airborne wind turbines - a report

Savitribai Phule Pune University

1. Introduction –

An Airborne Wind Turbine (AWT) is a design concept for a wind turbine with a

rotor supported in the air without a tower. Thus benefiting from more mechanical and

aerodynamic options, the higher velocity and persistence of wind at high altitudes,

while avoiding the expense of tower construction or the need for slip rings or yaw

mechanism. An electrical generator may be on the ground or airborne. [2]

Airborne wind turbines may operate in low or high altitudes, they are part of a wider

class of Airborne Wind Energy Systems (AWES) addressed by high-altitude wind

power and crosswind kite power. When the generator is on the ground, then the

tethered aircraft need not carry the generator mass or have a conductive tether. When

the generator is aloft, then a conductive tether would be used to transmit energy to the

ground.

Wind at high altitudes is almost constant and hence, is a vast energy resource than

surface winds. Since high altitudes have fast and more consistent wind blowing,

Airborne Wind Turbines (ATW) can generate more power compared to traditional

wind turbines. Further, ATWs make energy harvesting possible even at inaccessible

locations, such as offshore, but at lesser installation costs. Given these merits, the

airborne wind energy industry is uniquely positioned to contribute to the growth of

the overall wind industry.

One major disadvantage is that bad weather such as lightning or thunderstorms can

temporarily suspend use of these machines, probably requiring them to be brought

back down to the ground and covered. Some require a long power cable and, if the

turbine is high enough, a prohibited airspace zone. As of April 2014, no commercial

airborne wind turbines are in regular operation. [1][2]

1.1 Wind Energy -

Wind power is the use of air flow through wind turbines to mechanically power

generators for electricity. Wind power, as an alternative to burning fossil fuels, is a

renewable source of energy, produces no greenhouse gas emissions during operation,

K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering

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and uses little land. The effects on the environment are far less adverse than those of

non-renewable power sources.

Wind farms consist of many individual wind turbines which are connected to the

electric power transmission network. Wind power gives variable power which is very

consistent from year to year but which has significant variation over shorter time

scales.

With the growing increase in the demand for alternate sources of sustainable energy

worldwide, wind power is gaining importance across the globe. Wind energy

currently accounts for nearly half of the clean energy produced worldwide and is

predicted to grow 25% each year. [4]

1.2 History of Wind Power -

The first windmill used for the production of electricity was built in Scotland in July

1887 by Prof James Blyth of Anderson's College, Glasgow. Blyth's 10 m high, cloth-

sailed wind turbine was installed in the garden of his cottage and was used to charge

accumulators, to power the lighting in the cottage, thus making it the first house in the

world to have its electricity supplied by wind power.

In Cleveland, Ohio a larger and heavily engineered machine was designed and

constructed in the winter of 1887-88 by Charles F. Brush, this was built by his

engineering company at his home and operated from 1886 until 1900. The Brush wind

turbine had a rotor 17 m (56 foot) in diameter and was mounted on an 18 m (60 foot)

tower. The connected dynamo was used either to charge a bank of batteries or to

operate up to 100 incandescent light bulbs, arc lamps, and various motors in Brush's

laboratory.

With the development of electric power, wind power found new applications in

lighting buildings from centrally-generated power. Throughout the 20th century,

many people developed small wind stations suitable for farms or residences, and

larger utility-scale wind generators that could be connected to electricity grids for

remote use of power. Today wind powered generators operate in every size range


2

https://en.wikipedia.org/wiki/Incandescent_light_bulb

https://en.wikipedia.org/wiki/Charles_F._Brush

https://en.wikipedia.org/wiki/Cleveland,_Ohio

https://en.wikipedia.org/wiki/Accumulator_(energy)

https://en.wikipedia.org/wiki/Anderson's_College

https://en.wikipedia.org/wiki/Prof_James_Blyth

https://en.wikipedia.org/wiki/Scotland

https://en.wikipedia.org/wiki/Electric_power_transmission

https://en.wikipedia.org/wiki/Wind_farm


between tiny, up to gigawatt sized wind farms that provide electricity to national

electrical networks. [4]

1.3 Types of Wind Turbines –

Listed below are the different types of wind turbines- [1]

1. Horizontal Axis Wind Turbine (HAWT)

2. Vertical Axis Wind Turbine (VAWT)

3. Small Wind turbines

4. Airborne Wind Turbines (AWE)

1.3.1 Horizontal Axis Wind Turbine (HAWT) –

Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical

generator at the top of a tower. Small turbines are pointed by a simple wind vane,

while large turbines generally use a wind sensor coupled with a servo motor. Most

have a gearbox, which turns the slow rotation of the blades into a quicker rotation that

is more suitable to drive an electrical generator. [5]

Fig. 1.1 Construction of Horizontal Axis Wind Turbine


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Horizontal Axis Wind Turbine Advantages -

• Variable blade pitch, which gives the turbine blades the optimum angle of attack.

Allowing the angle of attack to be remotely adjusted gives greater control, so the

turbine collects the maximum amount of wind energy for the time of day and season.

• The tall tower base allows access to stronger wind in sites with wind shear

• High efficiency, since the blades always move perpendicularly to the wind,

receiving power through the whole rotation.

Horizontal Axis Wind Disadvantages-

• The towers and blades go up to 90 meters long are hence difficult to transport.

• Tall HAWTs are difficult to install, needing very tall and expensive cranes and

skilled operators.

• Massive tower construction is required to support the heavy blades, gearbox, and

generator.

• HAWTs require an additional yaw control mechanism to turn the blades toward the

wind.

1.3.2. Vertical Axis Wind Turbine (VAWT) -

Vertical-axis wind turbines (VAWTs) are a type of wind turbine where the main rotor

shaft is set transverse to the wind (but not necessarily vertically) while the main

components are located at the base of the turbine. This arrangement allows the

generator and gearbox to be located close to the ground, facilitating service and

repair. VAWTs do not need to be pointed into the wind, which removes the need for

wind-sensing and orientation mechanisms.[5]


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Fig. 1.2 Vertical Axis Wind Turbine

Advantages of vertical axis wind turbines (VAWT)-

• They are omni-directional and do not need to track the wind.

• The gearbox of a VAWT takes much less fatigue than that of a HAWT.

Should it be required, replacement is less costly and simpler, as the gearbox is easily

accessible at ground level.

• VAWT wings of the Darrieus type have a constant chord and are easier to

manufacture.

• VAWTs can be grouped more closely in wind farms, increasing the generated

power per unit of land area.

• VAWTs can be installed on a wind farm below the existing HAWTs; this will

improve the efficiency (power output) of the existing farm.[4]

• Research at has also shown that VAWTs can have an output power ten times

that of a HAWT wind farm of the same size.

Disadvantages of Vertical Axis Wind Turbines (VAWT)-

The blades of a VAWT are fatigue-prone due to the wide variation in applied

forces during each rotation. This has been overcome by the use of modern


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composite materials and improvements in design; the use of aerodynamic

wing tips causes the spreader wing connections to have a static load. The

vertically oriented blades used in early models twisted and bent during each

turn, causing them to crack. Over time, these blades broke apart, sometimes

leading to catastrophic failure. VAWTs have proven less reliable than

HAWTs. Modern designs of VAWTs have overcome many of the issues

associated with early designs.

One major challenge is dynamic stall of the blades as the angle of attack varies

rapidly.

1.3.3 Small Wind Turbines –A small wind turbine is a wind turbine used for micro-generation, as opposed to large

commercial wind turbines, such as those found in wind farms, with greater individual

power output.

1.3.4 Airborne Wind Turbines

Those Turbines which float in air, with a rotor supported in air, attached to a tether.

Futher discussion on AWE is given below.

Fig. 1.3. Airborne Wind Turbine


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2. Airborne Wind Energy Systems

A basic introduction of Airborne Wind Turbines (AWE) has been given above.

2.1 Types of Airborne Wind Turbines- [2]

AWTs 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). The different types of AWES are –

1. Ground-Gen systems - In which the conversion of mechanical energy into

electrical energy takes place on the ground.

2. Fly-Gen systems - In which such conversion is done on the aircraft in the air.

2.1.1 Ground-Gen Airborne Wind Turbine

In Ground-Generator Airborne Wind Energy Systems (GG-AWES) electrical energy

is produced exploiting aerodynamic forces that are transmitted from the aircraft to the

ground through ropes.

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.

The different types Of Ground- Gen AWES –

a) Leading Edge Inflatable (LEI) Kite.

b) Supported Leading Edge (SLE) Kite.

c) Foil Kite (design from Skysails)

d) Glider (design from Ampyx Power)

e) Swept rigid wing (design from Enerkite)

f) Semi-rigid wing (design from Kitegen)


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Fig. 2.1 Different Types of Ground-Gen AWES[2]

Working – [2]

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.

In these systems, the ropes, which are subjected to traction forces, are wound on

winches that, in turn, are connected to motor-generators axes. During the generation

phase, the aircraft is driven in a way to produce a lift force and consequently a

traction (unwinding) force on the ropes that induce the rotation of the electrical

generators. For the generation phase, the most used mode of flight is the crosswind

flight with circular or the so-called eight shaped paths. As compared to a non-

crosswind flight (with the aircraft in a static angular position in the sky), this mode

induces a stronger apparent wind on the aircraft that increases the pulling force acting

on the rope. In the recovery phase (Fig. 2b) motors rewind the ropes bringing the

aircraft back to its original position from the ground. In order to have a positive

balance, the net energy produced in the generation phase has to be larger than the


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energy spent in the recovery phase. This is guaranteed by a control system that adjusts

the aerodynamic characteristics of the aircraft and/or controls its flight path in a way

to maximize the energy produced in the generation phase and to minimize the energy

consumed in the recovery phase.

2.1.2 Fly-Gen Airborne Wind Turbine – [2]

In a Fly-Gen AWES, electrical energy is produced on the aircraft and it is transmitted

to the ground via a special rope (called tethers) 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.

Different types of Fly-Gen AWES –

a. Plane with four turbines, design by Makani Power.

b. Aircraft composed by a frame of wings and turbines, design by Joby Energy.

c. Toroidal lifting aerostat with a wind turbine in the center, called BAT

(Buoyant Airborne Turbine), design by Altaeros Energies.

d. Static suspension quad-rotor in autorotation, design by Sky WindPower.


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Fig. 2.2 Different Types of Fly-Gen AWES [2]

Working-a) Makani Power- this AWT takes off with the wing plane in a vertical position,

driven by propellers thrust. This flight mode is similar to a quadcopter flight and

rotors on AWT are used as engines. Once all the rope length has been unwound,

the AWT changes flight mode becoming a tethered flight airplane. In this second

flight mode a circular flight path is powered by the wind itself and rotors on AWT

are used as generators to convert power from the wind. During this phase the

cable length is fixed. In order to land, a new change of flight mode is performed,

and the AWT lands as a quadcopter.

b) Joby Energy -The main difference between Joby and Makani is that the tethered

airborne vehicle is a multiframe structure with embedded airfoils. Turbines are

installed in the joints of the frame. In Joby's concept, the system could be adapted


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to be assembled with modular components, constructed from multiple similar

frames with turbines. The power generation method and the take-off and landing

maneuvers are similar to those of Makani concept.

c) Altaeros Energies - Another project based on flying wind turbines in a stationary

position has been developed by Altaeros Energies, a Massachusetts-based

business. In this case, instead of using wings lift to fly, they use a ring shaped

aerostat with a wind turbine installed in its interior. The whole generator is lighter

than the air, so the take-off and landing maneuvers are simplified, and the only

remaining issue is the stabilization of the generator in the right position relative to

the wind. The aerostat is aerodynamically shaped so that the absolute wind

generates lift that helps keepinga high angle of altitude together with the buoyancy

force. After their energy production tests in 2012, Altaeros is additionally working

on multiple rotor generators with different lighter-than air craft configurations.

d) Sky Windpower Inc.- a different kind of tethered craft called ‘Flying Electric

Generator’ (FEG) which is similar to a large quadrotor with at least three identical

rotors mounted on an airframe that is linked to a ground station with a rope having

inner electrical cables. Take-off and landing maneuvers are similar to those of

Makani's and Joby's generators, but FEG operation as generator is different. Once

it reaches the operational altitude, the frame is inclined at an adjustable

controllable angle relative to the wind (up to 50°) and the rotors switch the

functioning mode from motor to generator. At this inclined position, the rotors

receive from their lower side a projection of the natural wind parallel to their axes.

This projection of wind allows autorotation, thus generating both electricity and

thrust. Electricity flows to and from the FEG through the cable. Sky Windpower

tested two FEG prototypes. They claimed that a typical minimum wind speed for

autorotation and energy generation is round 10 m/s at an operational altitude.

3. Wind Power – Locations and Availability

The wind power potential in Wind Speed Maxima (WSM) is dictated by how many

square meters of area perpendicular to the air flow can be swept by the AWE systems

per square kilometer of land. [3]


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Fig. 3.1

These figures show global maps of 21-yr average Wind Speed Maxima (WSM)

properties. Figs. 3.1 and 3.2 for January and July respectively.

Fig. 3.2


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4. Future Scope

Though many innovative designs and ideas are patented under this airborne wind

energy sector, commercialization of the technology ideas has not achieved great

success due to various technology and regulatory challenges, space constraints, noise

and aesthetics. A large number of players in the airborne wind turbine industry are

still in R&D phase, and a few others are in their prototype stage. Some of the

companies involved in AWT research have not protected their intellectual property,

while a few individual innovators and companies have patented their technology, yet

not commercialized their innovative ideas. [1]

4.1 Future in India

As of 31 January 2016 the installed capacity of wind power in India was 25,188 MW,

mainly spread across South, West and North regions. East and North east regions have

no grid connected wind power plant as of March, 2015 end. No offshore wind farm

are under implementation. However, an Offshore Wind Policy was announced in

2015 and presently weather stations and LIDARs are being set up by National

Institute of Wind Energy (NIWE) at some locations. [5]

But, there are no Airborne Wind Turbines, no R&D either. Looking at India’s

geographic locations and wind power availability, India could be an optimal location

to harness AWES. This could definitely be happening in the days to come.

5. Conclusion

High altitude wind energy is currently a very promising resource for the sustainable

production of electrical energy. The amount of power and the large availability of

winds that blow between 300 and 10000 meters from the ground suggest that

Airborne Wind Energy Systems (AWESs) represent an important emerging renewable

energy technology. In the last decade, several companies entered in the business of

AWESs, patenting diverse principles and technical solutions for their implementation.


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In this extremely various scenario, this paper attempts to give a picture of the current

status of the developed technologies in terms of different concepts, systems and

trends. In particular, all existing AWESs have been briefly presented and classified.

The basic 1474 A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51

(2015) 1461–1476 generation principles have been explained, together with very

basic theoretical estimations of power production that could provide the reader with a

perception on which and how crucial parameters influence the performance of an

AWES. [1][2]

In the next years, a rapid acceleration of research and development is expected in the

airborne wind energy sector. Several prototypes that are currently under investigation

will be completed and tested.

5. References-

1. Airborne Wind Turbines – A Technical Report by Scope e-Knowledge Centre

Pvt. Ltd. – July 2013

2. Airborne Wind Energy Systems: A review of the technologies

by - Antonello Cherubini, Andrea Papini, Rocco Vertechy, Marco Fontana

PERCRO SEES, TeCIP Institute, Scuola Superiore Sant'Anna, Pisa, Italy

Department of Industrial Engineering, University of Bologna, Italy.

3. Airborne wind energy: Optimal locations and variability

by Cristina L. Archer, Luca Delle Monache, Daran L. Rife

College of Earth, Ocean, and Environment, University of Delaware, Newark,

DE 19716, United States;

National Center for Atmospheric Research, Boulder, CO, United States

GL Garrad Hassan, San Diego, CA, United States

4. Wikipedia.org-

https://en.wikipedia.org/wiki/Wind_power_in_India

https://en.wikipedia.org/wiki/Airborne_wind_turbine

5. Open Energy Information –

http://en.openei.org/wiki/Wind_energy

6. Turbines Info-

http://www.turbinesinfo.com/types-of-wind-turbines/


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http://www.turbinesinfo.com/types-of-wind-turbines/

http://en.openei.org/wiki/Wind_energy

https://en.wikipedia.org/wiki/Airborne_wind_turbine

https://en.wikipedia.org/wiki/Wind_power_in_India