July 11, 2012 – A challenge facing renewable energy resources is the ability to store their energy when it is produced, then disburse it when needed. A team of researchers from Drexel University’s College of Engineering have taken up this challenge, and has developed a new method for quickly and efficiently storing large amounts of electrical energy.

The Drexel team has put forward a plan to integrate into the grid an electrochemical storage system that combines principles behind flow batteries and supercapacitors.

Batteries store a large amount of energy, but are relatively slow in discharging it and they have a limited lifespan, say the researchers, as compared to their counterparts, electrochemical capacitors (supercapacitors or ultracapacitors). Conventional supercapacitors provide a high power output with minimal degradation in performance for as many as 1 million charge-discharge cycles. But while the capacitor can rapidly store and discharge energy, it can do so only in small amounts as compared to a battery.

The obstacle in the way of using either a battery or 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, say the researchers. 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.”

The team’s research yielded a 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 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 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 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. When a larger capacity is desired, the tanks can simply be scaled up. 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.”

“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.”