Wind power, back-up power, and emissions

Large amounts of wind energy are already being reliably and cost-effectively integrated into the grid in the U.S. and around the world. In 2012, the Texas grid obtained 9.2% of its electricity from wind energy, with wind power providing more than 28% of the entire grid’s electricity at one point in time. Nearly 25% of the total electricity produced annually in Iowa now comes from wind energy. Xcel Energy in Colorado, during one hour in the spring of 2013, supplied 57 percent of its total demand for electricity from wind power. Similarly, European countries like Germany, Spain, Portugal, Denmark, and Ireland now obtain well more than 10% of their electricity from wind energy, with wind providing more than 45% of Spain’s electricity at one point.

How is this possible?

A large part of the answer is that grid operators are already very good at dealing with variability and uncertainty on the power system. Factories turning large electrical equipment on and off and millions of people changing their use of air conditioning and electric heating as the weather changes cause large fluctuations in the demand for electricity. Similarly, large changes in electrical supply can occur if a large conventional power plant experiences sudden outages due to a mechanical or electrical failure. As a contingency, the grid operator must always be prepared for the loss of a power plant or a transmission line and, through the use of any of a number of methods, they strive to minimize the effect of such disturbances on their customers.

Utilities must manage constant changes in supply and demand, which may or may not be predictable. Grid operators often deal with these changes by adjusting the output of flexible power plants such as hydroelectric dams or natural gas power plants that can ramp their power output up or down very quickly. In other cases the grid operator may purchase or sell power to a neighboring region, or it may use demand response resources – a strategy in which large electricity users like factories have agreed to reduce their electric power demand quickly on short notice in exchange for payment.

Wind output predictions

Though wind power is also variable, the changes in wind power output on a utility’s system tend to be gradual and predictable. If the wind turbines are spread over a large geographic area, changes in wind power output typically take an hour or more to change significantly. Numerous studies have evaluated the rate of change in output from wind power on utilities’ systems, a few of which are summarized in the table below. The recorded changes in power output are quite minor relative to the wind capacity on the systems studied. In addition, wind energy forecasters can now predict wind output hours and days in advance with an increasingly high level of accuracy and confidence, thanks to the use of advanced computers, weather models, and trained meteorologists.

Study
Wind Penetration Studied
1 Minute
5 Minute
1 Hour

Texas 2008

15,000 MW

6.5 MW

30 MW

328 MW

California 2008 

2,100 MW plus 330 MW solar

0.1 MW

0.3 MW

15 MW

7,500 MW plus 1,900 MW solar

1.6 MW

7 MW

48 MW

12,500 MW plus 2,600 MW solar

3.3 MW

14.2 MW

129 MW

New York 2005

3,300 MW

--

1.8 MW

52 MW

The fact that changes in wind output are slow and predictable has important implications for the cost and emissions associated with integrating wind. Slower changes can be dealt with through the use of non-spinning reserves – power plants that are not operating but are standing by ready to provide power within 30 minutes or so. Since non-spinning reserves are not operating, there is no fuel use associated with them standing by and available to be placed into operation. As a result, there are little to no emissions impact from having these reserves, and the cost of these non-spinning reserves is typically a few percent of the cost of the fast-response and higher-emitting spinning reserves. Keep in mind that, while a wind turbine is a mechanical device that can also fail abruptly, a single turbine represents only a small (typically <1%) fraction of the power being generated by the whole wind power plant. The probability that any two turbines in a wind plant will unpredictably fail at the same time is quite small.

Wind energy integration

Wind energy integration can be made even easier if our grid operating procedures are updated to accommodate variable resources. Many of the rules that govern grid operations were enacted when the fuel mix was dominated by coal, gas, nuclear, and hydroelectric, and these operating procedures haven’t kept pace as technology has improved and as the fuel mix has changed to include substantial portions of variable-source generation. As the Federal Energy Regulatory Commission (FERC) recently noted Order 764, many of these obsolete grid operating practices constitute discrimination against renewable resources that are trying to get on the grid.

Relatively simple reforms like better coordination of regional grid operations, dispatching generators at shorter time intervals, creating ancillary services markets that will incentivize flexible resources like demand response, and better use of wind forecasting in grid operations can greatly reduce grid operating costs and facilitate wind integration. As FERC noted, many of these reforms yield major benefits for consumers even if no wind capacity is on the system, simply by making the grid more efficient, so grid operators should be implementing these reforms anyway.

An incident in Texas in February of 2011 serves as a powerful example of how a large quantity of wind power is being reliably and cost-effectively integrated into the utility system. An unusually severe cold snap caused unexpected outages to over 50 coal and natural gas plants, but, contrary to initial reports, the wind power on the system reliably kept operating and avoided power interruption on the grid. The head of the Texas grid talks about the issue.