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ENERGY STORAGE
Fire Suppression Systems
This section explores three common fire
suppression systems for outdoor ESS
enclosures: automatic sprinklers, water
mist, and gaseous suppression systems.
Their respective advantages and limitations
will be compared.
Automatic Sprinkler System. Sprinkler
systems are the preferred method
for protecting ESS due to their superior
cooling capabilities, low cost, human
safety, and environmental friendliness.
While the rack frame may obstruct direct
water flow to the cells, sprinklers
can still effectively prevent a fire from
spreading to adjacent racks. However,
it is important to consider that sprinklers
may cause water damage following
discharge.
Lithium-iron-phosphate (LFP) and
lithium nickel manganese cobalt oxide
(NMC) are two common types of lithium-ion
batteries. According to a June
2019 research report titled " Development
of Sprinkler Protection Guidance
for Lithium-Ion Based Energy Storage
Systems " by FM Global, the minimum
sprinkler density required for ESS is
12.2 liters per minute per square meter
(Lpm/m2), or 0.3 gallons per minute per
square foot (gpm/ft2), for both LFP and
NMC batteries.
The report indicates that ESS systems
using NMC batteries pose a
greater fire risk than those with LFP
batteries, and it provides guidelines
for the required separation between
racks and the necessary sprinkler discharge
duration. For LFP batteries with
sprinkler protection, the minimum separation
is 0.9 m (3 ft.) from non-combustible
objects and 1.5 m (5 ft.) from
combustible objects. The discharge
duration must be at least 90 minutes.
Meanwhile, for NMC batteries with
sprinkler protection, the minimum
separation is 1.8 m (6 ft.) from noncombustible
objects and 2.7 m (9 ft.)
from combustible objects. Discharge
duration for NMC batteries is 45 minutes
multiplied by the number of adjacent
ESS racks, that is, racks with less
than the recommended separation for
combustible objects, but no less than
90 minutes.
FM DS 5-33 adopts the stricter NMC
requirements for separation distances
and
discharge
durations,
regardless
of whether LFP or NMC batteries are
used. FM-approved fire barriers can be
installed between racks to reduce water
demand. NFPA 855 specifies the same
sprinkler density as FM DS 5-33 but alDecember
2024 | POWER
lows for the use of an alternate density
if supported by large-scale fire testing in
accordance with the UL 9540A Installation
Level Test, Method 1.
For a standard 40-foot enclosure with
dimensions of 12.2 m (length) x 2.44 m
(width), the minimum required water
flowrate and capacity are calculated as
follows. Flowrate = 12.2 m x 2.44 m x
12.2 Lpm/m2 = 363.2 Lpm. Capacity =
363.2 Lpm x 90 min = 32,688 L (32.688
m3).
Water Mist System. A water mist
system, similar to sprinklers, uses water
but at high pressure to create a fine
mist with droplets smaller than 100
microns in diameter. This mist vaporizes
into steam, expanding more than
1,700 times in volume. Water mist suppresses
fires by cooling the cells, reducing
oxygen concentration, lowering
radiant heat transfer, and inhibiting the
exothermic reactions associated with
thermal runaway.
Water mist systems are categorized
by pressure into three types: Low pressure
(less than 12.1 bar [175 psi]), intermediate
pressure (12.1 bar to 34.5 bar
[175 psi to 500 psi]), and high pressure
(greater than 34.5 bar [500 psi]). NFPA
855 requires compliance with NFPA
750 for water mist systems. According
to NFPA 750, the system must provide
sufficient flow to meet the nozzle's discharge
density rating, with a minimum
discharge duration of 30 minutes. For
typical mist nozzles, the required water
density ranges from 1.0 to 2.0 Lpm/m2.
For a standard 40-foot enclosure, the
estimated minimum flowrate and capacity
are calculated as follows. Flowrate =
12.2 m x 2.44 m x 2.0 Lpm/m2 = 59.5
Lpm. Capacity = 59.5 Lpm x 30 min =
1,785 L (1.785 m3)
Please note that these figures represent
minimum requirements. The
actual flowrate and capacity should be
verified through large-scale testing to
ensure that battery temperatures are
reduced below the auto-ignition point
and the threshold for triggering thermal
runaway.
Compared to sprinkler systems, water
mist systems offer several advantages,
including lower water consumption and
smaller pipe sizes, which facilitate easier
installation in confined spaces. Additionally,
the fine mist reduces the risk of
equipment damage.
Gaseous Suppression System.
Common agents used in gaseous suppression
systems for ESS include inert
gases and FK-5-1-12 (NOVEC 1230).
www.powermag.com
Inert gases function by lowering the
oxygen concentration to suffocate the
fire and are environmentally friendly.
In contrast, FK-5-1-12 interrupts the
fire's chemical reaction rapidly. Both
agents minimize damage to electrical
equipment since they do not pose a
risk of short circuits, leave no residue,
and require minimal cleanup. To reduce
downtime due to replenishment delays,
the gas reservoir should be sized
to accommodate two discharges. Ensuring
proper airtightness in the enclosure
is crucial to prevent gas leakage.
NFPA 855 mandates compliance with
NFPA 2001 for the use of inert gases
and FK-5-1-12.
However, FM DS 5-33 does not
recommend gaseous suppression
systems for ESS due to their limited effectiveness,
lack of cooling, and potential
for reignition. Therefore, a combined
suppression system may be employed.
In the initial stage of temperature rise,
the electrical circuit should be cut off,
and the gaseous suppression system
activated to resist thermal runaway. If
this fails, a water-based suppression
system can be activated to fully extinguish
the fire.
Weighing the Pros and Cons
This study presents key findings and
recommendations for selecting fire suppression
systems for ESS in outdoor enclosures.
FM DS 5-33 and NFPA 855 are
recognized standards for the design and
installation of these systems.
A thorough analysis of three common
fire suppression systems was
conducted. The research suggests the
system with the highest suppression
effectiveness typically uses the most
water and may cause the greatest damage
to equipment. Still, the pros of these
systems often outweigh the cons. It is
important to note, however, that these
rankings are general; each system's
suppression effectiveness and water requirements
may vary based on individual
manufacturer designs.
To enhance safety, enclosures
should be equipped with vents to facilitate
gas release and prevent accumulation.
Since deflagration can
occur even in the presence of fire
suppression systems, the installation
of deflagration vents at the top of the
enclosure, compliant with NFPA 68,
can help mitigate the risk of enclosure
rupture during such events. ■
-Yung-Shun Yen and Kuan-Hung Chen
are industry experts with CTCI Corp.
21
http://www.powermag.com
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