Study investigates power generation from the meeting of river water and seawater

August 20, 2014 by Jennifer Chu
Pressure retarded osmosis (PRO) is a method of producing renewable energy from two streams of a different salinity. Credit: Jose-Luis Olivares/MIT

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 .

"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.

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. Credit: Leonardo Banchik

He and John Lienhard, the Abdul Latif Jameel Professor of Water and Food at MIT, along with Mostafa Sharqawy of King Fahd University of Petroleum and Minerals in Saudi Arabia, report their results in the Journal of Membrane Science.

Finding equilibrium in nature

The team based its model on a simplified PRO system in which a large semi-permeable membrane divides a long rectangular tank. One side of the tank takes in pressurized salty seawater, while the other side takes in or wastewater. Through osmosis, the membrane lets through water, but not salt. As a result, freshwater is drawn through the membrane to balance the saltier side.

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

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.

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.

Shown here is the maximum power that can be produced for a 4:1 seawater to river water combination. As the dimensionless area gets very large, the overall maximum power can be produced. Credit: Leonardo Banchik

"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."

The researchers came up with a model with which they could analyze a wide range of values for membrane size, permeability, and flow rate. With this model, they observed a nonlinear relationship between power and membrane size for large systems. Instead, 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.

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 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 . More research needs to be done to see whether it can be economically viable, but the science is sound."

Explore further: Study shows forward osmosis desalination not energy efficient

More information: Leonardo D. Banchik, Mostafa H. Sharqawy, John H. Lienhard V, "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, ISSN 0376-7388,

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not rated yet Aug 20, 2014
95 percent of a system's maximum power output can be generated using only half or less of the maximum membrane area.
Non-scalable, in other words, but possibly suitable for small scale projects. I like the given example of a wastewater treatment plant, because the advertising motto practically writes itself: PRO systems--where the shit really *does* hit the fan. Or powers it, at least.
not rated yet Aug 21, 2014
Some engineering approaches to a given problem will be feasible and effective. Some will not.

This is a fine example of 'not.'

Here's the thing. If all you want to do is harvest mechanical energy from flowing water, you just stick your turbines in the flow. But if you want to draw energy from molecular-level interactions, then by golly, crud in the system starts to matter. A lot.

Rivers move a lot of crud. They're pretty much the opposite of 'sterile.' So keeping the membrane clean is a big issue.

Osmosis is happening at the molecular level, which means it's easily blocked by crud. The membrane is thin and delicate. These characteristics make it fairly tough to engineer maintainable, reliable water purification systems, which operate on a much smaller scale than systems seeking to harvest energy from river-ocean interactions. Osmosis is doable for water purification, but it's sure not cheap. Scaling up to harvest river-ocean interactions is going to be real tough.
not rated yet Aug 21, 2014
Please, this idea is not a new invention from MIT. Is been in research projects in Netherlands since 2009. you can check the following websites to confirm it:
The first patent was from Israel in 1973. You can find it as Osmotic pressure energy.
Nowadays is a real project that will be upscaled in the afsluitdijk (the artificial barrier made by the dutch between ijsselmeer and the north sea).

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