Many decades ago, a much younger version of me was in the car with my father and brother, surfing the road on some errand. We were probably all in the front seat, and none of us were wearing a seat belt; They were simpler times. When we passed under an overpass, my father said, “Do you know why the overpasses on these roads are so high?” Me, six, certainly not, but it was clear that Dad did and had something to say about it, so we just shook our heads and waited for the lesson. “Because that’s how big nuclear missiles are.” He then went into an explanation of how the U.S. Interstate Highway System, Still in its infancy, was designed to make sure the military was able to move around the country, so flyovers were needed to allow passage. of trucks with large loads.
It was an interesting lesson at the time, and over the years I have continued to be impressed with the foresight and engineering that went into the interstate system here in the United States. It’s far from perfect, of course, and it’s only recently that system specs have begun to pinch things that seem totally unrelated to the dimensions of the overpass, namely the size and efficiency of wind turbines.
The higher the better
Outwardly, the ability to convert the kinetic energy of the wind into electricity would seem to have little to do with the design decisions made by civil engineers during the Eisenhower administration. But when it comes to wind power, size, or more specifically height, is important. The higher the center of a wind turbine is at ground level, the more consistent and faster the wind is. A study by the National Renewable Energy Laboratory mapped wind speeds at 110m and 160m above ground level (AGL) across the country. Compared to winds at 80m, roughly the average height of a large wind turbine these days, the difference is striking.
If wind turbines could be raised to 160m AGL, large stretches of the country could be used for wind power production. The NREL estimates that the highest turbines could generate up to 45% more electricity.
Low bridge ahead
Therefore, the need for taller turbines is clear, but building them taller creates other problems. The higher the tower, the wider the base must be, to support the enormous weight of the machinery on it and to resist the loads that the wind imposes on it. The current 80m turbine crop has base sections that are approximately 4.5m in diameter, which is slightly less than the minimum freeway overpass height (16ft, or 4.9m) specified for interstate highways. When placed in a low-boy trailer, or a tailor-made trailer for the job, the lower sections of the tower barely scrape underneath some flyovers, creating some interesting moments.
The obvious answer to the need for such large tower base sections is to forget about manufacturing them in factories and simply build them on site. And although that is possible, the construction economy goes against it. Factories that make such large parts are full of even bigger machines needed to make them. The workers there are skilled dealers who work under controlled conditions to produce precision parts that will withstand enormous loads. Expecting such precision processes to take place in the middle of a North Dakota cornfield in mid-February is a lot to ask.
Extrusion on site
However, it turns out that it is actually possible to establish a wind turbine tower factory in the workplace. General Electric, in cooperation with construction materials company LafargeHolcim and construction 3D printing company COBOD, has recently announced plans to 3D print the lower sections of wind turbine towers up to 200 meters high. Its first prototype, a ten-meter-high extruded concrete conical cylinder, was printed in late 2019 at COBOD’s Copenhagen facility, it was printed using the company’s BOD2 modular gantry printer.
The BOD2 printer was originally designed to print entire structures, from houses to small office buildings, on site with minimal need for specialized trades. Depending on how the 2.5m modules are configured for the X and Y axes, the BOD2 can print structures up to 14m by 50m, so it should be large enough to print the base sections even for the tower towers. larger wind turbines. Once the base sections are printed on site at a height where the diameter narrows to less than 4.5m, traditional precast sections will likely be transported to the site and assembled on top of the extruded pedestal.
Obviously, there is a lot of engineering to do to ensure that this hybrid approach will withstand the loads that such large wind turbines will experience. But it seems like a clever use of technology that, frankly, has always seemed to us to be a novice in terms of its ability to produce buildings that are attractive enough for people to really want to live and work in. Such a utilitarian app seems to be the perfect use case for large-scale 3D printing, and we can’t wait to see if it’s worth it in the long run.
