A new genre of electric power-generating stations could supply electricity for more than a half billion people by tapping just one-tenth of the global potential of a little-known energy source that exists where rivers flow into the ocean, a new analysis has concluded.

A report on the process - which requires no fuel, is sustainable and releases no carbon dioxide (the main greenhouse gas) - appears in ACS' journal Environmental Science and Technology.

Menachem Elimelech and Ngai Yin Yip explain that the little-known process, called pressure-retarded osmosis (PRO), exploits the so-called salinity gradient - or difference in saltiness - between freshwater and seawater.

In PRO, freshwater flows naturally by osmosis through a special membrane to dilute seawater on the other side. The pressure from the flow spins a turbine generator and produces electricity.

The world's first PRO prototype power plant was inaugurated in Norway in 2009. With PRO appearing to have great potential, the scientists set out to make better calculations on how much it actually could contribute to future energy needs under real-world conditions.

Elimelech and Yip concluded that PRO power-generating stations using just one-tenth of the global river water flow into the oceans could generate enough power to meet the electricity needs of 520 million people, without emitting carbon dioxide.

The same amount of electricity, if produced by a coal-fired power plant, would release over one billion metric tons of greenhouse gases each year.