Photovoltaic off-grid power generation system is a power generation system that uses the principle of photoelectric effect to convert solar energy into electrical energy. It usually consists of solar cell components, controllers, battery packs, DC/AC inverters, etc., as shown in Figure 1.
Figure 1 Principle diagram of photovoltaic off-grid power generation system
The function of the solar cell module is to convert solar energy into electrical energy, supply the load or charge the battery bank; the function of the controller is to protect the charging and discharging of the battery bank; the battery bank is used to store electrical energy; the function of the inverter is to convert DC power Convert to alternating current. At night or on rainy days, when the solar cell module cannot work, the battery pack supplies power to the load. It can be seen that the battery pack plays a very important role in the photovoltaic power generation system.
Characteristics of VRLA batteries for photovoltaic power generation systems
1 How batteries used in photovoltaic power generation systems work
The working mode of the battery can be divided into two types: recycling use and float charging use. If the battery is often in a state of frequent charging and discharging, that is, it is used cyclically; if it is often in a charging state, it is used in float charge, which can make up for the capacity loss caused by the battery’s self-discharge. VRLA batteries used in photovoltaic power generation systems are recycled.
2 Charging and discharging characteristics of batteries used in photovoltaic systems
The charging and discharging characteristics of batteries used in photovoltaic power generation systems generally have four points: (1) charging during the day and discharging at night and on cloudy and rainy days; (2) low charging rate, the average charging current is generally 0.01~0.02C, rarely reaching 0.1~0.2C ; (3) The discharge current is small, and the discharge rate is usually 0.004~0.05C; (4) One charging time is short, even if it is long, it is only about 10 hours. Photovoltaic systems rarely charge batteries completely and quickly, and batteries often remain undercharged.
3 Performance requirements for VRLA batteries used in photovoltaic power generation systems
Photovoltaic power generation systems are mostly built in remote mountainous areas, plateaus, and Gobi. The natural environment is very harsh, and the working environment temperature varies widely. Therefore, the following requirements are required for batteries in photovoltaic power generation systems: (1) Deep cycle discharge performance and long charge and discharge cycle life; (2) Strong overcharge resistance; (3) Strong capacity recovery ability after overdischarge; (4) ) Good charging acceptance ability; (5) When the battery is used in a static environment, the electrolyte is not easy to stratify; (6) It has maintenance-free or low-maintenance performance; (7) It should have good high and low temperature charge and discharge characteristics; (8) It can adapt to the use environment in high-altitude areas; (9) Each battery in the battery pack has good consistency.
Design method of lead-acid battery capacity for photovoltaic power generation system
To determine the battery capacity, you must first determine how much power the load connected to the system requires every day; secondly, how many days of power the battery needs to store based on climate conditions. When determining the battery capacity, the larger the capacity, the better. Excessive battery capacity will also cause problems. This is because when there is insufficient sunlight, the battery pack may remain in a partially charged state. This undercharged state will lead to increased sulfation of the battery, reduced capacity, and shortened life. The general calculation formula for battery capacity is
C=E·t/(D·η0·η1) (1)
In the formula, C is the capacity of the battery; E is the average daily power consumption of the load; t is the longest number of hours without sunshine; D is the allowable discharge depth of the VRLA battery; eta0 is the charging and discharging efficiency of the VRLA battery; eta1 is the inverter conversion efficiency.
Improvement of VRLA battery performance for photovoltaic power generation systems
Battery failure and short life are one of the reasons that hinder the promotion of photovoltaic power generation systems. The life of VRLA batteries will gradually shorten after being used in photovoltaic systems. The main factors that affect its life include: limited charging time, long-term undercharging; low current discharge; overcharging; temperature, etc. According to the special requirements of the photovoltaic power generation system for battery performance, combined with the above-mentioned factors affecting battery life, a series of performance improvements were made on the basis of the original VRLA battery. Specific improvement measures include the following aspects:
(1) Improve cycle life. In order to extend the cycle life of VRLA batteries, the corrosion layer formed by the grid alloy at the interface between the grid and the active material should have good conductivity, and the grid should have creep resistance. The battery design adopts tight assembly and appropriately increases the assembly pressure.
(2) Improve battery charging acceptance capability. For VRLA batteries, undercharging is more harmful to the battery than overcharging, so it is particularly important to improve the charging acceptance of VRLA batteries. High-stability expansion agents and conductive additives are added to the negative electrode paste formula to improve charge acceptance.
(3) Improve over-discharge performance. Reducing the specific gravity of the sulfuric acid electrolyte and adding special electrolyte additives can reduce corrosion of the plates, reduce the occurrence of electrolyte stratification, and improve the battery’s charge acceptance and over-discharge performance.
(4) Use a special safety valve. For plateau areas, due to the low atmospheric pressure, the safety valve pressure value is specially adjusted.
…Selection essentials and instructions for photovoltaic junction boxes
The main information to look at when choosing a photovoltaic junction box should be the current size of the module. One is the maximum working current, and the other is the short-circuit current. Of course, the maximum current that the module can output during the short-circuit current. The rated current of the junction box should be calculated according to the short-circuit current. If the safety factor is relatively large, if the junction box is calculated based on the maximum operating current, the safety factor will be smaller.
First of all, we must establish a condition, whether it is short-circuit current or maximum operating current, the nominal data of the component are values under standard test conditions, that is, the ambient temperature is 25°C, and the standard lighting Test data under the condition of intensity 1000W/m2 and AM1.5.
But we must be clear that when modules are shipped to different regions, the light intensity in different regions will change. In some areas such as Tibet, Ningxia, and Xinjiang, the light intensity may reach about 1700W/m2 during certain periods of time. At this time, the battery cells The current and voltage will change with the change of light intensity. I have done tests on this aspect. When the light intensity is from 1000-1300W/m2, the current has a linear upward trend.
Therefore, there is no clear selection standard for most component manufacturers. They may be empirical values. Some selection standards are 1.25 times the maximum operating current, and some selection coefficients are 1.3 or 1.4. But I think they are all There is no scientific basis.
The author believes that the most scientific basis for selection should be based on how the current and voltage of the battery cells change with the light intensity. You must understand the area where the components you are producing are used, and how much light is in this area when the light is the strongest, and then It should be more scientific to compare the curve of the current of the cell with the change of light intensity, check the possible maximum current, and then select the rated current of the junction box.
As for the wattage of the module, it is not accurate to equip me with a junction box with so many watts. If you choose a 156*156 chip as a module, no matter how many pieces your module consists of, all current module cells The connection methods are all connected in series, so the performance of the battery cells has determined the current of the module, and the voltage depends on the number of cells in the module in series. Therefore, the current of the module is an important parameter for selecting the junction box, and the open circuit voltage It’s not useless, it’s just a reference, because the open circuit voltage is only related to the reverse withstand voltage of the diode, which can be satisfied under normal circumstances. Because the power generation principle of crystalline silicon batteries is low voltage and high current, so be sure to pay attention to the wiring. The current of the box is a very, very important parameter! The most important thing about a junction box is its stability and reliability, so that it can be used safely for a long time. Therefore, the heat dissipation of the wiring and the junction temperature of the diode are very important. Only when the junction temperature is low and the structure of the junction box has good heat dissipation can the junction box work for a long time. Reliable security.
Low diode junction temperature has many benefits. On the one hand, it ensures the safety of the junction box itself, and on the other hand, it is related to the safety of components. Junction boxes are all certified. In terms of themselves, the materials used by everyone are similar, and the protection levels have been tested. As long as the supplier does not use materials and materials, there is no need to do it yourself when choosing a junction box. Environmental testing.
Therefore, it is enough for component manufacturers to pay attention to two aspects of indicators when selecting junction boxes:
The first aspect is the rated current temperature test results of the diode.
Because the junction temperature of the diode is too high, it will cause damage to the diode itself and reduce its service life. Everyone should know something about the life of the semiconductor. At the same time, it is also a great threat to the safety of the junction box. In addition, if the temperature is too high, it will last for a long time. Time may endanger the safety of components. For example, excessive temperature may cause the backplane to loosen, etc. very dangerous!
The junction box currently circulating in the market has a constant ambient temperature of 75°C and the nominal rated current passes for 1 hour, and the diode junction temperature is above 179°C. This is a very scary data. Friends who make components know that the temperature of component lamination should be around 150°C. Imagine how dangerous this temperature is at 179°C, because what we measure is the surface temperature of the diode box, and the temperature of the diode chip will be higher. The pins are all copper. The thermal conductivity of copper is much higher than the thermal conductivity of plastic packaging materials, so the temperature of the terminals will be very high. Currently, many junction box manufacturers reduce the junction temperature of the diodes inside the box. The design is very large, in order to conduct the heat of the diode operation to the terminals, which can meet the temperature requirements of the diode during TUV testing. However, because the heat is made of plastic housing, the heat is dissipated very slowly. In this case, it meets the TUV There is no problem during the test, because the test conditions are to pass the rated current for 1 hour, and the other is to pass 1.25 times the rated current for 1 hour. At this time, the junction temperature of the diode is not calculated. Many junction boxes currently on the market have already reached the junction temperature at this time. Above 200°C, the junction box itself may be deformed by heat, but at this time, the criterion for judging eligibility according to the standard is only the functionality of the diode and nothing else. However, during the actual application process of the component, the time when the shielding effect occurs is uncertain. Therefore, if the component is working for a long time, the heat cannot be dissipated and accumulates for a long time, which poses a huge potential threat to the safety of the junction box itself and causes the backplane to stick. There is also the possibility of loose joints.
The second aspect is to care about the overall resistance of the junction box, because this involves power loss. The test can be like this, connect the positive and negative poles of the connectors of the junction box together, and then use a microohmmeter to measure the overall resistance of the terminal heads on both sides. Resistance, this resistance value is the overall resistance of the junction box including the wires during long-term operation. The greater the resistance, the greater the power loss of the box, which is very unfavorable! At present, most of the 156 polycrystalline components of 240W and above use double-row diode designs. Since the junction temperature of the diode must meet the test when considering certification, the terminal block is designed to be very large, so the overall resistance of the junction box is relatively large. Normally, The resistance of the products currently on the market is greater than 13 milliohms. Such a large resistor power loss is also a very scary figure for components.
How to choose solar junction box diodes
- According to the power of photovoltaic modules, 150w, 180w, 230w, 310w?
- Other specifications of components
- What are the parameters of the diode, 10amp, 12amp, 15amp or 25amp?
- The most important point is, how much is the short-circuit current?
For this test, the diode selection depends on the following quantities:
Current (bigger is better)
Maximum junction temperature (larger is better)
Thermal resistance (smaller is better)
Pressure drop (smaller is better)
Reverse breakdown voltage (generally 40V is enough)
Job Wang（Mr）
Shenzhen Lolsolar New Energy Technology Co.,Ltd
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