Power Variability

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The short, medium and long term variations in wind speed shown in Figure 2.3 affect power system operation in different ways. This is sufficiently important, given the significant and growing proportion of wind capacity in a number of power systems, to merit more detailed discussion.

Variability from Second to Second

At low/medium wind speeds the electrical output from a single wind turbine could vary substantially. In Chapter 5 it will be shown that this may have a detrimental effect on the power system. However, when wind turbines are clustered in wind farms, there is physical spacing between them and the turbulence seen by each wind turbine is different and to a great extent uncorrelated. The electrical output from wind farms therefore exhibits substantially lower relative variability than that from a single wind turbine. At the planning stage, appropriate analytical studies are carried out to ensure that the variability expected from a wind farm at a particular site will not adversely affect the power system.

Variability from Minute to Minute

Figures 2.3 and 2.4 indicate that the character of wind is such that if the second to second turbulence is removed, the average wind speed from 10 minute period to 10 minute period remains effectively constant. In Chapter 3 will be shown that this 'persistence' nature of such averaged wind speeds is particularly important in integrating wind generated electricity in power networks. In practice the output of turbines can be regarded as uncorrelated on the timescale of minutes and as with the faster variations the affect of aggregation is to smooth out variations at these higher frequencies.

Variability from Hour to Hour and from Day to Day

Figure 2.9 shows the actual records of wind speed at 13 geographically dispersed wind sites in the UK. As expected from Figure 2.4, there is substantial variability at each location over

Site Specific and Average Hourly Wind Speed - UK

Surface wind speed, 72 hours, 50 or more records per hour (average trace), ms 1

Site Specific and Average Hourly Wind Speed - UK

Surface wind speed, 72 hours, 50 or more records per hour (average trace), ms 1

Figure 2.9 UK site specific and average hourly wind speed over 72 hours, with 50 or more records per hour. (Reproduced from Sinden, G.E., 2007, DPhil Thesis with permission of Environmental Change Institute, Oxford University Centre for the Environment)

1.00

0.90

0.10

0.00

0.70

0.10

0.70

200 400 600 800 1000 1200

Distance Between Recording Sites-km

0.00

200 400 600 800 1000 1200

Distance Between Recording Sites-km

Figure 2.10 UK onshore wind power correlation by distance between sites based on UK long term averages. (Reproduced from Sinden, G.E., 2007, DPhil Thesis with permission of Environmental Change Institute, Oxford University Centre for the Environment)

the 72 hour recorded period. Variability now becomes significant and effective integration of wind power in an electrical power network would have been problematic if the total resource were to fluctuate in sympathy with the fluctuations in one site. Fortunately this is not the case due to the geographic diversity of the sites. The benefits of geographic diversity are clearly illustrated by the bold line in Figure 2.9, which represents the average wind speed from all sites. This average is substantially smoother (i.e. exhibits much reduced variability) than the wind at the individual wind farm sites.

With large scale exploitation of the wind resource, wind farms are installed inevitably across a variety of geographically dispersed sites. As suggested above, this has a major beneficial effect in terms of overall variability. Just as the output from a wind farm has less short term variability than a single wind turbine due to their dispersion across the site, so the aggregate output from several geographically dispersed wind farms has less longer term variability than the output from a single wind farm. This reflects the fact that distant localities experience variations in wind due to shifting weather patterns that are time shifted in relation to one another, and also to an extent distinct. Figure 2.10 presents the correlation between pairs of onshore wind sites in the UK as a function of the distance between the sites, and demonstrates that sites very far apart exhibit low cross-correlation. The data from Reference [11] were recorded over a period of 15-20 years.

Seasonal Variability

Seasonal and monthly average wind speeds vary significantly over most of the world. Figure 2.11 shows the seasonal changes of monthly averaged wind speeds for Billings, Montana in the USA. The trend of higher wind speeds during winter compared to summer is typical of the Northern hemisphere. The figure also indicates that there is variability from year to year.

18.5 year average

J F M A M J JASOND Month

J F M A M J JASOND Month

Figure 2.11 Seasonal changes of monthly average wind speeds. (Reproduced from Reference [10] with permission of John Wiley & Sons, Inc.)

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Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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