What is the best way to move towards combined cycle efficiencies of 60% and above – effectively the four minute mile of gas turbine technology? There is no clear cut answer, and indeed the routes being taken by the major players seem to be increasingly divergent.
GE, with its H machine, appears to be the sole remaining proponent of extending steam cooling to gas turbine rotating parts. This is a complex and challenging business, but GE reports that it has been successfully demonstrated over a not inconsiderable operating period at commercial scale in the 50 Hz Baglan Bay plant in Wales – so much so that the rating of the 50 Hz System H has been upgraded from 480 to 520 MWe.
Mitsubishi toyed with the steam cooling of rotating parts in its version of the H a few years ago, but has not pursued the concept, and now confines steam cooling to stationary components. The company employs steam cooling in the combustor transition pieces of its established G machines as did Siemens-Westinghouse in its 60 Hz, W501G, turbines. In the G1 machine Mitsubishi extended steam cooling to the first row stationary blade ring, to achieve active clearance control. The first G1 has been operating for about 3 years at MHI's T-Point in-house test combined cycle plant at Takasago, while the first commercial manifestation of the G1 will be Portland General Electric's Port Westward facility, due to enter service in May 2007.
In Mitsubishi's new G2 machine, the steam cooling is further extended, to the second row blade as well. The lead G2 is scheduled for first firing this coming November, at TEPCO's Kawasaki plant, where three single shaft CCGT trains are due to be in service by mid 2009.
Alstom's GT24/GT26 machines are completely air-cooled, and indeed Alstom's current strategy is to extract the most it can from the existing platform rather than introduce new technology – perhaps reflecting an understandable tendency towards risk aversion in the wake of the problems that the GT24/GT26 units suffered in their early years of operation. The strategy seems to be working if the recent spate of sales - in Spain, Germany, Australia, Italy and the UK, for example – is anything to go by. Following the most recent upgrades (see p 35), the GT26 in 2-on-1 combined cycle configuration can deliver 59% efficiency, "a significant step in Alstom approaching 60% with the existing engine platforms", says the company.
Steam cooling of any kind is also studiously avoided in the new Siemens H class machine, which is designed to achieve over 60% efficiency. Even the concept of steam cooled combustion transition pieces, as used in the W501G, is eschewed, the rationale being to achieve maximum flexibility in operation, with the stated aim of competing "against steam cooled competitor products." Site preparation work has begun for the prototype Siemens H machine, which is to be hosted by E.On at its Irsching site in Germany, with first firing planned for late 2007. That will be an interesting time for enthusiasts of air cooling.
Meanwhile, looking to the longer term, a major R&D programme currently underway in Germany has set itself the even more ambitious target of achieving nothing less than 65% efficiency in combined cycle mode with an air cooled gas turbine, by 2025. The plan is to deploy a package of measures that potentially includes the use of "effusion" cooling in which cooling air is blown through "pores" in turbine blades, for example. This programme (described on pp 25-29) is charting new territory for gas turbine air cooling and is likely to yield some very interesting findings over the coming years.
James Varley, Managing Editor, MPS
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