Energy storage batteries and energy storage devices are indispensable components in the solar photovoltaic power generation system to store energy. Its important function is to store the energy of the photovoltaic power generation system and supply power to the load in the event of insufficient sunshine, night, and emergency. Commonly used energy storage batteries include lead-acid batteries, alkaline batteries, lithium-ion batteries, and supercapacitors, which are used on different occasions or products. At present, lead-acid batteries are widely used and have a history of more than 160 years since their development in the 1950s. At present, a variety of batteries have been derived, such as flooded lead-acid batteries, valve-regulated sealed lead-acid batteries, gel batteries, and lead-carbon batteries.
1. Working principle and basic structure
Lead-acid batteries are chemical energy storage devices that use lead and lead dioxide as positive and negative active materials and dilute sulfuric acid as the electrolyte. It has the characteristics of high energy conversion efficiency, long cycle life, high terminal voltage, strong safety, high-cost performance, and simple installation and maintenance. At present, it is the chemical power source of choice among various energy storage, emergency power sources, and starting devices. Important components of lead-acid batteries include:
1. Polar plates: Both the positive and negative plates are made of special alloy grids coated with active materials. The plates store and release energy during charging and discharging, so as to ensure the reliable capacity and performance of the battery.
2. Separator: It is an isolation medium placed between the positive and negative electrodes of the battery to prevent direct contact between the positive and negative electrodes of the battery and cause a short circuit. Different types of lead-acid batteries have different diaphragm materials. Valve-regulated lead-acid batteries are mainly AGM, PE, and PVC.
3. Electrolyte: The electrolyte of lead-acid batteries is dilute sulfuric acid prepared with distilled water, which can transfer ions between the positive and negative electrodes during the charge and discharge process, so the electrolyte must be free of impurities.
4. Container (battery cover): The container covered by the battery in which the electrolyte and the electrode plate are placed, which plays an important supporting role, and prevents internal materials from overflowing and external materials from entering the internal structure to pollute the battery.
2. Types and advantages
The working principle of lead-acid batteries is that electrical and chemical energy are converted into each other through electrochemical reactions. The electrode is mainly made of lead and its oxide, and the electrolyte is a storage battery with a sulfuric acid solution. Chinese: Lead-acid battery.
In the discharge state, the important component of the positive electrode is lead dioxide, and the important component of the negative electrode is lead.
In the charged state, the important components of the positive and negative electrodes are lead sulfate.
There are many types of lead-acid batteries used in photovoltaic energy storage systems. There are three types of lead-acid batteries, such as flooded lead-acid batteries, valve-regulated sealed lead-acid batteries, and lead-carbon batteries.
2.1 Flooded lead-acid battery
The sulfuric acid in the lead-acid battery electrolyte directly participates in the battery's charge and discharge reaction process. In traditional lead-acid batteries, the remaining space in the battery tank except for the plates, separators and other solid assembly parts is completely filled with sulfuric acid electrolyte, and the electrolyte is in a state of excess, so it is called a flooded battery. Sulfuric acid electrolyte. There is a cover on the top of the flooded battery, which can ventilate to prevent liquid from spilling. Since water will evaporate and decompose during use, it is necessary to regularly open the lid to replenish distilled water and adjust the electrolyte density, so it is customary to call it an open-lid battery. The rich liquid lead-acid battery is characterized by low price and long service life, but the disadvantage is that it needs frequent maintenance.
2.2 Valve-regulated sealed lead-acid battery
Valve-regulated sealed lead-acid battery, also known as the maintenance-free battery, is divided into AGM sealed battery and GEL gel sealed battery.
AGM battery uses pure sulfuric acid aqueous solution as the electrolyte, most of which exists in the glass fiber membrane, and part of the electrolyte is absorbed by the inside of the electrode plate. AGM sealed batteries have less electrolyte, thick electrode plates, and lower active material utilization than open batteries. Therefore, the discharge capacity of the battery is about 10% lower than that of open batteries.
Compared with current gel-sealed batteries, the discharge capacity of AGM sealed batteries is smaller. Compared with the flooded battery of the same specification, the price is higher, and it has the following advantages:
(1) The cyclic charging capacity is three times higher than that of lead-calcium batteries, and the service life is longer.
(2) It has high capacitance stability during the whole life cycle.
(3) Low-temperature performance is more reliable.
(4) Reduce the risk of accidents and environmental pollution (because the acid solution is 100% sealed).
(5) Simple maintenance, reducing deep discharge.
Gel sealed battery (ie GEL battery), gel lead-acid battery, is an improvement of the liquid electrolyte used in ordinary lead-acid batteries. The use of colloidal electrolyte instead of sulfuric acid electrolyte is superior to ordinary batteries in terms of safety, storage capacity, discharge performance, and service life. The electrolyte is composed of silica sol and sulfuric acid. The concentration of the sulfuric acid solution is lower than that of the AGM battery, and the amount of electrolyte is more than that of the AGM battery, which is equivalent to the flooded battery. This electrolyte exists in a colloidal state and is filled in the separator and between the positive and negative electrodes. The sulfuric acid electrolyte is surrounded by gel and will not flow out of the battery.
Gel sealed battery has the following advantages:
(1) The acid leakage rate is small. Gel-type gel batteries have no free electrolyte after the electrolyte gel, so the probability of acid leakage is much smaller than that of AGM-type batteries.
(2) Less water loss. Because it is more perfused than dilute sulfuric acid and has less water loss, the gel battery will not fail due to water loss.
(3) Effectively extend battery life. The injection of the colloid increases the strength of the separator, protects the electrode plate, and compensates for the defect of the separator contracting with acid so that the assembly pressure is not significantly reduced, which is also one of the reasons for prolonging the battery life.
(4) The colloidal lead-acid battery has strong resistance to overcharge.
(5) Strong recovery ability under heavy discharge. The colloid fills the gap between the separator and the electrode plate, reduces the internal resistance of the battery, and improves the charge acceptance.
Therefore, the over-discharge, recovery ability, and low-temperature charge-discharge performance of gel batteries are better than AGM batteries.
(6) Bulk lead-acid batteries have good self-discharge performance. Under the same sulfuric acid purity and water quality, the battery storage time can be extended by more than 2 times.
(7) The colloidal lead-acid battery has the obvious anti-vulcanization ability under severe power shortage. (8) The colloidal lead-acid battery has good post-discharge performance.
2.3 Lead Carbon Battery
A lead-carbon battery is a kind of capacitive lead-acid battery, which is a technology evolved from traditional lead-acid batteries. Adding activated carbon to the negative electrode of a lead-acid battery can significantly increase the life of a lead-acid battery.
Lead-carbon battery is a new type of super battery that combines the advantages of lead-acid batteries and supercapacitors.
(1) It not only gives full play to the advantages of instant large-capacity charging of supercapacitors but also gives full play to the specific energy advantages of lead-acid batteries and has a very good charge and discharge performance.
(2) The battery life is extended. Due to the addition of carbon (graphene), the sulfation of the negative electrode is prevented, the factors of the previous battery failure are improved, and the battery life is prolonged.
(3) The cost of electricity consumption has decreased. The electricity bill for lead-carbon batteries can be as low as RMB 0.5/kWh. On the basis of large-scale production, lead-carbon batteries are even expected to reduce electricity costs to less than 0.4 yuan.
Lead-carbon batteries are the most advanced technology in the field of lead-acid batteries, as well as the development focus of the international new energy storage industry, and have very broad application prospects. It is widely used in energy storage fields such as photovoltaic power station energy storage, wind power energy storage, and power grid peak shaving.
