IEEE Electrification - December 2021 - 20

battery system designs along with the state-of-the-art
hazard mitigation methods. We also summarize the development
of codes and standards to ensure safety and discuss
best practices for emergency preparedness.
Introduction and Background
Energy storage inherently has hazards associated with
inadvertent release of stored energy. This is true whether
the energy is stored as traditional hydrocarbon fuels, subject
to fires, as pumped hydroelectric storage, subject to
structural failures, or as electrochemical energy storage,
subject to thermal runaway and gas evolution. In the case
of hydrocarbon fuels, we are familiar with the hazards of
fuel fires, and the industry has developed safety systems
to address critical issues through design and globally
adopted safety standards. In the case of pumped hydro,
we reduced the possibility of dam failures through robust
engineering-designs and construction methods. However,
in the case of electrochemical energy storage, the use at
grid-scale is relatively new and safety issues are still
being analyzed.
With a growing need for energy storage in application
markets, such as reliability and capacity firming, the
pace of deployments of energy storage systems for grid
applications is accelerating. Deployment of BESSs as
large as 100 MWh is becoming common and the first
gigawatt hour-sized BESS plants are beginning to be connected
to the bulk electric grid, including a recently commissioned
system at Moss Landing near Monterey,
California. Hybrid systems such as solar + storage are
being built to replace other traditional generation assets
such as natural gas peaker plants. The majority of new
energy storage installations are using Li-ion batteries due
to their availability at large volumes and continued cost
reductions. In recent years, a lot of new manufacturing
capacity for Li-ion batteries has come online, primarily
geared for electric vehicles. This new manufacturing
capacity has led to significant cost reductions and helped
make Li-ion BESSs the technology of choice for grid energy
storage systems.
It is important to note that electrochemical systems
with well-established manufacturing and quality-control
systems have very low failure rates, well below one in one
million, but large numbers of cells will likely be employed
in grid-scale energy storage systems. For example, a typical
18650 Li-ion cell has a capacity of 10 Wh, and to build a
1-MWh storage plant requires almost a hundred thousand
cells. Statistically, there is a significant chance of a single
cell failure occurring in such large systems. The number of
cells and system-level components associated with largescale
energy storage systems leads to possibilities of a local
failure that is much greater than one in a million. Thus,
there is a need to provide system-level redundancies.
For large-scale energy storage with thousands of individual
cells arranged in packs and modules, there is an
additional concern with the propagation of a failure
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IEEE Electrification Magazine / DECEMBER 2021
event from one cell to multiple cells and possibly
through the entire system. This can occur through either
electrical or thermal transfer of energy. If a cell fails in a
manner that allows it to discharge its stored energy
through a short circuit, then electrical connectivity might
allow a much larger number of cells to also release energy.
This is particularly an issue for cells connected in parallel
to allow greater transfers of power. If a single cell
failure generates large quantities of heat, thermal propagation
of failure is possible as well. In these ways a single
point of failure can lead to a cascading series of failures
throughout the system.
With growing numbers of large battery fires involving
mostly Li-ion BESSs, there is a growing concern from
industry experts and regulators about potential safety
issues. Until recently, safety of energy storage systems
has been looked at almost exclusively through the narrow
lens of cell-level failures. While a greater understanding
of cell-level failures has been critical to the success of
rechargeable batteries in consumer electronics, the complexity
associated with the scale of energy in grid applications
necessitates consideration of a wider range of
system-level issues related to power electronics, to air
conditioning systems, and to fire suppression for large
energy storage systems and the surrounding physical
infrastructure. In addition, the kinetic behavior of cell-level
failure must consider the probability of propagation
and thermal runaway that is not indicated in smaller batteries
of the same chemistry.
The need for a comprehensive system-level approach
to safety, training, and preparation for first responders to
handle BESS failures is considered a priority by National
Fire Protection Association (NFPA), International Association
of Fire Fighters (IAFF) and other agencies representing
first responders. Recent examples of thermal runaway
incidents with grid-scale storage facilities and electric
vehicles show that first responders are not adequately
prepared to handle these incidents.
Complicating matters further are new types of electrochemical
energy storage technologies being connected to
the grid. These include Li-ion batteries with high energy
anode materials, Li-metal anodes, nonflammable electrolytes;
flow batteries using a variety of electrolyte materials,
advanced aqueous batteries including advanced lead-acid,
alkaline batteries like rechargeable zinc batteries, and
molten salt batteries. Each system has its own hazards
that vary with chemistry and design that must be managed.
The industry likely does not have an adequate
understanding of the operational safety of these systems.
Grid-scale systems are inherently complex, requiring not
only a large battery but also sophisticated power electronics,
energy management systems, thermal management
systems, transformers, and grid interconnection. Integration
of new battery types with these components is still in
progress, creating many potential points of failure. One
has to ask the question: what type of laboratory testing is
IEEE Electrification - December 2021

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