An Idea Charged Up at Zuccotti Park Pacing around his Brooklyn apartment, Tommy Mitchell paused to eye his machine with frustration. On top was an 85-watt solar panel that he realized he had just installed upside down. A solar-powered cellphone charger, it looks like an oversize white iPad on stilts, with eight charging wires sprouting like tentacles from the middle. Once charged, the machine, which sits on a huge battery, can run a whole week without sun, he said. There was no assembly manual because he invented the contraption. Mr. Mitchell, 25, became interested in sustainable energy after watching a T. Boone Pickens commercial about wind power about four years ago. "For some reason, it just struck me," he said. After studying neuropsychology at the Florida Institute of Technology and taking a job with a renewable energy company in New York, he founded a venture called Green Barrel Energy to produce charging stations. This is his first model, and staff members of the company that manages Bryant Park have been using it on a test basis. The idea for the stations hatched when Mr. Mitchell was strolling through the Occupy Wall Street protests in Zuccotti Park last fall. Curious, he asked a protester how he and others were keeping their cellphones and computers charged. The answer surprised him: a hot dog vendor's generator. "I was like, "Well that's awful,"" he said. That's when he began thinking about inventing a device that could harness renewable electricity in a public space without outlets. "It's so practical that you can see it," he said. Over the next week, Mr. Mitchell researched how campers rig up makeshift cellphone chargers far from civilization. Then he ordered individual parts from Amazon.com and built his own prototype. Invention in hand, he returned to Zuccotti Park and offered the services of his new device. He said it was an instant hit. While cellphone charging stations had been popping up in bars and airports, they usually charge a fee. And most stations require the user to bring a charger that is then plugged into the grid, which is largely powered by burning fossil fuels. Mr. Mitchell's station can charge phones at no cost to the user or the environment; in his view, it's a kind of solar holy grail. He's banking on the odds that event planners or park officials will pay to buy or lease his machines. Jonathan Koomey, a consulting professor for civil and environmental engineering at Stanford University, says that mobile electricity has emerged as a highly valuable commodity. At home, it costs about 12 cents a kilowatt hour to charge a battery, he said, but a portable AAA battery eats up about $400 a kilowatt hour. In other words, it can cost a lot to be on the move. Mr. Koomey said he hadn't heard of a station like Mr. Mitchell's. Neither had spokesmen for Verizon Wireless or Sprint. Still, solar charging is quietly gaining traction. Beth Ferguson, 34, made her first solar charging station when she was a graduate design student at the University of Texas at Austin. "I bought an electric bike on Craigslist and had no place on my college campus to charge it," Ms. Ferguson said. The charger, which she built as her thesis project, turned into a company called Sol Design Lab. Now based in San Francisco, the company designs solar energy hotspots using repurposed materials like old gas station pumps and abandoned pay phone booths. Sol Design Lab's machines have popped up on college campuses, on urban farms and in front of Austin City Hall, and there will soon be one at the Wellfleet Bay Wildlife Sanctuary in Mass. Music festivals also favor Ms. Ferguson's stations, which have appeared at South by Southwest, Bonnaroo, and Coachella. Last year her company charged 4,000 cellphones in four days at Bonnaroo, she said. The company has had several backers. Austin Energy donated $10,000 and Sol Design Lab's very first solar panel, she said. After Ms. Ferguson made her first prototype - "a gas pump with a solar panel and a battery" - South by Southwest microfinanced the first three machines. The city of Austin gave Sol Design Lab $11,000 toward solar public art projects, Ms. Ferguson said. Sol Design Lab employees also teach solar charging design classes at Hampshire College and the School of the Art Institute of Chicago. The company charges $5,000 to $70,000 a machine, depending on the project. "We're definitely growing and have more work than we can handle right now," said Ms. Ferguson, who says she receives 15 to 30 calls a month from prospective clients. When companies ask for a referral, though, she's at a loss. "I'm the only one I know doing mobile solar charging for festivals and urban centers and colleges," she said. Though a few larger companies like Sanyo make charging stations for electric bikes, Ms. Ferguson couldn't think of a large company that does solar cellphone charging on a massive scale. Although Mr. Mitchell said he had depleted his savings to start Green Barrel Energy and so far has no backers, he hopes to lease machines to host sites and to secure some corporate sponsors. Jerome Barth, vice president for business affairs for the Bryant Park Corporation, was intrigued by Mr. Mitchell's invention. "It seemed like an interesting technology," he said. Until Mr. Mitchell reached out to him, he said, he had not found anything on the market to meet the park's charging needs. The nonprofit park management company is "in conversations" with Green Barrel Energy about deploying more machines. In the meantime the prototype sits in a corner of the bustling park, charging staff members" phones.