High-altitude wind energy is a prospective resource for the long-term generation of electrical energy. Airborne Wind Energy Systems (AWESs) are important and trending renewable energy technology. This is because of the wide availability of winds blowing between 300 and 10000 meters above ground and their high power capacity. In the recent decade, some of the leading players in the worldwide airborne wind turbine industry including, Vestas Wind Systems, KiteGen Research, Kitenergy, SkySails Power, TwingTec, TU Delft, Ampyx Power, Enerkite, and Windlift, have joined the AWES sector, patenting various ideas and technical solutions for their application.
What is Airborne Wind Energy (AWE)?
High-altitude wind energy, also known as Airborne Wind Energy (AWE), is a renewable energy technology that generates electricity using airborne equipment. AWE, as opposed to traditional wind power, uses free-floating devices such as balloons, kites, and tethered wings held high in the air. Many well-established technologies in conventional ground-based wind energy turbines, such as generators, gearboxes, and grid-tied power converters, are also utilized in the AWE system.
The way these aerial wind turbines gather energy from the wind distinguishes them from their land-based or ocean-based counterparts. A tether line or a cable attaches the AWE device to the ground instead of a huge steel tower construction, and instead of long rotating turbine blades, specially designed aerofoil kites and wings sweep an area across the sky to capture wind energy. Further, these devices, such as a tethered wing or an aerofoil kite, are electromechanical systems that harness the kinetic energy of the winds circulating in the sky. Most airborne wind power devices are designed to fly in a crosswind or transverse direction, concentrating the wind's enormous power supply at medium to high altitudes of more than 200 meters. This is because the wind speed is often higher and steadier at altitudes above 200 meters than at lower altitudes, thus allowing the harnessing of these high winds as an energy source and creating electricity.
Furthermore, the lift and forces created by the wind at these heights are adequate to both sustain and power the flying apparatus. Because these airborne wind devices operate at a higher altitude and at higher wind speeds, more power can possibly be generated more consistently. Thus, with such high elevations and levels of power generation, these AWE devices can possibly be preferred over the typical wind turbine towers.
Working Principle of AWE
AWE is a wind energy system that uses flying blades or wings tethered to the ground. There are two primary concepts for converting wind energy into electricity are as follows:
Small propeller turbines with generators installed on the flying wing (Figure A) or
By having the wing or kite pull on the tether and the rope unwind from a ground drum, which drives the generator. This ground production approach necessitates reeling in the winch tether, resulting in a pumping or yoyo action (Figure B).
In the diagram, the onboard and off-board power generation has been showcased, where the kite (Airborne device) flies constantly across the wind, such that the onboard/off-board generators are responsible for power generation. In one case, the generated power is transmitted from the kite to the ground station via a tether (transmission lines) and extracted at the ground level. However, in another case, the electricity is generated at the ground station based on the in/out action of the tether connected with the kite.
Classification of AWE
Ground Gen: In this type of AWE classification, the electrical energy is created on the ground in a Ground-Gen AWES (GG-AWES) through mechanical work done by traction force. This force is carried from an airplane to the ground system by one or more ropes/cables. Further, this causes an electrical generator's motion, resulting in electricity production. Additionally, the GG-AWESs are subdivided into two types, i.e., fixed-ground station devices and moving-ground-station systems. Significantly, the ground station is fixed to the ground in the fixed-ground station devices. In the case of moving-ground-station systems, the ground station is not stationary but a moving vehicle.
Fly Gen: In this type of AWE classification, the electrical energy is generated onboard the aircraft during flight and delivered to the ground through a customized rope that incorporates electric wires in Fly-Gen AWESs. Further, the Fly-Gen AWESs convert this electrical energy using one or more specifically constructed wind turbines and store it in battery storage or transmit it to the required connections. Furthermore, the FG-AWESs are subdivided into two types, i.e., crosswind and non-crosswind flights. Significantly, these types of flights differ based on wind direction, i.e., wind perpendicular to the flight or non-perpendicular to the flight.
Present Deployments in Process
Ground Gen (Fixed Ground)
Delft University of Technology (TU Delft)
Professor Ockels, a former astronaut, initiated the first study on Airborne Wind Energy at Delft University of Technology in 1996. Delft University of Technology and Karlsruhe University of Applied Sciences have launched a collaborative initiative to continue constructing and testing a transportable 20 kW experimental pumping kite generator.
The Italian KiteGen Research (KGR) was one of the first businesses to test a Ground-Gen AWES prototype. KGR technology is based on a C-Kite that integrates onboard electronics with sensors and is controlled by two power ropes from a ground control station. In 2006, the prototype, known as the Kite Steering Unit 1 (KSU 1) was successfully demonstrated. Following several years of prototype testing, the company is working on the KSU 2. The ropes in this technique were coiled on special winches and propelled by a pulley system through a 20 m flexible rod to an arch-kite or a semi-rigid wing through a pulley system.
Kitenergy, an Italian firm created by a former KiteGen partner, is pursuing a similar concept by controlling a foil kite with two ropes. The company's prototype has a rated power of 60 kW. In 2012, a prototype accomplished 4 hours of continuous autonomous flying with no power production at the University of California, Santa Barbara.
SkySails GmbH is working on a kite-powered wind propulsion system for cargo ships. A few years ago, SkySails Power was formed to develop Ground-Gen AWES based on the technology utilized in the SkySails vessel propulsion system. Currently, two devices are in the works: a mobile AWES and an offshore AWES. SkySails' AWES is based on a foil kite controlled by a single rope and a control pod that adjusts the lengths of the kite bridles to steer the kite and change its angle of attack.
Twingtec, a Swiss business, is working on a 100 kW GG-AWES. After experimenting with soft and stiff wings, the team is working on automating takeoff and landing with a unique concept: a glider with integrated rotors with rotational axes perpendicular to the wing plane.
The Dutch Ampyx Power was the first to design a pumping glider generator. After multiple prototypes, the company is building and testing two 5.5 m 'PowerPlanes,' the AP-2A1 and the AP-2A2. The two officially registered airplanes are fully automated and equipped with cutting-edge avionics.
EnerKite, a German business, created a portable pumping kite generator with a rated continuous output of 30 kW. The ground station is mounted on a vehicle using a pivotal joint that allows for azimuthal rotations. The EnerKite prototype primarily employs a foil kite, although a delta kite and a swept stiff wing are also being investigated and tested. The aircraft has no onboard sensors and is controlled using three ropes. Currently, EnerKite is working on an automated launch and landing system for semi-rigid wings.
Windlift in the United States has a similar concept to Enerkite. SLE kites are used in their 12 kW prototype. They intend to market their goods to the military and remote places.
NASA researched wind energy harvesting from aerial platforms at Langley Research Center, after which they constructed an AWES, demonstrator based on a kite operated by two ropes and equipped with a vision-based system and sensors situated on the ground.
Ground Gen (Moving Ground)
Although it cannot be termed a moving-ground-station device, it is worth noting that the Laddermill idea proposed by former astronaut Professor Ockels in 1996 was the first continuous energy generation AWES concept.
KGR and Sequoia Automation bought the first moving-ground-station design based on a vertical axis generator in 2004. During operation, the lift forces are communicated to a revolving frame, which causes a torque to be generated around the primary vertical axis. The electric generator converts torque and spins into electricity.
KGR and the German business NTS offered an alternative system based on ground stations that travel on closed track loops. Beginning in September 2011, NTS tested a prototype in which four-rope kites are controlled by a vehicle that operates on a 400-meter flat-bed straight railway track.
Kitenergy proposed an idea based on a straight linear rail that is fastened to the ground via a pivotal joint. The rail direction is then altered perpendicular to the primary wind direction. The system's ground station is located on a wheeled vehicle that goes back and forth along the straight rail under kite traction forces from one side to the other. The electricity comes from electromagnetic rotating generators on the vehicle's wheels or linear electromagnetic generators on the rail.
Loyd's first mechanical concept
Loyd came up with one of the most renowned and old ideas for harnessing wind energy utilizing turbines on a kite, calculating that wind turbines mounted on a crosswind flying kite could create up to 5 times the power generated by identical turbines erected on the ground. Loyd's proposal envisions a reciprocating wind-driven device, similar to a multi-propeller plane, with a network of ropes connecting the aircraft to a base station.
Makani evaluated many AWES technologies over a decade, including Ground-Gen, single rope, multiple ropes, a moveable ground station on rails, soft wings, and stiff wings. Makani is currently working on a 600 kW prototype known as 'the M600.' Further, Makani intends to construct an offshore commercial version of the AWT with a nominal power of 5 MW and a wingspan of 65 meters.
Joby Energy Inc. is a US business working on an FG-AWES. According to Joby's proposal, the system may be erected using modular components, consisting of numerous comparable frames with turbines. The way of generating power, as well as the takeoff and landing operations, are comparable to those of the Makani concept.
Altaeros Energies, a Massachusetts-based company formed by MIT and Harvard alumni, created another proposal based on flying wind turbines in a fixed position. Instead of using wings lift to fly, they employ a ring-shaped aerostat with a wind turbine mounted within.
Sky Windpower Inc. proposed a new type of tethered craft called the 'Flying Electric Generator' (FEG) in 1986. This type was similar to a large quadrotor with at least three identical rotors mounted on an airframe and connected to a ground station via a rope with inner electrical cables.
Advantages of AWE
Less material means less environmental effect: CO2 footprint, aesthetic impact, and resource consumption.
Additional wind resources: harnessing greater global renewable potential
High full load hours imply more consistent power generation.
Low Levelized Cost of Energy: the possibility for decreased energy production costs.
Flexibility: simpler logistics, faster set-up
Scalability ranges from a few kW to many MW.
Mobile applications, repowering, and floating offshore are all new markets.
Disadvantages of AWE
As airborne gadgets and power connections fall to the earth, they may become separated or damaged, posing a safety risk.
If the gadget is retracted during inclement weather, no electricity is generated, and thus, the power balance will be affected.
Electrical energy losses, given the lengthy conducting wire that connects the aerial producing equipment to the ground, is a major issue. With the increase in altitude, the transmission losses increase significantly.
Acceptance of these big floating airborne devices over land and residential areas by the general public is a significant concern.
These autonomous devices face design and control issues in all wind and weather situations.
To fly in high altitude winds, kites and wings must be light and robust.
Thunder and lightning strikes (Bad weather) represent a significant threat to the destruction of any aerial item, specifically AWE.
Title: Wind installation comprising a wind turbine and an airborne wind energy system
Grant Date: 2021-05-11
Current Assignee: Vestas Wind Systems AS
The patent discloses a wind turbine system for harvesting and utilizing maximum wind energy and an airborne wind energy system for increasing the total power production.
As per the analysis, a conventional wind energy system is installed on a wind turbine site that includes electrical and mechanical components like rotors, blades, towers, etc. At the top of the tower, a nacelle (housing or a structure) is mounted, with each nacelle a rotor being rotatable about an axis of rotation is coupled. Further, this rotor is connected to the generator to convert the rotating rotor's energy into electrical energy for a power grid. The rotor, the generator, and the connections with the power grid are done via a power transmission line. Furthermore, an airborne wind energy system is connected at the top of the nacelle such as to increase the power production level of the installed system. Further, this airborne wind energy system is coupled to the wind turbine via a cable (tether) and comprises a separate generator that is electrically connected via the power transmission lines. Thus, such a system with the coupling of multiple wind energy sources allows for the harvesting of maximum power.
Conclusions and Future Scope
High-altitude wind energy is a prospective resource for the long-term generation of electrical energy. Airborne Wind Energy Systems (AWESs) are important and trending renewable energy technology. This is because of the wide availability of winds blowing between 300 and 10000 meters above ground and their high power capacity. In the recent decade, some of the leading players in the worldwide airborne wind turbine industry including, Vestas Wind Systems, KiteGen Research, Kitenergy, SkySails Power, TwingTec, TU Delft, Ampyx Power, Enerkite, and Windlift, have joined the AWES sector, patenting various ideas and technical solutions for their application. In the future, Airborne wind energy research and development is predicted to accelerate rapidly. Several prototypes that are currently being researched will be finished and tested on the basis of the key affecting parameters that include - flying mass, landing/takeoff, cables, altitude, movement curvature, cable electrostatic behavior, etc.
The compound annual growth rate (CAGR) of the airborne wind energy (AWE) market is estimated to be 9.4% from 2023 to 2030. This means that the AWE market is expected to grow by 9.4% every year on average over the next seven years. The ever-increasing need for power, particularly in rising economies such as China, Brazil, India, and Russia, has increased the demand for alternative energy sources.