Monitoring of mirror and sensor soiling with TraCS for improved quality of ground based irradiance measurements

Meteorological stations for solar irradiance measurements are mainly utilized for resource assessment of possible sites for future solar power plants and for thermal efficiency calculation and control of operating power plants. These stations consist of a solar tracker, two pyranometers and a pyrheliometer (MHP) for irradiance measurements. The accuracy of the MHP instrumentation is usually specified to be better than 2% if cleaned on a daily basis. However, soiling frequently exceeds other error influences significantly, reducing irradiance values and accuracy. Due to the high sensitivity to soiling shown by pyrheliometers, especially DNI measurements are affected. Reductions of measured DNI values exceeding 25% in only a few weeks are not unusual. In order to improve this situation, the soiling level of each individual sensor can be determined by following a special sequence of sensor cleaning and brief breaks combined with a close examination of the sensor responses. This allows for an approximate post processing correction of the irradiance data measured since the last cleaning (if recent). The corrections applied are cross- checked by means of an improved version of the TraCS asset. It can be used to control the sensor soiling correction procedure. The TraCS's improvement consists in rotating the mirror within its plane with the pyrheliometer thus scanning its surface instead of just viewing the same small spot on the mirror. Hence, a better accuracy of the mirror soiling level is achieved by deriving more reliable average values. Finally, the results of an examination of sensor soiling rates at several meteorological stations set up in the MENA region and cleaned following the described protocol is presented. This gives an idea about the range of regional differences in soiling rates to be expected in the North African region.