A new kind of generator could supply electricity for more than a half billion people using energy from rivers flowing into oceans, U.S. researchers say. The process, called pressure-retarded osmosis, requires no fuel, is sustainable and releases no carbon dioxide, researchers from Yale University reported in the journal Environmental Science & Technology. Researchers Menachem Elimelech and Ngai Yin Yip explain the little-known PRO process exploits the so-called salinity gradient -- the difference in saltiness -- between freshwater and seawater. In PRO, freshwater flows by osmosis through a special membrane to dilute seawater on the other side and the flow creates pressure that can spin a turbine generator and produce electricity. The world\'s first PRO prototype power plant was inaugurated in Norway in 2009. Elimelech and Yip say PRO power-generating stations using just one-tenth of the world\'s river water flow into the oceans could generate enough power to meet the electricity needs of 520 million people without emitting carbon dioxide. That amount of electricity, if produced by a coal-fired power plant, would release over 1.1 billion tons of greenhouse gases each year, they said.