Sunday, March 16, 2014

Towers of Power - Getting High Means Good Things

The US 2010 National Wind Map for 100 meters above the surface (land and water), from http://energy.gov/eere/wind/wind-resource-assessment-and-characterization and http://www1.eere.energy.gov/wind/pdfs/wind_speed_map_hi-res.pdf. Thanks to recent innovations in Low Wind Speed Turbines (LWST), any region with an average hub height wind speed of 6 m/s or greater now has commercial potential (though faster winds tend to give lower costs of electricity production). On this map, that means purple and blue is best, though brown-orange-pink are also decent prospects. About one third of the continental US now has decent prospects for electricity production via wind, including the huge "Patch of Awesome" stretching from Montana-North Dakota-Minnesota to Texas and New Mexico, which alone could power up the country AT LEAST 12 times over. Also, notice that the US portions of the Great Lakes are also mostly "in the purple" or better. And it turns out that a lot of the least windy part of the U.S. gets "decent" when heights in the 120 to 150 meter range are utilized.

There has been a lot of activity of late and a lot of news articles/corporate announcements with respect to an important trend in the wind turbine business - getting more power out of a given wind turbine at any given location. The trick is to "get high" but to do so in an economical manner.

There, did that get your attention…..

The trick to this is embodied in a bit of math. In general, wind speeds increase gradually and in a non-linear manner the further away from the ground, water or ice surface one goes. The simple way this is described is by the "power law", and the more mathematically correct way is via the "logarithmic wind shear profile". But they both come to a similar result, so let's go with the easy way. In the power law, the wind speed (u2) at a new height (h2) can be related to a known wind speed (u1) at a given height (h1) in the following way:

u2 = u1 * (h2/h1)^a (Eqn 1)

where a is the power law exponent. The value of a is related to the nature of the surface (a smooth surface like ice has a value of 0.1 or lower, while a rough surface (buildings, tall trees, hilly and uneven ground) can have a value of 0.25 to more. For a lot of the Great Plains, a has a value near 0.14, and it is often called the "one-seventh power law".

It turns out the the power that can be produced by the wind is proportional to the CUBE of the wind speed as long as moderate wind speeds are considered. If P1 is the power produced by a turbine at a wind speed u1, and P2 is the power made by a turbine at wind speed u2, then Eqn 1 can be arranged in this way:

P2 = P1 * (h2/h1)^3a          (Eqn 2)

If a has a value typical in WNY of 0.2, then the power obtainable with a taller tower is equal to the hight ratio raised to the 0.6 power (about the square root of the height ratio). But, since that probably does not mean much, consider a tower of 80 meters (the US standard these days) and one of 120 meters. In an area with a 0.2 power exponent, the same wind turbine nacelle/blade assembly could turn out 1.28 times as much power, and thus energy over the course of a year. And a 149 meter tower (routinely installed by Enercon for its wind turbines in Europe) would give rise to 1.45 times as much power production/energy output per year at a given site. For moderate wind speed regions (ESPECIALLY Western NY), this could be even greater, for a variety of reasons.

Of course, taller towers that work are more expensive to make and install than shorter ones. Taller towers also allow for bigger rotor diameters, and that also both makes more power (energy produced is also proportional to the SQUARE of the rotor diameter) but also costs more. Taller towers also have to be "tuned" to the turbine - they require greater stiffness, and since all the weight of the tower also has to be supported on the lower sections, the lower sections can have very thick walls. That means more metal for steel towers, which costs more money. And if you are thinking that this can get really complicated, you would be correct. Oh, and all this has to be done in a way to maximize the profit potential/minimize the production cost, too. The tower parts also have to be transportable, and readily installable with cranes that can be rented and which often are prices at $250,000 per month (even if it is only used for a day of actual parts lifting). Under optimal weather conditions, it only takes one day to install the tower sections, nacelle on top of the tower and the 3 blades to the tower, though finding optimal weather in a windy are is not always done without a lot of waiting…

In the last month, Vestas, GE and Siemens have all come out with new models of towers that allow their units to reach the faster winds present farther above the ground. The new markets being tapped are often forested regions in Germany and Scandanavia, and they are chasing after Enercon, a pioneer in tall tower design, manufacture and implementation. They all also DIFFERENT approaches, and also different from "hybrids" composed of a lower concrete section and an upper steel one. So, looks like there is competition in the tower biz, and some might work better than others depending on the region, wind regime and roads. For the next generation of >100 meter rotor diameters, steel towers are really only useful to around the 100 meter height, and after that, alternatives are needed. However, other companies - Gamesa, Acconia, Alstom, Nordex  and Senvion for example - also are promoting tall towers for their LWST.

Here is a summary of the basics:
1. Hybrids - prime users of these are Nordex and RE Power (now Senvion), who get tower heights in the 120 to 140 meter range via placing a "standard" steel tower (60 to 80 meter) on a lower reinforced concrete section. Here is an example of a Nordex 2.4 MW (57 meter long blades) unit on a 141 meter tower - the steel section is around 60 meters. This wind turbine is located in a heavily forested area (Bavaria?), and it reaches up to 200 meters (656 feet) when a blade is pointed straight up. This overcomes the large roughness length/power law exponent which happens when wind tries to pass over the trees in the forest.



2. All concrete towers
Enercon has been a leader in this area for over a decade. Their flagship E-126 x 7.5 MW turbine is SO BIG and SO HEAVY that a steel tower is both too flexible and too expensive (too many tons) to bother with - the nacelle alone weighs around 500 tons due to its huge annular generator (20 meter diameter, or about 66 feet). Most of the lower sections are delivered to the site in pieces (for example, three parts are made into a ring which is then lifted into place). The upper sections are delivered as entire rings about 4 meters tall. They built one of the largest factories in Europe to mass produce these "puzzles" - it has 300,000 square meters of factory floor space (about 1.3 million square feet). They are building a concrete tower factory in Welland, Ontario for the 77 turbines of the Niagara Region wind farm which should start going up this year. That project will use 3 MW turbines and tower heights between 124 to 135 meters and may be as large as 14.5 meters (47 feet) wide at their base. And they come with elevators for service personel. They may even be visible from the US….



3. Three sectioned base section
Vestas recently announced it has successfully its Large Diameter Steel Towers (LDST - see http://vestas.com/en/media/~/media/92670482644d4e5bb751ff6bd6f66a43.ashx). This consists of 3 steel sections that get bolted together on site to form the large diameter base. These sections can be easily transported by trucks and thus pass the "4.2 meter height barrier" and thus allow these to pass under most overpasses. The new wide base also allows thinner steel to be used, but because of the wider base diameter it is actually stiffer and lighter weight than conventional towers. The first one was installed in Germany last year. They will allow for 137 meter tall towers (three sets of these bases, each about 47 feet long, plus a conventional 94 meter "pipe style" tower…


4. GE just announced a really ingenious five sided lattice tower (space frame) that will allow a GE 2.75 MW turbine nacelle to sit 139 meters above the ground. While lattice towers are often used in radio and TV towers (light weight, easy to transport the parts, assembled on site), they have a problem due to the fact that birds like to perch on them, they are expensive to assemble and then there is the corrosion issue (they are exposed to the elements, and rust can set in). GE has solved the bird and weather problem by placing a weather resistant PVC coated polyester fabric around the tower. Due to the wide diameter base, the step up transformer and a high capacity battery can be located on the ground - the battery (50 kw-hr) gets rid of the need to tap the grid when the electromagnets in the generator need to get powered up (the generator cannot generate power unless its electromagnets are magnetized), and which is a serious pain on electrical grids ("current in-rush"). See http://www.greentechmedia.com/articles/read/Is-GEs-Space-Frame-Wind-Turbine-Tower-The-Future-of-Wind-Power


5. Lastly, there is Siemens Wind Power's entry in this field, which is a multi-segmented lower tower section (see http://www.energy.siemens.com/hq/en/renewable-energy/wind-power/wind-turbine-technology/tower.htm and http://www.turek2013.info/pdf/sunumlar/Bilgihan_Yasacan.pdf (pg 20-23)). The bottom section contains 14 segments

and 


As for those who like a touch of math, the correct expression for the variation of height with wind is:

R = U2/U1 = (Ln (h2/z0))/(Ln (h1/z0)) (Eqn 3)

where R is the ratio of wind speeds U2 and U1 at heights h2 and h1, while z0 is the ROUGHNESS LENGTH (in theory, the height above the ground where wind speeds are zero - but that is THEORY). The bigger the value of z0 the greater the resistance to flowing air because the surface is rougher.

You can calculate the value of a if you know R, h2 and h1 and also z0

a = (Ln R)/(Ln (h2/h1)            (Eqn 4)

z0 = Exp[(Ln ((h1^R)/h2))/(R-1)] (Eqn 5)

When a = 0.2, the value of z0 is around 0.658 meters, which is still considered relatively smooth surface. For a forest region, the value of a would be near 0.25, which corresponds to a roughness length near 1.79 meters. A turbine should be able to kick out 36% more electricity at 120 meters than it can at 80 meters for tower height.  And one at 150 meters of tower height, power output goes up by around 60% versus an 80 meter tower. And BTW, NY State is MOSTLY forested……

For a great explanation of what taller towers mean for forested and hilly lands, see http://windpower.org/download/1266/Vindforsk_Project.pdf

Anyway, if you want to make the world a better place and "take a bite out of crime" (let's assume that allowing Global Warming to manifest itself via CO2 pollution is a crime) and also take bite out of natural gas profits (assuming there are any these days), tall towers for wind turbines is one way to do it. Using tall towers means you only need 62% of the wind turbines that would otherwise be needed to completely power up NY State. And since it is Fukushima Day coming up, we could close down our six disasters in waiting in NY State that much faster (including the Ginna plant, which is a clone of Fukushima 1, the first 500 MW nuke to bite the big one three years ago. 

See, better living through wind turbines is quite possible….

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