Solar thermal collectors in the façade

In the main solar thermal systems are used to prepare hot water in small-scale plants. When it comes to applications in the field of solar space heating, large-scale plants in urban building projects, hotels and solar district heating networks, there are not always roof areas available which are sufficiently suitable for the installation of solar collectors. When installing these on existing roofs or joining them to flat roofs, the plants often form a foreign body since they are not an integral part of the architecture. For this reason solar plants are still rejected by some architects and town planners.

For a wide market penetration it is, therefore, necessary to develop collector systems which can be integrated in façades. As the development of façade systems for photovoltaic modules has shown, these open up a large and new market segment.
In this project system-, structural- as well as building physical basis theories have been elaborated. They served as a basis for constructional and aesthetically attractive solutions for the production of façade-integrated solar collectors without an air gap between the collector and the wall of the building for a backside ventilation of the collector.

Façade collectors are an integral part of the architecture as well as being an energy converter. Therefore architects and planners have to co-operate from the very beginning of a project to arrive at a successful conclusion. A survey among architects showed their wish of freedom in design regarding the integration of solar thermal collectors in façades.

When it comes to the design of a solar thermal system with vertical collectors less irradiation has to be taken into account compared to systems with collectors on an inclined surface. On the other hand a vertical collector has a better U-value than a tilted collector, because of the reduced heat losses of the collector due to the lower convection between the absorber and the glazing. If there is no thermal separation between the collector and the wall construction in the form of rear ventilation, the U-value is lowered even more because of the minimization of heat losses to the backside of the collector. This also leads to an improvement in the effective U-value of the whole wall during the heating season.

Two systems with façade integrated collectors were monitored to evaluate the systems’ thermal and humidity behavior. One test façade has been erected on a light weight wall construction and one on a brick wall to record the different behavior of the system collector/wall in different constructions. The results showed the different problems in the different systems: whereas the main question in massive wall constructions is the fixing of the collector without thermal bridges, it is the removal of the humidity in light weight wall constructions.
If the collector is mounted without an air gap for ventilation the construction must have the possibility to dry to the inside of the building. Therefore the inner layers of the wall must be open for vapor. When mounting collectors on massive walls it is important to take care of thermal bridges, otherwise heat losses of the building in the winter time are significant.