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Simple Physics Solutions to Storing Renewable Energy

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Source : PBS - Public Broadcasting Station (USA), NOVA program
Date: May 24, 2017

Link: http://www.pbs.org/wgbh/nova/next/tech/storing-renewable-energy/


When the long-awaited solar eclipse hit Europe on March 20, 2015, Germany held its breath. With more solar power capacity than any other nation, the country of 82 million was hit hard when up to 80% of its sunlight was cut off.

On any given day, as much as 40% of Germany’s electricity is provided by solar power, so the country’s four major electricity networks had spent months preparing for the drastic drop in solar electricity input as well as the rapid increase in solar power that resulted when the eclipse finally came to an end. Any drop could lead to blackouts, while a surge in the wake of the eclipse could trip circuit breakers or overwhelm transmission lines.

So engineers took measures to stabilize the grid and the electricity supply. They planned on drawing more heavily from fossil fuel, nuclear, and hydro plants, while also asking four aluminum smelters to dial briefly back their demand.

Then the eclipse happened. Immediately before, German solar panels were pumping out a combined 21.7 GW of electricity. At the eclipse’s peak, it dropped to 6.2 GW, and rebounded to 15 GW after the moon had passed. Despite the seesaw in production, the event passed without a major disturbance. The German power grid didn’t collapse as some had feared.

This August, the U.S. will face its own test when a total eclipse casts an arcing, transitory shadow more than 60 miles wide from Oregon to South Carolina. Even states that are untouched by the totality are racing to prepare for the drop in electricity production. California, for example, is bracing for a 6 GW surge in demand over supply during the event. The rate at which supply sags and eventually returns is anticipated to be two to three times the usual rate at which grid operators respond, according to the California Independent System Operator.

Eclipses are extreme versions of events that happen every day. The sun may duck behind a cloud, and the wind may stop blowing. That intermittency poses a challenge, but it’s not an unsolvable problem.

As renewable energy sources like wind and solar drop in price and become competitive with new fossil fuel plants—something which has already happened in Africa and China—grid operators will need better tools to balance supply and demand. More detailed and accurate weather forecasts will certainly help by predicting when and where sun and wind will be plentiful. But more than that, energy storage will play a key role keeping the grid flexible and adaptable, says Apurba Sakti, a research scientist at the MIT Energy Initiative focusing on energy storage technologies. “It helps you store at times of excess generation that you can then use later on when the sun in not shining or wind is now blowing,” he says. And that could keep the lights on as renewables ramp up.

Renewed Purpose

Germany’s North Rhine-Westphalia was once known as “the land of coal and steel.” Hundreds of mines once dotted the landscape, and while many have shut down, several still remain, including the old Prosper-Haniel coal mine that has been running for nearly half a century. It’s hard to miss the colossal, lime-green headframe that looms over the city.

But after next year, the tower will be conspicuous in its absence—when the tangle of steel will be replaced with wind turbines, an artificial reservoir, solar panels, and more. Last month, the German federal government, which has been working alongside private engineering firms since 2012, made plans to transform the coal mine into a giant battery. If the project proves successful, the site will provide the reliable energy that solar and wind can’t always promise to 400,000 homes.

The 600-meter deep mine creates an artificial elevation, which makes this coal mine a prime location for hydroelectric storage. Typically, so-called pumped storage plants operate by shuffling water between two reservoirs, one higher than the other. Excess electricity is used to pump water into the higher reservoir, where it sits until grid operators open the gates, sending the water downhill, through the turbines, and into the lower reservoir.

The Prosper-Haniel battery will work in a similar fashion, except the mine tunnels will replace the lower reservoir. When all the water has streamed through, it’ll be pumped back up to the top of the mine at night when the price is cheaper and demand is lower. “The German coal mine is interesting,” George Baker, president of Vcharge, a transactive electric storage technology company that provides market-based energy distribution optimization, says. “That’s a huge reservoir, not compared to natural reservoirs, but still.”

Baker says Germany’s wildly fluctuating energy crises has states like North Rhine-Westphalia looking for a large-scale solution to balancing its renewable energy. When the sun comes up or the wind starts blowing in Germany, supply often exceeds demand. As a result, prices tumble, often going negative, meaning that power stations need to pay people to use their electricity. Traditional fossil fuel power plants have a hard time producing power economically when demand ebbs and flows so frequently.

Batteries are an obvious solution, and though prices are likely to continue their steady march downward, they’re currently expensive. “Pumped hydro[electric storage] is at present the only commercially viable grid-scale storage technology,” Baker says. Pumped hydro is probably the most important asset in managing the balance of energy grids because it can go from generating energy quickly when needed to efficiently storing energy when electricity is cheap and abundant. Plus, it’s a proven technology, having been around since the 1980s.

But pumped hydropower isn’t a universal solution. For one, it needs a very particular kind of geography, like a reservoir at the top of a mountain or a disused coal mine. And two, water needs to be plentiful. So while pumped hydro works and works well in many cases, engineers are exploring other options to keep the lights on when the sun sets.

Up and Down and Up Again

More than 5,000 miles away from North Rhine-Westphalia, in southern California, there’s buzz about rather plain-looking railcars that don’t transport people or goods or really go anywhere at all. They’re owned by a Santa Barbara-based startup called Advanced Rail Energy Storage (ARES), and they’re another contender in the grid storage market.

When electricity is cheap, ARES electric locomotives draw power from the grid to haul heavy railcars up a hill. Once they’re at the top, the potential energy they embody acts as storage. When the grid needs electricity, the brakes release and the cars begin to inch downhill. The motors that hauled them up turn into generators, controlling the descent and generating power as they go. It’s essentially the same system that recharges the batteries of a Toyota Prius or Tesla Model S when the driver hit the brakes.

The rate at which energy is stored or recovered can vary, too, depending on the speed and quantity of the railcars. Overall, the concept is comparable to pumped hydro, but it uses weights and railcars instead of water. “Those are very old [and proven] concepts,” says Robert Armstrong, director of the MIT Energy Initiative, confirms.

AERS isn’t alone. “There’s a bunch of people who have been trying to use the fact that heavy stuff up in the air is a lot of storage potential energy to balance supply and demand in electricity markets,” says Baker. Energy Cache had a similar idea, though the company never moved beyond a small pilot project. Its “gravel on ski lifts” idea, as backer Bill Gates called it, was a system of buckets that would pick up gravel at the bottom of a hill then haul it to the top. When the grid called for energy, the buckets would pick the gravel up and let gravity move it back down the hill, powering an electrical generator as it went. Another company, Charroux, France-based Sink Float Solutions, is developing a system that would move concrete weights up and down the 13,000 feet of depth that lies between the ocean’s surface and floor.

For more visit the link provided at the top of the post.
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