Effect of shading in PV systems
Solar PV plants are composed of PV modules connected in various series and parallel combinations to provide optimum output .It is important to note that a single module expose to sun will behave different vis -a-vis a module which is part of a string connected through other component like Inverter ,Combiner box etc .. as part of solar PV systems. Hence it is important to take care various issues which effect generation at string level in the solar plant. shading is one of the major aspect while plant designing .The loses due to shading is as high as 3 to 5 % ,generally shading is seen in case the land is not properly levelized or the panels are not installed at a same level.Many times it is important to provide distance of 2-3 ft ,in order to avoid the linear shading as shown in the figure during morning and evening hours.
A typical PV module operated at around 35V and a number of modules (above 20) are connected in series to form a string. In case any of the module is subjected to shading, the exposed area of the solar cell due to shading will no longer generate power and the module become as a load(power consumer). The shaded module current will dictate the output current of the whole string. In extreme cases the shaded modules will generate excess heat due to reverse current flow. The excessive heat generated in certain area of PV modules leads to creation of the hot-spots. which is typically melting of a hole in cell material. In order to overcome this modules are equipped with 2 or 3 bypass diode’s which are located in the junction boxes. The bypass diode will bypass the cell current subjected to shading and protect the module failure due to hot spots. The problem of hot spot formation is significant in case of crystalline modules, while thin film modules as the softer reverse I-V characteristic, which do not allow hot spots to build up if an individual cell is shaded. Another advantage is that for glass-based modules, the films are deposited on the rear side of the front glass, so that the encapsulating polymer (e.g. EVA) lies behind the solar cells and cannot reduce the transmission of light to the cells. Thus the loss in output observed in some wafer-based modules simply cannot occur. We have been analyzing various plant performances ,it has been observed that even a small shading on a PV module string reduces the power output of the string from (2-4)%
For example in case of 20 module string of 280 W(35V ,8 A) module each, the total power output is 5.6Kwp,the voltage and current of this string are of the order of 720V and 8 Amps.When we checked a string with a single module partially shaded the current and voltages of the string were observed as 713V and 7.5 Amp and power output reduced to 5175 W. It shows that even a small shading on a 20 module string reduces overall power output by 4 %,so utmost care has to be taken at the designing stage of the plant to avoid the losses due to shading effect.