MIT Investigates Energy-Salt Nexus

August 28, 2014 By Karen Henry

MIT-Salt-energy-manageThere is the potential to harness a significant amount of renewable energy where the river meets the sea, according to a team of mechanical engineers at MIT.

The researchers evaluated an emerging method of power generation called pressure retarded osmosis (PRO) and found that such a system could potentially power a coastal wastewater treatment plant by taking in seawater and combining it with treated wastewater to produce renewable 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 researchers developed a model to evaluate the performance and optimal dimensions of large PRO systems. They 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.

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, MIT said, the streams’ salinity and flux will naturally change. To account for this variability, MIT developed a model that exchanges water across a membrane, similar to the way a radiator in a car exchanges heat between the air and a coolant.

With this model, they found that as the area of a membrane increases, the power generated increases to a point, after which it gradually levels off. While a system may be able to produce the maximum amount of power at a certain membrane size, it could also produce 95 percent of the power with a membrane half as large.

More research needs to be done to see whether such a system would be economically viable, MIT said.

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