The electrochemical flow capacitor technology, developed at Drexel, could be a solution to using renewable energy sources such as wind and solar power. |
In
the aftermath of the recent United Nations Rio+20 Conference on
Sustainable Development, the focus of many industrialized nations is
beginning to shift toward planning for a sustainable future. One of the
foremost challenges for sustainability is efficient use of renewable
energy resources, a goal that hinges on the ability to store this energy
when it is produced and disburse it when it is needed.
A
team of researchers from Drexel University’s College of Engineering has
taken up this challenge and have developed a new method for quickly and
efficiently storing large amounts of electrical energy.
The challenge of renewable energy
Electrical
energy storage is the obstacle preventing more widespread use of
renewable energy sources such as wind and solar power. Due to the
unpredictable nature of wind and solar energy, the ability to store this
energy when
it is produced is essential for turning these resources into reliable
sources of energy. The current U.S. energy grid system is used
predominantly for distributing energy and allows little flexibility for
storage of excess or a rapid dispersal on short notice.
The
Drexel’s team of researchers is putting forward a plan to integrate
into the grid an electrochemical storage system that combines principles
behind the flow batteries and supercapacitors that power our daily
technology.
Existing technology
Batteries
store a large amount of energy, but are relatively slow in discharging
it and they have a limited lifespan, or cycle-life, than their
counterparts—electrochemical capacitors, which are commonly called
“supercapacitors” or “ultracapacitors.”
Conventional
supercapacitors provide a high power output with minimal degradation in
performance for as many as 1,000,000 charge-discharge cycles. The
capacitor can rapidly store and discharge energy, but only in small
amounts compared to the battery.
The
obstacle in the way of using either a battery or a supercapacitor to
store energy in the grid is that energy storage ability is inextricably
tied to the size of the battery or the supercapacitor being used.
Supercapacitors, similar to lithium-ion batteries, are manufactured in
fairly small cells ranging in size from a coin to a soda can. Large
amounts of expensive material, such as metal current collectors, polymer
separators and packaging, would be required to construct a battery or
supercapacitor of the size necessary to function effectively in the
energy grid.
“Packing
together thousands of conventional small devices to build a system for
large-scale stationary energy storage is too expensive,” said Dr. Yury Gogotsi,
director of the A.J. Drexel Nanotechnology Institute and the lead
researcher on the project. “A liquid storage system, the capacity of
which is limited only by the tank size, can be cost-effective and
scalable.”
A grid energy storage solution
The
team’s research yielded a novel solution that combines the strengths of
batteries and supercapacitors while also negating the scalability
problem. The “electrochemical flow capacitor” (EFC) consists of an
electrochemical cell connected to two external electrolyte reservoirs—a
design similar to existing redox flow batteries which are used in
electrical vehicles.
This technology is unique because it uses small carbon particles suspended in the electrolyte liquid to create a slurry of particles that can carry an electric charge.
Uncharged
slurry is pumped from its tanks through a flow cell, where energy
stored in the cell is then transferred to the carbon particles. The
charged slurry can then be stored in reservoirs until the energy is
needed, at which time the entire process is reversed in order to
discharge the EFC.
The
main advantage of the EFC is that its design allows it to be
constructed on a scale large enough to store large amounts of energy,
while also allowing for rapid disbursal of the energy when the demand
dictates it.
“By
using a slurry of carbon particles as the active material of
supercapacitors, we are able to adopt the system architecture from redox
flow batteries and address issues of cost and scalability,” Gogotsi
said
In
flow battery systems, as well as the EFC, the energy storage capacity
is determined by the size of the reservoirs, which store the charged
material. If a larger capacity is desired, the tanks can simply be
scaled up in size. Similarly, the power output of the system is
controlled by the size of the electrochemical cell, with larger cells
producing more power.
“Flow
battery architecture is very attractive for grid-scale applications
because it allows for scalable energy storage by decoupling the power
and energy density,” said Dr. E.C. Kumbur,
director of Drexel’s Electrochemical Energy Systems Laboratory. “Slow
response rate is a common problem for most energy storage systems.
Incorporating the rapid charging and discharging ability of
supercapacitors into this architecture is a major step toward
effectively storing energy from fluctuating renewable sources and being
able to quickly deliver the energy, as it is needed.”
This
design also gives the EFC a relatively long usage life compared to
currently used flow batteries. According to the researchers, the EFC can
potentially be operated in stationary applications for hundreds of
thousands of charge-discharge cycles.
“This
technology can potentially address cost and lifespan issues that we
face with the current electrochemical energy storage technologies,”
Kumbur said.
“We
believe that this new technology has important applications in [the
renewable energy] field,” said Dr. Volker Presser, who was an assistant
research professor in the Department of Materials Science and
Engineering at the time the initial work was done. “Moreover, these
technologies can also be used to enhance the efficiency of existing
power sources, and improve the stability of the grid.”
This concept for energy storage was recently published in a special issue of Advanced Energy Materials
focused on next-generation batteries. The team’s ongoing work is
focused on developing new slurry compositions based on different carbon
nanomaterials and electrolytes, as well as optimizing their flow
capacitor design. The group is also designing a small demonstration
prototype to illustrate the fundamental operation of the system.
“We
have observed very promising performance so far, being close to that of
conventional packaged supercapacitor cells,” Gogotsi said. “However, we
will need to increase the energy density per unit of slurry volume by
an order of magnitude, and achieve it using very inexpensive carbon and
salt solutions to make the technology practical.”
