Shadings on Beam and Diffuse
First of all, when optimising sheds, one should keep in mind that the shading losses are of two kinds:
- The shadings which we usually visualize, are applied on the beam component. They apply especially on some periods of the day or the year, and often when the sun is not very "high" on the plane surface, therefore affecting hours of rather low production (there can be an exception with mutual shading of sheds, reinforced by the "electrical" effect of cell partial shadings).
- The shading on the diffuse component, which applies permanently with the same value when assuming an isotropic diffuse distribution. This shading factor is related to the part of sky which is "seen" by the collectors. Remember that in central Europe climates, the Diffuse fraction is of the order of 50% of the total incident irradiation !
In the shed disposition, the diffuse effect is particularly important, since the visible part of the sky hemisphere is limited to the front by the previous shed (affecting rather high incidence angles) and to the rear side by the plane itself. Due to its permanent effect, this is often the main part of the losses.
This situation is even much more sensitive with sun-shields.
General features and optimisation of sheds
With shed disposition, we should be aware that:
- The area occupation strongly depens on the collector tilt. For acceptable shadings, the "Limit shading"should be kept below about 18° to 20°. With 30° collector tilt, this implies that you can only install a collector area limited to 45% of the total available area.
- The mutual shading effect is also strongly dependent on the shed orientation: when not south, the morning or evening performances are much more affected.
The software offers two complementary tools for a better understanding of these situations:
- The beam shading graph, which shows the periods which are affected by the shadings. One can notice that with south orientation, the effects are rather limited to summer morning and evening. But as soon as you change the azimuth, the shading losses increase rapidly, and arise especially in regions where the incidence angles are high.
- The yearly yield graph shows the relative gain (with respect of the horizontal layout), as a function of the module tilt. This factor is based on the yearly global useful irradiation (beam and diffuse) falling onto the collectors. The curves show the pure transposition yield (as if there were only one plane) and the curve with mutual shadings, either "linear" and for electrical losses.
This graph can be drawn with two options:
- Either keeping the Incidence limit angle constant. In this case, we see that the collection/ground ratio is very sensitive to the plane tilt.
- Or keeping the Shed spacing (i.e., a chosen collection/ground ratio) constant.
In both cases, we see that the optimal tilt of sheds is lower than for a single plane. This is still more evident with non-south azimuths.
From these tools we can observe that:
With shed installations, choosing a rather low tilt is often a very good solution, which leads to acceptable losses of some few percent with respect to the optimum, with the following advantages:
- The installable power is much greater on a given ground area.
- The array orientation does no more affect on the performances: you can install the sheds according to the building geometry, special "south-facing" arrangements are no more useful.
- Module supports become more simple, cheaper and lighter, with less wind sensitivity.
- Architectural impact can be much more acceptable.
Nevertheless, the minimum tilt should be kept at some few degrees (2-3°) for module cleaning by the rain. Moreover, frameless laminates should be used in order to avoid dirt and mosses accumulations on the bottom side.