The Boston Globe By Linda Greenstein With the symbolic flip of a switch, clean, green electric power began flowing on Nov. 30 from a solar array built by nonprofit Citizens Energy Corporation atop a 19-acre-capped Superfund site straddling Dunstable and Tyngsborough. The site had previously sat vacant for 30 years. “It’s remarkable that on a hill where chemical sludge, metal toxins, and dirt and trash once mixed in a noxious cocktail, we can stand here today and instead see these 10,000 panels soak up the energy of the sun,” said Joseph P. Kennedy II, Citizens Energy chairman, at the ceremonial start of the project. Annually, the 10,000 solar panels will produce enough electricity to power 460 homes and prevent the release of 3,500 tons of carbon dioxide from nonrenewable power plants.