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• Electric school-bus batteries return surplus power to grid
• Stored energy in EV, home batteries can even out grid supply
• "Virtual power plants" reduce need for fossil-fuel backup

ROCKVILLE, United States - Maryland's Montgomery County already has the largest fleet of in the United States, but it also wants to use its vehicles as "batteries on wheels", sending power back to the grid to ease peak demand and help the clean energy transition.

As part of a broad green agenda, the large county just outside Washington is working to expand its fleet of 86 all-electric school buses to 1,400 within a decade and is installing rows of bright yellow chargers at school district facilities.

Those chargers hold a two-for-one secret. Not only can they refill a depleted bus battery in hours - they will also be able to route stored energy back to the electricity grid when needed.

Gregory J. Salois, director of the Montgomery County Public Schools Department of Transportation, noted that the United States has almost 500,000 school buses.

"If eventually you can get that massive idle power source to feed back into the grid ... it's phenomenal on the revenue side for whoever owns these buses, and it's great for folks that lost their power," he told Context.

It is a prospect that grid operators and analysts say will be critical to facilitating the green energy transition, as well as boosting local-level resilience by providing backup to a grid, a home, hospital or community center.

Such two-way or "vehicle-to-grid" (V2G) charging could also help even out regular peaks in electricity usage – such as at the end of the working day – which force utilities to keep fossil-fuel power stations at the ready to make up the gap.

The idea is similar to a water tower, where water is pumped up so it is available when needed later, said Michael C. Austin, lead technician with the Montgomery school transport department, standing in one of the district's new electric buses.

"The idea is that this is a giant tower of electricity," he said of the V2G project, as a nearby bus sped away quietly and surprisingly swiftly.

'Battery on wheels'

is already found in a few consumer electric vehicles, with several new models expected this year.

In February, the federal government that vehicle-charging stations funded by a new $7.5-billion program support V2G.

And in May, lawmakers in California proposed mandating that all new electric vehicles be .

While the U.S. Environmental Protection Agency's new $5-billion does not provide incentives for bidirectional charging, an agency spokeswoman said it was "of great interest".

Several states have rolled out a variety of V2G test projects for businesses and consumers, said Katherine Stainken, vice president of policy with the Electrification Coalition, a nonprofit that promotes EV adoption.

"We're starting to move beyond the pilot phase," she said. "This shiny object of the future is really becoming a reality."

School buses, in particular, have the potential to boost resilience in under-served communities that have been often hit hardest by climate disasters, Stainken said.

In January, the coalition co-produced a report that found a typical electric school-bus battery "could provide for nearly two days", if needed in an emergency.

School buses are unique in that they have "a very predictable life ... and that makes them perfect for electrification", said Sean Leach, technology director with Highland Electric Fleets.

"The idle time that each bus has – it can do grid service and be a more active member of its community," he said, referring to such buses as "a battery on wheels".

The company currently has deployments in four states, helping school districts, including Montgomery County, to electrify their fleets and use V2G to bring down costs.

Its first full V2G deployment was in the city of Beverly, Massachusetts, in 2021. That summer and the next, the project sent more than 7 to the local grid, using one or two buses, enough to power around eight U.S. homes for a month.

Jake Navarro, director of clean transportation with National Grid, which operates the local power network in Beverly, said it experiences 30 to 60 peak energy usage events per year, usually in June to September when the weather is hottest.

"As we see more electric vehicles adopted – particularly school buses, but others too, we'll see more and more interest in dynamic charging, including V2G technology," Navarro said.

"There's no question this will become more important."

Virtual 'sleeping giant'

Bidirectional charging involving EVs is part of a broader discussion around how the huge rise in battery use – including as power backup and solar power storage – can help stabilize electrical grids and boost energy resilience.

Together, such systems can form a powerful network of stored energy, sometimes referred to as a (VPP).

By the end of the decade, these "VPPs" could for U.S. utilities by 60 gigawatts and by 200 gigawatts by mid-century, the think-tank RMI said in a report this year.

Sunrun, a major solar and residential battery company with nearly 1 million customers, has formed multiple VPPs, supplying energy to utilities and sharing the revenue with its customers, said its policy director, Chris Rauscher, calling the set-up "the sleeping giant of distributed renewables".

The company is supporting a similar project in Puerto Rico, where regulators have ordered the local utility to create a VPP program to .

Rauscher said rooftop solar and battery systems on about 7,000 homes there will provide baseline energy for the island territory's grid - and will likely be extended to nearly every home where feasible.

He noted that VPPs and bidirectional EVs are not "off in the future", as many might think.

"They're two sides of the same coin," Rauscher said. "VPPs are here today, and we're operating them around the country. And bidirectional electric vehicles are nothing more than large batteries on wheels."

(Reporting by Carey L. Biron. Editing by Jon Hemming and Megan Rowling.)