Siemens HL: the bridge to 65%+ efficiency

About two years ago Siemens assembled a team to look in depth at likely future trends in the power market and to try and determine what the next steps should be for its large frame gas turbine technology. This was against a backdrop of rapidly declining renewables generation costs (as evidenced in recent auctions, for example) and substantial increases in solar and wind capacity annual additions (about 130 GW in 2016, projected to be around 130-250 GW/a in the coming years, compared with some 70 GW in 2011). There was also a shrinking market for gas turbines in the 10-MW-and-above size range (over 75 GW/850 units in 2011, down to about 50 GW/500 units in 2016) – although, interestingly, high-efficiency “jumbo frames” were accounting for an increasing share of the market – 35% in 2016.

The group was deliberately diverse, representing, for example, service, manufacturing, sales, marketing and strategy, as well as engineering.

Among the conclusions of its year long deliberations was that there would indeed a be a continuing role for large gas turbines, but customer requirements were changing, driven by: a growing share of fluctuating power from renewables; high LNG/gas prices in key markets (such as Asia); increasing competitive pressure from renewables with low LCOE (levelised cost of electricity).

The new gas turbine technology, would therefore need to deliver: fast load response capabilities and high ramp rates to stabilise the grid; higher efficiency; fast installation; low specific CAPEX; high power density.

The new HL class recently launched by Siemens – the “HL” intended to signify that it is seen as a “technology carrier” between the current H and a future L class capable of 65% combined cycle efficiency – aims to address these market requirements.

The HL is designed to achieve a net combined cycle net efficiency of over 63% (ISO conditions, natural gas fuel) and a GT ramp-up rate (for the 50Hz version) of about 85 MW/min (compared with corresponding figures for the H of about 61% and 55 MW/min (50 Hz version), 45 MW/min 60 Hz version). However, rather than a radical rethink, the emphasis has been on evolutionary development based on the H class, retaining (but building on) familiar and proven engine architecture such as: air-cooled four-stage power turbine (which has been the basis of Siemens gas turbines for over 50 years); Hydraulic Clearance Optimisation (providing higher efficiency at full load while facilitating immediate restart); steel rotor design employing Hirth serrations and central single tie rod to lock discs (which is service friendly, allowing on-site rotor de-stacking); and can annular combustion system. 

Among principle differences from the H:

• Increased turbine inlet temperature (about 100°C higher than that of the H), the main driver of increased efficiency.
• Modifications to the combustion system, in particular to help minimise the increase in NOx emissions that necessarily arises from the increased turbine inlet temperature. The new combustion system, called ACE (Advanced Combustion system for high Efficiency), has been extensively trialled at the Siemens burner high pressure test facility, the 100 million euro Clean Energy Center at Ludwigsfelde near Berlin. The modifications include improved fuel mixing by increasing number of premix burners per can from 8 to 25 and reduced residence time by shortening the burner transition pieces. The NOx emissions are 45 ppm (compared with 25 ppm for the H combustor), but with the addition of SCR this is reduced to 2 ppm. The combustion system modifications also contribute to enhanced fuel flexibility, part load capabilities and extended turn-down range (down to 30% of full power).
• Reduction in compressor stages from 13 to 12, while at the same time increasing pressure ratio, from 20:1 to 24:1 This is thanks largely to 3D optimisation of compressor airfoils (“3rd generation” rather than the “2nd generation” design used in the H). The number of variable guide vanes has also been reduced from three to two, further reducing complexity (with both the HL and H having one inlet guide vane).
• Improvements to turbine blade internal cooling features, to accommodate higher firing temperatures. “Super-efficient” internal cooling schemes have been employed, to the extent that directionally solidified alloys rather single crystal materials can be used, which improves maintainability. Blade life is improved, while reduced cooling air consumption contributes to increased efficiency.
• Large internally cooled free standing blades are used in stage 4 of the power turbine (as opposed to the uncooled shrouded stage 4 blades used in the H). This contributes to higher power output and allows exhaust temperature to be increased.
• Improved turbine blade thermal barrier coating to reduce spallation. This includes laser engraving of the barrier coating to reduce thermal stresses and introduction of a sacrificial layer on the surface of the TBC that is designed to remove metal dust residues left behind from manufacturing and arising from abradable metallic seals during commissioning. The TBC is also extended to seven airfoils in the HL, up from six in the H.
• Provision of a rotor air cooler, which allows use of the proven steel disc design. 

HL performance data is summarised in the tables. Two basic ratings are envisaged, the SGT-9000HL and the SGT-8000HL. The 9000HL will be available in 50 Hz and 60 Hz versions, with simple cycle installed capacity of 545 MWe and 374 MWe, respectively, the 8000HL is 50 Hz only, with a capacity of 453 MWe in simple cycle. Despite the higher power, physical dimensions of the HL are similar to those of the H, reflecting increased power density. Sixteen burners are employed for 50 Hz, twelve for the 60 Hz machine.

It is notable that the exhaust temperature of the HL is significantly higher than that for the H, 680°C vs about 630°C, which in turn means that in combined cycle mode higher steam temperatures are available in the HRSG, from about 580°C in the case of the H to above 600°C (perhaps 610°C) for the HL, contributing significantly to the high combined cycle efficiency.

Among business benefits identified by Siemens are the greatly increased operational flexibility and increased inspection intervals, 33 000 equivalent base hours (compared with 16 000 EBH for the H class) and 1250 equivalent starts (compared with 900 ES for the H).

Siemens is estimating an overall reduction in levelised cost of electricity of around 5% compared with the H in “high gas price markets”.

The new engines are designed to “plug into” MindSphere, Siemens’ cloud-based operating system for the Internet of Things, providing analytics to support operating engines.

Great efforts are also being made to ensure that the improved technologies to be incorporated in the HL (eg improved burners, turbine blades and barrier coatings) are downloadable into the existing Siemens operating gas turbine fleet, to help operators benefit from efficiency and performance improvements and improve their competitiveness.

Not suprisingly, with the industry’s somewhat chequered past record when it comes introducing new large gas turbines to the market, considerable attention is being paid to testing and validation of the HL.

This includes the burner testing at the Clean Energy Center (as already mentioned), prototype testing at Siemens’ Berlin test facility, and validation of an HL gas turbine in simple cycle mode under real power plant operating conditions at Duke Energy’s Lincoln County peaker site in North Carolina, USA.

The Lincoln unit is due to start up in early 2020, while the first commercial HL units are anticipated to enter operation by the end of 2020.