Gas turbines: put to the test

9th May 2016

A 7HA.01 turbine during assembly in Greenville
GE Power’s off-grid, full-speed, full-load test bed used 4,500 sensors as engineers pushed the machine to operate in stressful conditions
Harriet’s combined cycle efficiency exceeds 61%

Mark Egan reports on a facility where the world’s largest gas turbines prove their mettle

In parts of the world such as the USA and Brazil where electric current oscillates at 60Hz, there’s no larger and more efficient gas turbine than a machine that GE calls 7HA. So efficient, in fact, that when it swallows and burns 3.3 tons of air mixed with natural gas – equivalent to 23 tanker trucks – out comes a mere 6.3 fluid ounces of pollution, a volume slightly larger than a half-can of cola.

This is just one finding from three months of intense trials held at GE Power’s US$250 million testing centre in Greenville, South Carolina, USA, where a team of approximately 200 engineers pushed a version of the turbine labelled 7HA.01 well beyond the level of normal operation to pass validation testing. When the rigorous examination ended in November 2015, the turbine, nicknamed Harriet by GE employees, passed with flying colours.

Overall, validation tests rate the reliability of the latest technology to make it easier for projects using the machines to secure financing, insurance and trust among customers making the critical decision to buy the huge turbines.

The off-grid, full-speed, full-load test bed used 4,500 sensors as engineers pushed the machine to operate in stressful conditions, such as running the turbine at 110% of its rated speed, mimicking a power surge in Mexico or extreme heat in Saudi Arabia. The turbine was tested operating at ambient temperatures ranging from -37°C (-35F) to 85°C (185F) - far beyond what it would encounter in service.

Because the test bed is not connected to the grid, engineers can do things that might otherwise destabilise or damage the power network, such as replicating severe grid instability caused by the oversupply of power.

Jonathan Truitt, 7HA.01 product manager, says a crucial part of the testing was a new axial fuel staging (AFS) fuel injection technology that included 3D printed parts. The result was better performance and lower emissions than previous combustion technology.

The testing facility is so demanding that GE needed to supplement existing infrastructure in the city of Greenville to support it. The company built a dedicated gasworks to store 180,000 gallons of liquefied natural gas to feed the test bed, and North America’s largest railroad turntable to manoeuvre the turbines inside it.

In 200 hours of testing, engineers collected nearly 5 terabytes of data. That’s more than all the data generated by 500 GE 7F.03 turbines operating in the field for one year.

When used in a combined cycle power plant configuration that couples one gas turbine with one steam turbine, the 7HA.01 produces nearly 420MW of electricity - the equivalent power needed to supply more than 400,000 US homes. Another popular configuration, pairing two 7HA.01 turbines with one steam turbine, produces more than 840MW of power.

Harriet’s combined cycle efficiency exceeds 61% - the Holy Grail in the industry. Turning such a large percentage of fuel into energy can significantly lower power production costs.

The unit also achieved a ‘turndown rate’ of less than 25% of its full capacity while staying within emissions standards (versus an industry standard of 45%). Low turndown rate is critical because it allows operators to reduce output during off-peak hours, helping to save significant operating costs. It also gives them the flexibility to ramp back up quickly without having to stop and restart the turbine.

The unit also showed it can start and provide full power in less than 10 minutes (compared to a standard of 15-20 minutes), giving operators added flexibility to quickly meet changing grid demand. All this is important for incorporating into the grid intermittent renewable sources of energy like wind and solar power.

The testing is done in three phases. Validation puts the turbine through its normal operations and tests its efficiency and reliability. The demonstration stage studies the machine’s fuel and load flexibility, and pushes the turbine beyond its limits. The growth phase is used to try out new parts that may be used in subsequent turbines coming off the production line. “We blew away the target goals for the 7HA.01 in the validation and demonstration phases,” Truitt says. “Now, we’re taking what we’ve learned and modifying the test stand equipment and engine so we can really push the limits.”

Truitt says subsequent tests will use GE’s next-generation combustion system, which has been in development for years, and explore the limitations of the turbine compressor, the part that squeezes the air inside the machine. “We’re going to go well beyond what we specified for the machine for  - we’re going to push much more flow through it, fire it at higher temperatures, ramp it faster, start it up quicker. We’re going to push it to its limits to see where those limits truly are.”

The huge HA turbines come in two varieties. The 7HA is engineered for countries such as the USA and parts of Asia where electric current oscillates at 60Hz. The 9HA, which passed validation in 2014, is used in regions operating at 50Hz, such as Europe and most of the Middle East. The first 9HA will begin commercial operation in France in 2016 while the first 7HA gas turbines are expected to begin commercial operation in both the USA and Japan in 2017.

The Harriet turbines combine materials developed by GE scientists for supersonic jet engines and other advanced technology, such as aerodynamic blades made from single-crystal alloys and thermal barrier coatings. 

To date, 23 HA units have been ordered among 78 that have been technically selected for use in power plants being planned globally. GE spent more than US$1 billion developing the new turbines.

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