The concept of the window collector was setup by J. M. Robin et. Al. In /1/ an overview of the state of art is given.

To evaluate the thermal performance of the window collector, a numerical investigation is made. A simple model describing the thermal behaviour of the system is implemented under the TRNSYS environment. The model takes both passive and active solar gains of the window collector into consideration. A dynamic building simulation was carried out with a solar combi system for space heating and domestic hot water preparation. The goals of the simulation are the estimation of the possible yearly energy saving and its influence on the room temperature behind the collector.

The necessary collector parameters for this simulation are obtained from a collector test in accordance to EN 12975-2. The collector testing was performed with a first prototype of the window collector. For this testing the collector was treated like a conventional collector. The thermal efficiency of the window collector is of course the efficiency of a typical flat plate collector due to the fact that only half of the aperture area is covered by the absorber. The k-value obtained from this test method is over predicted and therefore not suitable for the window collector.

Under realistic conditions with the window collector integrated into a wall the heat losses will be much smaller than the calculated one taking the above mentioned k-value into account.

A one-family house at WQrzburg with a yearly energy demand of 12674 kWh served as a reference. A schematic description of the solar combi system with 750 litres of tank-in-tank storage is given below (Fig 2).

Comparison is done for the energy savings due to a wall-mounted conventional flat plate collector of 15 m2 and the window collector of the same area. The yearly energy saving was calculated as 22.2% for the flat plate collector and 19.2% for the window collector.

This is a remarkable result given that the absorber area of the window collector is only half that of the flat plate collector. The primary reason for this better performance of the window collector is its optically optimised design (distance of absorber tubes, reflector arrangement) under the prevailing irradiance condition on a vertical wall and the secondary reason is an additional passiv solar gain. On account of the special optical property of the window collector this passive solar gain is dependent on the incident angle

(IAM). Corresponding the IAM the collector shades the room located adjacent behind the window collector depending on the position of the sun. Ray-tracing studies are undertaken to evaluate this. The results of this study are depicted in Fig. 3.

The TRNSYS simulation described is not detailed enough to capture the interaction of the window collector and the room behind to calculate the influence on the indoor climate and comfort. The simulation results showed that even in summertime the room temperature may not rise above 25°C due to the shading effect of the window collector where as an ordinary window of the same size yields a temperature of 35°C.

A more detailed description of the simulation results is given in /1/

Despite the availability of the room temperature through TRNSYS simulation the comfort level inside the room is not fully discovered. The comfort level might be influenced by the possible high temperatures at the window collector. First measurements using infra-red camera showed temperatures of the inside glass surface up to 55° under stagnation condition corresponding to maximum a temperature at the absorber of 102°C. The effect of the window collector on the room climate still has to be investigated.