Article

Airborne Wind Energy Systems: A review of the technologies

Abstract and Figures

Abstract Among novel technologies for producing electricity from renewable resources, a new class of wind energy converters has been conceived under the name of Airborne Wind Energy Systems (AWESs). This new generation of systems employs flying tethered wings or aircraft in order to reach winds blowing at atmosphere layers that are inaccessible by traditional wind turbines. Research on AWESs started in the mid seventies, with a rapid acceleration in the last decade. A number of systems based on radically different concepts have been analyzed and tested. Several prototypes have been developed all over the world and the results from early experiments are becoming available. This paper provides a review of the different technologies that have been conceived to harvest the energy of high-altitude winds, specifically including prototypes developed by universities and companies. A classification of such systems is proposed on the basis of their general layout and architecture. The focus is set on the hardware architecture of systems that have been demonstrated and tested in real scenarios. Promising solutions that are likely to be implemented in the close future are also considered.
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Airborne Wind Energy Systems: A review of the technologies
Antonello Cherubini
a
, Andrea Papini
a
, Rocco Vertechy
b
, Marco Fontana
a,
n
a
PERCRO SEES, TeCIP Institute, Scuola Superiore Sant'Anna, Pisa, Italy
b
Department of Industrial Engineering, University of Bologna, Italy
article info
Article history:
Received 14 March 2015
Received in revised form
2 July 2015
Accepted 10 July 2015
Available online 31 July 2015
Keywords:
AWE
AWES
Review
Renewable energy
Kite power
Glider
High altitude wind
abstract
Among novel technologies for producing electricity from renewable resources, a new class of wind
energy converters has been conceived under the name of Airborne Wind Energy Systems (AWESs). This
new generation of systems employs ying tethered wings or aircraft in order to reach winds blowing at
atmosphere layers that are inaccessible by traditional wind turbines. Research on AWESs started in the
mid seventies, with a rapid acceleration in the last decade. A number of systems based on radically
different concepts have been analyzed and tested. Several prototypes have been developed all over the
world and the results from early experiments are becoming available. This paper provides a review of the
different technologies that have been conceived to harvest the energy of high-altitude winds, specically
including prototypes developed by universities and companies. A classication of such systems is
proposed on the basis of their general layout and architecture. The focus is set on the hardware
architecture of systems that have been demonstrated and tested in real scenarios. Promising solutions
that are likely to be implemented in the close future are also considered.
&2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1462
2. Availability of Airborne Wind Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1464
3. Classications of Airborne Wind Energy Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1464
4. Ground-Gen Airborne Wind Energy Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1464
4.1. Ground-Gen systems architectures and aircraft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1466
4.2. Fixed-ground-station systems under development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1467
4.2.1. KiteGen Research. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1467
4.2.2. Kitenergy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1468
4.2.3. SkySails Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1468
4.2.4. TwingTec . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1468
4.2.5. TU Delft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1468
4.2.6. Ampyx Power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1468
4.2.7. EnerKite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469
4.2.8. Windlift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469
4.2.9. New companies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469
4.2.10. KU Leuven . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469
4.2.11. SwissKitePower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469
4.2.12. NASA Langley. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469
4.2.13. Others. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469
4.3. Moving-ground-station systems under development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469
4.3.1. KiteGen Research. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469
4.3.2. NTS Energie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469
4.3.3. Kitenergy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469
4.3.4. Laddermill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1470
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/rser
Renewable and Sustainable Energy Reviews
http://dx.doi.org/10.1016/j.rser.2015.07.053
1364-0321/&2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
n
Corresponding author.
E-mail address: m.fontana@sssup.it (M. Fontana).
Renewable and Sustainable Energy Reviews 51 (2015) 14611476
5. Fly-Gen Airborne Wind Energy Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1470
5.1. Aircraft in Fly-Gen systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1471
5.2. Fly-Gen systems under development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1471
5.2.1. Loyd's rst mechanical concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1471
5.2.2. Makani Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1471
5.2.3. Joby Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1472
5.2.4. Altaeros Energies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1472
5.2.5. Sky Windpower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1472
6. Crosswind ight: the key to large scale deployment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1472
6.1. Crosswind GG-AWESs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1472
6.2. Crosswind FG-AWESs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1473
7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1473
7.1. Effect of ying mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1473
7.2. Rigid vs soft wings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1473
7.3. Take-off and landing challenge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1473
7.4. Optimal altitude. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1473
7.5. Angle of attack control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1473
7.6. Cables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1474
7.6.1. Polynomial curvature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1474
7.6.2. Unsteady analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1474
7.6.3. Tethers electrostatic behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1474
7.6.4. Aerodynamic drag opportunities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1474
7.6.5. Nearly zero cable drag. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1474
7.7. Business opportunities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1474
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1474
Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1475
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1475
1. Introduction
Advancement of societies, and in particular in their ability to
sustain larger populations, are closely related to changes in the
amount and type of energy available to satisfy human needs for
nourishment and to perform work [1].Lowaccesstoenergyis
an aspect of poverty. Energy, and in particular electricity, is
indeed crucial to provide adequate services such as water, food,
healthcare, education, employment and communication. To
date, the majority of energy consumed by our societies has
come from fossil and nuclear fuels, which are now facing severe
issues such as security of supply, economic affordability, envir-
onmental sustainability and disaster risks.
To address these problems, major countries are enacting energy
policies focused on the increase in the deployment of renewable
energy technologies. In particular:
Since 1992, to prevent the most severe impacts of climate
change, the United Nations member states are committed to
a drastic reduction in greenhouse gas emissions below the
1990 levels.
In September 2009, both European Union and G8 leaders
agreed that carbon dioxide emissions should be cut by 80%
before 2050 [2].
In the European Union (EU), compulsory implementation of
such a commitment is occurring via the Kyoto Protocol, which
bounded 15 EU members to reduce their collective emissions by
8% in the 20082012 period, and the Climate Energy Package
(the 202020 targets), which obliges EU to cut its own
emissions by at least 20% by 2020.
In this context, in the last decades there has been a fast
growth and spread of renewable energy plants. Among them,
wind generators are the most widespread type of intermittent
renewable energy harvesters with their 369 GW of cumulative
installed power at the end of 2014 [3]. Wind capacity, i.e. total
installed power, is keeping a positive trend with an increment of
51.4 GW in 2014. In the future, such a growth coulddecrease due
to saturation of in-land windy areas that are suitable for
installations. For this reason, current research programs are
oriented to the improvement of power capacity per unit of land
area. This translates to the global industrial trend of developing
Fig. 1. AWESs. Example of Ground-Gen (a) and Fly-Gen (b) AWESs.
A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 146114761462
single wind turbines with increased nominal power (up to
5 MW) that feature high-length blades (to increase the swept
area) and high-height turbine axis (to reach stronger winds at
higher altitudes) [4].
In parallel, since the beginning of 2000s, industrial research is
investing on offshore installations. In locations that are far enough
from the coast, wind resources are generally greater than those on
land, with the winds being stronger and more regular, allowing a
more constant usage rate and accurate production planning, and
providing more power available for conversions. The foreseen
growth rate of offshore installations is extremely promising;
according to current forecasts, the worldwide installed power is
envisaged in the order of 80 GW within 2020 [5].
In this framework, a completely new renewable energy sector,
Airborne Wind Energy (AWE), emerged in the scientic commu-
nity. AWE aims at capturing wind energy at signicantly increased
altitudes. Machines that harvest this kind of energy can be referred
to as Airborne Wind Energy Systems (AWESs). The high level and
the persistence of the energy carried by high-altitude winds, that
blow in the range of 200 m 10 km from the ground surface, has
attracted the attention of several research communities since the
beginning of the eighties. The basic principle was introduced by
the seminal work of Loyd [6] in which he analyzed the maximum
energy that can be theoretically extracted with AWESs based on
tethered wings. During the nineties, the research on AWESs was
practically abandoned; but in the last decade, the sector has
experienced an extremely rapid acceleration. Several companies
have entered the business of high-altitude wind energy, register-
ing hundreds of patents and developing a number of prototypes
and demonstrators. Several research teams all over the world are
currently working on different aspects of the technology including
control, electronics and mechanical design.
This paper provides an overview of the different AWES concepts
focusing on devices that have been practically demonstrated with
prototypes. The paper is structured as follows. Section 2 provides a
brief description of the energy resource of high altitude winds.
Section 3 provides a unied and comprehensive classication of
different AWES concepts, which tries to merge previously proposed
taxonomies. In Sections 4 and 5,anuptodateoverviewofdifferent
devices and concepts is provided. Section 6 explains why AWE is so
attractive thanks to some simple and well-known models. Finally,
Section 7 presents some key techno-economic issues basing on the
state of the art and trends of academic and private research.
Differently from other previously published reviews, this paper
deals with aspects that concern architectural choices and mechanical
design of AWESs. We made our best in collecting comprehensive
information from the literature, patents and also by direct contacts
with some of the major industrial and academic actors.
Fig. 2. Scheme of the two-phase discontinuous energy production for GG AWESs. (a) The energy generation phase occurs during the unwinding of the ropes as the aircraft
performs a crosswind ight. (b) The recovery phase is performed in order to minimize the energy consumed for the recovery.
Fig. 3. Scheme of three different concepts of moving-ground-station GG-AWES.
(a) Vertical axis generator: ground stations are xed on the periphery of the rotor of
a vertical axis generator. (b) Closed loop rail: ground stations are xed on trolleys
that move along a closed loop rail. (c) Open loop rail: ground stations are xed on
trolleys that move along a open loop rail.
A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 14611476 1463
2. Availability of Airborne Wind Energy
In the literature, the acronym AWE (Airborne Wind Energy) is
usually employed to designate the high-altitude wind energy
resource as well as the technological sector. High-altitude winds
have been studied since decades by meteorologists, climatologists
and by researchers in the eld of environmental science even
though many questions are still unsolved [7]. The rst work aimed
at evaluating the potential of AWE as a renewable energy resource
has been presented by Archer and Caldeira [8]. Their paper
introduces a study that assesses a huge worldwide availability of
kinetic energy of wind at altitudes between 0.5 km and 12 km
above the ground, providing clear geographical distribution and
persistency maps of wind power density at different ranges of
altitude. This preliminary analysis does not take into account the
consequences on wind and climate of a possible extraction of
kinetic energy from winds. However, the conclusions of these
investigations already raised the attention of many researchers
and engineers suggesting great promises for technologies able to
harvest energy from high altitude winds.
More in depth studies have been conducted employing complex
climate models, which predict consequences associated with the
introduction of wind energy harvesters (near surface and at high
altitude), that exerts distributed drag forces against wind ows. Marvel
et al. [9] estimate a maximum of 400 TW and 1800 TW of kinetic
power that could be extracted from winds that blow, respectively,
near-surface (harvested with traditional wind turbines) and through
the whole atmospheric layer (harvested with both traditional turbines
and high altitude wind energy converters). Even if severe/undesirable
changes could affect the global climate in the case of such a massive
extraction, the authors show that the extraction of only18 TW (i. e. a
quantity comparable with the actual world power demand) does not
produce signicant effects at global scale. This means that, from the
geophysical point of view, very large quantity of power can be
extracted from wind at different altitudes.
A more skeptical view on high altitude winds is provided in
Miller et al. [10] who evaluated in 7.5 TW the maximum sustain-
able global power extraction. But their analysis is solely focused on
jet stream winds (i.e. only at very high altitude between 6 km and
15 km above the ground).
Despite the large variability and the level of uncertainty of
these results and forecasts, it is possible to conclude that
an important share of the worldwide primary energy could be
potentially extracted from high altitude winds. This makes it
possible to envisage great business and research opportunities
for the next years in the eld of Airborne Wind Energy.
3. Classications of Airborne Wind Energy Systems
In this paper, the term AWESs (Airborne Wind Energy Systems) is
used to identify the whole electro-mechanical machines that trans-
form the kinetic energy of wind into electrical energy. 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 [11] (Fig. 1).
In a Ground-Gen AWES (GG-AWES), electrical energy is pro-
duced 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 xed-ground-
station devices, where the ground station is xed 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 con-
sumed. Among FG-AWESs it is possible to nd crosswind systems
and non-crosswind systems depending on how they generate
energy.
4. Ground-Gen Airborne Wind Energy Systems
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. As previously anticipated, GG-AWESs can be distinguished
in devices with xed or moving-ground-station.
Fig. 4. Control layout of crosswind GG-AWESs. (a) With on-board control actuators; (b) with ying control pod; (c) control through power ropes; (d) with additional
control rope.
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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 (Fig. 2). 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 ight is the
crosswind ight (Fig. 2a) with circular or the so-called eight-
shaped paths. As compared to a non-crosswind ight (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
energy spent in the recovery phase. This is guaranteed by a control
system that adjusts the aerodynamic characteristics of the aircraft
[12] and/or controls its ight path [13] in a way to maximize the
energy produced in the generation phase and to minimize the
energy consumed in the recovery phase.
Pumping kite generators present a highly discontinuous power
output, with long alternating time-periods (in the order of tens of
seconds) of energy generation and consumption. Such an unat-
tractive feature makes it necessary to resort to electrical rectica-
tion means like batteries or large capacitors. The deployment of
multiple AWES in large high-altitude wind energy farms could
signicantly reduce the size of electrical storage needed.
Fig. 5. Different types of aircraft in Ground-Gen systems. (a) LEI SLE (Leading Edge Inatable, Supported Leading Edge) Kite; (b) LEI C-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.
Fig. 6. Control of bridles tension. (a) Control bridles are attached to the leading and trailing edges of a LEI SLE kite. (b) A control pod can be used to control the ight
trajectory and angle of attack.
A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 14611476 1465
Moving-ground-station GG-AWES are generally more complex
systems that aim at providing an always positive power ow
which makes it possible to simplify their connection to the grid.
There are different concepts of moving-ground-station GG-AWESs
(Fig. 3) but no working prototype has been developed up to date
and only one prototype is currently under development (see
Section 4.3.2). Differently from the pumping generator, for
moving-ground-station systems, the rope winding and unwinding
is not producing/consuming signicant power but is eventually
used only to control the aircraft trajectory. The generation takes
place thanks to the traction force of ropes that induces the rotation
(or linear motion) of a generator that exploits the ground station
movement rather than the rope winding mechanism.
Basically, there are two kinds of moving-ground-station GG-
AWES:
Vertical axis generator(Fig. 3a) where ground stations are
xed on the periphery of the rotor of a large electric generator
with vertical axis. In this case, the aircraft forces make the
ground stations rotate together with the rotor, which in turn
transmits torque to the generator.
Rail generators(closed loop rail (Fig. 3b) or open loop rail
(Fig. 3c)) where ground stations are integrated on rail vehicles
and electric energy is generated from vehicle motion. In these
systems, energy generation looks like a reverse operation of an
electric train.
The following subsections provide an overview of the most
relevant prototypes of GG AWESs under development in the
industry and the academy.
4.1. Ground-Gen systems architectures and aircraft
In GG systems the aircraft transmits mechanical power to the
ground by converting wind aerodynamic forces into rope tensile
forces. The different concepts that were prototyped are listed in
Fig. 4; examples of aircraft of GG systems that are currently under
development are presented in Fig. 5. They exploit aerodynamic lift
forces generated by the wind on their surfaces/wings.
The aircraft is connected to the ground by at least one power-
rope that is responsible for transmitting the lift force (and the
harvested power) to the ground station. The ight trajectory can
be controlled by means of on-board actuators (Fig. 4a), or with a
control pod (Fig. 4b), or by regulating the tension of the same
power-ropes (Fig. 4c), or with thinner control-ropes (Fig. 4d).
Fig. 7. Groud-Gen AWESs. Summary of GG-AWESs.
A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 146114761466
There are also two GG concepts that are worth mentioning: one
uses parachutes which exploit aerodynamics drag forces [14,15],
the other uses rotating aerostats which exploit the Magnus effect
[16,17].
The most important aircraft used for GG systems are here
listed:
1. Leading Edge Inatable (LEI) kites [18] are single layer kites
whose exural stiffness is enhanced by inatable structures on
the leading edge (Fig. 5a and b). Mainly two kinds of LEI kites
are used in AWESs:
(a) Supported Leading Edge (SLE) kites [19] are LEI kites with at
least one bridle which supports the leading edge close to its
central part (Fig. 5a). In comparison with C-kites (that are
described in the following), the traction force of the central
bridles makes the wing at in its central region and this is
claimed to increase the wing aerodynamic efciency.
(b) C-kites, which are generally controlled by four main bridles
directly attached to extreme lateral points of the kite edges
(Fig. 5b). In pumping generators, the C-kite is held with
either one, two or three ropes. In generators with one rope,
the rope is connected to both the leading edge bridles,
while trailing edge bridles are controlled by a control pod
(i.e. a ying box with one or more actuators) attached to
the rope a few meters below the kite. The micro-winches
inside the control pod are used to steer the kite and control
the angle of attack. In case of two ropes, left bridles
converge in one rope and right bridles converge in the
second rope. The angle of incidence is xed and the kite
steers due to the difference in the ropes tension. In case of
three ropes, there is one rope for each trailing edge bridle
and one rope connected to the leading edge bridles. In this
case, kite steering and angle of attack can be controlled
from the ground.
The stiffened tube-like structure of LEI kites is especially useful
for take-off and landing maneuvers when the wing is not yet
supported by wind pressure. The ease of handling is very
appreciated also during small-scale prototyping and subsystem
testing. However LEI kites have severe scalability issues as the
tube diameter needs to be oversized in case of large wings.
2. Foil kites (also called ram-air kites) are derived from parafoils
[20]. These double-layer kites are made of canopy cells which
run from the leading edge to the trailing edge (Figs. 5c and 6b).
Cells (some or all) are open on the leading edge in a way that
the air inates all cells during the ight and gives the kite the
necessary stiffness. Bridles are grouped in different lines,
frequently three: one central and two laterals. With respect
to LEI kites, foil wings have a better aerodynamic efciency
despite the higher number of bridles and can be one order of
magnitude larger in size.
3. Delta kites are similar to hang glider wings. They are made by a
single layer of fabric material reinforced by a rigid frame.
Compared with LEI or foil kites, this kind of aircraft has a
better aerodynamic efciency which in turn results in a higher
efciency of wind power extraction (as discussed in Section 7).
On the other hand, their rigid frame has to resist to mechanical
bending stresses which, in case of high aerodynamic forces,
make it necessary to use thick and strong spars which increase
the aircraft weight, cost and minimum take-off wind speed.
Durability for fabric wings such as LEI, foil and delta kites, is an
issue. Performance is compromised soon and lifetime is usually
around several hundred hours [21].
4. Gliders (Fig. 5d) can also be used as GG aircraft. Like delta kites,
their wings are subject to bending moment during the tethered
ight. Gliders, and more generally rigid wings, have excellent
aerodynamic performance, although they are heavier and more
expensive. Lifetime with regular maintenance is several
decades.
5. Swept rigid wings are gliders without fuselage and tail control
surfaces (Fig. 5e). Flight stability is most likely achieved thanks
to the bridle system and the sweep angle.
6. Semi-rigid wings are also under investigation by the Italian
company Kitegen Research. They are composed of multiple
short rigid modules that are hinged to each other (Fig. 5f). The
resulting structure is lighter than straight rigid wings and more
aerodynamically efcient and durable than fabric kites.
7. Special design kites: Kiteplanes [22] and Tensairity Kites [23] are
projects developed by TUDelft (The Netherlands) and EMPA
(Research Center for Synergetic Structures, ETH Zurich), that
aim at increasing the aerodynamic efciency of arch kites
without using rigid spars.
4.2. Fixed-ground-station systems under development
This subsection provides a list of xed-ground-station GG
AWES which are summarized in Figs. 7 and 10.
4.2.1. KiteGen Research
The Italian KiteGen Research (KGR) was one of the rst
companies to test a prototype of Ground-Gen AWES [24].KGR
technology is based on a C-Kite integrating on board electronics
with sensor and is controlled by two power-ropes [25] from a
control station on the ground [26] (Fig. 4c). The rst prototype,
named KSU1 (acronym for Kite Steering Unit) [25,27], was suc-
cessfully demonstrated in 2006. After a few years of tests, the
company focused on the development of a new generator, named
KiteGen Stem, with a nominal power of 3 MW [28]. In this
system, the ropes are wound on special winches [29] and are
driven by a pulley system through a 20 m exible rod, called
stem, to an arch-kite or a semi-rigid wing. The stem is linked to
the top of the control station through a pivot joint with horizontal
axis. The most important functions of the stem are: (1) supporting
and holding the kite and (2) damping peak forces in the rope that
arise during wind-gusts. The entire control station can make
azimuthal rotations so the stem has two degrees of freedom
relative to the ground. The Stemconcept was rst patented in
2008 [30] and is now used by more and more companies and
universities.
At the beginning of the take-off maneuvers, the kite is hanged
upside down at the end of the stem. Once the kite has taken off,
the production phase starts: the automatic control drives the kite
acting on the two ropes, the kite makes a crosswind ight with
eight shapepaths; at the same time ropes are unwound causing
the winches to rotate; the motor-generators transform mechanical
power into electric power. The company aims at retracting the
cables with minimum energy consumption thanks to a special
maneuver called side-slipor agging[21]. Side-slip is a different
ight mode where the kite aerodynamic lift force is cleared by
rewinding at rst one rope before the other, which makes the kite
lose lift and stalland then, once fully stalled, both ropes are
rewound at the same speed and the kite precipitates ying
sideways. This maneuver can be done with exible foil kites or
semi-rigid wings. In this phase, the power absorbed by motor-
generators is given by rope rewind speed multiplied by the
resulting aerodynamic drag force of the side-slip ying mode.
This power consumption would be a small percentage of the
power produced in the production phase. After rewinding a
certain length of the ropes (less than the total rope length in
order to exploit only the highest winds) another special maneuver
A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 14611476 1467
restores the gliding ight and the aerodynamic lift force on the
kite. At this point one pumping cycle ends and a new production
phase starts.
KGR patented and is developing special aerodynamic ropes [31]
in order to increase their endurance and to increase system
performances. KGR also plans to use the Kitegen Stem technology
to produce an offshore AWES [32] since offshore AWESs are very
promising [33].
4.2.2. Kitenergy
Another Italian company, Kitenergy, was founded by a former
KiteGen partner and is also developing a similar concept by
controlling a foil kite with two ropes [34,35]. The prototype of
the company features 60 kW of rated power [36]. Kitenergy
submitted also a different GG-AWES patent [37] that consists in
a system based on a single motor-generator which controls
winding and unwinding of two (or more) cables and another
actuator that introduces a differential control action of the
employed cables. Another prototype developed by its co-founder,
Lorenzo Fagiano, achieved 4 h of consecutive autonomous ight
with no power production at University of California at Santa
Barbara in 2012 [38].
4.2.3. SkySails Power
The German company SkySails GmbH is developing a wind
propulsion system for cargo vessels based on kites [39]. A few
years ago a new division of the company SkySails Powerhas been
created to develop Ground-Gen AWES [40] based on the technol-
ogy used in SkySails vessel propulsion system. Two products are
under development: a mobile AWES having a capacity between
250 kW and 1 MW, and an offshore AWES with a capacity from
1 to 3.5 MW. SkySails' AWES is based on a foil kite controlled with
one rope and a control pod (Fig. 4b) which controls the lengths of
kite bridles for steering the kite and changing its angle of attack
[41]. Control pod power and communication with the ground
station is provided via electric cables embedded in the rope.
SkySails also has a patented launch and recovery system [42]
designed for packing the kite in a storage compartment. It is
composed by a telescopic mast with a special device on its top that
is able to grab, keep and release the central point of the kite
leading edge. When the system is off, the mast is compacted in the
storage compartment with the kite deated. At the beginning of
the launching operation, the mast extends out vertically bringing
the deated kite some meters above the ground (or the sea level).
The kite is then inated to have appropriate shape and stiffness for
the production phase. Kite take-off exploits only the natural wind
lift force on the kite: the system at the top of the mast releases the
kite leading edge, the pod starts to control the ight and the winch
releases the rope letting the kite reach the operating altitude.
While the energy production phase is similar to that of the KGR
generator, SkySails has a different recovery phase. Specically,
SkySails uses high speed winching during reel-in while the kite is
kept at the edge of the wind window. The kite is then winched
directly against the wind without changing the kite angle of
attack. Though it might seem counter-intuitive at rst, this kind
of recovery phase has proven to be competitive [43].
4.2.4. TwingTec
The Swiss company Twingtec is developing a 100 kW GG-
AWES. After having tried several concepts including soft wings
and rigid wings, the team is now tackling the problem of auto-
mating take-off and landing with an innovative concept: a glider
with embedded rotors having rotational axis perpendicular to the
wing plane. The rotors are used during take-off and landing. The
company plans to have the generator and power conversion
hardware inside a standard 20-foot shipping container in order
to easily target off-grid and remote markets. The AWES will supply
continuous and reliable electrical power thanks to the integration
with conventional diesel generators [44,45].
4.2.5. TU Delft
At Delft University of Technology, the rst research in Airborne
Wind Energy was started by the former astronaut, Professor
Ockels, in 1996 [46]. A dedicated research group was initiated by
Ockels in 2004 with the aim to advance the technology to the
prototype stage.
Recently, Delft University of Technology and Karlsruhe University
of Applied Sciences have initiated a joint project to continue the
development and testing of a mobile 20 kW experimental pumping
kite generator [47]. A main objective of this project is to improve the
reliability and robustness of the technology and to demonstrate in the
next months a continuous operation of 24 h. At present, they use the
third version of a special design LEI kite, co-developed with Genetrix/
Martial Camblong, of 25 m
2
wing surface area. Together with an
automatic launch setup [48], the wing demonstrated fully automatic
operation of their 20 kW system in 2012 [47]. Like SkySails' system,
this prototype is based on a single tether and an airborne control pod
(Fig. 4b) but they also control the angle of attack for powering and
depowering the wing during production and recovery phase, respec-
tively. An automatic launch and retrieval system for 100 m
2
LEI kites is
under development [49].
In the past, the research group tested several kinds of wings
such as foil kites and kiteplanes. TU Delft also tested an alternative
device for controlling the kite: a cart-and-rail system attached to
the tips of a ram-air wing and used to shift the attachment point of
the two bridle lines. By that system, the wing could be steered and
depowered with a minimal investment of energy. Ultimately, the
concept was too complex and too sensitive to deviations from
nominal operation [50].
4.2.6. Ampyx Power
The rst company that developed a pumping glider generator is
the Dutch Ampyx Power [51,52]. After several prototypes, they are
currently developing and testing two 5.5 m PowerPlanesthe AP-
2A1 and the AP-2A2 [53]. They are two ofcially registered aircraft
that are automatically controlled with state of the art avionics.
They are constructed with a carbon ber body and a carbon
backbone truss which houses onboard electronics with sensors
and actuators. Onboard actuators can drive a rudder, an elevator
and four aperons. One rope connects the glider to a single winch
in the ground station (Fig. 4a). Ampyx Power is actually one of the
few companies which has already developed an AWES [54] that is
able to automatically perform the sequence of glider take-off,
pumping cycles and landing. Take-off maneuver sees the glider
lying on the ground facing the ground station at some meters of
distance. As the winch starts exerting traction force on the rope,
the glider moves on the ground and, as soon as the lift forces
exceed the weight forces, the glider takes off. They also installed a
catapult for take-off and they have a propulsion system to climb
up. The glider ight is fully autonomous during normal operations
even though, for safety reasons, it can be occasionally controlled
wirelessly from the ground thanks to a backup autopilot. The
pumping cycles are similar to those of a kite. Glider landing is
similar to that of an airplane and is being equipped with an
arresting line so as to stop the glider in a right position for a new
take-off. During a test campaign in November 2012, the system
demonstrated an average power production of 6 kW with peaks of
over 15 kW (earlier tests showed peak in power production of
30 kW). Ampyx has started the design of its rst commercial
A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 146114761468
product: a 35 m wingspan AP-4 PowerPlane with a wind turbine
equivalentpower of 2 MW.
4.2.7. EnerKite
The German company EnerKite [55] developed a portable pump-
ing kite generator with rated continuous power of 30 kW. The ground
station is installed on a truck through a pivotal joint which allows
azimuthal rotations. EnerKite demonstrator uses mainly a foil kite, but
a delta kite and a swept rigid wing are also under investigation and
testing. The aircraft does not have on-board sensors and is controlled
from the ground with three ropes according to the scheme of Fig. 4d.
EnerKite is now developing an autonomous launch and landing
system for semi-rigid wings [56]. The company plans to produce a
100 kW and a 500 kW system [57].
4.2.8. Windlift
The US Company Windlift [58] has a concept similar to that of
Enerkite (Fig. 4d). Their 12 kW prototype uses SLE kites. They aim
to sell their product to the military and to off-grid locations.
4.2.9. New companies
The AWE community is constantly growing [24,59] and for
every company that goes out of business there are a few that are
born. Here are some startup companies that are worth
mentioning.
e-Kite was founded in 2013 in the Netherlands and developed a
50 kW GG-AWES (Fig. 4c) based on a direct drive generator. The
company is now building a 2-ropes rigid wing that will y at low
altitude [60,61].
Enevate is a Dutch 4-people startup that is mainly focused on
bringing the TU Delft GG-AWES to the next step towards a
commercial product [62].
Kitemill, Norway, started the development of a GG-AWES. The
company switched early on to a 1-cable rigid wing system with
on-board actuators (Fig. 4a) after having faced controllability and
durability issues with soft materials [63].
eWind solutions is a US company that is developing an
unconventional, low altitude, rigid wing GG-AWES [64].
4.2.10. KU Leuven
KU Leuven has been actively doing research in AWESs since
2006. After signicant theoretical contributions, the team devel-
oped a test bench to launch a tethered glider with a novel
procedure [65]. Before take-off, the glider is held at the end of a
rotating arm. When the arm starts rotating, the glider is brought to
ying speed and the tether is released allowing the glider to gain
altitude. They are currently developing a larger experimental test
set-up, 2 m long with a 10 kW winch.
4.2.11. SwissKitePower
SwissKitePower was a collaborative research and development
project started in Switzerland in 2009. It involved four laboratories
of different Swiss universities: FHNW, EMPA (Swiss Federal
Laboratories for Materials Science and Technology), ETH (Federal
Politechnique of Zurich) and EPFL (École Polytechnique Fédérale
de Lausanne). The rst prototypes, tested between 2009 and 2011,
were based on a C-kite controlled by one rope and a control pod.
The initial system worked according to the scheme of Fig. 4b,
similarly to KitePower and SkySails prototypes. In 2012, SwissKite-
Power developed a new ground station with three winches that
can be used to test kites with 1, 2 or 3 lines [66]. They also tested
SLE kites and tensairity kites. The project ended in 2013 and since
then FHNW is working in collaboration with the company
TwingTec.
4.2.12. NASA Langley
At Langley Research Center, the US space agency NASA con-
ducted a study about wind energy harvesting from airborne
platforms after which they developed an AWES demonstrator
based on a kite controlled by two ropes and having a vision-
based system and sensors located on the ground [67].
4.2.13. Others
In addition to the main prototypes listed above, there are
several other systems that have been built.
Wind tunnel tests of small scale non-crosswind generation, and
outdoor crosswind generation tests with a SLE kite [68,69] of
GIPSA-lab/CNRS, University of Grenoble.
Kite control project [70] of CCNR at Sussex University, UK.
EHAWK (Electricity from High Altitude Wind with Kites)
project [71] of Department of Mechanical Engineering of
Rowan University.
Kite powered water pump [72] of Worcester Polytechnic
Institute.
4.3. Moving-ground-station systems under development
In addition to pumping systems, a number of AWES concepts
with moving-ground-station have been proposed. Their main
advantage is the ability to produce energy continuously or nearly
continuously. However, only a few companies are working on
AWESs with moving-ground-station and there are more patents
and studies than prototypes under development. This subsection
provides a list of moving-ground-station GG-AWES which are
summarized in Figs. 7 and 10.
4.3.1. KiteGen Research
The rst moving-ground-station architecture which is based
on a vertical axis generator has been proposed back in 2004 by
Sequoia Automation and acquired by KGR [73].ThisAWES
concept is based on the architecture described in Fig. 3a. During
operations, lift forces are transmitted to a rotating frame indu-
cing a torque around the main vertical axis. Torque and rotation
are converted into electricity by the electric generator. This
system can be seen as a vertical axis wind turbine driven by
forces which come from tethered aircraft. There is no prototype
under development, but the concept has been studied in a
simulation [13] showing that 100 kites with 500 m
2
area could
generate 1000 MW of average power in a wind with speed of
12 m/s. The considered generator would have a 1500 m radius,
occupying a territory about 50 times smaller and costing about
30 times less than a farm of wind turbines with the same
nominal power.
4.3.2. NTS Energie
An alternative system based on ground stations that moves on
closed track circuits is proposed by KGR [74] and by the German
company NTS Energie und Transportsysteme [75,76]. Starting from
September 2011, NTS tested a prototype where 4-rope kites are
controlled by a vehicle which moves on a 400 m at-bed straight
railway track. They are able to produce up to 1 kW per m
2
of wing
area and they tested kites up to 40 m
2
[77]. The nal product
should have a closed loop railway where more vehicles run
independently.
4.3.3. Kitenergy
Another rail concept is proposed by Kitenergy [78] and it is
based on ideas published in 2004 in Drachen Foundation journal
[79]. The concept is based on a straight linear rail xed on the
A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 14611476 1469
ground with a pivotal joint. The rail direction is then adjusted
perpendicular to the main direction of the wind. The ground
station of the system is mounted on a wheeled vehicle which
moves along the straight rail, under the kite traction forces, back
and forth from one side to the other. The power is extracted from
electromagnetic rotational generators on the wheels of the vehicle
or from linear electromagnetic generators on the rail. The power
production is not fully continuous because during the inversion of
vehicle direction the power production will not only decrease to
zero, but it could also be slightly negative. Nevertheless the kite
inversion maneuver could be theoretically performed without the
need of power consumption.
4.3.4. Laddermill
Although it cannot be considered a moving-ground-station
device, it is important to mention that the rst concept of
continuous energy production AWES was the Laddermill concept
envisaged by the former astronaut, Professor Ockels in 1996 [46].
5. Fly-Gen Airborne Wind Energy Systems
In Fly-Gen AWESs, electric energy is produced onboard of the
aircraft during its ight and it is transmitted to the ground trough
one special rope which integrates electric cables. Electrical energy
Fig. 8. Different types of aircraft in Fly-Gen systems. (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, design by Altaeros Energies. (d) Static suspension quadrotor in autorotation, design by Sky
WindPower.
Fig. 9. Fly-Gen AWESs. Summary of FG-AWESs.
A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 146114761470
conversion in FG-AWESs is achieved using one or more specially
designed wind turbines.
A general classication of these systems is provided in this
section.
5.1. Aircraft in Fly-Gen systems
Besides the general classication between crosswind and non-
crosswind mode proposed in Fig. 10, FG-AWESs can also be
distinguished basing on their ying principles that are:
Wings lift: Achieved with a tethered ight of special gliders
(Fig. 8a) or frames with multiple wings (Fig. 8b).
Buoyancy and static lift: Achieved with aerodynamically shaped
aerostats lled with lighter-than-air gas (Fig. 8c).
Rotor thrust: Achieved with the same turbines used for elec-
trical power generation (Fig. 8d).
Aircraft in Fig. 8a and 8b y crosswind and harvest the relative
wind, while those in Fig. 8c and 8d y non-crosswind and harvest
the absolute wind.
There is also one FG concept that aims at exploiting high
altitude wind energy not by using aerodynamic lift. It uses instead
a rotating aerostat which exploits the Magnus effect [80,81].
5.2. Fly-Gen systems under development
This subsection provides a list of FG AWES which are summar-
ized in Figs. 9 and 10.
5.2.1. Loyd'srst mechanical concept
One of the most famous and old idea of exploiting wind energy
using turbines on a kite belongs to Loyd [6] who calculated that
wind turbines installed on a crosswind ying kite could be able to
generate up to 5 times the power produced by equivalent turbines
installed on the ground. He also patented his idea in 1978 [82].
Loyd's concept foresees a reciprocating wind driven apparatus,
similar to a multi propeller plane, with a plurality of ropes linking
the aircraft to a ground station.
5.2.2. Makani Power
After about twenty-ve years from Loyd's work, Makani Power
Inc. [83] has started the development of its Airborne Wind Turbine
(AWT) prototypes (as in Fig. 8a). In nine years, Makani tested
several AWESs concepts including Ground-Gen, single rope, multi-
ple rope, movable ground station on rails, soft wings and rigid
wings [84]. During these years, the company led several patents
where an electric and modern version of Loyd's idea has been
enriched with a tether tension sensor [85], an aerodynamic cable
[86], and with a new idea of a bimodal ight [87] that has been
invented to solve take-off and landing issues. In the bimodal ight
the AWT takes off with the wing plane in a vertical position, driven
by propellers thrust. This ight mode is similar to a quadcopter
ight and rotors on AWT are used as engines. Once all the rope
length has been unwound, the AWT changes ight mode becoming
a tethered ight airplane. In this second ight mode a circular
ight path is powered by the wind itself and rotors on AWT are
used as generators [88] to convert power from the wind. During
this phase the cable length is xed. In order to land, a new change
of ight mode is performed, and the AWT lands as a quadcopter.
Makani has developed and tested its 8 m, 20 kW demonstrator,
called Wing 7that showed the capability of fully automatic
operations and power production. After these results, in early
2013 Makani was acquired by Google. Makani is currently devel-
oping a 600 kW prototype, the M600. The M600 AWT has eight
turbines, each with ve propeller blades, and has a wingspan of
28 m. The prototype is now undergoing testing [89]. After M600,
Makani plans to produce an offshore commercial version of AWT
with a nominal power of 5 MW featuring 6 turbines and a
wingspan of 65 m.
Fig. 10. Classication of AWESs. The different AWESs concepts are listed here as explained in Section 3.
Fig. 11. Basic crosswind model. A simple and well known model for assessing the
power output of a crosswind Ground-Gen AWES [6,106,107].
A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 14611476 1471
5.2.3. Joby Energy
Founded in 2008, Joby Energy Inc. [90] is another US company
which is developing a FG-AWES. The main difference between
Joby and Makani is that the tethered airborne vehicle is a multi-
frame structure with embedded airfoils. Turbines are installed in
the joints of the frame (as in Fig. 8b). In Joby's concept, the system
could be adapted 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 [91,92]. Joby also patented
an aerodynamic rope for its system [93]. In 2009 and 2010, Joby
tested different small scale prototypes.
5.2.4. Altaeros Energies
Another project based on ying wind turbines in a stationary
position has been developed by Altaeros Energies, a
Massachusetts-based business led by MIT and Harvard alumni
[9496]. In this case, instead of using wings lift to y, they use a
ring shaped aerostat with a wind turbine installed in its interior
(as in Fig. 8c). The whole generator is lighter than the air, so the
take-off and landing maneuvers are simplied, and the only
remaining issue is the stabilization of the generator in the right
position relative to the wind [97]. The aerostat is aerodynamically
shaped so that the absolute wind generates lift that helps keeping
a 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 congurations.
5.2.5. Sky Windpower
Sky Windpower Inc. [98] proposed a different kind of tethered
craft called Flying Electric Generator(FEG) (as in Fig. 8d) 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 [99102]. Their concept was the
rst AWES to be tested in 1986 at University of Sidney [11,103].
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 deg)
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 ows 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 around 10 m/s at an operational altitude
of 4600 m [104]. Unfortunately the company went recently out of
business.
6. Crosswind ight: the key to large scale deployment
One of the most important reasons why AWESs are so attrac-
tive is their theoretical capability of achieving the megawatt scale
with a single plant. For example in [11] a 34 MW plant is envisaged
with a tethered Airbus A380, and many other publications present
theoretical analyses with MW scale AWES [6,33,105]. This scal-
ability feature is rare in renewable energies and is the key to
successful commercial development.
With reference to the extraction principles explained in Sec-
tions 4and 5, this section gives an introduction to the modelling of
crosswind ight, the most used ight mode in AWE. Modelling the
principle of crosswind ight is the rst necessary step towards the
understanding of AWESs and their potential. A well known basic
model is explained for the case of Ground-Gen and Fly-Gen
crosswind AWESs.
Only crosswind generation is analyzed because it was demon-
strated that it can provide a power one or two orders of magnitude
higher than non-crosswind generation [6]. AWESs concepts that
exploit crosswind power have therefore a strong competitive
advantage over non-crosswind concepts in terms of available
power and, therefore, in the economics of the whole system.
6.1. Crosswind GG-AWESs
This section explains how to compute the power output of a
xed-ground-station crosswind GG-AWES (Fig. 1a) during the
reel-out phase (Fig. 2a). As already introduced in Section 4,in
GG-AWESs, the recovery phase (Fig. 2b) represents an important
factor in the computation of the average power output but, for
simplicity, it is not considered in the following model.
The expression of the maximum power, P, for crosswind
Ground-Gen AWES can be derived following the analytical opti-
mization on the reel-out speed from [6] with the integrations from
[106] and [107]. The hypotheses are: high equivalent aerodynamic
efciency, steady-state crosswind ight at zero azimuth angle
from the wind direction, negligible inertia and gravity loads with
respect to the aerodynamic forces.
With reference to Fig. 11, the velocities are sketched in blue and
the forces are sketched in red. The velocity triangle at the kite is
composed by the three components V
k
,V
a
and V
n
w
: the aircraft
speed, the apparent wind speed at the aircraft and the wind speed
felt by the aircraft, respectively. V
r
is the reel-out velocity (i.e. the
velocity of the cable in the direction of its own axis) and V
w
is the
actual wind speed. V
n
w
is dened as V
n
w
¼V
w
cos θV
r
where
θ
is
the angle between tether and wind direction (that corresponds to
the angle of altitude in case of horizontal wind speed and aircraft
at zero azimuth). T
k
is the tether tension, Lis the wing lifting force,
Dis the drag force (i.e. the drag force of the aircraft D
w
plus the
equivalent cable drag force D
ce
). Notice that the velocity triangle
and the force triangle are similar because of the force equilibrium
at the aircraft, and therefore V
k
¼V
n
w
G, where Gis the equivalent
aerodynamic efciency [106],G¼L=Dthat is further explained in
Eq. (2). Assuming V
a
V
k
(valid thanks to the hypothesis of high
aerodynamic efciency) and imposing the equilibrium at the
aircraft, the tether tension is T
k
¼
1
2
ρV
n
w
2G
2
C
L
A, where
ρ
is the air
density. The power output can then be computed as
P
k
¼T
k
V
r
¼
1
2
ρðV
w
cos θV
r
Þ
2
V
r
G
2
C
L
A. This expression can be opti-
mized [6] in order to choose the reel-out speed that maximizes the
Fig. 12. Basic crosswind model. A simple and well known model for assessing the
power output of a crosswind Fly-Gen AWES [6,106,107].
A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 146114761472
power output, by setting dP
k
=dV
r
¼0. This simple optimization
leads to the choice of optimal reel-out speed V
ro
¼1=3V
w
cos θ,
and to the optimal power output, P
P¼1
2ρðV
w
cos θÞ
3
4
27G
2
C
L
Að1Þ
where Ais the area of the kite and Gis the equivalent aerodynamic
efciency, that is
G¼L
D¼C
L
C
D
w
þC
D
ce
ð2Þ
where C
L
and C
D
w
are the wing lift and drag coefcients, respec-
tively and C
D
ce
is equivalent cable drag coefcient. The expression
of C
D
ce
can be computed by equating the energy dissipated by the
distributed cable drag force to the energy that would be dissipated
by a concentrated equivalent drag force located at the top end of
the cable. This leads to R
r
0
nD
c
ðxÞV
c
ðxÞdx¼D
ce
V
k
where nis the
number of cables, ris the length of each cable, V
c
(x) is the velocity
in every point of the cable, V
c
¼V
k
x=r, and D
c
is the distributed
cable drag, D
c
¼
1
2
ρV
2
c
dC
?
where dis the cable diameter and C
?
is
the perpendicular cable drag coefcient. The expression for the
equivalent cable drag coefcient is therefore given as follows:
C
D
ce
¼
1
2
ρV
2
k1
4
C
?
nrd
1
2
ρV
2
k
A¼C
?
nrd
4A:ð3Þ
6.2. Crosswind FG-AWESs
This section explains how to compute the power output of a
crosswind FG-AWES (Fig. 1b) during the generation phase. As
already introduced in Section 5, unlike crosswind GG-AWESs,
crosswind FG-AWESs have the advantage of being able to produce
power without the need of a duty cycle for the recovery phase.
Similarly to what already done in Section 6.1, the expression of
the available crosswind power, P, for Fly-Gen AWES can be derived
following the analytical optimization on the ying generator drag
from [6] with the integrations from [106] and [107].
With reference to Fig. 12, using the assumptions and the
symbols dened in Section 6.1, it is possible to compute the
maximum power, P, for crosswind Fly-Gen AWES by combining
three expressions: the drag force from the ying generators
D
g
¼
1
2
ρV
2
a
C
D
g
A, the apparent wind velocity V
a
¼ðV
w
cos θÞC
L
=
ðC
D
þC
D
g
Þwhere C
D
¼C
D
w
þC
D
ce
, and the generators power
P
g
¼D
g
V
a
. Therefore P
g
¼
1
2
ρV
3
w
AðC
3
L
C
D
g
Þ=ðC
D
þC
D
g
Þ
3
. A simple opti-
mization dP
g
=dC
D
g
¼0 leads to the maximum value of power:
P¼1
2ρðV
w
cos θÞ
3
4
27G
2
C
L
Að4Þ
that is the same as the Ground-Gen case and occurs when the drag
of the ying generators is chosen to be C
D
g
¼C
D
=2. It is worth
noticing that the maximum power expressed in Eq. (4) does not
take into account the conservation of momentum and a more
rigorous procedure would yield a slightly lower value of P[108].
However, Eq. (4) correctly denes the upper bound for the
maximum power and is useful for rst assessments.
7. Discussion
In this section, we provide a brief discussion on some techno-
economic issues and topics that are considered to be relevant with
respect to the current development, trends, and future roadmap
of AWESs.
7.1. Effect of ying mass
In all AWESs, increasing the ying mass decreases the tension
of the cables. Since Ground-Gen systems rely on cables tension to
generate electricity, a higher mass of the aircraft and/or cables
decreases the energy production [107] and should not be
neglected when modelling [109]. On the contrary, increasing the
ying mass in Fly-Gen systems does not affect the energy
production even though it still reduces the tension of the cable.
Indeed, as a rst approximation, the basic equations of Fly-Gen
power production do not change if the aircraft/cable mass is
included and this is also supported by experimental data [108].
7.2. Rigid vs soft wings
A question faced by many companies and research groups is
whether rigid wings are better or worse than soft wings. On the
plus side for soft wings there are: crash-free tests and lower
weight (therefore higher power) because of the inherent tensile
structure. Conversely, rigid wings have better aerodynamic ef-
ciency (therefore higher power) and they do not share the
durability issues of soft wings mentioned in Section 4. It is unclear
whether one of the two solutions will prove to be better than the
other, but a trend is clearly visible in the AWE community: even
though a lot of academic research is being carried out on soft
wings, more and more companies are switching from soft to rigid
wings [110].
7.3. Take-off and landing challenge
Starting and stopping energy production require special take-
off and landing maneuvers as explained in Sections 4and 5. These
are the most difcult to automate and are requiring a lot of
research in private companies and academic laboratories
[49,55,65].
7.4. Optimal altitude
Another interesting question is how much is the optimal ight
altitude, i.e. how much are the optimal cable length and elevation
angle that maximize the power output. Increasing the altitude
allows to reach more powerful winds, but, at the same time,
increasing the cable length or the elevation angle reduces the
power output according to Eqs. (1)(4). Considering a standard
wind shear prole, the optimal ight altitude is found to be the
minimum that is practically achievable [108,111]. However, results
greatly change depending on the hypotheses and, for example, a
reduction in cables drag might lead to optimal ight altitudes
around 1000 meters [111]. More detailed and location-specic
analyses could be therefore useful to dene an optimal ight
altitude. Nowadays, many AWE companies are aiming at exploiting
low altitude winds with the minimum ight altitude set by safety
concerns. Only few companies and academic institutions are still
trying to reach high altitudes.
7.5. Angle of attack control
As shown in Eq. (1), the aerodynamics of the system is very
important to the power production. A rst aerodynamic design
should maximize the value G
2
C
L
.
A bridle system might be used to x an incidence angle so that
G
2
C
L
is xed at an optimal angle of attack. But it is easy to show
that an active control of the angle of attack is essential during the
production phase of Ground-Gen or Fly-Gen devices.
A variation of the angle of attack can be induced by a change in
the tether sag or in the velocity triangle. As for the tether sag, it is
A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 14611476 1473
possible to compute the variation of the nominal angle of attack
thanks to the model provided in [112]. Depending on the value of
the design parameters, the model would give a numerical value
between 7 deg and 11 deg for a large scale AWES. As regards the
velocity triangle, assuming controlled constant tether force (a
common constraint in a real power curve [108,50]) and computing
the effect of a different absolute wind speed on the angle of attack
with a simple velocity triangle, a variation of about 3.5 deg or 4
deg can be reasonably obtained.
Such variations of the angle of attack can decrease substantially
the power output or even make the ight impossible. For example,
using the values of the aerodynamic coefcients for an airfoil
specically optimized for AWESs [113], a steady state variation in
the angle of attack of just 72 deg can lead to a decrease in power
output between 5% and 42% with respect to the optimal angle of
attack. Real time control of the angle of attack in current AWES
prototypes from Ampyx and Makani [52,108] limits the angle of
attack variation to 72 deg in a real ight time history.
7.6. Cables
Cables for AWESs are usually made of Ultra-High-Molecular-
Weight Polyethylene (UHMWPE), a relatively low-cost material
with excellent mechanical properties [114] even though many
different materials are being used and studied [115]. The cables
can be 1, 2 or 3 and in some concepts they carry electricity for
power generation or just for on-board actuation. Each of these
choices has advantages and disadvantages, and, at the present
time, any prediction about the best tethering system would be
highly speculative. The tethers also represent a known issue in the
AWE community [116] because of wear, maintenance and
aerodynamic drag.
7.6.1. Polynomial curvature
It is worth noticing that Section 6describes a steady state ight
with straight cables under known tension, but for very long cables
this model is not accurate. For example, removing the straight
cable hypothesis gives a fourth order polynomial shape function
[112] for cables in steady state ight. The curvature is acceptable
for short round cables (C
?
¼1 and rup to 1 km), while it is highly
undesirable with extremely long round cables (e.g. r¼10 km).
Reducing the cable aerodynamic drag to 0.2 allows reasonable
curvatures with lengths of several kilometers (e.g. r¼5 km).
7.6.2. Unsteady analysis
Very long cables should be studied removing also the steady
state hypothesis in order to consider the fact that, in unsteady
ight, the lower part of the cables could reasonably move less than
in steady state ight, thus dissipating less energy.
7.6.3. Tethers electrostatic behavior
Some concerns have been raised regarding the behavior of
tethers in atmospheric environment [117]. An analysis performed
on dry and wet polyethylene ropes without inner conductors [14]
shows that non-conductive tethers will not trigger a ashover in
typical static electric elds of thunder clouds, however non-
conductive tethers are very likely to trigger a ashover when
subjected to impulsive electric elds produced by lightnings. It is
reasonable to say that AWESs will not work during thunderstorms
and that lightnings should not be an issue. However, an analysis
regarding the electrical atmospheric behavior of tethers with inner
conductors could be useful to understand the worst atmospheric
conditions to which conductive tethers might be exposed.
7.6.4. Aerodynamic drag opportunities
A reduction in the cable drag coefcient would likely lead to an
increase in power output by two or three times thanks to better
aerodynamic efciency, increased ight speed and higher opera-
tional altitude [111]. Several patents have been led to address this
issue [86,93,118] even though the reduction of cable drag by
means of e.g. fairing or streamlined cross-sections has not been
experimentally demonstrated yet. Because of the potential advan-
tages, funding opportunities might be available for concepts in
aerodynamic drag reduction [119].
7.6.5. Nearly zero cable drag
As regards the aerodynamic cable drag, two patented concepts
might provide an important improvement in the long term by
setting to zero the aerodynamic drag for the majority of the length.
They are worth being mentioned even though no prototype
already exists for both of them. The rst appeared in the very
rst patent about high altitude wind energy in 1976 [120] and
described the so-called dancing planesconcepts. Two Fly-Gen
turbines are held by a single cable, the top part of which splits into
two. Each turbine is tethered to one of these ends and follows a
circular trajectory so that only the two top parts of the cable y
crosswind through the air and the main single cable stands still
under balanced tensions. Several studies already exist for this
promising concept [121,122]. The second [123] describes a multi-
tetherAWES where three cables are deployed from three different
ground stations and are eventually connected to each other in
their top end. A single tether connects the top end of the three
cables to one kite that is then geometrically free to y crosswind
within a certain solid angle without moving the three lower cables
but only changing their inner tension. The further the ground
stations are spaced apart, the larger is the allowable solid angle.
For both of these concepts the result is the same: high altitude
crosswind ight is achieved with the longest part of the cables
completely xed in space, thus allowing to reach winds at very
high altitude with a relatively short dissipative cable length.
7.7. Business opportunities
To date, several tens of millions dollars have been spent for the
development of AWESs, which is a relatively low amount of
money, especially if one considers the scale of the potential market
and the physical fundamentals of AWE technology. The major
nancial contributions came, so far, by big companies usually
involved in the energy market [119]. The community is growing
both in terms of patents and in terms of scientic research [59].
But still, there is no product to sell and the majority of the
companies that are trying to nd a market t are now focusing
on off-grid markets and remote locations where satisfying a
market need can be easier at rst [110].
8. Conclusions
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. 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 briey presented and classied. The basic
A. Cherubini et al. / Renewable and Sustainable Energy Reviews 51 (2015) 146114761474
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 inuence the performance of an AWES.
In the next years, a rapid acceleration of research and devel-
opment is expected in the airborne wind energy sector. Several
prototypes that are currently under investigation will be com-
pleted and tested.
Acknowledgements
The authors would like to thank the numerous players in the
Airborne Wind Energy eld who gave us their feedbacks and
comments. This work was carried out with the nancial support of
Kitegen Research Srl and Scuola Superiore Sant'Anna. Finally,
special thanks to Anna Midori Ruggieri for providing some of the
illustrations.
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