100-millisecond Resolution for Accurate Overirradiance Measurements

Cloud enhancement of sunlight results in peak irradiance well exceeding extraterrestrial levels, even at high latitudes. Values above 1.8 kW/m² are possible in the equatorial regions. Recently, we detected bursts over 1.5 kW/m² in Northern Europe at latitude close to 60°N. Overirradiance events (intensities much higher than 1 sun) can last tens of minutes, as well as less than 1 s. They may have caused series arcing in photovoltaic modules, leading to fires and loss of property. The accurate measurement of short bursts requires sensors with response times on the order of milliseconds. The long response times of thermopile pyranometers smooth out important details of very short-lived peaks and may cause underestimations of more than 30%. However, sampling with very high resolution can result in very large data files. Thus, the question of an optimal sampling rate arises. This paper will show a method to answer this question. We propose two simple equations that relate the optimal sampling and averaging times to the second temporal derivative of irradiance at the narrowest peak. We studied records with 10-ms resolution from Southern Norway and arrived at an upper limit of 132 ms for accuracy level of 10 W/m².