Clearing the Air: Wind Power and Reliability
"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." -- Paul Bonavia, Chief Operating Officer, Xcel Energy
March 25, 2008
by Jeff Anthony, Manager of Utility Programs and Policy, AWEA
In its February issue of Windletter, the American Wind Energy
Association (AWEA) addressed questions about the reliability of wind
power in an attempt to get the facts straight. Wind power, often
regarded as too intermittent to be reliable as a major player in the
electricity generation game is in fact proving to be an important part
of the worldwide energy mix. In question and answer format, see the
answers below to some of the most pressing questions and misunderstood
issues about the reality of wind power today.
Can We Rely on Wind Power?
Yes. Wind power is currently supplying 48 billion kilowatt-hours (kWh)
of electricity annually in the U.S., powering the equivalent of over
4.5 million homes. Wind power is an important part of electric utility
generation portfolios. Yet some question whether wind power, being a
variable 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 a resounding "yes."
According to many utilities and reliability authorities, wind power can
readily be accommodated into electric system operations reliably and
economically.
High Wind Penetration and Reliable Operation
In Europe, Denmark receives over 20% of its electricity from wind
power, and in 2007 Germany received 7% of its electricity from wind
power. Both Spain and Portugal had periods in 2007 when wind power
provided over 20% of their electricity. In the U.S., Minnesota and Iowa
both get close to 5% of their electricity from wind power. These
examples provide real-world experiences with high penetrations of wind
power, as a valuable part of a utility generation mix that supplies
reliable electrical service to consumers without interruption.
Accommodating the Variable Nature of Wind Power
When wind isn't blowing, reliable electrical service is maintained by
turning up the output of other generators on the electric power system.
Electric utility companies serve as "system operators" that can be
thought of as air traffic controllers of the power system. System
operators can control, or dispatch, generators on their system such as
natural gas-fired and hydro generators. They have always actively
dispatched their systems in response to electrical demand, or load,
which varies randomly over the course of an hour or day. Wind behaves
similar to load in that it is "variable," meaning its output rises and
falls within hourly and daily time periods; and it 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 variation in load and
wind generation. System operators also keep generation in reserve,
called "operating reserves," which can be called on in case of a
shortfall. According to Paul Bonavia, Chief Operating Officer of Xcel
Energy, one of the nation's largest electric utility companies: "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 percent of the electricity on our system
from wind we forecast cost for operating system reserves of
approximately $5 per megawatt-hour, or roughly ten percent of the cost
of the wind energy. As we gain experience with wind we keep seeking
ways to achieve low integration costs."
Is Energy Storage Needed?
No, while it is natural to think that batteries or other storage
systems might be needed to supply steady power, it is not needed to
integrate wind energy into electric power systems. 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 used as a resource for the overall power system. It would
not be cost effective or efficient to couple energy storage resources
exclusively to individual wind plants.
Is Wind Less "Reliable" than Conventional Generation?
No. Conventional resources occasionally shut down with no notice, and
these "forced outages" require operating reserves. For example, a power
system that has 1,000-megawatt nuclear or coal plants will typically
keep 1,000 megawatts of other generation available, to be ready to
quickly supply electricity if a plant unexpectedly shuts down. 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.
As noted by Jon Brekke, Vice President of Member Services for Great
River Energy, a utility that operates in Minnesota and Wisconsin: "Wind
energy is a valuable part of our diverse and growing energy portfolio.
When partnered with other traditional generation resources, wind energy
is an effective way to provide reliable, clean and affordable power to
our member cooperatives.
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, which reduce the impact
of outages. For instance, there are 67 1.5-MW turbines at our Trimont
Wind Farm, so if one is down for maintenance only 1.5% of the total
wind farm's generating capacity is lost."
Changes in wind energy output are not instantaneous, as are
conventional generator failures. Because of the geographic diversity
inherent with large numbers of wind turbine installations, it typically
takes over an hour for even a rapid change in wind speeds to shut down
a large amount of wind generation. This is a significant benefit when
compared with the instantaneous tripping of conventional units. 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.
What Is the Cost of Wind Integration?
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 document prepared by the
Utility Wind Integration Group in coordination with the trade
associations of all three utility sectors (investor-owned, public, and
cooperative), the studies and experiences with utility wind integration
are summarized as follows:
• "Wind resources have impacts that can be managed through proper plant
interconnection, integration, transmission planning, and system and
market operations.
• System operating cost increases arising from wind variability and
uncertainty amounted to only about 10% or less of the wholesale value
of the wind energy.
• A variety of means — such as commercially available wind forecasting — can be employed to reduce these costs.
• In many cases, customer payments for electricity can be decreased
when wind is added to the system, because the operating-cost increases
are offset by savings from displacing fossil fuel generation."