Solar panels with windmills in background.

By Katrice R. Jalbuena

Harnessing the power of the sun and wind offers us access to an unlimited source of clean energy. Unfortunately, they’re not uninterruptable sources.

At night or when the sun is obscured by clouds and shadows, the power from a solar power system falters. When the wind doesn’t blow or when it’s blowing too strong, a wind turbine is brought to a stop. In order for more wind and solar energy to be integrated into the energy mix, there needs to be a way to store it.

With a proper storage system, the energy produced during peak hours – when the sun is shining or the wind is just right – can be kept and distributed as needed to the energy grid.

Engineers from Drexel University are looking to provide the grid what it needs with an electrochemical flow capacitor technology which combines the principles behind two energy storage techniques, the flow battery and the supercapacitor.

“Current technology for grid-level energy storage tends to be rather expensive, large undertakings,” said Dr. E.C. Kumbur told EcoSeed.

“The challenge that we undertook was that of finding a way to store energy from renewable sources, in a way that is both efficient, agile (rapidly store energy) and easily scalable to grid-level,” he said.

Dr. Kumbur is the director of Drexel’s Electrochemical Energy Systems Laboratory. Along with Dr. Yury Gogotsi, the director of the A.J. Drexel Nanotechnology Institute, Dr. Kumbur worked on a new storage technology that combines the advantages of two established technologies – the redox flow battery and the supercapacitor.

Supercapacitors

Supercapacitors can rapidly store and discharge energy and last a long time, but they can only store a limited amount. One would have to combine a lot of supercapacitors together to get a system that could handle the amount of energy needed for grid applications.

A flow battery is attractive for grid-scale applications as its energy storage capacity is easily scalable. A flow battery basically consists of two reservoirs in which an electrolyte liquid is stored. The flow of the electrolyte liquid from one reservoir to another creates a charge. The problem with flow batteries is its slow rate of charge and discharge.

The electrochemical flow capacitor combines the strengths of both technologies. It consists of an electrochemical cell connected to two external electrolyte reservoirs.

Small carbon particles suspended in the electrolyte liquid create a slurry of particles that can carry an electric charge. Uncharged slurry is pumped from its tanks through the cell, picking up energy stored through the carbon particles. The charged slurry is stored in the tanks until the energy is needed, then the process reversed.

“Essentially the unique aspect of this new technology is that it combines the advantages of a redox flow battery (i.e., scalable energy capacity) with those of a supercapacitor (i.e., long lifetime, high power density and rapid charging and discharging ability),” explained Dr. Kumbur.

According to him, the energy storage capacity of the EFC can be easily adjusted to suit the needs of the system by simply altering the size of electrolyte slurry tanks.

“Bigger tanks mean more storage and the system has the potential to be scaled up to MW levels,” said Dr. Kumbur.

The longer lifespan of the EFC – around 100,000 discharge cycles – will also allow operators to save time and money as the cells will not have to be replaced that often.

According to Dr. Kumbur, it is possible that the world might see an operational EFC installed in five to six years.

“At this point, the technology has primarily been explored at the laboratory scale. We are currently working to develop a bench-top demonstration unit to illustrate the full system operation and determine design guidelines for large scale applications,” he said.

Ongoing work on the technology will also include developing new slurry compositions based on different carbon nanomaterials and electrolytes, as well as optimizing the flow capacitor design.

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