h a l f b a k e r yBunned. James Bunned.
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As we all know, with all conventional forms of power, enormous amounts of heat energy is lost to the enviroment when the steam is condensed after powering the turbine. This steam is too cool to keep powering turbines, and yet it still contains (coal used in this example) 50% of the coal's chemical energy.
Why cant this heat be used to power a multi-stage flash distillation/desaination plant? (For my non-Australian readers, there is a huge drought in Australia). At the moment we're building a reverse-osmosis plant which will use 90MW...
Any thoughts ?
(btw this is my first post)
Desware
http://www.desware.net/des_D08.aspx
Interesting stuff [8th of 7, Aug 25 2008]
OK, I posted it
Vacuum_20Desalting
As mentioned in an annotation [Vernon, Aug 26 2008]
[link]
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All that heat to waste... |
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A factory I did some consulting for (I am an automation engineer) did a lot of cooling and had several cooling towers, costly things to run in the summer / cheaper in winter (factory is in New England, fairly cool winters). |
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Got some quotes for cogeneration, that is using their process waste to make power. In the end it cost more in capital and lost land than the cogeneration company could make on power, bad return on investment, better to let the heat go to waste. |
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Seems reverse osmosis desalinization doesn't benefit from heat, and boiling water is an expensive way to sweeten it- even with free heat. |
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Much better to find some way to get more electricity from the steam, and use that to power the pumps... but again, spendy infrastructure. |
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If you don't mind my asking- which plant are you thinking of? |
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No I wasn't suggesting that the heat from power stations be used in Reverse Osmosis, but rather for Multi-stage flash distillation (basically just boiling the water). |
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The Reverse Osmosis plant which is being built in Victoria will cost $4bn and use 90MW of power... The drought's so bad here we'll outlay THAT much money for water. |
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Which plant to use? Well, theres a 520MW coal fired power plant near Adelaide which is on the coast and actually uses seawater to cool it's steam. How expensive would it be to build a desal plant from there ? |
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I did some calculations this morning... |
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520 MW at 30% efficiency, losing 50% chemical energy to heat means that there is 867MW of heat lost in cooling. Flash Distillation plants (with heat recovery) use 27 KWh per KL of water... |
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This could produce 281 GL of water per year... almost twice than the proposed 90MW reverse osmosis plant! |
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Retrofitting heat recovery to existing plants is often not practical. The plant was designed to dump waste heat to ambient; if you run in heat recovery, the efficiency of the primary plant can be seriously compromised. |
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The best answer is probably a fission powerplant driving a combined desalination/electric generation plant. Nuclear plants do best when run at constant output. So, when electricity demand is high, couple the output turbines to the generators. As demand drops, couple them to mechanical pumps for the RO plant. And you can then design the downstream heat-recovery stages to get more water from flash evaporation. |
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This isn't heat recovery, though, is it [8th]? It's primarily water recovery. |
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I looked at the Australian program. It
looks like a stopgap measure at best.
Fact is that to produce enough water to
actually sustain the rural economy is
impossible. The government is doing
the equivalent of a rain dance in the
hope that Murphy's law will break the
drought. They have to do something or
they will get lynched but anyone can
see that you can't fight a dust bowl with
lawn sprinklers. |
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WcW, you're right... desalination plants can't run the rual economy... However, at least people will be aloud to have lawns and wash their cars... and perhaps take a little stress off the river system. |
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Pertinax, you're also right. Most power stations use fresh water cooling towers, this is why you see those huge steam plumes rising from them. All I am suggesting is that salt water be used to cool in a similar manner, and the steam (which should be potable or at least, desalinated water) condensed and carried away for storage and use... |
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WcW... The farmers here are close to lynching someone. |
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8th of 7... the idea here is to somehow utilise the vast amounts of low qualith heat lost in the cooling stages of poweplants. We already know how to use electricity to desalinate. |
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Could the steam, having been raised to a height then condensed, be used to generate supplementaty hydroelectricity? |
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Yes I have heard of them ... Then why arent they applied to power stations then? Anyway it would still be more efficient to use the the heat to desalinate rather than generate extra electricity then desalinate, wouldn't it ? |
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// it would still be more efficient to use the the heat to desalinate rather than generate extra electricity then desalinate, wouldn't it ? // |
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That depends on the relative efficiency of a RO plant and a condensation system. Some plants use butane to freeze sea water, concentrating the salt in the remaining liquor. |
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IF you have cooling towers, then by all means condense the steam in a closed-cycle system. |
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Well because the heat is relatively low tempreature, it's not the best for generating electricity... even with a stirling engine. |
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// IF you have cooling towers, then by all means condense the steam in a closed-cycle system // |
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You're still wasting the heat then... Just pump in seawater and condense the fresh water steam; putting it to good use. |
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Maybe I should move to Australia... |
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Cogeneration and using excess heat is easier said than done, sadly. |
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Domser, the 281 GL of water seems pretty good. The place I worked at had high expectations too- but the costs were too big. More towers because they are less efficient at cooling, and lost land for the cogen. |
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Maybe you mean: a new coal plant should flash distill water as a matter of course, and after the turbines use the result as drinking water. So always draw in ocean water, flash it, and outlet it immediately- nice because we don't try to cool the water for the coal process. Hard because we don't maintain a treated closed loop of water. |
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actually much harder than it sounds... |
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Bcrosby ... please note that calculation is based on my knowledge which is restricted to year 12 chemistry/physics. |
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This power plant near Adelaide condenses its steam by passing steam through pipes in the ocean... How about constantly pouring sea water over the hot pipes, that way the salt would be washed back into the ocean. Put a cover over all of this so the water condenses, then carry it away like hegelstone says. |
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Bcrosby ... can you please explain why the costs were too high and why efficiency was so adversely effected by the proposed heat recovery... |
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also, using the excess heat for flash desal and for power generation are two very different things... |
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power gen efficiency will be shot by low temps... costs will be high because of generators, copper, etc... |
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flash distillation only requires the water to be boiled , condensed away... and an inflow/outflow system devised. |
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// don't maintain a treated closed loop of water // |
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All steam turbine systems have a regime of keeping the working fluid ultra-pure, minimising TDS (Total Dissolved Solids) and blowdown requirements. Top-up is with deionised water. |
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The problem with a total-loss flash steam system, especially one using sea water, will be the huge requirements for flushing to stop the dissolved solids condensing; and flushing takes away the heat as well .... |
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not to mention the corrosion issues. Salt
produced by flash distilling saline forms
deposits that do not readily dissolve in
saline. Imagine that indestructible deposit
left on the bathtub by the leaky faucet
multiplied by ten thousand. Not a
workable system if you desire efficiency.
Hard real world facts. |
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Well if current flash distillation plants operate with steam-filled pipes there must be a way to do it efficiently... the only difference is the source of the steam.. isn't it ? |
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Ordinary desalination plants that use heat need it in more concentrated form than can be provided by most sources of waste heat. That's really the crux of the problem of trying to use waste heat for desalination. This is not to say that some UN-ordinary type of heat-using desalination plant can't use waste heat. I know of one such, that I independently dreamed up years and years ago. Some time after that I read about it in a magazine article, and have since assumed it is in wide use --except that when I search for it on the Web, I don't see enough details to be sure it is the same Idea. I guess that means I should go ahead and post it here at the HalfBakery some time. |
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You said MOST sources of waste heat, Vernon. The waste heat from a coal-fired power station is 50 % of the coal's chemical energy... that's more energy than electrical output. |
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[Domser], I specified "concentrated". If the waste heat from a coal-fired power plant was more concentrated than what it is, they'd be using it to make more power. When they design the plant they usually decide then how far to take on the Law of Diminishing Returns --and when they stop, that's when all the remaining heat is considered "waste". |
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One could route the waste heat through a tank of wet biomass or garbage. The wetness of the stuff decreases the energy that can be obtained burning it. The heat will help dry it. Then it can be burned with the coal. |
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Good point... then the water used for gardens? |
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Does anyone else have any ideas what all that waste heat could be used for... We've decided electricity gen is a no... Desal a maybe... Surely it must be good for something ! |
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If cold seawater was pumped past the hot steam flowing through pipes (as in a heat exchanger), the steam would still be rapidly cooled thus maintaining plant efficiency. |
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The hot seawater is then pumped into a small, covered, reservoir where a percentage will drift off as steam which can be condensed and collected. |
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The warm, slightly saltier, water in the reservoir will be continually pumped back in to the ocean. |
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Is this any improvement ? |
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These are the type of ideas I expect in halfbakery:
Those that make the world a better place to live in. |
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Thank you, kamathln. Unfortunately I don't yet have the knowledge or skill to really know whether or not this are viable- which is why I turned to Halfbakery. |
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Does anyone know the temperature of the steam after it has passed through the turbines ? |
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The temp of steam at the discharge of the steam turbine will be some comfortable margin above boiling (100C) because any condensation is very corrosive. Things are moving near the speed of sound (1000 km/hr or so) and water droplets erode impellors at those velocities. |
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So call it 120-140C. Not much of a deltaT above boiling to distill water. So don't consider that as an option. Also, that is an integral and very critical part of their process. They are not going to want complications and possible failures in that most important part of their process. The steam turbine and generator in a 500 MW will be the size of a large highway truck. Versus the rest of the coal plant which could be several hectares and 30 meters high. |
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The higher grade heat would be the exhaust of the flue gases. In a single-cycle gas turbine (we've got a 40MW one) the exhaust temp is 1100F/590C which is PLENTY to distill salt water (and you'd obvious use "cooling water" that had already been somewhat preheated (by 45F/25C above ambient in a coal plant I was in last week. |
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This absolutely is worth doing and, shooting from the hip, seems very economical if designed in from the start. Very likely also feasible even as a retrofit. I suspect the problems are more political and regulatory than technical or in the heat and energy balances.
For instance, if you cool the coal plant's exhaust gases, it won't disperse as far from the source. So the soot, mercury, etc, will land closer to the plant and upset the neighbors. The coal plant got their permits based on certain operating conditions and they don't want to rock the boat by revising their discharge stack parameters. |
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You're viewing all the savings that could go to society, I think. But the electrical utility is going to be most interested in a project in which they got most of the benefits. Yes, it is waste heat. No one is using it now. But to bring them to the table, you may have to pay $XX per MMBtu for something they currently throw away. I know of an existing co-gen project (very cold potable water was used to cool a power plant and the heated water prevents freeze-up in the buried water mains in a sub-arctic climate) that almost got scuttled when the two entities started trying to charge each other for the benefit they provided to the other. |
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Sometimes these things can happen when the two parties can examine their common interests and their rational minds are are larger than their egos. Or if some power from above knocks heads togethers to make a logical project like this happen despite the power plays and egos involved. |
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-David in Kenai
Registered engineer and Director of a electrical utility |
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How would you remove all that salt built up after distillation? |
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