Assessment of PV performance in Nordic conditions: Investigating the influence of spectral irradiance

Abstract

As PV becomes a significant part of the energy mix in the Nordics, it is important to investigate the factors affecting PV module performance in these conditions. This thesis aims to expand the knowledge on PV performance monitoring at high latitude with special focus on spectral irradiance. While broadband irradiance and temperature effects on PV performance are well discussed, a comprehensive understanding of the spectral impact across the world is missing. This thesis presents the main results from the outdoor performance assessment of fixed-tilt, flat-plate, commercially available solar PV technologies as well as the broadband and spectral irradiance resource assessment, from the two different monitoring stations of University of Agder, Grimstad, Norway. The spectral irradiance distribution at Grimstad is analyzed with the help of various spectral indices. The annual average general spectrum in Grimstad is blue-rich, with higher energy content in the blue range compared to the AM1.5G standard spectrum. The averaged photon energy (APE) indicates greater energy content than the reference spectrum at shorter wavelengths during summer and at longer wavelengths during winter period. A statistical approach is used to compare the indices under various sky conditions and APE is found to have lowest uncertainty, suggesting its use for further studies involving comparison of the spectral distribution across northern Europe. Furthermore, the impact of various atmospheric factors and design parameters influencing the spectral distribution is discussed using modelled spectra. The results show location-specific dependence of the spectral irradiance, with small yet significant difference in incident spectrum even in locations within the similar climatic conditions. Additionally, the spectral impact on commercially available crystalline silicon, CIGS and HIT module technologies is assessed and reported. The spectral factor is included in the performance and degradation studies of the PV modules. A spectral correction technique, in addition to the irradiance and temperature, is applied to the STC correction methods for performance assessment using a long-term measured data. In the Nordics, module technologies with smaller bandgap and wider spectral response range also experience small spectral gains, however, the overall spectral impact compared to the larger bandgap modules is modest. The performance degradation as well as the degradation of temperature coefficients are studied. Multi-crystalline modules deployed for 6 years in these conditions show a much smaller degradation rate compared to the global average, with the temperature coefficients remaining unaffected. The uncertainty in determining temperature coefficients is significantly reduced when spectral correction is accommodated for in STC corrections, providing an alternative method for determining temperature coefficients in absence of indoor measurement facilities, especially at high latitudes where spectral variations are considerable.