As California Integrates More Renewable Energy Resources, it Looks to New Technologies

Photo: AES Laurel Mountain Energy StorageCalifornia reached an important milestone in 2011 by satisfying over 20 percent of its electricity needs with renewable energy, according to the most recent quarterly report of the state’s Public Utilities Commission. The state is also on track to surpass its 2011 growth rate, with about 3,000 more megawatts of renewables slated to come online in 2012, the report says.

These achievements put California on track to attain its 2020 Renewable Portfolio Standard of 33 percent of electricity from renewable generation, a figure that represents the most ambitious target in the nation. But by virtue of its success, California has created technical challenges for the state’s electricity system.

Because weather conditions and time of day affect the amount of power generated by solar cells and wind turbines, high levels of these intermittent resources can strain the electricity grid, which must maintain an optimal frequency of 60 hertz (Hz) to operate reliably. If the grid fails to stay within this safe window, the electricity it transmits can damage or impair the functioning of equipment that taps into its power.

Even aside from the problem of solar’s and wind’s intermittency, system operators have needed to keep extra fossil-fuel plants online, ready to ramp up or damp down their power output in order to keep the grid’s frequency stable. But the extra demands that renewables place on that balancing process require new technologies, capable of responding more flexibly to sudden surges or declines in power.

The Northern California Power Agency (NCPA) took an important step in that direction with its new 300-megawatt natural gas plant, dedicated at the Lodi Energy Center on August 10.  The facility will not only be more efficient and cleaner than previous combined-cycle gas plants, but also will be able to vary its output rapidly over the course of a day so as to support the integration of more renewable resources.

“Up until now, if you had a gas unit, it was either very efficient but slow responding, or it was fast to respond but very inefficient,” California Energy Commission Chairman Robert Weisenmiller told the Sacramento Bee. “This type of technology, where you have both an efficient operation and a fast response, is a game-changer.”

In contrast to typical natural-gas plants, which require four to five hours to start up, and coal plants, which can take more than a day, the Lodi plant could be shut down at night, for example, when wind turbines are most productive, then begin generating power within ten minutes of starting back up and reach full base-load power production in under an hour. That kind of responsiveness was the goal that its manufacturer, Siemens, had in mind when designing the plant, said a company spokesman in a news release.

A similar natural-gas unit from Siemens, with a 758 megawatt rating, will also be constructed in Texas, the state which is number one in the nation for installed wind power.

In addition to its flexibility, the Lodi Energy Center plant will be environmentally friendly.  It will emit at least 20 percent less carbon dioxide than a typical natural-gas unit and about 70 percent less than the most efficient coal-fired plant, according to the NCPA. Furthermore, it will use reclaimed wastewater in its steam-generating and plant-cooling systems, thus relieving California’s highly stressed Central Valley water supply from an additional source of demand.

Alongside advanced natural-gas technology, California is exploring the viability of energy storage as an alternative approach to assuring the reliability of its electricity system when stressed by the intermittency of renewables, among other factors. The state enacted a landmark law in September 2010 that mandates the Public Utility Commission to determine if it is appropriate and cost-effective to set targets for utilities to procure energy-storage systems and, if so, to adopt an energy-storage procurement target by October 1, 2013. As part of this process, the California Energy Commission has sponsored workshops to help inform policy and awarded direct research grants to further the development of innovative technologies (for example, here and here).

The work carried out in the state complements a national initiative and grant program from the Department of Energy, along with federally funded research and development at the Sandia National Laboratories.

 — By Eleanor Saunders

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