Recalibration of SPN1 pyranometers against pyrheliometer and its relevance for the evaluation of concentrating solar process heat plants
Accurate determination of direct normal irradiance (DNI) is essential for evaluating and monitoring of concentrating solar power (CSP) and solar process heat (SPH) plants. Currently, a bi-axially tracked Pyrheliometer (PHM) is the most accurate measuring device, but requires high maintenance effort and is cost-intensive. As an alternative, the cost-effective pyranometer SPN1 was investigated by comparing highly resolved minute mean DNI measurements of two SPN1 devices to a reference PHM at the Institute for Solar Technology SPF in Rapperswil (Switzerland). To reduce systematic errors, a linear and a quadratic correction function have been applied to four years of measurements. They reduced the root mean square error RMSE from 48.8 W/m² (11.5% of mean PHM DNI = 426 W/m²) to 37.9 W/m² (8.9%) and 29.6 W/m² (6.9%), respectively. It is also shown that even recalibrations based on only one month of data already reduced the RMSE to maximum 33.4 W/m² (7.8%), if conducted between April and September. Considering the impact of solar elevation and DNI range on the SPN1 accuracy by bifurcating the correction functions only resulted in a negligibly small improvement of the RMSE. Finally, the influence of SPN1’s DNI measurement accuracy on the performance analysis of a real SPH plant in Switzerland was evaluated. After recalibration and correction of DNI values, the evaluation of monitoring data yielded annual efficiencies that were 15% and 12% higher in 2014 and 2015, respectively. It is recommended to recalibrate SPN1 devices over a minimum of one month against a PHM according to the methods presented in this paper.