From Wind Turbines to Concrete, the Incredible Bet on Gravity-Based Energy Storage

With a technology based on gravity and concrete, Energy Vault aims to store renewable energy more efficiently.

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The video has circulated on social networks: in a futuristic atmosphere, a massive crane 120 meters high uses its six arms to simultaneously move 35-ton concrete blocks at different levels in a tentacular concrete tower. This strange ballet is not aimed at constructing a city or an oversized building but... at storing electricity.

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Designed by the American-Swiss startup Energy Vault, in collaboration with the American Atlas Renewables and China Tianying, this concrete monster promises a storage capacity of 25 MW/100 MWh. Its goal is to meet the increasingly urgent storage needs as renewable energies expand. Because as capacities increase, prices become more volatile, penalizing consumers and investors when a lack of demand sends prices into negative territory.

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The approach is inspired by Pumped Storage Energy Transfer (STEP) stations, these gigantic interconnected lakes installed in the French mountains in the 1970s to "store" electricity from nuclear power plants overnight and release it during the day by pumping water to power dams. This new electricity storage system was implemented for the first time in China in 2023, in Rudong, near Shanghai. A prototype had also been developed in Switzerland in 2020, claiming an energy efficiency of 75%.

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The principle is technologically straightforward: whenever produced electricity exceeds demand or prices justify it, the crane hoists the concrete blocks to the top to convert kinetic energy into potential energy. To "discharge" it, the same concrete blocks are lowered to the ground. The speed then activates a turbine that generates electricity.

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Energy Vault also promises numerous other projects in China. In November 2023, the company, in partnership with the Chinese Tianying, announced agreements with five Chinese municipalities (Hebei, Jiangsu, Gansu, Jilin, and Xinjiang) to develop 1,160 MWh of batteries.

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To compete with lithium batteries that dominate the market, Robert Piconi, CEO of Energy Vault, highlights three advantages: the absence of cutting-edge technology and ease of implementation (this installation does not require specific geography like STEP), no degradation of storage capacities over time, and the environmental virtues of this battery as it does not use critical materials.

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However, these arguments are relative since concrete production emits several hundred kg of CO2 per cubic meter. Moreover, these massive installations may face strong opposition from local residents. Specialists also highlight the low energy density of these solutions, which require building imposing concrete walls to store the equivalent of a lithium battery of a few cubic meters. In other words, scaling up is very uncertain.

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The difficulty is also economic. While storage is promised a bright future, the volatility of prices linked to renewable production variations is not yet sufficient to remunerate such projects. "Battery storage systems should become key to the energy transition; however, questions about their profitability are slowing down their deployment. Given market conditions, energy market arbitrage is not a viable business model," note analysts at Rystad Energy.

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In 2022, the German giant Siemens Gamesa announced the halt of its project to build a demonstrator for electricity storage in volcanic rock. "The commercial market for large-scale and long-term storage has not yet emerged," argued the German group.