Is Texas a Wind-Power Success or Failure?

http://blogs.wsj.com/environmentalcapital/2009/08/06/is-texas-a-wind-power-success-or-failure/">Is Texas a Wind-Power Success or Failure?

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The Texas electricity authority, ERCOT, figures the state’s wind power capacity is only 8.7%. That means for every 100 megawatts installed in a wind farm, power authorities can only count on seeing 8.7 megawatts of electricity produced. That’s a lot less than the standard line that wind power in the U.S. produces at about 30% or 35% of its nominal capacity.

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... Getting a handle on how much power wind farms actually produce is tricky business. ERCOT itself has danced between estimates at low as 3% and as high as 16% in recent years, before settling—temporarily—on the 8.7% figure. Temporarily, because the Texas Generation Adequacy Task Force is “concerned” with how ERCOT arrived at that figure and still aims to determine “the true capacity value of wind.”

The picture isn’t much different in the rest of the country. Electricity regulators and utilities have tried to get a handle on how much juice wind power actually produces, and estimates vary widely—from as low as 5% to as high as 30%. Last year’s NREL report has all the details.

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The bottom line is that wind power is neither quite the laggard that Mr. Bryce makes it out to be, even in Texas—nor the panacea that many clean-energy advocates hope it will become. Things to keep in mind as the debate over America’s clean-energy revolution keeps simmering.

http://blogs.wsj.com/environmentalcapital/2009/08/06/is-texas-a-wind-power-success-or-failure/">There's more ...

Energy, Capacity, and Flexibility
Reliable and cost-effective operation of the electric grid requires a mixture of three types of resources: energy (electricity), capacity (ability to generate electricity at a certain point in time), and flexibility (ability to "turn up" or "turn down" electricity generation as needed). Each of the various types of power plants that generate electricity – nuclear, coal, gas, hydroelectric, wind and others – may specialize in providing one or two of these attributes, but no power plant excels at providing all three.

"Baseload" plants, a term typically applied to nuclear or coal-fired power plants, provide energy and some capacity. Interestingly, other types of power plants can provide these resources, often at costs competitive with baseload plants. Wind plants can produce energy just as well or better than nuclear or coal plants, while natural gas plants can provide capacity at lower cost than nuclear or coal plants. Thus, baseload power is only one of many ways to provide the power system with energy and capacity.

Moreover, baseload power plants provide almost zero flexibility, even though flexibility is a power system need that is just as essential as energy or capacity. In contrast, wind energy makes very valuable contributions towards ensuring that the grid has the right mixture of energy, capacity, and flexibility.

First, let us explore further what is meant by “energy,” “capacity,” and “flexibility.” Energy on the grid is a measure of power provided over time, and can be calculated by multiplying the amount of power used or generated by the time that it was used or generated. Thus, energy is measured in watt-hours, or more commonly kilowatt-hours (the unit used on household electricity bills) and megawatt-hours (1 megawatt-hour, MWh, is equal to 1,000 kilowatt-hours, kWh). For grid operators and planners, having enough energy largely means having enough fuel that can be converted to electricity, and having a diversity of fuels that will be available at a reasonable cost.

Capacity is a measure of power provided or used in a single instant, and thus is measured in watts, kilowatts, and megawatts. Operators of the electric grid must ensure that they have enough generating resources to provide the power capacity that will be needed at any point in time. Typically, grid operators think about capacity on years-ahead basis when they are deciding what power plants to build, on a day-ahead basis when they are deciding what power plants they should have ready to operate the next day, and on a real-time basis when they decide what power plants to operate.

Flexibility is the ability of power output, or capacity, to change over a given period of time. One can speak about the flexibility of a single power plant or the combined flexibility of all power plants on the grid. Flexibility is critical for accommodating changes in electricity supply and demand that occur, often unexpectedly, as power plants go offline or as consumers turn appliances on and off. Demand for electricity can vary by a factor of three or more depending on the weather and the time of day and year, which means that hundreds of gigawatts (GW) flexibility must be built into the power system. Flexibility can be measured over different time periods: e.g., a power system might have the flexibility to increase generation by 1 GW over 1 hour and 3 GW over 5 hours, with each capability being important for reliable system operation.

Specialization and the Division of Labor Among Power Plants
A power plant may specialize in providing one or two of these power system needs, but no power plant excels at providing all three. As a result, it is important to have a diversity of generation resources on the grid. Table 1 lists the ability of different types of power plants to provide the attributes of energy, capacity, and flexibility.

> table 1 available here: http://www.awea.org/pubs/factsheets/Baseload_Factsheet.pdf. <

As the table illustrates, wind excels at providing energy, as its fuel source is free. Wind also provides some capacity, typically in a ratio of about one unit of capacity for every two units of average energy output. A wind plant’s exact amount of capacity varies depending on a number of site-specific factors, as well as the time horizon being considered.

Wind plants can also rapidly and precisely reduce their output on command, giving them excellent flexibility for reducing supply. Flexibility to increase power supply is much more difficult for wind plants, as doing so requires holding the plant below its potential output, sacrificing a significant amount of energy that could have been produced for free.

Nuclear and coal plants, conventionally thought of as “baseload” plants, are remarkably similar to wind plants in that they are predominantly energy resources. Like wind, their fuel costs and operating costs are very low. Nuclear and coal plants are capable of providing capacity at a level close to their maximum output. Even so, no power plant can be counted on to reliably provide capacity at its maximum output, as all plants experience mechanical, electrical, or other failures from time to time and must go offline with little notice. For example, nuclear power plants in the southeastern U.S. have been forced to shut down, some for periods of several weeks, because summertime heat waves raised the temperature of the water in the rivers they rely on for cooling their steam generators. Wind energy, by contrast, uses no water.

Coal and nuclear plants have very little flexibility -- it is difficult for them to increase or decrease their output in response to commands from the grid operator. Changing the output of a nuclear or coal plant requires changing the amount of heat traveling through the plant’s steam system. The resulting temperature fluctuations can cause thermal stress to plant equipment, significantly increasing maintenance expenses and causing safety concerns. In fact, because of these safety concerns, Nuclear Regulatory Commission regulations largely prohibit nuclear plants from changing their output.

Natural gas power plants are generally the opposite of nuclear and coal plants, providing significant amounts of flexibility and capacity but very little energy. This is not because natural gas plants are incapable of generating large amounts of energy, but rather due to the fact that gas power plants typically have very high operating costs because, as a fuel, natural gas is generally more expensive than coal.

However, gas plants, particularly combustion turbine (CT) plants, do excel at providing capacity and at changing their output rapidly. Combined-cycle (CC) natural gas plants are more efficient and thus have lower operating costs than combustion turbine plants, but the tradeoff is that they are generally less flexible. Gas plants are also stellar for providing capacity whenever it is needed, with a plant’s capacity value typically many times higher than its average capacity factor. The comparisons in Figure 1 below of what resources provide the U.S. grid’s mix of energy and capacity illustrate how coal and nuclear plants are used predominantly to provide energy, while natural gas plants specialize in providing capacity and flexibility.

Never attribute to malice that which can be adequately explained by stupidity.

o 11:52 am August 6, 2009
o Michael Goggin, American Wind Energy Association wrote:

This article makes the common mistake of confusing capacity factor and capacity value. As the National Renewable Energy Laboratory study cited by the article explains, capacity factor is a measure of the amount of energy produced by a wind plant, while capacity value measures the amount of capacity a plant contributes towards meeting peak electric demand. 30-40% is the typical capacity factor of a wind plant, while 10-20% is the typical capacity value.

Wind plants are being built to provide large amounts of low-cost electricity to reduce the use of expensive fossil fuels and the pollution that results from their use. Folks in Texas can tell you that wind energy has already saved them billions of dollars by reducing the use of expensive natural gas and coal, while also reducing carbon dioxide emissions by millions of tons. Since reducing fossil fuel use and emissions are both issues of energy and not capacity, from that perspective the only metric that matters is the capacity factor of a wind plant. While a capacity factor of 35% may sound low to some, that is actually significantly higher than the capacity factors of other types of power plants. Natural gas plants typically have capacity factors of around 10%, hydroelectric plants are often around 25-30%. Even coal plants typically only have capacity factors that are in the 60-70% range.

For those who would like to delve a little deeper into the difference between energy and capacity, I would suggest this article:
http://www.awea.org/pubs/factsheets/Baseload_Factsheet.pdf