The race to achieve 60% efficiency from gas turbines in combined-cycle is gathering momentum with most of the major OEMs actively involved in achieving this goal. Drew Robb explores this ‘holy grail’ of combined-cycle gas turbine power plants.
Sixty per cent efficiency! Like breaking the sound barrier, running the four-minute mile or the search for the Holy Grail, many of the big gas turbine original equipment manufacturers (OEMs) are on a flat-out sprint to be the first to achieve demonstrable 60% combined-cycle efficiency.
GE, of course, announced the achievement of that lofty goal several years ago. But that turned out to be a little premature. These days, GE is a lot more conservative – by far the most conservative of the major OEMs – at least with regard to efficiency announcements.
Siemens and Mitsubishi Heavy Industries Limited (MHI), on the other hand, are claiming an imminent victory. According to Carlos Koeneke, technical director at Mitsubishi Power Systems Americas, the company’s J-class machine will provide in excess of 61% by 2011. In the meantime, the Japanese company leads the pack with 59.1% verified on an M701G2 gas turbine at the 1500 MW Tokyo Electric Kawasaki power station in Japan.
Figure 1: Siemens ‘H Class’ gas turbine is the latest in a long line of progression in combined-cycle technological developments.
Siemens claims that it can match this figure. Fischer states the Siemens F class in combined cycle operation scores in the 58% to 58.7% range at ISO conditions 1-on-1 configuration. Recently they reached well over 59% in Irsching 5 which is a combined-cycle gas turbine (CCGT) power plant in a 2-on-1 configuration based on two SGT5-4000F with a special optimized cycle design, In future, the coming H-class will offer above 60%,’ says Willibald Fischer, programme manager for the Siemens SGT5-8000H. After undergoing heavy testing at the Irsching prototype plant in Germany the machine will be released on the market. The company is so confident of its numbers that it is virtually guaranteeing 60% for current orders. ‘Based on careful evaluation of the test data gathered, Siemens Energy is now in a position to offer the simple-cycle SGT5-8000H up-rated to 375 MW at 40% efficiency and the SCC5-8000H 1S single-shaft, combined-cycle plant with an output of 570 MW at beyond 60% efficiency,’ says Fischer.
A GREAT LEAP FORWARD
Gas turbines were not always this efficient. The first simple cycle model, developed in 1939 had an 18% thermal efficiency. Its turbine inlet temperature was less than 540°C, with an exhaust temperature a little over 260°C. These days, we are talking about around 40% simple cycle efficiency, with turbine inlet temperatures of 1500°C and exhausts up to 630°C – and of course, on much larger machines.
CCGT, of course, represented a major leap northward. But they only gained real market traction in the early 1990s. By that time, developments in steam and gas turbine technology, as well as bleed over from the aviation field, had advanced enough to usher in the era of high efficiency turbines.
This was achieved by a combination of better alloys, coatings, combustors, compressor ratios, higher turbine inlet temperatures, better cooling, advanced heat transfer technology and a whole lot more. ‘Higher gas turbine efficiency is obtained not only through operation at higher turbine inlet temperatures and pressure ratios, but also with improved compressor and turbine aerodynamic designs, improved seals, better clearance control and larger engines,’ says Dale Grace, senior project manager in Electric Power Research Institute’s Generation Sector Combustion Turbine research.
The evolution of Siemens turbines serves as a case in point. By end of the 1980s, the E-class provided a 145 MW gas turbine coupled with a 80 MW steam turbine with approximately 50% net efficiency, such as at in Bang Pakong in Thailand.
‘The combined-cycle plant at Killingholme, UK, had achieved an efficiency of 52% in 1992,’ says Fischer. ‘In the last 15 years, the efficiency of combined-cycle power plants has been improved continuously.’ By the tail end of that decade, the F-class had risen to 56%. Incremental changes throughout the next ten years pushed the figure ever closer to the 60% goal (Figure 1).
‘Today, the most modern F-class is at 58.7% ISO at the high end. Nevertheless it can achieve beyond 59% as proven at Irsching 5 in Germany and Sloecentrale in the Netherlands for example,’ says Fischer. ‘This was attained by increasing firing temperature and optimization of the steam water cycle without hampering the flexibility through additional external cooling systems. In combination with increasing the compressor mass flow, the power output could be increased as well.’
|Figure 2: Mitsubishi’s ‘J Class’ gas turbine – on course for 60% efficiency in combined-cycle operation|
Advances in the water steam cycle, for example, have led to a 30-40°C increase in steam-temperature. A double pressure heat recovery steam generator (HRSG) was changed to a triple-pressure reheat steam turbine to push the F-class closer towards the elusive 60% mark.
Since 2001, when the F-class reached 58% at a combined-cycle plant in Mainz-Wiesbaden, Germany, the machine eked out tiny gains. With the new gas turbine, in the H-class, Siemens promises to have Irsching Unit 4 beyond 60% in 2011. Why the wait until 2011? Fischer cites the early F-class problems experienced by many manufacturers as they raced to be first-to-market with machines that were not quite ready for prime time. As a result, OEMs are going about their business in a more controlled manner and are being careful not to introduce the sale until it has been thoroughly tested.
The H-class has undergone 1500 hours of testing. Irsching Plant 4 is in the progress of being expanded to a single-shaft combined-cycle station for further testing before being handed over to E.ON Kraftwerke in 2011 for commercial operation.
TOPPING THE 61% BARRIER
Another company that is engaged in serious testing is MHI. Three of its M701G2 G-class turbines have been running at Tokyo Electric Power Company’s Kawasaki thermal power station. Each has a combined-cycle output of 500 MW, 59.1% efficiency (simple cycle, 39.5%), a 1500°C turbine inlet temperature, an exhaust temperature of 587°C.
While the G-class focused on steam cooling, a new upgrade known as the MS-501GAG uses an air-cooled combustor. It is a 1-on-1 combined-cycle arrangement. According to Koeneke, it has reached 59.2% efficiency. And once again, testing is ongoing. The G-series has an inlet temperature of 1500°C and keeps nitrogen oxides (NOx) at less than 15 ppm. In addition, MHI is working on the J-series turbine which is targeting 61% efficiency by 2011 (Figure 2). This machine has an inlet temperature of 1600°C. MHI claims that the M701J’s 670 MW will make it the largest gas engine in world. A 60 Hz M501J will also be available, which can hold 55% efficiency at 50% load. The J-series resumes the company’s love affair with steam cooling. It incorporates some of the technologies being developed as part of the Japanese National Project including advanced cooling, better turbine aerodynamics and lower thermal conductivity coatings. That project aims to eventually reach a turbine inlet temperature of 1700°C in order to attain efficiencies of 62–65%. Exhaust gases will be recirculated as part of a strategy to keep NOx levels down.
According to Koeneke, a major impetus behind 60% is the push to reduce carbon dioxide emissions. He believes that the current status of carbon capture technology and sequestration involves engineering and economic challenges that impair near future implementation on a large scale.
‘The best way to reduce carbon emissions in the near future is to replace existing old generation capacity with natural gas combined cycle with the highest efficiency possible,’ says Koeneke.
VIEWING THE SYSTEM AS A WHOLE
Alstom’s KA26-2 ICS (integrated cycle solution) combined-cycle reference plant was launched in 2007 with a net efficiency of 59%. This multi-shaft 2-on-1 facility has been used to develop various optimizations based on individual plant conditions. ‘We are commissioning combined-cycle plants today that will achieve around 59.5% plant net efficiency (based on ISO conditions),’ says Michael Ladwig, technical director of turbomachines at Alstom Power. ‘We are continuously improving all aspects of the KA26 plant and take part, with this combined-cycle plant model based around the Alstom GT26, in the race to achieve 60% net plant efficiency.’
According to Ladwig, Alstom focuses on optimizing plant performance rather than on component performance. The logic being that you have to view the system as a whole. Like the other OEMs, he calls attention to turbine inlet temperatures as the primary factor in efficiency.
In addition, increased steam temperatures and pressures, as well as improvements in the HRSG, steam turbines and generators have contributed to the increase in net plant efficiency of combined-cycle plants. ‘Efficiency is an important element in the profitability and environmental impact of a combined-cycle power plant,’ says Ladwig.
Alstom is going for a ‘one machine fits all’ philosophy whereby its GT24/26 can cope with the full range of operating conditions (see Figure 3). The GT24/26 can reach full load in less than 25 minutes and 160 MW in ten minutes by turning off one of the combustors. It has a lowload parking point which generates 10 ppm of NOx.
STEAM VERSUS AIR
Efficiency is clearly linked to turbine inlet temperatures and compressor pressure ratio. But a hike in temperature produces more emissions. Keeping those down requires effective cooling, which is why MHI tends to favour steam cooling for very high temperature applications because steam is a more effective medium for cooling.
GE is another OEM that seems to prefer steam for higher efficiency. A few years ago, it jumped the gun on announcing 60% but found it hard to back it up in the real world. These days, the OEM takes a more conservative view. Don Hoffmann, senior product manager for GE Power & Water repositions GE as follows: efficiency values approaching 58% are what GE is willing to stand behind for its F-class technology. The 60% efficiency rating is associated with GE’s most advanced gas turbine combined-cycle, the H System.
‘The major difference between the two technologies is that the H System uses steam cooling in the gas turbine which enables higher firing temperatures and higher efficiencies,’ says Hoffmann. ‘Rather than focusing completely on the 60% standard, GE’s strategy is to offer machines that meet a variety of operating profiles, not just efficiency. Each machine is designed to meet the needs of a customer’s specific operating environment.’
GE is saying it will meet 60% once the H System goes into full combined-cycle operation. This is planned at the Tokyo Electric Power Company’s Futtsu-4 plant within the next 12 months or so, though the company is remaining coy about exact dates. It’s not clear whether it has already conceded defeat to MHI and Siemens, or is quietly planning to steal their thunder.
The H System features closed-loop steam cooling to lift turbine inlet temperatures to 1430°C – 110°C higher than the F-class – and single-digit NOx levels. The compressor pressure ratio has been raised from 15:1 to 23:1. Steam cools the blades of the first two stages, with air cooling used on the third stage. The final stage goes uncooled. Single-crystal nickel alloys are used to withstand the first stages higher temperatures in conjunction with a thin ceramic coating. The downside of steam is that it is less flexible than air and harder to engineer into the gas turbine.
‘The air-cooled concept offers maximum added value by virtue of its higher operational flexibility – an essential prerequisite in the deregulated power generation market environment,’ says Fischer. Like GE, Siemens’ Fischer concurs that the race towards 60% has been tempered by the needs of flexibility, part load conditions and emissions compliance. Plants are now being required to start-up daily. Therefore, the waiting time until steam is ready and the added complexity in steam design inhibit system agility. ‘Steam might give you a little more efficiency, however we consider this the wrong solution for the market requirements for high flexibility and daily cycling, athough it presents more of a challenge to reach 60%,’ says Fischer.
Providing combined-cycle solutions with greater operational flexibility, FlexPlant 30 in the 60 Hz version, for instance, has an F-class boiler and F-class turbine with around 57% efficiency, the ability to start up daily and run 4000 to 8000 hours per year. Equivalent concepts are available and introduced in 50 Hz such as at Pont-sur-Sambre in France, and Sloecentrale in the Netherlands, reaching 30-40 minutes start up times at 250 starts/ year.
Ten years ago this type of CCGT would only have needed about 50 starts a year. But with more renewable energy connecting to the grid, it is used more for load shaving in the mid range. ‘Flexibility is more dominant in this setting than the last tenth of a % of efficiency,’ says Fischer. ‘If the wind disappears, you have to be at full load fast.’ Most industry experts agree.
‘The objective is not just to reach the high efficiency target, but to be able to operate a reliable plant,’ says Koeneke. ‘In other words, the main difficulty involves designing, building and debugging the new equipment so that the availability and reliability remain at the same level of preceding technology.’
‘Other operational capabilities such as rapid start-up, low emissions and good turndown are also demanded in the market,’ says Grace. Not everyone however, agrees that 60% is a sufficiently high goal. And, of course, there are many factors involved in how you calculate it. ‘To describe 60% efficiency, one must compare identical plants with the same configuration – either 1-on-1 or 2-on-1, with the same type of heat sink, same pollution controls, identical fuel composition and operating at the same ambient conditions,’ says Bechtel Fellow Justin Zachary.
‘Plants with once-through cold seawater heat sink, high methane fuel, no back-end selective catalytic reduction and operating below ISO have already achieved 60%. The devil is in the details rather than making the front page news with unqualified statements.’ He believes the industry’s focus should be on 63%.
|Figure 3: Alstom is going for a ‘one machine fits all’ philosophy whereby its GT24/26 can cope with the full range of operating conditions|
The four-minute mile barrier was broken by Roger Bannister in the 1950s. The following 50 years have seen the record decline by only 17 seconds. Outside of the professional elite, only a small percentage manages to break four minutes for the mile. And so it may be for power plants. ‘The majority of new combined-cycle power plants being in the 60% net efficiency range (actual site conditions) will not be achieved until after 2020,’ says Ladwig.
‘Everyone would love 60% but the cost is often prohibitive,’ says Scott Nolen, a product director at Rolls-Royce. ‘Not everyone needs a heavy-duty combined-cycle plant. Machines that can achieve baseload within ten minutes such as the Trent 60 are important for the peaking market and a lot cheaper.’
This article was first published in the March 2010 issue of Power Engineering International (www.peimagazine.com)