Electricity From Salty Water

BuzzSkyline writes "It's possible to produce energy by simply mixing fresh and salty water. Although chemists and physicists have long known about the untapped energy available where fresh water rivers pour into salty oceans — it's equivalent to 'each river in the world ending at its mouth in a waterfall 225 meters [739 feet] high' — the technology for exploiting the effect has been lacking. An Italian physicist seems to have solved the problem with the experimental demonstration of a 'salination cell' that creates power given nothing more than input sources of salty and fresh water. The researcher believes that this renewable, environmentally friendly energy source could be deployed in coastal areas and could provide another addition to the green-tech roster. A paper describing the technology is due to be published in an upcoming issue of the journal Physical Review Letters."

Comments

Double Duty?

[drrck]

So can we expect this to work in parallel with existing hydro power generation techniques?

Re:Double Duty?

[Wrath0fb0b]

I got the impression that the writer didn't understand, either. Can any of you chemists/physicists explain this phenomena in layman's terms for us?

Intuitively, anything that happens spontaneously (e.g. water falling down in a gravitational field) must be downwards in free energy or else it wouldn't happen (with any significant probability). So you know that when you pour together your rum and coke into a glass, the final state (uniform mix) must be lower in free energy than the initial state (rum on the bottom, coke on top).

Slightly less intuitively, you can understand it very simply with a lattice model of solution under the assumption that there are no energetic effects (true to first order). Imagine the solvent as a lattice in which each square/cube (2D or 3D, your choice) can be occupied by solute or not -- now count up the configurations that correspond to a mixed solution versus an unmixed solution. That difference is configurational entropy and drives it to seek the macroscopic state with the most microscopic realizations since, in the absence of significant energetic effects, every microscopic state is equally likely.

Of course, it's on wikipedia: http://en.wikipedia.org/wiki/Entropy_of_mixing

Re:Double Duty?

[James McP]

The article actually has an interesting addendum at the end that explains it, albeit in an interesting vernacular.

In short, salt water is ionic. A small initial electric charge is given to the two pieces of carbon (one positive, the other negative). The sodium and chlorine ions migrate to the respective carbon and thanks to the very high surface area of activated carbon, you get a very high quantity of ions. The water source then switches to fresh water. Elecrostatic force tries to keep the sodium & chlorine ions near the carbon but diffusion pulls them away. The work done to pull the ions away is what generates the power.

The inventor that it can generate as much as 1.6KJ / Liter of fresh water. If we diverted 10% of the Missisippi River's outflow into one of these facilities you get ~2.6GW of more or less continuous power. (Mississippi = 572,000 ft^3/s * 28.32 L/ft^3 x 10% x 1.6KJ = 2.6GJ/s = 2.6GW)

Re:Double Duty?

[The_mad_linguist]

Desalination plants produce brine, though, which has more salt than ocean water, so conceivably you could increase the efficiency by combining seawater and brine.

Some ideas

[drinkypoo]

One of the best places (potentially) to grow algae for biofuels is in the desert. You could pump seawater inland, and circulate it in pools. If you covered those pools with greenhouses (which could just be big clear balloons... or not-so-big ones, if you use arrays of small pools) and collected water they'd make you some fresh water, which could then be combined with incoming salt water to produce energy to help run the system, whether that would be the pumps, mixing devices which keep the pools circulating, or what ever else have you.

Another idea for the waste water produced from this process is to pump it inland and use it in the algae pools... so you can have coastal plants whose effluent is used to grow algae for carbon-neutral biofuels, and [optionally] to raise the water table in the desert.

Re:Economy is a Subset of Ecology

[ShadowRangerRIT]

Somewhat inaccurate. They have offered a reward to the first person to make in-vitro meat, where the meat is grown independent of the animal, economically viable. They oppose "unethical treatment," which is defined broadly enough to mean killing or confining an animal for virtually any reason. Bruce Friedrich, a spokesman for PETA, has said that if in-vitro meat were available, he'd eat it in a heartbeat. After all, no animal would have to suffer to provide it. It's a consistent position, which I respect.

Before anyone starts, I'm aware of hypocrisy in other areas (PETA pet shelters), but I'm addressing only their views on vegetarianism.