Important for a favourable micro-climate in a collector is the combination of ventilation rate and insulation material. The time-constant of the micro-climate in commercial collectors is much longer than the examples in table 1. A climatic cabinet was used for testing usual collectors. The ambient climate in the cabinet was set to 30°C and 95% rel. humidity (absolute humidity: 26 g/kg dry air). The collectors were pre-conditioned at 25°C and 45% rel.

humidity (absolute humidity: 9 g/kg dry air) at an inclination angle of 45°. The heat-transfer — fluid provided 40°C absorber temperature. The collector was moved into the cabinet fast and the rel. humidity in the air-gap was recorded. The results are compiled in figure 9a.

Figure 10: Frequency distribution of the humidity in the air-gap of different commercial collectors

The same collectors were exposed under simulated working conditions outdoors. The data from the measurement of the humidity in the air-gap are shown as frequency distribution

functions in figure 10. The ambient humidity peaks at high values of the relative humidity, mainly because of the night values. All collectors are drier.