If for some reason the current of one or more photovoltaic cells cannot flow, consider how the output of the array will change. As shown in Figure 1.
Since the components are connected in series by batteries, damage to one battery (or part of the battery is blocked) will reduce the current of the entire component. Similarly, if this component is part of an array, the current in the array will also be reduced.
If a battery is damaged, the rest of the array will force current through it, causing a significant temperature rise inside the battery and even further damage. This phenomenon is called the “hot spot” effect. In the extreme case of an open battery, the array output will be 0.
Diodes are semiconductors that allow current to flow in one direction, and the effects of the above conditions can be reduced by paralleling diodes.
1. Bypass diode
If the components are connected in series with batteries, the output power of the components will be reduced under the following conditions:
(1) The battery is defective.
(2) One or more batteries are blocked.
Even if the remaining battery runs under ideal working conditions and full sunlight, the output power will be reduced.
When a defective or occluded battery is working in an operating component or array, it will generate a reverse voltage across its two ends, as shown in Figure 2. When a reverse voltage appears, the diode can be used to provide an alternative path for current. This type of diode is called a bypass (shunt) diode.
The polarity of the battery/component in Figure 2 is the state during normal operation. If the battery/component is damaged or blocked, its polarity will be reversed so that the diode can be turned on and the string current will flow through the diode.
For most commercial crystalline silicon components, bypass diodes are not installed on every cell, although this should be an ideal situation. Many manufacturers configure one bypass diode for a string of 18 batteries, and two bypass diodes for 36 batteries. If the manufacturer does not provide bypass diodes, the recommended approach is to configure at least one bypass diode for each module in a photovoltaic array composed of modules in series. Figure 3 shows the actual junction box of a component, and you can see the diodes connected to the terminals.
Note: Many thin film photovoltaic modules have integrated battery bypass diodes.
Figure 4 shows the role of bypass diodes. In Figure 4(a), there is no bypass diode and defective battery, and the output voltage is xV. In Figure 4(b), when a component is open, the output voltage of the array without the bypass diode is 0. Two bypass diodes are installed in Figure 4(c), and the output voltage of the array is 0.5xV. In Figure 4(d), 4 bypass diodes are installed, and the output voltage of the array is 0.75xV. In general, if a component in the array is blocked (or defective), the more bypass diodes installed, the higher the output voltage of the component.
For Figure 4, if each component is operating at a voltage of 18V and used to charge a 48V battery pack, then the battery cannot be charged in Figure 4(b) (the output voltage is 0), in Figure 4(c) In ), the battery cannot be charged (output voltage is 36V), but in 4(d), the battery can be charged (output voltage is 54V). Therefore, if the component is shaded, the greater the number of bypass diodes, the greater the probability that the output voltage can charge the battery.
Defective batteries will accumulate heat in the case of reverse voltage. The main purpose of the bypass diode is to protect the battery pack from local overheating (hot spot effect) when a single battery has problems. Excessive heat will cause packaging or welding. The permanent destruction of the material will eventually lead to the replacement of the component.
2. blocking diode
Blocking diodes (also called series diodes or isolation diodes) conduct current during normal system operation and are used in series with components or strings. Its main function is to prevent the current from flowing back into the component at night, and to prevent the current from flowing into a defective parallel string. Figure 5 shows the placement of blocking diodes.
Whether it is necessary to install a blocking diode mainly depends on the photovoltaic technology and the electrical characteristics of the night. It is more common in the independent power supply system in the past, and is not often used in the current system.
3. How to choose a diode
In the selection of diodes, the following parameters are more important:
(1) The maximum current allowed by the diode in the forward direction (the maximum forward continuous current IF).
(2) The maximum critical voltage that the diode can tolerate in the reverse direction (reverse critical voltage UR).
Note: 6A (9A), 600V diodes are usually used as bypass diodes and blocking diodes.
What needs to be further considered is the forward voltage drop of the diode. For example, the voltage drop of a silicon rectifier diode at rated current is 0.6~0.7V, that is, the above diode will consume 3.6W (6A×0.6V) at a current of 6A. The voltage drop of the Schottky diode is only 0.2~0.4V, so if the voltage drop (power loss) is critical in the system design, a Schottky diode should be selected.