That camel has morphed into a the shape of duck over the past decade as California added around 13 gigawatts of solar power capacity. Solar generation booms at lunchtime and then disappears at sunset. This means that thermal plants have a moderate morning workout, are mostly idle in the day, and return at dusk to do the grid’s heavy lifting. They create a baseload curve that looks like the pert rump, sloping back and long, high neck of a right-facing duck.
The duck curve might sound jolly, but it is no fun for the operators of the state’s gas-burning plants. “Solar generation has made it even more challenging for utilities that provide baseload power to respond to the steeper peaks and troughs of California’s power demand,” says Alexander Pistner, an Atlanta-based senior product manager for GE Power.
He says many of California’s gas-burning plants were built long before solar and other renewable plants joined the state’s grid, and they were designed to stay on almost 24/7 as baseload power. They generally deliver their best possible fuel economy and lowest emissions when they are at high load, or when they are pumping out power uninterrupted for several days at a time. “Now they’re struggling to find the balance between coming offline completely and operating at minimum load,” Pistner says.
He explains that you cannot turn gas-burning power plants on and off like a light switch, or increase and decrease their load, without a negative impact on their efficiency and emissions. In this respect, they are similar to car engines, which generally burn much less gas per mile — and emit less — when purring at high speed on the highway compared to when they are stopping and starting in the city center.
But help is at hand for operators that are struggling to ride California’s growing duck curve. GE engineers have devised a neat shortcut to make its gas turbines deliver cruising-speed efficiency and emissions, even when they are being used like a city run-around. They have rolled out a turbine upgrade called in industry parlance 7F DLN2.6+ Flex, where DLN stands for Dry Low NOx (nitrogen oxide-based air pollutants), and 7F refers to GE’s turbine nomenclature.
The upgrade uses Axial Fuel Staging (AFS) technology — which splits the combustion of the turbine’s natural gas and superheated air into separate upstream and downstream zones. Derrick Simons, the Greenville, South Carolina-based expert in gas turbine technology who heads up the team that has designed the AFS system, says splitting combustion into two zones enables the injection of a relatively cool flame of burning gas and superheated air into the hotter downstream crucible. This bumps up the flame’s temperature, which controls the turbine’s emissions and increases its efficiency. “It works a bit like a jet’s afterburners — you can inject additional fuel and air downstream to optimize performance,” explains Simons, who has worked for GE for nearly two decades.
If you have ever used a Bunsen burner in a science laboratory, you know how to vary the flame between billowing yellow and roaring blue by regulating the airflow into the base of the barrel. The yellow flame is cool, fuel-rich and air-poor, whereas the blue, lean flame is fierce and hot because it burns with a higher ratio of air.
The same principle is at work in the turbine. When the turbine starts up, it burns gas in a cool, fuel-rich flame that is relatively inefficient and yields relatively high emissions of the toxic carbon monoxide (CO) gas. AFS technology increases airflow to the flame, boosting its temperature. This allows the turbine to deliver the excellent efficiency and ultralow emissions of baseload mode within minutes of joining the grid at sunset. It is a bit like an oven that instantly attains the perfect cooking temperature, rather than making the hungry user wait for it to warm up.
But engineers must maintain a delicate balance. The hotter upstream flame might reduce CO emissions at part-power, but it generally produces more NOx. This requires the injection of more fuel for a richer downstream flame to keep the NOx in check when operating at high load. “AFS technology allows us to operate in that sweet spot in terms of low CO and NOx emissions and high efficiency,” says Simons.
The upgrade also makes use of additive manufacturing, or 3D printing, to make key hardware components. “This includes the turbine’s fuel injectors and AFS’s air bypass,” says Simons. He adds that the precision of 3D printing squeezes extra efficiency out of the combustion process.
The biggest win from the 7F DLN2.6+ Flex upgrade is the improvement of the turndown capability of the gas turbine, which is jargon for the operational range at which it can deliver low emissions and excellent fuel economy. The upgrade is expected to help GE’s 7F turbines in California — which Pistner says are generating power 25% to 50% of the time — reduce their annual fuel consumption by around 25% and slash their yearly emissions by 20%. That is the equivalent of the emissions of around 3,000 cars per unit, per year.
The continued growth of renewables means there is huge potential for the upgrade. Worldwide, there are around 900 7F turbines in operation, of which approximately 200 are strong candidates for the upgrade, says Pistner. “The first priority is North America, then Latin America, then Asia — all of those regions are seeing increased renewables penetration,” he says. GE engineers have already upgraded four of its gas turbine ranges — the 7HA, 7EA, 9E and 9HA.02 offerings — with AFS technology.
“We continue to bring flexibility to these utilities as they adjust to a renewables-based world,” says Pistner. Put another way, California’s grid can transition from camel to duck without getting the hump or feeling down in the mouth.