California’s drive to add flexibility to its grid in response to expanding renewable generation took another step forward in September as NRG Energy commissioned two new fast-start units at its El Segundo Energy Center near Los Angeles. The two units, with a combined 550 MW capacity, represent the second Siemens Flex-Plant to go into commercial operation. The first, not surprisingly, is also in California, in Lodi near Stockton (a POWER Top Plant last year).

The combined cycle plant is capable of reaching 300 MW output in less than 10 minutes, and full power in under an hour, making it ideal to balance the substantial wind and solar generation in Southern California. The two units replace a 1960’s-era 335-MW steam boiler unit on the same site that was retired in 2001. Though NRG had initially received a license for slightly larger ocean-cooled baseload plant, it ultimately decided to proceed with a fast-start peaking unit.

Construction began in 2011 and was completed in mid-2013. NRG plans to replace the remaining steam boiler at El Segundo with another combined cycle plant within the next few years.

Nimble Power

The two 1 x 1 power islands each employ a Siemens SGT6-5000F gas turbine, an SST-800 steam turbine, an SGen6-1000A generator, and an SGen6-100A-2P generator. The single-pressure NEM USA DrumPlus heat recovery steam generator (HRSG) is engineered for fast starts and cycling, with a specially designed thin-walled steam drum that can handle rapid changes in output with the same design life as standard models (Figure 1).

F2 NRG fig 1
1. The fast-start plants HRSG is capable of rapid cycling through the use of a new drum design. Source: Tom Overton/POWER


Siemens also supplied the electrical equipment and the SPPA-T3000 power plant instrumentation and control system, as well as the engineering services and commissioning for the turnkey plant.

California is in the process of phasing out once-through cooling throughout the state, and the plant relies on an air-cooled heat exchanger rather than ocean water for cooling as did the old plant (Figure 2).

F2 NRG fig 2
2. An air-cooled heat exchanger replaces the once-through ocean water cooling system of the old steam boiler. Source: Tom Overton/POWER


The design of the new plant reduces potable water use by nearly 90%. Process water is reuse throughout the plant, and it is set up to operate as a zero-liquid discharge facility. It also employs comprehensive noise suppression technology (Figure 3).

F2 NRG fig 3
3. Because of the plant’s close proximity to residential areas and the beach—a popular spot with local surfers—the plant incorporates multiple methods to reduce ambient noise. The site is remarkably quiet even immediately next to the turbines. Source: Tom Overton/POWER


Ready for Renewables

“Our modernization efforts at El Segundo ideally complement California’s growth of intermittent renewable energy sources, such as wind and solar, in support of the State’s renewable portfolio standard,” said John Chillemi, President of NRG’s west region. “In addition, we have worked with the community to improve the visual aesthetics of the site by incorporating native landscaping, installing a new picturesque sea wall, and affording improved coastal views with the lower profile of the plant.”

The plant achieves a 49% efficiency rating, making it much more efficient than conventional peaking solutions. The plant is also environmentally friendly, as it produces 90% lower startup emissions compared to conventional combined cycle plants. It will supply its output to Southern California Edison under a 10-year power purchase agreement.

George Piantka, NRG’s director of environmental business, said the company envisions one of the plant’s roles as balancing growing renewable output in California, which is slated to reach 33% of total generation by 2025. The plant is permitted for up to 200 starts per year.

—Thomas W. Overton is POWER’s gas technology editor.