WIND ENERGY SYSTEMS - Cd3wd

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Chapter 5—Electrical Network 5–46 60 percent in 1953. It can also be seen that the effective capacity is slightly larger than the capacity factor for two years and smaller for the third year. The particular year of wind data thus makes a substantial impact on the results. The representiveness of the year or years selected must therefore be considered before any firm conclusions are drawn. The theoretical output of a MOD-0 located at Dodge City was determined for the years 1948 through 1973 in a attempt to determine the year-to-year variability[6]. The monthly energy production of each m 2 of turbine swept area is shown in Table 5.2 for each of the years in question, as well as the mean and standard deviation for all 26 years. The yearly standard deviation of 39.97 kWh/m 2 was computed from the yearly means rather than the sum of the monthly standard deviations. It may be seen that 1950 was two standard deviations below the mean. The only year worse than 1950 in this 26 year period was 1961 with 349.6 kWh/m 2 .The year 1952 is close to the 26 year mean, but the summer months are somewhat above the mean, which tends to improve the effective capacity in this summer peaking utility. The year 1953 is about one standard deviation above the mean. The only year better than 1953 is 1964 with 518.8 kWh/m 2 . We see that the three years selected cover the range of possible performance rather well. As more and better wind data become available, statistical limits can be defined with greater precision and confidence. In the meantime, for Dodge City winds, Kansas Gas and Electric loads, and a model 1500 kW wind turbine with 5 percent penetration, a reasonable estimate for capacity factor is a value between 35 and 50 percent. The corresponding estimate for effective capacity is between 30 and 55 percent of rated capacity. Other wind regimes and other load patterns may lead to substantially different results, of course. The effective capacity of the wind plant tends to saturate as more and more of the utility generation system consists of this intermittent random power source. The study assumes that the wind is the same over the entire utility area (no wind diversity) so all the wind generators tend to act as a single generator. The utility needs to have enough generation to cover the loss of any one generator, so penetration levels above 15 or 20 percent of total system capacity would not have much effective capacity. Large amounts of storage would be required to maintain system reliability at higher penetration levels. Figure 25 shows the variation in effective capacity with penetration for the four cases mentioned earlier. There is a wide variation, as might be expected, with Kansas Gas and Electric being the best and the Columbia River Gorge site of the Northwest Power Pool being the worst. The capacity factors for these two cases were almost identical, or about 45 percent for the wind years chosen. The reason for the low effective capacity at the Gorge site is that the bulk of the annual risk R a is obtained from just a few days in the Northwest Power Pool, and the wind did not blow on those days in the particular wind year selected. In a system like the Northwest Power Pool, wind diversity would be expected to improve the effective capacity, perhaps a significant amount. One test case showed that the combined output of wind turbines at both the Gorge and coast sites had a higher effective capacity than either site by itself. More computer studies are needed to determine the actual advantages of diversity. Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
Chapter 5—Electrical Network 5–47 Table 5.2 Theoretical Energy Production of MOD-0 Located at Dodge City (kWh/m 2 ) Standard 1950 1952 1953 Mean Deviation Jan. 31.6 34.1 40.2 36.3 3.85 Feb. 29.5 35.0 41.8 37.5 3.60 Mar. 38.5 41.0 42.8 41.9 3.79 Apr. 36.0 39.4 43.1 42.3 4.41 May 37.4 33.9 44.3 40.3 4.50 Jun. 38.9 43.5 50.5 39.7 6.72 Jul. 26.8 41.5 38.2 35.3 6.40 Aug. 22.9 36.4 37.3 34.3 6.55 Sep. 24.3 37.1 37.2 37.2 4.58 Oct. 31.3 31.1 41.0 36.7 4.57 Nov. 28.2 36.4 37.9 35.1 5.39 Dec. 22.2 30.5 39.7 37.8 6.27 367.7 440.1 493.8 454.4 39.97 It would appear from the limited data that effective capacities of wind turbines may vary from 10 to 60 percent for initial penetrations and from 5 to 45 percent at 5 percent penetration. Hopefully, wind diversity will raise the lower limit to at least 15 or 20 percent. We conclude, therefore, that wind turbines do have an effective capacity and that any complete cost analysis should include a capacity credit for the wind machines as well as a fuel savings credit. Figure 25: Wind power plant effective capacity versus penetration. The actual capacity displaced, D c , depends on the effective capacities of both the wind Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
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