the project

CEEGS (CO2 based electrothermal energy and geological storage system) is a cross-sectoral technology for energy transition, with a renewable energy storage system based on the transcritical CO2 cycle, CO2 storage in geological formations and geothermal heat extraction. 

It is a highly efficient, cost-effective, and scalable (small-to large-scale) concept for large-capacity renewable energy storage.

Extended capacity is obtained due to the underground system. It can be integrated into the grid, heating and cooling districts and industries. It also has the capacity for partial CO2 sequestration. The main objective of the project is to provide scientific proof of the techno-economic feasibility of the technology, raising the current low Technology Readiness Level from 2 to 4 by addressing gaps in the interface between surface trans-critical cycle and the subsurface CO2 storage.


From theoretical principles to models, simulations and processes in which advanced numerical simulations integrate reservoir behaviour, wellbore design and surface plant design;

From models and simulations to systems/experimental verification addressing CEEGS integration and efficiency in energy systems, with digital functional and laboratory models developed and components validated with results from the CO2 pilot-scale projects and;

Social, economic and sustainability assessments where social acceptance studies, Life Cycle Assessment (LCA) and Techno-Economic Analysis (TEA) tools evaluate impacts and concept deployment with renewables, hard-to-decarbonise industries, district heating and cooling, or in grid balance.

CEEGS Concept


First stage is to demonstrate the feasibility of the concept, solving the main challenge of the interface between the surface/subsurface parts, currently in TRL2. The rest of the components have higher TRLs. The project will set the interface and global concept into TRL 4. A 20 kW lab demonstration for the interface, TRL4, shall focus on the CO2 cycle and its operability. It will take three years. If successful, a second stage would build a demonstrator of 100 kW that integrates subsurface and surface components to reach a TRL6 in a second, 3-year project. A third stage 3-year project would impulse the technology up to a commercial scale.


Development of a proof of concept, Technology Readiness Level 4, based on the integration of models for system, components and energy system integration, a lab demonstration on a 20 kW power scale, as well as experimental data focusing on the challenges for the interface between surface components and underground systems, where there is the knowledge and technological gap.


It is a scalable system with the potential for high-performance, high-capacity energy storage. It can be used for electricity storage and production, heating and cooling, also providing partial CO2 sequestration. The concept minimises costs and surface impact, increases the energy storage capacity, and delivers energy in different forms, providing high versatility due to integration possibilities in multiple applications.


A novel concept integrating thermoelectric energy storage based on reversible transcritical CO2 cycles, for renewable energy, both in the form of electricity, thermal, and CO2 storage in geological formations.


A multidisciplinary consortium with expertise in energy (turbomachinery, processes, heat exchange, energy storage, thermal systems, etc.), geology (well designs, CO2 underground storage, etc.), and social sciences (risk perception, public engagement) with the support of leading European geology associations and industries in the energy sectors.

Key elements of the impact

Learn more about the project impact

The green and digital transition in Europe requires:

Decarbonisation of heating/cooling and electricity sectors through breakthrough technologies.

Large storage solutions for excess electricity generated, for example, through wind turbines and photovoltaic systems.

Storage solutions that are interoperable with the existing grids to utilise the energy infrastructure.

Enabling technologies for renewable energy production, in particular in the Mediterranean area and on islands.

Expected Results


A breakthrough energy storage solution based on transcritical CO2 cycle that combines geological storages of CO2 in natural reservoirs and new closed-loop CO2 cycle technology, with their integration at TRL4.

Definition of geological availably together with geological system design to guide exploration and feasibility.
New component designs and engineering solutions to make the CEEGS concept technological and economically viable. Solve open issues on TCO2/underground storage design.
Validation of technical assumptions and complete risk analysis, including social acceptance that fosters decision-making in a stage-gate process.

Draft a complete business model and business cases for stakeholders to trigger further public and private investments in the development (TRL 5-7) and demonstration (TRL 7-8) of the CEEGS technology.


Full set of technical benchmark data for component and system level as well as interoperability with the grid and renewable energy sources.
New knowledge on interface and load change effects on equipment and the CEEGS technology.
Knowledge on scale-up and commercialisation of CEEGS components in partnership with companies.
Scale up to a CEEGS Demonstration Plant level by engineering solution provider.
Full commercialisation of the CEEGS technology and roll out in Spain, Germany, and France.
Clear understanding of the process economics and feasibility including CAPEX and OPEX of the CEEGS system for different plant scales and geographical regions.



Demonstration of integrated CO2 underground storage will significantly increase energy storage operational efficiency. Global advancement of the knowledge on CO2 storage and closed CO2 cycles to stimulate further scientific discoveries.


Advance in a low cost-high capacity energy storage, capable of providing dispatchable renewable electricity, hence contributing to a clean energy system that is reliable, affordable and fair. Drive technical innovations at public research institutions and companies. New partnerships, products, and services as well as economic growth of equipment manufacturer, engineering service providers and energy companies across Europe. Triggering of new investments in the technologies. Contribution to the ‘security of supply’ due to grid stabilisation.


Contribution to informed political decision-making and stimulating public discourse through our profound communication activities. Support of legislative decision-making through scientific evidence. Support of areas where the CEEGS technology will be implemented.


Energy storage system for renewables dispatchbility, with low impact materials with safe and permanent storage of larger amounts of CO2 in geological settings.


The project integrates the knowledge and networks for a successful implementation in 3 years with a consortium with partners from 5 EU countries, with multidisciplinary skills on energy systems, energy storage, geology, geothermal systems and CO2 geological storage

The Consortium partners

University of Seville
The Centre for Research and Technology Hellas (CERTH)
European Federation of Geologists (EFG)
Helmholtz-Zentrum Dresden-Rossendorf (HZDR)
Spanish National Research Council (CSIC) (Agencia Estatal Consejo Superior de Investigaciones Científicas)
German Research Centre for Geosciences (GFZ)

Ellinika petrelaia monoprosopianonymi etaireia diylisisefodiasmou kai poliseonpetrelaioeidon kai petrochimikon

CIEMAT (Centro de investigaciones energéticas, medioambientales y tecnológicas)
ISC – Institute of Social Sciences (Instituto de Ciências Sociais da Universidade de Lisboa (ICS)
Lda (CONV)

Associated Entities

Geologica Belgica Luxemburga Scientia & Professionis (Belgium/Luxembourg)

Bulgarian Geological Society, Bulgaria (БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО)
Croatian Geological Society, Croatia (CGS – Hrvatsko Geološko Društvo)
Czech Union of Geological Associations, Czech Republic (CAEG – Unie Geologickych Asociaci)
Association of Greek Geologists, Greece (SEG – Syllogos Ellinon Geologon)
Hungarian Geological Society, Hungary (Magyarhoni Földtani Társulat)
Geosektionen (Sweden)
Italian National Council of Geologists (Italy, Consiglio Nazionale dei Geologi)
Polish Association of Mineral Asset Valuators (Polskie Stowarzyszenie Wyceny Złóż Kopalin)
Association of Portuguese Geologists, Portugal (APG – Associação Portuguesa de Geologos)
Serbian Geological Society, Serbia (Srpsko geološko društvo)
Slovenian Geological Society, Slovenia (SGD – Slovensko Geolosko Drustvo)
Official Spanish Association of Professional Geologists, Spain (ICOG – Ilustre Colegio Oficial de Geólogos)
Turkish Association of Economic Geologists, Turkey (Maden Jeologları Derneği, MJD)
National Association of Professionals in Geology and Mining, Romania (ANPGM, Asociația Națională a Profesioniștilor din Geologie și Minerit)
Ukrainian Association of Geologists, Ukraine
Geological Society of Estonia (EESTI GEOLOOGIA SELTS), Estonia
German Professional Association of Geoscientists, Germany (Berufsverband Deutscher Geowissenschaftler e. V.)
Institute of Geologists of Ireland

Industry Advisory Board

We are grateful to our Industrial Advisory Board for advising the CEEGS consortium on end-user needs, market trends and regulatory issues. 

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Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or CINEA. Neither the European Union nor the granting authority can be held responsible for them.