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Analysis of the Differences Between “Open-Door Burning” and “Closed-Door Burning” in Large-Scale Fire Tests for Energy Storage
In recent years, as global energy storage capacity has surpassed 500 GWh, energy storage fire accidents have become increasingly frequent. In 2024 alone, nearly 100 such incidents occurred, prompting heightened global concern over the safety of energy storage systems. Some regulatory bodies have proposed conducting large-scale fire tests on energy storage systems to assess how fire spreads within and between units when a single unit is fully engulfed in flames.
In this context, large-scale fire tests have become the ultimate litmus test for verifying the safety of energy storage systems. Recently, companies such as SUNGROW, e-STORAGE, REPT BATTERO, and HiTHIUM have publicly conducted “burn chamber” tests, bringing this critical phase of large-scale fire testing increasingly into the public eye.
Large-scale fire tests are a rigorous “final exam” for energy storage systems, and their importance is self-evident. They are not only a key means of testing the safety of energy storage systems in extreme fire scenarios, but also an important driving force for improving industry safety standards and ensuring the reliable operation of energy storage facilities. It is precisely for this reason that many manufacturers are competing to conduct such tests.
However, upon closer examination of these companies' testing processes, it becomes clear that their technical approaches, testing metrics, certification standards, and objectives vary significantly. Nevertheless, large-scale fire tests can generally be categorized into two main types: “open-door burning” and “closed-door burning.”
“Closed-Door Burn”
The “closed-door burn” refers to a large-scale fire test conducted under closed container door conditions, triggered by thermal runaway, to verify the fire resistance of energy storage systems equipped with venting plates or ventilation devices. This test is validated through simulation and modeling in accordance with standards such as NFPA 68 and NFPA 69.
“Closed-Door Burn” is based on a series of premises and assumptions. It relies on pre-set ventilation systems and explosion venting devices to ensure the timely release of flammable gases and control the spread of fire. In this scheme, the calculation of explosion venting area (NFPA 68) is related to the estimated maximum flammable gas generation rate and composition, and also depends on the normal operation of the ventilation system. The effectiveness of the ventilation system is also constrained by the actual gas generation rate and distribution, which is related to the number of modules triggering thermal runaway simultaneously within the energy storage system. However, under different actual operating conditions, factors such as the number of modules triggering thermal runaway and the location of the trigger source can lead to discrepancies between the actual gas generation rate and peak pressure and the design simulation conditions, thereby causing deviations in the calculated explosion venting area.
Furthermore, in the complex real-world operating conditions, extreme situations frequently occur. If the energy storage system experiences multiple failures rather than a single failure, this can impact the normal operation of ventilation and explosion venting. Under such circumstances, the test results will be highly uncertain.
“Open Door Burn”
“Open Door Burn” refers to a test conducted in an extreme fire scenario with ample oxygen supply, where the container door is actively opened (or even the explosion vent panel is removed), safety designs such as explosion venting and ventilation are abandoned, and the test relies solely on the passive fire protection design of the energy storage system. Its core objective is to verify the energy storage system's ability to autonomously resist fires and prevent thermal runaway propagation without external intervention, simulating the system's safety under the most severe fire conditions.
Taking HiTHIUM's testing as an example, the test was conducted under four extreme conditions: “Open-Door Burning + 15cm Extreme Spacing + Active Fire Suppression Shutdown + 100% SOC Fully Charged State,” successfully validating the energy storage system's safety protection capabilities under extreme combustion scenarios. Test results showed that after prolonged and intense combustion, the prefabricated container did not experience structural deformation or collapse, and adjacent containers also did not experience combustion or thermal runaway propagation. This achievement has undoubtedly set a new benchmark for large-scale fire tests of energy storage systems worldwide, effectively demonstrating the feasibility of the “open-door burning” approach in verifying safety performance under the most extreme conditions.
Fire Safety Strategies
“Open-door burning” and “closed-door burning” represent two important approaches and technical pathways for verifying the fire safety of energy storage systems.
The “open-door burning” approach involves testing under the most stringent conditions, relying entirely on the energy storage system's own passive fire protection design to withstand a fire, and can be considered a comprehensive safety solution.
The “closed-door burning” approach embodies the “controlled intervention” philosophy: it relies on engineering measures (ventilation, pressure relief, explosion venting, etc.) to guide and limit the development of a fire. It is a comprehensive solution that aligns with design expectations.
These two approaches are fundamentally different. “Closed-door burning” emphasizes design coordination, while “open-door burning” emphasizes the “extreme endurance of the product itself.” Each has its own advantages and disadvantages, as well as its own application scenarios.
Whether it is “closed-door burning” or “open-door burning,” the emergence of an absolutely safe testing scheme would undoubtedly be a major benefit for regulatory authorities. It would provide a solid basis for establishing unified and stringent safety standards for the industry, helping to standardize market order and promote the development of the energy storage industry toward greater safety and reliability.
From a current technical perspective, the “open-door burn” testing scheme, due to its simplicity and clarity of testing conditions, can more directly reflect the inherent safety performance of energy storage systems under extreme fire conditions, providing more persuasive data support for the safety assessment of energy storage systems.
Through the above analysis by the editor of the two large-scale fire test schemes, “open-door burning” and “closed-door burning,” it is evident that the “open-door burning” scheme has a clear advantage in terms of the rigor of the test and its ability to demonstrate the safety performance of the energy storage system. The details and assumptions of the operating conditions and boundary conditions in the “closed-door burning” scheme can significantly influence the results. Of course, in terms of testing difficulty, the “open-door burn” is harder to pass than the “closed-door burn”; however, the “open-door burn” is more explicit and straightforward, making its results more reliable and thus adopted by regulatory authorities.
There is no optimal solution for large-scale fire testing schemes. As the energy storage industry increasingly prioritizes fire safety, both the “open-door burn” and “closed-door burn” testing schemes have sparked extensive discussion within the industry. Regardless of which path the industry ultimately chooses—or whether both are adopted—their common goal is to enhance the inherent safety level of energy storage systems.
Manufacturers can choose based on product design and requirements. If you have any needs related to large-scale fire testing for energy storage, whether it's a customized combustion scheme or seeking optimization of passive fire protection design, please feel free to contact us. We will provide you with the highest quality and most professional support. We look forward to collaborating with you to contribute to the safe development of the energy storage industry.