Vanadium flow battery energy storage system with charge and discharge decoupling and control method thereof

Abstract

This invention discloses a decoupled charge-discharge vanadium redox flow battery energy storage system and its control method, belonging to the field of flow battery energy storage technology. Existing containerized vanadium redox flow battery energy storage systems suffer from problems such as a large number of individual cells, sequential charging and discharging, and low stack utilization. This invention uses four independent large storage tanks to store low-SOC cathodic solution, high-SOC cathodic solution, low-SOC anodic solution, and high-SOC anodic solution respectively, and configures two independent stacks dedicated to charging and discharging respectively, with the electrolyte flow direction controlled by a switching valve group. During charging, the low-SOC electrolyte flows from the charging tank through the charging stack and into the discharging tank; during discharging, the high-SOC electrolyte flows from the discharging tank through the discharging stack and back to the charging tank. The two stacks can operate independently and be periodically interchanged, achieving simultaneous charging and discharging. This invention significantly reduces the number of storage tanks, lowers the footprint and investment costs, achieves near 100% stack utilization, and a system overall efficiency of 78%-80%, making it particularly suitable for large-scale energy storage power stations with capacities of hundreds of MWh or more.

Description

Technical Field

[0001] This research relates to the field of flow battery energy storage technology. Specifically, it involves an architecture and charge / discharge control method for a 100-megawatt-level all-flow battery energy storage system. Background Technology

[0002] Currently, commercial vanadium redox flow battery energy storage systems generally adopt a containerized modular design. Each standard container contains an independent electrolyte storage tank (cathode and anolyte of approximately 45m³), a stack, two circulation pump sets, and auxiliary piping, control systems, thermal management systems, etc. Multiple containers are connected in parallel to form a 100MWh-level energy storage power station. For example, building a 100MW / 400MWh energy storage power station project requires approximately 400 individual 45m³ / MWh containerized energy storage systems, which highlights some significant problems in the current large-scale deployment of flow batteries. A search reveals that existing patents, such as CN104659396A (a vanadium battery system), propose setting up intermediate containers to temporarily store high-energy electrolyte to reduce self-discharge, but this still does not achieve the physical separation and reuse of the charge and discharge stacks, nor does it solve the system integration problem in large-scale energy storage scenarios. Summary of the Invention

[0003] The main technical problems solved by this invention are: 1. Solving the problem that vanadium battery charging and discharging cannot be performed in parallel; 2. Significantly improving the utilization efficiency of key equipment such as vanadium electrolyte and battery stack; 3. Improving the response flexibility of the vanadium battery system to grid dispatch; 4. Reducing the system complexity of large-scale energy storage power stations, reducing the footprint, and lowering investment costs; 5. Achieving decoupling expansion of electrolyte and battery stack, enhancing the scalability of the system. Explanation of technical terms:

[0004] Simultaneous charging and discharging: The same batch of vanadium electrolyte can perform charging and discharging operations simultaneously in the energy storage system without affecting each other.

[0005] Fuel cell stack rotation system: The charging fuel cell stack group and the discharging fuel cell stack group switch roles through valve switching according to a set cycle, so as to balance the working time of the two fuel cell stack groups and extend the overall life.

[0006] Electrolyte recycling: The electrolyte flows only between four storage tanks and two sets of fuel cells, with no external discharge and no waste, achieving 100% recycling.

[0007] Four-tank separation architecture: The system is equipped with four independent electrolyte storage tanks: cathode charging tank, anode charging tank, cathode discharge tank, and anode discharge tank, which completely isolates the electrolyte during the charging and discharging process at the physical level.

[0008] Charge and discharge stack: The charge and discharge stack used in this invention is the same as the stack used by domestic counterparts. It is a general-purpose stack that can both charge and discharge. In this invention, considering that charging and discharging can be operated independently, the charge and discharge stack only performs one of its capabilities, either charging or discharging.

[0009] Construct four ultra-large storage tanks, physically isolated according to cathode / anode and charged / discharged states: #1 tank Cathode charging tank Low SOC catholyte (uncharged state) ~9,000 m³ Tank #2 Cathode discharge tank High SOC catholyte (charged state) ~9,000 m³ #1 tank Anode charging tank Low SOC anolyte (uncharged state) ~9,000 m³ Tank #2 Anode discharge tank High SOC anolyte (charged state) ~9,000 m³ Note: The volume of a single tank can be adjusted according to actual site conditions and the specifications of abandoned storage tanks, as well as to local charging and discharging time requirements. For example, in some northern regions where there are only 5 hours of sunshine per day, a large number of photovoltaic systems are required to complete energy storage within 5 hours, but the released energy needs to be evenly distributed over 24 hours. In this case, the size of the storage tank and the number of fuel cells need to be adjusted unevenly.

[0010] fuel cell stack configuration (centralized layout, independent use) Configure one set of two fuel cell stacks, one for charging and one for discharging. The number of stacks should be configured according to a total power output of 100 MW (e.g., 100-200 sets, depending on the rated power of each stack). The two sets of stacks are connected separately, and the switching between the corresponding charging and discharging tanks is achieved through a unified set of pipes and valves. An automatic valve matrix is ​​installed at the inlet and outlet of the fuel cell stack to achieve flow direction switching. Taking the cathode side as an example, the piping topology is as follows: Outlet of Tank #1 (Cathode Charging Tank) → Main Liquid Supply Pump → Valve Group A → Cathode Inlet of Charging Stack → Cathode Outlet of Charging Stack → Valve Group C → Inlet of Tank #2 (Cathode Discharge Tank)

[0011] Normal discharge process: Outlet of Tank #2 (Cathode Discharge Tank) → Main Liquid Supply Pump → Valve Group B → Cathode Inlet of Discharge Stack → Cathode Outlet of Discharge Stack → Valve Group D → Inlet of Tank #1 (Cathode Charging Tank) Outlet of Tank #1 (Cathode Charging Tank) → Main Liquid Supply Pump → Valve Group B → Cathode Inlet of Discharge Stack (Discharge Stack becomes Charging Stack when energized) → Cathode Outlet of Discharge Stack → Valve Group C → Inlet of Tank #2 (Cathode Discharge Tank) Outlet of #2 tank (cathode discharge tank) → Main liquid supply pump → Valve group A → Cathode inlet of charging stack (charging stack becomes discharge stack when connected to grid) → Cathode outlet of charging stack → Valve group D → Inlet of #2 tank (cathode discharge tank) The topology of the anode-side pipeline is symmetrical to that of the cathode, connecting tank #3 (anode charging tank) and tank #4 (anode discharge tank).

[0012] (1) Charging mode: Valve group A is connected to supply liquid to tank #1, and valve group C is connected to return liquid to tank #2. Liquid flow direction: cathode charging tank → charging stack → cathode discharge tank.

[0013] (2) Discharge mode: Valve group B is connected to supply liquid to tank #2, and valve group D is connected to return liquid to tank #1. Liquid flow direction: cathode discharge tank → discharge stack → cathode charging tank.

[0014] (3) Switching actions: pump stop → valve switching (approximately 15-30 seconds), fuel cell stack switching to electricity, fuel cell stack switching to grid → pump start. The system is briefly offline during mode switching, but the frequency of charging and discharging switching in the daily scheduling of grid-side energy storage is usually measured in hours, so the brief offline does not affect the overall revenue.

[0015] 3.1 Charging Mode • Flow direction: 1# tank (cathode charging tank) + 3# tank (anode charging tank) → charging stack → 2# tank (cathode discharge tank) + 4# tank (anode discharge tank) • Result: The low-SOC electrolyte was converted into a high-SOC electrolyte after being reacted in the fuel cell stack, and was stored in tanks #2 and #4 respectively.

[0016] Tank status: Liquid levels in tanks #1 and #3 are decreasing; liquid levels in tanks #2 and #4 are increasing.

[0017] 3.2 Discharge Mode • Flow direction: #2 (cathode discharge tank) + #4 (anode discharge tank) → discharge stack → #1 (cathode charging tank) + #3 (anode charging tank) • Result: The high SOC electrolyte was converted into a low SOC electrolyte after being reacted in the fuel cell stack and returned to tanks #1 and #3 respectively.

[0018] • Tank status: Liquid levels in tanks #2 and #4 are decreasing; liquid levels in tanks #1 and #3 are increasing.

[0019] 3.3 Internal circulation saturation mode (optional) If a single pass through the fuel cell stack fails to bring the electrolyte to the target SOC (insufficient saturation), a small circulation loop can be added during charging mode: valve group D switches to return liquid to tank #1, allowing the liquid to circulate between tank #1 and the fuel cell stack until the overall SOC in the tank reaches the target, then switches back to tank #2 for storage. This mode is only used for initial charging or deep maintenance; daily operation primarily uses modes one and two.

[0020] Conclusion: This invention is suitable for large-scale all-fluid flow energy storage processes, with a tank volume of not less than 5000 m³ for storing the electrolyte. The charging stacks of this invention employ series and parallel connections. The discharging stacks of this invention are of the same model as the charging stacks, with a discharging time that is shorter than the charging time, approximately 67% of the charging time. This invention has been engineered at ***** headquarters, achieving simultaneous charging and discharging.

[0021] Experiments revealed that the overall efficiency of the system under normal operating conditions is 78-80%, which is about 1-2% lower than that of the container model. The main reason is the loss in pipelines and pumping. In the future, the pipeline layout and pumping process losses will be optimized to improve the system efficiency.

[0022] The experimental tanks used in this invention can be self-made PVC tanks, or they can be discarded oil tanks, LNG or CNG storage tanks, or discarded large water storage pools, etc. However, these discarded tanks and pools must be subjected to safety inspections before use to ensure their safe use. At the same time, necessary anti-corrosion work must be carried out on these reused tanks and pools, such as using PTFE or PE lining. After the anti-corrosion work is completed, it must be tested again. Only if it meets the usage requirements can it be used for the storage of vanadium electrolyte, thereby avoiding the occurrence of electrolyte pollution or leakage accidents. Attached Figure Description Figure 1 in the accompanying drawings of the specification is a simplified flowchart of this patent.

Claims

1. A vanadium redox flow battery energy storage system with decoupled charge and discharge and its control method, characterized in that, include Four storage tanks or containers are used to store low-SOC catholyte, high-SOC catholyte, low-SOC anolyte, and high-SOC anolyte, respectively. Two independently operating and interchangeable fuel cell stack systems exist. Each stack has two structures: an anode inlet, an anode outlet, a cathode inlet, and a cathode outlet. Switching valve groups A (A1) and B (B1) are connected to the two stacks, while valve groups C (C1) and D (D1) assist in liquid reflux. Switching valve groups A (A1) and B (B1) are connected to corresponding pumps, and the composition of the electrolyte entering and exiting the stacks is controlled by adjusting the valve group switches, thus achieving mutual backup for the stacks. When the charging and discharging stacks remain unchanged, the charging and discharging process is as follows: liquid in cathode tank #1 is pumped into valve group A and diverted to the charging stack. After reaction, it leaves the charging stack and passes through valve group C into cathode discharge tank #2. At this point, electrolyte is released from tank No. 2 and enters the discharge stack through valve group B. Under the action of chemical reaction and other factors, the electrolyte completes the discharge and is discharged from the outlet of the discharge stack, flowing back to tank No. 1 through valve group D. The anolyte operation process is similar to that of the cathode and will not be described in detail. When the charging stack performance is switched, that is, the performance and function of the charging stack and the discharging stack are interchanged. At this time, the cathode liquid operation process is as follows: The liquid in tank 1 enters valve group B through the pump and then enters the discharging stack from valve group B (at this time, it plays a charging role). After the charging is completed through chemical reaction, the electrolyte flows out from the discharging stack, passes through valve group C, and enters the cathode discharge tank 2. During the discharge process, liquid exits from tank 2, passes through a pump into valve group A, and then enters the charging stack (to perform the discharge function). After completing the chemical reaction, it is discharged from the stack and returns to tank 1 via valve group D. The anode operation process is similar to that of the cathode and will not be described again.

2. The vanadium battery system with simultaneous charging and discharging according to claim 2, characterized in that, The aforementioned large, independent storage container for holding uncharged cathodic and anodic solutions, as well as charged cathodic and anodic solutions.

3. The vanadium battery system with simultaneous charging and discharging according to claim 2, characterized in that, The independent charging and discharging stack is composed of multiple single stacks connected in parallel or series, and the power can be determined by increasing the number of stacks as needed.

4. The vanadium battery system with simultaneous charging and discharging according to claim 2, characterized in that, The switching valve groups A, B, C, and D have multiple valves or control keys, which can be pneumatically or electrically connected fluoropolymer-lined valve matrices.

5. The vanadium battery system with simultaneous charging and discharging according to claim 2, characterized in that, It also includes multiple control and detection systems such as valve group control unit and thermal system control unit. The valve group control unit is electrically connected to the valves A, B, C, D, as well as the anode circulation pump and cathode circulation pump, and is used to perform charge and discharge mode switching.

6. A charging and discharging control method based on the system according to any one of claims 1-5, characterized in that, Includes the following steps: Charging steps: The low SOC catholyte (tank #1) in the uncharged cathode storage tank and the low SOC anolyte (tank #3) in the uncharged anode storage tank are respectively introduced into the charging stack through the circulation pump and valve group. After the chemical reaction is completed in the stack, the catholyte enters the charged cathode tank (tank #2) and the anolyte enters the charged anode tank (tank #4). Discharge process: High SOC cathode and anolyte electrolytes are released from tanks #2 and #4 and enter the discharge stack through circulation pumps and valve groups, respectively. The chemical reaction is completed in the stack to release electricity. Then the cathode electrolyte flows back to tank #1 and the anolyte flows back to tank #3.

7. Simultaneous Charging and Discharging: The system is equipped with two independent fuel cell stacks, each independently handling charging and discharging. Initially, tanks #2 and #4 are empty, so the anode and cathode solutions from tanks #1 and #3 are used to charge the fuel cells and replenish them. Once tanks #2 and #4 are filled to one-third of their total volume, the discharging system can be started. By adjusting the number of charging and discharging fuel cells in operation, the charging and discharging balance of the system can be controlled, ensuring long-term stable operation.

8. Switching Modes. Currently, no fuel cell stack model has been developed that is fully suitable for unidirectional charging or discharging. Therefore, the fuel cell stack used in this system is a bidirectional charging / discharging stack, and each individual stack described in the invention simultaneously possesses charging and discharging functions. To extend the lifespan of the fuel cell stack and reduce the risk of accidents, the functions of the fuel cell stack can be interchanged by switching the copper drum valve and adjusting the fuel cell stack operating mode after the system has been running for a certain period of time.

9. The vanadium battery system with simultaneous charging and discharging according to claim 2, characterized in that, The charging stack is divided into multiple stack groups, and each stack group consists of a dozen or so electrolytic cells and auxiliary systems. The stack groups are connected in parallel, and the electrolytic cells are connected in series. The discharging stack is similar to the charging stack.

10. The number of fuel cells used for discharging is greater than the number of fuel cells used for charging, and the charging time is the same as the discharging time, which can meet the requirements of continuous and stable operation.

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