As an increasingly high proportion of energy grids are fed by renewable energy, developing storage solutions that can deal with intermittency in sustainably, safely and cost-effectively is key.
Lithium-ion batteries are still the frontrunner technology for large-scale energy storage, and their benefits are clear — high energy densities, relatively low maintenance and a rapidly dropping cost per kWh. But their drawbacks of limited lifespans, explosive failure modes and potentially precarious chains of component supply are equally well publicized.
What battery technologies and chemistries are making waves for stationary storage applications?
All-Iron Flow Batteries (RFB)
Redox Flow Batteries (RFBs) are hardly a new technology, but have received renewed interest in the past few years as grid energy storage solutions. Benefits include long lifespans, theoretically limitless scalability and long discharge times, however, they have been held back by their drawbacks including low energy densities, expensive component costs and in some cases toxic or dangerous electrolyte materials.
Energy Storage Solutions (ESS) have been working on developing and proving the commercial case for their all-iron flow battery which aims to solve several of these issues. In contrast to Vanadium flow batteries, the electrolyte materials are selected for their abundance, safety and low-cost — salt, iron and water. The battery can be transported “dry” and hydrated on site, also lowering logistics costs and improving mobility.
The non-corrosive electrolyte also allows for cheaper materials to be used for the power stack and other battery components. With a mild electrolyte pH (1 to 4) electrode reaction potential lower than the 0.8V carbon corrosion potential, all-iron flow batteries experience little electrode degradation — ESS’s modules experience minimal performance loss over 20 000+ cycles with approximately 70% peak round trip efficiency.
ESS are testing the business case under a contract with the U.S Army Corps of Engineers, with initial cost estimates at have set an estimated cost for their battery at $500/kWh. At this relatively early stage of development, the cost is certainly not attractive enough to compete with Li-on or even Vanadium flow on a wide scale but could be an ideal solution for smaller and/or remote grids.
Aqueous soluble ferrocenes (RFB)
Researchers from Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have made great strides in developing flow batteries using aqueous soluble organic electrolytes. These have the advantage of being non-corrosive and non-toxic — not only are they safer, but the component parts can be made of cheaper, less durable materials.
Previous attempts using viologen in a neutral electrolyte solution had been promising in terms of power, but the molecules were degrading too quickly to be useful. By modifying the molecular structure of viologen, researchers were able to make it more resilient to decomposition and serve as the negative electrolyte.
The researchers then took this knowledge and applied it to ferrocene, a molecule great for storing charge, but insoluble in water. By modifying the ferrocene molecules using a similar process to that used for the viologen molecules, the researchers were able to dissolve ferrocene into an aqueous solution to make the positive electrolyte.
With this, the Harvard team were able to engineer an exceptionally long lasting battery, losing only 1% of its capacity every thousand cycles. The non-corrosive electrolytes would also allow a substantial reduction in cost for the other components of the battery. This research was only published in February 2017 but the technology offers great potential for safe, stable and low-cost energy storage.
These battery technology and chemistry innovations are at the cutting edge of research, and it is yet to be seen how they will scale to commercial and utility energy storage applications. And with Elon Musk hinting at a big battery breakthrough, do any of these newcomers have the potential to really contend with Li-on in the energy landscape? Give your opinion at the Energy Storage World Forum discussion.
Image: Sumitomo Electric