Safety of Grid Scale Lithium-ion Battery Energy Storage Systems

Page | 030

– 29 – June 5, 2021
Appendix 3: Shortcomings of Existing Engineering Standards for Li-ion BESS
The July 2020 report for the Arizona Public Service by Dr D Hill [8] provides a comprehensive discussion of existing engineering standards relating to BESS, and how they are inadequate to address the known hazards of “thermal runaway” incidents in Li-ion BESS. This was the failure mode leading to the explosion at McMicken, Arizona.
Unfortunately, when the UK’s first “mega-scale” solar plant and battery storage site was granted approval in May 2020, this paper had not been published. The Cleve Hill solar developers cited one standard, UL 9540A [3]. This is also cited by some insurance and risk consultants [20].
It is important to be clear that nothing in UL 9540A addresses thermal runaway, and as a test method standard, it can provide no “safety certification” for Li-ion BESS.
Specific criticisms made in the Hill/DNV report include the following:
1.
2.
3.
UL 1973 allows for the complete destruction of a BESS and the creation of an explosive atmosphere so long as no explosion or external flame is observed. An installation can do all these things but still “pass” UL 1973. At McMicken one rack was completely destroyed and an explosive atmosphere created but no flame or explosion occurred until first-responders opened the cabin door.
UL 9540A is merely a test method for generating data. It does not define any “pass/fail” criteria for interpreting results. Specifically, it does not address cell-to-cell cascading in thermal runaway, nor the evolution of a potentially explosive atmosphere. It does not even prescribe that the cell-to-cell cascading rate be measured.
It allows that thermal runaway may proceed to an entire rack (as at McMicken) and offers testing of fire suppression systems (which operated correctly at McMicken but cannot prevent thermal runaway, and did not prevent an explosion).
Presentation of data generated under UL 9540A to an “AHJ” (Authority Having Jurisdiction) does not translate to a succinct understanding of potential risks.
NFPA 855 [21] does require evaluation of thermal runaway in a single module, array or unit and does acknowledge the need for thermal runaway protection. However, it assigns that role to the Battery Management System (BMS). Yet thermal runaway is an electrochemical reaction that once started cannot be stopped electrically. It is uncontrollable by electronics or switchgear, only by water cooling.
The evolution of engineering and safety standards has not yet incorporated the lessons of experience arising from the McMicken explosion [8] or explosion incidents in the UK like the Liverpool explosion and fire of 15 September 2020 [1]. Compliance with existing standards does not prevent such incidents happening again.
Articles in the industry press3 do now recognise and discuss the problem of thermal runaway but make proposals such as:
Such statements betray a dangerous misunderstanding. Batteries cannot be “shut down”,
except by complete discharge, which cannot be done quickly. Taking cells “out of circuit” is useless;
thermal breakdown and runaway will still occur.
“If off-gases can be detected and batteries shut down before thermal
runaway can begin, it is possible that fire danger can be averted”.
3 https://www.energy-storage.news/blogs/preventing-thermal-runaway-in-lithium-ion-energy-storage-systems