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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."
So can we expect this to work in parallel with existing hydro power generation techniques?
It would be interesting if this could make desalinization more energy efficient. After you finish desalinization you end up with clean water and very salty water. If you mixed the less salty sea water with your now very salty water, could they recover part of the huge amounts of energy that desalinization requires?
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This is actually really interesting! Think about it. We've been limited to solar cells for a long time for producing electricity, and those have limitations we are constantly struggling against. But... Now, you can make a simple isolated enviroment consisting of water and salt. Design it such that fresh water runs down from a resivoir into a lower resivoir with salt. Expose the lower resivoir to sunlight, and use the greenhouse effect to speed up the evaporation of the water. Direct the vapors up to the upper reservoir, where they precipitate out, and flow back down! Thus, we generate electricity and use the sun to separate the two components to repeat the cycle. (plus if you want, you can capture the heat from the condenser, for even more energy) Not something you could put in your car, but on a large scale I bet this could work. Similar to large steam powered plants.
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.
"You're right," Fisheye says. "I should have set it on 'whip' or 'chop.'"
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.
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Here's the problem. Gravity is only interested in moving stuff in one direction, down. At some water will have to move against the gradient. Does the process produce enough energy to do that? If it does, how much water do you need in how much space, but just as importantly, what is the rate of production?
Oh, we'll put it on the coast, people say. Do the mixing reservoirs have access to the ocean? Good luck with the tides.
I think you need to consider this more deeply.
You need two water sources, fresh and salty. The chambers are flushed alternately from each source. The only way this happens naturally is if you use the tides. Which gives you a total of 1 cycle a day.
Nature is not going to do all of the work. At some point, water needs to be moved against a gradient.
He needs a pump in the lab. If you used a river delta, where there's a natural water flow, you only need a series of diversion gates; one to let fresh water in, one to let seawater in.
He states that in theory you could capture 1.6KJ/Litre of water. 1 Liter water =~ 1 kg. Assuming you want 1m of elevation difference (it simplifies the math and that much head is a pretty solid flush), that's 9.81 J/liter at 100% pump efficiency. Assuming ~30% total pump efficiency, you're at ~30 J/liter. Now say that he has to pump both fresh and salt water, so it takes a total of 60 J of pumping per liter of fresh water reaction. If his system can reach 50% theoretical efficiency (0.8 KJ/liter) then you'd generate ~0.74 KJ/L of fresh water.
Using my example above of diverting ~10% of the mississippi river, Mississippi = 572,000 ft^3/s * 28 L/ft^3 x 10% x 0.74 KJ =~ 1.2GJ/s =~ 1.2GW.
I'd say a 1.2GW power plant is a pretty nifty goal.
Of course at the moment he's generating 0.00005J/liter in his proof-of-concept unit, so that 50% theoretical may be lofty, but with ~60 J/L of overhead, he starts positive power production at 4% theoretical. Not sure what percentage he needs to produce more power than manufacturing the the carbon and other plant facilities requires.
I've been on slashdot so long I'm starting to get out of touch with the cool stuff if it ain't on slashdot.