Are Flexible Generation Plants Performing as Expected?

Highly flexible, fast-ramping, fast-cycling combined cycle plants hit the market with a big splash a few years ago. But are they performing as advertised? Though the few operational plants are still new and still learning, the initial results are encouraging.

The Lodi Energy Center (LEC) is a 296-MW 1 x 1 combined cycle plant in Lodi, Calif., just north of Stockton and east of the San Joaquin River delta (Figure 1). From the outside, there's little to distinguish it from the many other combined cycle plants large and small that power the California Independent System Operator (CAISO) grid.

1. Fast start for a fast starter.The Lodi Energy Center in California was the first U.S. plant to employ Siemens' Flex-Plant technology. In the first two years, it's totaled 380 starts and achieved 94% availability. Courtesy: Siemens

On the inside, though, there's much to set this plant, which began commercial operations in November 2012 and earned a POWER Top Plant award that year, apart from its older brethren. LEC was one of the first plants in a new generation of combined cycle facilities specifically designed for fast starts and fast ramping while maintaining both high efficiency and low emissions.

LEC is operated by the Northern California Power Agency (NCPA) and owned by NCPA and a coalition of local public power agencies in the area. A turnkey plant delivered by Siemens, which refers to the design as a Flex-Plant 30, it's built around Siemens' SCC6-5000F gas turbine, which is paired with a Nooter Eriksen triple-pressure reheat heat-recovery steam generator (HRSG) equipped with a once-through Benson high-pressure section, high-capacity steam attemperation, and full-capacity steam bypass systems.

LEC also utilizes innovative piping warm-up strategies, a Siemens SPPA-T3000 control system and steam turbine stress controller, and optimized plant stand-by using auxiliary steam to maintain vacuum. The plant has as many analyzers and system drains as a conventional 3 x 1 plant.

Another Siemens Flex-Plant, NRG Yield's two-unit, 550-MW El Segundo Energy Center, came online in 2013; two more, Panda Power Funds' Temple I and Sherman plants (both 2 x 1 758-MW plants), started up in Texas in 2014.

Designed for intermediate to continuous cycling duty, LEC manages efficiencies above 57% with startup times that are as short as half those of earlier plants due to the integration of fast-start features. Ramping at 13.4 MW/minute from a cold start, the plant can reach 150 MW output in a little over 10 minutes, and the fast ramp rate means it reaches CO compliance in 23 minutes and NOx compliance in 40 minutes.

That's performance that is becoming critically important with continually increasing amounts of renewable generation being added to CAISO. California already has the nation's highest state renewable portfolio standard, 33% by 2020, and Governor Jerry Brown announced in January that he would seek to raise it even further, to 50% by 2030. That means the state's gas-fired fleet will be called upon to back up an enormous amount of variable wind and solar generation.

But Does It Work?

All that, at least, was the intent. But are LEC and the new highly flexible plants like it living up to the hype?

The question is not an idle one. A 2012 study by the National Renewable Energy Laboratory and Intertek APTECH found that shifting to faster ramping and startups from a baseload role resulted in considerably higher operational and maintenance costs for typical combined cycle plants. Worse, these costs increased with greater penetration of renewable generation into the energy mix.

In the case of LEC, at least, according to Plant Manager Michael DeBortoli, the answer is an unqualified yes.

"It has lived up to our expectations," he told POWER in an interview in January. "So far, the plant has been running very well. We cycle a lot and have a lot of starts and stops almost on a daily basis, and everything has been running fine."

DeBortoli confessed some concern with being what amounted to a guinea pig for the new design. "Being the first one in the country with this new technology," he said, "I thought we were going to encounter a few hiccups, but the plant has been operating to expectations."

LEC has been a workhorse since it came online. From November 2012 through the end of January 2015, the plant racked up an impressive 380 starts despite two planned outages. Over that period, it's achieved 94% availability.

The plant is regularly being ramped from 165 MW minimum load to maximum output. "That's basically achieved within a 10-minute interval," DeBortoli said. "The gas turbine is being run at the max ramp rate."

And it's being done without any excessive wear and tear on the HRSG. "The Benson technology, which is the once-through HP section, we have not had any issues on that," DeBortoli said. "All of our bypass valves have been working very well."

In the Benson once-through natural-circulation design, the drum is replaced by a thin-walled external separator. The change allows for higher temperature transients and simpler chemistry control.

Rafael Santana, LEC maintenance manager, said the number of problems has been surprisingly small. "We did encounter some minor hiccups with our HP turbine control valve," he said. "But it's not the fast-start plant that caused that issue, but rather a manufacturing defect, probably aggravated by the number of starts."

The gas turbine as well is performing admirably.

"We have had multiple planned outages to conduct inspections, and in the most recent inspection of the turbine in November, all the components looked pristine, so they were returned back to operation instead of replacing them."

Though greater demands are placed on the plant with the additional cycling, the demands on the plant staff are not unusual. "I wouldn't say there is much difference," Santana said. "In terms of normal maintenance and intervals of operation, it's the same."

Minor Growing Pains

As with any plant, there was a learning curve. Siemens provided remote monitoring of LEC for the first year, using networked instrumentation to track the operating parameters from its service center in Orlando, Fla. This enabled Siemens to give LEC feedback any time anything wasn't operating in an optimal fashion.

"There were some growing pains in the beginning," Jeremy Lawson, LEC plant engineer said, such as properly tuning the water levels in the HRSG up to the bypass valves.

There were a few little hiccups with the HRSG, with one of the lower acoustic baffles coming loose due to the cycling regime and a lack of support. There was also a minor leak in one of the tube bundles in the #1 preheater at the tube support a few months after operations began. Both problems were corrected and have not recurred.

The plant also encountered high-temperature trips of the steam turbine (ST) across the intermediate-pressure exhausts. This was resolved through faster ST starting and loading.

DeBortoli's staff also found some ways to reduce the plant's already low CO emissions. Working with Siemens, the plant staff made adjustments to the fuel flow and positioning of the inlet guide vane (IGV). "We ended up keeping our IGV closed until we got to 50 MW."

These tweaks, plus boosting the ramp rate to its 13.4 MW/minute maximum, helped cut CO emissions by 350 pounds per start.

But all in all, these were minor challenges.

"Out of the box, there is just a little bit of dialing in so the operators understand what works and what doesn't," Santana said, "but overall I think that we haven't really changed anything major with the logic or the operation."

DeBortoli concurred: "The overall process is pretty much where we want it to be. Not ongoing improvements from here on out, but just really minor things."

Moving On

With the uncertainty and changes in the California energy market, particularly the drought that is challenging hydroelectric generation, DeBortoli said LEC expects to continue seeing demand for its fast-ramping capabilities going forward. "The market condition is very dynamic right now, so we don't know what's going to happen in the next couple of months."

Last July, General Manager Ken Speer likened NCPA's experience with LEC to "driving a Ferrari rather than a Chevy." LEC and its sister unit in El Segundo appear to have hit the ground running when it comes to meeting the role they were intended for. a-

- Thomas W. Overton, JD is a POWER associate editor.

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