Efficiency versus flexibility: Advances in gas turbine technology

An update on the latest advances in gas turbine technology including the new breed of high-efficiency gas turbines, such as the Siemens Energy’s SGT5-8000H turbine, Mitsubishi Heavy Industries’ J-series machine and GE Energy’s H System, as well as the more modest but equally important aeroderivative industrial gas turbines.

Paul Breeze, UK

Over the past 20 years gas turbines configured as combined-cycle power generation units have become one of the mainstays of the power generation industry, particularly in the liberalized electricity markets. This popularity has been driven by a number of factors including the relatively low cost of natural gas during at least part of this period, the low cost of gas turbines and the speed with which gas turbine-based power plants can be installed.

Another key attraction of the combined-cycle power plant is energy conversion efficiency. A good modern plant can approach 60 per cent fuel to electricity conversion efficiency while the best coal fired plants will only achieve around 47 per cent efficiency.

During the last decade carbon dioxide emissions have also become a factor in the equation; gas turbines burning natural gas emit less carbon dioxide into the atmosphere per unit of electricity than coal fired plants, and so this can assist governments and utilities to meet more stringent emission requirements.

During this same period the power generation market has seen significant consolidation and this has affected the manufacturers of gas turbines. Major names such as Westinghouse and ABB have disappeared from the large gas turbine marketplace and today the only companies manufacturing gas turbines in the 200″400 MW range are GE, Alstom, Siemens, Ansaldo Energia and Mitsubishi Heavy Industries (MHI).

Meanwhile, aeroderivative manufacturers such as Rolls-Royce, Pratt & Whitney, as well as GE, concentrate on small and medium-sized turbines (2-60 MW) alongside companies such as Kawasaki Heavy Industries and Solar Turbines. 

Efficiency versus flexibility 

Gas turbine design has advanced enormously in the last 20 years and progress continues. In the past higher efficiency was the primary aim of gas turbine development. Around 1990, typical combined-cycle efficiency was 50 per cent. By 2010 the best installations were above 59 per cent.

Efficiency remains important to all manufacturers, but there has been a significant shift towards products that can operate more flexibility in load following and grid support roles. Whereas before combined-cycle plants were seen as baseload plants, now their role is equally likely to be for intermediate service and to provide ancillary services.

A significant part of this shift is due to the increasing importance of variable output renewable capacity such as wind and solar power to grids across the world. As the proportion grows, and turbine manufacturers are expecting these technologies to contribute 20 per cent to 30 per cent of grid capacity in Europe by 2020, so the combined-cycle plant, along with energy storage and hydropower, will be required to provide the main back-up for this intermittent capacity.

Efficiency and flexibility are not complementary. In order to obtain the flexibility that is required for this type of service, some efficiency must be sacrificed. The main manufacturers are responding to this by offering complementary heavy gas turbine lines, one for ultimate efficiency and one that offers greater flexibility. Even so, high efficiency remains the equivalent of the space race within the gas turbine world, though one where gains are becoming increasingly difficult to find. 

The quest for high efficiency 

For the past decade an important target for all the large gas turbine manufacturers has been to achieve 60 per cent efficiency in a combined-cycle plant under standard (ISO) conditions. Although several have now come close, that goal has yet to be achieved.

Click to Enlarge
Siemens expects its H-class gas turbine to achieve 60 per cent efficiency in combined-cycle mode

Efficiency is important because it affects both operating costs and carbon emissions, both of which are important concerns for plant operators today. Achieving high efficiency requires optimization across the whole plant, including gas turbine, heat recovery steam generator (HRSG) and steam turbine. Among these three, however, gas turbine performance appears to be the key to pushing efficiency to 60 per cent and beyond.

A gas turbine comprises three main components, a compressor, a combustion chamber and an expansion turbine integrated into a single unit. Thermodynamic efficiency of this unit depends on both the pressure and temperature drop of the working gas, air, from the turbine stage inlet to outlet. Increasing one or both can be used to increase efficiency. For this, all three components play their part.

The optimization of compressor design can be used to provide a higher pressure at the turbine inlet and this is being exploited in high efficiency designs. Thus GE has increased the gas turbine pressure ratio in its H system turbine to 23:1, aiming for 60 per cent efficiency. The same pressure ratio is used by MHI in its J-series, but the higher turbine inlet temperature results in efficiency in excess of 61 per cent. Siemens SGT5-8000H has a pressure ratio of 18.2, slightly higher than its SGT5-4000F. Meanwhile, Alstom’s turbines have operated since the mid 1990s with a pressure ratio of about 30:1.

The combustion chamber controls the temperature of the gas entering the turbine so changes here can affect overall efficiency. While raising flame temperature is relatively straightforward compared to other changes, combustor designs have tended to stabilize and all the main companies now have proven designs which they utilize across their turbine ranges.

This leaves the turbine as the component where the greatest gains in efficiency are likely to be made, though gains here are becoming hard to win. Modern computer design has led to the optimization of blade profiles to gain maximum efficiency so there is little that can be done there. Thus designers are forced to exploit the one area left to them, turbine gas inlet temperature. Since thermodynamic efficiency depends on the overall temperature drop across the turbine, raising the inlet temperature should equate to an increase in efficiency.

The combustion gas temperature for GE’s H-class gas turbine, one of the 60 per cent contenders, puts it in the 1500 à‚°C class, roughly 100 à‚°C higher than the company’s F-class machines, which constitute the main large frame GE turbines in use today. Inlet temperature of the Siemens’ new H-class machine, which is also hoping to achieve above 60 per cent efficiency, has not been released.

However, both are likely to be topped by MHI. The latter’s J-series turbine boasts an inlet temperature of 1600 à‚°C, 100 à‚°C above that of the same company’s H-series machines and higher than any other product to date. This machine, which is being tested in Japan today, is being marketed as capable of 61 per cent efficiency.

At these temperatures, the materials used to manufacture gas turbine components are being pushed to their limits. Blades, vanes and other hot gas path components are commonly made from nickel-based alloys cast in single crystal form to increase strength and resistance to fracture and deformation. Although not by MHI for their large machines. Single crystal blades do exhibit improved properties and MHI has used them successfully in their small units, but the single crystal form is expensive and has associated repair challenges so, says MHI, do not justify their use in large frames.

These alloys will start to melt at anywhere between 1200 à‚°C and 1400 à‚°C. In order to prevent the consequent damage at the temperatures experienced within the gas turbine inlet, these components must be coated with thermal barrier coatings (TBCs) and then cooled internally so that the actual metal does not approach its melting point. Current TBCs are based on ceramics that are typically made from zirconia doped with various other elements to create a heat resistant layer with very low thermal conductivity that can maintain a 300 à‚°C temperature drop to the metal beneath.

The TBC slows the rate at which heat is transferred to the metal beneath. However in order to prevent the temperature rising too high, the component must also be cooled internally, either with air or steam. Air cooling is the most common type in use. This requires moderately hot air which is extracted from the gas turbine and cycled to cool the hot path components.

However, since this air is ‘stolen’ from the gas turbine it has the effect of lowering overall efficiency. Steam cooling, in which steam from the steam cycle of the combined-cycle plant is used to cool the components internally does not have the same efficiency cost. This is the solution adopted by MHI, and by GE for their high-efficiency G-class and H-class machines. However Siemens are using air cooling for their latest H-class turbine and Alstom’s turbines are also air cooled.

Temperatures may be able to go higher still. MHI is taking part in a Japanese government project which has the aim of increasing the turbine inlet temperature to 1700 à‚°C. The J-series turbine has already exploited some of the developments from this programme but there are expected to be others yet to be used.

Overall efficiency of these large industrial gas turbines in simple cycle mode is between 38 per cent and 42 per cent. Higher efficiency can be achieved. The best available today appears to be GE’s LMS100 with a simple cycle efficiency of 46 per cent. However, this is at the expense of a low gas outlet temperature which makes it less suitable for combined-cycle applications. 

Steam cycle improvements 

In addition to improvements in gas turbine efficiency there are gains to be made within the steam bottoming cycle of a combined-cycle plant. The highest efficiency combined-cycle plants involve close integration of gas and steam cycles and optimum efficiency is achieved by balancing the two cycles effectively.

As with the gas turbine, steam turbine efficiency depends on pressure and temperature of the working fluid, in this case steam. Raising the overall flue gas input temperature to the HRSG can be used to increase both. This requires a higher gas turbine outlet temperature and so must be balanced against any fall in gas turbine efficiency.

Click to Enlarge
Mitsubishi’s J-series is designed for combined-cycle efficiency of about 61 per cent LHV Source: MHI

A higher overall gas turbine operating temperature will help raise efficiency in both components but as seen above techno-logical limits appear to be approaching here. The HRSG design will also play a part in overall efficiency, while the type of HRSG may also affect another critical operational criteria for modern combined-cycle plants, flexibility. 

Flexible plant and divergent product lines 

Power plant flexibility involves a number of important parameters. A flexible plant must be able to start-up quickly, so time from start-up to full load will be important. It must be able to ramp both up and down to meet different load demand levels with ease and speed. Efficiency must be maintained, not only at full load but at part load down to 50 per cent or lower. Emissions must also be kept low when operating at part load.

In addition, companies are looking at ways of ‘parking’ a combined-cycle plant overnight so that it ticks over at low load. This avoids the necessity of shutting the plant down, which has both speed and maintenance penalties, or continuous operation at say 50 per cent load, which will often not be the most economical option. One of the technological advances which may need to be sacrificed for flexibility is steam cooling of the inlet stage gas turbine components. Maintaining the cooling steam is vital during ramps but requires slower ramp speeds due to the complexity and overall integration of gas and steam cycles.

However, abandoning steam cooling is likely to result in a lower gas turbine inlet temperature and hence lower overall efficiency. This is part of the reason why some companies offer more than one product line, one using steam cooling for high efficiency and another with air cooling for greatest operational flexibility.

GE has six of its high-efficiency H-class combined-cycle machines operational, one at Baglan Bay in Wales, three in Japan at a Tokyo Electric Power plant and two in California. According to GE, these are all operating at baseload and though none has achieved the magic 60 per cent efficiency, GE believes it has shown the machine to be capable of reaching this efficiency.

At the same time the company has recently launched a new version of is 60Hz 7FA turbine which features 57.5 per cent efficiency in combined-cycle mode but fast start-up and ramp rates and a better turn-down capability.

One of the keys that GE has identified for fast combined-cycle start-up is to decouple the gas turbine from the steam turbine bottoming cycle. This enables the gas turbine to start quickly without the need to hold output while the steam cycle components heat up. It has also targeted emissions-compliant ramping as an important attribute and expects the gas turbine NOx emissions to be below 9 ppm over a wide load range.

GE claims the new 7FA will be capable of 75 per cent output in ten minutes during hot start from an overnight shutdown and a ramp rate of 30 MW per minute. Overall gas turbine output is 211 MW. GE also has a 7FB gas turbine, launched several years ago to take advantage of some H-class technologies in an F-class machine.

This is now being positioned as an IGCC turbine and has been rebranded as the 7F-Syngas. Two of these are to be installed at a 618 MW IGCC facility at the Edwardsport power plant in Indiana owned by Duke Energy.

Although efficiency is a primary development driver, all-round operational flexibility is a key feature of Alstom’s product line. It markets its gas turbines as part of integrated combined-cycle packages and this rather than the simple cycle gas turbine is its primary product.

The company’s KA26 combined-cycle power plant for the 50 Hz market is marketed today with an efficiency capability of over 59 per cent, while using air-cooling of its turbine blades. Several KA26 units in Europe, including the KA26 plant at Emsland in Germany, have achieved above 59 per cent efficiency and the company believes 60 per cent is potentially possible.

According to product manager Mark Stevens the KA26 technology utilizing the GT26 gas turbine can still maintain an efficiency of about 53 per cent per cent at around 50 per cent plant load.

In addition, the gas turbine will typically maintain NOx emissions at below 15 ppm when operating at this load point and even below due to sequential combustion.

In addition, the KA26 is capable of being parked at a very low load, typically in the range of 20 per cent plant load with close to baseload emissions, as an alternative to shutting down completely and with lower fuel consumption than running at higher ‘minimum loads’. From this very low load point, the KA26 is able to ramp back up to full load within 15-20 minutes, Stevens said.

As with GE, the company hopes the KA26 and KA24 products will appeal to utilities and power plant operators who expect to operate in both the power delivery and ancillary services markets.

Meanwhile, MHI has pinned its colours squarely on high efficiency. Its high efficiency J-series turbine builds on experience with the company’s G-class machines which use steam cooling, though only in stationary parts such as combustion liners and blade rings. In the J-series, this is promised to lead to 61 per cent efficiency.

Toshishige Ai of MHI says the quest for high efficiency has particular value in Japan; the country has to import all its fossil fuel so efficiency is recognized as extremely important from both a cost and an emissions perspective. Nevertheless, MHI has recently acknowledged that flexibility is also important and has developed a fully air-cooled version of the G- series turbine. As for the J-series, six units have already been sold, with the first starting up in 2013.

Siemens has developed the SGT5-8000H (50 Hz), an air-cooled gas turbine with a generating capacity of 375 MW, which is expected to exceed 60 per cent efficiency in combined-cycle configuration. Single shaft combined-cycle output is 570 MW.

There is also an SGT6-8000H, a 60 Hz version rated at 274 MW and six of these have been ordered by Florida Light and Power with the first start-up in 2013.

Meanwhile, Siemens is also paying attention to flexibility with its own fast-cycling concept, development of which began in 2002. This has led to the introduction of features such as a once-through HRSG to eliminate the high-pressure drum, which in a more conventional HRSG takes a considerable time to reach the desired temperature. Adding a stack damper helps reduce the rate of heat loss during shutdown, allowing faster hot start.

These techniques are being applied to combined-cycle plants based on the company’s current F-class turbines. According to Siemens, two plants, one at Pont sur Sambre, France and the other at Irsching, Germany have demonstrated the capability of reaching full load after overnight shutdown in 30 minutes, with efficiencies of over 58 per cent and over 59 per cent respectively.

Siemens has been looking carefully at grid support requirements too, and has developed methods to maintain power output in the event of a drop in frequency as well as remaining in stable operation during islanding. Ansaldo Energia, whose turbines are based on Siemens ” the company was a licensee until 2004 ” is pursuing a similar path to other companies in regard to flexible operation.

New low NOx burners have recently been introduced, life-cycle analysis is used to improve efficiency, increase power output or increasing maintenance intervals and dual-fuel switchover operation has been improved. However, the company has no current plans to develop its own 60 per cent plus efficiency combined-cycle system yet. 

Aeroderivative developments 

While the largest heavy duty gas turbines and combined-cycle plants often attract the greatest attention, there remains a market for smaller units. An important player here is Rolls-Royce, which has recently introduced a new version of its RB211 industrial gas turbine called the RB211-H63.

Click to Enlarge

Rolls-Royce’s RB211-H63 will be ready to ship from the first quarter of 2012 Source: Rolls-Royce

This unit is rated at 44 MW with a wet low emissions (WLE) combustion system and 38 MW with a dry low emissions version. However, the company believes that uprating to 50 MW will be possible in the future. The 44 MW, WLE version of this turbine will have an open cycle efficiency of 41.5 per cent with a compression ratio of 25.1:1 and will be available in 2012 with the 38 MW version available for delivery in 2013.

As with large turbine manufacturers, Rolls-Royce is aiming for flexibility. In this case it claims the capability of reaching full power from start-up in ten minutes. Stated NOx level is 25 vppm. 

How high can efficiency go? 

The ambitious Japanese National Project for high efficiency gas turbine development is aiming to develop the technology to achieve a turbine with an inlet temperature of 1700 à‚°C and an efficiency of 62 per cent to 65 per cent in combined-cycle operation.

Both targets still remain out of reach in commercial machines, although MHI appears to be approaching both in its latest product. Going beyond these targets is likely to require new ceramic materials to replace the metal components in the inlet stages of gas turbines.

The emphasis for many companies is shifting towards flexibility and it is here that much development work is likely to concentrate in the coming years. So while efficiency will remain a key parameter, advances in this area over the next 20 years look likely to be slower than they have been over the previous 20 years. 

More Power Engineering International Issue Articles
Power Engineering International Archives
View Power Generation Articles on PennEnergy.com

No posts to display