OSLO — The oil and gas exports that made Norway rich are also key contributors to climate change.

Norway may have an answer for that.

The country's mountains, lakes and rivers could eventually be turned into something like a giant battery — storing power generated by wind farms and solar cells elsewhere in Europe, then sending electricity back when renewable output slumps.

The idea is to use excess power generated by renewables to pump water upstream behind dams. When the electricity is needed, the water would be released, rushing past turbines as the energy is turned back into useful electricity.

The concept appears particularly attractive for the EU, which will redesign its electricity market over the next years to reflect the growing share of renewables in the system. The European Commission is expected to make its proposals on the topic by the end of this year.

The country would be able to deliver a lot of power "when the sun doesn’t shine and the wind doesn’t blow, in times where Europe needs huge volumes fast," Norway's energy minister Tord Lien said.

The "green battery" idea excites Maroš Šefčovič, the European Commission vice president in charge of the energy union.

Using Norway’s hydro dams to ensure stable energy supply in countries that rely on renewables is “the cheapest and perhaps most environmentally-friendly solution," says an expert.

"Norway, despite its vast hydrocarbon reserves, develops its renewables and energy storage capacities in an impressive way," Šefčovič said.

Norway is already one of the world’s top hydro power producers, able to cover almost all its own electricity needs with what's generated from the country's abundant lakes and glaciers. That, in turn, allows Norway to export almost all of its oil and gas.

Šefčovič, who traveled to the country earlier this year, was particularly taken by the 1.2 gigawatt Kvilldal power plant. Impressed, he declared that the dam "has a bigger storage capacity than would the Chinese wall plastered on both sides [with] the most sophisticated batteries of today."

A complex operation

Norway is already taking some early steps in shifting from supplying only its own 5 million people with electricity to becoming a crucial part of the EU's energy system.

The country exports hydro power to the Netherlands and exchanges renewables with Sweden and Finland, and there are plans for similar green exchanges with Germany and the United Kingdom within the next five years. But the marquee project is with Denmark, a country that relies heavily on wind energy. Norway imports Denmark's excess wind-generated power and sends hydro-generated electricity back when the wind isn't blowing, allowing Denmark to rely on renewables for about 40 percent of its electricity needs.

"You can clearly see that the Danish system, with so much wind and very little base load, would not be able to operate without hydro power,” said Auke Lont, the CEO of Statnett, the state-owned company that operates Norway’s grid.

While Norwegian companies are eager to boost power links with Europe, some officials caution against doing too much too quickly, arguing there are still kinks to be worked out in the grid. But advocates say it has enormous potential.

Using Norway’s hydro dams to ensure stable energy supply in countries that rely on renewables is “the cheapest and perhaps most environmentally-friendly solution," said Kaspar Vereide, a professor at the Norwegian University of Science and Technology.

A longtime advocate of the green battery idea, Vereide designed a way to upgrade the country’s dams so their reservoirs can fill and drain much faster. He calculates that it would cost about €6 billion to upgrade 20 of Norway's hydroelectric plants to act as more efficient energy stores; that would give them a capacity of about 20 GW of power.

The crucial element is getting Norwegian electricity out of Norway and to other parts of Europe, something that requires expensive undersea cables.

Norway has more than 900 hydro power plants that provide about 95 percent of the country’s domestic electricity needs — and officials say it could produce even more.

“We are holding back production," said Christian Wilhelm Rynning-Tønnesen, CEO of Norway’s state-owned hydro company Statkraft. “We can increase our ability to produce a lot in a short period of time.”

The crucial element is getting Norwegian electricity out of Norway and to other parts of Europe, something that requires expensive undersea cables allowing electricity to be sent back and forth as needed.

The country is scheduled to connect with Germany by 2020, once the €1.5-2 billion NordLink cable is operational, and by 2021 with the U.K. through the €2 billion NSN Link. And Norway could easily build more cables, according to Rynning-Tønnesen.

"All the projects connecting Norway with European countries through bi-directional interconnectors really bring benefits to all," Šefčovič said.

Šefčovič said the green battery idea would be an important element of the North Sea Offshore Grid Initiative, a project involving 10 countries which aims to develop an offshore electricity grid linking wind farms and other renewable energy sources.

Norwegian NIMBYs

Within Norway, however, there has been some resistance from consumer groups, which argue that building more infrastructure and trading more hydro power with other countries will raise electricity prices home as power is shunted to foreign markets.

Statkraft’s Rynning-Tønnesen dismissed such concerns, saying the impact for consumers would be marginal. Besides, he said, this is a win-win deal for Norway as it would import wind-powered electricity at “extremely low prices” while exporting hydro power at a higher price in return.

Other Norwegian officials tried to pour a little cold water on the most optimistic projections.

“Let’s face it, we're huge on gas. We're large on [hydro]. But we can’t power Europe ourselves" — Tord Lien, Norway's energy minister

“I don’t believe in the expression ‘green battery’ because it creates the image of Norway supplying Europe with energy,” Lien, the energy minister, said. "I would call it 'capacity companion.'

“Let’s face it, we're huge on gas. We're large on [hydro]. But we can’t power Europe ourselves," he added.

And Lont, who is ultimately responsible for the working of the grid, cautioned prudence.

"Before we embark on the next project, we think it's wise to gain experience on how these interconnections interfere with the system,” he said.

Even the Danish connection took some adjustment. Turbines stop working if the wind blows harder than 25 meters per second and Statnett needs to be more nimble as it switches — sometimes very quickly and at very high volumes — from importing to exporting power to Denmark.

"That means the whole system in Norway needs to be able to cope with that," Lont said.

The idea that Norway could bolster its neighbors with excess — and clean — hydro power is a much-discussed topic in green energy circles, with science and technology journalists debating the specs online and officials discussing the politics at environmental conferences and meetings.

The green battery idea could become reality but "not tomorrow," Lont said. "You don’t want to create a revolution. You want to do it as an evolution.”