Two ways are possible to optimise the micro climate in solar collectors, as discussed above. Changing the ventilation rate either by increasing or decreasing the size of the ventilation holes yields only small changes in the wetness behaviour, as shown in figure 11a while a change of the insulation materials as a much bigger impact (figure 11b). The optimisation of different collectors displayed in figure 9a by changing the ventilation rate resulted in the wetness behaviour shown in figure 9b. The unchanged reference collector (No. 7) serves as reference for the eye.

Attention has to be paid to the position of the ventilation holes. Usually diagonal ventilation (see figure 2) is more effective when dispersed ventilation holes along the edges. The effect is clearly diminished if insulation materials cover the ventilation holes (see figure 12).

Figure 11 : Impact of changes on the micro-climate on the humidity response function of collectors

2. Conclusion

The micro climate in ventilated flat plate collectors is dominated by the moisture adsorption of the insulation material or other parts of the frame and by the ventilation rate. Reliable measurement procedures for the ventilation rate and for the micro climate have been developed. They are suitable tools for optimisation of the micro climate by selecting the appropriate insulation materials and ventilation rates.

Further work is necessary for quantifying the properties of the insulation material, it’s degradation behaviour and the geometrical dimensions of the collector.