India’s ambition of creating 500 GW of renewable energy capacity by 2030 cannot be achieved without large-scale storage systems. It is generally accepted that conventional battery systems cannot provide an affordable solution. In the search for alternatives, a bright candidate has been the “flow battery”, which is modular and therefore easily scalable.
As detailed earlier in these columns, in flow batteries the energy is stored in two liquid electrolytes in separate reservoirs. When you charge, the energy supplied causes electrons on the electron-poor side to move to the electron-rich side – like taking up water – creating a potential difference. During discharge, the reverse happens – electrons flow from the electron-rich side to the electron-poor side. (The flow of electrons is electricity. Gaining electrons is a “reduction reaction”, losing electrons is “oxidation”, hence “redox”.) The conversion of chemical energy into electrical energy occurs in a cell, which is divided into two half-cells by a membrane. Of course, you need both electron-rich and electron-poor electrolytes to begin with – that’s the science of electrolyte preparation. Usually, vanadium is used as an electrolyte, because the metal exists as four ionic species – with two, three, four or five protons (positively charged) more than the number of electrons (negatively charged).
Flow batteries bring significant benefits. First, they are modular – you can keep increasing the energy storage capacity by simply increasing the volume of the electrolyte reservoirs. If you want more power, just increase the number of membrane stacks. The separation of storage capacity and power is a notable advantage. RP Deshpande, an expert, compared it to an internal combustion engine in cars. Engine size determines horsepower; if the manufacturer wants the car to travel a longer distance on a single tank of fuel, he only has to increase the size of the fuel tank.
Second, they can store energy for long durations, which is important for the seasonal wind industry. Giant tanks can hold large amounts of electrolytes to store more energy. However, the big obstacle to vanadium redox batteries has been the high cost of vanadium, which accounts for more than a third of the system cost.
But now Professor Kothandaraman Ramanujam from IIT-Madras has developed a lead-based flux battery. Ramanujam narrates Quantum that the battery is working well and is ready for sale.
The use of lead in energy storage is old; lead-acid batteries are ubiquitous. Everyone knows how to handle lead.
Moreover, the metal is cheap and available in India.
So what was holding back lead redox flow batteries? Ramanujam says that in ‘soluble lead redox flow batteries’ the big challenge was the growth of dendrites, which are filament-like spikes or extrusions, which could create a short circuit by connecting the two electrodes and giving the electrons a minus- resistance path.
Ramanujam’s team managed to “successfully circumvent” the problem of dendrite growth. “Using additives, dendrite growth was attenuated up to 50 mAh per cm² surface capacity, when the battery was cycled at 50 mA per cm² current density. The capacity achieved is nearly five times greater than that of lead-acid batteries,” Ramanujam said. The battery has been tested for over 500 charge-discharge cycles. Ramanujam is seeking funding for a pilot plant.
Is this the breakthrough the industry is waiting for? It looks like it, but only a pilot plant can give a clearer picture. Flow battery technology is also being refined elsewhere in the world. Attempts have been made to replace vanadium, the active material for energy storage, with iron (Yang Shi, Chinese University of Hong Kong), manganese (Prof Ingo Krossing, University of Fribourg), manganese and sulfur (Dr Barun Chakrabarti, University of Warwick). Just as there are multiple lithium-ion battery chemistries, perhaps there is an application-based role for all of these emerging technologies.
“Flow batteries are in the early commercialization stage and are just beginning manufacturing scale-up,” says Dr. Rahul Walawalkar, Founder and Chairman of the Indian Energy Storage Alliance and Chairman of the Global Energy Storage Alliance, adding: “We anticipate that batteries will be a key part of the solution mix as we move towards net zero. Walawalkar, however, cautions against focusing too much on cost reduction while ignoring other factors such as round-trip efficiency.
April 24, 2022