Treatment of the Diffuse component

Basic hypothesis: isotropic diffuse

The determination of the shading effects on the diffuse component is based on the hypothesis that the diffuse is isotropic. This means that the array receives the same irradiance whatever the direction of the sky.

This assumption is realistic for covered and semi-covered conditions. But not completely true when the sun is shining, as the "circumsolar" (bright enhancement in a cone of 5° around the sun, due to humidity and aerosols) is accounted within the diffuse component in usual weather data.

Calculation as an integral

PVsyst will evaluate the shading factor for the diffuse as an integral of the shading factor, calculated for each direction of the space. This integral is performed over the sky vault, as "seen" by the collector plane, i.e., the orange slice limited by the collector plane and the horizontal plane. This integral uses the table of the shading factors calculated previously for the beam component, for any direction of the beam (not only the possible sun's positions).

This integral, named "shading factor for diffuse" is not dependent on the sun's position, but only on the geometry of the system. It is therefore constant over the whole year, and even independent on the latitude. It is calculated once at the beginning of the simulation, and is applied to the diffuse component at each time step of the simulation.

For tracking systems, the shading factor for diffuse should be recomputed for each position of the trackers. In practice, it is evaluated for several orientations of the trackers at the beginning of the simulation, and interpolated between these values during the simulation. With backtracking, there are no mutual shadings for the beam component, but the shading loss on the diffuse is significant. Note that by default, large tracking systems are approximated for the sake of the diffuse shadings computation. This may lead to some inaccuracies in some cases; for these, the approximation should be removed, and one should calculate with all trackers.

Global calculations on diffuse

The shading factor on the Far shadings is computed the same way. We have to apply also a similar calculation to the IAM (Incidence angle Modifier).

However, these integrals are not independent: when the direction is below the horizon line, you have no irradiance contribution for the calculation of the near shadings nor the IAM.

Therefore for each direction, we should evaluate the product of the Far shadings, Near shadings and IAM loss as parameter for the integral. Along with the last irradiance loss (soiling, independent on the direction), this leads to a constant global loss factor for diffuse, used for the calculation of the "Effective diffuse" component in the simulation.