Pumped energy transfer stations (STEP)

Also called turbinage pumping, the pumped energy transfer station (STEP) is a hydroelectric installation consisting of two basins. Located in a mountainous area, this station is very useful in order to fill temporary shortages of solar and wind energy. SirEnergies tells you more about this hydroelectric installation.

Pumped energy transfer station: What is the purpose of a WWTP?

Developed in the 1970s, a WWTP consists of two basins located at different altitudes in mountainous areas. This hydroelectric station makes it possible to realize An energy storage to redistribute it during periods of significant demand. It requires a significant drop in altitude as well as a large volume of water due to a low energy density. Significant investments are also required.

In France, there are 6 main pumped energy transfer stations. The largest WWTP is located on the Grand'Maison site in Isère. It has a volume of 14.3 million m³.

In order to raise water into one of the basins, the WWTP draws it at off-peak hours from a lower basin so that the reservoir, also called an altitude lake, is filled up upstream. The water is then Turbinated during peak hours.

The electricity then harvested will be served during a peak period, i.e. when the demand for energy is very high. This STEP makes it possible to regulate electrical production and consumption on the electrical network. During peak consumption, production costs are also reduced.

In terms of the technique used for stationary storage, it is the transfer of energy by pumping hydraulic which is the most popular. Because of its energy-consuming nature, a pumped storage station is not recognized as producing energy from renewable source.

How does a STEP work?

A pumped energy transfer station is located between two basins used as water reservoirs. When consumption is low, part of the electricity is taken from the electrical network in order to pump water from the lower basin to the high-altitude lake.

This restraint will allow store energy which can thus be used in periods of high demand. During hours of very high consumption, the water retained in the upper basin makes it possible to Make an alternator and a turbine turn in order to distribute electricity on the grid.

You should know that this system is very energy-intensive since it consumes between 15 and 30% of energy, which is colossal. STEPs can be classified into two sub-categories:

• “pure” WWTPs: they are based on a closed cycle operation where the external supply of water is almost zero,
• “mixed” WWTPs: they receive an external supply of water and can pump and turbine it between 4 and 5 times before returning it to its source of origin.

Whether they have a daily or daily intake, STEPs are only used as a last resort in order to securing the electrical network. In a sense, it is therefore a question of additional electrical capacity in order to complete the network and to fill the gaps in wind and solar energies.

Source: Energy Encyclopedia

Pumped energy transfer station: what are the challenges?

A STEP generates a lot of discussions, whether in terms economic or ecological.

Where can we find STEPs?

While a WWTP previously had to be located in mountainous areas due to a significant difference in altitude and colossal volumes of water, new hydroelectric installations are beginning to be created, as in Haute-Normandie.

These projects are called STEP Marines. These installations must be located along the coastline with a coastal drop of about a hundred meters in height for the upper reservoir. The sea will then serve as the lower basin. Nor should the site be protected by coastal law, which protects the various coastal areas from abuses linked to real estate speculation.

The maintenance of the STEP

These hydroelectric installations have a good lifespan. These installations can be counted on for forty years. However, due to increased demand on these stations, particular attention should be paid to the reliability of materials and the maintenance of these devices. In particular, the turbine must be the first part that requires special care.

The profitability of pumped-storage stations

Located between 70 and 85%, it is estimated that WWTPs have very good electrical efficiency. For example, France produces between 6 and 7 TWh per year. It is therefore important to pay particular attention between the moment when the water is pumped during periods of low consumption and the moment when the water is turbined.

For the electrical installation to be profitable, the purchase of electricity must be made at low cost and the sale must be carried out at high rates in order to cover operating costs and amortization.

The operation of STEP systems according to prices

Turbinage pump stations are very useful for smoothing prices at times of peak consumption. They also use excess energy from the grid if it is produced in too much quantity.

The least expensive technique is the energy storage by STEP. However, you should know that pumped energy transfer stations are obliged to pay the TURPE (Tariff for the Use of Public Electricity Networks) when they are final consumers of electricity.

What are the future goals for the STEPs?

By 2025 or 2030, the Multiannual Energy Programming (PPE) wants to see an increase of 1 to 2 GW in the production capacity of French WWTPs.

This shows how important energy storage is with these hydroelectric installations, both in France and around the world. In fact, these installations represent no less than 97% of energy storage capacities. In order to improve the performances of these turbinage pump stations, the IEA (International Energy Agency) suggested two innovations to be implemented by 2050:

• the development of marine WWTPs linked to offshore production units,
• the use of hydroelectric devices with variable speed, which will have perfect control of the volumes of water that are turbined or pumped.

Particular attention should also be paid to the cost of an installation, which can be an obstacle to the development of pumped energy transfer stations.

In any case, a STEP is a very useful device for energy storage. Many solutions need to be implemented in order to develop these installations.

Do not hesitate to consult our article to know The operation of a hydroelectric dam.