Over the last few years, the wind power market has exploded. Wind energy is becoming one of the fastest growing renewable energy sources – and for good reason. It is not an exclusive that our current fossil-fuel based energy system is unsustainable. Planet Earth is suffocating. Various international summits have been issuing warnings for some years now, asking to drastically reduce global greenhouse gas emissions. In this race against time, the volume of the global wind farm more than doubled between 2010 and 2015, headed by China and the United States.[1] Europe now ranks third with Germany and Denmark as market leaders.

Although there is a skyrocketing rate of implementation, onshore wind turbines may already be reaching limits as sustainable technology. Despite proven economic profitability and environmentally-friendly impact, the reluctance remains strong at times. More than ever, the dispute flares up over the disfigurement of the landscape, over the noise, and the ground vibrations that ever-greater wind turbine blades may cause. Similarly, the conflict of both land and airspace use through jamming issues with aviation and weather radars, the collateral damages to avian fauna or the devaluation of property, patrimonial and tourism assets, is still up for debate. Besides, conventional wind turbines are not fully carbon-free, being made of steel and cement, whose global production is responsible for more than 5% of the world emissions. For each 2.5 MW wind turbine, the volume of concrete foundation amounts roughly to 400m3.

An intermittent source of energy

Nevertheless, the fundamental challenge is probably the shortfall due to wind power variability and regularity. As an intermittent source of energy, onshore wind energy raises problems of balance between economic management and technical operation to networks. Rotor blades are only productive 20% of the time and most of a traditional wind turbine’s costs are due to the rest of the structure. Furthermore, after some decades in exploiting operating areas, the remaining wind potential inexorably declines, forcing to fall back on less competitive zones, with more capricious windy conditions and greater difficulty to hardwire at high voltage.

All these existing barriers have not escaped to investors, always looking for promising investments. It becomes clearer why the offshore wind energy is also quickly growing. Although still hesitant, this market is currently deemed as a more suitable technology for wind energy production. With more favorable and stable winds offshore, combined to a less pronounced wind gradient, the benefits of higher capacity factors are expected. Of course, important problems are still to be solved before deploying massively this technology. Issues such as turbine design, load management, grid integration, transport of heavy components, power plant’s assembly and better storage capacities are still being dealt with.

Wind is variable but predictable

Researchers are looking at more efficient, flexible and competitive alternatives, leading to a new recent wave of innovation in Airborne Wind Energy Systems (AWES). Even so the first mathematical description of kite energy systems was published by Miles L. Loyd in his work, Crosswind Kite Power, in May 1980. The concept of this disruptive technology consists in replacing most of a conventional wind turbine, namely the tower and its rotor, by tethered flying devices. It creates an extraordinary opportunity to harness wind energy at higher altitude where winds are stronger while drastically reducing materials costs, minimizing visual, noise, and environment impact, also increasing the wind energy potential to more isolated operating areas.

At high and very high altitude, the theoretical wind potential amounts to 18,000 TW per year.[2] Exploiting only 1% of those winds would be sufficient to power the world’s growth energy demand.

Consequently, many projects have already been developed and start-ups have proliferated over the past decade. Different shapes and materials are now considered for the kite, from fixed wings like a glider airplane to soft wings as used by surfers. So far, two major types of AWES can be distinguished.

A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 1461-1476

The first kind of prototype is a movable model, tethered by one or two strong flexible cables to the ground and works as a kite up to 450 meters in altitude. The kite’s crosswind motions in circles or figure-8 patterns at high speed pulls the rope that turns a turbine on the ground, generating power. So, wind kinetic energy no longer cranks the rotor blades but it is the kite’s up and down motion which generates a high traction force through the sized rope driving the heavy generator at ground level. Afterwards, this electric generator transfers produced electric power to the network or storage batteries.

A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 1461-1476

Among the variants of this movable AWE prototype, one of them replaces the kite by a powered glider airplane of carbon fibre modelled on propelled aircraft. Launched from ground stations by rotors integrated on the high-performance wing, the airplane is driven in circles by the wind, just like a traditional wind turbine’s blade. Other AWES use a power-generated drone whose direction and operating speed are controlled. Its propelled system enables the aircraft to take off, fly and land autonomously, with or without wind, by utilizing a vast array of sensor suites which provide the autopilot with critical information to perform the task safely. This model is specifically designed for the floating offshore wind market, pushing the capacity factor from less than 50% up to 70%, as AWES are flying at higher altitude where winds are stronger.

The second AWES’ type consists in a tethered stationary airborne platform connecting an aerostat with an innovative modern flight control system to reliably deploy energy but also communication up to 600m above ground. This system is composed by a huge helium-filled balloon made in canvas and equipped with three blades in composite material to benefit from the Magnus effect. Due to the wind, those blades drive the aerostat spinning around. The generator integrated at the rotation axis recuperates and transforms the kinetic energy into power. Then, on the ground, the power is fed into the grid or batteries from the tethered cable. This technology truly helps to compensate the intermittent and low power wind at the ground.

A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 1461-1476

In conclusion, higher winds, lighter base, and lower costs turn out to be undeniable assets for the AWES that may be one of the lasting levers for the energy transition without intermittent power production. If those game-changing technologies still are in their earlier stages, making cost-effective renewable energy requires a radical change. Projected to be cheaper than the mainstream forms of both on- and offshore wind power, AWES could produce more than twice the energy of conventional wind turbines. Indeed, this state-of-the-art technology enables to reduce costs by 60% thanks to its lower development, operational, and maintenance costs. AWES also appear to be more compact by decreasing the amount of construction materials by 80%, making it much easier to transport with standard containers, and then to install on-site. The upcoming airborne energy sector will be, no doubt, the next key player in combating climate change. Experts such as Dr Peter Harrop and Mr Raghu Das expect the AWES to appear on the horizon by 2025 in the energy market.

Fany Touitou – Content Editor at Greenfish
Quentin Lancrenon – Project Analyst at Greenfish

[1] According to studies of US National Academy of Sciences and Natural Climate Change Journal.
[2] According to the Global Wind Energy Council.