Engineers are striving to develop new techniques for the construction of taller wind turbines in order to enhance their ability to harvest energy at higher altitudes.
Wind speed rises in direct relation to altitude, while wind power in turn increases with the cube of velocity.
The US National Renewable Energy Laboratory estimates that increasing the height of wind turbines from the current average of 80 metres to 140 metres would achieve a radical increase in America’s wind power potential.
According to NREL’s figures, the 60-metre height difference would make wind power cost-effective on around 615,000 square kilometres of US soil, equivalent to 1,800 gigawatts of unlocked energy potential.
Achieving this height increase involves considerable difficulty, however, as it requires an attendant increase in the width and mass of the base of wind turbine towers, which makes delivery of the facilities on existing transportation infrastructure unwieldy.
The US Department of Energy estimates that transportation costs for a single 100-metre tower, which must be conveyed from factory to farm site in four separate pieces, could run as high as $180 per mile.
“If you…increase the height to 100 metres or above, the base becomes so big that you cannot actually transport [the turbine tower] because it won’t clear the vertical clearance on bridges,” said Sri Sritharan, Iowa State University’s Wilson Engineering professor in civil, construction and environmental engineering.
Sritharan is head of ISU’s Hexcrete project, which seeks to solve the challenge of building higher wind turbines by using concrete in lieu of steel for their towers.
The design that Sritharan has produced consists of a hexagonal structure made from high-strength concrete which is readily capable of achieving heights in excess of 100 metres – at least 20 metres greater than the current average height for US wind turbines of 80 metres.
Sritharan’s hexagonal design has a flat surface on each of its sides, making it far easier to transport than the tubular steel structures currently used, and can be borne using standard semi trucks.
General Electric has developed a solution which simply involves altering the design of wind turbine towers to make them easier to transport and simpler to build.
Its Space Frame Tower is essentially a steel lattice structure which is wrapped in a translucent fabric, conferring it with a solid appearance while also permitting the ingress of ambient light from outside.
The lattice structure consists of steel beams which can be transported in standard 12.2-metre shipping containers or flatbed trailers, as opposed to the huge and unwieldy segments of existing tubular towers.
The Space Frame Tower can achieve heights of 139 metres – just a fraction shorter than the dimensions advocated by NREL. It can also be assembled on any project site within a time frame comparable to those for tubular steel towers of commensurate size.
Massachusetts-based start-up Keystone Tower Systems is also using the method of on-site assembly to facilitate the construction of taller towers, but by means of an innovative manufacturing technology instead of a new structural design.
Company founder Eric Smith has developed a technique for the manufacture of steel tower tubes which borrows extensively from spiral-welding technology that has long been employed by the oil industry for the on-site construction of pipes.
The technique involves feeding trapezoidal flat steel plates into machines that roll and weld them into tubes. Smith has altered the technology so that the resulting tubes possess just a gradual taper instead of a distended conical shape, making then ideal for the construction of turbine towers.
The raw materials for the towers could be delivered to wind farm sites with tremendous ease, given that they consist primarily of flat steel plates. Smith believes that the technique could also achieve massive cost reductions, requiring a tenth of the labour per ton compared to existing tower factories for just half the price.