Diodes are two terminal electronic component which allow current to flow in one direction. This diode can be used to block the flow of electric current from other parts of an electrical circuit. These types of silicon Diodes are basically known as Blocking Diodes during the usage with a solar panel. Bypass Diodes are connected in parallel with either a single or a number of solar cells to prevent the solar cells overheating by sunlight. By providing a current path around the bad cell, these types of diodes also protect the partially shaded solar cells from burning out. Blocking diodes are used differently than bypass diodes.
Bypass diodes in solar panels are connected in “parallel” with a solar cell or panel to shunt the current around it, whereas blocking diodes are connected in “series” with the PV panels to prevent current flowing back into them. Therefore, blocking diodes are different than bypass diodes. These types of diodes are physically the same in most cases. In order to serve various purposes, they are used in different way.
Usage of Bypass Diodes in Solar Photovoltaic Arrays
Here, diodes with green color are “bypass diodes”, one in parallel with each solar panel to provide a low resistance path. Bypass diodes in solar panels and arrays safely carry this short circuit current. On the other hand, diodes with red colors are known as the “blocking diodes”, one in series with each series branch. These blocking diodes are also known as a series diode or isolation diode, ensure that the electrical current only flows in one direction “OUT” of the series array to the external load, controller or batteries.
The reason for this is to prevent the current generated by the other parallel connected solar panels in the same array flowing back through a shaded solar cell and also to prevent the fully charged batteries from discharging or draining back through the array at night. Therefore, when several solar panels are connected in parallel, blocking diodes are used in each parallel connected branch.
Blocking diodes are basically used in solar photovoltaic arrays when there are two or more parallel branches or there is a possibility that some of the array will become partially shaded during the day as the sun moves across the sky. The size and type of blocking diode used depends upon the type of solar photovoltaic array.
Two types of diodes are available as bypass diodes in solar panels and arrays. One is the PN-junction silicon diode and another is the Schottky barrier diode. Both diodes are available with a wide range of current ratings. Forward voltage drop of the Schottky barrier diode is about 0.4 volts/ On the other hand, the PN junction silicon diodes have the voltage drop of 0.7 volt for a silicon device.
This lower voltage drop helps to save one full solar cell in each series branch of the solar photovoltaic array. Because of dissipating less power in the blocking diode, the array becomes more efficient. During manufacturing solar panel, most manufacturers add both bypass and blocking diodes in their solar panels.
