From the basic circuit of the solar cell array, it can be known that the solar cell array is composed of a string of battery modules, a backflow prevention element, a bypass element and a junction box.
(1) Solar cell module string
Solar cell module string is a circuit composed of solar cell modules connected in series in order to meet the needs of output voltage. When the performance of each battery module is consistent, the series connection of multiple battery modules can increase the output voltage of the square array proportionally without changing the output current.
In the circuit, the solar cell module strings are connected in parallel with each other by means of backflow preventing elements. The parallel connection of components is to increase the current required to obtain the gain.
If the performance parameters of all the battery components forming the square array are quite different, the operating current of each battery component in series will be limited by the component with the smallest current, and the output voltage of each parallel battery component will also be limited by the battery component with the lowest voltage. So the total efficiency of the square matrix will be lower than the sum of the efficiencies of all the individual components.
(2) Hot spot effect and bypass diode
①Hot spot effect
When sundries such as leaves and bird droppings fall on the solar cell, the solar cell that is shaded alone will not only be unable to generate electricity, but also the resistance will increase significantly, and all the voltage of the series-connected circuit will be added to this battery unit. The current flowing through the high-resistance unit will make it heat up, especially under strong sunlight, the temperature will rise sharply, so the unit cell or even the entire solar cell module is damaged (burned out dark spots), which is The so-called hot spot effect (also known as the heat island effect). In addition, the resin filled at and around the shaded solar cells will change color while the hot spot effect is generated, and the protective film will also expand.
In order to prevent the heat island effect, the solar cell module is generally placed obliquely so that leaves and the like cannot be attached, and a bird-proof needle is installed on the solar cell module. In order to prevent the solar cell from being damaged due to the hot spot effect, a bypass diode with unidirectional conductivity is generally connected in parallel between the positive and negative electrodes of the solar cell module to prevent the generation of the heat island effect.
It should be noted that a bypass is to open up another path in addition to the original official path. Resistor bypass is to open up another path through the electrical appliance. If the positive and negative poles of the solar cell power supply are directly connected with a wire, it is undoubtedly a short circuit, and the solar cell may be damaged. The so-called bypass in the solar cell array is to connect a diode in parallel next to each component. When the component connected in parallel with it produces a hot spot effect, a forward bias will be formed at both ends of the bypass diode of the component to make the diode conduct, and a large current will occur. It will leak through the bypass diode, thus preventing the solar cell module from being burned out, and it will not affect the power generation of other normal modules.
Bypass diodes are generally installed directly in the module junction box. According to the power of the module and the number of battery cells, 1-3 diodes are generally installed. When the components are used alone or in parallel, there is no need to connect diodes. If the number of components in series is not large and the working environment is good, bypass diodes can also be considered.
Commonly used diodes are basically silicon rectifier diodes. In order to prevent the bypass diode from being damaged by breakdown, generally the reverse peak breakdown voltage and maximum operating current of the bypass diode should be more than twice the maximum operating voltage and operating current.
③Anti-reverse current diode (anti-reverse charging diode)
When the solar cell module or square array is not generating electricity, if the current generated by the battery flows in turn to the solar cell module (so it means reverse charging), it will not only consume energy, but also its current will be damaged. For solar cell modules, diodes are used to prevent this reverse current effect. This diode is called an anti-reverse current diode and an anti-reverse current element, also known as an anti-reverse charging diode.
The installation of anti-reverse flow elements on each string of solar cell modules is also to prevent the effect of reverse current. In the solar cell array, if a string of modules is blocked by leaves or birds, the string of solar cell modules cannot generate electricity, which will definitely cause the voltage imbalance between the string of modules in the solar cell array. When this imbalance reaches a certain value, it will receive the current of other battery strings, that is, the current of the high-voltage branch will flow to the low-voltage branch, forming a reverse current, and even causing a square array The overall output voltage decreases.
In addition, if a battery is installed on the DC output circuit of the solar cell array, the solar cell will become the load of the battery at night, and the battery will discharge to the solar module at night until the battery is fully discharged, and the solar cell array will be charged to the battery. The electricity is wasted in vain. Therefore, anti-backflow diodes should also be installed in such circuits.
The anti-backflow diode has a forward voltage drop, and it will consume a certain amount of power when connected in series in the circuit. Generally, the voltage drop of the silicon rectifier diode used is about 0.7V, and the high-power tube can reach 1~2V.
In the independent photovoltaic power generation system, some photovoltaic controller circuits have been connected to anti-reverse current diodes, that is, when the controller has anti-reverse charging function, the module output does not need to be connected to diodes.