Soiling impact on direct normal irradiation measurements

Abstract

Accurate and reliable solar radiation measurements are necessary to plan, finance and operate solar power projects. Soiling of sensors is an important source for underestimation of solar radiation. Earlier studies showed that especially pyrheliometers measuring direct normal irradiance (DNI) suffer from high susceptibility of dirt, while Rotating Shadowband Irradiometers (RSI) showed almost a factor of 10 better soiling behavior. This study extends the experiences by more extensive comparisons at another site in Spain and one site in Germany both covering one year. For the first time the soiling behavior of four different pyrheliometer makes are compared in parallel. At the Andasol 3 CSP plant in Southern Spain daily soiling rates vary between 0.1% and 0.4% with a mean of 0.14% for pyrheliometers, while RSI soiling rate on average was only 0.035%. At the site near Freiburg, Southern Germany, the soiling rates of pyrheliometers vary between 0.2% and 0.6% with a mean of 0.4%, while RSI soiling is about 0.1% per day. Thus, the resistance to soiling of the RSI in our experiments is only about a factor of 4 better than for pyrheliometers. This is significantly less than found earlier but still a good reason to better apply RSI instruments in case pyrheliometers cannot be cleaned with high frequency - ideally daily, but at least weekly or more in more dusty regions. In agreement with other studies we find a high dependence of soiling rates on the location and its characteristics. Comparisons between various pyrheliometer types show that pyrheliometers from various manufacturers show similar high soiling rates. But a protection cap lessens the soiling susceptibility, as the only pyrheliometer without such a hood shows significantly higher soiling rates. The specification of daily soiling rates presumes a linear behavior with time. From a longterm test where cleaning was omitted over a period of about five months it is shown that after several weeks saturation effects reduce the soiling process. From analysis if this experiment we conclude that the soiling of radiometers follows an exponential dependency. Until a total soiling of around 4% is reached a linear soiling rate is a good approximation.