What exactly is shading in the context of solar power plant?
Solar PV systems comprise of a number of solar panels wired into arrays depending on the wattage requirement. Each of these panels, in turn, compose of several solar PV cells that are the basic units involved in capturing energy from the sun and converting them into electricity. Now, if a shadow is cast on even just one part of the solar panel in your array, the output from the entire system could be potentially compromised. This is called shading of PV panels.
What can cause shading?
Shading is usually caused due to clouds, environmental obstructions such are trees or nearby buildings, self-shading between panels in parallel rows, dirt, dust and other debris like bird droppings, etc.
These shading effects may be static due to the position of the obstruction or in some cases dynamic, for example, shadow cast by moving clouds.
How does it affect performance of the PV systems?
Solar arrays are usually divided into “strings” of solar panels .And one string can contain more than one solar panel. This is usually done to make the output compatible with the rating of the inverter.
If shade from a tree or chimney is cast on even one panel of the string, the output of the entire string can almost be zero for as long as the duration of the shade. This is because the panels are wired together in such a way that the output is reduced to level of current passing through the weakest panel. If there is a separate, unshaded string, it will continue to produce power as usual. The effect of shade on the entire system depends on how the panels are wired together.
How is this problem combated?
A few measures can be taken to minimise the effect of shading of PV systems. They are listed below:
- Careful positioning of PV systems
This is the most obvious solution to handle the problem of shading. It is extremely important to consider all times of the day for all seasons of the year when working out whether a nearby object can cast a shadow on your system. Probability of nearby trees which may grow tall enough or buildings that may come up in future also needs to be considered before finalising the location of PV systems.
- Using bypass diodes for each cell of the solar panel
With this technology, the power output from the solar panels isn’t brought to zero just because one single cell is shaded. Here the shaded cells are simply bypasses and not allowed to effect the output of the entire panel. The power output of the panel might reduce, but will not be directly based on the power output of the lowest performing cell.
- Using novel technologies like micro inverters
Micro inverters are attached to each and every solar panel and they convert DC electricity to AC immediately, avoiding the effect of power losses from shaded panels. But micro inverters currently available in the market in most cases are not economically viable.