Siemens’ ‘H’ class GT is being advertised as setting new benchmarks in terms of performance and operating economy. With a construction and development cost of r500 million, it will also be the world's largest and most powerful gas turbine.

The go ahead for Siemens’ 340 MWe H type gas turbine, to be jointly developed by Siemens and the utility E.On, has been announced. This time the gas turbine system, designated the SGT5-8000H (SCC5-8000H in the CC version), will not be developed on the test bed independently of its combined cycle boiler system but on an E.On Energie power plant site at Irsching near Ingolstadt in Bavaria.

The prototype, a development of the F class V94.3A (aka SGT5-4000F), will be rated initially at 340 MWe for the type tests. Much of the enhanced combined cycle performance of over 530 MWe and better than 60 % thermal efficiency will no doubt derive from the advanced vertical tube Benson HRSG originally demonstrated at the Cottam combined cycle demonstration site in England. That we know the expected GT output can be contrasted with the case of GE’s ‘H’ machine, whose GT output has never been revealed on the grounds that (because of the steam cooling requirement) it would never be operated without a steam section. GE would only say that the nominal CC output was 480 MWe, with 520 and 530 MWe outputs at 6.7 °C being achievable. This 520 MWe figure has now re-emerged as GE’s new rating for the ‘H’.

The new turbine is said by Siemens to be ‘based on the best of the Siemens [V943A] and Westinghouse [W501 F and G class] designs’, although since Siemens insist that it is to be purely air cooled, and since a major innovation of the Westinghouse 501 G class turbines lay in their partly steam cooled design (in the transition piece), it is not easy to suggest what the Westinghouse contribution might be.

E.On Kraftwerke GmbH, a subsidiary of E.On Energie, will take over the plant, and put it into commercial use once the unit has been validated in successful trials. The additional steam cycle is to be fitted when E.On Energie take over and put it into commercial operation. Provisional schedules suggest 2007 for the prototype and 2011 for first commercial operation.

H class efficiency

Measures expected to lead to the targeted efficiency increase are said to include an advanced sealing system for low leakage cooling air, possibly a reference to HCO (see below), the use of (unspecified) advanced materials to allow an increase in the firing and exhaust gas temperatures, and a new compressor with newly designed blading. This last is likely to be the legacy of upgrade work on the Mainz-Weisbaden V94.3A machine over several years which resulted in slightly re-designed compressor blades and vanes on stages 1 and 2, and on inlet guide vanes, for increased mass-flow. The same process also produced a modification of row 4 turbine blades, re-staggered to reduce aerodynamic losses at increased mass flow.

The main increase is likely to come from increased mass and energy flow inherent in the high pressure, high temperature combined cycle process, incorporating a new Benson boiler designed for the higher mass flow and exhaust gas temperature of the new engine.

The 8000H is the first new frame developed by Siemens and Westinghouse after the merger, and is said to ‘combine the best features of the existing product lines and technology advancements.’ But it will not include steam cooling of blades, nor steam cooling of the combustor transition piece. This latter was a technology developed by Westinghouse under DoE funding, but now owned by Siemens and allegedly being pursued by Mitsubishi under its licensing of the W501.

Siemens will have their own reasons for staying with air cooled blades but the advantage cited is that it allows faster start up and cycling, a barely veiled reference to the more complex steam cooling design developed by GE.

Other features

Siemens claims that what it calls its simpler operational concept with less complexity in engine and parts will lead to lower O & M costs, improved start up and turn down capability and more flexibility in on-line scheduling.

Reductions in CO2 emissions of some 40 000 t/a compared to today’s technology are also claimed, which will count for substantial emissions trading benefits. State of the art instrumentation to be used on the test plant will include Siemens’ new on-line thermal barrier coating monitoring system.


Siemens first demonstrated its hydraulic clearance optimisation (HCO) system to control turbine blade-tip clearances, resulting in increased power and efficiency, in 2001. After a successful run in the test bed engine in Berlin the first commmercial application was installed at Kraftwerke Mainz-Wiesbaden on a V94.3A of 2001 vintage.

However good the thermal balance of the engine, the blade clearances are bigger than they need to be once the engine is fully heated because they are designed for hot restart conditions which is the most critical operation mode. Furthermore, clearances need to be large enough to account for casing ovalisation during heat up.

The HCO uses this potential by shifting the rotor along in the compressor direction after the engine is fully heated, which, since the casing is cone shaped, reduces the radial clearances by about 1 mm. The resulting efficiency gain in the turbine of about 0.35 % points is far larger than the loss in the compressor section, owing to slightly increased clearances, of about 0.15%points.

During validation in 2003 GT power was increased by about 1.5 MW at constant fuel consumption, translating as a GT efficiency increase of about 0.3 % points. As the additional power is actually taken out of the exhaust energy, the impact on the combined cycle performance is slightly lower but still close to 1 MW more power and between 0.1 and 0.2 % points of higher efficiency depending on the boiler characteristics.