The wall construction consists from the inside to the outside of a gypsum plaster, expanded clay bricks (Leca-bricks), cellulose insulation and the fagade collectors. The four collector modules have been mounted with a backside of oriented strand boards (OSB), the insulation of the collector is mineral wool. The absorbers of the collectors are coated with black solar varnish and selective collector coatings in blue, green and grey. The black solar varnish is a standard coating for absorbers, whereas the coloured coatings have been developed in the project.

Because cellulose was used as an insulation material, condensation at the backside of the collector would damage the insulation material. According to the supplier of the cellulose, small amounts of condensate are acceptable, but in any case permanent condensation has to be avoided.

Calculations with the simulation program WUFI [1], developed by the Fraunhofer Institute for Building Physics, have shown that a relative humidity of up to 90% is encountered at the backside of the collector — that means no condensation was encountered with the program. Nevertheless, it was recommended to install the system in winter but only start operation in spring, so that in winter the collector will reach higher temperatures and the wall can dry out
to the inside of the building. This reduces the risk of condensation, which could harm the insulation material.

A major positive effect of a non-ventilated fagade collector is the reduction of the effective U — value. The effective U-values of the wall are calculated from the real ambient temperature, the room temperature and the heat transfer through the wall. This value will change with the conditions and differ in most cases from the static U-value, which is used to characterise a wall and must comply with legislative regulations.

With “Instationar”, a program developed by AEE INTEC to solve transient heat transfer equations, the effective U-values of the wall construction with and without collectors have been calculated for a winter day with high (hemispherical solar irradiation in the wall plane: 4814 Wh/m2.d — primary collector loop operating) and low (hemispherical solar irradiation in the wall plane: 434 Wh/irF. d — primary collector loop not operating) solar irradiation respectively [2]. The results show the positive effect of a fagade integrated collector. The effective U-value of the wall with the collector is up to 77% lower on a winter day with high solar irradiation compared to the static U-value. An improvement of 21% can still be reached on a day with low solar irradiation. The effect of a lower effective U-value is a reduction of transmission heat losses through the wall. The following table gives an overview of the calculation results for the wall constructions with and without integrated collectors. Separate columns are used for absorbers with black solar varnish and the absorbers which are coated with the selective colours developed in the project.

Finally, the maximum temperatures which can occur during stagnation in January (incidence angle close to perpendicular) have been calculated using the program “WandMax” which was developed in the project by AEE INTEC. The results show that no temperatures occur which could harm the used building materials. A maximum temperature at the backside of the collectors of approx. 35°C was calculated.

To investigate the real hygro-thermic processes within the wall, the most important parameters have been monitored and analysed. These parameters were the outside temperature and relative humidity, temperature and relative humidity between the glass cover and the absorber, behind the collector backside, between cellulose insulation and Leca-bricks and inside the room.

The analysis of the data has shown a very good correlation with the simulations done in advance. The relative humidity at the most critical point for condensation — the back side of the collector — has not exceeded values of 85% in the documented period from December 2003 to February 2004. Also the temperatures within the wall construction show good correlation with the results of all simulation programs. At the end of February, a decrease of the relative humidity at the backside of the collector can be seen. At the same time, the relative humidity between insulation and LECA-bricks increased. This could be a first indication of the drying process towards the inside of the building as it is expected.

The mounting of the collectors onto the wall has been investigated for thermal bridges causing heat losses to the outside using THERM [3].

Gypsum plaster

Leca bricks 20 cm

Collector backside OSB

Collector insulation Absorber Glass cover

Figure 6: Wall construction — System Korneuburg

Figure 6 shows the top view of a cross-section of the wall with the construction used to mount the collector. A wooden beam has been mounted to the wall with oriented strand boards at each side. At the front side a second wooden beam is used. The collector frame is connected with screws to the latter mentioned wooden beam. The gap between the OSBs is filled with an insulation material. Figure 7 is the side view of the mounting construction and illustrates the temperature distribution with an outside temperature of -12°C and an inside temperature of 20°C. The two mounting screws can be seen, but there is almost no influence on the temperature distribution.

The results have shown that the mounting construction is well optimized and no additional heat losses will occur.

Summarizing, the data clearly show the suitability of the wall construction for the integration of non-ventilated fagade collectors.