Imagine generating electricity from the energy a redwood tree exerts when it sucks water up 300 ft. (91 m) to its highest needles. Is there any greener juice than that?
Stein Erik Skilhagen thinks about this sort of thing all the time for Norway's state-owned power company, Statkraft, where he's the head of osmotic power.
Yes, osmotic power. Of course you remember studying osmosis. But just in case you've forgotten, it's the passage of a liquid from a region where it's highly concentrated through a semipermeable membrane to a region of lower concentration, raising the latter's volume. The by-product of that rising volume is energy.
Skilhagen isn't focusing on the osmosis that drives the tree's impressive lifting. Rather, he's looking into salt water's osmotic tug on freshwater wherever the two meet, as they do all over the world. Harness the pull that salt water has on freshwater the salt water has a lower concentration of water and hence attracts the freshwater and you've got osmotic power, also called salinity-gradient power. It's a promising carbon-free renewable energy that could one day help marginalize fossil fuels. And unlike intermittent wind and solar, osmosis would work around the clock.
"We critically need more green energy in the world," says Skilhagen. "Osmotic can be a valuable contributor. You can make electricity from the combination of freshwater and seawater." Statkraft says there's enough global osmotic potential to generate 1,700 terawatt-hours of electricity a year, roughly half the E.U.'s consumption.
Statkraft has proved the concept at a prototype plant opened in November 2009 in Tofte, Norway, along the Oslo Fjord. Water from the Tofte River falls into a vessel separated from salty fjord water by a thin, permeable membrane. The membrane lets the freshwater force its way through to the enclosed saltwater side, where pressure builds, pushing water through a pipe to drive a turbine.
But the plant generates less than a watt of energy per square meter of membrane, well below the 5 watts that Skilhagen says would make it cost-effective. Statkraft is yanking out its original polymer membrane and replacing it with a thinner one that could triple output. That would be a significant advance but would still fall short of 5 watts. So Skilhagen expects to test a third membrane soon. He needs one strong enough to separate salt water and freshwater, but permeable enough to let them mix easily. "The less the resistance the better," says Skilhagen, who thinks Statkraft can commercialize the process by 2015.
For all its promise, osmosis remains a solution in search of a technology to deliver it. One obstacle: there are few membrane manufacturers. There's Germany's Microdyn-Nadir, Yale University spin-off Oasys Water and Arizona's Hydration Technology Innovations. "That needs to expand," says Michael Flynn, senior research scientist at NASA's Ames Research Center at Moffett Field in California. "If somebody comes up with a better membrane, we'll buy it."
NASA, for its part, is developing osmosis primarily for water treatment on spacecraft but is interested in its electricity too. Flynn thinks Statkraft's progress will lead to more users, which would lower costs through economies of scale. He calls Statkraft's prototype "a game changer."
Other companies are also trying to change the game. In Japan, Kyowakiden Industry Co. is operating a prototype osmotic plant in partnership with Tokyo Institute of Technology and Nagasaki University. The Canadian utility Hydro-Québec has identified 12 gigawatts of osmotic potential along the St. Lawrence estuary, James Bay and Hudson Bay, which would add 25% to its capacity. But Denis Faubert, general manager of Hydro-Québec Research Institute, says the utility won't open an osmotic plant until 2020 at the earliest.
Skilhagen notes that if more power companies pursue osmotic, more membrane makers will emerge. He advocates government subsidies for osmotic, much like those for wind and solar, which helped create fully functioning industries. Statkraft has spent about $25 million, 30% of it from the Norwegian government and the E.U. In the U.S., however, the window for funding osmotic energy has closed. "We're back to our normal budgets now," says Alan Hoffman, a senior analyst in the Energy Department's Office of Energy Efficiency and Renewable Energy, referring to poststimulus realities. "But I'm glad there are people doing it."
The technology could also benefit from osmotic desalination, which taps similar membranes to deliver clean water. Skilhagen foresees 30 osmotic power plants by 2030. The number could be higher with more development funds. If only those came through osmosis.