Gas turbines have come a long way since the first design to produce a positive power output was constructed in 1903. Today, models with simple cycle capacities greater than 410 MW are available with combined cycle efficiencies greater than 64%. Furthermore, there are good reasons to be optimistic about even more advanced technology in the future.

Kennedy Maize and Aaron Larson

The Greek power system provides an interesting energy transition case study. Lignite, the lowest-ranked coal in terms of heat content, was for many years the leading source of power generation in Greece. In fact, the International Energy Agency (IEA) reports that nearly 72% of Greece’s electric power was generated from lignite in 1990.

Like many countries around the world, the Greek government has acted in response to climate change to reduce carbon dioxide emissions, and lignite is a prodigious producer of these. Therefore, Greece aims to eliminate most lignite-fired power plants by 2023, and all units by 2028.

Greece has made great strides in reducing its lignite usage. The IEA says only about 13% of Greece’s electric power generation came from lignite in 2020, while natural gas, which accounted for less than 0.3% of generation in 1990, supplied almost 39% of Greece’s power in 2020.


Adding a State-of-the-Art Gas Turbine

In an industrial area in northeastern Greece, a power plant is under construction that exemplifies just how and why gas turbine technology is supplanting coal, both economically and to the benefit of the environment. It is the Komotini plant, a gas-fired combined cycle plant, producing power from a Siemens Energy SGT5-9000HL gas turbine, coupled with a heat recovery steam generator (HRSG), to produce 877 MW of electric power. It’s notable that the Siemens Energy gas turbine has an efficiency of up to 64%, compared to lignite plant efficiencies that often have percentages in the low 30s.

1. Keadby 2’s SGT5-9000HL gas turbine is shown here being delivered to the combined cycle project’s site in the UK. Courtesy: Siemens Energy

Construction on the Komotini project started in 2021, and the station is scheduled to enter commercial operation in 2024. The heart of the project—the SGT5-9000HL gas turbine—is an advanced air-cooled, heavy-duty machine launched by Siemens Energy in May 2020. The first 50-Hz SGT5 9000HL gas turbine was delivered by Siemens Energy to the Keadby 2 combined cycle power project in the UK (Figure 1).

Replacing high-emission coal with cleaner gas-fired generation is a logical step in the energy transition, while not forestalling the growing share of renewable energy. The Komotini plant is expected to reduce annual CO2 emissions by up to 3.7 million tons compared to a conventional Greek lignite plant.

Other benefits of relying on gas power through the energy transition revolve around its load balancing and dispatch capabilities, something lacking in most renewables and nuclear power. Furthermore, many gas turbine manufacturers, including Siemens Energy, are developing upgrades to designs that will allow using e-fuels in the future, such as 100% hydrogen (H2 ), which means generating companies can avoid burdening customers with stranded assets.

2. Siemens Energy’s HL-class consists of two engines: The SGT5-9000HL for 50 Hz, and the SGT6-9000HL for 60 Hz. A modular design approach allows for a substantial number of common parts. The HL-class is designed for high operational flexibility. Courtesy: Siemens Energy

Derived from proven Siemens Energy H-class technology (see sidebar), the SGT-9000HL (Figure 2)—available in both 50-Hz and 60-Hz models—is the most powerful gas turbine design available from Siemens Energy’s extensive gas turbine portfolio. In fact, Duke Energy announced on Aug. 23, 2022, that its Lincoln Combustion Turbine Station in North Carolina, powered by the first 60-Hz SGT6-9000HL, which Siemens Energy delivered to the site in November 2019, has been certified with the official Guinness World Records title for the “most powerful simple-cycle gas power plant” with an output of 410.9 MW.

“The key to helping our customers in the energy transition is not just by providing high power output but doing it with the highest efficiency and flexibility possible. Our HL turbine, with the benefit of advanced combustion and cooling technologies, leveraging additive manufacturing, advanced coatings, and more, makes a difference for customers and for the environment. And the HL turbine can ramp up quickly, to provide power to the grid when renewables aren’t available. It’s an exciting frontier to be part of!” said Hans Thermann, head of global HL-Class Portfolio Management at Siemens Energy.


The Roots of Gas Turbine Technology

In 1791, English inventor John Barber (1734–1793) took out a British patent (Obtaining and Applying Motive Power, & c. A Method of Rising Inflammable Air for the Purposes of Procuring Motion, and Facilitating Metallurgical Operations) that described all of the essential elements of a gas turbine engine. While Barber’s basic design was sound, it was technologically impractical at the time.

It took more than a century for Barber’s concepts to be put to practical use in the real world. In 1903, Jens William Aegidius Elling, a Norwegian engineer, researcher, and inventor, built the first gas turbine to produce more power than was needed to run its own components. Elling’s first design used both rotary compressors and turbines to produce about 8 kW. He further refined the machine over subsequent years, and by 1912 he had developed a gas turbine system with separate turbine unit and compressor in series, a combination that is still common.

Gas turbine technology continued to be enhanced by a number of innovative pioneers from various countries. In 1930, Britain’s Sir Frank Whittle patented a design for a jet engine. According to a Massachusetts Institute of Technology history of gas turbines, Gyorgy Jendrassik demonstrated Whittle’s design in Budapest, Hungary, in 1937. In 1939, a German Heinkel HE 178 aircraft, with an engine designed by Hans von Ohain that used the exhaust from a gas turbine for propulsion, flew successfully. The same year, Brown Boveri Co. installed the first gas turbine used for electric power generation in Neuchatel, Switzerland. Both Whittle’s and von Ohain’s first jet engines were based on centrifugal compressors.

Siemens and Westinghouse industrial gas turbines were both based on axial compressors. The Siemens design stems from the Junkers JUMO 004 gas turbine, which entered production in 1942. It had an eight-stage axial compressor, a single-stage turbine, and six combustor cans. The Westinghouse WE19 gas turbine, meanwhile, entered production in 1943. It had a six-stage axial compressor, single-stage turbine, and eight three-can combustors.

Siemens began developing its first industrial gas turbines in 1948. Some features of those early turbines are still seen today. Among the long-lasting design elements are the center tie rod, stacked discs, and mechanical torque transmission keys or teeth. In 1956, Siemens released the VM 2 gas turbine, which featured a water and steam-cooled rotor. Additionally, the original VM 2 design had ceramic vanes, which were air cooled.

Through the years, significant improvements were made to materials, cooling, design technology, and validation testing. The advances have resulted in increases in efficiency and reliability.

In 1961, Siemens released the first single-case gas turbine—the VM 51, which was a 13-MW unit. The design concept, which is still in use today, included a single casing, two bearings, a single tie bolt, and Hirth gear couplings. The following year, Siemens released the VM 80, a 23-MW two-case engine that had a large external combustor with ceramic heat shields. It had an efficiency of 32%, which was notable at the time.

To address the distributed market size, Siemens Energy has spent the past 20 years investing heavily in a much broader range of gas turbine applications (Figure 3). The company’s gas turbine portfolio also includes many options below 100 MW, which constitutes small and medium gas turbines, as well as aeroderivatives.

“Siemens Energy’s wide gas turbine portfolio makes a perfect fit for the variety of applications where gas turbines are used,” said Ulf Radeklint, head of Gas Turbine & Future Innovation for the company’s Industrial Gas Turbines business. These applications include simple cycle and combined cycle electricity generation, combined heat and power including process steam generation, and direct mechanical drives for a variety of purposes including compressors. Some of these gas turbines have multiple shafts for a more versatile power-to-speed range.

3. Siemens Energy began producing gas turbines at the company’s Huttenstraße site in Berlin, Germany, in 1972. The ETSA 1 turbine, shown here, was the first power plant turbine delivered from the facility. Courtesy: Siemens Historical Institute

One of the major technological advances of the past 20 years has been the introduction of additive manufacturing (AM), which has been used in serial production of components, such as for SGT-800 burners. One of the main benefits of AM is that more advanced geometries can be used for improved fuel flexibility, as well as for the reduction of welding operations, reducing both lead time and cost.

Today, Siemens Energy offers cutting-edge technology in all of its gas turbines. Important features of these gas turbines include such things as air cooling for maximum efficiency; fast-start capability, which allows more flexible operation; variable stators on compressors for optimal part-load operation and lower starting stresses; and aero foils that are removable for maintenance with the rotor in place, among others. Furthermore, the most efficient Siemens Energy gas turbine today can reach efficiency as high as 64% in combined cycle mode—a far cry from units built in the 1940s that struggled to achieve 17% when operating in simple cycle mode.


The Rise of Hybrid Power

While some people advocate for a total shift from fossil fuels to renewable energy, particularly wind and solar power, that option has practical problems. Wind and solar fluctuate, often unpredictably. As a result, they can contribute to grid instability, as has been seen with solar in California, and more recently with wind in Texas during this summer’s heat wave. In the Lone Star State, a heat blanket stifled wind, which was unable to generate more than 10% of its nameplate capacity for an extended period, bringing the Texas grid to near collapse.

That’s where a more balanced approach, including gas-fired power, comes in. The transition to much greater use of renewables requires new building blocks. Among these building blocks: energy storage, conventional power generation, carbon capture, grid stabilization, and smart control systems. While some of these elements deliver the needed energy services independently, when combined, they create a “hybrid power plant.” Siemens Energy defines a hybrid plant as combining at least two elements of renewables, energy storage, and thermal power generation.

“The market for gas turbines is transitioning toward decarbonized fuels (stored energy) and from primary energy supply to primary capacity supply (fewer hours). Baseload engines are shifting operation mode to intermediate peak, peaking, and backup power plants. Gas turbines being the base for one of the cheapest ways to add capacity on the market. Another important aspect is providing grid stability when there is a high penetration of renewables,” said Ulf Radeklint, head of Gas Turbine & Future Innovation for Industrial Gas Turbines with Siemens Energy.

“Batteries are well-suited for providing power during short time periods, but they are a very expensive alternative if they have to provide power for many hours (or even days or weeks). Gas turbines operated on hydrogen and/or biofuel can be a very competitive alternative when backup power is needed for longer time periods,” said Christian Andersson, a gas turbine expert with Siemens Energy.

“A study by Chalmers University of Technology in Sweden has shown that gas turbines can be expected to be especially competitive in energy systems with a high share of wind power,” Andersson said. “The reason for this is that the load variations have relatively long durations and are irregular, in contrast to systems dominated by solar power, which have short and more regular variations, which is suitable for batteries.”

A central element of a hybrid plant is a gas turbine capable of firing fossil-free fuels, such as hydrogen or biofuels. Major gas turbine original equipment manufacturers, including Siemens Energy, are committed to providing new gas turbines with the capability of firing 100% hydrogen by 2030 or earlier. Many designs are already capable of co-firing between 30% and 75% hydrogen, and some smaller units are capable of firing on 100% hydrogen today. Since 2021, Munich-based international consulting and standards organization TÜV SÜD has offered a “H2 -readiness” certificate. Notably, Siemens Energy was the first company to obtain the certification.

In Leipzig, Germany, Siemens Energy is working with the city government to build on the site of a former lignite-fueled plant an example of what hybrid power could look like. The project will provide electricity and district heating, with two hydrogen-ready SGT-800 gas turbine engines installed. The turbines will initially run on 30% hydrogen and 70% natural gas, but eventually will be entirely fueled with hydrogen. The project will include an electrolyzer to produce H2 that can be stored and used for power generation and high-temperature heat.

The SGT-800 is a success story in its own right (Figure 4). When first introduced in 1998, the gas turbine carried a 43-MW nameplate capacity. The engine proved to be a highly reliable design. Yet, engineers continued to improve upon the design in evolutionary steps built on fleet experiences, and by 2019, the machine’s capacity had been increased to 62 MW. Furthermore, efficiency of the SGT-800 rivals that of much larger units, including some 250- MW gas turbines.

The SGT-800 has a strong track record of performance. Several developers have featured it in multiple-unit projects, and many owners value its flexibility to operate at part-load. The design has a 75% hydrogen capability at present, but is on a roadmap to 100% by 2025 or sooner. It can also be operated on biofuel. An example of this will be deployed by Stockholm Exergi in a backup plant to be fueled with 100% liquid biofuel by 2025.


Deep Decarbonization

Green fuels and carbon capture are complementary, and both are necessary to accelerate the path toward full decarbonization. While natural gas generation produces much lower levels of CO2 emissions than coal, some countries, are examining carbon capture, utilization, and storage (CCUS) programs to mitigate CO2 from gas-fired generation.

4. The SGT-800 supports combined cycle and cogeneration installations with excellent efficiency and outstanding steam-raising capability. Designed for flexible operation, it is perfectly suited for base and intermediate load as well as grid support. Courtesy: Siemens Energy

In the UK, a consortium that includes Siemens Energy is participating in a “Front-End Engineering Design” (FEED) study for carbon capture at a gas power plant in Teesside, scheduled for completion by the end of 2022.

Any electrical energy project in the future will likely include a tailored mix of technologies. Finding the optimal technology mix is a challenge. Getting the various elements working together to achieve net zero is one of the reasons why Siemens Energy offers Energy System Design (ESD), a service to study all available levers for a customer’s situation and recommend appropriate approaches.

Looking beyond 2030 and toward the global goal of net zero carbon emissions by 2050, there is “deep decarbonization,” using renewables, green hydrogen, and coupling economic sectors. The idea is to spread renewable power to all energy-intensive parts of the economy, such as buildings, mobility, industry, or agriculture.


Turning Brownfields Green

As the energy transformation has been moving forward, attention is getting focused on how to use sites of former plants that have been decommissioned or abandoned, widely known as “brownfield” sites. That’s a circumstance facing many electric generators, particularly those with old coal-fired plants, as the world economy moves to a net zero carbon environment.

Often, brownfield transformations offer advantages over starting with a green field. Repurposing existing assets can be cheaper than building new ones in terms of time (it can be completed faster and usually requires fewer permits) and of sustainability (repurposing emits less CO2 and doesn’t occupy new land). Meanwhile, keeping existing qualified staff can increase acceptance in local communities and beyond. Some examples of brownfield opportunities include:

Coal-to-Gas. In the short term, coal generation is converted to natural gas, drastically reducing CO2 emissions. In the mid-term, “H2-ready” plants, later fully converted to hydrogen or biofuels, reduce emissions further. In the long run, these gas power plants will be an essential component for a completely decarbonized energy system.

Brownfield Engine Exchange. Existing gas-powered plants can be equipped with new, more efficient turbines. Existing steam cycles can remain in place, while reducing NOx, CO, and CO2 emissions relative to power output. In the European Union, the target is to increase energy efficiency by 32%, while reducing emissions by 40% by 2030.

Rotating Grid Stability Conversion. Reliable, stable grids require inertia. In conventional fossil power plants, large spinning turbines typically provide this essential service. As many of these are being phased out, “rotating grid stabilizers” can ensure grid stability. These stabilizers are large pieces of existing spinning machinery and usually a flywheel. Connected to the grid and spinning continuously, they provide all-important inertia. It is not very difficult to convert core parts of existing power plants into such stabilizers.

Hybridization of Power Plants. Many of today’s power plant sites eventually will be turned into hybrid plants, integrating various functioning elements into power generation.


Beyond Traditional Boundaries: Distributed Energy Generation

A major trend in the power industry is the movement away from centralized energy supply to generating close to where power is needed. This is an interesting reversal of what generally occurred in the 20th century, where centralized utilities most often located large generating plants in remote sites and moved the power to market over wires.

The term of art for this move away from central station generation is “distributed energy.” Siemens Energy describes this as devices below 100 MW used to generate electricity, steam, and heat for industrial customers and municipalities.

In the centralized past of large-scale generation, the focus was cheap electricity supply, driven in part by economies of scale. Today, and going forward, the aim is to be sustainable, meaning carbon neutral with a strong focus on renewables, energy efficiency, and supply matched to demand. This paradigm shift can be clearly seen by changes in public opinion, national and international regulations, and carbon pricing.

To aide implementation of reduction goals, more than 2,000 companies around the world have joined the Science Based Targets Initiative (SBTi), a coalition of the Carbon Disclosure Project (CDP), United Nations Global Compact (UNGC), World Resources Institute (WRI), and the World Wide Fund for Nature (WWF) to reduce their emissions. Siemens Energy is among the firms supporting the organization. The company is committed to becoming climate neutral for scope 1 and 2 (internally generated emissions) by 2030, using only 100% renewable electricity for its own operations by 2023.

Deep decarbonization, the long-term goal for the world to reduce effects on the climate, must transcend the electric power sector. It must also transform all other industries. This requires deep linkages among the power sector, industrial energy and power, and other energy-consuming sectors, including transportation and buildings large and small.

But today, rotating equipment, that is, turbines, remain at the heart of industrial heat and power generation. Some generators aim to burn 100% green hydrogen in time, which produces no carbon emissions. All Siemens Energy gas turbine models can burn levels of hydrogen in the fuel mix. As previously mentioned, the SGT-800 has recently demonstrated 75% hydrogen co-firing, clearing the way toward 100%.

A current example of what the future could look like—the convergence of multiple sector energy production—is in Shanghai, China. In 2015, the Shanghai Orient Champion pulp and paper plant began an energy transformation. The company produces face tissues, paper and kitchen towels, toilet paper, and paper rolls for the printing industry. Paper production is considered one of the five most energy-intensive industries. Before 2015, the company, which has the capacity to produce 140,000 tons of paper annually, had operated its eight paper machine production lines with grid power and externally generated coal-fired high-quality steam.

In line with Chinese policy, the paper company management decided to invest in its own clean power generation and explored the option of clean and efficient gas-fired combined onsite heat and power generation. Paper production requires a reliable and highly flexible power supply that can always adapt to load shifts. Making paper also requires steam for the drying process. To avoid downtime, the company was also looking for a solution that was particularly service-friendly.

After researching various turbine types and manufacturers, the paper company chose the Siemens Energy workhorse SGT-300 gas turbine. In 2017, it ordered two SGT-300 single-shaft turbine packages, with a total electric capacity of 15.8 MW. Fired up in 2018— the first onsite energy production in the Chinese paper industry—the units are working well. After several months of trouble-free and stable operation, the company completely canceled its external power and heat supply.

“There are good reasons to be optimistic about the future for gas turbines. We expect that gas turbines will have an increasingly important role to support renewable energy sources by providing backup power, and act as an enabler for adding more renewable power to the grids while keeping the grid stable,” said Andersson. “But the role of gas turbines will not only be to support other renewable energy sources, it will also be to provide renewable power by operating on renewable fuels such as hydrogen and biofuels.”

Kennedy Maize is a veteran energy reporter and a frequent contributor to
POWER. Aaron Larson is POWER’s executive editor.