Onshore wind developers are as keen as their offshore counterparts to maximise the capacity and efficiency of their projects by installing more powerful turbines with longer blades on taller towers. But one of the biggest obstacles to their ambitions remains the difficulty and expense of transporting these huge components over land.
As a rule of thumb, it takes 11 heavy truck deliveries for the construction of a single 2MW turbine. These are not off-the-shelf commercial vehicles either, but purpose-built to cope with the long and heavy loads, using bespoke trailers that cost $500,000 apiece. Even countries with a sophisticated transport infrastructure can struggle to accommodate the hundreds of deliveries these vehicles will make during the wind-farm construction process. The problems are far more acute in emerging markets, such as India and Latin America, where large regions with good wind resources remain essentially inaccessible by conventional methods.
Few people in the industry expect anything other than slow, incremental development in wind-turbine road transport. The main drive has been towards more modular designs for turbines and towers, smaller and lighter to transport if often more expensive to construct, which can then be assembled on site. Segmented blades, still at the pioneering stage, would help too. But a report published in January by the US National Renewable Energy Laboratory (NREL) indicates the difficulty of transporting the largest-diameter components, notably blade roots and tower bases, will restrict most onshore projects to 3-4MW turbines for the foreseeable future.
One company thinks there is an alternative. Los Angeles-based Aeros, best known for manufacturing airships for military and commercial applications, is developing a technologically advanced and innovative aircraft that it believes will revolutionise the transport of large, heavy items, including wind-turbine components.
The Aeroscraft is not an airship, at least not in the conventional sense. The ability of airships to carry cargo has always been compromised by the problems of ballast. If an airship is delivering, say a 10-tonne cargo to a location, it will require 10 tonnes of ballast to keep it on the ground while the cargo is offloaded. The alternative would be to release a large quantity of helium, but helium is an expensive and non-renewable resource.
To get round this problem, the Aeroscraft has adopted a system similar to that used by submarines. It is lighter than air when flying, but it can compress a certain amount of its helium, which it pushes into fabric tanks under pressure. The higher density of the compressed gas makes it heavier than air and allows the craft to land. Aeros calls this buoyancy management system COSH, which stands for control of static heaviness.
Another factor that distinguishes the Aeroscraft from traditional airships is its propulsion system. The prototype, which successfully completed tethered testing in September 2013, was powered by three vectored electric-diesel motors, which give it vertical take-off and land (VTOL) capabilities. Landing cushions, rather the traditional undercarriage of heavier-than-air craft, allow it to land on rough ground.
According to Aeros spokesman John Kiehle, the firm is planning to develop two configurations for production aircraft, first the ML866 with a 66-tonne payload and then the ML868, which will have a lifting capacity of 250 tonnes, sufficient to transport all major components of a 1.5MW-class turbine (nacelle: 56 tonnes, tower: 71 tonnes, and blade assembly: 36 tonnes) in one delivery.
Aeros plans to make four of the smaller model and 18 of the larger one, targeting the energy industry and military markets. "We believe this initial fleet will grow considerably based on needs from the energy (and other) industries," says Kiehle, who points to 2017-18 as the earliest date the Aeroscraft would be operational.
Asked how wind developers would contract to use these craft, Kiehle says: "Aeros's strategy is to seek written forward commitments for a majority of the initial fleet on long term (longer than one year in duration) timecharters, and reserve the remaining initial fleet's capacity for the spot market." The aircraft will generally be made available on a fleet lease access model rather than being sold, says Kiehle.