Fast-ramping peakers support solar… and bridge the way to net zero
Solar generation is one part of Duke Energy’s clean energy transformation strategy to cut carbon emissions in half by 2030 and achieve net-zero carbon emissions by 2050.
As a top US renewable energy provider on track to own or purchase 16,000 MW of renewable energy capacity by 2025, Duke Energy is well on its way to achieving its carbon reduction goals.
However, like other utilities with similar goals, continued investment in natural gas infrastructure and major advancements in technology, including hydrogen integration, will be needed.
Duke Energy’s regulated fleet serves 7.9 million customers in six states and generates 51,000 MW of capacity, including 14,597 MW of carbon-free generation from its hydro, nuclear and solar plants.
The company uses natural gas peakers to manage day-to-day and seasonal demand spikes, especially when renewables fall short in supplying energy because the sun isn’t shining.
That’s part of the reason why Duke Energy entered into a unique agreement with Siemens Energy to allow Siemens Energy to test its newest advanced 402 MW natural gas combustion turbine, the generation SGT6-9000HL, at Duke Energy’s Lincoln Combustion Turbine Station in North Carolina. Regulators approved the project in 2017, and construction started in 2018.
The 746-acre Lincoln site is located outside of Charlotte, North Carolina, and is a half-hour’s drive north of the Siemens Energy manufacturing plant.
The new unit was built on 60 acres at the site, taking advantage of the location’s proximity to the grid and natural gas sources.
First fire of the giant turbine took place in April 2020, successfully delivering power to the Duke Energy grid a month later. The initial firing confirmed that the engine and the auxiliary systems –including natural gas supply and the lube oil, control and start-up systems – all worked as designed.
Testing milestone complete
Siemens Energy has surpassed 7000 hours of operational testing of the new unit.
“This first-year milestone marks clear progress toward realizing our company’s goal to achieve net-zero emissions by 2050,” said Kevin Murray, vice-president of project management and construction at Duke Energy.
“We needed an additional 400 MW of peaker capacity at our Lincoln site to support our use of renewables, and this turbine is already helping us serve our customers, even though it’s still in its testing phase.”
Though the new unit is capable of operating in combined-cycle mode with an efficiency of more than 64%, the four-stage, 340-tonne SGT6-9000HL natural gas turbine operates at the Lincoln site as a simple-cycle combustion peaker.
Murray calls its 85 MW-per-minute ramp-up rate “exceptional” when compared to the 10- to 20-MW ramp-up rates of Duke Energy’s existing fleet.
“When the sun stops shining, and we see big swings in our load, our operators can quickly start this unit to compensate and maintain the reliability of our grid,” Murray said, noting that the unit can also operate effectively at a 28% turn-down to provide even more operating flexibility.
In addition, the advanced combustion system of the SGT6-9000HL natural gas turbine can fire at hotter temperatures, use mixed fuels and reduce emissions.
Currently, it runs on natural gas but has the capability of cofiring hydrogen. Most recently, it successfully tested a mixture of natural gas and No. 2 fuel oil.
In 2024, when testing is completed and Duke Energy takes over the turbine’s ownership and operation, the unit will be part of 16 other natural gas turbines at the site, each operating at 75 MW.
“When we look at a new generating asset, we consider performance, efficiency, emissions and maintenance cost over the entire life cycle, so we look out 30 years,” Murray said.
“This next-generation turbine delivers all of that to the benefit of our customers, the environment and our shareholders. It’s a triple-win for all our stakeholders.”
According to Diego Caso, Siemens Energy’s testing and validation director at the Lincoln site, the HL-class of natural gas turbines – the SGT6-9000HL for 60 hertz and the SGT5-9000HL for 50 hertz – represents millions of person-hours in design, engineering and testing to date.
Conceptual design started in 2015 and was built on the company’s knowledge and experience drawn from its prior classes of natural gas turbines. Prototype testing in Berlin, Germany, was conducted in 2019 in an off-grid setup, and then on-grid testing at Duke Energy started in 2020.
Both Murray and Caso attribute the successful testing and operation of the new HL-class turbine to the cooperation between their respective engineering teams.
“From site preparation through construction, installation, commissioning and this first phase of testing, our teams have worked shoulder to shoulder, each learning a lot from the other,” Murray said.
Advanced blade geometries, coatings, combustion systems and sensors
At Lincoln, the testing team is evaluating all kinds of combustion configurations for the greatest fuel and operating efficiencies.
“For example, with its advanced design and technologies, such as rotor blades that are 20% larger than previous generations, this system requires less cooling air through the flow path components,” Caso said.
“This reduces the turbine inlet temperature while maintaining a high-power output and low emissions.”
The turbine components were all designed and engineered using 3D digital twin software. This approach enabled greater visualization and hypothetical stress-testing of various geometries before initial fabrication.
Then, in manufacturing, not only were traditional casting and machining used to build the components but also additive manufacturing.
The latter gave metallurgy engineers the flexibility to explore different alloy combinations in building various parts to withstand extreme conditions, such as making the combustion system capable of burning hydrogen fuel mixtures.
While a fully validated turbine has 200-300 standard instruments, Caso’s testing and validation team hooked the unit up to more than 6,000 additional sensors with wires running from each sensor to a wide range of data monitoring systems.
Testing is conducted and monitored by Siemens Energy’s engineering staff at the Lincoln testing facility and company engineers elsewhere in the world.
They also have access to a digital twin. The engineers use this data to achieve even higher efficiency while also addressing the needs of power generation requirements for different markets and the ambient conditions of different geographies around the globe.
“For example, we have installed and are using an exhaust heat exchanger for air preheating to adjust the compressor inlet temperatures, so we can mimic much hotter ambient temperatures for customers in equatorial regions and places like Saudi Arabia and Southeast Asia,” Caso said.
“We’re also using the device for fuel gas heating to test different combustion variables.”
The operational activities at the Lincoln site are the final step in Siemens Energy’s three-step testing and validation process for the SGT6-9000HL.
It is also testing a 50-hertz version of the engine in a combined-cycle SGT5-9000HL power plant at Keadby 2 in the UK.
“A lot of the testing of the 60-hertz engine at the Lincoln site is directly applicable to the 50-hertz engine because they are identically scaled,” Caso said.
“Though the road to 2024 may seem long now, we’re proving this turbine will be a reliable addition to Duke Energy’s fleet. Together, our companies’ engineering teams are continually learning how to take best advantage of the turbine’s next-generation design for the benefit of Duke Energy’s customers.”
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