Mass electricity storage by thermal geodoublet & CO2 as working fluid – SeleCO2

To do so, the scientific and technical program has been divided in 4 work packages:
- WP0 is led by ENGIE and dedicated to project coordination,
- WP1 is led by ENGIE and is dedicated to the technical and economic evaluation of the project: study of the value chain of electricity and of competing storage technologies will help define the target cost for the SELECO2 project as well as its positioning in the value chain.
- WP2 is coordinated by BRGM and is dedicated to modeling of different components of the process as well as its integration in a global model. This WP is fundamental for the project. Sizing of rotating machines, thermal transfer in the exchanger and in the thermal storages are a challenge when dealing with supercritical CO2. Once these elements are characterized, optimization of operational parameters will be realized. This will address the constraints of various elements of the value chain of the concept industrialization defined in WP1.
- WP3 is coordinated by CEA and is dedicated to the conception of an experimental set-up to study supercritical CO2 flow behavior. This WP will help define various parameters and behavior models of supercritical CO2 that are needed to the process modeling developed in WP2. According to the results of WP1 and WP2 regarding system behavior, tests will be defined.

The first significant result of the project was on the positioning of the CO2 technology in the value chain of electricity. Preliminary results showed that SELECO2 technology would position on optimization of production and of distribution and transport networks. Capacity market associated with energy supplier could also be a potential market for SELECO2 technology.
Thanks to preliminary findings on system sizing (thermal storage, rotating machines,…) and optimizations on the choice of material for the SELECO2 project led by the partners – and validated by numerical simulation, investment costs of the technology were reduced significantly. Thanks to these recent improvements, technology could be seen as a competitive technology within this high growth potential market.
From an experimental point of view, a test loop was designed in order to assess supercritical CO2 behavior during heat exchanges occurring at the wall (thermal flow implemented) as well as thermal penetration within the rock mass (diffusive flow).

The final perspective of the project is to assess the technical, economical and environmental feasibility of this innovative process for electricity storage, which will use a thermal geological doublet with supercritical CO2 as heat transfer fluid. Proving the competitiveness of this kind of solution would help remove some barriers frequently encountered for more mature technologies such as PSH and STEP.
Some of the barriers such as thermal exchanges or working cycles still need to be overcome by the end of the project. Particular attention will be paid on design and sizing of the cold storage as well as finding a way to reduce the technology costs.

Papers on the project results are currently under consideration and will be prepared by the partners.

Peak power consumption control and the development of renewable energy are the two challenges faced by most industrialized countries.
To meet these challenges, the massive electricity storage technologies will be called to play a growing role. However, as the more mature technologies such as PHS (Pumped hydraulic storage) or the CAES (Compressed Air Energy Storage) are facing environmental, geological and economic constraints that limit their growth or their deployment around the world.
Faced with these constraints, Thermo-electric energy storage electricity (TEES) has the advantage of presenting a higher energy density and escape geological and geographical constraints.
This context has motivated the launch of the SeleCO2 project which aims to conduct preliminary research and development in the implementation of innovative technology of storage of electricity based on the use of a thermal geodoublet with CO2 as working fluid.
The principle of SeleCO2 process this is based on a thermodynamic cycle using CO2 to store electric energy in the form of cold and heat in two thermal geostocks in a shallow rocky massif.
The device includes two cylindrical volumes in a massive rock out-of water, called geostocks (the 'cold' at -55°C and the other 'hot' to +95°C maximum) connected by a circuit of CO2 which are established a heat pump and a ORC turbine (Organic Rankine Cycle). Each geostock is composed of a set of vertical geothermal heat exchangers in which circulates the cooling fluid (CO2), which exchanges heat with the rock mass through the wall of the heat exchanger.
During loading phase, electricity is used to transfer heat from the low temperature geostock to the high temperature geostock thanks to a heat pump. When the geodoublet is 'full' the temperature difference between the two geostock is maximum.
During unloading phase, the thermal transfer between the high temperature geostock and the low temperature one is converted into mechanical energy by thermal machinery (turbine-compressor system) that drives a generator producing electricity.
The energy storage capacity corresponds to the energy accumulated in the two geostocks as well as the energy accumulated within the significant volume of CO2 contained in geothermal heat exchangers.
This storage method allows to consider efficiencies of high cycles, the use of a heat pump for the load to compensate the low thermal mechanical energy conversion efficiency in the discharge. In addition, CO2 under critical conditions allows to reduce the size of the system.
Based on preliminary studies, the overall electrical performance expected is between 50 and 70%, for 10 MW power combined and a large thermal capacity (on the order of 1 GWh). Moreover, the process allows storing energy during more than 100 hours which is very unusual for an energy storage technology.
Given this characteristics, SeleCO2 technology can be considered as a low cost energy storage technology. Consequently the economic, industrial and environmental potential of the technology could be very interesting.
Given the uncertainties associated with this innovative technology, the project will focus in a first phase of 6 months (marked by a milestone) to investigate more precisely the main technico-economic characteristics of the technology.
These preliminary results will condition the second phase of the project where it is proposed to conduct extensive research work to improve the understanding and modeling of heat transfer between CO2 in its various phases, with the exchangers and rock mass. Then the modeling of the various turbomachineries and the storage process over a cycle will be performed.
The project will be coordinated by GDFSUEZ which will bring expertise in the field of energy storage to optimize the positioning of SeleCO2 technologies on the future markets.