The bypass diode is a standard accessory in photovoltaic modules, which is used to eliminate the hot spot effect caused by reverse bias due to mismatch, which can lead to power reduction and damage to photovoltaic modules. Therefore, the design and corresponding detection of a reliable bypass diode device are crucial.
Mismatch effect in photovoltaic modules
Mismatch effect refers to the phenomenon that occurs when interconnected solar cells or photovoltaic modules do not have the same performance or operate under different conditions.
When the parameters of one photovoltaic cell in a photovoltaic module are significantly different from those of others, a mismatch phenomenon will occur.
The impact and power loss caused by mismatch effects depend on the operating point, circuit structure layout, and parameters of the affected solar cells in the photovoltaic module.
Because most photovoltaic modules are in series, series mismatch is the most common type of mismatch encountered. Series mismatch can be divided into short-circuit current mismatch and open circuit voltage mismatch, among which short-circuit current mismatch is the most common and serious. It is usually caused by shadows or faults in the photovoltaic cells themselves.
Hot spot effect
1. When the local photovoltaic cells of the photovoltaic module are obstructed by objects such as leaves, dust, bird droppings, etc;
2. There is a defect in one of the photovoltaic cells in the photovoltaic module, and the power generation efficiency is lower than other photovoltaic cells;
3. Poor welding during the manufacturing process of photovoltaic modules and the PID potential induced attenuation effect in the later stage;
Once the total operating current of the photovoltaic module exceeds the short-circuit current of these “problematic cells”, the voltage of these photovoltaic cells will be biased into the load, thereby consuming the electrical energy of other “normal photovoltaic cells” inside the photovoltaic module.
Therefore, the ‘problematic photovoltaic cell’ will be transformed into an energy consuming component and heated by the energy generated by the same string of photovoltaic cells, resulting in local overheating and the generation of hot spot effect.
If the heat generated by the hot spot effect is too large, it may cause local yellowing, burning, bulging and delamination, and even glass breakage on the back panel of the photovoltaic module. In severe cases, it may also cause a fire.
Effective methods to reduce the hot spot effect
By using bypass diodes at both ends of each string of photovoltaic cells, the damage caused by thermal spot effect to photovoltaic modules can be effectively avoided.
Working principle of bypass diode:
The diode is connected in parallel with the solar cell and in opposite directions. In normal operation, the voltage of each photovoltaic cell is forward biased, so the voltage of the bypass diode is reverse biased, which is equivalent to an open circuit and has no effect.
When one photovoltaic cell in the series is obstructed or experiences a mismatch effect, the voltage is reverse biased. At this time, the bypass diode acts as a wire, playing a shunt role.
The photovoltaic voltage generated by other normal series photovoltaic cells can still be utilized by external loads, thereby reducing the risk of thermal spot effect on the entire photovoltaic module.
Therefore, the design and corresponding detection of a reliable bypass diode device are crucial for photovoltaic modules.
