EnergyThe power of salt
Where the river meets the sea, there is the potential to harness a significant amount of renewable energy, mechanical engineers say. 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 would take in river water 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.
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.
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 would take in river water 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 percent 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 river water.
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.
“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,” Banchik says.
