When people talk about an “all of the above” approach to energy they’re usually referring to the sources we know – gas, hydro, nuclear, solar and wind. But a steady stream of emerging alternatives promises to take advantage of natural processes to produce zero-carbon electricity for untold millions. The latest entrant in this sounds-too-good-to-be-true energy sweepstakes: pressure-retarded osmosis, a kind of reverse water desalination that kicks off energy instead of consuming it.
Scientists at Yale University recently published an analysis of the process in Environmental Science and Technology. They suggest that it could provide power for half a billion people just by taking advantage of the mixing of fresh river water as it flows into the salty sea at the river’s mouth.
Osmosis: a refresher
To understand this new energy source, you’ll need to recall high school science lessons on osmosis: if two solutions are divided by a semi-permeable membrane, the less concentrated liquid will move into the more concentrated liquid until the two form an equilibrium. This is how plants draw water from the soil, and why potatoes shrink when they are boiled in salt water.
Power from pressure-retarded osmosis could flow all day and night, unlike intermittent solar and wind.
The same process occurs when freshwater from rivers empties into salty oceans. The river water is evenly distributed into the sea. If a semi-permeable membrane could be situated amidst the mixing, however, the energy associated with that exchange could be harnessed. And unlike with solar and wind power, which are intermittent sources, the power would flow all day and all night.
Researchers first struck upon this hypothesis in the 1970s, but their ideas exceeded the technological capacities of the time; a membrane the size of a college campus would be needed to harness energy of any value. In past years, technological advances allowed labs and companies to begin designing economically viable membranes with a greater energy-harnessing efficiency, reigniting the field.
“If you have freshwater on one side and salty seawater on the other side with a membrane in between, because of the chemical potential between them, the water will flow from the fresh to the sea water side,” said Menachem Elimelech, a professor of environmental and chemical engineering at Yale University who co-authored the paper. “We convert osmosis into mechanical work to create energy,” he said.
Pressure-retarded osmosis relies upon pressure from the osmotic flow to push the river water through specially designed membranes that spin a turbine generator and produces electricity. The system can be thought of as a reverse of the way water desalination plants work, since the energy of mixing is equivalent to that of separation.
Like dark fingers, cold ocean waters reach deeply into the mountainous coastline of northern Norway, defining the fjords for which the country is famous. Flanked by snow capped peaks, some of these ice-sculpted fjords are hundreds of meters deep. This scene was acquired by the ASTER instrument on NASA’s Terra satellite on April 17, 2002.
Norway test case
In 2009, Norway built the first prototype pressure-retarded osmosis power station. Norway’s plant demonstrated that the system could produce electricity, and since then the membrane technology has improved to become more economical and efficient. “In our lab, we produce small scale membranes,” Elimelech said. “In principle, someone can upscale the membranes into a larger system relatively easily.” He predicts a marketable system will be available in two or three years.
A power source that could potentially meet the needs of 520 million people.
Because rivers flow continuously into the ocean, pressure-retarded osmosis would yield a constant source of electricity. The researchers calculated that about 157 gigawatts of renewable power could be harnessed by channeling only a tenth of the world’s river water discharge, taking into account losses and inefficiencies. That figure is the equivalent electrical consumption of 520 million people based on the average global electricity use of about 300 watts per capita, according to the study. Producing the same amount of electricity through coal-fired power plants would release over a billion metric tons of greenhouse gases each year. And unlike, renewables that are located far from population areas, large cities are often situated around rivers mouths such as the Mississippi and Hudson rivers.
Many challenges remain, such as designing a system that prevents membranes from becoming clogged with dissolved organic material contained in the river water. Environmental factors must also be taken into account. Engineers would need to locate plants away from sensitive areas, like estuaries, and take into account potential disruption of the water’s natural flow. With proper environmental assessments and planning, though, Elimelech thinks the system can be a viable source of alternative power.
“Because there are so many other renewable energy options, this one did not get as much attention as wind and solar,” he said. “But I think it’s getting there, and lots of people are starting to realize its potential.”