Electricity Storage, Reality and Perspectives:
Feasible at Small and Medium Scales, Out of Reach at Large Scale
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The idea that the issue of electricity storage has been solved or is about to be solved seems to have taken hold widely. Mistakenly. While electricity storage is available at small and medium scales, up to relatively large scales (around a hundred GWh) and while improvements at these scales are still expected (mainly concerning batteries), it is unfortunately not available at very large scales (mass storage). Yet, mass storage would be indispensable to store the electricity demand of at least one very cold winter day as a substitute to missing intermittent production (windless day). Indeed, it is 1 800 GWh/day that would have to be stored. This represents ... 18-fold the total storage capacity of France's existing PHES (Pumped Hydroelectric Energy Storage) plants; these can generate 5 GW instantaneous power and they can store 100 GWh energy. And PHES plants are, today, the most efficient industrial scale storage facilities available. This substantiates the extent of the challenge.
What can be done to do better? PHES plants along with storage of compressed air in very large underground cavities are and will remain insufficient. There is an unbeatable reason for this, pertaining to the laws of physics: the energy density of these forms of energy is insufficient. Only chemical energies, such as synthetic gases, hydrogen (produced by water electrolysis to avoid emitting CO2) and methane (produced by hydrogen methanation) present energy densities that would be capable of satisfying the needs. But the very small overall efficiencies of these conversion chains [electricity → synthetic gas → electricity] increase the cost of the electricity retrieved to such a degree that their economic model is not viable. There is a potential for improvement but the overall efficiency will still be limited because of the large number of energy transformations involved. Considerable investment cost reductions would also be necessary so that the economic viability of these solutions is quite unpredictable in the near term. Increased R&D in promising storage solutions, whether already identified or groundbreaking, remains a necessity but does not allow, today, to place mass storage capacities on the horizon when making strategic evaluations, in particular for the present PPE (Programmation pluriannuelle de l'énergie).