Effects of local atmospheric conditions on plane-of-array solar irradiance

Estimating spectral plane-of-array (POA) solar irradiance on inclined surfaces is critical for photovoltaic and concentrated solar systems design, operation and performance assessment. This work uses a previously developed line-by-line Monte Carlo radiative transfer model to theoretically simulate the anisotropic distribution of solar irradiance in the atmosphere under various local atmospheric conditions. The Monte Carlo model statistically produces the angular distribution of solar irradiance without any angular discretion, thus computes high fidelity POA on surfaces of any orientation. The effects of aerosols, low altitude clouds and high altitude contrails on POA are quantified using the radiative model.

Under cloudless skies, due to the strong forward scattering nature of aerosols, the diffuse irradiance is anisotropic, where the zenith and azimuth spreads of ground-level solar intensity is confined within ±30° and ±45° of solar zenith and azimuth angles, respectively. Therefore surfaces facing the Sun receive the most direct and diffuse solar irradiance. With the presence of clouds or contrails, multiple scattering events are more common, resulting in larger zenith and azimuth spreads of ground-level solar intensity. Under optically thin clouds or contrails (optical depth at 497.5 nm is smaller than 1.0), the POA for surfaces facing the circumsolar region is reduced by less than 5% while POA for surfaces facing the non-circumsolar regions is increased. Under optically thick clouds (optical depth at 497.5 nm is greater than 5.0), nearly all photons have been scattered multiple times, resulting in a nearly isotropic irradiance field. The POA then becomes less surface azimuth-dependent and a function of surface tilt angle alone. In sum, the optical properties of aerosols, clouds and contrails not only affect the magnitude of solar intensity on the ground but also modify the angular distribution of the intensity for POA calculations.