Going higher is not the problem, but more height demands thicker steel walls in the base section and wider base diameters. As result, tower sections are getting heavier, larger and more difficult and costly to transport because special permits and equipment are needed. Northstar intends to break this cycle through technology advances drawing partly from older wind technology. The basic concept looks similar to today's standard tubular towers. But most of the sections are not one-piece tubes but rather modular pieces of curved steel plate that are bolted together on the project site. Unlike the internal horizontal bolting flanges common to today's tubular towers, these units have special vertical flanges that bolt the sections together from the inside and spread the load through the whole tower, not just passing it through the bolts as is the case with horizontal flanges.
Hansen, who is the grandson of the founder of Denmark's Vestas, says these flanges are common in bridges and were common in the wind industry decades ago, including use by wind turbine makers NEG Micon (now part of Vestas), Japan's Mitsubishi, and Germany's Tacke (now part of the GE wind business). "Those machines used these connections and none of them have ever failed a bolt and never been retightened," says Hansen. Micon's 750 kW units with 65 metre towers were the last to use the flange design because it became cumbersome to have someone holding a nut on the outside of the tower while a bolt was tensioned from the inside. Northstar's intellectual property is a new design and method of holding the outside nut during tensioning so everything is put together from inside the tower, minimising worker exposure to open heights and weather. "We're revitalising this way of putting towers together. It's not revolutionary, it's the next evolutionary step to enable this continuation of size in the wind business," Hansen says.
The result is a tower that uses 10-12% less steel and which costs about $35/mile to transport using standard flatbed trucks instead of $128/mile for conventional oversize transport, he adds. Smaller cranes can also be used on site. Total savings of 10-15% are possible compared with using conventional towers, Hansen argues. The Germanischer Lloyd-certified towers will be offered in three sizes: 80, 95 and 110 metres. The theoretical engineering limitation is over 200 metres, says Hansen.