Neither Statkraft nor Wetsus expects to crank out more than just a few kilowatts—enough to boil water—with their initial experiments. Instead, they plan to demonstrate what could be scalable, commercially viable energy production as well as determine if salt power endangers the health of source-water estuaries. Statkraft estimates salt power's worldwide electricity-generating potential at up to 1,700 terawatt-hours, or about 10 percent of global demand.
Salt power is attractive for several reasons: For one, unlike renewable energy technologies for harvesting solar or wind power, salt systems are not dependent on the weather and could provide baseload (constant, predictable) electricity like that supplied by coal, natural gas and nuclear energy. "The river water is flowing into the sea 24/7, so you have a constantly available source of energy," Skilhagen says. He also points out there are no emissions besides brackish water, which swirls in the river's mouth anyway.
Unlike conventional hydropower, a saline power plant does not require damming off a waterway, and it may require less infrastructure than, say, riverbed-mounted turbines or floating fleets of generators for equivalent tidal and wave power. A salt power plant, including its membrane stacks, turbines, cleaning facilities and offices, could actually be located in a riverside industrial complex's basement, for example, or constructed underground within a riverbank with pipes extending into the waterway, Statkraft's Skilhagen says—a big advantage for incorporating such facilities into already-developed, populated coastal areas.
Membrane design and performance remain the biggest hurdles for both Statkraft's and Wetsus's approaches. The membranes must be made more efficient, durable and resistant to microbial buildup, or so-called biofouling. Pretreating the pumped-in water by filtering out organic matter and river-borne debris helps, but this critical step consumes energy and is expected to be expensive, Skilhagen says, adding that it is too early to know just how expensive.
Accordingly, experts remain cautious about salt power's prospects. "Both these methods are promising and certainly worth researching," says Ari Seppala, a mechanical engineer and thermodynamicist at the Helsinki University of Technology in Finland, "but both may still need a breakthrough innovation before commercialization." Seppala sees no physical or chemical showstoppers standing in the way of producing much-improved membranes, although he also points out that a better, membrane-free method could yet be found for exploiting salinity differences to yield electricity.
Another key uncertainty: a salt-power plant's effect on the local aquatic environment. "This is a completely new process that has not been tested on this scale," says Menachem Elimelech, a professor of chemical and environmental engineering at Yale University. "I doubt there will be no [environmental] impact at all." If the ecological footprint is minimal and the membrane technology advances, however, he adds that salt power could be a "significant renewable energy option."