(Report produced by: Dongfang Securities)
Energy storage system: To ensure safety and improve efficiency, multiple technological routes are blooming
The electrochemical energy storage system consists of two parts: the DC side and the AC side. The DC side is the battery compartment, including equipment such as batteries, temperature control, fire protection, combiner cabinets, containers, etc., while the AC side is the electrical compartment, including energy storage converters, transformers, containers, etc. The battery on the DC side generates direct current, and in order to interact with the power grid, AC/DC conversion must be carried out through a converter.
Classification of energy storage systems: centralized, distributed, intelligent cluster, high-voltage cascade, and distributed
According to the electrical structure, large-scale energy storage systems can be divided into:
(1) Centralized: Low voltage high-power boost type centralized grid connected energy storage system, with multiple clusters of batteries connected in parallel to PCS. PCS pursues high power and high efficiency, and is currently promoting the 1500V solution.
(2) Distributed: A low-voltage low-power distributed boost grid connected energy storage system, where each cluster of batteries is connected to a PCS unit, and the PCS adopts a low-power, distributed layout.
(3) Intelligent string type: Based on the distributed energy storage system architecture, innovative technologies such as battery module level energy optimization, battery single cluster energy control, digital intelligent management, and fully modular design are adopted to achieve more comprehensive applications of energy storage systems.
(4) High voltage cascaded high-power energy storage system: Single cluster inverter with batteries, directly connected to the power grid with a voltage level of 6/10/35kV or above without a transformer. The capacity of a single unit can reach 5MW/10MWh.
(5) Distributed: Multiple branches are connected in parallel on the DC side, and a DC/DC converter is added at the outlet of the battery cluster to isolate the battery cluster. After the DC/DC converters are gathered, they are connected to the centralized PCS DC side.
Energy storage technology roadmap iteration revolves around safety, cost, and efficiency
Safety, cost, and efficiency are key issues that need to be addressed in the development of energy storage. The iterative core of energy storage technology is also to improve safety, reduce costs, and improve efficiency.
(1) Security
The safety of energy storage power plants is a concern of the industry. The potential safety hazards of electrochemical energy storage power plants include electrical fires, battery fires, hydrogen explosions when exposed to fire, and system abnormalities. Tracing the causes of safety issues in energy storage power plants can usually be attributed to thermal runaway of batteries. The causes of thermal runaway include mechanical abuse, electrical abuse, and thermal abuse. To avoid safety issues, it is necessary to strictly monitor the battery status to avoid the occurrence of thermal runaway triggers.
(2) * Rate
The consistency of battery cells is a key factor affecting system efficiency. The consistency of the battery cell depends on the quality of the battery cell, the energy storage technology scheme, and the working environment of the battery cell. As the number of battery cell cycles increases, the differences in battery cells gradually manifest. The differences in the actual working environment during the operation process will exacerbate the differences between multiple battery cells, highlight consistency issues, pose challenges to BMS management, and even face safety risks. Mismatch between battery modules in series: The available capacity of the battery cells in series can only reach the capacity of the weak battery module, making it impossible to fully utilize the capacity of other batteries. Parallel mismatch between battery clusters: The available capacity of battery clusters on the parallel link can only reach the capacity of weak battery clusters, making it impossible for other battery capacities to be fully utilized. The difference in internal resistance of the battery causes circulation: The circulation of the battery causes the temperature of the battery cell to increase, accelerates aging, increases system heat dissipation, and reduces system efficiency. In the design and operation plan of energy storage power plants, the consistency of batteries should be maximized to improve system efficiency.
(3) Low cost
The cost of energy storage systems is related to initial investment and cycle life. The aging and degradation of battery materials, charging and discharging systems, battery operating temperature, and monomer consistency can all affect the cycling life of the battery. When the temperature difference between the batteries inside the container exceeds 10 degrees, it will shorten the battery life by more than 15%. The difference in temperature rise between modules can also lead to a shortened overall system lifespan. The energy storage system should improve its cycle life by optimizing charging and discharging methods, reducing temperature differences between systems, and improving battery consistency.
Energy storage integration technology roadmap: gradually iterating topology solutions
Centralized solution: 1500V replacing 1000V becomes a trend
With the development of centralized wind and solar power plants and energy storage towards larger capacity, DC high voltage has become the main technical solution for cost reduction and efficiency improvement, and energy storage systems with DC side voltage increased to 1500V have gradually become a trend. Compared to traditional 1000V systems, 1500V systems increase the withstand voltage of cables, BMS hardware modules, PCS, and other components from no more than 1000V to no more than 1500V. The 1500V technical solution for energy storage systems comes from photovoltaic systems. According to CPIA statistics, the market share of domestic photovoltaic systems with a DC voltage level of 1500V in 2021 is about 49.4%, and it is expected to gradually increase to nearly 80% in the future. A 1500V energy storage system will be beneficial for improving its compatibility with photovoltaic systems.
Looking back at the development of photovoltaic systems, achieving a DC side voltage of 1500V can reduce AC/DC side line losses and transformer low-voltage side winding losses through higher input and output voltage levels, improve the efficiency of power plant systems, increase the power density of equipment (inverters and transformers), reduce volume, and reduce workload in transportation, maintenance, and other aspects, which is conducive to reducing system costs. Taking the 1500V photovoltaic system solution released by TBEA in 2016 as an example, compared to traditional 1000V systems, the efficiency of the 1500V system has been improved by at least 1.7%, the initial investment has been reduced by 0.1438 yuan/W, the number of equipment has been reduced by 30-50%, and the inspection time has been shortened by 30%.
The performance of the 1500V energy storage system scheme has also improved compared to the 1000V scheme. Taking the Sunshine Power Supply scheme as an example, compared to the 1000V system, the energy density and power density of the battery system have increased by more than 35%. For power plants with the same capacity, there are fewer equipment, and the cost of battery systems, PCS, BMS, cables and other equipment has significantly decreased. Infrastructure and land investment costs have also decreased simultaneously. According to calculations, compared to traditional solutions, the initial investment cost of the 1500V energy storage system alone has been reduced by more than 10%. However, at the same time, as the voltage of the 1500V energy storage system increases, the number of batteries connected in series increases, making it more difficult to control consistency. The requirements for DC arc risk prevention protection and electrical insulation design are also higher.
Distributed solution: high efficiency, mature solution
The distributed scheme is also known as AC side multi branch parallel connection. Compared with centralized technical solutions, the distributed solution transforms the DC side parallel of the battery cluster into AC side parallel through a distributed series inverter, avoiding parallel circulation, capacity loss, and DC arc risk caused by DC side parallel connection, and improving operational safety. Simultaneously changing the control accuracy from multiple battery clusters to a single battery cluster results in higher control efficiency.
Shandong Huaneng Huangtai Energy Storage Power Station is the world's first 100 megawatt level decentralized control energy storage power station. The Huangtai Energy Storage Station uses batteries from Ningde Era and a PCS system from Shangneng Electric. According to calculations, after the energy storage power station is put into operation, the overall battery capacity utilization rate of the station can reach about 92%, which is 7 percentage points higher than the current industry average. In addition, through decentralized control of battery clusters, automatic calibration of battery state of charge (SOC) can be achieved, significantly reducing the workload of operation and maintenance. The efficiency of grid connection testing is as high as 87.8%. From the current project quotation, the cost of a decentralized system is not higher than that of a centralized system.
The distributed solution has high efficiency and limited cost increase, and we estimate that its market share will gradually increase in the future. At present, Ningde Times and Shangneng Electric are selected as the equipment for the 100 megawatt operating power plants. Compared to centralized schemes, 630kW or 1.725MW centralized inverters need to be replaced with low-power series inverters. For inverter manufacturers, if they have series inverter products and strong research and development capabilities, they can quickly enter distributed schemes.
Intelligent string scheme: one package, one optimization, and one cluster, one management
Huawei's intelligent cluster solution addresses three main issues in centralized solutions: (1) capacity degradation. In traditional solutions, the use of batteries has a significant "short board effect", where battery modules are connected in parallel. When charging, one battery cell is fully charged, charging stops, and when discharging, one battery cell is discharged, discharging stops. The overall lifespan of the system depends on the short lifespan of the battery. (2) Consistency. In the operation and application of energy storage systems, there are deviations in battery consistency due to different specific environments, resulting in exponential decay of system capacity. (3) Capacity mismatch. Parallel connection of batteries can easily cause capacity mismatch, and the actual usage capacity of batteries is much lower than the standard capacity.
The intelligent stringing solution solves the three problems mentioned above in centralized solutions through the design of stringing, intelligence, and modularity: (1) stringing. Using an energy optimizer to achieve battery module level management, using a battery cluster controller to achieve inter cluster balance, and distributed air conditioning to reduce inter cluster temperature differences. (2) Intelligence. Apply advanced ICT technologies such as AI and cloud BMS to internal short circuit detection scenarios, predict battery status using AI, and adopt a multi model linkage intelligent temperature control strategy * charging and discharging status * optimization. (3) Modularization. The modular design of the battery system allows for individual disconnection of faulty modules without affecting the normal operation of other modules within the cluster. Modularize the PCS design, so that when a single PCS fails, other PCS can continue to operate, while when multiple PCS fail, the system can still operate.
High voltage cascade scheme: * scheme without parallel structure
The high-voltage cascade energy storage scheme is designed through power electronics to achieve a grid connected voltage of 6-35kV without the need for a transformer. Taking the Xinfengguang 35kV solution as an example, a single energy storage system is a 12.5MW/25MWh system, and the electrical structure of the system is similar to high-voltage SVG, consisting of A, B, and C three-phase components. Each phase contains 42 H-bridge power units paired with 42 battery clusters. There are a total of 126 H-bridge power units and 126 battery clusters in the three-phase system, storing a total of 25.288MWh of electricity. Each cluster of batteries consists of 224 cells connected in series.
The advantages of the high-voltage cascade scheme are reflected in: (1) safety. There are no parallel cells in the system, some batteries are damaged, the replacement range is narrow, the impact range is small, and the maintenance cost is low. (2) Consistency. The battery packs are not directly connected, but are connected through AC/DC, so all battery packs can be controlled for SOC equalization through AC/DC. The interior of the battery pack is only a single battery cluster, and there is no parallel connection of battery clusters, so there will be no current sharing problem. The battery cluster achieves balanced control between battery cells through BMS. Therefore, this scheme can greatly utilize the capacity of battery cells, and can install fewer battery cells under the same grid connected power on the AC side, reducing initial investment. (3) * Rate. Due to the lack of parallel operation of battery cells/clusters in the system, there is no short board effect, and the system life is approximately equivalent to the life of a single battery cell, which can greatly improve the operational economy of energy storage devices. The system does not require a step-up transformer, and the actual system cycle efficiency on site reaches 90%.
The high-voltage cascade scheme, as a new technological route, needs to be validated through operation. (1) In terms of technology, on the one hand, the high-voltage cascade scheme has a voltage of 35kV for each phase, and the electromagnetic environment is harsh, posing higher requirements for BMS control. On the other hand, the high-voltage cascade scheme involves parallel connection on the AC side, selecting multiple H-bridges for connection. ABC three-phase AC has multiple H-bridges in series for each phase, reducing stability. To improve stability, redundant design is necessary. If one H-bridge fails, it can be switched to a bypass circuit. (2) In terms of operation, the DC and AC sides of the 35kV energy storage system are placed in the same position, which increases the difficulty of operation and maintenance and poses certain safety risks. The penetration rate of the current high-voltage cascade scheme is still low, and it needs to be verified for stability and stability through multiple projects.
From the perspective of project price, the energy storage project quotation of the high-voltage cascade scheme is similar to that of traditional projects. In April 2022, the joint venture of Jinpan Technology and Tianjin Ruiyuan Electric won the bid for the Zhongguang Nuclear Power Hainan Baisha Bangxi 25MW/50MWh Energy Storage Project, with a bid price of 64.999166 million yuan and a unit price of 1.30 yuan/wh.
Distributed scheme: DC isolation+centralized inverter
The distributed scheme, also known as the DC side multi branch parallel connection, is based on the traditional centralized scheme. A DC/DC converter is added at the outlet of the battery cluster to isolate the battery cluster. After the DC/DC converters are gathered, they are connected to the DC side of the centralized PCS, and 2-4 PCS are connected in parallel to a local transformer. After being boosted by the transformer, they are connected to the grid. By adding DC/DC isolation in the system, the DC arc, circulation, and capacity loss caused by DC parallel connection are avoided, greatly improving the system's safety and efficiency. However, due to the need for the system to undergo two-stage inverters, it has a negative impact on system efficiency.
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