In the early days of photovoltaic power generation systems, open lead-acid batteries were generally used as energy storage devices. However, open lead-acid batteries need to be treated with acid and water, and the acid solution is liable to pollute the environment. This is not conducive to unattended use and environmental protection. In recent years, valve-regulated sealed lead-acid batteries (VRLA) have been widely used in China's telecommunications, electric power, railways, and other industries due to their seals that do not leak acid or corrode equipment to pollute the environment. Today's photovoltaic system batteries are almost all VRLA batteries. The main functions of VRLA batteries are twofold: (1) The battery can store excess energy or provide energy to the load at night or when it is cloudy and the power of the PV array is inconsistent with the load. (2) Operating characteristics of the solar cell module. Affected by solar irradiance, temperature, etc., the load often can not be near the ideal working point. The battery has a clamping effect on the working voltage of the solar cell, which can guarantee that the load is near the ideal working point.
The composition of photovoltaic power generation system
Photovoltaic power generation system is a power generation system that converts solar energy into electrical energy using the principle of photoelectric effect. It is usually composed of solar cell modules, controllers, battery packs, and DC/AC inverters.
The function of the solar cell module is to convert solar energy into electrical energy, supply the load to work or charge the battery pack; the role of the controller is to protect the battery pack from charge and discharge; the battery pack is used to store electrical energy; the role of the inverter is to direct current Convert to AC. In the night or on rainy days, when the solar cell module fails to 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.
VRLA battery characteristics for photovoltaic power generation system
1 Working methods of batteries for photovoltaic power generation systems
The working mode of the battery can be divided into two types: cyclic use and float charge. Frequently in the charge and discharge working state, that is, recycling; often in the charging state is used for floating charge, can make up for the battery capacity loss caused by self-discharge. VRLA batteries for photovoltaic power generation systems are used in a cyclic manner.
2 Battery charging and discharging characteristics of photovoltaic system
The charging and discharging characteristics of a photovoltaic system battery generally have four points: (1) charging during the day and discharge at night and overcast and rainy days; (2) low charging rate, the average charging current is generally 0.01 to 0.02C, and rarely reaches 0.1 to 0.2C. (3) discharge current is small, discharge rate is usually 0.004 ~ 0.05C; (4) a short charge time, even if only about 10h long. Photovoltaic systems rarely fully and quickly fully charge batteries, and batteries tend to be undercharged.
3 Performance Requirements for VRLA Battery for Photovoltaic Power Generation System
Photovoltaic power generation systems are mostly built in remote, remote mountainous areas, plateaus, and Gobi. The natural environment is very harsh and the temperature of the working environment varies greatly. Therefore, there are the following requirements for a battery in a photovoltaic power generation system: (1) It has a deep cycle discharge performance, and has a long charge and discharge cycle life; (2) High over-charge resistance; (3) Excellent capacity recovery capacity after overdischarge; (4) ) Good charge acceptance; (5) When the battery is used in a static environment, the electrolyte is not easy to layer; (6) Has maintenance-free or less maintenance performance; (7) should have good high and low temperature charge and discharge characteristics; (8) It can adapt to the use environment of high altitude areas; (9) The consistency of each battery in the battery pack is good.
Lead-acid battery capacity design method for photovoltaic power generation system
Determine the battery capacity, we must first determine the amount of power required to load the access system every day; Second, according to the weather conditions, the battery needs to store the number of days of power. When determining the battery capacity, it is not that the larger the capacity, the better. The oversized battery capacity also causes problems. This is because when the solar radiation is insufficient, the battery pack may remain partially charged. This undercharged state will result in increased sulfation of the battery, reduced capacity, and shortened lifespan. The general formula for the battery capacity is C=E·t/(D·η0·η1) (1) where C is the battery capacity; E is the daily average power consumption of the load; t is the maximum number of hours without sunshine D is the allowable depth of discharge for the VRLA battery; η0 is the charge and discharge efficiency of the VRLA battery; η1 is the inverter conversion efficiency.
Performance Improvement of VRLA Battery for Photovoltaic Power Generation System
The failure of the battery and the short life time are one of the reasons that hinder the promotion of the photovoltaic power generation system. After the VRLA battery is used in a photovoltaic system, the lifespan will be gradually shortened, and the factors that affect its life are mainly: the charging time is limited, the long-term undercharging, the low-current discharging, the overcharging, and the temperature. According to the special requirements of the photovoltaic system photovoltaic system on battery performance, combined with the above factors that affect the battery life, based on the original VRLA battery based on a series of performance improvements. The specific improvement measures include the following aspects:
(1) Improve cycle life. In order to prolong the cycle life of VRLA batteries, the conductivity of the corrosion layer formed at the interface between the grid and the active material of the grid electrode should be good, and the grid should have creep resistance. The battery design adopts tight assembly and the assembly pressure is appropriately increased.
(2) Improve battery charge acceptance. For VRLA batteries, undercharging is more harmful to batteries than overcharging, so it is especially important to increase the charge acceptance of VRLA batteries. In the negative lead paste formula by adding high stability expansion agent and conductive additives to improve the charge acceptance.
(3) Improve over-discharge performance. Reduce the proportion of sulfuric acid electrolyte, and add a special electro-hydraulic additive, can reduce the corrosion of the plate, reduce the generation of electro-hydraulic delamination, and improve the battery's charge acceptance and over-discharge performance.
(4) Use a special safety valve. For the plateau region, the pressure value of the relief valve is particularly adjusted due to the lower atmospheric pressure.