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Energy storage technology in photovoltaic power generation system
- May 30, 2018 -

The energy storage technology in photovoltaic power generation system is an important technical measure to transfer peak power, develop low-voltage electricity, optimize resource allocation, and protect the ecological environment. In our country, the promotion and application of energy storage technology has just started. Although the promotion and application is very small, it has obvious benefits and great potential.


        1. The role of energy storage technology in photovoltaic power generation systems


The energy storage technology is particularly suitable for photovoltaic power generation systems of renewable energy. Due to the instability of renewable energy sources, it cannot be operated continuously. Therefore, energy storage technology plays a very important role in photovoltaic power generation systems. The role of energy storage technology in photovoltaic power generation systems is as follows:


1) Load regulation. The energy storage device can be charged during the low load period of the power system and discharged during the peak load period.


2) Load tracking. Superconducting energy storage systems, battery energy storage systems, and flywheel energy storage systems through the power electronics interface can quickly track changes in the load, thereby reducing the need for large generators to track the load.


3) The system is stable. The rapid change of the active power and reactive power output by the energy storage device can effectively dampen the power and frequency oscillations in the system.


4) Automatic power generation control. Accumulator with AGC can effectively reduce the area control error.


5) Rotary kinetic energy storage. An energy storage device with a power electronic interface can rapidly increase its power output, and can be used as rotational kinetic energy in the power system, reducing the need for rotational energy of conventional power systems.


6) VAR control and power factor correction. An energy storage device with a power electronic interface can provide rapidly changing reactive power while rapidly providing active power.


7) Black boot capability. The energy storage device can provide photovoltaic power generation equipment for island operations with the electrical energy required for startup.


8) Increase the efficiency of power generation equipment to reduce its maintenance. The ability of the energy storage device to track the load enables the photovoltaic power generation system to operate in a constant output power state, allowing its power generation equipment to operate at a high-efficiency operating point, thereby improving the overall power generation efficiency, maintenance intervals and service life of the power generation equipment.


9) Delayed the system's need for new transmission capacity. Energy storage devices are installed in appropriate areas of the system and they are charged during the power outage period, thereby reducing the peak load capacity of transmission lines and effectively increasing the capacity of transmission lines.


10) Delayed the system's demand for new generation capacity. When the energy storage device flattened the peak load, the need for the peak capacity of the system was reduced.


11) Improve the effective utilization of power generation equipment. In the peak period of electricity consumption, the power output by the energy storage device can increase the total capacity of the system.


2. Battery storage


Due to the nature of natural resources, renewable power sources have obvious intermittent and fluctuating power output when they are used for power generation. Their changes are random and can easily impact the power grid. In severe cases, they can cause power grid accidents. In order to make full use of renewable energy and guarantee its reliability, it is necessary to timely control and suppress such energy changes that are difficult to predict accurately. The energy storage device is used to solve this problem. Battery storage system consists of battery, inverter, control device, auxiliary equipment (safety, environmental protection equipment) and other components.


The battery pack is an energy storage device in an off-grid solar energy (000591) photovoltaic power generation system. It converts the direct current converted from the solar radiation from the solar cell array into chemical energy for storage. Since the input energy of the solar photovoltaic power generation system is extremely unstable, it is generally necessary to configure the storage battery so that the load can work normally. The electric energy generated by the solar cell is stored in the form of chemical energy in the storage battery. When the load needs to supply electricity, the storage battery converts the chemical energy into electric energy to supply the load.


The characteristics of the battery directly affect the working efficiency, reliability, and price of the solar photovoltaic system. The choice of battery capacity generally follows the following principles: First, under the premise of being able to meet the load power consumption, the electrical energy produced by the solar cell modules during the day should be stored as much as possible, and at the same time, the electrical energy needed by the electrical load during the scheduled continuous rainy days should be stored. The capacity of the battery is affected by the amount of electricity required for the end load and the duration of the sunshine (power generation time). Therefore, the battery watt-hour capacity and ampere-hour capacity are determined by the predetermined load demand power and the continuous no-sunlight time, so the performance of the battery directly affects the operating characteristics of the solar photovoltaic power generation system.


Comprehensive analysis of the characteristics of various batteries, because of lead-acid batteries have a good price, and energy density can also meet the requirements of system design, so among these batteries, cost-effective lead-acid batteries are most suitable for photovoltaic power generation systems, Lead-acid batteries have the longest history and are still widely used. Lead-acid batteries were invented by Plante in 1859 and have a history of more than 150 years. For more than a hundred years, the technology, structure, production, performance and application of lead-acid batteries have been continuously developed. The development of science and technology has brought vitality to ancient lead-acid batteries.


The lead-acid battery has undergone major reforms in modern times, and its performance has greatly improved. The main sign is the Valve-Regulated Lead Acid Battery (VRLA) battery developed in the 1970s. GatesEnergyProductsInc, the United States, pioneered the development of lead-acid batteries by introducing ultra-fine glass fiber suction-type hermetic sealing technology. In the past ten years, the performance of bipolar VRLA batteries and horizontal electrode VRLA batteries has been further improved. In the bipolar VRLA battery, a bipolar electrode with positive and negative active materials on both sides of the powerful thin plate is introduced to greatly reduce the internal resistance, thereby greatly increasing the specific energy and the charging rate. The VRLA battery has high energy, low cost and long service life. (10 years), large capacity (2 times that of ordinary lead-acid batteries), no leakage, safety, no pollution, recyclable, maintenance-free, easy to use. For newly developed bipolar and horizontal VRLA batteries, the C/3 discharge specific energy of ≥50Wh/kg shows excellent performance.


3. Colloidal lead-acid battery


Colloidal lead-acid batteries are simply batteries using colloidal electrolytes. Colloidal lead-acid batteries are one type of lead-acid batteries. The simplest method is to add a gelling agent to sulfuric acid so that the sulfuric acid electrolyte becomes colloidal. The difference between colloidal lead-acid batteries and common lead-acid batteries lies not only in the change of electro-hydraulic to gel-like, but also in the development of the electrochemical properties of electrolytes, as well as their application in grids and active materials. For example, the use of non-solidified aqueous colloidal lead-acid batteries, from the electrochemical classification and characteristics of the same colloidal lead-acid batteries. Another example is the use of polymer materials in grids, commonly known as ceramic grids, which are characteristics of colloidal lead-acid batteries. Recently, some laboratories have added a targeted coupling agent to polar plate formulations, which has greatly improved the reaction utilization of the active materials on the plates.


The colloidal lead-acid battery is a sealed structure, electrolyte gel, no leakage, no acid mist and no pollution due to charge and discharge. It is an environmentally friendly product that is vigorously promoted and applied by the country. The most important characteristics of colloidal lead-acid batteries are: the discharge curve is straight, the inflection point is high, the specific energy, especially the specific power, is more than 20% larger than that of common lead-acid batteries, and the service life is generally about twice longer than that of ordinary lead-acid batteries. Strong ability; self-discharge is small, resistant to storage; over discharge recovery performance is good, large current discharge capacity increased by more than 30% than ordinary lead-acid batteries; good low temperature performance, high temperature stability, meet the 65 °C or even higher temperature environment requirements; Long life, can reach 800 ~ 1500 charge and discharge times, the unit cost of industrial costs than lead-acid batteries, high economic efficiency.


Colloid quality and filling process have an important influence on the quality of colloidal lead-acid batteries. The design, manufacturing process and application conditions (especially charge-discharge processes) of colloidal lead-acid batteries all restrict the performance of colloidal lead-acid batteries. The properties of the colloid must be compatible with the structure of the battery and the conditions of use. The structure and the conditions of use of the sealed battery are conducive to the stability of the gel, and the properties of the gel make the performance of the sealed battery more perfect. Modern excellent colloidal lead-acid batteries are valve-regulated lead acid batteries (VRLA), and colloidal lead-acid batteries made from unmodified semi-finished products of ordinary lead-acid batteries are also controversial.


Colloidal filling, gel stability and ensuring battery capacity are the three key technologies for colloidal lead-acid batteries. The colloidal lead-acid battery produced by the German Sunshine Company has a very low colloidal viscosity and is filled with a colloidal lead-acid battery at atmospheric pressure. Even large-sized colloidal lead-acid batteries fill colloidal lead-acid batteries like perfused dilute sulfuric acid. The gel is fully gelled in the battery, showing a homogeneous paste gel inside and outside of the polar group. During the entire life of the colloidal lead-acid battery, there is no liquefaction phenomenon. This is difficult to do for colloidal lead-acid batteries produced in China. . Suntech's technology is the world's most advanced, its Dryfit series of colloidal lead-acid batteries is safe and reliable, long life, is the world's most excellent colloidal lead-acid batteries. However, the specific energy and large-current discharge of the colloidal lead-acid battery produced by Sunshine Company are inferior to AGM-VRLA batteries (ie, valve-regulated sealed lead-acid batteries using ultra-fine glass fiber membranes). In addition, the solar company's plate conversion process is complex, and the production cycle is long. Some varieties of colloidal lead-acid batteries need to be shipped through 10 cycles of charge and discharge to reduce the production efficiency and increase the product cost, which is not conducive to large-scale product development. Compete with the market.


Valve-regulated sealed lead-acid batteries hold the electrolyte in two ways. One is to fix the electrolyte by AGM. The other type is a colloidal structure in which the electrolyte is fixed by a colloid. However, C&D Technologies of the United States combines two methods to fix the electrolyte, called composite technology. In the definition of a colloidal lead-acid battery, only the electrolyte is gel-like (more intuitively, it is known as jelly-like). There is no provision for the use of the separator. Therefore, as long as the gel is used to fix the battery of the electrolyte, It can be called colloidal lead-acid battery.


Regardless of the use of liquid silica and fumed silica, the principle of gelation is the same, and there is a difference in particle size and purity between them. Therefore, after adding the battery, the performance of the battery has a relatively large impact. The strength of the gel is proportional to the content of silica and the amount of acid, and the greater the strength, the less likely it is to hydrate and break.


The internal resistance of the battery is proportional to the content of silica in the colloid, so the high rate performance (3C and above) of the colloidal lead-acid battery is worse than that of the AGM-VRLA battery of the same structure, but the rated capacity is lower than that of the AGM-VRLA of the same structure. The battery is 5 to 10% larger. Colloidal lead-acid batteries using a dedicated separator such as PVC-SiO2 or phenolic resin have a smaller rated capacity than AGM-VRLA batteries due to the content of silica. If using PVC or PE as a separator, the silica content must be quite high to form a stable colloid. The colloidal lead-acid battery produced by the composite technology has a float life of 1.5 to 2 times that of the AGM-VRLA battery of the same structure, and the cycle capacity can be increased by 20%.


Most of the world's leading companies that produce lead-acid batteries produce colloidal lead-acid batteries, such as: German Sunshine, Hagen, American DEKA, Trojan, Exide, SEC, etc., but Japan's YUASA does not produce colloidal lead-acid batteries, but Its UXL series batteries have colloidal components, its main role is to reduce the electrolyte layering phenomenon. In terms of applications, mainly in solar energy, power batteries, etc., its market is relatively large, the price is about 20% higher than AGM-VRLA batteries.