August 21, 2014 – According to a team of mechanical engineers at MIT, there is the potential to harness a “significant amount” of renewable energy where the river meets the sea.

The researchers evaluated an emerging method of power generation called pressure retarded osmosis (PRO) in which two streams of different salinity are mixed to produce energy. In principle, a PRO system takes in riverwater and seawater on either side of a semi-permeable membrane; through osmosis, water from the less-salty stream would cross the membrane to a pre-pressurized saltier side, creating a flow that can be sent through a turbine to recover power.

The MIT team has now developed a model to evaluate the performance and optimal dimensions of large PRO systems. In general, the researchers found that the larger a system’s membrane, the more power can be produced—but only up to a point. Interestingly, 95% of a system’s maximum power output can be generated using only half or less of the maximum membrane area.

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,” Banchik says. “You can save money if you identify the membrane area beyond which there are rapidly diminishing returns.”

Banchik and his colleagues were also able to estimate the maximum amount of power produced given the salt concentrations of two streams: the greater the ratio of salinities, the more power can be generated. For example, they found that a mix of brine (a byproduct of desalination) and treated wastewater can produce twice as much power as a combination of seawater and riverwater.

Based on his 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.

—With files from Jennifer Chu, MIT News Office

Photo 1: Pressure retarded osmosis (PRO) is a method of producing renewable energy from two streams of a different salinity. Illustration Jose-Luis Olivares/MIT.

Photo 2: In this simplified PRO system, permeate from a dilute feed stream enters a concentrated draw stream in a pressurized state via osmosis, after which useful power can be extracted from the draw-permeate mixture. Image Leonardo Banchik/Elsevier B.V.

Photo 3: Shown here is the maximum power that can be produced for a 4:1 seawater to riverwater combination. As the dimensionless area gets very large, the overall maximum power can be produced. Image Leonardo Banchik/Elsevier B.V.