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Originally Posted by RustyLugNut
As I mentioned earlier in the thread, a gas turbine ( Brayton cycle ) can feed a steam turbine ( Rankine cycle ) to exceed the 42% Carnot cycle theoretical limit. It is done regularly on some of the Navy ships ported here in San Diego. I am quite sure it is much more widespread in its use.
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About the most it could possibly be is 25% as that is the amount of energy that is derived from natural gas and oil. Once again that is the most it could be not the likely amount of US energy production.
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A friend of mine is a mechanical engineer in the installation and retrofit of these systems. He says these combined cycle systems can reach and exceed 50% efficiency even when using the gross lower heating value of the fuel oil. Use of the relatively low temperature output steam is found in crew comfort heating and sea water purification, among other processes.
On land, both General Electric and Westinghouse have Combined Cycle generating facilities that exceed the Carnot limit even when energy for fuel cleaning steps are taken into account. These plants are in the range of 250 - 500 MW and above.
The plant that RedDevil toured may have been an early triple cycle plant.
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Not according to their website its not. They say its a straight coal powered steam plant
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The gas turbine ( at roughly 1400 deg C ) feeds the bottoming cycle steam turbine ( 800 deg C ) which then feeds a final bottoming cycle Kalina type turbine ( ammonia/water refrigerant at 200 - 300 deg C ). The final bottoming cycle will only capture a few percent of the wasted heat energy, but it can push the combined cycle efficiency well over 50% - thus allowing for losses through transmission - and the claimed delivery efficiency.
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it may be possible to design a plant like this but nobody has yet. If you can name an example I would like to know of it.
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Current developments are attempting to add bottoming cycles that use refrigerants that expand in the 100 - 200 deg C range to absorb more lost energy. Finally, the application of Sterling engines to the low 100 deg C exhaust stream is being looked at.
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and these low temperature stages might extract an additional 3 to 7% due to the low operating temps. Also a bottoming cycle tends to reduce the efficiency of the upper cycle. At some point a bottoming cycle no longer helps
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Yes, it all adds cost and complexity. But, as most emissions are based on an amount per KWH produced, and greater efficiency adds profit, power generators will move in this direction.
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They aren't likely to spend 50% more to get 10 % more output. The binary mercury vapor cycles proved to be uneconomical to build and operate in the past. Its not like we will see more of them in the future as they are the type of design that could use nuke or coal which provides better than 60% of US electrical needs currently.