Hold in store

In 2020, Dishant Mishra hit a financial roadblock while developing hyper-storage wind turbine technology. After a Master's in wind energy from the Carl von Ossietzky University of Oldenburg in Germany, Mishra started a company named Baud Resources in 2017 to develop wind energy technology with in-built gravity-based energy storage. Mishra conceived a wind turbine system that would store excess electricity by lifting heavy objects against gravity when wind was available and dropping them to rotate generators and generate electricity when wind was not available. He had started with research funding from the European Commission, but the money dried up due to the coronavirus epidemic. Some of his mentors then suggested that he discard wind turbines and focus on storage.

Baud was then incubated at the Indian Institute of Technology (IIT) Kanpur, from where it received ₹4 lakh for R&D. Mishra tweaked his plans and developed a gravity-based, inclined storage prototype in 2023. The system was similar to a pumped hydro system for energy storage, where water was pumped uphill into a reservoir when excess electricity was available and released downhill through turbines to generate electricity when the demand was high. Baud's equipment could be used with sand, coal ash, construction debris, or any locally available waste. "The challenge with pumped hydro systems is that they're not very scalable," says Mishra. "They need specific geography; they need a water dam, so a lot of things have to come together to make it happen."

A pumped hydro energy storage system requires land acquisition, for which there is no clear legal framework. Energy Vault, a major manufacturer of gravity-based energy storage systems, claims its technology can store energy at $0.05/kWh (kilowatt-hour), while pumped hydro storage costs $0.17/kWh. While start-ups such as Baud Resources and U.S.-based Advanced Rail Energy Storage are betting on inclined, rail-based systems that offer scalability and modularity, some other start-ups, including the U.K.-based Gravitricity and Green Gravity in Australia, are using vertical designs to save space. When electricity is in surplus, the motors in these systems raise materials, such as sand or concrete blocks, to a higher elevation. When electricity demand rises, the masses are lowered in a controlled manner, causing motors to operate in reverse as generators, converting mechanical energy to electricity.

Such systems are safe and have longer life spans than widely used energy storage systems such as batteries. Unlike lithium batteries, which store energy for 5-6 hours, these can store energy for up to 10-12 hours without degradation, as mechanical energy does not degrade that rapidly over time. This makes them a better cushion against electricity volatility caused by aberrant weather or increased demand. When such projects are developed in remote locations on cheaper land, they are more economical, too. Mishra says that their first small plant will store energy at ₹4/kWh, while the medium and larger plants will bring the cost down to ₹3 and ₹1.5/kWh, respectively. This makes such systems considerably cheaper than lithium batteries, where the cost of energy storage is ₹8-10/kWh.

The cost of renewable energy generation has fallen significantly over the past decade, but the high cost of energy storage remains the main challenge in the green energy transition. Economies are seeking technologies that can store large amounts of energy in a scalable, affordable, sustainable, and safe manner. Researchers and start-ups are developing and innovating different modes to store energy: mechanical routes like pumped hydro, gravity-based storage, compressed air storage, flywheel, electrochemical storage via batteries and supercapacitors, in the form of heat via molten salts, phase change materials and in chemicals such as hydrogen, synthetic fuels, ammonia and methanol.

Industry observers believe that each of these technologies will have its place in a world powered by clean energy. The choice of energy storage technology depends on the duration (short, medium, or long), the balance between how quickly energy needs to be delivered and how much energy needs to be stored, and the required efficiency. It also depends on cost, location and safety considerations. Batteries are the best option for quick response and short- and medium-duration storage in urban areas where compact systems are required. Mechanical storage systems are best suited for large, grid-level storage of medium- and long-duration energy in remote locations where land is available. Thermal energy storage is suited for industries that need heat instead of electricity. Chemicals such as hydrogen, ammonia, and methanol can store energy for long periods, lasting a few seasons. 

India currently has six working pumped storage plants. Four more are under construction, and 17 others are in the planning stage. While most pumped storage projects are being developed by State governments, private developers such as Greenko, Adani Green Energy, and JSW Energy are also developing pumped hydro projects. Other promising options include Compressed-Air Energy Storage (CAES), where excess power is used to compress air into underground caverns or tanks. When energy is needed, the pressurised air is released to drive turbines that generate electricity. Scientists at IIT Madras have run numerical simulations to determine the basic process parameters suitable for a small-scale CAES system across different application scales.

Researchers and start-ups are also developing batteries with different chemistries. Roorkee-based Indi Energy, Pune-based KPIT Technologies, and Pune-based Rechargion Energy have developed sodium-ion batteries. A team from IIT Madras, led by Aravind Kumar Chandiran, has created a mechanically rechargeable zinc-air battery with a LEGO block design that can be easily repaired. Chennai-based Ramcharan Company has designed solid-state sodium silicate batteries, with a high cycle stability of more than 3,000 cycles. 

Meanwhile, multiple methods for storing and utilising heat energy are being tested. Cold chain start-ups, such as Inficold, are converting excess renewable energy to ice for cooling applications. Bengaluru-based Voltanova has proprietary carbon blocks that absorb heat coming from solar thermal collectors, retain it at high temperatures and release it when required through a heat transfer fluid. The heat can be either used directly for industrial applications or converted to electricity. New Delhi-based deMITasse Energies' system stores heat from the Sun in chemicals and regenerates it when required.

While mechanical storage systems, such as the one developed by Baud Resources, can store large amounts of energy for longer periods, batteries are the mainstay of urban energy storage systems because they are portable, compact, and offer fast, flexible, and modular solutions. Lithium-ion batteries are currently the poster child of energy storage. The technology matured early and has seen a 90% price reduction since 2010. As lithium is a critical mineral, start-ups and scientists are developing battery chemistries that are free of critical minerals. 

Electrochemical batteries, such as lithium-ion and lead-acid batteries, have remained the leaders due to their high performance, which is required in electric vehicles and electronics. However, they fall short when used for stationary storage. "Of course, 4-5 hours can be done with lithium, but when you are looking for a round-the-clock renewable, you would need something with 8-10 hours or even higher hours of storage," says Arjun Bhattarai, Co-founder and CTO of Singapore-based VFlowTech.

In 2013, when Bhattarai, a mechanical engineer, had to choose a PhD topic, he opted for flow batteries, which store energy in liquid electrolytes rather than solid electrodes like lithium batteries. Vanadium flow battery technology was maturing at the time, but had low efficiency and a high cost. Bhattarai had worked on improving the performance of vanadium flow batteries. These batteries store energy by changing the charge state of vanadium ions in liquid tanks and release it back when needed. While the energy is stored in electrodes in electrochemical batteries, it is stored in the electrolyte in flow batteries. So, the storage capacity can be increased by adding more electrolytes.

After his PhD from the Singapore-based Nanyang Technological University, Bhattarai started VFlowTech in 2018 with Avishek Kumar, who had finished a PhD from the National University of Singapore. The company's product, PowerCube, has a life span of 25 years and can store energy for up to 24 hours. This battery has been installed at the Gujarat-based Pandit Deendayal Energy University. Other installations are underway at Hindalco and the Defence Research and Development Organisation. The company has also started a manufacturing wing in Palwal, Haryana.

India does not mine vanadium, the primary raw material of these flow batteries. The company is therefore looking to extract or recover vanadium from secondary sources such as steel slag, spent catalysts from refineries, and fly ash from coal power plants. A project with IIT Delhi is developing a method to extract vanadium from refinery waste. As vanadium is a critical mineral, scientists and start-ups are also developing flow batteries that are free of vanadium. Chennai-based start-up Tharam-Thiran Green Energy Flow is developing an iron-sulphur redox battery (also see Ideas that flow well). The Noida-based Offgrid Energy Labs has developed a zinc-bromine flow battery. For now, however, vanadium flow batteries remain the mainstay of long-term energy storage. 

While batteries currently dominate the energy storage space, the need to store energy over long periods — from weeks to months, and sometimes years — has kept the world's interest in hydrogen alive. Hydrogen can help to decarbonise the steel, cement and chemical sectors, among others. In steel manufacturing, it can replace coal as a reducing agent, and be used to produce high-temperature heat in the cement and chemical sectors. Hydrogen can also replace diesel and jet fuel in shipping and aviation. However, the hydrogen economy is riddled with challenges in hydrogen production, storage, transport and utilisation. Given its role in energy transition, many countries, including India, have started hydrogen energy missions.

Commercially, hydrogen is produced by combining steam with methane (CH₄), which produces carbon dioxide and water. Researchers are looking at producing hydrogen by splitting water using electricity from clean sources such as renewable or nuclear energy. It needs fresh water, which is increasingly becoming scarce. Sundara Ramaprabhu, Emeritus Professor of Physics at IIT Madras, feels that this issue can be resolved if an electrolyser system can use seawater, rainwater, or groundwater for hydrogen production. "Seawater contains a lot of minerals, mainly sodium, potassium chloride, that are very salty and can easily corrode the electrolysers," says Ramaprabhu.

In a regular electrolyser, fresh water is split by the electricity and oxygen and hydrogen ions are produced at the anode. Hydrogen ions then pass through a membrane and turn into hydrogen gas at the cathode. Both the anode and cathode are coated with catalysts to enable these reactions. When using seawater, the chloride ions from the salts compete for electrons to form chlorine, which combines with hydroxyl ions to form hypochlorite, which corrodes electrodes. To overcome this, Ramaprabhu's team has developed metal oxide catalysts that selectively favour an oxygen evolution reaction at the anode to prevent a hypochlorite formation., This ensures that electrons at the anode split water and do not oxidise chloride. Such systems can be easily coupled with floating solar panels or solar panels at seaside or offshore wind projects to produce hydrogen remotely. The technology is currently at Technology Readiness Level (TRL) 4.

Different configurations of high-performing hydrogen electrolysers are being developed worldwide. Bengaluru-based Newtrace has created "membrane-less" electrolysers to make green hydrogen without the need for rare-earth metals. Selvaraj Kaliaperumal, Professor at the Pune-based National Chemical Laboratory, has developed a high-performing 3-kilowatt anion exchange membrane hydrogen electrolyser that produces 1.2 kg of pure hydrogen a day. Bengaluru-based Ossus Biorenewables has a different model: it produces green hydrogen from industrial waste carbon in bioreactors. Similarly, Varanasi-based Aranayak Fuel and Power has developed reactors that convert biomass waste into hydrogen.

Hydrogen is sometimes called the Swiss Army knife of decarbonisation because of its many uses. While it enables energy storage, it is an energy carrier that can be transported in fractions and be used for electricity generation via fuel cells in vehicles as well. It is used to produce ammonia, burned for heat and used as a catalyst or raw material in industries. Ramaprabhu believes that sound investments in hydrogen technology over the next 5-10 years will build resilience into future energy systems based on renewable energies.

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