The PSH is a storage unit that helps to maintain energy security supply and stability of the network, said Eesti Energia Developments communications manager Lennart Komp.

The plant's design is unique as it utilizes mining waste and abandoned oil-shale mine tunnels in its construction. Eesti Energia said oil shale or coal mines have never been used as water reservoirs for PSH plants before.

"In addition to maintaining supply security, the facility promotes the use of renewable energy, aids in achieving waste-free production, and reuses industrial sites," said Margus Vals, a member of the management board of Eesti Energia.

"In light of the planned connection to the continental European electrical system by 2026 at the latest, it is critical that the Baltic States establish the appropriate energy markets and production or storage facilities to maintain the security of supply, as sustainably and affordably as possible," he added.

In agreement with Elering, the transmission system operator for Estonia's electrical network, the new 225-megawatt PSH plant will be market-ready by 2025-2026.

The PSH power plant is a huge storage facility made up of two interconnected reservoirs, one of which is higher in elevation than the other. The upper reservoir will be built on a waste rock structure, while the bottom reservoir being the closed mine.

When the energy system is powerless, water flows down from the higher reservoir through pipes to an electric turbine, which converts the kinetic energy of flowing water into electrical energy. The water is then discharged into the lower reservoir.

When energy is cheap, water is pumped back up, and the process is repeated as necessary.

Renewable energy production is unpredictable; the projected and actual output of wind and solar farms vary significantly, Vals said.

"Even a couple of hours before production, the projected output can vary significantly from reality. To deal with this, the power system must be able to generate electricity as quickly as possible in order to maintain the balance of consumption and production along with the frequency. One of the technologies that enables such a trick is a modern pumped-storage hydroelectric power plant, which is able to load itself up and down in a matter of minutes," Vals explained. 

This type of plant could be introduced in other countries, where the topography prevents the development of a conventional PSH plant and where there are mines that are closed or about to close.

Eesti Energia is also proposing to utilize mining waste that was formed during the oil shale enrichment process in the construction of the plant.

The project is now in the environmental impact assessment stage, with preliminary construction plans underway.

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