When the Portuguese power grid needs more electricity, a large multinational energy company releases millions of liters of water from a dammed reservoir.
Stanley Reed and Matilde Viegas traveled to Ribeira de Pena in northwestern Portugal to explore and photograph the project extensively.
When Portugal’s electrical system needs a boost, a signal activates a power station buried deep in a hillside in the country’s scrubby, pine-clad north. Inside the man-made cave, valves ten feet in diameter suddenly open, allowing water from a reservoir four miles away to flow through four massive turbines.
Up close, the spinning turbines make a deafening noise. At full power, they generate enough electricity to rival a nuclear reactor.
This is the heart of a massive hydroelectric project reshaping a rugged river valley about 105 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 feeder stream — and the three resulting reservoirs stretch for nearly four square miles.
“These are my pyramids,” said David Rivera Pantoja, the project manager, who has worked on the project for nearly 15 years.
But the 1.5 billion euro complex of concrete, tunnels and water is not only huge. It also provides an answer to one of the most difficult questions facing renewable energy.
Hundreds of billions of dollars are spent worldwide on solar and wind energy. But when the sun goes down or the breeze dies down, where does the electricity come from? Iberdrola’s mammoth project — which uses water and gravity to generate power on demand and then pumps the water back to the upper reservoir when rates fall — is part of the solution.
The concept of storing energy in the form of water on top of a mountain has been around for more than a century, but interest waned in the 1990s, when plants that burned natural gas became the go-to source for call power, which causes the price differences between peak and off-peak power.
Now, however, a kind of global renaissance in the technology is taking place, 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 an energy flow that is less stable than a traditional power plant powered by coal, natural gas or nuclear reaction.
The continued shift to renewables and away from fossil fuel power plants creates a need for other sources of electricity that can help bridge the gaps.
“You can’t just have sun and wind,” said Fabian Ronningen, an analyst at Rystad Energy, a consulting firm. “You need something to balance.”
By tapping a reservoir and using the water to run underground turbines, engineers can generate renewable energy on demand. The rise and fall of the dammed water serve as visible markers of the process taking place.
A facility such as this one on Portugal’s Tâmega River stores energy in the form of water when there is a strong wind or on sunny days, then releases it to flow, generating electricity and lowering the water level in the upper reservoir as energy is consumed . less abundant and more expensive.
It’s like a huge battery, but one that produces much more electricity for a longer period of time than the large electricity storage facilities used for similar purposes. And by reversing the power from the turbines to pump the water back into the tunnel, it can be charged endlessly.
Iberdrola executives say plans by governments in Europe and elsewhere to increase wind and solar power are leading to increased demand for facilities like those on the Tâmega.
Pumped-storage plants can also essentially provide energy insurance to install even more clean energy generation resources, aiding efforts to tackle climate change, analysts say. For example, Iberdrola plans to install a large wind farm nearby.
Because pumped-storage plants are so useful in keeping a power grid running, they are finding favor in many countries, including China, India, and Australia. Various proposals are also making their way into reality in the United States.
But such large projects also entail substantial disadvantages. In Europe, the ability to build such massive facilities may be limited by high costs, long lead times and opposition from environmentalists and local residents who object to river valley flooding. And the flooding from dams can damage riverine habitats of fish, birds and plants and flood antiquities.
Moreover, the better locations already have dams, so it was quite unusual for such a large complex as the Tâmega to start in a Western European country. “This was quite an exceptional project,” said Martin Burdett, news editor of the International Journal on Hydropower and Dams. And the construction work is not yet completed.
For that reason, power companies can focus on upgrading existing hydro plants with pumps and other equipment so they can continue to reuse water that is lost when it flows through a conventional hydro plant.
“We will turn them into something better suited for the future,” said Ivar Arne Borset, a senior vice president at Statkraft, a Norwegian company that is one of the world’s leading hydropower operators.
But as the climate warms, southern European countries like Portugal are becoming increasingly vulnerable to drought, including a brutal last summer that reduced Portugal’s hydroelectric output by more than 50 percent compared to 2021.
“It’s a very expensive way and a very destructive way to produce energy,” said João Joanaz de Melo, an associate professor in the Department of Environmental Sciences at NOVA University in Lisbon.
Despite this, the Tâmega project received a seal of approval from the European Investment Bank, the European Union’s lender, which has provided a €650 million loan.
“The investment for this project will reduce the Iberian market’s dependence on fossil energy and 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 government agency, the Instituto de Crédito Oficial, kicks in a new loan of 400 million euros. Portugal has also promised an annual payment of €12.74 million for 10 years as an investment incentive.
Construction of the Tâmega facility was a long and difficult process that involved much more than garnering financial support. Iberdrola won an auction for the site in 2008, paid just over €300 million to use the site for 70 years, then negotiated compensation for the 50 or so homes that would be flooded by the reservoirs. The company agreed to pay €50 million for roads, playgrounds and sports facilities to compensate local governments. And the company has agreed to environmental improvement projects, such as planting trees on a piece of land similar to what the dams flooded.
To ensure that the power plant has enough water even during drought, Iberdrola built two dams on the Tâmega, about ten kilometers apart. These flood defenses will all have turbines so that they too can generate electricity and income when water is released.
The company also dammed a small river that flows into the larger one, creating a third reservoir high on a plateau about 600 meters above the valley floor. A four-mile-long, 24-foot-wide water tunnel connects this man-made lake to caverns dug deep underground where turbines and other equipment have been installed.
For most of its length, the tunnel is level, but as it approaches the power plant, it begins to descend and eventually plummets vertically, creating tremendous water pressure. When all four turbines are running, water flows through them at 42,000 liters per second. High above, at the reservoir, the water level slowly drops. An hour of release pulls the level of the upper reservoir, which is about 100 feet deep when full, down about two feet.
Construction continues at the facility. One of the dams, the Alto Tâmega, will not be completed until 2024.
However, the underground power plant is already in operation. There, a team of technicians works in an underground world. This installation has started producing electricity and is still undergoing final adjustments.
As a system that can reuse water, it has so far proved to be more resilient in times of drought than conventional dams. Mr Rivera says the facility was running near full capacity in the fall, releasing water when electricity prices rose and returning it when they fell. The same water went “up and down, up and down” all day long, he said.
Iberdrola says last summer’s severe drought and low water levels at traditional dams have increased use of the storage system. The company also says periods of both high and low electricity generation from wind and solar should increase demand for the reservoir. The more the turbines are used, the more revenue they bring to Iberdrola, executives say.
The power plant is installed in a huge cavernous room. Technicians in yellow vests and hard hats are sometimes present to perform maintenance, but are not always required. The turbines are switched on and off from Madrid, where Iberdrola is located.
And so, after 15 years, Mr. Rivera has almost completed his monument. While countries like Portugal are running out of space to build conventional dams, water is such a useful medium for storing electricity that it seems almost inevitable that more of these centers will be built.
But in the future there may not be many projects as big as this one in Europe. Innovations are likely to reduce water loss and make the facilities less impactful on the environment. Mr Burdett of the International Journal on Hydropower and Dams said existing hydropower stations will be upgraded to save water. He added that depleted mine shafts and excavations near the sea were all considered alternatives to damming rivers.
