By Ed Caryl
The California Independent System Operator (ISO) manages the high-voltage wholesale power grid for the state of California. On their web site, they have several links including one to yesterday’s hourly breakdown of power usage and sources. The figures below are for May 25 2011.
Figure 1. The hourly breakdown of renewable power fed to the California grid. Source: California ISO.
In California, most wind power is generated in three high wind locations: Altamont pass east of San Francisco, Tehachapi pass east of Bakersfield, and San Gorgonio pass near Palm Springs. Notice the huge drop in the wind farm output centered on 10 AM local time. The wind power output at all the wind farms dropped from over 1800 Megawatts to less than 200 Megawatts in less than six hours. Solar power picked up about 400 Megawatts of that, but solar had a glitch of it’s own at about 5 PM, when a cloud obscured the sun at the major solar plant in the Mojave Desert. The grid had to replace this power, just when the load was reaching maximum in the middle of the day. Where did the backup power come from? As you can clearly see in figure 1, none of the backup power came from a renewable source.
Figure 2. The hourly breakdown of all electrical power production in California on May 25th 2011. Source: California ISO, same link as above.
California must import up to 33% of its power, most from thermal and nuclear plants in other southwest states, and some from hydroelectric dams in the Pacific Northwest. As you can see from the plot in figure 2, most of the slack when the wind died came from thermal power plants in California and imported power from other states. Nuclear plants are difficult to throttle up and down, so most of that make-up power came from thermal sources (fossil fueled).
To be available on a moments notice, (or even on a few hours notice) thermal power plants are on “standby” status. In most cases this means that they have turbines already turning, feeding minimal power to the grid, so that they can be “throttled up’ quickly. The fastest responding are the natural gas powered plants. Coal powered plants take a bit longer to be fired up.
Planning is a big part of managing a power grid. The load can be predicted with good accuracy. Even fluctuations in temperature affecting load are predicted more than 24 hours in advance. But wind is more difficult, and clouds over solar plants more difficult yet. Today’s wind, for instance, is fluctuating over a scale of minutes, giving power output fluctuations of 200 Megawatts in 30 minutes. This must be giving the operators headaches.
California plans to build renewable power resources to the tune of 33% by 2020. The Pacific Northwest already has grid problems with Oregon and Washington wind farms. California will need three to five thousand Megawatts of reserve fossil fueled or hydroelectric plants to back up the renewable power resources. Given the May 25th 2011 wind power glitch, that may be low.