POWER March 2020 - 28

ENERGY STORAGE
Flow Batteries: Energy Storage
Option for a Variety of Uses
Energy storage is becoming increasingly important to the power industry. Lithiumion
battery technology has been implemented in many locations, but flow batteries
offer significant benefits in long-duration usage applications and situations that require
regular cycling throughout the day.
Tisha Scroggin-Wicker, PE, and
Kieran McInerney, PE
T
he chemistry behind flow batteries
has long been proven in the power industry
and most analysts agree they
are ideal for long-duration energy output
with very low degradation of components
within larger, utility-scale deployments.
With life spans reaching up to 30 years,
depending on the electrolyte chemistry,
flow batteries may provide unrivaled cost
certainty versus other emerging storage
technologies on the market. Though flow
batteries currently represent a higher
upfront capital investment than a similarsized
lithium-ion configuration, they become
more competitive when evaluated
on a total cost of ownership over a 20to
30-year lifecycle. Moreover, costs are
dropping for flow batteries as technology
advances and manufacturing efficiencies
are implemented.
In the utility space, flow batteries are
best suited for longer discharge durations
(six hours or more) in megawattscale
power increments. Certain use
cases favor flow batteries over other
storage types. For applications where
multiple charge/discharge cycles are required
each day, flow batteries are available
within milliseconds as loads dictate
and they can quickly recharge from a variety
of available power sources. In fact,
depending on tank configurations, flow
batteries can discharge and recharge simultaneously,
providing power capacity
or voltage support almost indefinitely. Attributes
of flow batteries include:
■ Demonstrated 10,000-plus battery
cycles with little or no loss of storage
capacity.
■ Ramp rates ranging from milliseconds
for discharge if pumps are running, to
a few seconds if pumps are not.
■ Recharge rates for flow batteries also
are reasonably fast.
■ Wide temperature ranges for operation
and standby modes compared to
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lithium-ion options.
■ Little or no fire hazard.
■ Chemistries that pose limited human
health risk due to exposures.
■ Easy scale-up of capacity by adding
electrolyte volume (although that may
involve more tanks and piping).
How Flow Systems Work
Though there are dozens of different
types of flow batteries, only about 10 to
12 specific chemistries appear ready for
commercial applications. All operate on
the same basic principle of incorporating
liquid electrolyte to function as a source
of direct current (DC) electricity that runs
through an inverter for conversion to alternating
current (AC) power.
In a redox flow battery, catholyte and
anolyte are stored in separate tanks, and
pumps are used to circulate the fluids
into a stack with electrodes separated by
a thin membrane. This membrane permits
ion exchange between the anolyte
and catholyte to produce electricity. The
power produced is dependent on the
surface area of the electrodes, while the
storage duration is a function of the electrolyte
volume. For some technologies,
the power and energy can be scaled independently,
allowing for an easily customizable
battery.
In a hybrid flow battery, electroactive
material is deposited on the surface of
the electrode during the charge cycle
and then dissolved back into the electrolyte
solution during discharge. For hybrid
technologies, the storage duration is a
function of both the electrolyte volume
and the electrode surface area. While
most hybrid technologies can achieve
durations of six to 12 hours, power and
energy are not fully decoupled.
Flow batteries can be configured as
both a single tank, usually for smaller applications,
or as a dual tank, usually on a
larger footprint. The single-tank systems
typically feature zinc or other metal batteries,
while dual-tank systems require
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electrolyte comprised of saltwater, iron,
vanadium, or other minerals.
Flow battery system designs change
depending on the application and project
size. Behind-the-meter commercial systems
are commonly kilowatt-scale packaged
units that can fit into a typical utility
room. For distribution applications in the
1-MW to 5-MW range, containerized and/
or modular solutions exist with varying
levels of scalability depending on the storage
duration requirements. Utility-scale
designs in development may have millions
of gallons of electrolyte storage, so the industry
is trending toward large quantities
of stack modules headered together and
piped to large, field-erected tanks.
Power stacks and balance-of-system
components, such as piping, pumps, seals,
cooling systems, and control instrumentation,
require more routine maintenance
than lithium-Ion configurations. However,
if routine maintenance guidelines are followed,
flow battery performance should
not degrade within the project lifetime.
When the operations and maintenance
(O&M) costs are compared to lithium-ion
capacity augmentation costs required to
offset performance degradation, flow battery
annual costs are less expensive.
Use Cases
Though specific power markets and load
factors will vary widely throughout the
world, flow batteries can perform several
use cases. More importantly, a flow battery
may have greater use case flexibility
than lithium-ion systems designed for a
specific application.
Lithium-ion
systems
designed
for
deep discharge will exhibit greater performance
degradation (with potential
warranty implications) if they are cycled
multiple times per day or used for different
applications such as frequency
response. Because flow battery performance
doesn't degrade, there are fewer
limitations on use cases once the system
is installed. More importantly, a flow batPOWER
| March 2020

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POWER March 2020

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