Kind of Blue
The world of waters is divided between salty and fresh. That's no accident; a great deal of energy, provided free by the sun, goes into evaporating seawater and dropping it into lakes, ponds, glaciers, and aquifers. At heart this process is an electrical transformation, as sodium and chloride ions (the elements of salt) are filtered out to create reservoirs of high and low salinity. That salinity difference represents an energy gradient, a potential for ions to flow. One can think of the earth as a giant battery: one pole is called "lakes and rivers," the other is called "the sea."
Wherever the two waters meet, the gradient equalizes and energy is released. Imagine a 680-foot-high waterfall at the mouth of every river and a turbine dam generating power from it; an equivalent amount of energy, scientists calculate, is freed through salinity mixing. So why not harvest it? That has been the promise of "blue energy" since the 1950s. After decades of techno-tinkering, it's finally seeing the light. Two years ago, Norway's state electricity company fired up a prototype salinity-power plant along the North Sea coast; another has sprung up in Holland. And in May, a team of Stanford University researchers announced a still-more-efficient approach for generating blue energy. It won't solve the world's power woes, but blue energy's limitations turn out to be as enlightening as its potential.
Generating energy from a mix of freshwater and salt water entails corralling the ions within. In the Norwegian plant, salt water sits on one side of a semi-permeable membrane, and freshwater is drawn through by osmosis. This raises the hydraulic pressure on the saltwater side, which drives a turbine to generate electricity. In the Dutch plant, the freshwater and salt water are channeled in close proximity through a series of tiny cells, across two membranes: one permits only the salt water's positive sodium ions to pass through; the other allows only the negative chloride ions to pass. Separating the ions creates a potential difference: electricity. The output of each plant is meager -- two to four kilowatts, about enough to run a fridge -- but both are viewed as significant steps toward larger-capacity plants. Norway anticipates that blue energy will eventually supply at least 10 percent of the nation's power.
Earlier this year, Yi Cui, a nanomaterials scientist at Stanford, proposed a design that does away with membranes altogether. It's basically a stationary battery -- in practice, a series of tiny ones -- consisting of two specially made electrodes, one positive and one negative, across which freshwater and salt water would flow in alternating stages. In the process, the sodium and chloride ions would be drawn out, to be stored or used as electricity. "The idea is, you charge the battery in freshwater and then discharge it in seawater," Cui says. Because seawater contains 60 to 100 times as many ions as freshwater, the battery generates far more electricity than what's needed to charge it. Cui imagines a power plant stationed near the mouth of a major river, where it could pull in both the salt water and freshwater it needed; its only discharge would be salty and fresh water, at a normal temperature.
In theory, tapping the salinity gradient of all the rivers on earth could supply as much as two terawatts, or about 13 percent of global energy demand. In practice, blue energy will be a far smaller player, for reasons having little to do with technology and everything to do with geography. Rarely does freshwater empty directly into salt water; most rivers are tidal estuaries, and their salinities often mix across hundreds of miles. Pumping the two waters to a single spot will require more energy than the amount extracted, says Richard Seymour, the head of ocean engineering research at the Scripps Institution of Oceanography. He puts more stock in wave power, at least for the masses. Still, he says, there are places where blue energy might make sense. In Norway, freshwater falls sharply into saltwater fjords. Twenty-six percent of Holland is below sea level; its existence depends on the pumps, drainage ditches, levees, and dikes that direct the Rhine into the sea. Climate change will heighten that imperative, as the snowpack melts and more freshwater inundates the nation. "The availability of excess freshwater is unique," Seymour says. "That might be a situation where they could afford to do it."
Blue energy isn't everyone's answer, but that's okay. Over the past century, we've come to think about energy in monolithic terms: Coal. Gas. Oil. One Source Fits All. Maybe part of the shift toward clean and renewable means learning to diversify -- to consider a wider portfolio of smaller, more local sources, blue or otherwise, that are better suited to their communities. Turning on a light switch would become more like a trip to the greenmarket and less like an encounter with Big Ag and all the environmental costs it obscures. Let a thousand powers bloom.