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Proper Sizing of Battery Energy Storage Systems

Energiavarastojen Mitoitus

Expert Insights from Tiina Turpeinen, Business Area Manager, Energy Storages, Celltech Oy

The Increasing Role of Battery Storage in Renewable Energy

Companies in the renewable energy industry are increasingly exploring integrating battery energy storage systems (BESS) into their mix of wind, solar, and hydroelectric power generation. These versatile storage solutions, mostly based on lithium-ion technology, serve multiple purposes. They help balance power consumption and production within the grid, bridge production shortfalls, act as emergency power sources, and safeguard hydroelectric plant turbines during startup. Tiina Turpeinen emphasises, “To ensure that an energy storage system meets its intended purposes as efficiently as possible, precise sizing is critical.”

Key Questions for Defining a Battery Energy Storage System

“When planning for a BESS, it’s vital to contemplate the desired services from the battery and the amount of energy intended for storage,” says Tiina Turpeinen. “Is the goal to support multiple uses or to participate solely in reserve markets? Do we want to leave room for adding new services in the future? What energy capacity is expected from the battery system towards the end of its lifespan?” Turpeinen lists some of the key questions.

Oversizing Based on Depth-of-Discharge

The more services expected from a BESS, the greater the capacity needs to be to accommodate simultaneous demands. Turpeinen points out, “Accounting for the inevitable ageing of batteries due to calendar time and usage, which reduces their energy output over time, is essential in planning. Consider the desired end-of-life scenario and size the battery system accordingly.” Additionally, to extend the life of energy storage systems, batteries typically are not discharged from 100% to 0% but operate within a range, such as between 10% and 90% state-of-charge levels. Therefore, batteries must be accordingly oversized for 80% Depth-of-Discharge (DoD).

The Impact of Desired Operational Duration

The required size of a BESS also depends on how long the system is expected to operate before needing a recharge. “How long should the backup power last? A system designed for two hours of use without charging is physically significantly larger than one intended for an hour’s use, requiring more space and incurring higher costs,” Turpeinen explains.

The number of Charge-Discharge Cycles

Another critical consideration is the number of daily or annual charge-discharge cycles the battery is expected to perform. Partial versus complete cycles affect the total energy transferred and, consequently, the wear on the battery. Charging a lithium-ion battery involves the movement of lithium ions between the cathode and anode. The anode (negative electrode) interacts with the Li+ ions, resulting in potential microcracks of the anode structure and consumption of Li+ ions and the electrolyte over time. “The number of cycles significantly influences the battery’s lifespan and the required nominal energy capacity,” Turpeinen adds.

For instance, a two-hour battery system designed for one daily cycle expected to deliver 50 MWh at the end of its life must have a nominal energy capacity of over 70 MWh. A similarly sized system planned for two daily cycles, starting with an energy capacity of 50 MWh, would need to be sized up to approximately 85 MWh. “Since the battery system represents the most significant cost component of energy storage and its characteristics also influence the choice of inverter equipment between the battery system and the grid, accurate sizing is paramount. It ensures the battery energy storage system’s functionality, suitability, and safety,” Turpeinen concludes.

Embracing the Future with optimally sized BESS

In summary, accurately sizing battery energy storage systems is not only a technical requirement but also essential for successfully integrating renewable energy sources into our power grids.

Dealing with the intricacies of energy demand, battery lifespan, and operational capabilities demands a forward-thinking strategy and a deep understanding of the changing energy landscape. By prioritising precise sizing and flexibility in design, we can ensure that these critical systems not only meet the energy requirements of today but are also capable of adapting to and thriving in the energy paradigms of the future. From this perspective, the role of battery energy storage goes beyond mere functionality, becoming a cornerstone of our transition to a more resilient, efficient, and sustainable energy ecosystem.