UL 9540A TEST METHOD FOR BATTERY ENERGY STORAGE SYSTEM

What is the UL 9540A Test Method?

UL 9540A is a standard for the safety of energy storage systems and equipment and was developed by UL as a test method for evaluating thermal runaway fire propagation in battery energy storage systems and is widely recognized by the relevant authorities.

Authoritative US industry codes such as the American Electrical Code (Chapter 706), the US Residential Code (Chapter R327), the US International Fire Code (Energy Storage Chapter), and the National Fire Protection Agency's NFPA 855 standard all impose UL9540A listing requirements on energy storage systems.

The results of the UL 9540A test can provide the buyer of an energy storage system, as well as local architects and fire departments, with a safety assessment in the following:

• Installation guidelines for energy storage systems.

• Ventilation requirements for energy storage systems.

• Effectiveness of fire protection systems.

• Strategies for the fire department.

How to Conduct a UL 9540A Test.

UL 9540A test method consists of four stages of testing, which are the cell level test, module level test, unit level test, and installation level test.

The Cell Level Test is carried out on the smallest individual battery cell of the BESS. An effective and repeatable method is used to force a battery cell into a thermal runaway in a pressure vessel.

During thermal runaway, the gasses generated by the cell shall be gathered. Gasses collected during thermal runaway are analyzed for composition, and the synthetically replicated gases are used for the lower flammability limit test (LFL), maximum explosion pressure test (Pmax), and burning velocity test.

The Module Level Test is carried out on the battery module under a smoke collection hood. The numerous cells within the module are forced into a thermal runaway in the same methodology used in the cell level test. The thermal runaway cells shall create a condition of cell-to-cell propagation within the module.

During the module thermal runaway, the generated gas shall be collected by the hood and sampled for analysis. The analysis includes chemical heat release measurement (based on the oxygen consumption principle), FTIR spectrometer for vent gas composition analysis, FID hydrocarbon content analysis, hydrogen gas content analysis, and smoke release measurement.

The Unit Level Test is conducted with BESS units installed as described in the manufacturer’s instructions and the test configuration should correspond to the design of the BESS. For the indoor floor-mounted BESS units, an initialing BESS unit should be placed with the target BESS unit and the instrumented wall around it as per the manufacturer’s instruction. The initiating BESS unit shall contain components representative of a BESS unit in a complete installation, and a hood is mounted above the initiating BESS to collect the combustion gas for further analysis. The initiating BESS unit should be forced into a thermal runaway in the same methodology used in the module level test that creates a condition of the module to module propagation within the BESS unit.

During the BESS unit thermal runaway, the generated gas shall be collected by the hood and sampled for analysis. The analysis includes convective heat release measurement, chemical heat release measurement (based on the oxygen consumption principle), FTIR spectrometer for vent gas composition analysis, FID hydrocarbon content analysis, smoke release measurement, and instrumented wall measurement of temperature and heat flux.

The Installation Level Test is used to assess the effectiveness of the fire and explosion mitigation methods for the BESS in its intended installation, including the effectiveness of sprinklers or fire protection plans. The installation level testing does not apply to residential use BESS. The installation level test should be conducted like an unit level test but with a sprinkler system and flame indicator above the initiating BESS unit.

UL 9540A is a graduated test method, from the smallest cell unit to the complete BESS system in the installation form. Once the present level of testing has been complied with, the next level of testing is not required.

Also, test methods and results are also continuous, e.g. the thermal runaway parameters from the cell test will be used as thermal runaway parameters for the module test, and the thermal runaway parameters from the module test will be used as thermal runaway parameters for the unit and installation level test. This ensures the repeatability and reproducibility of tests and at the same time significantly improves the safety of the tests.

01 Cell Level Test

The Cell Level Test is applicable to the battery cell used in a battery energy storage system (BESS), the thermal runaway of the battery cell is forced in a repeatable way in a pressure vessel. The method & parameters of the thermal runaway of the battery cell will be applied to the module level test.

Collect the gas produced by the thermal runaway of the battery cell and analyze the gas composition and content using Gas Chromatography (GC).

Use the synthetically replicated gas to conduct the lower flammability limit test (LFL test) at the ambient temperature & at the cell venting temperature, maximum explosion pressure test (Pmax test), and burning velocity test.

The cell level test requires the following 5 sets of equipment: pressure vessel, GC, gas flammability limit tester, explosion pressure tester, and burning velocity tester.

The corresponding reported data of the equipment are listed in Table 1.1.

Table 1.1 Equipment used in UL 9540A cell level test

The performance criteria of cell level test are listed in Table 1.2.

Table 1.2 Performance criteria of UL 9540A cell level test

Equipment Involved

02 Module Level Test

The Module Level Test is carried out on the base of the cell level test and is applicable to individual complete modules used in BESS.

The test is carried out under a smoke collection hood and uses the same thermal runaway method as the cell level test.

The test uses continuous online analysis methods to obtain the data of heat release, smoke release, gas composition and content, hydrocarbon volume, and hydrogen volume from the thermal runaway of the module, and mass measurement before and after the test.

The module level test requires the following 4 sets of equipment: heat release calorimetry, FTIR spectrometer, hydrocarbon content analyzer (FID), and hydrogen gas analyzer.

The corresponding reported data of the equipment are listed in Table 2.1.

Table 2.1 Equipment used in UL 9540A module level test

The performance criteria of module level test are listed in Table 2.2.

Table 2.2 Performance criteria of UL 9540A module level test

03 Unit Level Test

The Unit Level Test is carried out on the base of the module level test and is applicable to a complete BESS unit.

The tests are divided into eight conditions according to the different usage scenarios of BESS. The test configurations, data collected, and performance requirements are different for different scenarios. It needs to be carried out in accordance with the manufacturer's guidelines to select the appropriate test configuration.

Take the example of an indoor floor mounted used BESS, the BESS system should be installed under a smoke collection hood. Uses continuous online analysis to obtain data of heat release, smoke release, gas composition and content, and hydrocarbon volume, from the thermal runaway of the BESS unit. Plus an instrumented wall to obtain the temperature and heat flux.

The unit level test is developed on the module level test with the following changes:

1) Remove mass measurement.

2) Increase convective heat release measurement.

3) Increase instrumented wall (temperature & heat flux measurement).

4) Remove the hydrogen content measurement.

The corresponding reported data of the equipment are listed in Table 3.1.

Table 3.1 Equipment used in UL 9540A unit level test

The performance criteria of unit level test are listed in Table 3.2.

04 Installation Level Test

The Installation Level Test is appropriate for units intended for indoor & non-residential use and assesses the effectiveness of sprinklers or fire protection plans.

The test is carried out as unit level test but with the following changes (for test method 1) :

1) Increase a sprinkler system.

2) Increase a cable tray as the flame indicator.

3) Sampling port on the ceiling instead of the exhaust hood.

4) Remove heat release measurement, and smoke release measurement.

5) Increase hydrogen content measurement.

The corresponding reported data of the equipment are listed in Table 4.1.

Table 4.1 Equipment used in UL 9540A installation level test

The performance criteria of unit level test are listed in Table 4.2.

Summary

UL 9540A provides a systematic test method for evaluating the safety of a battery energy storage system in the event of a thermal runaway happened.

The tests are conducted at different levels to determine the safety of the energy storage system and provide lots of data for the manufacturer to improve the energy storage system, for the user to configure the appropriate fire protection system, and for the fire department to develop suitable strategies.