18 July 2012—Grid-scale energy storage and renewable energy are a natural fit. When the wind blows strongest and the sun shines brightest, if all that power can’t be used right away, then it is usually simply lost. Storing it for calm periods or a cloudy day is a crucial part of many future energy plans.
A number of technologies—like flywheels, sodium sulfur batteries, and compressed-air storage—will eventually contribute to that storage, but these all have drawbacks: Some don’t store enough energy, some discharge it too slowly, and some lose their storage abilities after a few years of use. But new research out of Drexel University, in Philadelphia, suggests a technique that combines some of the best aspects of two disparate ideas—flow batteries and supercapacitors—that could hit the sweet spot of scalable and affordable energy storage.
The Drexel device, called an electrochemical flow capacitor (EFC), consists of an electrochemical cell—two electrode compartments separated by a membrane—that’s connected to two pairs of external reservoirs. Each reservoir contains a slurry of highly porous carbon particles suspended in a liquid electrolyte medium. When charging the system, the electrodes are polarized and slurry flows past them, gathering a double layer of charge on its way to the reservoirs. “Inside the cell, energy is stored capacitively at the surface of the carbon particles,” similar to what you’d see in a supercapacitor, says Emin C. Kumbur, who led the research at Drexel along with Yury Gogotsi.
The charged slurry can be stored in the reservoirs until it is needed, when it is pumped back through the cell and discharged. A similar flow concept is used in flow batteries, which store energy in a chemical reaction as electrolytes flow past. “Operationally, the system behaves much like a redox flow battery,” Kumbur says. “Fundamentally, however, the energy is stored in the same way as conventional supercapacitors.”
The primary advantage of the carbon-based slurry supercapacitor approach is that it does not wear out as quickly as other energy-storage technologies. Batteries—be they lead-acid, lithium-ion, or redox flow—are limited by the fact that the chemical reactions eventually degrade the battery’s chemicals and materials. Supercapacitors have much longer lifetimes without any such chemical reactions and degradation. According to the paper in Advanced Energy Materials by Kumbur and his colleagues, the EFC would, like a supercapacitor, be expected to last on the order of 100 000 or more cycles. Redox flow batteries and lithium-ion batteries, meanwhile, might last less than 15 000 cycles.
However, supercapacitors can store only small amounts of power and don’t scale easily. The EFC mirrors a redox flow battery, however, in its ability to store utility-scale amounts of energy. By building bigger reservoirs for the slurry, it is possible to increase energy storage. “Additionally, systems based on aqueous electrolytes are safe and environmentally friendly, unlike most batteries,” Kumbur says.
Power Sludge: Electrolyte carrying carbon particles is key to storing energy in a flow capacitor.Photo: Drexel University
Jun Liu, a laboratory fellow at Pacific Northwest National Laboratory who was not involved in the work on the EFC, says the idea has great potential to reduce manufacturing costs as well, thanks to the use of a liquid electrolyte instead of solid materials. Liu notes, though, that obstacles remain before this idea can be commercialized; among the problems are the need to better understand and control the fluid’s properties and to improve the charge transfer inside the cell.
Also, Liu says, “right now the energy density is still very low and not comparable with redox flow batteries, not to mention regular lithium-ion batteries.” The Drexel researchers agree, saying they are aiming for an increase in the energy density by an order of magnitude.
Kumbur and his colleagues think flow capacitors could help alleviate what may soon become a pressing need for electricity storage. A landmark renewable-energy report from the National Renewable Energy Laboratory (NREL) released in June asserted that the United States could easily support the sourcing of 80 percent of its electricity from renewable technology. Storage would have to be a big factor: Getting to 80 percent renewables would require a jump from the 20 gigawatts of storage available today (almost all is pumped hydropower) to as much as 150 GW.
The NREL study does not include yet-to-be-realized storage tech and avoids the inclusion of capacitors in its analysis. Still, it acknowledges that related technology could play a role: “[Capacitors’] low energy capacity has restricted their use to short time-duration applications. A major research goal is to increase their energy density and increase their usefulness in the grid.”
For the moment, though, the EFC is still firmly in the lab. Kumbur says his group is working to create a benchtop demonstration of the technology to attract investors to the idea. “It is very difficult to forecast when this technology may be implemented in an industrial application, but it is possible that we may see an operational unit installed in the next 5 to 10 years,” he says.
About the Author
Dave Levitan is a science journalist who contributes regularly to IEEE Spectrum’s Energywise blog. In our June 2012 issue he reported on what’s behind the persistent gap between record-setting solar cells and what comes off the manufacturing line.