New, efficient industrial gas turbines coming: Siemens, GE, full report

At the heart of the latest combined-cycle gas turbine plants from GE and Siemens lies innovation, not just gigantic size. A key enabler of increased thermal efficiency is turbine design with higher temperature combustion and exhaust gases. Link to photo of Siemens unit.

08/08/2008


Technology advances in industry often proceed at a slow, conservative pace. This applies more so to physically very large and complex systems, like the new-generation stationary gas turbines coming online. These weigh up to 440 metric tons—that’s nearly a million pounds of hardware! But at the heart of the latest combined-cycle gas turbine (CCGT) plants lies innovation, not just gigantic size.

For example, the newest gas turbines apply single-crystal materials, super alloys, special thermal coatings, and—in one design—closed-loop steam cooling to reach 60% thermal efficiency and beyond. Moreover, the turbine systems are said to produce fewer emissions than conventional CCGT plants. Even substantially higher efficiency is possible in a cogeneration plant, where excess generated heat is put to use for process or domestic heating, in addition electric power generation.

A key enabler of thermal efficiency is turbine design with higher temperature combustion and exhaust gases. However, this requires special cooling at the hottest section of the turbine. GE Energy uses steam cooling in its H system turbine that operates as hot as 1,430 °C (2,606 °F). Siemens Power Generation has opted for all air-cooling, because this method is considered simpler than steam cooling and offers more design flexibility by avoiding dependence on the steam cycle, according to Siemens. ( Photo, Siemens tour: Largest gas turbine: 2,838 sensors, 90 GB data per hour of testing .)

Use of advanced materials also enables higher temperature operation. GE’s H turbine uses single-crystal materials in first-stage blades and vanes that endure extreme temperatures over a long service life. Similarly, row 1 turbine blades of Siemens’ SGT5-8000H machine are made of specialized high-temperature alloy material to combat long-term effects of high-temperature and stress (creep deformation).

Actual performance

GE Energy and Siemens have a history of gas turbine expertise. Earlier smaller turbines from each company have recorded thermal efficiencies in the 57-58% range in combined-cycle plants, so the 60% design goal is realistic.

Hard operating data is still scarce, however. Although GE turbines at Baglan Bay power station in South Wales (U.K.) has generated power for some time, it served as a gas turbine technology validation site. Still, the plant is said to have reached “just under 60%” efficiency.” Additional CCGT plants and further operating experience will validate efficiency levels.

Actual thermal efficiency depends on site ambient conditions and balance of plant (BOP) equipment configurations, according to GE. “Specific site conditions and BOP configuration at Baglan Bay were not optimized to achieve 60% overall plant efficiency,” as reported in GE Energy in March 2008. “Data gathered at Baglan Bay support the technology’s capability and GE has offered the 9H System with a 60% efficiency guarantee where site conditions are appropriate.”

Extensive testing

Extensive, predefined testing is necessary to ensure that turbine performance meets design specs, along with reliable, long-term operation associated with power systems. With several different technology levels being validated, the long development cycle needed for these turbines—from first firing through commercialization—becomes evident.

Proof of the gas turbine comes after its incorporation into the combined-cycle system, notes Phillip Ratliff, director of next-generation gas turbines at Siemens. This is scheduled in phase 2 of development for the Irsching CCGT plant in Southern Germany (see Advancing Technology column, Hunt for more than 60% thermal efficiency, Control Engineering August 2008 ). “The idea was to build a gas turbine for the most efficient and operationally flexible combined-cycle power plant,” says Ratliff. “However, the gas turbine is a major contributor to the eventual plant’s success; in fact it’s the driver for the high-temperature steam cycle.”

Ratliff is confident about the technological success of Siemens’ CCGT system, but has some concern about economic issues, such as cost of fuel and high-temperature materials used in the plant. In fact, fuel cost makes up the biggest expense to run a gas-fired power plant.

And that’s the main reason for developing CCGT power plants of the highest efficiency.

Frank J. Bartos, P.E ., Control Engineering consulting editor
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