Combined gas and steam
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Combined gas and steam (COGAS) is the name given to marine compound powerplants of gas and steam turbines, the latter are fed with steam generated with the heat from the hot exhaust of the gas turbines. This way some of the otherwise lost energy can be reclaimed and the specific fuel consumption of the plant can be decreased. Large (land based) electric powerplants built using this combined cycle can reach conversion efficiencies of over 58%.
- For more details on this topic, see Combined cycle.
If the turbines don't drive propeller shafts directly and instead a turbo-electric transmission is chosen, the system is also known as COGES.
COGAS differs from other combined marine propulsion systems in that's not intended to operate on one system alone. While this is possible, it won't operate efficiently this way, as it's with Combined diesel and gas systems when they run solely on diesel engines. Especially COGAS should not be confused with Combined steam and gas (COSAG) power plants, which employ a traditional, oil-fired boilers for steam turbine propulsion for normal cruise and added gas turbines for faster reaction times and higher dash speed.
COGAS has been proposed as upgrade for ships that use gas turbine as their main (or only) engine, e.g. in COGOG or COGAG mode, like the Arleigh Burke class destroyers, but currently no naval ship uses this concept. Hower some modern cruise ships are equipped with COGES. E.g. Celebrity Cruises' Millennium and other ships of her class use turbo-electric plants with two General Electric LM2500+ gas turbines and one steam-turbine.
