How far is the lithium battery from becoming the "group pet" of energy storage?
No matter which form of energy storage plays the same role in the application of power grid (generation, transmission, distribution and utilization), it is not only an effective means to improve the reliability and toughness of power grid. The application of energy storage in power grid began in the early 20th century and has a history of more than 100 years. The most traditional and largest application scale of energy storage is pumped storage. At present, it is developing towards electrochemical distributed energy storage mode.
Function of energy storage device in Smart Grid
Driven by electric vehicles and other related industries, the scale of lithium battery industry expands rapidly and the price decreases rapidly. The application of lithium battery as distributed energy storage system is gradually increasing. By the end of 2020, the cumulative scale of electrochemical energy storage projects put into operation in China was 1072.7mw, of which lithium battery energy storage projects accounted for about 60%.
Under the development background of building flexible smart grid, energy storage is one of the effective means to improve the toughness, adjustability, flexibility, intelligence, independence and reduce coupling of power grid. From the current optional energy storage elements, lithium battery is a more appropriate choice, especially for user side energy storage. The advantages of lithium battery as an energy storage element, such as high energy density and long life, are gradually emerging. However, at present, the application of lithium battery energy storage on the user side is growing slowly, and there needs to be a breakthrough in the economy, safety and applicability of the energy storage system itself.
1、 Can lithium batteries save money
At present, the energy storage of lithium battery on the user side does not have much other benefits except cutting peak and filling valley to earn the price difference between peak and valley, and the way of economic return is single. The economy of peak shaving and valley filling depends more on the regional peak valley electricity price, which shows the distribution area of domestic peak valley price difference. From the survey, the price difference between peak and valley in Beijing is the largest, more than 1 yuan. Therefore, the application of energy storage in peak and valley arbitrage has the most advantages. The next are Tianjin, Shanghai, Jiangsu and Guangdong. Generally speaking, the first tier cities and coastal provinces are more active in implementing peak valley electricity price, so the user side energy storage will have better economy.
It shows the yield of peak shaving and valley filling of energy storage project (the abscissa is kwh investment cost, the ordinate is the yield of the whole life cycle, and different color curves represent the change of peak valley price difference). The lithium battery model considers the charge and discharge efficiency, depth and capacity decline, according to 4500 cycles, plus 1500-2000 echelon utilization cycles, and 20% residual value is considered in the calculation. It can be seen that even if this ideal algorithm is adopted, the rate of return is not very considerable (the rate of return appears only when the peak valley price difference reaches 0.7 yuan). In addition, at present, the quality of lithium batteries in the energy storage industry is uneven, so it is difficult for owners and integrators to make an intuitive judgment on battery performance. The number of cycles of lithium batteries in the market is in the range of 3000 ~ 6000 (even if the manufacturer promises that the number of cycles is higher, if it is only used for peak shaving and valley filling, the cycle life of the battery may not be reached, and the calendar life may have reached, which will also reduce the service efficiency of the battery), and there is still great doubt whether the actual operation is true. All these have increased investors' doubts about the economy of lithium battery energy storage project and increased the difficulty of the application of lithium battery energy storage on the user side.
2、 Will lithium batteries be safer
Lithium battery is an energy body, which is characterized by high energy density, so it itself has safety risk. The higher the energy density, the greater the safety risk. At present, the domestic lithium battery energy storage industry basically excludes ternary batteries, and it is also reasonable to think that lithium iron phosphate batteries are more appropriate (the energy density of ternary batteries is higher than that of lithium iron phosphate batteries, and the safety risk of ternary batteries is also greater than that of lithium iron phosphate batteries).
It is necessary to improve the quality of lithium battery and reduce the risk of battery out of control. At present, the safety of lithium battery energy storage is more focused on the lithium battery itself, but the energy storage system is not only lithium battery, but also other system equipment and protection equipment. Its safety is also worthy of attention. At present, more owners and integrators focus on the battery itself, but ignore the peripheral equipment and system integration design. For example, the investigation results of an energy storage accident in Shanxi show that the cause of the accident is the continuous discharge of the top of the fixing bolt to the shell, not the battery itself. South Korea summarized the causes of 23 energy storage system accidents into four aspects: poor electric shock protection system, poor management of operation environment, careless installation and poor management of integrated control and protection system (see Polaris energy storage network - battery does not carry the pot! South Korea put forward four improvement measures for the causes of energy storage power station accidents). All these show that in addition to selecting high-quality lithium batteries, there is still a certain probability hidden danger in the safety of its energy storage system, which needs to be guaranteed by a more perfect and systematic system integration design.
3、 Is lithium battery applicable
Firstly, from the perspective of attributes, the application of energy storage system in power grid is more defined as a production unit. For user side energy storage, its characteristics are closely related to user side operation behavior, showing the characteristics of many points, wide range, large quantity and dispersion. It is a user side equipment that can participate in regulation. If only one production unit is defined, there will be many application constraints.
At present, the specifications for the construction, operation and maintenance of lithium-ion battery energy storage system have been established one after another. However, due to the emergence of lithium-ion battery energy storage system as a new thing, the relevant guiding principles and basis need to be further improved. For example, the national standard gb51048-2014 code for design of electrochemical energy storage power station mainly defines energy storage as an independent production unit. At present, the design of user side energy storage project is difficult to meet the specification requirements. The national standard GB / t36558-2020 general technical conditions for electrochemical energy storage system of power system requires that the energy conversion efficiency of lithium-ion battery energy storage system should not be less than 92%, but it is difficult for manufacturers to achieve. The efficiency of domestic Kelu electronic cl5231f energy storage system is 88%, and the efficiency of Tesla PowerPack energy storage system is 88% ~ 89%.
The application of lithium battery in the energy storage system on the user's side, in addition to the reference specifications of lithium battery energy storage itself, involves more cooperation with the existing buildings and equipment on the user's side, whether the relevant design specifications match the existing specifications of the building, whether the relevant protection can rely on the existing protection measures of the building, etc. these parts lack reference basis in both design and practical application. The above work involves multi-disciplinary knowledge integration, and it is necessary to strengthen cross professional exchanges and cooperation.
In conclusion, as a new application mode, the efficiency and energy advantages of lithium battery energy storage have been widely recognized. However, the research on the application of lithium battery in user side energy storage engineering is still insufficient, the support of relevant standards and specifications needs to be supplemented, and further research is needed on its attribute definition, economic benefits, system design and construction and operation guidance.
