Safety of Grid Scale Lithium-ion Battery Energy Storage Systems

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– 20 – June 5, 2021
stored energy equivalent to twenty Tesla Model S cars, it is clear to see that a much greater amount of water would be needed.
The actual amount of water required will depend on the energy storage capacity per cabin which, in the case of Sunnica, is still unstated. Some simple estimates are, however, made below. The requirements suggested to date by the Local Authorities for the Sunnica installation are completely inadequate and, if not addressed, would leave Suffolk FRS without the means to control a major BESS “fire”.
Taking a storage capacity of 10 MWh in just one of the Sunnica cabins (see Appendix 1), a complete thermal runaway accident in such a BESS would release that stored electrochemical energy, plus an indeterminate quantity of heat from combustion of hydrocarbon polymer materials or inflammable gases evolved from them. Such Total Heat Release may be up to twenty times the amount of the stored electrochemical energy in the BESS [11].
The thermal capacity of water is 4.2 kJ kg-1 K-1 or in kWh terms, about 1.17 kWh m-3 K-1. If heated from 25 °C to boiling point about 87.8 kWh m-3 of thermal energy is required.
Hence the water volume required to absorb 10 MWh of released energy without boiling is about 114 m3 or 30,000 US gallons, the same amount as required in practice to control a fire in a single Tesla Model S car with a mere 100 kWh battery, 100 times smaller than a 10 MWh BESS.
The quantity suggested by the Local Authorities’ joint response is 228 m3 (1900 L min-1 for 2 hours), twice the above estimate, which would naively be sufficient for a 20 MWh BESS fire. However, from the experience of recent BEV fires, it could be insufficient by a factor of 100.
No such calculations were presented in the Examination of the 700MWh Cleve Hill BESS [6].
2. “Clean agent” fire suppression systems are a common fire suppression system in BESS, but are totally ineffective to stop “thermal runaway” accidents. The McMicken explosion was an object lesson in this: the installed “clean agent” system operated correctly, as designed, on detection of a hot fault in the cabin [8]. There was no malfunction in the fire suppression system. But it was completely useless because the problem was not a conventional fuel-air fire, it was a thermal runaway event. Only water will serve in thermal runaway.
Indeed in the McMicken explosion the “Novec 1230” clean agent arguably contributed to the explosion by creating a stratified atmosphere with an air/Novec 1230 mixture at the bottom and inflammable gases accumulating at the cabin top.
The most probable cause of the explosion was mixing caused by the opening of the door by first responders. The explosive mixture contacted hot surfaces and ignited [8].
3. A further recommendation of the Hill/DNV report [8] into the McMicken explosion is for a means of controlled venting of inflammable gases before first responders attempt access. In the Local Authority response to the Sunnica consultation, ventilation is listed as a BESS requirement [22] but the reason given, bizarrely, is “to control the temperature” – at which ventilation or air- conditioning (also listed) would be totally ineffective, lacking any significant thermal capacity.
The critical reason for controlled ventilation is the removal of inflammable gases before an explosive mixture forms. Deflagration panels (to decrease the pressure of explosions that do occur) are also recommended.