Towers

“The Higher the tower the greater the power” -The Parris-Dunn wind machine company from 1937

Selecting the right tower for your location is probably the most important decision you will make after choosing your wind turbine… and the most important consideration is tower height. As a general rule, a turbine’s output increases substantially with the height of its tower. Also, if a tower is too low, turbulence from surrounding obstructions will decrease power production considerably.

The above image shows how obstructions can have an effect well downstream of a prevailing wind.   The effect of the building extends to a distance of 20 times the height of the building and to an elevation of 2 times the height of the building.  So, for instance, a 30 foot high building could create turbulence up to 60 feet high and for a distance of 600 feet from the building.  A few general rules for siting a turbine in prevailing winds are:

  1. A turbine upwind of an obstruction should be sited at least twice the distance of the obstruction’s height.
  2. A turbine downwind of an obstruction should be sited at least 10 times the distance of the obstruction’s height or more depending on tower height.
  3. Minimum turbine height should be twice the height of obstructions upwind.

Another general rule is simpler – the bottom of the blades should be at least 30 ft higher than any obstructions within 300 ft. Therefore, if you have a 40 foot building nearby, and your turbine has a blade diameter of 20 ft, your MINIMUM tower height should be 80 ft.  40 + 30 + (20/2) = 80

Or even more simple – get the tallest tower your budget allows and that makes economical sense. According to Mick Sagrillo, one of the foremost experts in small wind technology,  the most common mistakes people make with wind energy are using

  1. too  short a tower,
  2. too short a tower, and
  3. too short a tower.

One last thing to consider regarding the height of obstructions like trees – over the 30-50 year life cycle of a wind system, trees will grow, the tower won’t!

Power = 0.5 x Swept Area x Air Density x Velocity3

Ouch, that looks technical! Basically, for our purpose here, it states that the power produced from a wind turbine increases with the cube (power of 3) of the velocity, or speed of the wind. Therefore, a change in wind speed from 8 mph to 10 mph will actually increase the power in the wind by 100%!

We know that wind speed and power increase with height, but how high should you go to get the biggest BANG for your buck? Looking at the figure to the right, a turbine will generate 100% more power on a 100 ft tower as the same turbine on a 60 ft. tower. Go to 120 ft and you’ll get 150% more power!

For our typical rural customers in Central and North Texas, the best investment scenario is a 100 or 120ft tower for a 10kW turbine…. in West Texas, in most cases an 80 ft tower will do just fine.

Towers fall into two categories: freestanding, or self-supporting and guyed.

Freestanding, or self-supporting towers are just that – freestanding. The classic windmill and the large commercial turbines of today as in this picture are both classified as freestanding.

Guyed towers in contrast employ several far-flung anchors and support cables to achieve the same result, and are by far, the most common choice for turbines sized 10kW and below.

There are advantages and disadvantages to both guyed and self-supporting towers – mainly costs.

Guyed towers include a mast, guy cables and earth anchors and offer the best compromise between strength, cost, ease of installation and appearance. The only real disadvantage is that they take up more space as you can see in the photo on the right.

To give you some idea as to the space required, the distance from the tower to the earth anchors to which the cables are attached, shouldn’t be less than half the height of the tower.  For example, Bergey recommends a ‘guy radius’ of 50 ft for its 84 ft tower.

In the self-supporting category, there are two main types of towers; the most common for small wind is the truss or lattice tower, so called because it resembles the latticework of a trellis. The other is a tubular tower, or monopole, which you see almost exclusively on medium sized (100kW – 1MW), and larger turbines.

Lattice towers have 3 legs with diagonal bracing. One advantage of freestanding towers is that they take up a lot less room than guyed towers.  For a turbine the size of the Bergey Excel 10 kW, a typical foundation for a 100 ft tower is a concrete block about 4′ deep and 16’ square.  The tower base itself is about 12’ across.  For some soil conditions it is preferable and less costly to use a pier type foundation.

Tubular towers are generally considered to be more aesthetically pleasing than lattice towers, and this is especially true when viewed in the foreground. But from a distance, lattice towers are less ‘visible’ as you tend to see through them, especially in silhouette. The main drawback for using tubular towers for for small wind is cost – they are significantly more expensive than guyed and lattice towers, reducing the customer’s return on investment.

 

Bergey 10kW on ARE Monopole

However, there is good news on the tubular towers front as new entrants to the rapidly growing market for small wind systems, design, engineer and manufacturer innovative tower raising and foundation systems. One of these companies is American Resource & Energy out of Minneapolis, MN. The company is revolutionizing the industry with their motorized and manual tower raising systems and ‘mobile’ foundation.

 

 

It may come as a surprise that the turbine itself generally represents about 50% or less, of the total installed cost of a wind system (assuming a 10kW turbine on a 100’ self-supporting lattice tower). Most of the balance is spread over the tower, foundation, accompanying electronics, wiring, crane hire and labor. And the biggest chunk of this will be on the tower.

The foundation for the tower is the next costliest piece of a wind system, and can vary greatly depending on your choice of tower and the foundation required based on the soil composition or topography. Get Natural Energy Wind & Solar will advise on the range of options based on your preference and requirements – in some cases a soil sample will be sent to the tower manufacture for analysis.

Get Natural Energy Wind & Solar has access to a wide range of towers, including manufacturing our own in some cases, and is experienced in a number of foundation technologies.

After reading all the information on this page, the main take-away should be this:

Higher is better, no two installations are the same, and we’ll give you expert advice on your unique circumstances.