Renewable energy is at the centre of the transition to a less carbon-intensive and more sustainable energy system. Renewables have grown rapidly in recent years, accompanied by sharp cost reductions for solar photovoltaics and wind power in particular. The future development of wind power presents a significant opportunity in terms of providing low carbon energy. “By 2020, wind power could prevent more than 1 billion tonnes of carbon dioxide from being emitted each year by dirty energy – equivalent to the emissions of Germany and Italy combined,” said Sven Teske, Greenpeace senior energy expert.
From Caribbean islands to the windswept coasts of northern Europe, a new way of generating renewable energy is taking shape, harnessing the wind with kites, drones and other airborne devices. Airborne wind energy (AWE) is an umbrella name for concepts that convert wind energy into electricity with the common feature of autonomous kites or unmanned aircraft, linked to the ground by one or more tethers. AWE systems offer several potential advantages over conventional wind turbines. They have low capital costs, due to the small amount of material than tower-based turbines, can be deployed faster due to relatively simple construction and can harness stronger and more consistent high-altitude winds prevalent above 200 m altitude. Researchers in this nascent field are working on a dizzying array of devices, including kites, wings, drones and even a set of spinning, sky-borne hoops that are being developed by a Scottish firm called Windswept and Interesting.
“Wind turbines on the Earth’s surface suffer from the very stubborn problem of intermittent wind supply,” said KAUST atmospheric scientist Udaya Bhaskar Gunturu, in a release put out by the university. This has led researchers and energy companies worldwide to look upwards and explore the possibility of the strong and reliable winds at high altitudes. Flying a wind turbine on a kite — with the electricity being delivered to the ground through its tether — may seem an unlikely scenario, but several companies worldwide are already testing prototype systems.
Tethered kites could potentially offer the flexibility to vary the altitude of the turbines as wind conditions change. Current technology would most likely allow harvesting wind energy at heights of two to three km, but there is also a lot of wind even higher than that. The researchers found that the most favourable regions for high-altitude wind energy in West Asia are over parts of Saudi Arabia and Oman. A German partnership is working to develop a fully automated airborne wind energy system, according to announcement by utility Energie Baden-Wuerttemberg AG (ETR:EBK), or EnBW, which is participating in the initiative.
As part of the research project, called SkyPower100, the partners aim to develop and test a 100-kW flying system by 2020. The system should work autonomously and start, land and stow the kite by itself. It will be tested for several months, during which time the consortium wants to gain clues of how to scale the technology into the megawatt class on shore. Future offshore use is also considered. According to the announcement, high-altitude wind technology could become a pioneering supplement to conventional wind energy.
The project is funded by the Federal Ministry for Economic Affairs and Energy (BMWi) and coordinated by SkySails Power GmbH, which is responsible for the development, production, installation and testing of the pilot device. Other partners include EWE Offshore Service & Solution GmbH and the Institute of Propulsion Systems and Power Electronics of the Leibniz University Hannover.
Classifications of Airborne Wind Energy Systems
Several different concepts are currently being pursued and convergence towards the best architecture has not yet been achieved. A possible classification of AWE systems is shown , including several specific implementations.Ground-gen concepts are based on the conversion of mechanical into electrical energy at ground level, while fly-gen concepts are based on the conversion in the air, onboard the airborne unit.
AWESs are generally made of two main components, a ground system and at least one aircraft that are mechanically connected (in some cases also electrically connected) by ropes (often referred to as tethers). Among the different AWES concepts, we can distinguish Ground-Gen systems in which the conversion of mechanical energy into electrical energy takes place on the ground and Fly-Gen systems in which such conversion is done on the aircraft.
In a Ground-Gen AWES (GG-AWES), electrical energy is produced on the ground by mechanical work done by traction force, transmitted from the aircraft to the ground system through one or more ropes, which produce the motion of an electrical generator. Among GG-AWESs we can distinguish between fixed-ground-station devices, where the ground station is fixed to the ground and moving-ground-station systems, where the ground station is a moving vehicle.
In a Fly-Gen AWES (FG-AWES), electrical energy is produced on the aircraft and it is transmitted to the ground via a special rope which carries electrical cables. In this case, electrical energy conversion is generally achieved using wind turbines. FG-AWESs produce electric power continuously while in operation except during take-off and landing maneuvers in which energy is consumed. Among FG-AWESs it is possible to find crosswind systems and non-crosswind systems depending on how they generate energy.
Fixed-ground-station GG-AWES (or Pumping Kite Generators) are among the most exhaustively studied by private companies and academic research laboratories. Energy conversion is achieved with a two-phase cycle composed by a generation phase, in which electrical energy is produced, and a recovery phase, in which a smaller amount of energy is consumed.
Most ground-gen concepts drive a drum-generator module in pumping cycles, alternating between traction and retraction phases to generate electricity. Fly-gen concepts use onboard wind turbines with continuous electrical energy output and a conducting tether to deliver this energy at ground level. Of the fly-gen solutions, crosswind systems can generally produce more power (1-2 orders of magnitude higher) than non-crosswind systems.
Peschel – whose company Kitepower spun out of a research group at TU Delft in the Netherlands – notes that its biggest kite, with an area of 100 m2 and a generating capacity of 100 kW, fits into a large surf bag and can be launched by two people in 20 minutes. Fixed-wing AWE systems are bulkier, but not by much.
The benefits could increase when deploying AWE systems on floating offshore platforms. Compared to a conventional, tower-based offshore floating wind turbine, a tethered AWE system is subject mainly to tensile rather than bending loads, potentially reducing the cost of large stabilising subsea structures and ballasting.
A potential reduction in weight (with possibility of further reduction) could reduce the capital expenditure (CAPEX) of platforms and subsea structures; and the reduced size of the devices allows for rapid installation at a lower cost. Important technical challenges of AWE systems are :
• High complexity. The operation of AWE systems crucially depends on a fast-feedback control based on a quite complex set of distributed sensors and actuators that must guarantee fully autonomous flight over long periods of time.
•Lack of proven reliability and operational hours. Existing technology demonstrators still rely on supervised operation, especially in the take-off and landing phases, and most of the developed systems are not fully autonomous. Depending on the technologies, currently achieved operating times vary between 2 and 3 h and almost 24 h of autonomy.
• Limited knowledge. All the predictions of economic potential and environmental benefits of AWE are based solely on calculations of aerodynamics and mechanics during tethered flight. So far, these calculations have not been fully validated using experimental data. It is not certain that after full development, the technology will yield the promised energy conversion performance. Consequently, the impact and feasibility of scaling up to utility scale generation has not been deeply and rigorously assessed. There is also a need for more research on wind resources/conditions between 100 and 1000 m height, with a better description of the atmosphere in general for sustainable energy systems.
There are still several technical problems to address such as: the durability of flexible materials, greater design convergence, and water erosion testing of materials. Apart from space restrictions (a no-fly zone is necessary given the altitudes in which these devices would operate), regulation, social acceptance, safety and the potential of harm due to lightning strikes and storms are challenges which need to be addressed
Recent interesting technology and research trends include: drones and multi-drone concepts, , advanced aerodynamic modelling, electronics and sensors that allow tethered devices to be controlled autonomously, systems for autonomous take-off and landing, and high-lift multi-element aerofoils. The projected power for large scale operation is up to 25 MW/km2.
The majority of implemented development platforms is in a nominal power range up to 20 kW. A notable outlier and currently the most prominent project is the M600 which was developed by Makani Power in the USA and which has been tested onshore in California and Hawaii. After acquisition by Shell the company announced it would start offshore operation off the coast of Norway . Competitor Ampyx Power based in the Netherlands is on track for flight testing a 250 kW rigid wing system designed for pumping cycle operation on a test site in Ireland, developed by E.ON.
A possible scale-up range beyond 1 MW is feasible, but even if this power level is achievable, size could be challenging e.g. due to aero-elasticity problems . When implementing these systems, it would also be important to consider not only the power per square metre wing area but also all the surrounding area, which varies for different technologies. Moreover, the scalability would require longer cables and higher altitudes. At the present time, ground generation technology is not necessarily more advanced than flying generation systems.
Multiple Drone Airborne Wind Energy Systems
Experimental evidence and physical models show that the performance of flying generators is significantly affected by the aerodynamic dissipation due to drag of the traction cables. Such dissipation sets an upper bound to the effective operating altitude and limits the scalability of the nominal power of the devices.
In this context, Multiple Drone Airborne Wind Energy Systems (MD-AWESs) represent an effective solution that could introduce radically new perspectives in the field of airborne systems. An MD-AWES is a crosswind architecture which features multiple drones that are connected to the ground with a single shared cable. This aims at significantly reducing the aerodynamic drag of the cables and thus providing a huge potential gain in techno-economic performance.
The specific challenges of this technology that still need to be investigated are:
• Layout/Architecture choice: the best architecture in terms of number of drones, connections to ground and between drones, type of drones.
• Control: during generation and during the two most critical phases of take-off and landing.
• Design: structural design of drones, flight dynamics, stability, etc.
Autonomous take-off and landing systems
Fully autonomous take-off and landing is one of the current technical bottlenecks in the development of AWE technology. Except for vertical take-off and landing systems, this functionality has not been fully demonstrated. Moreover, the possibilities and constraints are very different between soft kites, where a fixed or telescopic mast is usually envisaged to support the wing at take-off, possibly with additional support of a drone, and rigid aircraft, where linear or rotational launching concepts have been proposed.
In a recent project at ABB Corporate Research, a fully autonomous, linear take-off system in compact space for a rigid tethered aircraft has been proved. However, long-term extensive testing in all wind conditions would be required, and the landing phase has not been experimentally investigated yet. For soft kites, automatic launch and landing concepts exist, however their reliability and full-scale applicability have not been fully proven. The main challenge lies in the low speed of the aircraft during take-off and landing, which results in less controllability, coupled with the short tether length and the uncertainty of environmental conditions.
Over the next few years, AWE developers hope to fly their devices for longer periods, with less human intervention. Kitepower’s mini-AWE park in Curacao will operate automatically under the watchful but remote eyes of technicians in Delft, who will use sensor data to decide when to land the kite for maintenance and re-launch by a local ground crew. Another company, SkySails Power, has already deployed its kite as an auxiliary generator on a catamaran operated by an environmental charity, Race for Water. In 2020 it plans to offer commercial, ground-based “plug and play” AWE units of up to 500 kW.
At present, AWE requires significant fundamental academic research to get to a required level of maturity, but there is some small-scale commercial investment taking place in the development and testing of devices. Large commercial players are starting to take an interest in the technology and it can be foreseen that this will increase once a level of reliability can be demonstrated.