IEEE Electrification - December 2021 - 82

VIEWPOINT
should approach US$30-US$40/kWh.
The acceptable degradation over
3,000-5,000 cycles would enable a
reasonable 10-15-yr service life from
daily cycling.
The second family, termed moderate
performance, ultra-low cost, trades
off some performance characteristics
with dramatically lower costs. This
family would enable the weekly-toseasonal
durations required for the
second stage of grid decarbonization
as well as the most demanding resilience
needs. The performance
ranges for this family could have
round-trip efficiencies of approximately
50% or slightly less, which
would be offset by upfront capital
costs below US$10/kWh. Given that
these resources would only occasionally
be cycled, acceptable degradation
over hundreds or dozens of
cycles could still enable a sufficiently
long service life.
Any new technology would need
to overcome the advantages of the
current dominant technology: Li-ion.
For Li-ion batteries in 2021, their
long-term performance is well characterized,
the supply chain is developed
and expanding, and their costs
continue to decline (albeit at a slower
pace). Absent external intervention,
performance and supply chains
take time to establish. Therefore, the
most promising path for a new technology
to displace Li-ion is to demonstrate
a significant cost advantage
for a given application.
High Performance, Low Cost
The technologies in the high-performance
category are likely to favor
those that store energy in higherquality
forms, such as mechanical or
electrochemical. Chemical
or
mechanical energy can be readily
converted to and from electricity at
relatively high efficiencies, which can
improve the cost profile for systems
that cycle frequently.
Pumped storage is the most wellestablished
gravitational-mechanical
system. Gravitational systems that
82
use pulleys and railcars have also
been explored. In many cases, these
mechanical systems incorporate
exceptionally low-cost storage media
(like rocks or water) but have conversion
equipment or geographic
deployment constraints that can
drive up costs elsewhere in the system.
Although these systems may
have competitive levelized costs,
many markets today lack the structures
to invest in an asset with a lifetime
of 50-plus years.
The other major class of high-performance
technologies include
emerging electrochemical batteries.
Most of these candidates use a flowbattery
architecture (as opposed to
individually packaged cells), where
energy and power can be scaled
independently. Innovators are forging
an entire new class of electrochemistries
that use low-cost materials like
iron, zinc, or Na, bypassing the Li-ion
supply chain entirely. Theoretically,
most of these chemistries have
attractive cost profiles, although the
material cost of containment (i.e.,
tanks) may establish an upper bound
in the tens of hours for which these
technologies would be cost-effective.
A few of these technologies are currently
manufactured at the scale
that would maximize their costreduction
potential.
Moderate Performance,
Ultralow Cost
The technologies in the ultralowcost
category are likely to favor storage
mediums that utilize lowerquality
energy forms, i.e., heat.
These technologies can use very
low-cost materials as the storage
medium, such as concrete or molten
salt. Although some technologies
were envisioned to use heat
from concentrating solar or other
sources, their very low costs enable
grid electricity a viable input as a
heat source. After being stored in an
insulated medium or through an
endothermic reaction, thermal
energy typically requires a heat
IEEE Electrification Magazine / DECEMBER 2021
engine for conversion back to electricity,
which limits its electricityonly
efficiency. To be suitable for
very long-duration applications,
innovators are examining methods
to best insulate the heat, reducing
self-discharge over time.
Because these technologies have
low, round-trip efficiencies, their
optimal competitive use may be for
longer (beyond tens of hours) durations,
where other technologies
become impractical or too expensive.
Some of these technologies still
require earlier-stage R&D, especially
for innovations around materials.
The other technologies incorporate
largely off-the-shelf components. For
the most part, these technologies still
need to be demonstrated and manufactured
at scale.
The Case for Diversity
To date, there does not appear to be
any candidate storage technology
that simultaneously exhibits every
desirable characteristic, including as
high efficiencies, high cycle life, low
cost, or geographic portability. Therefore,
it is difficult to envision that any
technology in existence or development
could follow a pathway to
dominance that semiconductors or
LEDs exhibit in their respective
domains. A more likely outcome is
that a diversity of storage technologies
will coexist to serve the variety
of energy-balancing needs, from distributed
to centralized architectures,
and from hourly durations to weekly
or even seasonal.
Concluding Currents
In some ways, the future of storage as
a perpetually diverse industry is
already here. For example, researchers
and commercial vendors are
already piloting hybrid pumped
hydro and battery systems, where the
battery reduces mechanical stresses
by offloading very short-term variations.
Although other industries like
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