Six years ago, US-based SunEdison, then a big player in solar energy that also had operations in India, said it would buy 100 MW worth of batteries from another US company called Imergy. These batteries would power hundreds of mini grids in India. SunEdison was doing it as part of its CSR activities. This deal had one uniqueness — the type of the battery used. The mini grids were to be powered by ‘redox flow batteries’, an area that Imergy was a specialist in.
Then SunEdison went bankrupt and its mini grids project in India dissipated into the void. If it had succeeded, it would have given a fillip to redox battery adoption in India. It didn’t, and that storage technology lost mindshare.
But today, there seems to be a redox redux. Indian laboratories are busy working on this; and there is renewed curiosity in the industry.
How it works
The redox battery is a fairly mature technology. Simply put, energy is stored in two liquid electrolytes in separate tanks. When you charge, the energy supplied urges electrons from the electron-poor side to move to the electron-rich side — like taking water uphill — creating a potential difference. During discharge, the reverse happens — electrons flow from the electron-rich side to the electron-poor side. Flow of electrons is electricity. (Gaining electrons is a ‘reduction reaction’, losing electrons is ‘oxidation’, hence ‘redox’.) The conversion of energy from chemical to electrical happens in a cell, which is split into two half-cells by a membrane. Of course, you need electrolytes that are electron-rich and electron-poor to start with — that is the science of preparing electrolytes. Usually, vanadium is used as an electrolyte, as the metal exists in four ionic species — with two, three, four or five (positively charged) protons more than the number of (negatively charged) electrons.
The beauty of the system is that you separate energy storage from power (electricity generation). If you want to store more energy, you simply increase the size of the tanks; if you wish for more power, you just add more cells in the cell stack — it is completely modular. A German company, RWE, recently announced that it is working on a huge, multi-gigawatt-hour redox flow battery system, where the electrolytes will be stored in giant underground salt caverns. The system can power 75,000 homes for one full day.
Redox batteries come handy in large storage applications, especially in maintaining grid stability, which assumes importance in the context of increasing renewable energy penetration. You can store excess electricity from wind and solar farms and supply it when required. Further, they can fully discharge before needing recharge, and last really long — over 20 years. Since, unlike conventional batteries, scaling up is simpler and cheaper, you can absorb huge quantities of energy. Another German company, Schmid, is building a 3-GWhr storage system for Saudi Arabia — in comparison, the biggest conventional storage systems are in tens of MWhrs.
In India, scientists have begun making redox flow batteries in labs. Anil Verma, professor at IIT-Delhi, has developed a 0.5-kW redox flow battery and installed it as a mobile and laptop charging station, to collect data for further research. He told Quantum that starting afresh is not like reinventing the wheel, because no matter how advanced redox flow technology is, challenges exist. “The technology has great potential and more research is required,” he said, adding that his team has come up with a market-ready redox battery of improved performance and lower cost. He also stressed that India should have indigenous capabilities.
Similarly, Sreenivas Jayanti, professor at IIT-Madras, and his student Ravindra Gundlapalli have built one with a power rating of 2.5 kW and have developed an operating protocol and design criteria for a 10-kW device. Since redox batteries are easily scalable, growing them from lab to industry levels is limited only by factors such as availability of raw materials and technical manpower.
Elsewhere in the world, research is happening at a frenetic pace, mostly in developing cheaper and more efficient electrolytes. Search is on for organic (carbon compound) electrolytes. Last year, the University of Southern California announced that it had developed electrolytes with iron sulphate and an organic material called AQDS (anthraquinone disulfonic acid, which can be made from carbon dioxide), which are “inexpensive” and “attractive for storing energy from solar and wind farms”, according to Sri R Narayan, the team leader. The new wonder material graphene (one-atom-thick sheet of carbon) is being tested for electrodes; scientific literature speaks of other materials such as nitrogen-doped mesoporous carbon and hydroxylated carbon fibres for electrodes.
The relatively high cost of small-scale redox flow batteries has been hampering their widespread adoption. “Commercialisation of vanadium flow batteries is limited by its high cost,” says a literature of IIT-Madras. About 35 per cent of the battery cost is of the active material, vanadium. Verma, who is close to incorporating a start-up, says he is onto making 5-20 kWhr (meaning, the battery will be fully discharged in four hours) — the device would cost ₹3-3.5 lakh. This works out to $230 a kWhr, slightly higher than lithium-ion batteries. In contrast, USC says its iron sulphate-AQDS battery would cost $66 a kWhr.
Verma says there is an immediate business case for replacing diesel gensets at industries and conventional batteries in telecom towers with flow batteries. Jayanti told Quantum that redox flow batteries are the “best candidates” for grid balancing, as more and more energy comes from wind and solar. Both the academics say that they are seeing renewed interest in flow batteries from Indian industry. As such, it may only be a matter of time before redox flow batteries become ubiquitous.