No-one following the growth of the wind turbine industry over the last two decades will have failed to have noticed the trend towards larger and taller turbines. While this might seem to just be the natural order of things [after all, "bigger is better", right?] – it might be a useful exercise to pause for a moment and to determine why this might be so.
First, some data: as the figure below shows, twenty years ago, the largest commercial wind turbines were producing around 0.5 MW of electrical power, with a rotor blade diameter of approximately 40 m [130 feet] and a tower height of a little over 50 m [165 feet]. Ten years ago the largest turbines were producing perhaps 2 MW, the rotor diameter had doubled to around 80 m [260 feet] and the turbine nacelle [the "pod" at the top that contains all the electromechanical components] sat perched on a tower now 100 m [330 feet] tall. If we fast forward to the near-present day, we would see that last year saw the start of production of turbines generating as much as 7 MW of power, with rotor diameters of over 125 m [410 feet] and tower heights in the order of 115 m [380 feet].
I’ll skip the usual comparisons of these lengths and heights to the length of [American] football fields. You get the picture - these are big mechanical structures.
There are some obvious advantages to being able to produce more power per wind turbine installed, mainly related to economies of scale. Installing 10 X 5 MW turbines instead of of 20 X 2.5 MW turbines means half the towers to be installed, half the nacelles to be placed at the top of the towers, and in theory half the crane time required to get the job done.
Before we turn to the guts of the turbine itself though, sitting so high above the ground, let’s look for a moment at the turbine blades and tower. The larger these structures have become, the more difficult it is to put them into place. Have some sympathy for the poor truck drivers who have to drive the vehicles that carry those enormous rotor blades, 60 m [200 feet] long, into some of the less accessible places on the planet. So why go to all that bother? Why not just build a bigger, beefier bunch of machinery at the top of the tower instead?
Well, we can do that, and it is in fact being done [see my next post in this series for some examples]. However, it turns out that the amount of energy that a wind turbine can extract from the wind, is proportional to the total area across which the blades of the turbine will sweep. This means that longer blades will, in theory, enable us to generate greater power. Of course, you can’t have these blades coming perilously close to the ground as they move, so the center of rotation has to be placed further above the ground, in order to be able to accommodate the increase. So that’s one reason why the blades are getting longer and the towers are getting taller.
There is another key reason though, for the increased tower heights. If you have the time and patience to look through the mathematics involved, you’ll see that the energy that our wind turbine can extract from the wind, is proportional to the cube of the speed of the wind that the turbine experiences. This means that, all other things being equal, a doubling of the wind speed, for example, would lead to eight times the amount of energy being extracted at the first wind speed. Wind speeds generally increase as you get further away from the surface of the planet and of course, a taller tower helps us to get there. There are also considerable variations in average wind speed across the globe, and this “cubing effect” helps to explain why seemingly small differences in wind speed from site to another, can have significant ramifications on the viability of a project and its estimated payback period.
A number of recent studies have shown that, setting the technical and logistical challenges aside – and I know that that’s a big aside! – based on average wind speeds at elevated positions, there is enough wind energy potential world wide to provide all of the planet’s energy needs several times over. In the USA alone, a 2005 study showed that there was enough offshore wind energy potential to replace all the conventional power stations in the entire USA!
One final comment: in most stories in the media on renewable energy production, the writer will frequently compare the total power to be produced from a particular installation, to the number of homes that this energy could comfortably power. What’s interesting to me is that this actually depends on where on the planet those houses just happen to be and so needs to be made clear in any given case. For example, a not untypical 5 MW wind turbine produces enough power for the needs of 1,500 average-sized single family homes in the USA. Were we to re-locate the aforementioned wind turbine to the European Union [EU], we would have enough power for 2,500 average-sized EU single family homes. Going even further afield to China, we would see that 5 MW is enough power for 10,000 average single family homes in that country. We can see, therefore, that the impact of a single wind turbine significantly depends on the location and context in which it is being used.
In my next post, the third in this series, I will discuss the evolution of large wind turbine electromechanical machinery, and how permanent magnets are now a crucial element in dealing with the significant mechanical and electrical challenges that come with building these larger turbine systems. If you missed the first post of the series, you can find it here.
Some of the content of these articles is drawn from a paper that I presented at the Magnetics 2008 Conference in Denver, Colorado, titled “Going Green: The Growing Role of Permanent Magnets in Renewable Energy Production and Environmental Protection“. You are welcome to download a copy of the presentation from here.