Solar irradiance measurements for the monitoring and evaluation of concentrating systems

Precise solar radiation measurements are essential for monitoring and evaluation of concentrating solar systems. In this paper the quality of irradiance measurements from the low cost systems SPN1 and Rotating Shadowband Irradiometer (RSI) are compared to reference measurements from a Pyrheliometer, a Pyranometer and a Shadow Ring Pyranometer. A graphical and statistical analysis of the measurement differences has been conducted. The investigation is based on data of nearly one year and focuses mainly on deviations regarding the Direct Normal Irradiance (DNI). Furthermore, estimations for the expected errors are given. It is shown, that the mean deviation of DNI measurements from the RSI are rather small. However, large short time variability can lead to considerable measurement inaccuracies. The SPN1 shows a strong non-linear bias that should be corrected.