Wind Systems Integration Basics

This page explains how power is balanced in the electricity grid and how various energy sources contribute to the U.S. generation mix. The page then explains how wind energy operates and is managed on the grid. Some of the following documents are available as Adobe Acrobat PDFs. Download Adobe Reader.

Electricity Grid and Generation Mix

Impact of Variable Power Sources

Power system is designed to handle tremendous variability in loads
Wind adds to that variability
System operators must balance loads plus resources (within statistical tolerance)
Key implication: it is not necessary or desirable to match wind's movements on a one-to-one basis.

Insights about Wind Integration

Aggregation reduces variability
Geographic dispersion reduces operational impacts
Large balancing areas reduce impacts
Use of wind forecasting in the power system control room reduces operational impacts and costs.

In an electricity grid, electricity consumption and production must balance at all times; any significant imbalance could cause grid instability or severe voltage fluctuations, and cause failures within the grid. Total generation capacity is therefore sized to correspond to total peak demand with some margin of error and allowance for contingencies (such as plants being off-line during peak demand periods). Operators will generally plan to use the least expensive generating capacity (in terms of marginal cost) at any given period, and use additional capacity from more expensive plants as demand increases¹.

No individual fuel is capable of providing the energy to meet all of our nation's electricity demands. Fuel diversity is key to affordable and reliable electricity. A diverse fuel mix protects electric companies and consumers from contingencies such as fuel unavailability, price fluctuations, and changes in regulatory practices ². The quantity of capacity or operating reserves needed is determined by the grid operators on an aggregate basis, based on the variability of load and the scale and characteristics of resources on the grid. Rules for operating the electricity market compensate generators for the benefits they bring and ensure that enough reserves exist to keep the lights on. Electric generation resources in a region contribute in different ways to cost minimization and reliability, as well as resource diversity and environmental factors ³.

U.S. Generation Mix

An image of an Edison Electric Institute chart showing the nation's fuel mix. The PDF provides the latest statistics.

A map of the United States showing electricity generation mixes by census region. The PDF provides the latest statistics.

Electric companies use a diverse mix of fuels to generate electricity:

coal
nuclear
natural gas
hydropower
renewables
oil

Edison Electric Institute provides a chart showing the nation's fuel mix (PDF 64 KB).

In 2008, American wind farms generate about 1% of U.S. electricity supply.

A U.S. Department of Energy report analyzes a path to reaching 20% wind power by 2030 ⁴.

Regional Generation Mixes

Different regions of the country rely on different fuel mixes. Several factors influence an electric utility's decision to use particular fuels such as the price and the availability of supply. Edison Electric Institute provides a map of the United States (PDF 270 KB) arranged by census region to illustrates the diversity of fuel use and shows how the electricity generation mixes in various regions of the country differ. The map further demonstrates that major changes in the generation mix could have economic and reliability impacts, especially on a regional basis.

Types of Generation and Capacity Factors

Capacity factors vary greatly depending on the type of fuel that is used and the design of the plant ⁵. A generator's capacity factor is a ratio of the total energy output relative to the theoretical sustained peak output. Wind generators' average capacity factor falls in the 20% to 40% range depending primarily on their location ⁶.

Basload Generators (coal, nuclear)

Baseload generators have capacity factors of more than 80% ⁷.

Baseload plants have the lowest costs per unit of electricity because they are designed for maximum efficiency and are operated continuously at high output. Geothermal plants, nuclear plants, coal plants, and bioenergy plants that burn solid material are almost always operated as baseload plants ⁸.
Peaking Generators (diesel generators and gas turbines)

Peaking generators have capacity factors from 0%-10% ⁹.

They are prepared to support the rest of the grid, but is relatively expensive. Some of these may operate only a few hours per year or up to a several hours per day ¹⁰.
Intermediate Generators

Intermediate generators, or load following power plants, are in between baseload and peaking power plants in efficiency, speed of startup and shutdown, construction cost, cost of electricity and capacity factor ¹¹.

Intermediate generators provide spinning operating reserves, ramping up and down quickly to balance load and generation, reacting to the variability of load and stepping up in the event of unscheduled generation outages ¹². Intermediate generators include hydroelectric power plants and steam turbine power plants that run on natural gas or heavy fuel oil, although heavy fuel oil plants make up a very small portion of the energy mix ¹³.

Compensation

Every generator connected to the grid receives a different combination of revenues for contributing energy, capacity, and "ancillary services" (which includes operating reserves). Baseload generators will receive substantial revenues for energy and capacity, but many provide no operating reserves. Peaking generators may receive most of their revenues in capacity and ancillary service markets and little for energy generated. Wind is primarily an energy resource, getting paid for the energy it contributes to the grid which does not need to be generated by fossil fuel combustion; wind also receives modest capacity value recognizing its statistical contribution to reliability ¹⁴.

Wind Power on the Grid

Wind power is by its nature variable, and as a result, it differs from the majority of generation supplying the electric grid. Aspects of this variability are often cited as shortcomings, but wind energy can also be viewed as an additional generation source to our national electric grid with its own set of contributing characteristics.

Intermittency and Back Up

A highlighted portion of Karl Pfirrman's interview with Citizens for Pennsylvania's Future.

"PJM on Wind (PDF 107 KB)," E³, Citizens for Pennsylvania's Future; Vol. 9, No.5, December 5, 2007.

A cited shortcoming is that wind power will not be as regularly and reliably available at system peak times as most other generators.

Like other resource-limited generation, such as the run-of-river hydroelectric generators that have contributed to the supply mix for nearly a century, wind contributes less to meeting peak demand.

It is also argued that wind power requires additional backup resources by other generation on a one-to-one basis.

Karl Pfirrmann of the PJM Interconnection, the entity charged with operating the largest electric operating system in the country, explains that if wind generation is available, it is invariably included in the supply mix because wind generators have very low operating costs and thus can accept virtually any market price. When wind generation is available, it is almost always the most expensive power (in this case coal or natural gas-fired generation) that is either reduced or not brought on line (PDF 107 KB).

Grid Operations and Management

The PJM system is so large, approximately 1,270 sources of generation interconnected to an extensive transmission grid, that it will be able to integrate a good deal of wind — even unscheduled wind generation — into the system without operational difficulties, and the wind generators themselves are carrying costs associated with their variable nature.

Penetration

Wind energy "penetration" refers to the fraction of energy produced by wind compared with the total available generation capacity ¹⁵.

In 2008, American wind farms generate about 1% of U.S. electricity supply.

A U.S. Department of Energy report analyzes a path to reaching 20% wind power by 2030 ⁴.

In 2006, the Utility Wind Interest Group produced a report (PDF 123 KB) in cooperation with the American Public Power Association, Edison Electric Institute, and National Rural Electric Cooperative Association. Together, these three utility industry associations represent nearly 100 percent of the utilities in the United States. The report showed that there aren't any fundamental technical barriers at the present time to wind penetrations of up to 20 percent of system peak demand.

Predictability

Pfirrmann explains that wind is not as variable as people may think. PJM's experience shows that, if a wind generator is operating at a certain level at present, there is an 80 percent probability that it will be operating within ±10 percent of that level one hour from now. And, there is a 60 percent probability that it will be operating within ±10 percent of that level five hours from now.

Wind power forecasting methods are used, but predictability of wind plant output remains low for short-term (hourly or daily) operation. Future wind forcasting models will help system operators to better predict the output from the wind generators.

More Information

An image of the cover of the publication.

Wind Power — Clean AND Reliable (PDF 85 KB)

A factsheet prepared by the American Wind Energy Association. It covers the topics: Can we rely on wind power?, high wind penetration and reliable operation, accommodating the variable nature of wind power, Is energy storage needed?, Is wind less "reliable" than conventional generation?, and What is the cost of wind integration?

20% Wind Energy by 2030: Wind, Backup Power and Emissions (PDF 85 KB)

A factsheet prepared by the American Wind Energy Association. In the context of 20% wind energy and climate change, the factsheet addresses the "no reduction in emissions" myth and the "backup power" myth. It provides an overview of the power grid's operations, explains spinning reserves and non-spinning reserves, and discusses how wind energy operates and is managed on the grid.

References

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