Controlling airborne wind turbines remains the greatest challenge to the technology becoming widely accepted.
Despite their green credentials, wind turbines seem to be loved and loathed in equal measure. Loved because they're 'helping the planet' loathed by the people living near them who regard them as a blot on the landscape. It's this ambivalence towards them that has given rise to the NIMBY (Not In My Back Yard) acronym coined by the media in response to collective objections to proposed wind farms in recent times.
Aside from ruining a community's cherished view of a landscape (or seascape), the technology's detractors are also usually keen to point out that these behemoths only earn their keep when there's actually some wind - which is rarely all the time. But now technology is emerging that counters both objections, by lofting the turbines into the air.
Broadly, designs for airborne wind turbines (AWTs) fall into two types, blimps and kites. The technology is still at an early stage, however, with the first commercial systems not expected to appear for at least a couple of years, so there's a wide range of designs being proposed and evaluated around the world. But the work of the founder members of the Airborne Wind Energy Consortium - Joby Energy, Magenn Power, Makani Power and Sky WindPower - gives a good snapshot of the different approaches.
Joby, Makani and Sky are all developing kite-based designs, while Magenn is using a helium-filled blimp, all of which are anchored to the ground via a winch-operated conductive tether. Whatever the design though, control systems play a crucial role. It is, however, more simple in the Magenn blimp.
The Magenn Air Rotor System (MARS) will operate at the lower altitudes, at about 200-300m. Since lift comes from its helium envelope there are no control surfaces to adjust to account for changing wind conditions. The design also exploits the Magnus effect, which causes the blimp to behave like an aerofoil, providing additional lift and keeping the blimp stabilised and positioned within a controlled and restricted airspace.
As Anthony Pizarro, director of corporate development at Magenn Power, explains: 'Control is simple because the MARS self-aligns in the wind. A suite of sensors conveys performance- and maintenance-related data via the Internet to the device's owner, and although the control and monitoring systems are not redundant, a failure would not cause catastrophe - helium-inflated airships tend to be very stable and do not suddenly drop, even when damaged.'
This blimp approach works best at lower altitudes, in winds of lower velocity; further up, however, a blimp's inherently high drag becomes a handicap in the stronger winds, so it's here that kite-based designs offer better efficiency. This is where control becomes a challenge.
Allister Furey is conducting research at Sussex University's Centre for Computational Neuroscience and Robotics (CCNR) into adaptive control systems for kite-based systems, and he says' 'It's a demanding control problem. The idea is to fly them as perpendicularly as possible, and in loop trajectories as much as possible.
'There's the issue of aerobatics in a changing wind environment, as well as that of flight behaviours in different weatherconditions. So controllers need to be robust within the range of expected conditions; they must also be able to cope with, say, wet or frozen wings and ageing wing materials.
'What you're trying to achieve is autonomy in a wide range of unpredictable conditions. You also need to maintain control in case part of the system fails - maintaining safety of operation is obviously key,' he says.
'CCNR is looking specifically at kites made from flexible material, or rigid and flexible components. Because of this flexing though, you can't create an analytical model, so the challenge is how to control such a system that can't be modelled and yet to a high degree of fidelity.'
Joby Energy declined to release details of its control technology but its website describes a multi-wing structure supporting an array of turbines connected to motor-generators that produce thrust during take-off and generate power during crosswind flight, at heights of about 400-600m.
Orientation in flight is maintained by a computer system that adjusts the aerodynamic surfaces on the wings and differentially controls the speeds of rotors.
As Furey explains: 'In a kite-based design, you can change the angle of the wing but you have very poor control authority. The Joby craft though, for example, has multiple rotors so it gives better authority.'
Makani Power's current design, the M1, looks rather like a monoplane, although it's similar in operation to Joby's craft. It has a rigid wing though, so control is more akin to that of a UAV.
Makani's CEO, Dr Corwin Hardham, explains some of the control system's salient points. 'An autonomous control system pilots the Makani AWT and is responsible for maintaining a stable flight path while maximising power output, by calculating the wing's position and heading using sensor data - barometric pressure, GPS location, wing attitude and so on - and adjusting the control surfaces to maintain the correct flight path,' he says.
'Shortly after launch, control of the wing is switched from the ground pilot to the onboard control system, which navigates according to a pre-described circular path. To improve the signal-to-noise ratio in a range of conditions, we have several sensor sets. This enables us to measure both small and large dynamics with good fidelity.'
Sky WindPower calls its 'rotorcraft' design a Flying Electric Generator (FEG), and again the approach is essentially the same as with Joby and Makani, although without flying a looped path. The ultimate goal here is to deploy the technology at very high altitudes, up to 5,000m or so, controlling the craft's attitude autonomously by integrating information from GPS, gyroscope and 'other sensing means', according to the company's PJ Shepard.
These and other practical solutions are tantalisingly close. As Furey says: 'Everything's there, it just needs to be integrated and refined so that it will work autonomously for months on end, and account for all foreseeable types of failure. So a lot of flight testing still needs to be done.'