STEPs, these gigantic batteries that will become crucial for the electrical system

2023-07-22 05:55:00

How does a STEP work?

A WWTP makes it possible both to produce electricity using hydraulic energy, but also to store this energy. It systematically rests on two basins, with an upper water reservoir and a lower water reservoir. During consumption peaks, in the morning and at the end of the day around 7 p.m., the water descends from the upper basin to the lower basin where the hydraulic power station is located. For example, at the Grand’Maison dam located in Isère, the second most powerful in Europe, the water travels 7 kilometers through galleries dug in the mountain, then falls 1,000 meters to strike the power plant’s turbines at 400 km / h, which activate a rotor which transforms mechanical energy into electrical energy. The device then makes it possible to provide the network with a power of 1,800 megawatts (MW), the equivalent of two old-generation nuclear reactors, in the space of just 15 minutes.

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Conversely, when there is too much electricity on the network, due for example to significant production from solar and wind farms and relatively low consumption (which happens in particular at night and on weekends), the hydropower plant brings the water up to the upper basin. This action consumes a lot of electricity, which makes it possible to absorb excess electrons on the network, while constituting a stock of water that can be used later to produce electricity during a future peak in consumption. At the Grand’Maison dam, the storage capacity can vary between a few megawatts and 1,300 MW, depending on the needs of the network. WWTPs are thus often compared to huge batteries. Apart from a small percentage of regulatory flow returned to the watercourse on which the hydropower plant is installed, the water circulates permanently in a loop. Several cycles of turbining and pumping can thus follow one another during the same day.

How many STEPs are there today in France?

Today, France has six hydraulic dams equipped with WWTPs. These are under concession and all are operated by EDF. They represent 5 gigawatts (GW) of installed capacity: Grand’Maison (1800 MW) in Isère, Montézic (910 MW) in Aveyron, Sup

er-Bissorte (730 MW) in Haute-Savoie, Revin (720 MW) in the Ardennes, Cheylas (460 MW) in Isère, La Coche (380 MW) in Savoie.

Why are WWTPs expected to play an increasingly important role in the future?

According to the Futurs Énergétiques report by the electricity transmission system operator RTE, solar and wind power (onshore and offshore) might represent just over half of the electricity mix by 2050 in the event that 14 new second-generation EPR reactors are built (scenario N2). Hypothesis taken up by Emmanuel Macron during his speech in Belfort in February 2022.

However, these energy sources are intermittent. This means that they only produce electricity when there is light and wind. Thus, when these factors are not present, they do not produce electrons. They can thus produce a lot when consumption is low, on weekends, for example. Or, on the contrary, not to produce when the electricity demand is very high. They are said to be uncontrollable, unlike nuclear power or even coal and gas power plants (both of which emit a lot of CO2, on the other hand), which can be adjusted to consumption needs. However, to ensure the proper functioning of the electrical network and avoid a blackout, the production of electricity must at all times correspond to the consumption of electricity.

The electrical network will therefore need more and more flexibility tools to ensure this tricky balance, hence the importance of WWTPs, which make it possible to absorb the excess electricity and reinject it at the desired time. RTE thus estimates that 8 GW of STEP will be needed by 2050, ie 3 GW more than current capacities. The current energy roadmap provides for 1.5 GW of additional STEP by 2030-2035.

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What is blocking?

Today, EDF estimates to be able to develop these additional 3 GW following 2035. However, “in the absence of visibility on the future of the legal regime for hydraulic concessions in France, EDF cannot make investments to develop its hydraulic fleet”, explains the group, which is pleading for the ownership of the dams now under concession to be returned to it.

Indeed, since 2015 Paris has been in dispute with Brussels, which demands to open up to competition the operating concessions of the expired dams. Since then, a status quo has settled: France does not open the concessions to competition, but does not make new investments in these facilities either… Which has largely slowed down the development of hydroelectricity in recent years, when it represents 11% of the tricolor electricity mix.

For now, the electrician is content to make small improvements. For example, at the Grand’Maison dam, whose concession still runs until 2060, four groups of the hydraulic power station have been renovated, enabling them to gain an additional 14 MW each. “Our goal is to improve the hardware so that it requires less maintenance to increase the utilization rate”, explains Maxime Pradel, dam operation coordinator.

EDF has a much bigger project in the cards: boosting the capacities of the Montézic STEP, by adding a new 460 MW power plant, representing a potential investment of more than 500 million euros. When will this huge project be able to see the light of day? The answer is obviously complicated. Because, in addition to the dispute with Brussels, EDF faces a wall of investments. This week its CEO, Luc Rémont, estimated that it would be necessary to more than double the amount of annual investments to 25 billion euros, while the group’s debt peaks at 65 billion euros…

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