A great deal has been published about the characteristics of industrial-scale wind power, covering a range of points of view. This paper is a case study of some of the approaches and considerations that can be used in the analysis of such papers.
The subject of this case study is a recently published paper by Charles Komanoff who takes the wind proponent view. In general Komanoff:
- Uses emotive and pejorative language in referring to those of an opposing opinion. On the other hand, those with the same views as his are treated with complimentary descriptions. Both are indicators of a questionable position that warrants closer analysis.
- Does not sufficiently distinguish between very small and larger wind penetrations
- Provides incomplete treatment of wind’s capacity value.
- With respect to backup requirements for wind plants, focuses on the normal operating reserves within a small wind penetration jurisdiction, Pennsylvania, without addressing the extensive and continuous amount of mirroring/shadowing backup required to render wind’s highly volatile output useful in higher wind penetration jurisdictions.
- Appears to depend upon a report by Gross et al, which is itself not convincing. See Appendix A.
- In dealing with the load following capability of nuclear and coal-fired plants, focuses on the ability of these to ramp output up and down without incurring stress and reliability problems in normal operating conditions (often cycling once per day). In the case of coal plants, in addition to greater reliability problems, he does not address the fossil fuel and emissions impact of large and frequent output variations during the day if used as wind shadowing/mirroring backup. In the case of nuclear plants, any ability to perform in this capacity is questionable and ignores the lack of any opportunity for reducing fossil fuel use or CO2 emissions.
At the beginning Komanoff quotes what he wrote a few years ago in a magazine article:
[S]ince wind is variable, individual wind turbines can’t be counted on to produce on demand, so the power grid can’t necessarily retire fossil fuel generators at the same rate as it takes on windmills. The coal- and oil-fired generators will still need to be there, waiting for a windless day. But when the wind blows, those generators can spin down.
The underlying assumption, which appears to be the basis for most of the following arguments, is that when the wind blows it does so steadily, and when it ceases it remains so for long periods. Unfortunately it does not, and this is at the heart of the issue, which can be summarized as follows:
- On days of little or no wind, wind plants make their most useful contribution of any period by allowing the other generation plants to be utilized in their normal mode of operation, which is their most efficient in terms of fossil fuel use and CO2 emissions.
- On days when wind is available, even in moderate amounts, it must be remembered that the electricity output variation of wind turbines is related to the cube of the wind speed, that is any change in wind speed by a factor of two is magnified by eight times (and 3 times the wind speed change produces 27 times the change in electricity output). Significant variations can occur over the period of a fraction of an hour, ranging over the full scale of wind plant capacity, and greater variations can occur over multiple wind plants. As a result wind output must be “mirrored” or “shadowed” by conventional generation means to render it useful.
Wind output volatility is a major factor in the discussion about the effect on the wind mirroring/shadowing backup plant. Even on days of moderate wind, significant, random variations in wind output occur, but the variations are most violent during high wind periods. Because the conventional plants have to mirror wind’s volatility, different plant types are required (e.g. increased use of OCGT plants, which are more responsive but less efficient than CCGT plants) and those involved are forced to operate at a lower level of average production, increasing their costs per megawatt-hour (MWh). As they are being used in a more inefficient mode than normal operations, they consume more fossil fuel and produce more CO2 emissions per MWh. This is like a car in continuous stop/start speed-up/slow-down city traffic as opposed to steady highway driving. This mode of operation can totally offset any reductions claimed in fossil fuel consumption and CO2 emissions as a result of the presence of wind.
Assertions made in this paper will undoubtedly be challenged and this is appropriate. The purpose is not to declare a winner but to illuminate the necessary considerations.
By Kent Hawkins, BSc (EE)
July 6, 2009
Download original document: “Case Study on Methods of Industrial-Scale Wind Power Analysis”
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