IEEE Electrification - December 2021 - 25

Aqueous Battery Systems-
Safety Aspects
Currently, Li-ion batteries are the
dominant electrochemical energy
storage devices on the grid, as such,
they have received the bulk of attention
for hazard mitigation through
codes, standards and safety training.
However, the hazards from other battery
systems, particularly aqueous
based systems, need to be considered
as they are increasingly utilized for
grid applications. Aqueous batteries
are defined as batteries that use
water as their supporting electrolyte.
This broad category includes chemistries
ranging from classic lead-acid to
newly designed rechargeable zincmanganese
oxide with formats spanning
typical cylindrical cells to redox
flow batteries (RFBs). Aqueous batteries
are generally safer than Li-ion systems
given their more stable aqueous
electrolyte versus a flammable organic
electrolyte and active species that are generally relatively
stable transition metals. There are still inherent risks for
aqueous batteries that must be managed. Safety concerns
for aqueous batteries will vary based on their chemistry
and design. The three primary risks are gas evolution,
electrolyte leakage, and thermal runaway.
Gas evolution can occur through different routes that
It is important to
understand that
many systems are
being designed to
handle a battery cell
fire gracefully and
that after some
cleaning, part
replacement,
inspections, and
testing, these
systems may
continue to operate.
from corrosion resistant materials
that can tolerate the electrolyte for
the lifetime of the system. Additionally,
secondary containment needs to
be put in place to hold any spill or
leaks that may occur. Loss of containment
may occur through buildup
of pressure from gas evolution, or an
explosive reaction from accumulat -
ed gases. Secondary containment
should be able to withstand such
forces, so electrolyte is not leaked
into the surrounding area. An example
of a loss of containment scenario
is shown in Figure 5.
Thermal runaway is a common
risk for lead-acid and other aqueous
batteries like nickel metal hydride and
nickel cadmium systems. RFBs and
zinc-based systems have yet to show
thermal runaway risks but cannot be
definitively ruled out as a possibility.
As with Li-ion systems, thermal runaway
in aqueous batteries involves
depend on the system and how it is operated. In aqueous
systems, hydrogen evolution can occur at all anodes
through electrochemical mechanisms. The rates and occasion
of hydrogen generation changes with the chemistry
and operational conditions. Hydrogen presents an explosion
and flame hazard if it builds up beyond 4% of the gas
mix. However, there are well-documented ways to manage
the risk since hydrogen evolution in lead-acid batteries
has been studied for decades. Other gases can evolve
depending on the system in question. Most notably,
mixed-acid electrolyte vanadium RFB systems (electrolyte
contains sulfuric and hydrochloric acid) can produce Cl2
gas and zinc-bromine hybrid RFBs can produce Br2 gas
from their respective catholytes. Both gases are highly
toxic and corrosive, presenting new safety considerations.
For example, Cl2 gas has a 60-min exposure limit of 3 ppm
and an exothermic reaction with H2 gas that can be initiated
by a minimal ignition source.
The risk of electrolyte leakage depends on the system.
So-called starved systems have minimal amounts
of electrolyte, reducing the leak risk, but flooded batteries
and RFBs contain significant amounts of electrolyte
that can leak. The electrolyte for these systems are often
highly concentrated acids or bases and very caustic.
Consequently, primary containment needs to be made
excessive heat generation, gas evolution, and even explosions.
Due to their more stable electrolyte and lower energy
density, these events are typically less energetic than in
Li-ion systems, but they still can pose significant risks. A
recent NFPA report highlighted 14 significant events from
stationary lead-acid systems since 1993 that resulted in
damage to the surroundings, significant injury to personnel,
and one notable case where a battery fire ignited a
methane pocket in a mine that led to 13 deaths
For all of the risks described previously, the scale and
system design will determine the potential impact of an
event. In a large bank of lead-acid or other nonflow aqueous
batteries, thermal runaway may be able to ignite other
cells, like how Li-ion thermal runaway can propagate.
Adding more cells to the system increases the potential
size of the resulting fire or electrolyte leak. Grid-scale RFBs
consist of large tanks of electrolyte (tens of thousands of
gallons per MWh) connected to an electrode stack. Consequently,
significant amounts of gas can be generated
quickly in these systems, which can cause an explosive
reaction leading to loss of electrolyte containment. The
resulting electrolyte spill could involve thousands of gallons
of hazardous material.
Nonbattery ESS-Safety Aspects
Basic safety evaluations are still critical even from nonbattery
energy storage technologies that have been around
for decades. Here, we consider pumped-storage hydropower
(PSH) and flywheels as examples. PSH plants store
and generate energy by moving water between two reservoirs
at two different elevations. During times of low electricity
demand, excess energy is used to pump water to an
IEEE Electrification Magazine / DECEMBER 2021
25
IEEE Electrification - December 2021

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