Wired has an interesting article on a man called John Craven who has a plan to produce a great deal of electricity, among other benefits, by exploiting the temperature differential between the cold water deep in the ocean and the warmer water above it.
The scheme sounds utopian, to say the least, and certain elements sound particularly crazy (chilling the roots of plants to produce more crops, anyone), but can a more scientifically qualified Doper give me a more accurate analysis of his plan? Why will it / won’t it work? What has the Wired article failed to mention?
Hrm, ok. Makes sense to me. But i’m not exactly qualified for any real analysis of this.
I haven’t a clue as to why he needs cold water. It seems that warm water pumped into a vaccum chamber would be all that is needed for the energy generation. I suppose the cold water could generate additional energy condensing the steam into a pool which can then be used to turn a turbine via gravity.
I wonder what environmental impact this system would have. He’s basically airconditioning the world with all that water he’s pumping from the ocean. Is it enough to lower ambient temperature? would it generate winds?
The plant thing is odd, but if the article is telling the truth it seems to be working. 3x yeild is a statistically significant output!
I also wonder how well his system scales. You can’t just expand a vaccum chamber. I’d imagine you’d have to build a bunch of them in series. And who the hell cleans them? There must be mucho salt(I guess that can be sold too) in each of those things after each evaporative cycle.
The whole point is to use the temperature difference between surface water and deep-sea water to generate power. Heat alone does not do any work, you also need a “cold”.
Currently, power is usually generated by heating water to boiling temperature, using the steam to turn the turbine and using lukewarm water (cooled in the big cooling towers) to turn the steam back into water. Sounds like Craven’s system does the same thing at lower temperature and pressure - heat from the lukewarm surface water boils the water, turns the turbine and then is condensed by the very cold deep-sea water.
I’m a bit confused by some aspects of it (what creates the vacuum, and wouldn’t it be a bad idea to boil seawater, what with all the impurities?) but it doesn’t violate the laws of thermodynamics or anything like that. I’m sure it can “work” in the sense that it would generate some power. The real question is whether it can be done cheaply enough to compete with other methods of power generation. And I don’t think anyone can answer that without more work.
[QUOTE=scr4]
The whole point is to use the temperature difference between surface water and deep-sea water to generate power. Heat alone does not do any work, you also need a “cold”.
Currently, power is usually generated by heating water to boiling temperature, using the steam to turn the turbine and using lukewarm water (cooled in the big cooling towers) to turn the steam back into water. Sounds like Craven’s system does the same thing at lower temperature and pressure - heat from the lukewarm surface water boils the water, turns the turbine and then is condensed by the very cold deep-sea water.
[quote]
Um, I still don’t see why you need cold water.
Pump warm water into vaccum chamber
Since theres no counteracting air pressure the heat in warm water is sufficient to vaporize the luke-warm water.
steam rises and turns turbine.
Allow steam to escape to atmosphere.
wheres the cold water come in(um maybe they help run the compressors that create the vaccum?)
Because vacuum chambers take a lot of energy to operate, and warm water costs energy to make. In the immortal words of the satellite test techs in L.A., “maintaining a vacuum chamber sucks.” Water is nearly incompressible, so at those depths the compression is significant enough to make a difference, and the water pushes back very hard. Therefore if you run a pipe from those high-pressure depths to atmospheric pressure (a scant 14 lbs/sq. in) then you get all of that compressed water leaping up your soda straw to escape. He’s talking about moving a column of water that weighs tons, and moving it vertically against gravity several thousand feet, and getting all of that mass and distance practically for free. The thermal difference between the cool water and ambient air temperature lets them pull water vapor out of the air cheaply, which is a big benefit. Just the ability to air-condition a tropical island and provide fresh water is huge in itself.
For the record I don’t see how he’s going to generate energy, either.
The idea has been around for a long time. It was called OTEC (ocean thermal energy conversion, if memory serves. It works, but you have to really scale up to get useful energy out of it. Essentially it’s very low efficiency solar power, but the sea acts as a VERY large area collector for you.
It comes in two flavours, closed cycle and open cycle. With closed cycle, you use a working fluid such as ammonia. The warm surface water vapourises the ammonia, which runs through a turbine, and the cold deep water condenses it back.
With open cycle, you vapourise the warm water directly. Here’s a verbal schematic of how it works:
Firstly, imagine a floating hollow tower in the sea, at least 35ft high, with a closed-off top. Start sucking the air out of the tower so that the warm 25 deg. C surface water rises up it. Eventually the pressure in the tower headspace is so low that the boiling point of water falls to 25 deg. C, so the water boils and fills the headspace with low-pressure steam, also at 25 deg. C.
You can’t just vent this steam through a turbine to atmosphere, because the atmosphere is at higher pressure - it blows backwards through your turbine and pushes the water back down the tower.
Here’s the trick, where you need the cold water. You build a second tower beside the first, and suck water up it, but this water is drawn from the deep ocean and so is at about 4 deg. C. Keep sucking the water up and the same thing happens - the pressure becomes so low that the water boils at 4 deg. C . The pressure will be lower than that in the warm tower.
Then, you link the two headspaces of the towers together with a tube, and put a turbine in the tube. The warm steam is at higher pressure than the cold steam, so it runs through the turbine into the cold tower and condenses when it contacts the cold water. This maintains a lower pressure on the cold side. Meanwhile, more water is turning to steam on the hot side, maintaining the higher pressure there, and the process runs continuously. All the evaporation on the warm side is drawing heat out of the hot water, and the warm steam condensing on the cold side is warming up the cold water, so you have to keep replenishing them from the surface and the depths respectively to stop the temperature difference from dissappearing.
A real plant would use a heat exchanger rather than a cold tower, so the condensed water would be drinkable. This system was used in Hawaii to produce fresh water from seawater, although it produced no net power. All the power generated was used to run the pumps drawing water up from the deep ocean. To get net power, you have to scale up.
There was talk for a while about building something like a 6km high tower partially in the North-Sea over here with the Ammonia / closed system solution that would generate an output that would provide energy for the whole of the Netherlands with some to spare to sell to other countries. It was technically considered feasible, but practically just too plain scary - for instance to have such a high tower, with planes and such flying about …
Still, it’s a cool system. You see oceans of water move all over the planet because of naturally occuring temperature differences, so it’s definitely a feasible method of energy production. I imagine you could even reinforce the system by adding solar and/or wind-power on top.
I can’t see that being the same thing as OTEC. There’s no reason at all why you’d need a 6km tall tower, and the North Sea has cold surface waters. Do you have a link to any details?
The article lacks technical detail, but I see some obvious problems with this sequence. You generally use high pressure fluid to drive a turbine. If you want to use water vapor generated in a vacuum chamber to drive a turbine, you have to have a higher vacuum on the other side of the turbine. You can’t just exhaust to the atmosphere since the pressure gradients want to drive flow the other direction. You could drive a turbine by letting the atmosphere flow backwards through this system into your vacuum chamber, but you’ll never get back as much as you expend generating the vacuum (at least with any pump/turbine process I’ve seen). You’d be better off scrapping the system and just using the power you would have used to generate the vacuum.
Is this true? Most of my fluid dynamics was external flows and it’s been a very long time since I dealt with pipes, but my intuition would be that no matter how deep you sink your pipe, the water in that pipe would just rise to the level of the surrounding surface. The pressure at depth would be sufficient to fill the pipe to the ambient surface, but not make it squirt out above the ambient surface. This means that when you apply a pump to that pipe, the energy required is a combination of the energy required to pump from the ambient surface (i.e. as if your intake were just under the water surface, not at depth) plus the energy to overcome friction in the long pipe. But you don’t have to supply the potential energy of the rising column because the pressure at depth is doing that.
Compare three situations:
[ol]
[li]A pipe from the surface of the ocean to a depth of 5000 feet. In this case, water in the pipe levels with the surface and your pumping requirement is what I mention above.[/li][li]A pipe from the ocean surface up a 5000 foot hill. In this case, the external pressure does no good and your pump has to supply 5000 feet worth of potential energy.[/li][li]A pipe from 5000 feet in the ocean into a STP reservoir at the same depth. In this case, the pipe doesn’t have a 5000 foot column of water in it to offset the pressure at depth like we had in case #1, so the water will squirt out with enormous pressure. But in this case you’d also have to expend enormous energy to maintain your STP reservoir at that depth[/li][/ol]
That’s right; the water pressure at depth squirts the water in your pipe all the way up to sea level, but no further; pumping it out is only harder than pumping water from sea level because of friction in the pipe and (initially) because you’re accelerating a large column of water up to pumping speed.
So you can get really cold water and run it through a system that extracts usable energy from the temperature differential (which could be as simple as a peltier effect semiconductor, although I suspect this wouldn’t be as efficient as other methods. Then you can use a portion of the energy extracted to run the pump. This isn’t perpetual motion and there’s no violation of thermodynamics; all you’re really doing is exploiting the movement of heat input from the sun to a colder heat sink.
Instead of pumping up the cold water (and having to deal with impurities), you could drop a heat exchanger down there and use a closed loop for the coolant. You’d have to insulate the pipes running up and down, so that you don’t gain too much warmth on the way up.
Right there is the point you are missing. The steam was generated at vacuum. It won’t escape to atmosphere…atmosphere will “escape” into your exhaust distroying the vacuum.
In order to flow through the turbine, the exhaust must be a lower pressure (deeper vacuum) than the inlet. The power produced depends on how large of a pressure difference. The boiling (=condensation) point of the working fluid (which need not be sea water, nor even water at all) depends on temperature. Lower exhaust temperature equates to lower exhaust pressure equates to more available power.
You are of course correct about pumping water from depth - there is no potential energy change in bringing it to the surface. Pipe friction is an issue - to get reasonable power out of these systems, you have to build them big. One design I saw involved a fibreglass fabric pipe 10m in diameter. You hang an electric pump off its bottom end with a power cable coming down from the surface, then push the water up through it, inflating it.
Mangetout - a Peltier system isn’t very efficient, but I don’t see why you couldn’t have your hot junctions at the surface and your cold junction at depth, with long cables. Ditch the pumps and turbines altogether!
Because you’re right, and right is always worthwhile? Sorry I missed that bit in your post. I did read it, but missed it’s relevance to Harmonix’s error. Kevbo just repeated the same thing, so I guess we all need to be more careful (or tolerant of dupes).
It’s just a bunch of thermocouples wired in series. You can seperate the junctions as much as you like. Can’t see any theoretical obstacles, just practical ones - pathetic efficiency, long conductors etc.
More of the “SOS”
Just provide a bit more funding to perfect the process and well have py in the skie fpr everyone. Utopia here we go!, or is it come?
Always in development never in full blown commercial operation.
Always problem$, too much to do so little time. etc. etc.
They’re wired in series, yes, but the hot and cold is at the end of each one, like this:
=========== <hot side
=== === ===
P N P N P N <Semiconductor blocks
---= === === =--- <Terminal connections here
=========== <cold side
In order to separate the hot and cold junctions you’d have to find a way to break the whole system in half, which would mean splitting the device in half horizontally (or manufacturing it that way, of course), then linking each of the P chunks with its counterpart at the other end, like this:
=========== <hot side
=== === ===
P N P N P N <Semiconductor blocks
| | | | | |
| | | | | | <Lots and lots of really long wires
| | | | | |
P N P N P N <Counterpart semiconductor blocks
---= === === =--- <Terminal connections here
=========== <cold side
So your cable linking the half of the device at ambient temperatures would have to be linked by a massively parallel ribbon cable down to the chilled on on the ocean floor.