Study of soiling on pyranometers in desert conditions

This work presents a study of the degree of soiling on pyranometers in Qatar, a region with high amounts of dust in the atmosphere. The experiment is performed during several months including a dusty season. It consists of using two pyranometers to measure the global horizontal irradiance: one of the pyranometers is considered as the reference sensor and is cleaned on a daily basis, and the other pyranometer is used as the testing one, on which different frequencies of cleaning are tested. In order to evaluate the losses in the measurements due to dust/dirt accumulation on the sensor dome, the ratio of the daily average of the global horizontal irradiance measured by both sensors was calculated and analysed in function of the time passed after the last cleaning of the testing sensor. From day to day, the changes in this ratio are between 0.3% and0.5% per day depending on the soiling environment of each season, with increasing changes towards the summer. The relative errors (RMSE and bias) of the output of the testing sensor as compared to the reference one were also studied and show a linear increase with the time passed since the last cleaning, for the period under study. Lastly, a (post-processing) method for correction was deducted empirically from the correlation between the measurements (soiled and reference). By applying the correction, reductions of 87% in the relative bias error value and 70% in the relative RMSE value, were obtained. This study helps to develop an adapted quality assurance program for sensors installed at remote sites in the desert, including an adapted cleaning schedule which takes into account the local environmental conditions