In September, the first modular elements that make up the gravity storage system based on pumped hydro will be installed in the 500-meter deep wells at the Nuraxi Figus site.
Partnership between the Sardinia Region and Energy Vault for the conversion of the site with gravity energy storage
A hybrid gravity energy storage system of 100 MW, located within a former underground coal mine in Sardinia. The project will start in September at the Nuraxi Figus site, owned by Carbosulcis. The mine will be decommissioned by the end of 2026 and converted into a technology hub with minimal environmental impact. Also, thanks to the partnership with Energy Vault, a Swiss company specialising in grid-scale sustainable energy storage solutions.
From coal to hybrid gravity accumulation
The Sardinian plant will combine 20 MW of gravity energy storage in mining shafts with 80 MW of battery storage on the surface. How does it work in practice? Energy Vault’s solution combines long-established pumped hydro energy storage technology with an innovative gravity energy storage technology developed by the company, which has the main characteristic of modularity.
At the heart of Energy Vault’s EVO technology is a modular system for storing water in prefabricated modules shaped like trees. Without, therefore, requiring the construction of dams for water storage. The modules are installed inside the mine shafts, 500 meters deep, at various heights, and connected to the pumping turbines via forced conduits. Water is pumped into the modules according to the need to store or generate energy.
Some details of the gravity energy storage system still need to be refined. At the moment, the Sardinia Region—owner of the extraction site—and Energy Vault are considering a system capable of operating for four hours a day. However, a final decision is still awaited, the details of which will depend on the technical and commercial requirements set by Terna.
A green hub to stabilize the Sardinian electricity grid.
According to Energy Vault, the project in Sardinia is the second of its kind after that of a green microgrid in Northern California, Calistoga PG&E, where the company has paired green hydrogen with lithium batteries. The conversion of Nuraxi Figus, one of the largest coal mines in Europe with reserves of 2.5 billion tons of sub-bituminous coal, will support the industrial and economic transition of the area, which has been significantly affected by the abandonment of coal production.
The installation of the first modular elements is scheduled for September 2024, while the testing of the underground component of the hybrid gravity energy storage system is expected to be completed in 2025. Once operational, the site will help stabilize the island’s electrical grid, aiming to distribute renewable energy to meet high demand during peak hours, while also promoting further local use of renewable production in Sardinia.
“The accumulation of energy is a fundamental component for better utilization of renewable energies 24 hours a day in Sardinia.” “We consider this important collaboration with Energy Vault a fundamental step in the development of the Technology Hub, which aims to promote innovation by repurposing a former mining site and strengthening the concept of ‘smart transition’ towards renewable energies in an area like Sulcis Inglesiente, which has been heavily impacted by the exit from fossil fuels, such as coal,” emphasizes Francesco Lippi, CEO of Carbosulcis.
