absorb ample energy and store it temporarily in a storage unit. If none of that energy is removed for a while, the collectors are at risk of overheating. This is because the storage unit can no longer absorb energy. An intelligent absorber layer protects Viessmann flat-plate collectors against this risk.

The difference between standard absorbers and switching absorbers

Viessmann's patented ThermProtect technology prevents further energy absorption once a certain temperature has been reached, when the solar cylinder is fully heated. If the switching temperature is exceeded, the crystalline structure of the absorber layer changes, increasing the rate of heat radiation many times over, and reducing collector output. The stagnation temperature thus drops significantly, preventing the formation of steam. When the collector temperature falls again, the crystal structure returns to its original state. The solar energy can then be absorbed as before. The change in the structure of the crystalline surface can be restored indefinitely, i.e. it is fully reversible. ThermProtect also enables more generous dimensioning of the entire surface when compared to conventional flat-plate collectors.


In summary, high performance Viessmann Vitosol 200-FM and Vitosol 100-FM flat-plate collectors are characterised by the following:

ThermProtect guards against overheating at high outdoor temperatures and with low heat dissipation
High solar coverage
Suitable for central heating backup and domestic hot water heating
High leak-tightness due to circumferential curved frame with glass retaining strip
Quick and easy assembly thanks to stainless steel corrugated pipe push-fit connectors
Suitable for different installation versions: on-roof, flat roof, in-roof and facade
Suitable for horizontal and vertical installation
Attractive design with option for customisation when the Vitosol 200-FM is painted in RAL colours
UV-resistant and corrosion-resistant materials for all components

Flat-plate collector product range

Flat-plate collectors from Viessmann allow various installation options. They can be installed both horizontally and vertically. Installation on pitched roofs is also possible. When replacing a boiler in a residential building, a combination package of two flat-plate collectors and a solar cylinder is often the ideal solution.