Supercapacitors: What and Why?

By Riley Yaylian

Renewable
electricity generation technologies like wind and solar, both of which are
still in the early stages of commercial development, are growing rapidly. Both of
these technologies, however, are intermittent in nature and cannot provide steady
power without some form of energy storage.

The storage technology that most
folks are familiar with is batteries, specifically, lithium-ion batteries like
the ones in your mobile devices and the Tesla
Powerwall
. The Powerwall and other utility-scale
energy storage devices take electricity and store it for later use when the
wind isn’t blowing or the sun isn’t shining.

But there is another,
lesser-known technology that can complement Li-ion batteries: the supercapacitor. While a
supercapacitor operates under the same electrochemical principles governing
Li-ion batteries, it is an altogether different beast. The main advantage of a
supercapacitor is that it has a high power density, meaning a small device can deliver huge
amounts of energy in a small amount of time without damaging the device. This
comes from the usage of high surface area electrodes that allow easy motion of
the ions used in the pertinent reactions, which also leaves the electrodes
unperturbed by the reactions and thus enables virtually infinite cycle life (greater
than one million cycles).

The same mechanisms that give
high power density, however, result
in low energy density, meaning that
little energy can stored in a small
device. For this reason, supercapacitors are sometimes used to complement high-energy
density Li-ion batteries, a pairing that gives high energy density and high power
density.

So what are the applications of
supercapacitors and their pairing with batteries?

Supercapacitors are most useful
in storage applications where batteries cannot respond quickly enough to the
electrical demands placed on them or would be damaged in doing so. For
instance, in grid applications electricity demand may suddenly spike, requiring
an instantaneous response from the battery or supercapacitor. The battery may
not have the power capabilities to meet this demand for a few seconds and may
suffer damage or inefficiencies in doing so. A supercapacitor can provide the necessary
burst of energy for a few seconds while the battery catches up and then takes
over.

Another application is in the
regenerative braking system in hybrid and electric vehicles. While braking, the
electric motor of a hybrid or electric vehicle can essentially spin in reverse
and capture the kinetic energy lost in braking and store it in the battery.
However, the power generated can reach many kilowatts, a high power not easily
handled by the battery system. A supercapacitor can be used in conjunction with
the battery, absorbing the high power and then feeding it to the battery at a
more manageable rate and thus preserving the cycle lifetime of the battery.

Lastly, supercapacitors excel in
applications with heavy stop-start loads, such as municipal buses. These
applications, and others not mentioned, are poised to create increased demand
for supercapacitors. Further research and development may see this
oft-overlooked device take some market share from the mighty Li-ion battery.

Riley
Yaylian is a volunteer at the The Climate Center and has a BS in
Materials Science and Engineering from UC Davis, and an MS in Materials Science
and Engineering from UC San Diego.

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