The intermittency of renewable energies is the main obstacle to their large-scale deployment. To ensure more consistency in supply, many rely on the use of energy created during periods of overproduction. However, this avenue involves energy storage techniques which have yet to be proven.
The 1is Last December, former senior officials and specialists in energy issues signed an open letter to denounce the massive development of renewable energies. Among the grievances stated are the cost of subsidies mainly benefiting private producers, the inflation of electricity prices and the loss of energy sovereignty induced by the importation of materials.
Meeting the challenge of intermittency
The signatories also deplore the fluctuation in electricity production due to the dependence of renewable energies on weather conditions. Can the storage of these energies, during periods of electricity overproduction, compensate for these variations in production to adjust it to demand? The various current storage processes include electrochemical storage (lithium-ion batteries), storage by hydraulic pumping (STEP[1]), or even hydrogen storage.
The island of El Hierro and its mixed results
In 2015, the island of El Hierro, located in the Canary Islands, inaugurated a wind farm coupled with a STEP. Two water retention basins filled with pumps were dug to supply the hydraulic power plant. This system was to replace the old electrical plant with Diesel generators.
With its strong wind potential and its volcanic relief offering the necessary height difference, the island seemed to be a site of choice. The surplus produced by the wind turbines was to be used to pump water from the lower basin to the upper basin in order to produce electricity when there was not enough wind. The project received international media coverage hastily calling El Hierro a “100% autonomous island.” Today, the cost/benefit ratio of the STEP is disappointing since it is the diesel turbines which still largely compensate for periods without wind. The contribution of this structure to the island’s electricity supply only reached 42% in 2016. The project, mainly fueled by a policy of subsidies, cost 5.5 times more than the cost of installing wind turbines alone .
In addition, hydraulic pumping as well as the creation of artificial reservoirs can disrupt local aquatic ecosystems.
Hydrogen or lithium-ion battery storage
Excess electricity produced can be stored in the form of hydrogen using electrolysis, the product of which will be reinjected into the conversion chain. The cost of this technique is very high. The kWh of electricity returned is in total almost ten times more expensive than that produced by a nuclear reactor.
In addition, the efficiency of the hydrogen chain is estimated at around 25% when that obtained with battery storage is 70%. The rate of energy loss during storage and release approaches 70% for hydrogen while it is less than 10% for the lithium-ion battery.
Hydrogen, an extremely light gas, requires technologies increasing its density to reduce the volume of storage. By maintaining extremely low temperatures, hydrogen changes to liquid form, increasing its energy density. However, as liquid hydrogen tends to gasify when temperatures increase, insulating materials must be developed to prevent leaks.
Hydrogen can also be stored in gaseous form in natural reservoirs such as underground cavities. Since 2021, Storengy[2] is developing a pilot device in a salt cavity. In 2023, the French group Vallourec innovates with its underground and vertical gaseous hydrogen storage system for large-scale applications.
Concerning battery storage, the company TagEnergy announced, on December 3, the launch of the construction of the future largest battery energy storage platform in France. This will benefit from Tesla Megapack technology.
[1] Pumped energy transfer stations
[2] A subsidiary of Engie
