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Old 07-02-2013, 07:30 PM   #52 (permalink)
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He may have toured a combined cycle plant.

Originally Posted by ConnClark View Post
I'm calling BS on the 50% efficiency claim! peak steam cycle efficiency with today's top of the line super heated steam plants is 40% maximum. That is about the thermal dynamic limit. To exceed that you have to go to a binary vapor cycle (biggest was just 40MW) or Kalina cycle (of which are currently only small scale up to 3.6 MW).
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. 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. 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.

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.

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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