Long-term behavior, accuracy and drift of LI-200 pyranometers as radiation sensors in Rotating Shadowband Irradiometers (RSI)

Kurzfassung

Rotating Shadowband Irradiometers (RSI) are frequently used for solar resource assessment at remote sites due to their significantly higher robustness for soiling, their lower power and maintenance requirements and their cheaper acquisition and operation in contrast to pyrheliometer on tracker systems. The primordial lower accuracy of their photodiode sensor, usually a LI-200 pyranometer from LI-COR Inc., is mainly caused by restrictions of their spectral sensitivity and temperature dependence. However accuracy is notably increased by application of corrections to the raw sensor response. Thus, finally a coincidence of DNI measurements from RSIs with high-precision pyrheliometer measurements within 15 W/m2 (root mean square deviation for 10 min averages) for actual values, less than 3% for daily DNI and within approximately 1.5% of the monthly and annual sum is reached. Within this contribution, the long-term behavior of the LI-COR sensor is examined with regard to the drift of the photodiode sensitivity. This is analyzed from recalibrations of 30 sensors after one to four years and from long-term studies lasting from one to several years. If a significant drift appears, the corresponding uncertainties can be reduced through recalculation of the previous measurement data for the total measurement campaign. Furthermore, studies about the coincidence and deviation of the responses for global, diffuse and direct irradiance of RSI measurements between several individual RSIs and to reference measurements from high-precision thermopiles are presented for different time resolutions.