Miniera d’Energia : transforming a coal mine in Sardinia into a hybrid energy storage hub using gravity technology : case study.
Date: 2025-07-22
Institutions: Directorate-General for Energy
Subjects: Sardinia, case study, climate change policy, coal mining, energy storage, energy technology

Case Study
Miniera  
d’Energia  

Transforming a coal mine in Sardinia 
into a hybrid energy storage hub 
using gravity technology 

Energy2

EUROPEAN COMMISSION 
Directorate-General for Energy 
Directorate B – Just Transition, Consumers, Energy Security, Efficiency and Innovation
Unit B.1 - Consumers, Local Initiatives, Just Transition

EC-ENER-JUST-TRANSITION@ec.europa.eu

European Commission  
B-1049 Brussels 

ISBN: 978-92-68-25053-2 

doi: 10.2833/0839536 

MJ-01-25-034-EN-N

Manuscript completed in July 2025
1st edition
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MINIERA D’ENERGIA TRANSFORMING A COAL MINE IN SARDINIA INTO A HYBRID ENERGY STORAGE HUB USING GRAVITY TECHNOLOGY INITIATIVE FOR COAL REGIONS IN TRANSITIONC A S E   S T U D Y

3

Miniera d’Energia: Transforming a coal mine in Sardinia 
into a hybrid energy storage hub using gravity technology 

The Miniera d’Energia is a repurposing project for a decommissioned underground coal mine 
in Sardinia, Italy that aims to use the shafts for an innovative gravity-based battery energy 
storage system. Energy Vault, a global energy storage company together with Carbosulcis, the 
local mining company wholly owned by the Region of Sardinia works together on creating a pilot 
hybrid storage system that can provide former underground coal mines with a new purpose.  

D E S C R I P T I O N

Location: Carbosulcis coal mine, Sardinia, Italy  

Type of action: Renewable energy infrastructure 
transformation and repurposing of a coal mine 

Actors: Public company, private company, regional authority

Financing conditions: EU funding, private funding, state aid

CARBOSULCIS COAL MINE

K E Y   P O I N T S

A P P R O A C H

•  Hybrid energy storage system combining 
gravity-based and battery technologies 
using existing coal mine shafts.

E N A B L I N G   C O N D I T I O N S

•  Availability of mine shafts and infrastructure; 

•  National framework conditions and EU funding support;

•  Close collaboration between stakeholders;

•  Year-long process of exploring energy alternatives.

A C H I E V E M E N T S

•  Applied gravity energy as a completely 

new technology at pilot stage; 

• 

Improved their administrative capacity 
to secure external funding.

K E Y   C H A L L E N G E S

•  Alignment of coal phase out date with project timelines;

•  Proving economic viability and technical feasibility; 

•  Ensure future work opportunities for the existing workforce.

MINIERA D’ENERGIA TRANSFORMING A COAL MINE IN SARDINIA INTO A HYBRID ENERGY STORAGE HUB USING GRAVITY TECHNOLOGY INITIATIVE FOR COAL REGIONS IN TRANSITION4

A   V I S U A L I S A T I O N   O F   T H E   P I L O T   M E M B R A N E   P O D S   T O   B E   U S E D   F O R   T H E   G R A V I T Y   

E N E R G Y   S T O R A G E   I N   T H E   S U L C I S   R E G I O N ,   I T A L Y . 
Source: Energy Vault

Introduction
The transition to climate neutrality does not only require 
the installation of wind and solar power generation, but 
also suitable energy storage solutions. In Sardinia, Italy, 
there are plans to transform the Nuraxi Figus coal mine into 
a carbon-neutral technology hub (see also box). The project 
integrates a hybrid energy storage system that combines 
gravity-based battery storage technologies to stabilise the 
island’s power grid and facilitate the local use of renewable 
energy. As one of the first real-world applications of 
its kind, the gravity storage system makes use of the 
underground shafts of the former coal mine and benefits 
from existing energy infrastructure.  

The central mechanism of the energy storage system is a 
lift that transports heavy weights up or down – either to 
generate power or to store energy, similar to a traditional 
pumped hydro system. The site combines 20 MW of 
gravity storage within 500-meter-deep mine shafts and 
80 MW of lithium-ion battery storage, as this was 
seen to be most beneficial to serve its main purpose of grid 
stabilisation for renewable energy management.

For the project, the municipality-owned mining company 
Carbosulcis collaborated with the US company Energy 

Vault, incorporating the first commercial scale deployment 
of their EV0 modular pumped hydro system. The system 
uses prefabricated membrane pods shaped like trees 
(see picture) to store water on ground level and inside the 
mine shafts. The EV0 system connects several of these 
water storage modules to pump turbines via penstocks. 
With a peak power output of 100MW, the storage system 
is rather small compared to the GW-scale energy storage 
needs that will be required for balancing a system that 
is based on renewables. However, it confers the benefit 
to provide flexible energy storage, either for minutes or 
years. The gravity system is set to work in conjunction 
with a lithium-ion battery storage solution, called G-Vault, 
to enhance grid flexibility and provide ancillary services 
such as support for frequency regulation and voltage 
control. Energy Vault plans to complete the EV0 system 
prototype by 2025, with full operations beginning in 
2027-2028. The company will then take full ownership 
and operation of the site, while the mine itself will stay in 
the ownership of the public mining company. 

Energy Vault and Carbosulcis are currently refining the 
system's operating parameters with a target of a 4-hour 
discharge duration for the gravity energy storage facility, 
meaning that the system will be capable of delivering 
its full rated power continuously for four hours. The final 

MINIERA D’ENERGIA TRANSFORMING A COAL MINE IN SARDINIA INTO A HYBRID ENERGY STORAGE HUB USING GRAVITY TECHNOLOGY INITIATIVE FOR COAL REGIONS IN TRANSITIONspecifications will be defined based on the technical and 
commercial requirements set by Terna, Italy's national grid 
operator, under the MACSE capacity market framework (see 
below for more information). This process will determine 
the system's capacity and flexibility, as well as its role in 
supporting grid stability and meeting peak demand.

The gravity storage project forms an integral part of the 
Carbosulcis Energy Hub, a comprehensive, multi-phase 
initiative aimed at transforming the Nuraxi Figus and Seruci 
mining areas into a center for renewable energy production, 
storage, and innovation. 

Phase 1 will involve the installation of a 10 MWp solar 
photovoltaic (PV) system at the Nuraxi Figure site, along 
with a 10 MWh lithium-ion battery storage system. This 
phase is designed to enhance energy management and 
provide grid-balancing services. 

Phase 2 will expand the energy hub with an additional 10 
MWp PV system at Seruci site and the full deployment of 
the integrated battery storage infrastructure, increasing the 
project’s overall capacity to store and dispatch clean energy. 

The addition of three 4.2 MW wind turbines in Phase 3 
is expected to produce around 24 GWh of electricity each 
year. This will further diversify the energy mix and reinforce 
supply resilience.

In parallel, the project is advancing environmental studies 
for an underground Adiabatic Compressed Air Energy 
Storage (A-CAES) system and developing digital platforms 
for smart grid management and Virtual Power Pakn  
(VPP) integration. 

Once fully operational, the Carbosulcis Energy Hub will 
supply approximately 9GWh/year for internal consumption, 
displacing conventional electricity procurement and 
generating annual savings of over EUR 750,000. Surplus 
energy production is estimated at 53 GWh/year from solar, 
wind, and battery systems, which will be sold under long-
term agreements to the Sardinian Regional Authority (RAS) 
at market-aligned rates, generating a projected EUR 32.7 
million in revenue over 10 years. Additionally, the project 
will create a minimum of 23 direct jobs, with broader 
indirect employment expected through supply chain and 
maintenance activities. 

5

Other economic diversification activities 
at the Nuraxi Figus mine

In addition to the energy storage project presented in 
this case study, Carbosulcis has introduced a range 
of projects to diversify its business as part of its 
transition strategy away from coal use for energy. 

The former mining shafts are not only used for the 
energy project, but also have been identified as an 
ideal location for purification of the gas argon that 
is used for scientific experiments on dark matter. 
As part of the ARIA project, a collaboration of 
the National Institute of Nuclear Physics, Princeton 
University, the University of Cagliari and Carbosulcis 
developed a 350m cryogenic distillation column to 
produce very pure Argon-40, commonly used for… 
As technological spin-offs, the plant also started to 
produce stable isotopes of commercial interests, 
such as 3 C,  15 N, and  18 O, which are used, for 
example, in medicine. 

In collaboration with the University of Cagliari, 
Carbosulcis is also involved in the SPIRULINA 2.0 
project, which aims to utilise hot water from the 
mine to grow spirulina algae to be used as food, 
health supplements or cosmetics. The research 
includes the cultivation of 13C-labeled CO2, 
produced through ARIA’s isotope separation, for 
applications in medical and pharmaceutical sectors. 

Another activity at the mine is the FeDe project, 
which aims to desulfurize the coal produced at the 
mine and repurpose the by-product as an organic 
fertiliser for agriculture. This process began during 
the mine's operation and could potentially continue 
using leftover coal.

Lastly, the Ulysses project aims to transform 
part of the Carbosulcis mine into an underground 
research laboratory focused on geological storage 
and methane recovery from coal beds. 

To support Sardinia's energy transition, Carbosulcis 
has additionally also proposed an 8.08 MWp solar 
photovoltaic (PV) plant on a reclaimed landfill 
site. This solar installation is expected to generate 
14.1 GWh annually and is a part of the company’s 
broader vision to transform the former coal mining 
area into a renewable energy and technology hub.

Read more on the Carbosulcis website (in Italian): 
https://www.carbosulcis.eu/

MINIERA D’ENERGIA TRANSFORMING A COAL MINE IN SARDINIA INTO A HYBRID ENERGY STORAGE HUB USING GRAVITY TECHNOLOGY INITIATIVE FOR COAL REGIONS IN TRANSITION6

Overall, providing Battery Energy System investors with 
key downside risk protection as well as ensuring highly 
bankable projects given a strong credit counterpart (the 
government backed system operator Terna).

Close collaboration between Carbosulcis and Energy 
Vault, as well as the regional authority of Sardinia is 
seen as another enabling condition for project realisation. 
The current planning is the outcome of a year-
long process of exploring energy alternatives, 
and especially of how to make use of existing energy 
infrastructure for the time proceeding coal mining. As such, 
the deep shafts have been treated as an asset rather than 
a liability that need to be dealt with. To be able to explore 
innovative solutions, Carbosulcis also signed several 
partnerships with universities and public authorities for 
research and development projects, that led to the gravity 
storage project, but also other business opportunities at the 
site (see box above).

Achievements 
Even though the gravity energy storage project is still 
in its pilot stage and final proof of concept is still to be 
seen, the activities at the mine provide some valuable 
lessons learnt about how a (public) mining company 
can actively shape a coal phase out process, by 
initiating alternatives for energy production and storage, 
collaborating with international companies and research 
institutions and  doing outside-the-box thinking. Notably, 
these activities can also happen in a rural area without 
the benefit of proximity to larger metropolitan areas or 
industrial hubs that often make it easier for companies to 
diversify their businesses. 

The public company also improved their 
administrative capacity to secure external funding, 
which happens to be crucial for any future transition 
project development. 

Further reading
 Energy Vault. Energy Vault Gravity System.  
https://www.energyvault.com/products/g-vault-
gravity-energy-storage

 RSE Research (2024). The Electricity Storage 
Capacity Procurement Mechanism.  
https://www.rse-web.it/wp-content/
uploads/2024/05/08_MACSE-inglese.pdf

 Aurora (2024). The role of power storage systems 
and investment opportunities in Italy.  
https://montel.energy/assets/03-aurora.pdf

Key challenges 
The development of the Miniera d’Energia hybrid energy 
storage system faces several key challenges. These 
mainly stem from technological development issues, 
permitting processes and alignment with project 
timelines (the mine is expected to close in 2026, this is 
when the gravity storage pilot stage should be finished), 
and meeting regulatory and market requirements. 

In addition, the Italian energy storage market is slowly 
becoming more competitive, with multiple projects 
competing for funding under the MACSE procurement 
mechanisms (see below). To be eligible to receive funding, 
the Miniera d'Energia project had to demonstrate 
economic viability and technical feasibility, which 
was not considered possible initially due to the novel 
technology approach. As such, the decision was taken 
to first prove technology readiness via a pilot before 
expanding to full-scale implementation.  

As mining activities will cease after 2026, a key interest 
of the mining company is to ensure future work 
opportunities for the 116 workers. Despite the energy 
storage project providing some new jobs, only very few 
could be recruited from the existing workforce due to 
different skills requirements. This has similarly been 
the case with other business diversification processes 
at the mine (see box) and remains a key challenge for 
Carbosulcis and the region as a whole. 

Enabling conditions 
One of the key enabling conditions for the hybrid 
energy storage project is the availability of existing 
infrastructure at the Nuraxi Ficus coal mine. The site 
includes 500-meter deep mine shafts, a core precondition 
for the gravity energy storage application, 15 kilometres 
of underground tunnels, and extensive surface facilities 
covering over 200 hectares. 

Financial national framework conditions and EU 
support has been another critical enabling condition to 
realise the project. The European Commission's approval 
of a 17.7 billion EUR state aid scheme for energy storage 
projects, along with Italy’s new Mechanism for Acquiring 
Clean Energy Services (MACSE) was seen as the framework 
that made investments into the new storage system 
a business case. In short, the MACSE scheme provides 
key downside risk protection for infrastructure investors, 
ensuring long-term investment viability by offering 
competitive contracts with internal rate of return exceeding 
9% with minimal exposure to energy market volatility. 
Moreover, the MACSE business case is not exposed to 
merchant risks1 as it offers fixed payments (€/MWh) for 15 
years and is partially adjusted for 20% of yearly inflation. 

1  Merchant risk refers to the financial risk to which a project or asset is 

exposed when its revenues depend on selling electricity at fluctuating market 
prices rather than receiving fixed payments or tariffs.

MINIERA D’ENERGIA TRANSFORMING A COAL MINE IN SARDINIA INTO A HYBRID ENERGY STORAGE HUB USING GRAVITY TECHNOLOGY INITIATIVE FOR COAL REGIONS IN TRANSITIONGETTING IN TOUCH WITH THE EU

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