When the Portuguese electric power grid needs more electricity, a large multinational power company releases millions of gallons of water from a dam reservoir.

Stanley Reed and Matilde Viegas traveled to Ribeira de Peña in northwest Portugal to locate the project and photograph it extensively.

When Portugal’s electrical system needs a boost, a signal activates a power plant buried deep in a mountainside in the country’s bushy, pine-covered north. Inside the man-made cave, valves, nine feet in diameter, suddenly opened, allowing water to flow through four giant turbines from a reservoir four miles away.

Up close, the spinning turbines make an ear-piercing noise. At full power, they generate enough electricity to rival a nuclear reactor.

It’s the center of a massive hydroelectric project that’s reshaping a rugged river valley about 65 miles 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 Tamega River and one on a feeder stream – and the three resulting reservoirs spread over about four square miles.

“This is my pyramid,” said project manager David Rivera Pantoja, who has been working on the project for nearly 15 years.

But the 1.5 billion euro ($1.6 billion) complex of concrete, tunnels and water isn’t just massive. It is also providing an answer to one of the most complex questions facing renewable energy.

Hundreds of billions of dollars are being spent around the world on solar power and wind power. But when the sun goes down, or the winds calm down, where will the electricity come from? Iberdrola’s massive project – which uses water and gravity to generate electricity on demand, and then pump the water back into an upper reservoir when rates drop – is part of the solution.

The concept of storing energy in the form of water on top of a mountain has been around for over a century, but interest waned in the 1990s, when plants burning natural gas became a well-known source for on-call electricity. , the value difference between shaving peak and off-peak power.

Now, however, there is a global renaissance of sorts in the technology known as pumped storage.

What has changed is the rapid development of clean sources of electricity, such as wind and solar farms, in countries such as Portugal. While these technologies churn out electric power free of greenhouse gas emissions, they produce an energy stream that is less stable than a conventional power plant powered by coal, natural gas or nuclear reaction.

The ongoing shift away from renewable energy sources and fossil fuel plants is creating a need for other sources of electricity that can help bridge the gap.

“You can’t just have solar and wind,” said Fabian Rönningen, an analyst at Rystad Energy, a consulting firm. “You need something to balance.”

Tapping a reservoir and using its water to spin underground turbines allows engineers to create renewable energy on demand. The rise and fall of the dam’s water serve as visual markers of this process.

One such facility on Portugal’s Tamega River stores energy in the form of water when the wind is blowing on strong or sunny days, and then lets it flow, generating electricity and raising the water level in an upper reservoir. Falling occurs when energy is less abundant and more expensive.

It’s like a giant battery, but a battery that produces far more electricity for longer periods of time than larger power storage facilities used for similar purposes. And it can be recharged endlessly by reversing the flow of the turbines and pumping water back into the tunnel.

Iberdrola executives say plans by governments in Europe and elsewhere to increase wind and solar power mean more demand for facilities on Tâmega.

Analysts say pumped storage plants could provide energy insurance to install even more sources of clean energy generation, aiding efforts to combat climate change. Iberdrola, for example, plans to set up a large wind farm nearby.

Because pumped storage plants are so useful for keeping the power grid humming, they are being favored in many countries, including China, India and Australia. Several proposals are also making their way to reality in the United States.

But even such large projects bring considerable downsides. In Europe, the scope for building such massive facilities may be limited by the high cost, long duration, and opposition by environmentalists and local residents to flooding of river basins. And flooding from dams can damage river habitats and archetypes of fish, birds and plants.

Furthermore, there are already dams on the better sites, so it was quite unusual to go ahead with a large complex such as Tâmega’s in a Western European country. “It was quite an extraordinary project,” said Martin Burdett, news editor for the International Journal on Hydropower and Dams. And the construction work is not completed yet.

For this reason, energy companies may focus on upgrading existing hydroelectric facilities with pumps and other equipment so that they can reuse the water that is lost when it flows through a conventional hydroelectric dam.

“We will turn them into something suitable for the future,” said Ivar Arne Borset, a senior vice president at Norwegian company Statkraft, one of the world’s leading operators of hydroelectric installations.

Still, as the climate continues to warm, southern European countries like Portugal are being hit by droughts, including a brutal one last summer that slashed Portugal’s hydroelectric production by more than 50 percent over 2021. reduced.

“It’s a very expensive way and a very destructive way to produce energy,” said João Jonaz de Mello, an associate professor in the Department of Environmental Sciences at the University of Nova in Lisbon.

Despite this, the Tâmega project won a seal of approval from the European Investment Bank, the EU’s lending arm, which has provided a loan of €650 million.

“The investment provided for 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 emailed message. The bank also wanted to stimulate the local economy and create jobs.

A Spanish government body, the Instituto de Credito Oficial, is kicking in another loan for €400 million. Portugal has also promised an annual payment of €12.74 million for 10 years as an investment incentive.

The construction of the Tâmega facility was a long and difficult process that involved much more than obtaining financial backing. Iberdrola won an auction for the site in 2008, paying a little over €300 million to use the site for 70 years, and then negotiated compensation for the approximately 50 homes that would be inundated by the reservoir. The company agreed to pay €50 million for roads, playgrounds and sports facilities to compensate local governments. And the company has agreed to environmental enhancement projects such as planting trees on comparable expanses of land compared to the floodplains in the dams.

To ensure sufficient water for the power plant, even during droughts, Iberdrola built two dams about six miles apart on the Tamega. Each of these barriers will have turbines so that they too can produce electricity and revenue when water is released.

The company also dammed a small river that flows into the larger river, forming a third reservoir on a plateau about 2,000 feet above the valley. A four-mile, 24-foot-wide water tunnel connects this artificial lake to hollow caverns deep underground where turbines and other equipment are installed.

For most of its length, the tunnel is flat, but as it approaches the power plant, it begins to drop and eventually falls vertically as water pressure builds up. If all four turbines are running, the water rises at a rate of 42,000 gallons per second. Higher up, on the reservoir, the water level gradually decreases. The one-hour release draws down the level of the upper reservoir, which is about 100 feet deep when full, by about two feet.

Construction continues on the facility. One of the dams, Alto Tamega, will not be completed until 2024.

However, the underground power plant is already operational. There, a team of technicians work in an underground world. The facility has commenced power generation while undergoing final adjustments.

So far, as a system that can reuse water, it is proving to be more resilient in times of drought than traditional dams. Mr. Rivera says the facility ran nearly full in the fall, releasing water when power prices rose and sending it back when power dropped. He said that the same water went “up and down, up and down” all day.

Iberdrola says last summer’s severe drought and low water levels in traditional dams have increased the use of the storage system. The company also says that periods of both high and low electricity generation from wind and solar should increase demand to tap into the reservoir. Officials say the more often the turbines are used, the more revenue they will generate for Iberdrola.

The power plant is set up in a vast cavernous space. Technicians in yellow vests and hard hats are sometimes present to perform maintenance but they are not required all the time. The turbines are switched on and off from Madrid, where Iberdrola is located.

And so after 15 years, Mr. Rivera is almost done with his monument. While space to build conventional dams is running low in countries such as Portugal, water is such a useful medium for storing electricity that it seems almost inevitable that more of these stations will be built.

But there may not be more such large projects in Europe in the future. The innovations are likely to reduce water loss and make facilities less destructive to the environment. Mr. Burdett of the International Journal on Hydropower and Dams said existing hydroelectric plants would be modernized to conserve water. He said empty mine shafts and digging near the sea were all being considered as alternatives to damming rivers.

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