Where the river meets the sea, there is the potential to harness a significant amount of renewable energy, according to a team of mechanical engineers at MIT who evaluated the method of power generation called pressure retarded osmosis (PRO), in which two streams of different salinity are mixed to produce energy.


The team developed a model to evaluate the performance and optimal dimensions of large PRO systems and found that the larger a system’s membrane, the more power can be produced — up to a point: 95 percent of a system’s maximum power output can be generated using only half or less of the maximum membrane area.

Pressure retarded osmosis (PRO) is a method of producing renewable energy from two streams of a different salinity. Illustration: Jose-Luis Olivares/MIT

Leonardo Banchik, a graduate student in MIT’s Department of Mechanical Engineering, says reducing the size of the membrane needed to generate power would, in turn, lower much of the upfront cost of building a PRO plant. “People have been trying to figure out whether these systems would be viable at the intersection between the river and the sea. You can save money if you identify the membrane area beyond which there are rapidly diminishing returns.”


Based on those calculations, Banchik says that a PRO system could potentially power a coastal wastewater-treatment plant by taking in seawater and combining it with treated wastewater to produce renewable energy. “Here in Boston Harbor, at the Deer Island Waste Water Treatment Plant, where wastewater meets the sea … PRO could theoretically supply all of the power required for treatment.”

Finding equilibrium in nature


“Nature wants to find an equilibrium between these two streams,” Banchik explains.


Banchik says that while others have modeled the power potential of PRO systems, these models are mostly valid for laboratory-scale systems that incorporate “coupon-sized” membranes. Such models assume that the salinity and flow of incoming streams is constant along a membrane. Given such stable conditions, these models predict a linear relationship: the bigger the membrane, the more power generated.

But in flowing through a system as large as a power plant, Banchik says, the streams’ salinity and flux will naturally change. To account for this variability, he and his colleagues developed a model based on an analogy with heat exchangers.

“Just as the radiator in your car exchanges heat between the air and a coolant, this system exchanges mass, or water, across a membrane,” Banchik says. “There’s a method in literature used for sizing heat exchangers, and we borrowed from that idea.”


Still, if PRO systems were to supply power to Boston’s Deer Island treatment plant, the size of a plant’s membrane would be substantial — at least 2.5 million square meters, which Banchik notes is the membrane area of the largest operating reverse osmosis plant in the world.

“Even though this seems like a lot, clever people are figuring out how to pack a lot of membrane into a small volume,” Banchik says. “For example, some configurations are spiral-wound, with flat sheets rolled up like paper towels around a central tube. It’s still an active area of research to figure out what the modules would look like.”

“Say we’re in a place that could really use desalinated water, like California, which is going through a terrible drought,” Banchik adds. “They’re building a desalination plant that would sit right at the sea, which would take in seawater and give Californians water to drink. It would also produce a saltier brine, which you could mix with wastewater to produce power. More research needs to be done to see whether it can be economically viable, but the science is sound.”


Citation: Leonardo D. Banchika, Mostafa H. Sharqawyb, John H. Lienhard, 'Limits of power production due to finite membrane area in pressure retarded osmosis', Journal of Membrane Science, Volume 468, 15 October 2014, Pages 81–89 DOI: 10.1016/j.memsci.2014.05.021