A picture from 2011 of a Senvion LWST from a few years ago (a 3.4 MW rated unit with a mere 104 meter rotor diameter placed on a128 meter tall tower) - https://elsa.senvion.com/anon/public_folder. It makes the high voltage/high power transmission tower look small by comparison, and that is probably at leaf 160 feet (50 meters) above the ground…. This company just came out with a 3.4 MW unit with a 140 rotor diameter x 3.4 MW unit, in theory able to tap 81% more air moving through it’s swept rotor area (https://www.senvion.com/global/en/wind-energy-solutions/wind-turbines/3xm/34m140-ebc/) than the 104 meter model. That’s some serious cost of electricity production reduction…. However, based on similar scale ups, only 2/3 of this is likely to be realized, which is still a 54% increase in energy output for an identical wind resource.
Over the last couple of years, Low Wind Speed Turbines have now become THE dominant form of new wind turbines that are being sold, especially outside of China (but even in China they are becoming a big factor in the wind biz). Those installing turbines have done the rather simple math surrounding these - for essentially the same installation costs and cost of the turbine, a LWST versus a “medium” or a “high speed” turbine is a better bargain. For essentially the same price/cost, a LWST makes more electricity over the course of a year. This makes the cost to generate the electricity less. And if the cost of the product (electricity from those wind turbines) is more or less fixed and the cost to make it drops, well, that’s certainly better than the situation where the cost is rising and the price stays the same or the cost is constant and the price is dropping.
For example, consider the 3.4M140 (2015) and the 3.4M104 (2011) turbines. The main difference is that the older unit has a rotor blade that is 50.5 meters long (~ 166 feet) while the new one is is 68 meters long (about 225 feet). Since the mass of these is more or less proportional to the 2.4 power of the length, the new blade weighs about twice as much as the older, smaller one (probably less, as some tricks of the trade have been learned in how to make these blades). If the labor to make these blades is roughly the same as for the smaller ones, but the mass is about twice as much, the cost to make that blade is mostly the cost of epoxy resin and fiberglass cloth.
An estimation of the cost breakdown for a turbine can be seen in this graph (from 2012):
Using a turbine like the 3.4M104 (a bit ahead of its time in 2011), the blades constituted about 20% of the total cost. The new, seriously more humongous turbine would be more costly to make, mostly as a result of having blades weighing around 30 tons each and not 15 tons. This means that the longer bladed turbine would cost about 20% more than the smaller turbine, and that the blades are now about 33% of the cost of the turbine. But, a 20% increase in cost to get potentially an 54% increase in potential energy output. That’s not too shabby…
Another aspect of these longer blade turbines is that a lot more land deemed windy enough now exists. For example, instead of searching wide and far for the windiest spot of land, the places to search for reasonably windy lands near electrical transmission lines. A recent study by the US Dept of Energy shows the amazing increase in land area that is made viable for wind energy via LWST in combination with taller towers… The “possible” windy area jumps from 1.2 million km^2 to 2.7 million km^2 (110 meter tall towers) to 4.2 million km^2, an increase of 225% to up to 350% at a 40% gross capacity factor.
Going to a large rotor diameter almost necessitates taller towers. For example, putting a 70 meter radius blade on an 80 meter tower would mean that the blade at the low point (10 meters above the ground) in its rotation is getting very little energy to tap while at its high point (150 meters above the ground) a lot of wind pressure would push on the end of the blade. The blade would begin to wobble and set up a horrendous vibration - pushed hard at the top and hardly at all at the bottom. It would be far better to have the blade experience at least decent wind speeds for most of its rotation. With a 130 meter tower, the minimum distance to the ground would be 60 meters, while at the top it would be 200 meters (656 ft above the ground),
The wind resource at a given spot is usually evaluated at the center point of the rotor - also called the hub height. For an area with trees and/or some hills/buildings, hub height matters a great deal. This is because there is a logarithmic relationship between the height and the wind speed. For regions characterized by a surface roughness of 1 meter (very typical for NY State), here is a table relating average wind speed to hub height, as well as the cube of the ratio of wind speeds relative to speeds at an 80 and also 100 meter heights.
Hub Height Wind Speed Ratio, WS cubed
80 meters 6.00 m/s 1.00 0.86
100 meters 6.31 m/s 1.16 1.00
120 meters 6.55 m/s 1.30 1.12
130 meters 6.66 m/s 1.37 1.18
140 meters 6.77 m/s 1.43 1.23
150 meters 6.86 m/s 1.49 1.28
Tall towers really go good with large bladed wind turbines. They also go good with local manufacture of the “extra boost” portions of these towers, because most of these are made of reinforced concrete sections that are easily transported on trucks, but due to the weight and their ease of manufacture, making these local makes sense. It turns out that steel is just too flexible - even when 1” thick - when it is in the form of a turbo with a base diameter of around 145 feet (~ 4.2 meters), which is as large as can be transported and still fit under bridges that cross over a highway. At present, the nearest concrete tower plants are in Welland, Ontario and in Quebec (to make towers for Enercon turbines), but these are easy to set up. It’s just that the US has not caught up with what has been common in Europe for more than a decade. But hey, better late than never….
BTW, at 6 m/s the likes of a LWST can put out 35% of its capacity on an average basis - see page 8 of http://nozebra.ipapercms.dk/Vestas/Communication/Productbrochure/2MWbrochure/2MWProductBrochure/. A 6500 MW-hr/yr output is 37% of the rated capacity of a 2 MW unit (100% would be 17532 WM-hr/yr), and that's at a 6 m/s hub height wind speed. Not bad...
These days, LWST are now being used in faster wind locations, too, pushing net yields towards the 50% level and higher. As a result of more electricity made for a slightly larger investment, the costs of electricity production form these newer turbines has dropped very dramatically. After all, the bulk of the cost goes like this:
Cost = Capital Cost * Fixed Charge Factor/(Energy made per year) + O&M
Cranking out more annual energy production makes it less expensive to generate electricity.
For example, let’s say the installed capital cost for a 2 MW turbine is $5 million, and the Fixed Charge Factor is 8%/yr but the net output is 40% and the O&M cost is $10/MW-hr. The cost to make this electricity would be $67/MW-hr. But if that turbine used a taller tower to get to a 50% net output (but it cost an additional $500,000), the cost to make that electricity drops to $60/MW-hr.
In the US, most wind turbines are placed on 80 meter tall steel towers, though in some cases towers in the range of 95 to 100 meters are used. That is the limit of how large conventional steel towers can be, because taller towers require a larger base diameter, and a larger base diameter cannot be transported over roads due to the height of overpasses. Towers taller than 100 meters will have to use some other arrangement.
An easy solution is to place a conventional 80 meter tower on a “non-conventional base” - such as one made of multiple steel panels that can be bolted together, or concrete sections. For example, Vestas now has their “Large Diameter Steel Tower” (LDST) system that allows heights of 119 to 137 meter towers to be utilized. But there are a number of ways to make a usable tall tower for a wind turbine. The trick is to do it at a low cost. After all, the turbine owners are expected to make a profit, and this is still a cut-throat business when methane is being sold for less than the cost to make it.
Anyway, the really tall turbines are not needed everywhere - the usual indication is either the presence of trees or hills or both. But in much of the parts of the world where a lot of people live and where it’s not a desert - such as the eastern part of the US - that applies. It also applies when wind speeds are not that great, and especially where there is a hefty wind shear due to a rough surface (especially because of trees!). In fact, there’s the best rule of thumb - if there are trees present, tall turbines (120 meters and taller) are in order. It’s just what you do in this century, especially if you want to forestall Global Warming or at least make a valiant attempt at living without pollution sourced electricity.
So, for much of the US - here’s a map to ponder. With this, the quest as to how to power up America - at least the electricity portion - with renewable non-pollution based electricity - is done, except for some true believers for whom fax don’t mean much at all. The answer was to how to do it fastest and at the lowest cost is via wind turbines. It would take $2.5 trillion worth (maybe less) to do it, and a decade or less if we just bother to do it And for buffering/short term storage, these turbines mesh nicely with pumped and deferred hydroelectric storage. But then some people want to research this subject to death, while others just want no solution to come about (even though it has). Oh well, read it and weep. Or maybe read it and weep because our country shows no indication of bothering to choose this path - we just won’t be allowed to choose this approach of Low Wind Speed Turbines on Tall Towers.
Oh well, on to Paris 2015 for the Climate Talks, all talk and so little action….
Map from http://apps2.eere.energy.gov/wind/windexchange/windmaps/resource_potential.asp which shows the land area of the US with a 35 % gross capacity factor or better that can be tapped for LWST on 140 meter towers (lower for fast wind areas). Cool, eh?