IEEE Electrification - December 2021 - 33

the final installation-level test is performed with fire suppression
and other safety measures in the design to
assess the effectiveness of preventing unit-to-unit propagation
and preventing or reducing deflagration risks.
Conducting tests according to UL 9540 A can be timeconsuming
and expensive, particularly if installation-level
testing is required. However, the wealth of information
provided, including video recordings, is invaluable in
allowing the AHJ to make informed decisions on proposed
installations and for fire protection engineers to design
fire-suppression and/or deflagration systems.
Influence of Safety Standards on Battery Design
As ESS deployments have shifted to multihour applications
such as the replacement of peaking generation, a
system's footprint has become progressively more important,
causing containerized Li-ion battery systems to be
designed with increasing energy density. Figure 4 shows
the rise in energy density over time for containerized Liion
battery systems offered by one company: Saft. All of its
systems use Li-ion chemistry based on nickel oxides,
including Li nickel cobalt aluminum oxide and lithium
nickel cobalt manganese oxide (NMC).
The earlier container designs included relatively small
modules that incorporated thermal barriers between large
cylindrical cells, and those barriers were effective at preventing
cell-to-cell propagation of TR. A standard FSS
using a clean agent, Novec 1230, is installed to handle any
electrical fires that may occur, and its cooling effect provides
an additional safety margin against propagation. For
the 2020 container design, the installed energy is maximized
using close-packed pouch cells in large modules.
This arrangement makes it difficult to avoid propagation
of TR. A clean-agent FSS extinguishes any visible flame
but is unable to eliminate ongoing propagation deep within
the modules. In this case, the primary FSS is supplemented
by a water FSS.
The easiest way to implement a water FSS is to have
a dry-pipe system with a connection for the fire
department and to provide firefighters with status information
and a decision tree for when to use the system.
Ideally, the connection point would be located a safe distance
away, although that may create issues for service
access to the containers. Moreover, a manually actuated
system may be problematic from an effectiveness standpoint
as it requires human intervention within the period
between actuation of the clean-agent FSS and reignition
because of ongoing propagation. Testing on the 2020 container
design indicated a maximum interval of 90 min for
the fire department to arrive on site, assess the situation,
consult documentation, and activate the water FSS. A
manual system would also present problems with certification.
An automatic system would meet certification
needs but would require a high-volume source of water to
be available at each container, which can present problems
at remote sites.
Considering such issues, the newer codes and standards
are having a significant influence on battery system
design for ESSs. The cells that use Li-iron phosphate (LFP)
chemistry, which is more resistant to TR, are becoming the
standard, despite their lower energy density. (The adoption
of LFP is also helped by lower cost.) Module manufacturers
are taking extra steps to limit cell-to-cell
propagation of TR within their units. Beyond limiting
propagation, battery enclosures must be designed to avoid
or mitigate the buildup of potentially explosive gases
released during a TR event.
One of the challenges in UL9540A fire testing is the
interpretation of the performance requirements of the
module-, unit-, and installation-level tests. The intention
is to understand the performance under propagating TR
conditions, yet different chemistries respond differently
when in TR, and the test itself may introduce some variability
in battery response compared to field conditions,
depending on the way in which TR is initiated.
For example, most NMC chemistries produce sparking
and often flames when TR is initiated using an external
heater, as in the UL 9450 A test. These flames can
(a)
(b)
(c)
Figure 4. The evolving energy density in 20-ft containerized systems. (Source: Saft America Inc.; used with permission.) (a) 2012: 0.6 MWh, (b)
2017: 1.2 MWh, and (c) 2020: 2.5 MWh.
IEEE Electrification Magazine / DECEMBER 2021
33

IEEE Electrification - December 2021

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