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| August 2010 |
Insight/Opinion |
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Wind farming – Understanding the wind baseload portfolio effect
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 Wind farms in the Netherlands. Credit J. Limson |
While wind farms offer the promise of accessible technology for addressing alternative energy needs, one of the first questions is invariably “what happens if the wind does not blow”. In this article we examine how a portfolio of wind farms, spread over a country the size of South Africa can answer this question.
All wind turbines operate on a principle of converting wind energy into electrical energy by rotating a turbine and spinning electrical generation equipment. Different turbines are designed to handle different wind speeds, but generally a turbine will perform best in a reasonably brisk wind. Placing turbines in a place where there is a high likelihood of wind naturally maximises electrical output.
A wind turbine has a rated capacity – for instance a 3MW wind turbine will be a turbine which can, at a specific optimum wind speed produce 3MW of electricity. At lower wind speeds it will produce less than this output. If you take an average of the output of a wind turbine over a time period, such as a year, that gives you the percentage yield achieved. In an ideal situation, wind turbines will exceed a yield of 30% of their rated capacity, although in many cases developers will settle for sites which give a yield as low as 22-25%.
If one looks at a fuelled power station, it can be allowed to produce a constant output of power. Hence a 1500MW coal, nuclear or gas powered plant will produce a constant output of 1500MW – this can be fed into a grid, and, because it is quite predictable that there will be 1500MW of power, one can trust this as a base load – the basic guaranteed power available on the grid. Because a single wind farm has a less predictable output, a single wind farm cannot produce a base load.
If however, as is suggested for South Africa, a series of wind farms is spread across the country, the likelihood of a specific base load of power being produced can be increased to the point where a wind base load, produced by a portfolio of wind farms becomes guaranteed.
Wind speeds shift across the country, and from day to day, as is clear in the weather forecast below. Consider for example the effects of a cold front moving through the country from Cape Town in the west to Port Alfred in the east. As it moves it alters wind patterns along the coast, meaning that some wind farms would perform at 100% and some at lower percentages. Accessing wind energy then from a diverse portfolio makes sense. Using forecasting techniques, the accuracy of the predicted available power improves as well. The South African Wind Energy Association has calculated that if 30 000MW (approximately 10 000 turbines) of windfarms were installed in a geographically dispersed fashion across South Africa that these would be able to produce a guaranteed minimum output of 7000MW of electricity. This is nearly 1 and two thirds the output of the new ESKOM coal fired Medupi powerplant.
In the interests of developing a sustainable energy sector for South Africa a diverse portfolio of wind farms spread strategically across the country could mean that a wind base load can be realised.
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