Compressed Air energy storage for the electrical grid – SACRE

Though the Huntorf compressed air storage was a success, CAES did not experience the rapid development expected in the 80’s. However, when their power range is considered, they appear as the only credible alternative to pumped hydroelectricity plants. Recent changes in the technical and economical context can drastically modify this situation.
On one hand, « adiabatic » concept, in which the heat generated in the compression system is stored, should increase the energy efficiency to 70%, especially when improvements are searched for systematically (compressor train, expander, heat storage materials, design of the heat exchanger) and when underground voids are available in the vicinity of the electric grid.
On the other hand, electric energy storage makes intermittent sources of energy, e.g. windmills, more attractive, and environmental concerns lead to a more complete assessment of storage benefits, including carbon balance and optimal management or dimensioning of the electric grid.
The first part of the SACRE project includes a static modelling of the French electric grid, which will allow optimal implementation of storages. A typical day will be modeled, allowing for a more precise dimensioning of storage main characteristics. Benefits will first be assessed through a marginal approach; however a more sophisticated method will also be used in which two equilibrium models, with and without CAES, will be compared; possible gains will include smaller investments for peak consumption and reduction of CO2 emissions.
The second part of SACRE deals with air storage techniques in salt caverns, aquifer layers, mined caverns or above-ground tanks. A map of available voids or suitable geological formations will be drawn, together with an assessment of the range of costs of a stored KWh. A detailed conception study will be performed: well architecture, large casing diameter, corrosion by salty air, and environment protection concerns (cavern abandonment). Experience drawn from the Huntorf storage proves that severe although tolerable rock spalling must be expected. Numerical computations will allow extrapolation of this result to different depths, cavern shapes and features of the P-T cycle; a rock-mechanics testing program will be inferred from these computations.
Operating an adiabatic CAES also means storing heat in severe conditions – 650°C, 70 bars, salt particles in the air. For heat storage material, ceramics originating from waste treatment will be considered. This innovation might be a breakthrough. Exchanger architecture will be optimized and materials will be fully characterized. This effort will provide data for the design of a 1 m3 prototype. Extrapolation to scale 1 will be discussed together with possible advantages of a multi-storied storage system and with selection of the storage material container.
Assembling of the various technical parts of the system will be made through the study of the behaviour of air during pressure build up and decompression. During the first year of the project, a preliminary design of an advanced adiabatic system will be made; it will be used as a backbone for the following of the project; it will allow assessment of the results obtained in the course of the project and it will be continuously updated as the project proceeds.