Modular towers and the quest for stronger wind

Taller towers for wind turbines make sense in many ways. For instance, an 80-m tower can let modern 2 to 3-MW wind turbines produce more power than if  installed at 60 m, and taller towers will let larger turbines enter the market. Taller towers also allow putting turbines in less turbulent winds, thereby decreasing their wear and fatigue.

For many turbine manufacturers, however, towers are an afterthought and have been a relatively low-tech part of wind turbines. But as designers configure taller conventional towers, their limitations become more obvious. Tower designs are now producing an:

• Increased interest in reducing their cost. The tower-cost portion of the overall wind
turbine is moving up from 10% towards 20% of system cost.
• Increased attention to cutting tower transportation costs
• Increased attention to the interaction between tower and turbine
• A focus on reducing weight
• Higher steel prices over the past two years have further forced OEM’s to focus on
tower optimization.
• Tower manufactures have not focused enough on the wind industry because it has
been considered cyclical, making dedicated plants too risky. As a consequence, few
tower manufactures can handle the volume and size demands of today’s wind turbine
towers.
• Larger machines will need taller towers.

A mathematical rational
The Midwest, which is experiencing a build-out of wind turbines, typically has higher wind-shear values than the 0.14 rule-of-thumb used elsewhere. In many places, especially the upper Midwest, shear values are around 0.3 and are conservatively sustained from 0.22 to 0.26. Using an average 7 m/s wind speed at 65 m and a shear value of 0.24, turbine output from a typical 2.5 MW machine installed at 80 m reaches 8.2 million kWh per year. Increasing the height to 100 m lets the turbine produce close to 1 million kWh per year more. With a Power Purchase Agreement of $0.06 per kWh, a 20-m-taller tower produces an additional annual value of $60,000. This should give developers one way to justify additional spending.

However, conventional 100-m towers in the U.S. are relatively costly and in many instances they nearly double the cost of an 80-m tower. That does not include increased transportation costs associated with large tower sections. Simply increasing the height of a conventional welded tubular tower may not be the most cost effective way to reach the greater wind speeds.

An alternative tower design will be needed. Modular designs of 100 m provide one solution to the cost problem. It will be available as an option to wind-farm developers over the next few years.

A closer look
Towers with a continuous taper or an increasing taper are the most efficient way to handle wind-turbine loads. The design from my Northstar Wind Towers, for example, uses field-assembled panels to eliminate transportation restrictions. The design allows adding tower panels to increase tower diameter and height. Increased diameters allow for thinner tower wall thicknesses, resulting in a more efficient use of steel, thus lowering weight and cost. Flanges at the tower top and base allow for a conventional interface with the turbine and foundation. Flanges use the same mounting criteria as conventional towers. However, the increased bottom diameter presents new options when designing the foundation. For instance, wider foundations require less depth, thereby eliminating the need for costly embedment rings often used in conventional foundations.

Conventional towers also have a diameter up to only 4.2 meters. Thus, there is a limited amount of area under the flange to conduct the load to the foundation. With increased loads from the turbine and a static diameter, it is necessary to distribute the load over a larger area. This is done with a case embedment piece that goes a few meters into the foundation and sticks up over it. The piece helps distribute a conventional tower’s load over a larger surface area inside the foundation, but also adds about $40,000 more to each foundation. Such additions are not incurred in modular towers because they distribute load over a larger base-flange area. The same load over a larger area produces lower loading (psi).
Modular towers must be assembled before erection. Our design uses slip-critical or “friction” connections for site assembly, a method used in many wind-turbine towers. These tried-and-true friction connections are widely used in bridges and high-rise buildings where post inspection is limited.

In addition to reducing manufacturing time and cost, an added benefit of friction connections is that tension tools need no longer be calibrated (sometimes an hourly task) because the method is not based on torque which further reduces risk related to installation error while minimizing preassembly work.

An advantage of a friction connection is that tensioning relies on a turn-of-the-nut system and not torque. This enables a much more precise bolt tensioning. Flanges in conventional towers use larger bolts that rely on a specific torque. This means that the torque “guns” have to be calibrated a couple of times a day, especially in changing weather and pressure conditions. Panel construction, combined with bolted connections also reduces the amount of welding to 10% to 15% of that required by a conventional tower. The 100+ meter towers will have a similar appearance to conventional tubular towers that have been in use for the last two decades.

The Northstar tower is roll formed and painted in a way similar to today’s tubular towers. The design’s modularity and smaller size of individual components allows using state-of-the-art and automated painting equipment.

Increasing demands on shipping companies that can haul large traditional towers have placed a growth constraint on the industry. Modular towers, however, ship on standard trailers. Lower tower sections bolt together in the field. Top sections ship preassembled and require no additional field attention. The average to-site transportation cost of a modular tower is 65% to 75% less than a comparable tubular tower shipped the same distance. With primary manufacturing locations planned in the Midwest, Southwest, Pacific Northwest and Atlantic Northeast regions, transportation distances to most North American installations will be cut significantly.

What’s possible
The tallest tower the company has designed is a 120 m version for a 2+MW machine. A preliminary design is done for a 140-m version that will hold a 4+MW machine with a 140-m rotor diameter. The practical limit at the moment is the availability of cranes capable of lifting turbines to these heights. One study suggests the modular design capable of getting a turbine to over 165 meters.

Why Taller Modular Towers Make Sense

A modular turbine tower is one way to get around the shortcomings of conventional solid section designs.

Peder M. Hansen

Executive Vice President

Northstar Wind Towers

Blair, Nebraska

Taller towers for wind turbines make sense in so many ways. For instance, an 80 m tower can let modern 2 to 3-MW wind turbines produce more power than at 60 m and taller towers will let larger turbines enter the market. Taller towers also allow putting turbines in less turbulent winds, thereby decreasing their wear/fatigue.

True, there are added costs associated with taller towers, and decisions makers will have to consider the gains from the new-generation towers against the increased power production and power price.

A few figures

The Midwest, which is experiencing a tremendous build-out of wind turbines, typically has higher wind-shear values than the 0.14 rule-of-thumb used elsewhere. In many places, especially the upper Midwest, shear values are around 0.3 and are conservatively sustained from 0.22 to 0.26. Using an average 7m/s wind speed at 65 m and a shear value of 0.24, turbine output from a typical 2.5 MW machine installed at 80 m reaches 8.2 million kW/hr per year. Increasing the height to 100 m lets the turbine produce close to 1 million kW/hr per year more. With a Power Purchase Agreement of $0.06 per kW/h, a 20-m-taller tower has an additional annual value of $60,000. This may not sound like much, but it gives developers one way to justify additional spending.

Conventional 100-m towers in the U.S. are relatively costly and in many instances they nearly double the cost of an 80-m tower. And that does not include soaring transportation costs associated with large tower sections. Simply increasing the height of a conventional round, welded tubular tower may not be the most cost effective way to reach the greater wind speeds.

Turbine manufacturers continue to evaluate alternative towers. The design will be available for purchase as an option by developers over the next few years.

How it’s done

Towers with a continuous taper or an increasing taper are the most efficient way to handle wind-turbine loads. The design from my firm uses field assembled panels to eliminate transportation restrictions. The design allows adding tower panels to increase tower diameter and height. Increased diameters allow for thinner tower wall thicknesses, resulting in a more efficient use of steel, thus lowering weight and cost. Flanges at the tower top and base allow for a conventional interface with the turbine and foundation.            Flanges use the same mounting criteria as conventional towers. However, the increased bottom diameter creates new options for the foundation design which makes it possible to construct wider foundations resulting in less depth and thereby eliminating the need for costly embedment rings often used in conventional foundations.

Modular towers must be assembled before erection. Our design uses slip-critical or “friction” connections for site assembly, a method used in many wind-turbine towers. These tried-and-true friction connections are widely used in bridges and high-rise buildings where post inspection is limited. Panel construction, combined with bolted connections also reduces the amount of welding needed to 10% to 15% of that required by a conventional tower.    The 100+ meter towers will have a similar appearance to conventional tubular towers that have been used for the last two decades.

In addition to reducing manufacturing time and cost, an added benefit of friction connections is that tension tools need no longer be calibrated as the method is not based on torque which further reduces risk related to installation error while minimizing preassembly work.

The Northstar tower is roll formed and painted similar to today’s tubular towers, only on a modular scale. The design’s modularity and relatively smaller size of individual components allows using painting equipment that is in-line, automated, and state-of-the-art.

Increasing demands on shipping companies that can haul large traditional towers have placed a growth constraint on the industry. Modular towers, however, ship on standard trailers. Lower tower sections bolted together in the field. Top sections are shipped preassembled and require no additional field attention. The average to-site transportation cost of a Northstar tower is 65% to 75% less than a comparable tubular tower shipped the same distance. With primary manufacturing locations planned in the Midwest, Southwest, Pacific Northwest and Atlantic Northeast regions, transportation distances to most North American installations will be cut significantly. Taller towers are a great benefit to developers and owners of wind power plants and will let the industry continue its growth, in turbine size and total MW deployed.

A few more production figures

The additional generation capacity from a 2.5-MW turbine at 100 m versus 80 m can average 1 million kW/hr per year more, adding $60,000/year per turbine, in revenue. This equates to a payback in less than a five years, or an additional 20 million kW/hr potential over the life of the turbine. This example assumes a total value of $1.2 million in additional revenue per turbine or $48 million additional revenue over 20 years for a 100 MW wind-power plant.