Rivers flowing into the sea offer vast potential as electricity source

Apr 18, 2012
Rivers flowing into the sea offer vast potential as electricity source

A new genre of electric power-generating stations could supply electricity for more than a half billion people by tapping just one-tenth of the global potential of a little-known energy source that exists where rivers flow into the ocean, a new analysis has concluded. A report on the process — which requires no fuel, is sustainable and releases no carbon dioxide (the main greenhouse gas) — appears in ACS' journal Environmental Science & Technology.

Menachem Elimelech and Ngai Yin Yip explain that the little-known process, called pressure-retarded osmosis (PRO), exploits the so-called salinity gradient — or difference in saltiness — between freshwater and seawater. In PRO, freshwater flows naturally by osmosis through a special membrane to dilute seawater on the other side. The pressure from the flow spins a turbine generator and produces electricity. The world's first PRO prototype power plant was inaugurated in Norway in 2009. With PRO appearing to have great potential, the scientists set out to make better calculations on how much it actually could contribute to future energy needs under real-world conditions.

Elimelech and Yip concluded that PRO power-generating stations using just one-tenth of the global river water flow into the oceans could generate enough power to meet the electricity needs of 520 million people, without emitting carbon dioxide. The same amount of , if produced by a coal-fired power plant, would release over one billion metric tons of greenhouse gases each year.

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More information: “Thermodynamic and Energy Efficiency Analysis of Power Generation from Natural Salinity Gradients by Pressure Retarded Osmosis” Environ. Sci. Technol., Article ASAP. DOI: 10.1021/es300060m

Abstract
The Gibbs free energy of mixing dissipated when fresh river water flows into the sea can be harnessed for sustainable power generation. Pressure retarded osmosis (PRO) is one of the methods proposed to generate power from natural salinity gradients. In this study, we carry out a thermodynamic and energy efficiency analysis of PRO work extraction. First, we present a reversible thermodynamic model for PRO and verify that the theoretical maximum extractable work in a reversible PRO process is identical to the Gibbs free energy of mixing. Work extraction in an irreversible constant-pressure PRO process is then examined. We derive an expression for the maximum extractable work in a constant-pressure PRO process and show that it is less than the ideal work (i.e., Gibbs free energy of mixing) due to inefficiencies intrinsic to the process. These inherent inefficiencies are attributed to (i) frictional losses required to overcome hydraulic resistance and drive water permeation and (ii) unutilized energy due to the discontinuation of water permeation when the osmotic pressure difference becomes equal to the applied hydraulic pressure. The highest extractable work in constant-pressure PRO with a seawater draw solution and river water feed solution is 0.75 kWh/m3 while the free energy of mixing is 0.81 kWh/m3—a thermodynamic extraction efficiency of 91.1%. Our analysis further reveals that the operational objective to achieve high power density in a practical PRO process is inconsistent with the goal of maximum energy extraction. This study demonstrates thermodynamic and energetic approaches for PRO and offers insights on actual energy accessible for utilization in PRO power generation through salinity gradients.

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User comments : 10

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hyongx
1 / 5 (1) Apr 18, 2012
This is interesting, but just a thermodynamic proof-of-concept.
And also, using 10% of the world's fresh water river flow to power the needs of Less Than 10% of the world's population seems unimpressive.
Lurker2358
1 / 5 (1) Apr 18, 2012
This is interesting, but just a thermodynamic proof-of-concept.
And also, using 10% of the world's fresh water river flow to power the needs of Less Than 10% of the world's population seems unimpressive.


Not to mention costing more than it's worth by restricting access for river transports. Even with some sort of "lock" mechanism with double gates, you'd spend nearly as much energy accommodating transports as the darn thing produces.
Deathclock
2.3 / 5 (3) Apr 18, 2012
This is interesting, but just a thermodynamic proof-of-concept.
And also, using 10% of the world's fresh water river flow to power the needs of Less Than 10% of the world's population seems unimpressive.


Why? That means we could almost switch entirely to river flow power generation for all of our energy needs... that seems very impressive to me.
Deathclock
2.3 / 5 (3) Apr 18, 2012
Even with some sort of "lock" mechanism with double gates, you'd spend nearly as much energy accommodating transports as the darn thing produces.


No, you wouldn't.
antialias_physorg
5 / 5 (2) Apr 18, 2012
Even with some sort of "lock" mechanism with double gates,

You don't need any of this. There is no reason why this needs to be built accross the entire breadth of a river delta (nor whether it cannot be built under the waterline where river deltas are deep enough to allow ships to pass over it.)

And also, using 10% of the world's fresh water river flow to power the needs of Less Than 10% of the world's population seems unimpressive.

The point is: it's free. There is really no reason NOT to do this. It's not like fresh water will be wasted by this. It will flow into the sea anyhow.
Lurker2358
not rated yet Apr 18, 2012
A_P:

Typically, rivers actually have to be dredged, because silt builds up in them and large ships can't pass any more. In many cases they literally only have 2 or 3 feet of clearance from the bottom of the river!
TopCat22
not rated yet Apr 18, 2012
Thinking they can tap 10% of the worlds rivers is insane. If they said 0.0001% maybe that is considered possible ... but the cost will be ridiculously prohebitive for the small amount of energy generated b ased on their own numbers.
Deathclock
2.3 / 5 (3) Apr 18, 2012
A_P:

Typically, rivers actually have to be dredged, because silt builds up in them and large ships can't pass any more. In many cases they literally only have 2 or 3 feet of clearance from the bottom of the river!


Woe is us, whatever could we do to get around such a conundrum?

Considering that we have space probes in the heliopause and can just about create our own life forms through genetic engineering I would think a little human ingenuity could solve this easily enough such that these energy harvesters AND ships could share the river.
antialias_physorg
5 / 5 (3) Apr 18, 2012
River deltas are usually broad. The path that ships take is somewhere in the center. They don't go all over the place.

That leaves the vast majority of the delta open for buiding these kinds of powerplants.

Add to that that many major cities have been located at river deltas (for obvious reasons) and you have a win-win-win-win scenario.
RealScience
3 / 5 (2) Apr 18, 2012
I first read in the early 1970s that the osmotic pressure of fresh water into average-salinity seawater is equivalent to a 600-foot waterfall at the mouth of every river (its actually a bit more than that). It has been 40 years since then, and only in the last five years has there been real progress on tapping this enormous potential energy source.
But it is not easy - today's membranes are barely up to the task. A multistage process would be gentler on the membranes and on the ecosystem as well (brackish estuaries are among the most productive ecosystems, so having multiple stages of progressively saltier water kilometers apart should be built into the planning from the start.

High potential, but not easy engineering...