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.
ü 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.
ü In principle, a PRO system would take in river water and seawater on either side of a semi-permeable membrane.
o The researchers have now developed a model to evaluate the performance and optimal dimensions of large PRO systems.
o 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.
ü The team based its model on a simplified PRO system in which a large semi-permeable membrane divides a long rectangular tank.
ü One side of the tank takes in pressurized salty seawater, while the other side takes in river water or wastewater. Through osmosis, the membrane lets through water, but not salt.
ü As a result, freshwater is drawn through the membrane to balance the saltier side.
ü As the freshwater enters the saltier side, it becomes pressurized while increasing the flow rate of the stream on the salty side of the membrane. This pressurized mixture exits the tank, and a turbine recovers energy from this flow.
According to a researcher, 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.
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