Pumped-storage power plants: STEPs

STEPs (Stations de transfert d’énergie par pompage) are large infrastructures capable of storing electricity thanks to two water reservoirs, one of which is located high up.

  • To generate electricity, water is allowed to flow from the upper to the lower reservoir, turning a turbine that transforms this kinetic energy into electricity.
  • To store electricity, water is pumped from the lower basin to the upper basin.

The principle and performance of WWTPs

The principle behind pumped-storage power stations (P.S.T.P.s) is quite simple: when you want to store electricity, you use it to pump water from one basin to another higher up. Then, when you need it, you pump the water back down, this time to turn a turbine that generates electricity, as in a conventional hydroelectric dam. This is also known as “pumped storage”.

WWTPs can be either “pure”, i.e. operating in a closed circuit, or “mixed”, i.e. receiving natural water flows as well. In the latter case, the plant is no longer just an electricity storage facility, but also a production facility in its own right.

Efficiency is quite high for a storage device: around 70 to 85%.

The price and storage capacity of STEPs

[To be completed]

1m3 of water at an altitude of 100 m has a potential energy of 0.272 kWh

History and current status of WWTPs

In 2019, there were 157,994 MW installed worldwide (I don’t have the stored energy figure). The top 7 are China (30,290MW), Japan (27,637 MW), the USA (22,855 MW), Italy (7,685 MW), Germany (6,364 MW), Spain (6,117 MW) and France (5,837 MW). The most powerful plants are Bath County in the USA (3,003 MW), Huizhou (2,448 MW) and Guangzhou (2,400 MW) in China.

In France, there are 6 main WWTPs in operation offering significant back-up capacity to the national power grid (in order of turbine power):

  • Grand’Maison in Isère, with a turbine capacity of 1,790 MW, is the 7th largest power plant in the world;
  • Montézic in Aveyron (910 MW);
  • Super-Bissorte in Savoie (730 MW) ;
  • Revin in the Ardennes (720 MW);
  • Le Cheylas in Isère (460 MW)
  • La Coche in Savoie (330 MW).

An interesting infrastructure is El Hierro, a small island in the Canary Islands, Spain. Inaugurated in 2014, the El Hierro hydro-wind power plant combines a wind farm with a peak power of 11.5MW, which can feed a STEP capable of transporting water over a 700m gradient and producing 11.3MW. It has decarbonized on average half of the electricity used by the island’s 8,000 inhabitants, the rest being produced by an oil-fired power plant.

Innovative projects in this field include:

  • StepSol, which proposes to combine STEP and photovoltaic production.
  • Rhenergise is proposing to replace water with a mineral-rich liquid called R-19, whose density is 2.5 times higher, thus reducing the head required (=200m) and enabling the UK, for example, to have its own.(source)
  • We could consider drowning the Vallée d’Abondance in Haute-Savoie to build a gigantic STEP capable of storing 3TWh. This would require the relocation of up to 4,000 people and create a 21km² lack. This is the Grande Abondance project dreamed up by Benjamin Laredo, a student engineer. It would cost between 30 and 50 billion euros. (source)

The future: marine and underground STEPs?

An interesting approach, particularly to combine with offshore wind power, is to install a power station at the foot of a cliff and a reservoir at the top. The lower basin would simply be the sea. A 30MW test in Okinawa, Japan, was inconclusive.

Innovative projects in this field include:

  • A Californian start-up,Oceanus Power & Water, is exploring the possibility of marine STEPs. Together with EDF, it is developing a facility in northern Chile.