Effect of shading on solar PV system

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The Effect of shading always shows impact on generation of the solar PV system. In this regard, I am giving you brief about shading and its effects on generation of the solar system.

Soft shading can be described as simply lowering the intensity of the irradiance levels, without causing any form of visible separation of shaded and unshaded regions. A great example of soft shading would be due to cirrus or stratus clouds evenly blocking out some, but not all of the sunlight. Soft shading cast on a PV cell will cause the cell’s current output to proportionally drop. As long as there is enough light (~50W/m2), the voltage output of the cell will remain unchanged and only the current output will diminish. The voltage of the PV cell depends more on temperature and the electron band-gap in the materials than on the light itself.

Hard shading is created when a physical object, such as a telephone pole, or tree is physically obstructing the sunlight, creating obvious visible regions of lit and unlit cells on the array. It’s a lot more difficult to describe how hard shade affects a PV cell, as the physical geometry of the shade comes into play. As long as there is a solid strip or channel of illuminated material between two electrodes on a PV cell, there will be some electric current. The current is proportional to the surface area of the cell that was illuminated and the shape of the shadow does not appear to matter as much as the area of the shadow. However, certain shapes of shading would create narrow areas of illumination in which the current would squeeze and be focused in the narrow illuminated portions of the cell, creating areas of extremely high temperature, known in the industry as “hot spots.” These hot spots in very rare cases have been known to cause burn-outs and small fires within modules, as they may have the current from an entire module being pinched into a very tiny area of solar cell.qwe

At the module level, several solar cells are connected in series, in order to increase their output voltage. Soft shade, cast on a module will still allow voltage to be generated, but less current will flow from the module. Hard shade cast on a portion of the area on a solar module will open the circuit, causing the voltage generated by the module to drop.

Strings of modules are again connected in series, where the current must be the same throughout all components. That means that without the bypass diodes, any shade on any cell in the string would cause the entire string to stop producing power entirely. Such a devastating loss of power had to be avoided, so typically three diodes are placed in along the solar cells. The diodes are placed in such a way they will allow current to flow through them only if the solar cells they bypass are shaded and opened. Since the diodes have a negligible voltage drop, there are very little losses induced by the diodes, so the only real loss from a shaded group of cells is whatever voltage they were providing.

It is important to note that voltage mismatch cannot occur on a single-string solar array, as there are no parallel string connections with which to create an “imbalance.” A solar array consisting of only one string of modules can only have current-mismatch effects applied to it. Casting hard shade on a single string will drop the voltage of the string, but the inverter will detect this drop and quickly adjust, seeking the new maximum power point.

As for the effects of these two different mismatches on the inverter, several outcomes are possible, depending on the software behavior of the inverter. The PV curve of the entire array exists as the series sum of the modules and the parallel sum of the strings. A shadow moving over the surface of several modules over time has the effect of constantly changing the PV curve from one smooth peak to more of a mountain range.As the peaks of the PV curve in the inverter change from the shade, the electronics that track the maximum power point can become confused or lost, causing the inverter to choose to operate for long periods of time well outside the optimal output range. This can cause significant loss of power output and eventually annual energy yield. There have been notable advancements in the methods and electronics of the inverters, as well as the birth of new technologies in order to deal with the complications brought on by shade. 33

As the peaks of the PV curve in the inverter change from the shade, the electronics that track the maximum power point can become confused or lost, causing the inverter to choose to operate for long periods of time well outside the optimal output range. This can cause significant loss of power output and eventually annual energy yield. There have been notable advancements in the methods and electronics of the inverters, as well as the birth of new technologies in order to deal with the complications brought on by shade.

 

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