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Guru
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Advanced Rankine Cycle???

11/27/2012 4:35 PM

Basic question: what happens (advantageously) if the area below the work integral in the T/S diagram is now in play? To do this, one would have to avoid condensing the steam at the steam turbine exhaust. To see the diagrams I am referring to, use

"Rankine cycle" as a search term, and click on the wikipedia link. There are others if you want a different source. Also here is a link.

http://en.wikipedia.org/wiki/File:Rankine_cycle_with_reheat.jpg

the following patent by Ernst Korting of Vienna, Austria (notice the date) describes a means of condensing steam with water (which apparently could be condensate water from other means.)

I propose that if even as much as 50% of the steam turbine exhaust could be captured in water otherwise condensed, the evaporative loss of water (cooling tower) can be reduced tremendously, and this is the largest use of water in steam electric generation.

Patent number: 141361
Filing date: May 23, 1873

There are numerous patents dealing with jet condensers, distinct from surface condensers. I think the idea would be to utilize a number of jet condensers to capture as much of the steam exhaust as possible, but remember that it will take about 12 to 15 pounds motive fluid to capture one pound of steam. All of this condensate circulating through this type of condenser would require the largest part of it to be cooled in a natural draft tower (dry tower). There would still be an immense heat loss.

Can anyone find a way around this? It does sound a bit like trying to beat thermodynamics, which can't be done. Even if cooling off 15/16 of the water (motive and condensed) and conserving 1/16 of the heat, while eliminating most of the high temperature exhaust penalty, and eliminating the need for vast amounts of water for evaporative cooling, does the cost outweigh the benefits in areas where water resources are scant? Another thought: solar power towers experience problems with cooling tower drift - eliminated in this concept.

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Guru

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#1

Re: Advanced Rankine Cycle???

11/27/2012 9:40 PM

The cost does outweigh the benefits.

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Guru
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#2
In reply to #1

Re: Advanced Rankine Cycle???

11/28/2012 9:46 AM

Expensive yes, so what you are saying there is just too much pumping energy wasted?

So, then, in arid water with little to no water resources, the best way to run the steam Rankine cycle will be with air condenser only?

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Guru

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#3

Re: Advanced Rankine Cycle???

11/28/2012 6:00 PM

The OPs presentation is a bit limited - as I understand - to that diagram only. Well, if my memory serves, it limits conversion efficiency into the low-mid 30%. A second stage helps lifting it into low 40%s. Obviously, condensation is in play then, partially. The cooling tower is dry then, by necessity. A counterflowing heat exchanger will recoup a large fraction of the phase change heat. As no sytem is without losses, a deep well with water treatment system is needed, to provide the low mineral water for the closed system.

At that point only detailed economic study can decide if this, or a completely dry gasturbine is the right fit.

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#4
In reply to #3

Re: Advanced Rankine Cycle???

11/29/2012 9:47 AM

I ran into a couple of educational presentations yesterday about the Rankine cycle and Carnot cycle as a "real" cycle. There is an issue in any Rankine cycle without superheat, where two-phase flow can cause problems with the steam turbine final stages. With superheated steam, and better with steam staged re-heat this is easily overcome, along with steam extraction to regenerative feedwater heaters.

Perhaps in the future, where superheat is not a possibility (due to the source I can only presume), designs incorporating the boundary layer turbine (Tesla Turbine) will be of more utility since two-phase flow does not appear to be a problem in such machines. Albeit, with such a system operating nominally as a two-phase Carnot cycle, the best efficiency is ~28% for a 300oC source.

http://web.mit.edu/16.unified/www/SPRING/propulsion/notes/node63.html

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#5
In reply to #3

Re: Advanced Rankine Cycle???

11/29/2012 9:57 AM

The company I am with is apparently looking at moving away from combined cycle units, and also away from our older fleet of conventional gas-fired steam turbines, toward a newer model gas turbine having two-stage compression with an intercooler and water injection for NOx control. This helps with (1) the elevation penalty, and (2) intake temperature penalty.

I have already recommended to the key decision makers that we should avoid direct cooling heat exchanger on the intercooler, since the high temperature there will result in a problem with the control of scale and corrosion. We will have available a nice new cooling tower with about half its capacity available to cool this off, but I have suggested we opt for a secondary closed loop cooler with much higher water quality than that in the cooling tower. The loop would be at a much lower temperature based on volume and flow, and thus would be cooled by the cooling tower using a second heat exchanger. This is more expensive than just air cooling the intercooler, but with lower final temperature.

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