airborne wind turbines - a report
TRANSCRIPT
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
1
Savitribai Phule Pune University
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
K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering
2
Savitribai Phule Pune University
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
K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering
3
Savitribai Phule Pune University
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]
K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering
4
Savitribai Phule Pune University
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
K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering
5
Savitribai Phule Pune University
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
K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering
6
Savitribai Phule Pune University
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)
K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering
7
Savitribai Phule Pune University
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
K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering
8
Savitribai Phule Pune University
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.
K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering
9
Savitribai Phule Pune University
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
K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering
10
Savitribai Phule Pune University
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]
K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering
11
Savitribai Phule Pune University
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
K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering
12
Savitribai Phule Pune University
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.
K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering
13
Savitribai Phule Pune University
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/
K. K. Wagh Institute of Engineering Education and Research, Nashik, Mechanical Engineering
14