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Shading calculations
In order to evaluate the shading losses during the simulation, we have to treat each of the three irradiance components of the input meteo data in the appropriate manner:
•For the Beam component, we will define a Shading factor which depends on the sun position.
•For the Circumsolar component, we apply the same shading factor as for the beam. However the Circumsolar component will not produce electrical shadings.
•For the Diffuse component, we will define a factor as an integral over all sky directions. This will result in a Shading factor for diffuse, which is independent on the sun position.
•For the Albedo component, we will perform an integral according to the near obstacles on the ground. This will result in a Shading factor for albedo, independent on the sun position.
When applying these shading calculations in the hourly simulation, we observe two kinds of losses:
The Linear shading losses are computed from the Shading factors described above, and represent the irradiance deficit on the PV array.
At each time step, the simulation will evaluate the shading loss on the Beam, Circumsolar, Diffuse and Albedo contributions of the irradiance input data. These losses will be available in the results:
ShdLoss Global linear shading loss (or irradiance shading loss), total of the 4 contributions,
ShdBLss Loss on the beam component,
ShdCLss Loss on the circumsolar component,
ShdDLss Loss on the diffuse component,
ShdALss Loss on the albedo component.
The Electrical shading losses is a result of electrical mismatch when interconnecting shaded and unshaded PV modules as an array.
For instance, the global current in a string of modules (or cells) is driven by the cell producing the lowest current. And if the current imposed in the string is higher than the maximum current of a shaded cell, the by-pass diode of the shaded sub-module will be activated.
In PVsyst, there are 3 ways to compute the electrical losses:
•In the unlimited sheds (or trackers) orientation, the electrical effect may be evaluated in a simple and reliable way, using a 2D analytic calculation.
•a calculation named "According to module strings", which is an approximation giving an upper limit to the potential mismatch loss. As this represents a maximum, PVsyst proposes a parameter "Fraction for electrical loss" to decrease the magnitude of the electrical shading losses. Note that for regular row-based arrangements (regular mutual shades), this factor should be left as 100%.
•a detailed calculation named "Module Layout", which gives a more accurate evaluation involving the exact placement of each PV module within the 3D construction, as well as its location in the electrical system. This calculation is based on the combination of the IV curves of the components forming the PV array.
During the simulation, the electrical loss is accumulated in the variable named ShdElec.
NB: The electrical losses only apply to the Beam component. The diffuse irradiance comes from all directions of the sky, and the irradiance inhomogeneities are not sufficient to create significant mismatch losses (and we don't have any means for evaluating them). For similar reasons, we also consider that the Circumsolar component does not produce electrical losses.
NB: The main impact of the module orientation (portrait or landscape) is on the electrical loss. In the module layout tool, you have to define it explicitly. With the option "according to module strings", you will define the "partition" sizes accordingly.
Optimizers at the module level (or better at sub-module level) may recover a part of the mismatch losses due to shades on a string.
However this recovery does not represent the full electrical losses: when a few cells are shaded in a sub-module, the by-pass diode will be activated for short-circuiting this sub-module in the string. Therefore:
•The production of the remaining illuminated cells in the sub-module, as well as the production of the remaining diffuse part, is lost,
•The voltage across the diode (≲ 1V), times the current in the string, represent an additional power loss, the diode will heat-up.
For regular shading situations, such as mutual shadings in regular row arrangements, optimizers at the string level (or better at module or sub-module levels) may recover part of the loss due to the mismatch between strings in parallel.
Gains due to eventual reflections on near reflective surface, usually specular, cannot be calculated by PVsyst.
However, though they are sometimes spectacular, these effects have negligible energetic consequences: they are in general involved only for very special hourly periods, and in the presence of the beam component. Moreover, their effect on the real output of a sizeable PV field remains negligible, when considering that to benefit from it, a complete string of cells in series should be uniformly illuminated by this supplementary supply (the production of a chain is indeed limited by the production of the weakest cell). In the same way, the reflection on the back of a shed, on the one hand only intercepts a small efficient part of the beam component, and on the other hand only illuminates a non-homogenous band in the lower part of the collector.