Stay cool – GE’s H-class gas turbine

GE has introduced a new all-air-cooled version of its H-class gas-turbine technology. Called the HA (the A standing for air-cooled), the first machine off the production line, a 9HA (50Hz) version, went on its way from GE's gas-turbine factory in Belfort, France, to its $185-million Greenville, South Carolina, gas-turbine test bed. Here, fitted with additional instrumentation, the gas turbine will undergo about 14 weeks of exhaustive full-scale full-load testing. The test cell is not grid connected, allowing in-depth exploration of gas-turbine behaviour at full load and beyond - equivalent to more than 8,000 operating hours with a field prototype, with a wider operating envelope than would be encountered in the field.

The second HA machine off the production line at Belfort, also a 9HA, will be deployed at EDF's Bouchain FlexEfficiency 50 combined cycle plant in France, which is currently under construction - and is GE's lead FlexEfficiency 50 project - with commercial operation scheduled for 2016.

A number of other GE HA projects are in the pipeline, including an order for six 7HA (60Hz) gas turbines from Chubu Electric in Japan and an order for a 9HA from another Japanese utility. There is also a "tremendous amount of bid activity", says Vic Abate, president and CEO, power generation products, GE Power & Water.

Turn up the heat

In GE terminology, "H class" is essentially defined as a firing temperature in excess of 2,600°F (about 1,426°C) and up to about 2,900°F (1,600°C), with a pressure ratio of 21:1, in conjunction with a four-stage power turbine. By contrast, E class refers to firing temperatures of about 2,000-2,200°F and F class to 2,300-2,600°F.

GE commercially introduced its H-class technology ten years ago; the lead project being a 9H at Baglan Bay in the UK, followed by three more 9H units at Tepco's Futtsu plant in Japan and two 7H gas turbines at the Inland Empire Energy Center in the US.

But these GE H-class gas turbines, which have now amassed around 200,000 operating hours, says GE, employed steam cooling of turbine components, including, remarkably, rotating parts. The new HA has greatly increased generating capacity and can achieve combined cycle-net efficiency of around 61.5% in ISO conditions, employing air cooling only - a technology strategy adopted by Siemens some years ago for its H-class gas turbine and an option offered by Mitsubishi for its G and J series machines.

The ability to achieve such high efficiencies with just air cooling derives from advances in technology on a number of fronts, including blade aerodynamics with concepts imported from aero-engine practice; better understanding of heat transfer; improved design of hot-gas-path components to reduce temperature and stress gradients; more effective deployment of air by using more intricate cooling flows (for example, employing several thousand tear-drop shaped holes where 500 circular holes might have been used in the past); improved thermal barrier coatings; and improvements derived from the wealth of operating experience amassed with the existing all-air-cooled gas turbine fleet.

H division

While the current HA gas turbine does not employ any components made with additive manufacturing (aka 3D printing), the technique has been used to speed up the HA prototyping process and thereby accelerate introduction of the new machine. In the future, however, additive manufacturing is likely to be used to 'print' ever more intricate parts; in particular, to achieve further improvements in internal-blade-cooling paths and cooling-air management. This, combined with other advances such as the use of ceramics in aerodynamic 3D blading, higher firing temperatures (with the possibility of 3,000°F (around 1,700°C) now under consideration), better sealing and further-improved heat transfer, should enable 63% combined cycle efficiency to be achieved by the end of the decade.

And looking beyond that, Vic Abate, VP renewable energy at GE, believes that 65% combined cycle efficiency may be achievable with air cooling alone. Nevertheless, he doesn't think it is entirely the end of the road for steam cooling.

"I wouldn't say steam is done for ever... the physics of steam makes it still justified," he says.

And GE has built up a considerable body of knowledge in its application to gas-turbine cooling, which could be revisited at some point in the future. However, steam cooling adds cost and complexity, and requires increased maintenance resources; it also reduces operational flexibility (not as important ten years ago when large combined cycle units were seen as predominantly baseload machines), so "the next chapter will be air", says Abate.

First rate

The 50Hz and 60Hz machines come in two versions, .01 and .02. The .02 version in each case has a higher generating capacity thanks to a bigger compressor. The 50Hz gas turbines have 16 can annular combustion chambers, while the 60Hz machines have 12. The highest-rated machine of the new platform, the 9HA.02, at over 470MW in simple cycle, leads the industry in output in terms of unit size, GE believes, and is certainly larger than the Siemens all-air-cooled H machine - the 50Hz version of which is rated at about 375MW in simple cycle (although another contender for the world's largest gas turbine must still be MHI's M701J, also rated at about 470MW).

Vic Abate says he "feels very good" about GE's ability with the HA portfolio of power plants to be in a position to offer "blocks of power in sizes appropriate to market requirements", notably in the context of coal and nuclear retirements. The high efficiencies have also "caught the eye of customers in high-fuel-cost markets", he says, while, in addition, the machines have the operational flexibility, fast start capability, ramping performance and turn down characteristics that power plant operators are increasingly insisting upon.