May 15, 2008

Regulating wind power into a dispatchable resource

By Robert D. Castro, University of Southern California, and Fernando Pardo

Take it to the bank

The ability to bank wind energy has multiple and far-reaching implications. Operationally, the major effect is to set a value on wind power capacity. One of the major shortfalls of using wind as a source of electricity is that wholesale-market dispatchers cannot rely on wind capacity being available to meet load (see sidebar “Loss of wind forces Texas to brink of blackout”).

Loss of wind pushes Texas to brink of blackout

The Electric Reliability Council of Texas (ERCOT) narrowly missed a blackout on February 26 by activating the second stage of its emergency electric curtailment plant (EECP) and curtailing industrial loads to compensate for a steep drop in wind power capacity and a drop in frequency on the ERCOT grid just as the evening demand was increasing.

According to an ERCOT press release issued the following day, wind production dropped more than 1,700 MW in the middle of the afternoon of February 26 to 300 MW by early evening, when ERCOT activated its demand response procedures. Within 10 minutes, industrial customers shed 1,100 MW of load, helping ERCOT avoid having to shed more load or to initiate rolling blackouts to avoid a broad system failure. Most of the interruptible loads were restored after roughly 90 minutes. “Preliminary reports indicate the frequency decline was caused by a combination of events, including a drop in wind energy production at the same time the evening electricity load was increasing, accompanied by multiple power providers falling below their scheduled energy production,” the statement said.

Bad timing

A 2007 study of the ERCOT system by General Electric concluded that the daily behavior of wind is “anti-correlated” with load, meaning that wind drops off sharply as the morning load builds, and builds up strongly as the nighttime load falls. This effect is most pronounced in late spring and summer, the study said.

The GE analysis, which relied on ERCOT data, also concluded that adding wind generation to the Texas grid increases the need to boost the percentage of overall power plant capacity set aside to provide ancillary services, which ensure that voltage levels and other reliability factors are maintained at optimal levels.

The report, which modeled scenarios assuming differing levels of installed wind capacity, found that if Texas were to install 15,000 MW of wind capacity, more than 75% of it would be available for use only 10% of the year.

“It’s obvious around here that the wind blows when we don’t need it as much, and that at peak times it’s not something we can rely on,” said ERCOT spokeswoman Dottie Roark the day after the grid emergency. “Wind works best in conjunction with other generation.”

Transmission trouble

ERCOT reported in January that wind provided only 2.9% of the 307 million MWh the state consumed in 2007. Wind’s share of the total Texas energy mix continues to climb, however. In 2006, wind met 2.1% of demand, while in 2005 wind energy amounted to 1.4% of the total MWh consumed in the state.

Texas, the nation’s wind generation leader, currently has 4,300 MW of installed wind capacity; projects totaling an additional 3,400 MW in capacity have signed contracts with prospective transmission providers. However, the state has only 5,000 MW of transmission capacity available to bring wind energy from west Texas--where virtually all of the wind capacity is located--to load centers in the eastern part of the state.

“Our biggest issue with wind is transmission congestion,” Roark said. “Obviously, we already have more installed capacity available than transmission capacity, and we have more than 40,000 MW of proposed wind projects in the queue.”

Though no one in ERCOT expects all of the 40,000 MW will be built, Texas clearly has a pressing need to build transmission to meet the growing demand for power in the state.

—By Chris Holly (cholly@accessintel.com), a reporter with POWER’s sister publication, The Energy Daily (www.theenergydaily.com).

Wind is typically assigned a capacity value of about 10% of total installed wind farm capacity. That is, for every 100 MW of wind turbines installed, the utility’s energy control center typically expects about 10% to be on-line at any given time. Some utilities do not assign any capacity value to wind; they use spinning reserve or fast-start generation to compensate for any shortfalls. For example, in Texas, ERCOT purchases these as “ancillary services.” Others use wind power to store energy for later use as on-peak capacity, for example by pumping water into a storage facility and then dispatching the hydroelectric energy during peak-demand periods.

Economically, the benefits of banking wind energy are enormous. Wind is capricious, but in most places it is stronger, and therefore capable of producing more power, at night, an off-peak time. Its value to a utility ranges from $15/MWh to $50/MWh. But in California, where wind speeds are higher than average, the typical wind energy price of over $70/MWh often causes negative cash flow for a utility. Even accounting for the value of RECs (whose prices are estimated to climb as high as $30/MWh), a 50% loss per MWh would not be atypical.