RIBEIRA DE PENA, Portugal – When the Portuguese electricity system needs a boost, a signal turns on underground power plant deep in a hill in the north of the country, covered in scrub and pine trees.

Inside the man-made cave, valves 3 meters in diameter suddenly open and allow water from a reservoir located 6 kilometers away to start flowing through four huge turbines.

Up close, the turbines make a deafening noise.

At full power, they generate as much electricity as a nuclear reactor.

This is the heart of a vast hydroelectric project that is reshaping a rugged river valley about 65 kilometers east of Porto, Portugal’s second largest city after Lisbon.

In addition to the underground power plant, Iberdrola, the Spanish energy giant, has built three dams in the area – two on the Tâmega River and one on a tributary stream – and the resulting three reservoirs span nearly 6 square kilometres.

“These are my pyramids,” says project director David Rivera Pantoja, who has been working on them for nearly 15 years.

But the €1.5 billion ($1.6 billion) concrete, tunnel and water complex isn’t just huge.

It also answers one of the most controversial questions of the renewable energy.

Worldwide, hundreds of billions of dollars are spent on solar and wind energy.

But when the sun goes down or the breeze dies down, where will the electricity come from?

Iberdrola’s giant design — which uses water and gravity to generate power on demand, then pumps water into the upper reservoir when rates drop — is part of the solution.

The concept of store energy in the form of mountaintop water has been around for more than a century, but its interest waned in the 1990s, as natural gas-fired plants became the on-demand energy source, narrowing price differences between the peak power and quiescent power. peak hours.

Now, however, there is something of a worldwide renaissance underway in this technology, known as pumped storage.

What has changed in countries like Portugal is the rapid growth of clean energy sources, such as wind and solar farms.

While these technologies produce electricity without greenhouse gas emissions, they generate a less stable flow of energy than a traditional power plant fueled by coal, natural gas or a nuclear reaction.

The continued shift to renewable energy sources and the move away from fossil fuel power plants is creating a need for other sources of electricity to help fill the gap.

“You can’t just have solar and wind power,” says Fabian Ronningen, an analyst at consultancy Rystad Energy.

“You need something to balance.”

Exploiting a reservoir and using its water to drive underground turbines allows engineers to create renewable energy on demand.

The rises and falls of dammed water serve as visible indicators of the process that is taking place.

A structure like this one on the Tâmega River in Portugal stores energy in the form of water when the wind blows hard or on sunny days, and then lets it flow, generating electricity and lowering the water level in the upper reservoir when the energy is less plentiful and more expensive.

It’s like a huge battery, but it produces much more electricity for a longer time than large electricity storage plants used for similar purposes.

And reversing the flow of the turbines to pump water into the tunnel allows for endless recharging.

Iberdrola executives say plans by European and other governments to increase wind and solar power mean greater demand for facilities such as the Tâmega.

Pumped storage plants can also provide, in essence, energy insurance to install even more sources of clean energy generation, contributing to the effort to tackle climate change, analysts say.

Iberdrola, for example, plans to install a large wind farm nearby.

Because pumped storage plants are so useful for keeping the electricity grid running, they are gaining popularity in many countries, for example China, India and Australia.

​Several proposals are being implemented in the United States.

But projects of this magnitude also have significant drawbacks.

In Europe, the possibilities for building such large structures may be limited by high costs, long delivery times, and opposition from environmentalists and local residents who oppose flooding of river valleys.

Additionally, flooding from dams can damage riverine habitats for fish, birds, and plants, and inundate antiques.

Furthermore, the best sites already have dams, so it was rather unusual for a complex as large as the Tâmega to be successful in a Western European country.

“This is an outstanding project,” says Martin Burdett, editor-in-chief of the International Journal on Hydropower and Dams.

And the work is not finished yet.

Therefore, energy companies can focus on retrofitting existing hydroelectric plants with pumps and other equipment so that they can continue to reuse the water that is lost when it passes through a conventional hydroelectric dam.

“We will transform them into something more suitable for the future,” says Ivar Arne Borset, vice president of Statkraft, a Norwegian company that is one of the world’s leading operators of hydroelectric plants.

However, as the climate warms, southern European countries such as Portugal are increasingly vulnerable to droughts, including last summer’s brutal reduced Portuguese hydroelectric production more than 50% compared to 2021.

“It’s a very expensive and very destructive way of producing energy,” says João Joanaz de Melo, an associate professor in the Department of Environmental Sciences at NOVA University in Lisbon.

Despite this, the Tâmega project has obtained the green light from the European Investment Bank, the credit arm of the European Union, which has granted a loan of 650 million euros.

“The planned investment in this project will reduce the Iberian market’s dependence on fossil energy, as well as carbon dioxide emissions,” the European Investment Bank said in an email.

The bank also wanted to stimulate the local economy and create jobs.

A Spanish public body, the Instituto de Crédito Oficial, will provide another loan of 400 million euros. Portugal has also promised an annual payment of €12.74 million over 10 years as an investment incentive.

The construction of the Tâmega plant was a long and difficult process which involved much more than obtaining financial support.

Iberdrola won an auction for the site in 2008, paying just over 300 million euros to use it for 70 years, and then negotiated compensation for the approximately 50 houses that would have been flooded by the floods.

The company has agreed to pay €50m for courses, playgrounds and sports facilities to compensate local governments.

And the company has agreed to environmental improvement projects, such as planting trees on an area of ​​land comparable to what the dams have flooded.

To guarantee sufficient water for the plant, even during a drought, Iberdrola has built two dams on the Tâmega, separated by around 10 kilometres.

Each of these barriers will have turbines so that they can also produce electricity and income when the water is released.

The company also dammed a small river that flows into a larger one, creating a third reservoir atop a plateau about 2,000 feet above the valley.

A water tunnel 6 km and 7 meters wide connects this artificial lake with the caves dug underground where the turbines and other equipment are installed.

For most of its length, the tunnel is level, but as it approaches the power station, it begins to descend and eventually plummets vertically, creating enormous water pressure.

If all four turbines are running, water flows at 42,000 gallons per second.

Above, in the tank, the water level slowly drops.

An hour of emptying lowers the level of the upper tank, which is about 30.5 meters deep when full, by about 60 centimetres.

The works continue.

One of the dams, the Alto Tâmega, will not be finished until 2024.

The underground plant, however, is already in operation.

There, a team of technicians works in an underground world.

This plant has started producing electricity, although the last adjustments are still in progress.

So far, as a system capable of reusing water, it is proving more resilient in times of drought than conventional dams.

Rivera says the facility operated at near full capacity in the fall, pumping water when electricity prices went up and turning it back on when they went down.

The same water “was going up and down, up and down” all day, she says.

Iberdrola says last summer’s severe drought and low water levels in traditional dams have increased usage of the storage system.

The company also says periods of high and low power generation from wind and solar should increase demand to tap into the reservoir.

The more the turbines are used, the more revenue they will generate for Iberdrola, its managers say.

The plant is installed in a vast cavernous space.

Sometimes there are technicians in yellow vests and helmets to do the maintenance, but they aren’t always needed.

The turbines are turned on and off from Madrid, where Iberdrola is based.

After 15 years, Rivera has almost finished his monument.

Although countries like Portugal are running out of places to build conventional dams, water is such a useful means of storing electricity that it seems almost inevitable that more such centers will be built.

But it is possible that there will not be many projects in Europe of this size in the future.

The innovations are likely to reduce water loss and make structures less destructive to the environment.

Burdett, of the International Journal on Hydropower and Dams, said existing hydroelectric plants would be retrofitted to conserve water.

He added that spent mine shafts and excavations near the sea are being explored as alternatives to damming rivers.

c.2023 The New York Times Society