My question is about the grid and wind power. If we were to add wind power into the generation mix, would the grid still be as reliable as it is today? -- Alan T., Portland, Maine

It's human nature to make assumptions based on everyday experience: from personal experience, for example, most of us know that the wind isn't blowing all the time, at least in most places with which we're familiar. But science and reality, we also know, often have a way of overruling our assumptions.

Nevertheless, some ask, can wind power, being a variable resource-meaning it generates electricity when the wind is blowing-be relied upon as a significant part of a system that provides reliable electricity to consumers without interruption. In fact, based on a growing body of analytical and operational experience, the answer is a resounding yes. According to many utilities and reliability authorities, wind power can readily be accommodated into electric system operations reliably and economically.

In Europe, Denmark receives over 20% of its electricity from wind energy, and in 2007 Germany got around 7% of its electricity from the wind. Both Spain and Portugal had periods in 2007 when wind energy provided over 20% of their electricity-in fact, Spain recently set a single-day record with over 40% of its electricity coming from its wind farms. Here in the U.S., Iowa leads the nation in the percentage of electricity it gets from wind, at 5.5%, while Minnesota is not far behind with 4.6%.

These are examples of how high penetrations of wind power can be a valuable part of a utility's "generation mix" that supplies electricity, reliably and without interruption, to consumers.

How it's done

How is this already being accomplished if, as everyone knows, "the wind doesn't blow all the time"? First, it's important to remember that the electrical grid is an amazing technical achievement of the 20th century, requiring operators to balance system demand for electricity with the system production of electricity from generation facilities (power plants), on a real-time basis-24-7, 365 days a year. This real-time need to balance load (i.e., demand, or the electricity users are requesting) with the supply of electricity, makes electricity very unique, but dealing with that uniqueness also provides the system with tools to allow grid operators to adjust the system when the output from wind power plants change over the course of a few hours. This built-in flexibility allows system operators to manage the existing variability of load-and additional variability introduced by large amounts of wind power added to the system.

So, even here in the U.S., where wind contributes only a bit over 1% to our electricity needs, those who manage the grid, known as system operators, are already accustomed to working with not only an ever-changing electricity supply but demand (power usage) that is constantly ebbing and flowing as well. They do this by controlling, or "dispatching," generators that are the most agile at ramping up and down, such as natural-gas and hydroelectric plants.

Wind energy output behaves similarly to load in that it is variable, meaning its output rises and falls within hourly and daily time periods. Moreover, wind is "non-dispatchable," meaning its output can be controlled only to a limited extent. Reliable electrical service can be maintained by system operators dispatching generators up and down in response to variations in both load and wind generation. And with or without wind on their system, operators also keep generation in reserve to meet any shortfalls that may arise. Demand can spike with little notice, and fossil plants can-and do-trip offline unexpectedly.

Such events occurred in Texas on February 26, when system operators implemented their "demand response" program, under which power is cut to end users who are paid to voluntarily have their electricity shut off during load peaks or supply drops. The event received some media attention and, with Texas leading the nation in installed wind power capacity, some of those reports pointed the finger at the renewable resource as a possible culprit. The event, however, by no means can be characterized as a problem with wind's reliability. Over the 40-minute period preceding the load curtailment, wind generation did decline by 80 MW compared to what it had been scheduled to contribute to the grid; however, during that same period, non-wind generation decreased by 350 MW relative to its schedule, while load unexpectedly spiked to a level that was 1,185 MW more than what had been forecast.

There are several takeaways from that event. For one, wind forecasting, a key component to getting the most out of wind in a reliable manner, something that is in the early implementation stages in Texas, was not in use at the time. Texas, incidentally, is now accelerating its implementation of wind forecasting.

In addition, the incident provides an example of how all generation sources sometimes fail to meet their scheduled output; as previously mentioned, system operators are already expert at making necessary adjustments when unexpected events arise. Finally, it should be noted for those not familiar with demand response programs that curtailment is one of the tools that system operators have in their toolkit to use-and in this case, the tool worked: no one involuntarily lost power, and those customers (likely industrial and other large-load users) are no doubt enjoying the lower electricity rates they are paying for participating in the program.

Conventional resources occasionally shut down with no notice, and these "forced outages" require operating reserves. A power system that has a 1,000-MW nuclear or coal plant will typically keep 1,000 MW of other generation available to be ready quickly in the event one of those large plants unexpectedly shuts down. Such shutdowns are typically the most challenging events with which a utility system operator must cope. With the kind of flexibility that the grid already possesses, large amounts of wind power can be added without increasing the total amount of reserves that are needed.

Understanding breeds a desire for more

Xcel Energy is a major investor-owned utility in the U.S., serving some 3.3 million customers. Xcel Energy leads all utilities in the amount of wind it is delivering to its customers. One of the top people responsible for making sure the lights stay on for all those people and businesses in Xcel's territories is Chief Operating Officer Paul Bonavia. Here's what he has to say about wind:

"Wind energy is an integral piece of our power supply portfolio. It provides a hedge against fuel price volatility associated with other forms of electric generation. Our studies and experiences show that wind energy integrates effectively and reliably into our power systems with regional market operations to mitigate the impact of wind variability. In these cases even with 25% of the electricity on our system from wind we forecast cost for operating system reserves of approximately $5 per megawatt-hour, or roughly 10% of wind energy. As we gain experience with wind, we keep seeking ways to achieve low integration costs."

For more information-and specific studies-on how wind can be added reliably and at relatively little additional cost, go to www.uwig.org.