California-based grid storage specialist Energy Vault and Italian coal miner Carbosulcis are developing a 100MW hybrid gravity energy storage system. The system, which will combine modular gravity storage and batteries, will play a key role in the conversion of Sardinia away from coal mining and towards Net Zero energy production.

The energy storage system will be deployed within 500-metre deep mine shafts along with the VaultOS proprietary energy management software. It will be play a key role in the Sardinia Autonomous Government’s targeted conversion of the coal mine to a Carbon Free Technology Hub. The Government, which wholly owns Carbosulcis, plans that the availability of low/zero emissions energy will be a catalyst to attract new industrial and technological activities.

The Hybrid Energy Storage solution employs Energy Vault EV0 gravity technology through a new water-based, modular pumped hydro application. The proposed system combines long-established pumped hydro energy storage technology with Energy Vault’s innovative gravity energy storage technology, allowing the partners to repurpose the underground features of the site as a retired coal mine.

The Hybrid Energy Storage solution is designed to optimise and capitalise on the specific topology of the site and the 500-meter-deep mine shafts. The parties recently executed a land lease agreement. Installation of the first modular gravity components is set to begin in September 2024. Testing of the underground component of the Hybrid Energy Storage System is expected to be completed in 2025.

Carbosulcis’ Nuraxi Figus site, located on the island of Sardinia in Italy, is currently set to be retired by the end of 2026. The Sardinia Regional Government aims to convert the site into a Technology Hub with a minimum environmental impact. This is planned to help the industrial and economic reconversion of an area heavily affected by the exit from coal production.

The Hybrid Energy Storage System will also help to stabilise the island’s power grid. The Government plans to dispatch renewable energy to help meet the high demand during peak load hours while encouraging further local use of renewable generation in Sardinia. The proposed site would be wholly owned and operated by Energy Vault.

“We are very excited about the innovative energy storage combined solution proposed by Energy Vault, including underground gravity energy storage technology, that we are testing together and that can become one of the solutions to support our project to convert our mine into a new Technology Hub. This project will maximise the local use of renewable energy, and at the same time provide services that will improve the stability of the power grid in Sardinia,” said Francesco Lippi, Chief Executive Officer (Amministratore Unico), Carbosulcis. “We are deeply appreciative of the partnership that Energy Vault has demonstrated thus far, and their customer-centric approach to the design of energy storage solutions has already proved extremely valuable. Energy storage is a critical component to better utilising renewable energy around-the-clock on the island of Sardinia. We see this important collaboration with Energy Vault as a vital step in the development of the Technology Hub, which is intended to promote innovation, by recovering a former mining site and strengthening the concept of “smart transition” to renewable energy in an area like the Sulcis Inglesientes heavily affected by the exit from fossil fuels, like coal.”

“We are proud to collaborate with Carbosulcis for such an important project. The conversion of the coal mine in Nuraxi Figus to a Technology Hub powered by renewable energy will help the Sardinia region to economically revitalise one of its areas most affected by the exit from fossil fuels. We believe that this exciting application of our energy storage solutions, including a new use for our gravity energy storage technology – deploying it underground – could hold vast potential for future applications,” said Marco Terruzin, Chief Commercial and Product Officer, Energy Vault. “Discovering and researching new uses of our technology demonstrates the broad versatility and potential of our unique and innovative portfolio of energy storage solutions. We see a future where our technology helps power the world with renewables. Whether it’s on an island in Italy, by a wind farm near Shanghai, China, providing back-up power for a town in California, or supporting the grid in Texas, Energy Vault’s technology is actively being deployed in a wide range of commercial applications and geographies around the world.”

EV0 is part of Energy Vault’s G-VAULT Gravity Energy Storage System (GESS) portfolio and was announced in May 2024 along with other new gravity storage system products.  The new design is a water and vessel-based “modular pumped hydro” solution that is uniquely addressing applications for underground deep mine shafts, in this case a 500-meter-deep shaft at one of the largest coal mines in Italy, which is now under closure. The mine with reserves amounting to 2.5 billion tons of sub-bituminous coal, used to be one of the largest coal mines of its kind in Europe and the world. This is the second deployment of a hybrid energy storage system by Energy Vault after the Calistoga PG&E project in northern California, the largest green microgrid in the world, which pairs green hydrogen with lithium-ion batteries.