Shading analysis

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The shading analysis is part of the planning phase of a photovoltaic system .

In a shading analysis, all shadows that can fall on the photovoltaic modules of a system at different times of the day or year are calculated with the help of a sun path indicator or a technical device developed for this purpose. A shading analysis is used to predict the performance of a (planned) photovoltaic system. A shaded module has a negative impact on the overall performance of a string . It can therefore be better for performance not to equip shaded areas with photovoltaic modules or to combine all shaded modules in a string.

background

Since the voltage of solar cells is very low (approx. 0.5 V), up to 72 units are connected in series in a photovoltaic module. In addition, in a photovoltaic system, several modules are usually connected in series in order to increase the voltage further, so that the grid-connected system can be more easily adapted to the mains voltage of 230 V.

If a solar cell or part of a module is shaded, this also reduces the performance of all other modules in this string . In this case, however, less of the voltage drop of the shaded module is decisive, but that the module passes or hardly no more current and as a switched reverse bias diode behaves. As the current in the shaded module increases, so does the current in the entire string. This is also referred to as the “garden hose effect”: if you press the hose shut in one place, less water comes out of the back.

Types of shade and countermeasures

Temporary shading
Temporary shading can result from dirt (leaves, bird droppings) or environmental influences (snow). These shadows are not to be neglected, especially if you live near an industrial area or a forest. Dirt can cause performance losses of up to 10%. In order to keep pollution as low as possible, the solar modules must have the highest possible self-cleaning capacity. Washing off dirt and grime when it rains is called self-cleaning. Sufficient self-cleaning takes place from an inclination angle of the modules of 20 °, but this also depends on the surface of the modules. You can also remove dirt from the system by hosing it down with a hose or wiping it off with a damp cloth, but you should not rub the PV system dry to avoid scratches.
Location-related shadows
Depending on the location, shadows can be caused by buildings or trees. Power lines can also cast a shadow across the roof. Chimneys, sloping ceilings, antennas or lightning rods can also cause shading. These are particularly critical because they are very close to the photovoltaic system. It is therefore often economically viable to move the antenna to the other side of the roof. You should always try to prevent all shadows as far as economically feasible and technically possible. If trees have to give way, the city's approval is often required from a certain size.
Self-shading
The modules themselves create their own shadow. This happens especially with elevated systems. Elevated systems are mostly used for flat roofs and are necessary to get an optimal slope. The shading can be reduced here by sufficient distance between the module rows and the inclination angle of the modules.
Countermeasures to reduce the effect of shading
All newer solar modules available on the market have at least one, usually three bypass diodes . On the one hand, these protect shaded solar cells from overheating and, on the other hand, enable them to be bridged. Modern inverters with a so-called global peak MPP control are able to regulate the voltage of a solar module string (the series connection of several solar modules) down to such an extent that the partially shaded cells are completely bridged and the garden hose effect described above no longer occurs. Another way to reduce the loss of performance due to partial shading is to connect the individual solar modules in parallel. However, this creates very large currents, so that this variant is generally only used in very small photovoltaic systems.

Tools

A horizonoscope is a quick and easy tool to determine on-site at which times of day and seasons obstacles cast a shadow on a point.

literature

  • "German Society for Solar Energy. Guide to Photovoltaic Systems." , VWEW-Verlag Frankfurt 2005 (3rd edition)
  • Jans-Joachim Geist: "Photovoltaic systems - plan, assemble, test, maintain." , Elektor Verlag Aachen 2007 (3rd edition), ISBN 3-89576-191-5