By Jeff Anthony and Michael Goggin, American Wind Energy Association
Wind power currently supplies 48 billion kilowatt-hours (kWh) of electricity annually in the United States, powering the equivalent of more than 4.5 million homes. The U.S. wind energy industry shattered all previous records in 2007, with 45 percent growth and more than 5,400 MW of generating capacity installed (Figure 1). Wind farms accounted for about 30 percent of all new power generating capacity added in the United States in 2007. No other technology is installing zero-emissions power generation on such a scale today.
This growth has been spurred by utility companies’ increasing recognition that wind energy offers a variety of important benefits and can be reliably integrated at low cost. Paul Bonavia, Chief Operating Officer of Xcel Energy, said wind energy is an integral piece of the utility’s 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 percent of the electricity on our system from wind, we forecast cost for operating system reserves of approximately $5 per megawatt hour, or roughly 10 percent of the cost of the wind energy. As we gain experience with wind, we keep seeking ways to achieve low integration costs,” Bonavia said.
Some question whether wind power, being a variable, non-dispatchable resource (meaning it generates electricity when the wind is blowing, not on demand) can be relied upon as part of a system that provides reliable electricity to consumers without interruption. Based on a growing body of analytical and operational experience, the answer is “yes.”
Many utilities and system operators have already found that large amounts of wind power can be reliably and economically integrated with the grid. Denmark receives more than 20 percent of its electricity from wind power and other countries in Europe produce significant amounts of their electricity from wind power as well (Table 1). Both Spain and Portugal had periods in 2007 when wind power provided more than 20 percent of their electricity. In the United States, Minnesota and Iowa both get close to 5 percent of their electricity from wind power. These examples illustrate that wind power can be a valuable part of a utility generation mix that supplies reliable electrical service to consumers without interruption.
Accommodating Wind’s Variable Nature
When wind output decreases, reliable electrical service is maintained by turning up the output of other generators on the electric power system. System operators can dispatch generators on their system such as natural gas-fired and hydroelectric generators. They have always actively dispatched their systems in response to electrical demand, which varies randomly over the course of an hour or day. Demand, like wind, is variable and non-dispatchable. Some generation is always kept in reserve to accommodate unexpected increases in load (demand) or to respond to the sudden loss of large conventional generating plants or transmission lines.
Conventional resources, such as baseload coal-fired plants or nuclear plants, occasionally shut down with no notice, often in fractions of a second; as a result, system operators are required to maintain operating reserves to address these “forced outages.” The power system can still be operated perfectly reliably in this fashion. Thus, “reliability” is not specific to any single generation facility; rather it is measured on a system-wide basis. In addition, unlike other sources of generation that can go offline in 1/60th of a second, wind’s output tends to decrease gradually over a matter of hours, giving system operators more time to respond to changes.
In fact, as more wind turbines are installed, the aggregate variability of their output decreases. This is because larger wind plants are built by installing more, rather than larger, wind generators. Those individual machines must be spaced out, resulting in a decrease in variability speed because of the time it takes for a “wind event” to cross the turbine field. Aggregating several wind plants further reduces wind variability because of the greater distances between individual wind plants. The United States’ large size and diverse wind resource makes it likely that shortfalls in wind output in one region are compensated for by higher output in other regions. In addition, wind forecasting tools that warn system operators of pending major wind output variations are becoming widely used and better integrated into system operations.
Jon Brekke, vice president of Member Services for Great River Energy, a utility that operates in Minnesota and Wisconsin, said, “Wind energy is a valuable part of our diverse and growing energy portfolio. Geographic diversity of wind energy helps even out the variability of wind energy in the regional market.” In addition, wind farms are typically made up of many individual turbines that reduce the impact of outages. For instance, there are 67 1.5-MW turbines at Great River Energy’s Trimont Wind Farm, so if one is down for maintenance “only 1.5 percent of the total wind farm’s generating capacity is lost,” Brekke said.
It can be said that wind power is as reliable as any other form of electric generationbut that the variable nature of its output does require some special considerations.
Balancing Variability and Flexibility
The key to integrating wind power in electricity grids is to recognize that there are already a number of sources of variability on the system today and while wind power adds other sources of variability, the challenges to integrate it are incremental. This is not unlike the challenges system operators had to adjust to in the 1960s and 1970s to integrate large, baseload nuclear power stations that had previously not been part of the operating scheme. Wind power is best integrated into large regional markets for electricitya small utility that balances its own system in a real-time manner in a small geographic footprint is likely to face challenges in integrating a large amount of wind power (see “Austin’s Windy Limits,” page 68). But for a system that is robust, interconnected to other control areas and has a good mix of resources, wind power can be integrated readily and at low cost. As previously noted, this is already happening in several European countries and experience is being gained in a number of regions in the United States.
Recent work by the U.S. Department of Energy (DOE) points out that several sources of variability in an electric system exist, including:
- System loads (customers)
- Variable generators (wind and solar)
- Schedule and dispatch errors
- System outages.
According to the DOE research, the key to balancing these sources of variability are the inherent sources of flexibility within that same system: dispatchable generators, dispatchable loads and energy storage.
The solution is for system operators to use the inherent system flexibility to match the variability in the system at a given time, something they already do on a continual basis. Because wind power has limited dispatchability and has a variable output over time, to accommodate higher wind penetration levels system operators may find it useful to take steps to increase their system’s flexibility or manage the variability of wind. As pointed out in the recent DOE work, a number of options exist to give system operators the tools they need to integrate larger amounts of wind power. These solutions and tools include:
- Reducing wind variability through accurate forecasting, wind output management and wind curtailment.
- Utilizing grid flexibility through existing flexible generation, balancing aggregation, markets and contracts, demand response and energy storage.
Is Energy Storage the Key?
Some people question whether energy storage must be developed before large amounts of wind power can be added to the energy mix. The answer is “not necessarily.” While it is natural to think that batteries or other storage systems might be needed to accommodate the variable output of wind plants, they are not necessary at the present time. The power system essentially already has storage in the form of hydro reservoirs, gas pipelines, gas storage facilities and coal piles that can provide energy when needed. Storing electricity is currently significantly more expensive than using dispatchable generation. In the future, through advances in technologies such as batteries and compressed air, energy storage may become cost-effective. The prospect of plug-in hybrid electric vehicles holds great promise because the expense of their batteries would be covered by their fuel cost savings and they could provide many megawatts of storage for the overall electrical power system. This would allow wind power and other renewable energy resources to displace consumption of foreign oil. Still, energy storage will best be viewed as a resource for the overall power system to be deployed when it is the most cost effective source of needed flexibility. It would not be cost effective or efficient to couple energy storage resources exclusively to individual wind plants.
Wind Integration’s Cost
To address wind energy’s variability, some incremental generation may be required for system balancing. While this is not a reliability issue, it can add a modest amount to the overall cost of electricity service. The costs of this generation include the costs of keeping the generators available and ready to operate and the fuel costs of operating them. The exact costs depend on the mix of generation on a given system and various other factors. In a condensed five-page summary of these studies by the Utility Wind Integration Group written in 2006 (in coordination with the Edison Electric Institute, the American Public Power Association and the National Rural Electric Cooperative Association), the wind integration studies performed to date were summarized by the following conclusions:
- Wind resources have impacts that can be managed through proper plant interconnection, integration, transmission planning and system and market operations.
- On the cost side, at wind penetrations of up to 20 percent of system peak demand, system operating cost increases caused by wind variability and uncertainty amounted to about 10 percent or less of the wholesale value of the wind energy. These conclusions will need to be reexamined as results of higher-wind-penetration studiesin the range of 25 percent to 30 percent of peak balancing-area loadbecome available. However, achieving such penetrations is likely to require one or two decades.
- In the next one to two decades, changes are likely to occur in both the makeup and the operating strategies of the nation’s power system. Depending on the evolution of public policies, technological capabilities and utility strategic plans, these changes can be either more or less accommodating to the natural characteristics of wind power plants.
- Wind forecasting tools and other technologies that are commercially available can be employed to reduce system operating costs associated with wind variability.
- Since wind is primarily an energy, not a capacity, source, no additional generation needs to be added to provide back-up capability provided that wind capacity is properly discounted when determining generation capacity adequacy. However, wind generation penetration may affect the mix and dispatch of other generation on the system over time, since non-wind generation is needed to maintain system reliability when winds are low.
- Wind generation will also provide some additional load carrying capability to meet forecasted increases in system demand. This contribution is likely to be up to 40 percent of a typical project’s nameplate rating, depending on local wind characteristics and coincidence with the system load profile. Wind generation may require system operators to carry additional operating reserves. Given the existing uncertainties in load forecasts, studies indicate that the requirement for additional reserves will likely be modest for broadly distributed wind plants. The actual impact of adding wind generation in different balancing areas can vary depending on local factors. For instance, dealing with large wind output variations and steep ramps over a short period of time could be challenging for smaller balancing areas, depending on the specific situation.
Wind Power’s Future
Wind energy use has grown significantly across the United States and globally in recent years in response to increasing recognition of its environmental, economic and energy security benefits. Although many regions have already successfully integrated large amounts of wind energy with their electric grids, there is still a great deal of misunderstanding about wind and grid integration. Variability, similar to that introduced by wind energy, is nothing new for system operators, who have always accommodated random variations in load and the sudden loss of conventional generating units while maintaining reliability. Moreover, greater use of wind forecasting tools and the increasing geographic diversity of wind energy facilities continue to reduce the impact of wind’s variability on the grid. With reasonable changes, grid operators can tap the flexibility of different components of the electric grid to reliably integrate large amounts of wind energy at low cost.
Authors: Jeff Anthony is the manager of Utility Programs and Policy for the American Wind Energy Association (AWEA). He joined AWEA in March 2007. He is responsible for supporting utilities in their efforts to integrate and adopt wind power as a mainstream generation technology. Michael Goggin is an electric industry analyst for AWEA. He works to promote changes in transmission rules and operations to better accommodate wind energy in the power system while maintaining system reliability.