Most collectors are equipped with special openings for enabling a controlled ventilation. Low — cost collectors or collectors with wooden frames are not really tight and allow considerable uncontrolled ventilation. Both contribute to changes of the micro-climate.

A procedure for measurement of the ventilation rate must take into account both. The working group Materials for Solar Thermal Collectors (MSTC) of the Solar Heating an Cooling Programme (SHCP) of the International Energy Agency (IEA) developed a method for Ventilation rate measurements and tested it by performing a round robin test of the same collector in several laboratories: IBE (DK), TNO (NL), SPF (CH), ISE (D). The method was to measure the air-flow-rate through an additional opening in the back-plane of the collectors while applying a positive or negative pressure difference in the order of some Pascal between collector and ambient. The results of the different labs are shown in figure 3. The good

agreement proved the applicability of the procedure. The variation of the ventilation rate of different commercial collectors is shown in figure 4. The ventilation rate was normalised by relating it to the volume of the collector in order to enable a comparison.

The function of pressure and flow rate follow the expected parabolic shape (figure 4a). The ventilation rate was defined as the flow-rate in collector-volumes per hour at a pressure — difference of 1 Pa. The collectors with the high flow-rates were untight wooden constructions. The values are usually not reached in operation. The flow-velocity measured by a micro­anemometer in front of a ventilation hole of a controlled ventilated collector during stagnation conditions (displayed in figure 5) follows the air temperature measured in the gap between absorber and glazing. The total air exchange during this sunny winter day was about 7 volumes per day. The air-exchange during night-time was nearly 50% of the day-time, with reversed direction, of course, because of the relative cooling in the clear nights. This might cause accumulation of moisture in the collector, when the collector temperature is below the dew-point and/or the insulation material was dried before.

Figure 4: Measurement of the ventilation rate of different collectors