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This paper represents an expert view from Europe of future emerging technologies within the wind energy sector considering their potential, challenges, applications and technology readiness and how they might evolve in the coming years. These technologies were identified as originating primarily from the academic sector, some start-up companies and a few larger industrial entities. The following areas were considered: airborne wind energy, offshore floating concepts, smart rotors, wind-induced energy harvesting devices, blade tip-mounted rotors, unconventional power transmission systems, multi-rotor turbines, alternative support structures, modular high voltage direct current generators, innovative blade manufacturing techniques, diffuser-augmented turbines and small turbine technologies. The future role of advanced multiscale modelling and data availability is also considered. This expert review has highlighted that more research will be required to realise many of these emerging technologies. However, there is a need to identify synergies between fundamental and industrial research by
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Renewable and Sustainable Energy Reviews
journal homepage: www.elsevier.com/locate/rser
Future emerging technologies in the wind power sector: A European
perspective
Simon Watson
a,
, Alberto Moro
b
, Vera Reis
b
, Charalampos Baniotopoulos
c
, Stephan Barth
d
,
Gianni Bartoli
e
, Florian Bauer
f
, Elisa Boelman
b
, Dennis Bosse
g
, Antonello Cherubini
h
,
Alessandro Croce
i
, Lorenzo Fagiano
i
, Marco Fontana
j
, Adrian Gambier
k
, Konstantinos Gkoumas
b
,
Christopher Golightly
l,1
, Mikel Iribas Latour
m
, Peter Jamieson
n
, John Kaldellis
o
,
Andrew Macdonald
p
, Jimmy Murphy
q
, Michael Muskulus
r
, Francesco Petrini
s
, Luca Pigolotti
e
,
Flemming Rasmussen
t
, Philippe Schild
u
, Roland Schmehl
a
, Nafsika Stavridou
c
, John Tande
v
,
Nigel Taylor
b
, Thomas Telsnig
b
, Ryan Wiser
w
a
Delft University of Technology, Wind Energy Section, Faculty of Aerospace Engineering, Delft, 2629, HS, the Netherlands
b
European Commission, Joint Research Centre, Ispra, Italy
c
University of Birmingham, UK
d
Centre for Wind Energy Research ForWind, Oldenburg, Germany
e
University of Florence, Italy
f
Technical University of Munich, Germany
g
RWTH – Center for Wind Power Drives, RWTH, Aachen University, Germany
h
Sant’Anna University of Pisa, Department of Civil and Industrial Engineering, Italy
i
Politecnico di Milano, Dipartimento di Scienze e Tecnologie Aerospaziali, Italy
j
Università degli Studi di Trento, Department of Industrial Engineering, Italy
k
Fraunhofer IWES, Bremerhaven, Germany
l
Brussels, Belgium
m
Centro Nacional de Energías Renovables (CENER), Spain
n
University of Strathclyde, UK
o
University of West Attica, Greece
p
Offshore Renewable Energy Catapult, UK
q
University College Cork, Ireland
r
Norwegian University of Science and Technology, Norway
s
Sapienza Universita’ di Roma, Italy
t
Technical University of Denmark, Denmark
u
European Commission, DG RTD, Brussels, Belgium
v
SINTEF Energy Research AS, Norway
w
Lawrence Berkeley National Laboratory (LBL), USA
ARTICLE INFO
Keywords:
Wind power
Wind energy
Emerging technology
Technology readiness level
Renewable energy
ABSTRACT
This paper represents an expert view from Europe of future emerging technologies within the wind energy sector
considering their potential, challenges, applications and technology readiness and how they might evolve in the
coming years. These technologies were identified as originating primarily from the academic sector, some start-
up companies and a few larger industrial entities. The following areas were considered: airborne wind energy,
offshore floating concepts, smart rotors, wind-induced energy harvesting devices, blade tip-mounted rotors,
unconventional power transmission systems, multi-rotor turbines, alternative support structures, modular high
voltage direct current generators, innovative blade manufacturing techniques, diffuser-augmented turbines and
small turbine technologies. The future role of advanced multiscale modelling and data availability is also con-
sidered. This expert review has highlighted that more research will be required to realise many of these emerging
technologies. However, there is a need to identify synergies between fundamental and industrial research by
https://doi.org/10.1016/j.rser.2019.109270
Received 22 November 2018; Received in revised form 16 June 2019; Accepted 10 July 2019
Corresponding author.
E-mail address: S.J.Watson@tudelft.nl (S. Watson).
1
Independent Consultant.
Renewable and Sustainable Energy Reviews 113 (2019) 109270
1364-0321/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/BY/4.0/).
T
correctly targeting public and private funding in these emerging technology areas as industrial development may
outpace more fundamental research faster than anticipated.
1. Introduction
The future development of wind power presents a significant op-
portunity in terms of providing low carbon energy. It also presents
several challenges. It needs to be cost competitive compared with the
use of fossil fuels and other competitor renewable energy sources, most
notably solar photovoltaics. The expectation now is that developments
should be subsidy-free. The wind turbines of today have seen rapid
developments in their underlying technology in order to increase their
competitiveness. Nevertheless, we can expect further developments in
the next few years and it is instructive to consider potential future di-
rections.
This paper is a review of future emerging technologies (FETs) in the
wind power sector based on the opinion of European experts from a
range of relevant technology areas. Although the technologies need not
be considered exclusive to Europe, the review draws heavily on the
European experience, particularly European projects. A large number of
the FETs identified originate from academic work within universities
and research institutes. Some of the technologies identified are being
developed by university spin-outs or start-up companies. A few areas
identified are the result of work by larger industrial organisations. What
is presented is a summary of information solicited before, during and
after a workshop organised by the European Commission. The purpose
of this review is to consider different aspects of FETs in wind power
such as Technology Readiness Level – TRL (summarised in Appendix A)
and the potential advantages and challenges that may ultimately
characterise FET development.
The criteria used to define a relevant FET in this case are:
Technology related to wind energy supply and conversion;
A radically new concept, not achievable by incremental research on
mainstream technologies;
Technology at an early stage of development, i.e. TRL should not be
greater than 3.
The novelty of some of the technology areas here identified as
emerging may be debatable as some have already been developed in
other sectors or have been demonstrated to a higher technology
readiness level (TRL) for smaller-scale devices; however, even if not
radically new, application in the wind sector and at a larger scale is still
considered to be at a low level of development and shows future pro-
mise. Complex applications including several technologies, some of
them having a TRL greater than 3, have been included when the system
integration of technologies has not advanced beyond 3. Although it
could be argued that there are a number of supporting technologies, e.g.
energy storage, innovations in network connections, etc, that are still at
a low TRL level and might be included in this review, it was decided to
restrict the scope to technologies which are directly relevant to wind
power. Including a wide range of supporting technologies would have
made this review too broad and indeed topics such as energy storage
would warrant a review of their own.
The aim of this paper is to gather, organise and highlight knowledge
and information. This is not intended to be a road map for the future of
wind power but rather a view of future emerging technologies which
are believed to have potential. The review does not pretend to be ex-
haustive, but it is believed that it does nevertheless represent a wide
range of innovations that show promise in the future. For each tech-
nology there is a discussion of the main advantages and disadvantages,
the state of the art, an assessment of the present TRL and possible de-
velopment trends which are considered either ‘slow’, ‘average’ or ‘fast’.
‘Average’ is considered where the technology achieves a TRL+1 within
4–5 years. ‘Slow’ expects a longer timescale and ‘fast’ a shorter one [1].
Values of typical nominal power are also reported, together with an
estimate of the potential for scale-up.
The information is presented in sections for each of the identified
FETs. Each section gives a brief technical description and the degree of
development, challenges and potential including scaling, present TRL
level and projected TRL trend. To support the TRL assessment the most
recent guidelines were used [2]. The TRL indicator is a relatively new
concept in the field of energy technologies so can sometimes not be
unequivocally defined, especially for FETs at an early stage of devel-
opment. When assessing complex systems, the TRL of the least devel-
oped components and/or of the integration issues that present new
challenges are considered. The assessment of TRL level along with
present scale and future projected target power is summarised for all
the identified technologies at the end of this paper in Table 1. It should
be stressed that as this review concentrates on technologies which are
at a low TRL at present, there is no attempt to present a current or
future projected levelised cost of energy (LCOE) of any of these tech-
nologies. This would be very difficult to do and would be highly
speculative at this point in time.
The following sections structure the review into different categories,
namely: future wind generation technologies, future technologies
which will support these forms of wind power generation and future
List of abbreviations including units and nomenclature
aaxial induction factor
AWE Airborne wind energy
BTC Bend twist coupling
CAPEX Capital expenditure
CAWT Cross axis wind turbine
CC Circulation control
C
p
power coefficient
C
t
thrust coefficient
DAWT Diffuser augmented wind turbine
FET Future emerging technology
HAWT Horizontal axis wind turbine
HVDC High-voltage direct current
Hz Hertz
IPC Individual pitch control
km kilometre
kV kilovolt
kW kilowatt
m metre
MD-AWES Multiple drone airborne wind energy system
MoDaR Morphing downwind-aligned rotor
MRS Multiple-rotor system
MW Megawatt
OWC Oscillating water column
STC Spar-torus-combination
SUMR Segmented ultralight morphing rotor
TLP Tension leg platform
TRL Technology readiness level
VAWT Vertical axis wind turbine
VIV vortex-induced vibrations
WEC Wave energy converter
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
2
knowledge that will be required to fulfil the potential of such power
generation. A short conclusion is then provided at the end to summarise
the findings and suggest what is required for some of these future
technologies to be realised.
2. Future wind generation technologies
2.1. Airborne wind energy
Airborne wind energy (AWE) is an umbrella name for concepts that
convert wind energy into electricity with the common feature of au-
tonomous kites or unmanned aircraft, linked to the ground by one or
more tethers [3]. AWE systems offer several potential advantages over
conventional wind turbines. They require less material than tower-
based turbines, have the potential to be manufactured at lower cost, can
be deployed faster and can harness stronger and steadier winds by
flying at higher altitudes.
Several different concepts are currently being pursued [4,5] and
convergence towards the best architecture has not yet been achieved
[6]. A possible classification of AWE systems is shown in Fig. 1, in-
cluding several specific implementations.
Ground-gen concepts are based on the conversion of mechanical
into electrical energy at ground level, while fly-gen concepts are based
on the conversion in the air, onboard the airborne unit. Most ground-
gen concepts drive a drum-generator module in pumping cycles, al-
ternating between traction and retraction phases to generate electricity.
Fly-gen concepts use onboard wind turbines with continuous electrical
energy output and a conducting tether to deliver this energy at ground
level. Of the fly-gen solutions, crosswind systems can generally produce
more power (1-2 orders of magnitude higher) than non-crosswind
systems [7].
The main advantages claimed by these concepts are low capital
costs, due to the small amount of material used, a relatively simple
construction and installation and a higher capacity factor, due to
stronger and more consistent high-altitude winds prevalent above
200 m altitude [8–10].
The benefits could increase when deploying AWE systems on
floating offshore platforms. Compared to a conventional, tower-based
offshore floating wind turbine, a tethered AWE system is subject mainly
to tensile rather than bending loads, potentially reducing the cost of
large stabilising subsea structures and ballasting [11]. A potential re-
duction in weight (with possibility of further reduction) could reduce
the capital expenditure (CAPEX) of platforms and subsea structures;
and the reduced size of the devices allows for rapid installation at a
lower cost. Important technical challenges of AWE systems are [6]:
High complexity. The operation of AWE systems crucially depends on
a fast-feedback control based on a quite complex set of distributed
sensors and actuators that must guarantee fully autonomous flight
over long periods of time.
Lack of proven reliability and operational hours. Existing technology
demonstrators still rely on supervised operation, especially in the
take-off and landing phases, and most of the developed systems are
not fully autonomous. Depending on the technologies, currently
achieved operating times vary between 2 and 3 h and almost 24 h of
autonomy.
Limited knowledge. All the predictions of economic potential and
environmental benefits of AWE are based solely on calculations of
aerodynamics and mechanics during tethered flight. So far, these
calculations have not been fully validated using experimental data.
It is not certain that after full development, the technology will yield
the promised energy conversion performance. Consequently, the
impact and feasibility of scaling up to utility scale generation has
not been deeply and rigorously assessed. There is also a need for
more research on wind resources/conditions between 100 and
1000 m height, with a better description of the atmosphere in gen-
eral for sustainable energy systems.
There are still several technical problems to address such as: the
durability of flexible materials, greater design convergence, and water
erosion testing of materials. Apart from space restrictions (a no-fly zone
is necessary given the altitudes in which these devices would operate),
regulation, social acceptance, safety and the potential of harm due to
lightning strikes and storms are challenges which need to be addressed
[12,13]. Space restrictions could cause difficulties to implement wind
farms of airborne devices, creating some doubt as to the economic
viability of this technology in densely populated areas such as Europe.
AWE could be initially exploited for a niche market where the com-
petition of existing mainstream turbines is weaker [14]. For example,
AWE could be an interesting solution for small/isolated systems, e.g.
kite on truck, to power desalination plants, pumping farm water,
military applications, etc.
Recent interesting technology and research trends include: drones
and multi-drone concepts, as outlined in Section 2.1.1, advanced
aerodynamic modelling, electronics and sensors that allow tethered
devices to be controlled autonomously, systems for autonomous take-
off and landing, as outlined in Section 2.1.2, and high-lift multi-element
aerofoils. The projected power for large scale operation is up to 25 MW/
km
2
[15].
The majority of implemented development platforms is in a nominal
power range up to 20 kW. A notable outlier and currently the most
Fig. 1. Classification of AWE systems, adapted from Ref. [7]. N.B. Not included are concepts based on rotary mechanical power transfer to the ground.
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
3
prominent project is the M600 which was developed by Makani Power
in the USA and which has been tested onshore in California and Hawaii
[16,17]. After acquisition by Shell the company announced it would
start offshore operation off the coast of Norway [18]. Competitor
Ampyx Power based in the Netherlands is on track for flight testing a
250 kW rigid wing system designed for pumping cycle operation on a
test site in Ireland, developed by E.ON [19].
A possible scale-up range beyond 1 MW is feasible, but even if this
power level is achievable, size could be challenging e.g. due to aero-
elasticity problems [17]. When implementing these systems, it would
also be important to consider not only the power per square metre wing
area but also all the surrounding area, which varies for different tech-
nologies. Moreover, the scalability would require longer cables and
higher altitudes. At the present time, ground generation technology is
not necessarily more advanced than flying generation systems.
The present TRL of AWE is viewed as being between 3 and 5 because
of reliability problems of the current prototypes. Present devices have
typically only a few hours of autonomous flight. The various AWE
technologies, supported by the necessary investments, are envisaged to
reach the commercialisation stage in roughly 10 years. The TRL evo-
lution trend is considered ‘slow’ not only because of challenges yet to be
solved, but also because of a general current lack of long-term invest-
ment.
At present, AWE requires significant fundamental academic re-
search to get to a required level of maturity, but there is some small-
scale commercial investment taking place in the development and
testing of devices. Large commercial players are starting to take an
interest in the technology and it can be foreseen that this will increase
once a level of reliability can be demonstrated.
2.1.1. Multiple Drone Airborne Wind Energy Systems
Experimental evidence and physical models show that the perfor-
mance of flying generators is significantly affected by the aerodynamic
dissipation due to drag of the traction cables. Such dissipation sets an
upper bound to the effective operating altitude and limits the scalability
of the nominal power of the devices.
In this context, Multiple Drone Airborne Wind Energy Systems (MD-
AWESs) represent an effective solution that could introduce radically
new perspectives in the field of airborne systems [20–22]. An MD-
AWES is a crosswind architecture which features multiple drones that
are connected to the ground with a single shared cable (see Fig. 2). This
aims at significantly reducing the aerodynamic drag of the cables and
thus providing a huge potential gain in techno-economic performance.
Preliminary studies of MD-AWES have demonstrated their feasi-
bility. The concept has been investigated by numerical simulation
[20–23] and by experimental testing under controlled conditions [23].
More specifically, the multi-drone concept has been proven in models
based on experimental evidence taken on single drone systems, sug-
gesting a TRL of 2–3 for this FET. Their potential upscaling could en-
visage single units of multi-megawatt class. This could set new per-
spectives from the point of view of the techno-economic effectiveness of
AWE.
The specific challenges of this technology that still need to be in-
vestigated are:
Layout/Architecture choice: the best architecture in terms of
number of drones, connections to ground and between drones, type
of drones.
Control: during generation and during the two most critical phases
of take-off and landing.
Design: structural design of drones, flight dynamics, stability, etc.
2.1.2. Autonomous take-off and landing systems
Fully autonomous take-off and landing is one of the current tech-
nical bottlenecks in the development of AWE technology. Except for
vertical take-off and landing systems, this functionality has not been
fully demonstrated. Moreover, the possibilities and constraints are very
different between soft kites, where a fixed [24] or telescopic mast [25]
is usually envisaged to support the wing at take-off, possibly with ad-
ditional support of a drone [26], and rigid aircraft, where linear or
rotational launching concepts have been proposed [27].
In a recent project at ABB Corporate Research (see Fig. 3), a fully
autonomous, linear take-off system in compact space for a rigid teth-
ered aircraft has been proved [28]. However, long-term extensive
testing in all wind conditions would be required, and the landing phase
has not been experimentally investigated yet. For soft kites, automatic
launch and landing concepts exist, however their reliability and full-
scale applicability have not been fully proven. The main challenge lies
in the low speed of the aircraft during take-off and landing, which re-
sults in less controllability, coupled with the short tether length and the
uncertainty of environmental conditions. The estimated TRL is TRL 2 or
3 depending on whether part of each approach has been experimentally
tested or not.
2.2. Offshore floating wind concepts
The main innovation of floating wind concepts, compared with
mainstream offshore fixed structure mounted turbines, lies with the
floating support system. These floating structures have no foundation
on the sea-floor, but are instead based on either semi-submersible,
tension leg or spar platforms, kept in place by different mooring and
anchoring systems [29].
The development of floating wind structures has grown out of ex-
isting fixed structure technology (Fig. 4). There is therefore potential
Fig. 2. Left: illustration of the working principle of a multi-drone system. Right: possible strategy for take-off and landing [23].
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
4
for optimisation of floating offshore systems. This optimisation would
be possible through the integrated design of the platform and the wind
turbine. The more specific, tailored design of wind turbines for offshore
floating platforms, including downwind rotors, high tip speed ratio
operation and possibly two bladed rotors, could have an impact on the
cost of the platform and the whole floating system.
There currently exist a great variety of ideas under development
that may ultimately realise lower costs than fixed structure solutions:
catenary moored semi-submersible platforms [30], the tension leg platform
(TLP) which has a smaller and lighter structure, but requires a design
which increases stress on the tendon and anchor system, (see e.g. Ref.
[31] for design considerations); and the spar-buoy [32], which is more
suited to deeper waters (> ∼80 m) [33].
There is a high potential for offshore wind power in deep waters.
However, there are difficulties involved in its exploitation. In deep
water (60–300 m), there are higher wind speeds, but also higher cabling
and mooring costs. Nevertheless, it has been shown that floating de-
signs could achieve lower levelized costs compared with bottom
mounted designs due to their lower sensitivity to cost increase with
water depth [34]. Floating wind technology is important for countries
like the USA (particularly the West coast [35]) and the east coast of
Japan [36] which have long coastlines and steeply shelving seabed
bathymetries. In these countries, it would be expected that the most
relevant technical advances of floating wind technologies will occur.
Floating-turbine design allows for lower transportation and in-
stallation costs and lower assembly costs since for some concepts the
whole setup (both platform and wind turbine) can be assembled on land
and transferred offshore. The difference with existing technology is that
at present, assembly is at sea, risking unstable and logistically complex
conditions and costly weather downtime. The turbine and foundation
for offshore structures comprise approximately 50% of the initial
CAPEX cost [37]. The installation and foundation construction of this
technology can be dramatically reduced, opening the path for more
economic offshore wind energy.
Research and development are necessary in areas related to: addi-
tional fatigue loading, possibly reduced by advanced control and in-
novative design; material and design improvement of mooring and
anchoring systems; and platforms (dynamics, size and weight) [38]. In
order to reduce weight and costs of semi-submersible structures, which
are currently made of steel, there will likely be a need for new materials
for floaters. Holistic design of platforms would benefit from further
research, such as on high voltage dynamic cables, motion control and
active mooring systems. Advanced modelling tools and advanced con-
trol systems for pilot installed wind turbines are elements that require
further research and development [39,40]. Ecosystem compatibility,
symbiosis and societal acceptance are also areas to be studied. The lack
of knowledge and practical studies lead to assumptions about floating
structure forces and tensions that need to be fully assessed. In general,
Fig. 3. Left: Sketch of the dynamical model. Right: Picture of the small-scale prototype built at ABB Corporate Research.
Fig. 4. Offshore platforms.
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
5
differences in the environmental impacts are mainly at the installation
stage such as the noise from pile hammering. This can be significant for
monopiles and jackets, although new approaches could mitigate the
impacts such as the use of bubble curtains [41]. The other main chal-
lenge is in decommissioning: for fixed-bottom turbines, current practice
is to cut the structure just below the ocean floor, i.e. a section of the pile
remains in the seabed [42], whereas floating platforms can be towed
away, and any drag anchors fully removed.
Floating structures may have to be designed to operate and survive
extreme environments, which can significantly impact both capital and
operational expenditure. Many of the challenges associated with oper-
ating a floating wind farm in deep-water Atlantic conditions have not
yet been considered. In general, structural design of floating wind
turbine platforms depends on installation location, which needs to be
fully analysed and the platform constructed accordingly, restricting
industrialised mass production. There is, however, the potential for
modular fabrication with assembly in the port which needs to be in-
vestigated.
Although currently several offshore floating turbines have been in-
stalled in Europe, there are few, if any, wind turbine designs designed
specifically for floating concepts. Considering the development trends,
Vertical Axis Wind Turbines (VAWTs) could fill a market niche due to
their low speed and high torque. If more research is carried out in this
field, these turbines could become relevant again. VAWTs are in-
herently less efficient, but hybridisation with floating devices could be
advantageous. A detailed study of the potential aerodynamic and
structural integration of VAWTs with floating platforms has been car-
ried out [43–45]. However, from a practical point of view, the in-
tegration of a VAWT with a floating platform is considered at a low TRL
at present. The TRL of the most advanced floating offshore horizontal
axis wind turbines (HAWTs) is between 8 and 9 [46] since spar-buoy
and semi-submersible designs are already being built and tested at large
scale. However, TLP designs have not yet reached this level of maturity.
Most of the implemented technologies have a power of about 6 MW,
such as the Siemens turbines used in the Hywind project [47]. The TRL
differs if new concepts or components are integrated into the whole
system. There are such projects which at present are at a TRL of 4–5 or
even lower. For floating platforms designed for very deep waters and
hybrid concepts (see Section 2.2.1) the TRL is still at a very low level
with only a slow evolution expected.
Fixed offshore wind has shown that larger turbines generally result
in lower costs so floating offshore wind would benefit from larger
turbines. Theoretically, turbines could be scaled up to at least 20 MW,
though future scaling-up difficulties may restrict development to
around 10 MW.
Given that a small number of commercially-led demonstrators have
now been built, it can be foreseen that larger commercial players will
lead the advance of this technology based on the need to develop
deeper waters in certain parts of the world. Nonetheless, public re-
search investment will be needed to help support future demonstration
projects before a commercially viable concept can be realised.
2.2.1. Floating hybrid energy platforms
The maximisation of energy production from offshore sites is cri-
tical, both in relation to keeping costs down and minimising environ-
mental impact. This is especially important for the more exposed sites
in Atlantic regions, which the wind industry will be moving into in the
next decade. Hybrid energy platforms take advantage of synergies and
compatible aspects of different energy types or even different tech-
nology types within the same industry [48].
An example of combining wind with wave energy is the Blackbird
system [49]. This consists of a Storage Base Anchored Uniaxial Hybrid
VAWT with a Wave Energy Converter (WEC) supported on a fully
submerged Tension Leg Buoy. This hybrid unit consists of a synergy
between VAWT, WEC, horizontal sea level generator, submerged WEC
system and a single mooring line with integrated cable, moored to the
seafloor via a drilled anchor/suction caisson storage concept.
The European (EU) project MARINA [50] examined more than 100
hybrid concepts, from which three were chosen for more detailed ex-
amination. These three concepts were the Spar-Torus-Combination
(STC), the Semi-submersible Flap Combination (SFC), and a large Os-
cillating Water Column (OWC)-array floater. The STC and the SFC
concepts add wave energy converters to an existing floating wind tur-
bine, while the OWC array concept is an integration of a wind turbine
with a very large V-shaped floating platform [51].
Another type of hybrid is the wind–wind system. The development of
multi-converter platforms exploits conventional and airborne wind
energy converters on the same platform. Multi-turbine floating plat-
forms are already being developed, e.g. SCDnezzy [52], presenting a
clearer path forward, whereas various types of airborne wind technol-
ogies are still in an early stage of development. Airborne wind is itself
facing several challenges as mentioned earlier. Assessment of whether
tethered airborne systems will work on floating platforms is needed.
The design of a stable floating platform allowing efficient operation is
critical. In some cases, the platform needs to be very large and, given
the lower power generation of kites (at present 100 kW though this
could be scaled), it impacts significantly on the cost of energy. Com-
bining AWE with a platform for a conventional wind turbine, if tech-
nically feasible, could have beneficial impacts such as better exploita-
tion of the wind resource.
The benefits of hybrid platforms lie in the synergies between the
different forms of energy production. The combination of elementary
technologies on a single platform may have the potential for higher
overall production levels and to share infrastructure, e.g. platforms,
cables, substations, etc. In addition, resource analyses for wave energy
sites have generally shown that viable sites also have a high wind en-
ergy resource. Wave energy tends to be more predictable and less
variable than wind energy with the peaks in wave energy production
trailing the peaks in wind energy production. This will have the ad-
vantage of smoothing combined wind/wave production overall thus
increasing energy market value [53]. Hybrid platforms could operate
efficiently at most wave energy sites, but floating platforms in-
corporating only a wind generator can be unstable. The use of the STC
helps to stabilise the system, which constitutes a major advantage.
Several projects, some of them being in pre-commercial stages, are in
place and still have a higher levelised cost of energy than conventional
fixed offshore wind, since for the latter the learning curve is based on
more years of experience.
These hybrid devices present, compared to the single floating de-
vices, an additional set of challenges and development needs due to
greater complexity and reliability problems. The MARINA project has
highlighted the challenges associated with the hybrid wind-wave plat-
form design, by presenting a comparative numerical and experimental
study for the STC and SFC concepts. It was found that for these con-
cepts, wave energy technology contributed less than 10% of the total
energy production of the hybrid platform. In addition, for some com-
binations, the inclusion of a wave energy converter tended to destabi-
lise the platform, going against the fundamental principles of the
platform. For instance, the spar is designed to be hydrodynamically
transparent but this is no longer the case when a point absorber is
added. The large OWC array had good energy balance but the large
pitch motion of the floating platform gave rise to operational and sur-
vival problems.
The most developed technology at present would seem to be the
hybrid platform with floating wind and wave energy [54], such as the
P80, developed in the framework of the Poseidon project (Fig. 5). The
P80 project (from the company ‘Floating Power Plant’) is the upgraded
version of the P37 device. P37 is a hybrid wind-wave floating scaled
device that has been tested in North Sea conditions having 33 kW of
installed wind power and 50 kW of installed wave power. The wave
energy converters are heaving/pitching flaps that must always be
aligned towards the incident waves in order to maximise production.
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
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The P80 was tested [55] at 1/50 scale (TRL of 4, possibly 5). It aims at a
single wind turbine of between 5 and 8 MW and a 2–3.6 MW wave
energy converter. The actual power values are low, e.g. the floating
power plant P37 has a nominal power lower than 100 kW.
Floating hybrid wind turbines face similar challenges as single tur-
bine floaters, with the additional challenges associated with a more
complex system. One of the challenges is the lack of knowledge re-
garding hybrid dynamics and interaction with the floater. For these
devices to work there has to be a clear synergy between technologies.
Wave energy conversion devices are generally at a lower TRL in
comparison with single wind devices, lowering the global TRL of hybrid
systems. As mentioned previously, the TRL of floating wind turbines is
between 8 and 9, while the TRL assessment of several hybrid projects is
between 1 and 5. An example of low TRL technology is the Blackbird, a
concept system with a TRL of 1–2. The STC technology is estimated to
have TRL of 2–3, while systems like the P37 mentioned above, using the
SFC technology, could have a higher TRL, i.e. 4–5.
In terms of scaling-up potential for hybrid devices, it can be ex-
pected that values should be similar to the those of single technology
floating devices, i.e. of the order of 10 MW.
There is a question regarding the synergy of the technologies, i.e. is
it preferable or desirable to combine devices rather than have them
operating separately? The main value of these combinations may not
only be additional power generation but the fact that, for example,
wave energy converters can work as dampers by reducing platform
motion and loads if the system is optimally designed. This has the po-
tential for reducing capital and possibly operations and maintenance
costs which in turn could reduce LCOE. Overall, the development of
these technologies is associated with high values of investment which
represents a challenge.
2.3. Smart rotors
Larger rotor blades make it necessary to consider blade/rotor con-
cepts that can adjust themselves to non-homogeneous wind flow like
gusts, turbulence spots, shear, etc. For very long blades, i.e. greater than
70 m, it is very hard to define the optimal operational point, since the
inflow situations may vary quite a lot along the blade. Therefore, a local
optimal blade setting, i.e. adjusted to the flow on a scale of metres or
tens of metres, makes sense. This could reduce loads, increase or
smooth out power output or help in wind turbine or wind farm control.
Devices which integrate this type of concept fall into the category of
smart rotor technology [56]. This concept could incorporate both active
and/or passive load alleviation systems. These are described in the
following section.
2.3.1. Passive and active control systems
Passive load alleviation systems are not controlled by operators or
automatic systems. They can be distributed along the blade span, e.g.
Bend Twist Coupling (BTC), or placed in specific regions of the blade
[57]. These technologies can use anisotropic (in the case of BTC),
elastomeric (coating) and multi-stable materials [58]. Given their
characteristics, carbon fibre composites and 3-D printing are essential
for the future development of these technologies.
BTC allows a twist of the blade caused by a primary bending de-
formation [59]. Modular and articulated rotor blades have also been
the subject of study, such as the Segmented Ultralight Morphing Rotor
(SUMR) which employs a Morphing Downwind-Aligned Rotor (MoDaR)
technology. During extreme weather conditions, these blades can fold
together reducing the risk of damage. This concept is intended to be
implemented in blades of over 200 m in length for 50 MW wind turbines
located in areas with harsh climate conditions. In the USA, a 13.2 MW
concept with a TRL = 4 has been developed [60].
The low drag vortex generator is another type of passive system. This
technology consists of a small vane typically attached to the suction
side of wings or blades, where it causes local mixing in the boundary
layer and thereby can delay or prevent flow separation [61]. It can be
used in to reduce or mitigate separation in the root region and to pre-
vent erosion of turbine blades. Turbine blades with a low drag penalty
can be designed with add-ons installed in the outer blade sections, in
order to reduce the effect of erosion or roughness changes on the
blades. This is the subject of research as part of the European Demo-
Wind initiative through the Offshore Demonstration Blade project [62].
These passive devices can be included in the design process from the
beginning, thanks to modern multi-disciplinary analysis and optimisa-
tion approaches or can be implemented in existing blades. They may
also be integrated with different types of active control systems.
Blades with movable parts can be considered both passive and ac-
tive systems. In the passive case, the slats and/or flaps are coupled
using only springs without the use of actuators or control systems.
Trailing-Edge Flap Control, which is currently being demonstrated at MW
scale, Leading Edge Slats [63] and Moving Tips are examples of active
control systems [64].
A specific type of active control system is circulation control (CC) on
wind turbine aerofoils. This delivers compressed air from special slots
located on the blade surface. Compressed air dynamically adjusts the
aerodynamic performance of the blades, and can essentially be used to
control lift, drag, and ultimately power. This system has been shown to
exhibit high levels of control in combination with an exceptionally fast
response rate [65]. One example is the technology developed by Ko-
hana (see Fig. 6), where a 100 kW prototype was tested.
This rotor ejects air from the suction side surface towards the
leading edge to cause separation. This is used as a high bandwidth, fast
response control system reducing both extreme and fatigue turbine
loads. Another application is to use the ejected air directed towards the
trailing edge allowing the generation of enhanced lift. In this case, the
rotor blades have, especially on the outboard rotor, elliptical aerofoil
sections of very low solidity which are structurally efficient and light-
weight. Because of this, they exhibit reduced lift and drag loading when
idling in extreme winds. The rotor can be ∼30% larger in diameter
within the same load envelope of a standard design [66].
Individual Pitch Control (IPC) is a multivariable control which uses
feedback and feedforward based on estimated wind speed which can
Fig. 5. The hybrid wind-wave platform project P80. Courtesy of Floating Power
Plant.
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
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control power production and alleviate loads experienced by wind
turbines through pitching the blades individually [67].
Another important feature related to active control systems is the
use of the rotor as a wind sensor, which can allow drive and control
systems to use the data they themselves provide, e.g. Refs. [68,69].
2.3.2. Degree of development, challenges and potential of smart rotors
The main potential of smart rotor technology would be in enabling
and scaling up to larger wind turbines (> 20 MW). Scaling up turbine
size is seen as a viable option to reduce LCOE as the balance of plant
required represents a smaller fraction of the overall capital cost per unit
of installed capacity. The synergy between passive load alleviation,
active control and the rotor-as-a-sensor technology will enable reduc-
tions in weight and fatigue loads. It may also contribute to reducing the
cost of energy, extending the length of the blades, increasing the swept
area and improving capacity factors. These technologies can be used to
re-blade existing operating wind turbines and provide better manage-
ment of the load during peak usage. Another potential advantage is in
improving annual energy production in the latter years of their life
without affecting the loads on other sub-components, such as nacelle
and tower. Furthermore, the design life of turbine blades can be sig-
nificantly enhanced by modifying their shape. Potential relevance is for
wind turbines in general, especially large turbines, both land-based and
offshore.
Vortex generators have been used on blades since the 1980s. They
can slightly increase the power output. The concept and its benefits,
well known in aeronautics and road transport, are still being studied for
wind power applications [70,71]. The main benefits lie in mitigation of
blade wear and erosion prevention. The erosion control aspect is a new
insight arising from this technology. The fact that this can probably be
implemented in existing blades is considered a big advantage.
Combining passive and active control is possible, as well as applying
these technologies for both VAWTs and HAWTs [72]. The challenges
are to fully understand the interaction of these technologies with tur-
bulence, control and turbine dynamics. These are all non-linear pro-
blems with a high degree of complexity. Lifetime and cost of movable
components and actuators may also present some challenges. It may be
challenging to find or develop techniques which can stand the required
millions of load cycles with minimal maintenance. The development of
equipment and materials to build such blades, and at the same time
endure fatigue loading, is another important challenge.
There is a need to improve the measurement of the dynamic wind
inflow to drive the active control systems and allow the effective use of
smart rotor technology [73]. This can be through better processing of
conventional wind speed measurements from such as nacelle-mounted
cup anemometers and wind vanes or through the use of remote sensing
data from lidars or spinner-mounted sonic anemometers [74]. The use
of high-fidelity computational fluid dynamics (CFD) codes could be
significant in defining the 3D wind field approaching the rotor. The
measurement and processing of strain on the blades also has an
important role to play as input to smart rotor solutions [75].
Developing efficient pumping systems and high efficiency variable
speed fans, is one of the challenges for CC systems. Fan technology is
extremely well developed but predominantly for operation at relatively
constant loads. The development of economic designs of very low so-
lidity elliptical aerofoils, with accurate manufacture of the slots for air
delivery, is a particular challenge. The use of carbon in the elliptical
blade sections may prevent undue flexibility that could result from the
sections being optimally of very low solidity. Effective aeroelastic
modelling of the circulation-controlled rotor and integration of a rea-
sonably controllable pressurised pumping system also need R&D re-
sources, although no fundamental problems are anticipated.
Optimisation of control will obviously be of major importance and will
involve new challenges around pumping power costs, load regulation
and energy capture. The added cost to rotor systems of CC is not yet
known but an outline cost of energy calculation indicates that there can
be net benefit even if rotor costs are escalated by a factor as great as 4.
Outline calculations suggest that pumping power demand is typically
lower than 10% of the overall energy generated. By expanding the rotor
size without increase in loading it is possible to obtain more annual
energy output than a standard rotor. Technology is established but
substantial further work on integrated design and structural blade as-
pects is required. CC system technology is heavily patented, even if
many patents are generic. Since research in this field is mainly per-
formed by the industry it is difficult to correctly estimate its TRL (2–3 in
the public domain, maybe higher).
The IPC and BTC, individually and together, have been wind tunnel
tested, which would imply a TRL of at least 4. Depending on how the
blade is designed, the TRL of BTC may vary. In the more mature cases,
the TRL of BTC is 5–7, since some specific technology has been tested at
large scale, but it has not reached the market yet. Some manufacturing
techniques are not viable at the moment; several components have to be
hand-made since there is not an industrial manufacturing process.
If a solution for the challenges is found, the TRL evolution can
follow an average to possibly fast track if there is disclosure of some of
the confidential information required. For most of these technologies,
the expected time-scale for scaling up applications is of the order of a
decade. The BTC technology should reach the market in 5 years. Most
advanced turbines being developed now have a nominal power of
12 MW and the scalability can go to 10–50 MW. Some of these tech-
nologies have to compete with each other for funding and this could
influence the pace of the TRL change.
Although industry is leading the development of some of the smart
rotor technologies and patents have been filed, several innovations are
still very much at the fundamental research stage. At this point, it is
difficult to say which technologies are likely to succeed in the longer
term and significant public research money will still be required to
assess some of the more speculative innovations.
2.4. Wind turbine with tip-rotors
This conceptual technology consists of wind turbines where the
traditional torque transmission by gearbox and generator is substituted
by a fast-rotating rotor/generator mounted on the tip region of each
blade (see Fig. 7). While conventional turbines extract power at a free
wind speed of around 10 m/s by conversion of torque, the tip-rotor
converts power at around 70 m/s. The concept can be designed for both
two- or three-bladed turbines [76].
The efficiency of the tip rotors to convert power could be close to
100% at low tip speed ratios, as the usual Betz limitation of 59% does
not apply for the moving tip rotor [77]. This is because it is the thrust of
the tip rotors which is providing the reaction torque to the primary
rotor causing them to extract power as they move. It can therefore be
shown that in the idealised Betz model case [77], the ratio of the power
extracted by a tip rotor to that extracted by the primary rotor is
=C C a/ (1 )
p t
, where
Cp
is the power coefficient of the tip rotor,
Ct
is
Fig. 6. Circulation control system. Courtesy of Kohana Technologies Inc.
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
8
its thrust coefficient and
a
is the axial induction factor from idealised
actuator disc theory. If the tip rotor were itself optimised, then
=a1/3
.
This would imply the overall efficiency of the tip rotor to be the optimal
efficiency of the primary rotor multiplied by
(1 1/3)
, i.e.
× =(16/27) (1 1/ 3) 0.395
. If the overall efficiency were determined
by each rotor operating at the Betz limit, i.e.
× =(16/27) (16/27) 0.351
,
it can be seen that this configuration has the potential to slightly exceed
this value. However, this would necessitate relatively slow-moving and
large tip rotors which would defeat the object of the design so in
practice any design would sacrifice efficiency somewhat to have rela-
tively low weight, high speed tip rotors [77]. It has been demonstrated
by Siemens that direct drive generators for e-propellers for aircraf can
reach a performance of 5 kW/kg [78]. This suggests a two-bladed
10 MW turbine could have two generators in the tip-region with a
weight of 1000 kg each. This value is considered relatively low in
comparison to the blade weight. The drive train for a typical 10 MW
turbine will have a weight of the order of several hundred tonnes. This
significant mass reduction has the potential to reduce capital costs.
Since wind turbines with tip-rotors do not require a main shaft or
gearbox, this technology aims at cost and weight reduction, being parti-
cularly beneficial, for example, for floating offshore concepts. It may be
possible to make the main rotor with a fixed pitch, downwind, free
yawing, teetering rotor and possibly with extended pitchable blade tips.
Noise and erosion of the tip-rotors are the two main challenges as-
sociated with the anticipated high speed of the rotor, together with
centrifugal forces on the tip rotors. The aerodynamics and aeroelastics
of this concept are more complex than for conventional turbines,
whereby specific rotor blade shapes and materials need to be developed
and investigated.
The concept is more suitable for very large wind turbines, since
larger turbines have fewer challenges related to the centrifugal forces
on the rotors. This technology may be more appropriate for the offshore
environment, given the additional noise and visual impact compared
with conventional single rotor turbines.
Concerning the degree of development of this technology, no pro-
totype has yet been constructed or tested. The TRL of such a concept is
thus 1–2 with a possible scalability of the same order of magnitude as
for a conventional 10 MW offshore turbine. A slow TRL trend is
anticipated since investors are reluctant to fund the technology due to
its radical and high-risk nature.
This form of technology is clearly at a very fundamental level of de-
velopment with only very basic paper concepts produced. There is the
need for detailed concept studies to be publicly funded in order to assess
the challenges and potential for this form of wind power generation device
before any industrially-led developments can be expected.
2.5. Multi-rotor wind turbines
To improve efficiency and reduce overall loads on a wind turbine it
is possible to replace a large single rotor with a multiple-rotor system
(MRS), as shown in Fig. 8. This innovative solution could allow a large
power system (20 MW or more) to be installed at a single site by means
of a high number of standardised rotors. As mentioned above, scaling
up is seen as a key factor in overall cost reduction.
With individual control of rotors, it is possible to respond to a tur-
bulent wind field across the device, allowing for more efficient gen-
eration and with the potential to alleviate loads. Possible advantages of
the MRS can be to mitigate structural and material problems associated
with the scaling up to a large device. There is also the possibility of
yawing without the requirement for a separate mechanism.
Optimum rotor size is likely to be determined by operations and
maintenance logistics rather than aerodynamics. Overall design opti-
misation is interactive with aerodynamic, electrical, loading con-
siderations and other factors. Turbines can be clustered to reduce some
electrical costs, but the independent operation of each turbine max-
imises overall load reduction [79,80].
A major advantage of this technology is in the standardisation [80].
The production process of smaller rotors could be industrialised and
could have lower costs while present production methods of large
turbines require customisation. Moreover, in the case of malfunctioning
of one rotor this does not imply any interruption of energy production
from the working rotors of the array. For the same total swept area as
an equivalent single rotor, power efficiency can increase.
Within the framework of the EU INNWIND project, a 20 MW multi-
rotor concept was modelled (TRL = 2). This MRS design (45 rotors with
440 kW power each) was compared to a reference single 20 MW turbine
[79,80]. The MRS power production was estimated to be 8% higher
than a single rotor system of the same overall swept area [80].
Fig. 7. Schematic of a possible demonstration scale 11 kW turbine with tip-
rotors.
Fig. 8. Vestas multi-rotor wind turbine. Photograph courtesy of ZX Lidars.
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
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Detailed designs for maintenance are needed to optimise logistics
and evaluate operations and maintenance costs more reliably.
Reliability studies have shown that, while there can be a large main-
tenance advantage in having much smaller components [81,82],
avoidance of the use of jack-up vessels is critical to have effective design
to avoid minor failures. The outline concept within the INNWIND
project was to have a built-in top-level travelling crane that could
handle each rotor nacelle system automatically and lower to base level.
The modelling of an MRS is challenging and needs more develop-
ment. Aerodynamics are an important area which need further research
[83]. The design tools for turbulent wind loading and aeroelastic be-
haviour of a multi-rotor system are at a very preliminary stage. Active
control and structural vibration alleviation present particular chal-
lenges. There is a need to improve design, modelling and reliability due
to the increased number of components.
In terms of scaling up the size of a wind turbine and reducing costs,
this machine offers higher power extraction efficiency on average, po-
tential fatigue life extension and reduction in material scaling problems
[84]. There has been work, e.g. Ref. [85] to develop three and seven
rotor concept systems for small scale applications. The most advanced
European project exploiting this technology is a 900 kW four rotor-
turbine developed by Vestas. This project appears to have a TRL of 5–6.
The TRL of most multi-rotor turbines is between 2 and 4. Scalability
should be of the order of 20 MW but it could reach higher levels. Fur-
ther work on modelling and complete system design is needed, after
which the technology could evolve at a fast pace.
As there has been a commercial demonstrator of this technology as
mentioned above, there is clearly potential for industrial funding of
research to bring this technology to maturity, supported by public
funding to tackle some of the more fundamental challenges associated
with aeroelastic design and control.
2.6. Diffuser augmented wind turbines
Diffuser Augmented Wind Turbines (DAWTs), also known as wind lens
or shrouded wind turbines, are HAWTs which possess a diffuser-type
structure resembling a funnel, able to collect and concentrate the ap-
proaching wind. The diffuser can be modified by adding a broad ring or
brim around the exit point and an inlet shroud at the entrance, thus
creating a ‘lens effect’. This design increases diffuser performance and it
has been demonstrated as producing increased power compared to con-
ventional turbines, for a given turbine diameter and wind speed [86].
There could therefore be a potential for LCOE reduction compared with an
un-shrouded machine, but only if the cost of the diffuser is less than the
cost of making the rotor larger in order to provide the equivalent power
output. This remains a significant challenge. Other more tangible potential
benefits are a possible reduction in airborne tip noise or the opportunity to
make the machine less visible by embedding it within a building structure,
though structure-borne noise remains a challenge.
DAWT devices are at a semi-commercial level of development in
Japan, with power ratings of the order of tens of kW. The most ad-
vanced projects have been developed by Kyushu University, who have
studied several configurations, from single DAWTs to multi rotor sys-
tems (see.Fig. 9), with a maximum tested power of 100 kW [87]. In
addition, tests on floating platforms were performed in Hakata bay.
Multi-rotor DAWTs appear to display convincing performance with the
typical challenges of MRS, enhanced by complex interactions with the
diffusers [87]. These interactions with optimal spacing can be very ben-
eficial but perhaps due to unsteady effects, the geometrical symmetries do
not always correspond to flow field and rotor performance similarity.
Structural loading is a challenge for cost effectiveness of all diffuser aug-
mented systems, single-rotor or multi-rotor. Whether there is the potential
to have larger compensating energy gains and lighter total rotor and na-
celle systems with multiple rotors still remains a topic of research.
Although this technology is not likely to be significantly cheaper
than the non-ducted technology, it could be suitable for a niche-market.
The TRL is around 5–6, with a scaling up target of 1 MW. The TRL
evolution is expected to be ‘average’.
This type of technology will be more suitable for smaller scale appli-
cations and it is not expected that there will be major industrial investment
in associated research in the near future. Public funding for this tech-
nology needs to target niche applications such as the urban environment.
2.7. Other small wind turbine technologies
The following non-mainstream concepts are briefly reviewed,
namely, wind turbines based on magnetic levitation,innovative vertical
axis and a cross axis design:
Magnetic levitation (also MagLev) wind turbines use full-permanent
magnets to attempt to eliminate friction through levitation of the
blades [88] (see Fig. 10(a)). This technology is quite advanced
(TRL = 8) for small power applications (∼kW). One of the major
challenges of this technology is the low techno-economic efficiency
and the limited suitability for scaling up [88].
Vertical axis wind turbines (VAWTs), as the name suggests, have a
vertical shaft around which the rotor turns. In most recent appli-
cations, these have been developed for offshore applications [89] as
shown in Fig. 10(b). VAWTs, with intrinsically lower optimum ro-
tational speeds, are penalised by producing relatively high torques
with cost and mass implications for the drive train. An interesting
idea to avoid this problem is using secondary rotors which can ex-
ploit the tip speed of the VAWT rather than dealing with very low
shaft speeds and very high torque.
The Cross Axis Wind Turbine (CAWT) extracts wind energy from
airflows coming from the horizontal and vertical directions [90] (see
Fig. 10(c)). The CAWT is not necessarily a novel concept but in-
novative designs are emerging, perhaps interesting for niche appli-
cations, particularly important in the context of urban use of wind
energy.
Most of these technologies are for low power applications (‘small
wind’), being suitable for urban environments, perhaps embedded in
buildings. It is envisaged that there could be applications for niche
markets. Cost reductions here could result from more efficient designs,
cheaper manufacturing and materials, cheaper installations and
Fig. 9. 10 kW multi-rotor DAWT system installed in Japan. Courtesy of Kyushu
University and Riamwind Corporation.
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
10
economies of scale.
The main challenges of these technologies are not only technical,
but also the noise, aesthetics and societal acceptance need to be ad-
dressed. Moreover, in some cases there are apparent legislative chal-
lenges [92,93].
At the present time, it is difficult to assess these technologies in terms
of TRL or power generation scale. In terms of future research investment,
similar comments can be made as for the ducted technology, although
some of the concepts described above are at an even earlier stage of de-
velopment and will need public funding to assess their full potential.
2.8. Wind induced energy harvesting from aeroelastic phenomena
Air flow-induced vibrations of mechanical systems can be exploited to
extract energy, when specifically designed to experience large-amplitude
oscillations. The mechanical system has to be combined to work with sui-
table energy-conversion apparatus, such as electromagnetic or piezoelectric
transducers. This type of technology will not be used for large-scale gen-
eration, but for applications where a small amount of autonomous power is
required, e.g. wireless sensors or structural health monitoring. These energy
harvesting devices have possible applications in urban settings and for en-
ergy harvesting at small and micro-scales. The LCOE for such devices will
remain high compared with the much larger scale wind power generators,
but for very small scale applications, may be cost effective.
Among fluid-structure-interaction phenomena [94], those con-
sidered suitable for energy harvesting applications include: (i) dynamic
instability of classical flutter [95], (ii) interference between vortex-in-
duced vibrations (VIV) [96,97], and, (iii) dynamic instability or galloping
[98–100].
Flutter-based devices involve a rigid, streamlined model (a simple
flat plate or aerofoil) of finite length, which is elastically suspended to
oscillate along two degrees of freedom: heaving (cross-flow translation)
and pitching (rotation). The energy extraction is activated in the
heaving motion component, being less sensitive to a damping incre-
ment. Linear generators, typically solenoids, are used (see Fig. 11).
Depending on the application, different governing parameters can be
selected. The design configuration can be adapted to specific operating
ranges of flow speed.
VIV and galloping-based devices involve a rigid, finite-length model
with a bluff cross section for the generation of vortices. The model is
schematically shown in Fig. 12 for the case of ocean current-induced
vibrations showing that a bluff body can be elastically suspended to
oscillate in the cross-flow direction only. This principle, with appro-
priate tuning of the system, could equally be used to harvest energy
from the wind.
For these technologies, the energy harvesting performance mainly
depends on:
the flow speed at which the device starts to operate (cut-in velocity),
which should be as low as possible;
the motion amplitude, which should be as high as possible;
the damping level, which dictates the proportion of energy flowing
on to the next conversion device.
The last point is critical, since the added damping due to energy
harvesting can attenuate the flow-induced vibrations, i.e. the Scruton
Number may increase, leading to the vanishing of vortex shedding
[101]. Electromagnetic transducers have a better performance when
working with large-amplitude motions at low frequencies, while pie-
zoelectric transducers perform better at higher frequencies.
Research has mainly focussed on flapping foils, with either semi-
active control or fully-passive motion; in the latter case, classical-
flutter-based devices [102]. The efficiency predicted through theore-
tical analyses or computational simulations is high (up to 30–35%) and
may be competitive with other technologies. However, the efficiency is
currently lower than 5%, with a low TRL and a power per unit length of
2 W/m, obtained with a 10-cm wide plate oscillating at about 2 Hz in a
uniform flow at 10 m/s [97].
The technology based on flutter can be considered as a fully-passive
version of the more studied flapping-foil technology not requiring an
Fig. 10. Examples of: (a) magnetic levitation; (b) innovative vertical axis (Courtesy of DTU wind energy) and (c) cross-axis wind turbine [91].
Fig. 11. Flutter vibration energy harvester [95].
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
11
active control mechanism to operate. Scientific research on the post-
critical regime of flutter still needs development. The TRL for flutter-
based devices is 2–3. These devices are still at laboratory scale, where
the conversion apparatus is often simulated through dampers. Effective
prototypes that can also generate energy are still to be developed. This
technology has potential applications in urban environments (see
Fig. 11) and turbulent situations, since preliminary tests have shown
that high levels of turbulence intensity and large turbulence scales have
little influence on the system response [102]. There have been some
conceptual projects. An example is the piezo-tree, a tree-shaped device
with piezoelectric leaves based on fluttering technology. The concept
presented several challenges such as very low power output and pro-
blems of electrical integration. Future research should focus on the
aerodynamic properties of the cross section and on optimisations such
as the reduction of mass ratio, to further enhance the amplitude of the
motion and to widen the operative range. From the perspective of real
applications, devices that contain a single oscillating system or an array
of them can both be conceived. However, installations of multiple
systems still require investigation of possible interference effects.
VIV excitation only devices have typically a narrow operational
range, are generally unsuited to variable flows, and are very sensitive to
design [94,96]. Based on this concept, the company Vortex Bladeless is
developing and testing demonstrators including a 100 W nominal
power device, the Vortex Tacoma [103]. Devices based on the inter-
ference between VIV and galloping present a larger operational range.
Recent studies have explored the interference of vortex-induced vi-
bration and galloping, with results that can be positively exploited for
low-power energy harvesting systems [104]. An example project (on
the upper scale of TRL) is the European Space Agency funded project
“piezoTsensor” [105].
The TRL of VIV-galloping devices is estimated to be 2–3, and for VIV
3–4 with a slow trend for development. Some devices have a nominal
power of 2 W/m, with others of the order of 0.1 W/m, therefore power
upscaling is likely to be low.
Because of the niche nature of these technologies, much of the research
is still at the academic level. There is little commercial development at
present. The challenge will be to channel public research funding into
those niche applications which show the highest level of promise.
Fig. 12. Device exploiting the vortex induced vibration principle from an ocean current [96].
Fig. 13. Compliant tower concepts for the offshore wind industry: (a) Slender monopile (‘dumb tower’), (b) Guyed tower, (c) Buoyant tower, (d) Articulated buoyant
tower, (e) Tower with mass trap, (f) Compliant piled tower. Courtesy of W. de Vries [109].
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
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3. Future supporting technologies
3.1. Alternative support structures for wind turbines
Current support structures for fixed bottom-mounted offshore wind
turbines to date consist mainly of monopiles, although a number of
gravity base solutions and jackets have been employed, as well as other
concepts (see Fig. 13). A number of unconventional designs have been
suggested as alternatives in the literature such as suction bucket jackets
and suction caissons [106], but others have not been studied well en-
ough to date. Alternative fixed bottom support structures, self-rising
lattice towers and new materials for towers and support structures are
discussed in this section. These have the potential to reduce LCOE by
reducing the volume and/or the mass of the material required and by
making installation simpler and cheaper.
With the rapid acceleration of offshore wind energy, it can be ex-
pected that research in this area will be industrially led, supported by
public sector funding where appropriate.
3.1.1. Alternative fixed bottom support structures for offshore wind
A number of alternative fixed bottom-mounted support structure
concepts exists. Here we focus on two of the most promising alternative
support solutions. A first option is the use of a compliant structure (see
Fig. 13) that has its first eigenfrequency below the main wave fre-
quencies, in contrast to the usual design where it is higher. Such a wind
turbine is a compromise between a fixed and floating structure, a
concept also referred to as a semi-floater. The structure attracts rela-
tively low hydrodynamic loads and is therefore attractive, but due to its
complexity, it has not been a popular choice so far [107]. The con-
nection to the foundation needs special attention and might need to rely
on novel solutions such as an articulated joint [108].
Another example is the use of a full lattice tower, replacing the
usual wind turbine tower with a multi-member braced structure. This is
also used for jackets or as part of certain semi-floater designs as shown
in Fig. 13 e-f. Bolted lattice towers were the preferred solution for small,
onshore turbines, before the circular tower began to dominate [110].
Part of the reason for this was the high stiffness (reducing vibrations)
and the ease of assembly using bolted connections, whereas drawbacks
are the many connections, the difficulty of safe access to the turbine and
aesthetics, the latter being less of a concern offshore. Although welded
cylindrical towers have solved most of these problems since the 1980s,
lattice structures could be more suitable in building taller wind turbine
towers, since they benefit from being robust enough with much less
material used, while giving the flexibility in transportation of the cross-
sections on site and their ease of mounting.
The innovation of these concepts would be in their use offshore. In
this case, a number of challenges will need to be met, including: miti-
gating the risk of corrosion, using different profiles and connection
methods than onshore and the development for use with much larger
turbines than the technology has been used for before [111].
This concept is attractive to address structural frequency response
problems that can be seen with circular towers for very large wind
turbines which become ‘too soft’ [112], but also in terms of reducing
the weight of the wind turbine system or providing a more efficient
connection to the foundation, i.e. removing the need for a dedicated
transition piece.
3.1.2. Self-rising towers
The production of wind energy could increase with larger wind
turbines on higher towers. Lattice structures could be more suitable for
building taller wind turbine towers, even if there is a lack of very high
cranes for erection. Self-rising lattice towers are suitable for horizontal
axis wind turbines both in onshore and offshore applications, con-
structed by raising each tower subsection from the prior lower tower
section. Their advantage is that there are no large cranes necessary for
the installation of the tower. Tower subsections can be mounted and
lifted to their final position with the aid of frames and the use of small
size cranes and/or cables [113].
The two main EU projects on this self-rising concept are
HyperTower [113–116] and SHOWTIME [117,118]. The HyperTower
project aims to optimise the design of self-rising lattice towers that are
ideal for onshore wind farms while the SHOWTIME project focuses on
the design of hybrid towers. These hybrid towers are suitable for off-
shore locations where the bottom lattice part is connected to the tubular
shell upper part by means of a transition piece that is carefully designed
to sustain both wind and wave loads.
3.1.3. New materials for towers and support structures
Current wind turbine towers and support structures are constructed
from steel and/or concrete. The steel is typically the same grade as used
in the construction industry. Higher grades of steel can offer better
structural performance (strength, buckling resistance) and lead to
lighter structures. Hybrid solutions [119] using both steel and concrete
and the use of alternative materials such as wood, aluminium [120] or
especially composites, e.g. reinforced materials, or sandwich structures
[121], can offer similar advantages in performance.
3.1.4. Degree of development, challenges and potential of alternative
support structures
The advantage of alternative fixed bottom-mounted support struc-
tures is based on more efficient connections between tower and foun-
dation. A full lattice structure is quite strong even for high towers while
monopiles become too soft when taller, with 20 MW turbines or larger
anticipated in the future. This technology offers a more integrated de-
sign and flexibility. Similarly, the compliant support structures follow a
different strategy to reduce wave loading and resonance problems,
thereby leading to a lighter structure. Lattice structures are lighter than
tubular versions and provide opportunities to transport tower parts to
site more easily for assembly and installation.
This technology poses severe challenges, as the system behaviour is
of critical importance for a wind turbine. The main challenges related to
these concepts are understanding the complex dynamics, especially in
the interaction with the rotor, challenges related to manufacturing, e.g.
reducing welding costs, safe access to the turbine and installation
techniques, particularly offshore.
The TRL of these alternative fixed bottom-mounted support struc-
tures is estimated to be TRL 1–3, since most of the research has been
limited to concept studies and structural analysis based on numerical
simulations, e.g. Refs. [108,122]. An exception is the 2B6 downwind
6 MW wind turbine that has been tested since 2015 in an onshore de-
monstrator project. This particular concept is therefore at TRL 7–8.
Lattice towers have been employed commercially at TRL 9 for onshore
turbines up to 2.5–2.75 MW (with the Fuhrländer FL2500 and the GE
2.75–120 turbines). However, there is a non-trivial challenge to adapt
these designs for offshore applications and new turbines that are much
larger, potentially scaling up to at least 30 MW, and which will behave
much more dynamically. This requires new, innovative solutions for the
structural design, the control, and the installation.
The use of micro-cranes for self-rising wind turbine towers could
allow assembling such lattice towers on-site with lower installation
costs. Since the small sections can be transported with conventional
vehicles, in contrast to those required for long tubular tower sections,
the transportation costs are also reduced. This technology is still at
conceptual and testing stage, with no commercial structure designs
released in Europe, though a hybrid monopile-lattice self-erecting
tower is under development by Nabrawind of Brazil. For these reasons
the present TRL of this technology is 2–5, though there is an ongoing
project to build a demonstration telescopic tower as part of the EU
Elican project which would be at a higher TRL. The idea of using a
telescopic tower to simplify the installation of an offshore monopile
wind turbine has been studied, though a number of assumptions were
made that would need to be addressed for a viable solution to be
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
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developed [123].
To allow the development of this technology, more research would
be required on materials and suitable beam sections. There are plans to
carry out laboratory tests on cross sections, using performance data to
build tower numerical models. Considering the current rotor tech-
nology and the design of this kind of support structure to date, a limit of
the order of 30 MW is estimated.
Hybrid composite solutions for towers and support structures offer a
lighter and stronger structure, especially important for onshore wind
energy. Additional advantages include shorter assembly times and ea-
sier transport. This can influence turbine design positively, leading to
assembly cost reductions, even if offset by currently higher material
costs. Some drawbacks that need to be considered are related to man-
ufacturing, since more complex processes, welding and surface treat-
ments would be required to produce such composite materials. Because
such materials are not widely in use, there are more uncertainties re-
garding structural safety, e.g. fatigue behaviour, compared to standard
designs. Consequently, there is a conservative view regarding the cur-
rent design standards. The use of novel materials may require mod-
ification of these standards and guidelines. There are insufficient in-
centives to study the strength and behavioural limits of these materials.
Some concepts are at a preliminary stage of study (TRL = 2), or at
an intermediate level, e.g. the Hexcrete concrete tower concept [124]
which has been validated in the laboratory (TRL = 4). Other concepts
are almost commercial, e.g. the Elisa self-installing tower developed
during the Elican project [125] was recently installed at the PLOCAN
test site with a 5 MW wind turbine (TRL=6–7). For the latter, the aim
is to reduce costs to make them economically viable. These concepts,
with weight reduction, could find a potential use for offshore and
floating applications.
The fact that TRL level is judged to be low in some cases is because
the implementation and adaptation of these support structure tech-
nologies on wind turbines is new. Small changes in one component can
have significant impact on the overall wind turbine design that need to
be fully understood before higher TRLs can be achieved. For all the
technologies, the development rate would be considered as ‘average’.
3.2. Unconventional power transmission for wind turbine rotors
Current wind turbines generate mechanical power from aero-
dynamic forces on the rotor, which turn a shaft that drives an electro-
mechanical generator. Although the efficiency of the electro-mechan-
ical conversion is high, the variable rotational speed requires a fre-
quency converter to connect each turbine to the power grid. Other
challenges include reliability problems with mechanical gearboxes or,
in the case of gearless turbines, the weight of the directly driven gen-
erator as turbines become increasingly large.
It is possible to replace the mechanical gearbox using a hydrostatic
transmission system [126,127]. Such a system also has the possibility to
provide continuous speed, torque and power control, removing the
need for a converter. Alternatives to conventional electro-mechanical
generators are hydraulic transmission systems (see Fig. 14) or com-
pressed-air technology [128,129].
The main advantages of these technologies are lighter nacelles and
the possibility to include energy storage options in the case of hydraulic
and compressed air systems. Lighter nacelles reduce the amount of
material required for the support structure thus reducing capital cost.
Integrated storage options increase the value of the energy converted.
Both of these factors have the potential to reduce LCOE. The main
challenge is the reduced efficiency of hydraulic or compressed-air sys-
tems compared to electrical conversion, which could be addressed by a
central conversion plant collecting power from several turbines.
However, the reliability of such a complex system is still a challenge.
Material and design challenges also need to be addressed.
The most advanced project was a Scottish-based turbine with 7 MW
of nominal power [131]. This seems to have a well-established TRL
(4–7), having already been tested using a scaled prototype in an op-
erational environment. The development ceased soon after Mitsubishi
Heavy Industries formed a joint venture with Vestas to focus on the
improvement of the more conventional Vestas 8 MW turbine, latterly
scaled up to 9.5 MW. The risks and uncertainties connected with the
new technology might have outweighed the benefits. However, it is
likely that the technology might be offered as an option for the new
turbine at a later stage. There are other projects exploiting this tech-
nology; some of them never went beyond the concept stage (TRL 1–2),
others were tested at laboratory scale, with some simulations for full
size wind farms (TRL = 3–4), with power not exceeding 1 MW. An
Fig. 14. Schematic of a hydraulic transmission system for wind turbine with
energy storage [130]. Fig. 15. Overview of a modular series connected converter [133].
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
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example of a protype which is currently being developed is the 500 kW
machine developed in Delft, i.e. the Delft Offshore Turbine [132].
This technology only makes sense when deployed at large scale. The
potential for scaling up is around 100 MW but the TRL trend is slow.
3.3. Modular HVDC generators
High-Voltage Direct Current (HVDC) systems can bring advantages
to offshore wind energy transmission by eliminating capacitive losses in
the underwater power cables. The modular high voltage generator
concept could produce HVDC electricity by connecting in series, recti-
fying and increasing the tension of different sections of the electric
wiring of the wind generator's stator (see Fig. 15).
A voltage output of 100 kV and higher is possible with this type of
configuration [133,134]. This concept has been verified experimentally
using a 45 kW prototype with three generator segments and three
converter units in series [133].
The modular HVDC generator concept has the potential to reduce
the size and weight of the electrical machines used to generate energy,
to increase the voltage and to rectify the power produced by offshore
wind. This system could provide direct HVDC production, instead of
relying on more complex systems needing AC generators, AC/AC
transformers and AC/DC converters. When DC transmission systems are
required, fewer conversion steps could result in lower losses, lower
investment cost and higher system reliability. All of these factors could
contribute to a reduction in LCOE. This system might increase the en-
ergy yield from a wind farm by 3%–7%. With higher ‘up-time’ of the
turbines, it could be possible to keep producing electricity even when
sub-modules fail.
The main challenges consist of: improving power/weight and
power/volume ratios; achieving full hardware and software protection
and reach efficiencies higher than 95; % and managing the voltage
control of DC modules. Installation can be challenging, as can: un-
wanted magnetic coupling of the stator coils due to modularisation; the
need to explore new stator designs to achieve proper electric field
control and avoid induced capacitive currents and; thermal cooling.
The mechanical robustness of the stator may be compromised when it is
divided into modules due to the sub-harmonic fields in the machine.
Due to the low power output of existing prototypes and their lim-
itations, the present TRL of this technology is 3 and it is difficult to
assess TRL trend and scaling potential at this stage of development.
The limited work in this area thus far suggests that public sector re-
search will be initially required to drive the development of this technology.
3.4. Innovative blade manufacturing techniques and materials
Blade manufacturing techniques may be relevant to the future
performance of wind turbines and in terms of improving component
lifetime. New solutions such as automated manufacturing, either invol-
ving fibre composite laminate laying [135] or additive 3D printing processes
for both moulds and blades [136] would allow technology-driven cost
reduction in blade manufacturing, while reducing the uncertainty, i.e.
manufacturing tolerance, in the process. Further and quicker adapta-
tion to specific customer needs under the so-called ‘Industry 4.0’
paradigm would also become possible, as well as fast testing of new
aerodynamic shapes. Sensors and actuators are expected to play a major
role in future wind turbine blade manufacturing for enhanced mon-
itoring and smart rotor designs, e.g. BTC.
An alternative material to fibre-glass wind turbine blades is fabric-
based materials. They could significantly reduce production costs and
weight of the blades. This technology uses tensioned fabric wrapped
around a spaceframe blade structure, that is, a truss-like, lightweight
rigid structure, replacing the current clam shell wind turbine blade
design [137]. The blade structure would be completely modified, al-
lowing for easy access and repair to the fabric to maintain standard
wind turbine performance (see Fig. 16).
New polyurethane based materials such polyurethane prepreg
sheets and fibreglass/polyurethane foam preforms [138] can be used to
produce lighter, stronger and longer blades, compared to the current
commercial epoxy-based versions. A key property of wind turbine
blades is the inter-laminar fracture toughness. The incorporation of
multi-walled carbon nanotubes into polyurethane composites can
double the fracture toughness of epoxy blades [139].
The production of wind turbine blades is still quite a manual and in-
volved process [140]. Automation has the potential to simplify production
and cut costs. Producing blades with 3D printing processes could be highly
beneficial in this regard. Since these alternative manufacturing techniques
are applications of existing technologies this should not be considered a
radically new technology. This would also lead to incremental cost re-
duction. However, it can be observed that these manufacturing techniques
are used for small scale structures while application in the wind power
sector would normally require much larger scales so at present TRL is still
around 1–3 with an average evolution.
New materials for blades have a lot of potential, particularly at
larger scale. Appropriately designed new materials could contribute to
weight reduction and increased stiffness for a typical 70 m blade. A
wider spectrum of materials is needed. A new fibre which could be
better adapted to wind turbine blades than existing carbon fibres would
be extremely valuable for the wind power sector.
The US ARPA-E program funded a project on fabric-based blade
material and the challenges of this technology were subsequently re-
ported [137]. There are still several challenges to be addressed with
regard to suitable new textiles, such as how the material behaves,
particularly its fatigue proprieties and performance and thus the TRL of
this technology is 2–3 with a slow TRL evolution. The most advanced
ongoing project in Europe is the textile-based ACT Blade tested in the
Offshore Renewable Energy Catapult laboratories [141].
Significant basic research will be required in order to accelerate de-
velopments in this area, though with blades a relatively high cost item and
future challenges in scaling, this should be seen as a priority area.
4. Future knowledge requirements
4.1. High-fidelity multi-scale integrated models for complex wind inflow
Within the wind power sector, there is still the need for accurate and
appropriate models for better understanding of wind inflow funda-
mental physics. Wind energy systems operate in highly unpredictable
turbulent conditions. This can lead to unsuitable adaptations of wind
power plants, turbine design and operation related to wind resource
variability. Accurate turbine and layout design as well as turbine con-
trol strategies require high-fidelity models of scales varying from
∼1 mm to > 1000 km. To provide confidence in model accuracy, va-
lidation against observations will be important through benchmarking
Fig. 16. Illustration of a fabric blade prototype tested in the Offshore
Renewable Energy Catapult laboratories. Courtesy of ACT Blade Ltd.
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
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studies and the use of remote sensing data, e.g. Refs. [142,143].
High-fidelity models should be improved for a number of reasons,
e.g. they should be capable of providing better estimates of the wind
energy potential of large wind farms than at present; they should be
able to better predict the forces experienced by turbines in operation to
ensure that future larger machines are properly designed; they should
be able to better forecast the wind on multiple timescales to allow smart
control of wind farms to manage output and loads and to facilitate the
integration of wind energy into networks.
High-fidelity multi-scale integrated models for complex wind inflow
should be able to accurately predict the behaviour of a wind energy
system and estimate its lifetime. They should be suitable to optimise
design and management of wind power plants by fully capturing the
interaction with and between turbines through their wakes in areas of
varying terrain complexity. Ultimately, this would help to significantly
reduce the cost of energy [144,145] and would require application of
supercomputing capacities, as well as extensive verification and vali-
dation through high-resolution measurement campaigns.
Simple computational models are commonly used for representing
different temporal and spatial scales, ranging from kilometres for
weather phenomena to millimetres for the boundary layer of the blades
[147], as depicted in Fig. 17. High-fidelity multi-scale integrated
models present several challenges related to the high complexity of the
interactions. There is still a significant lack of knowledge, so research is
needed on models coupling the different scales [148]. There is work in
this area, e.g. the U.S. Department of Energy is developing a multi-year
research initiative, Atmosphere to Electrons [149], which targets a
better understanding of the complex physics governing electricity
generation by wind plants.
An integrated multi-scale interdisciplinary approach is new but
important if we are truly to understand the interaction of wind gen-
eration technology with the environment and advance its design ac-
cordingly. Modelling efforts at different scales for wind inflow have
been performed but only in separate specialised research communities
and thus the level of development of truly integrated multi-scale models
is still at a TRL of 3 and the TRL development trend is still slow.
Fig. 17. The complexity of the wind flow from large regional scales to small scales. With permission of the National Renewable Energy Laboratory [146].
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
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4.2. Wind energy databases and big data analysis
A large quantity of wind energy data has been generated in recent
years, with a wide variety of variables routinely collected from hun-
dreds of sensors on experimental and turbine installed devices. Long
term wind simulations made with numerical weather prediction
models, computational fluid dynamics model simulations and mea-
surement campaigns are further sources.
Data-sharing from renewable energy industry sectors, e.g. turbine
manufacturers, operators and utility companies, data merging and data
mining need to improve for both wind farm operational efficiency and
assessment methodologies [150]. However, typically there is a gap in
the information flow between the wind resource assessment community
and operational producers and operators or utility companies.
Factors, such as real wake effects, anomalous component behaviour
and misalignment could be identified, checked and corrected if the vast
resources of data were suitably exploited [151]. In addition, under/over
estimation of energy yield during wind farm design project prospecting
and during design phase could be quantified.
Through the gathering of data, it could be possible to bring down
turbine operation and maintenance costs and at the same time prevent
knowledge loss, i.e. empirical knowledge of operators is lost when they
retire. Tools to achieve this may be available but they are not optimised
to solve such problems. Wind energy data analysis requires wind re-
source assessment experts, wind farm operation experts, mathemati-
cians, statisticians, physicists, engineers, meteorologists, IT specialists,
etc., to work together.
Distilling the complex data stream into real knowledge poses tech-
nological and scientific challenges. Not only data are needed but also
appreciation of the kind of data required, how to analyse them and an
understanding of the right questions to ask.
Although, ‘big data analytics’ has been driven heavily by industry
who are starting to make available commercial offerings in this sector,
the TRL of big data analytics still has a wide range from 3 upwards. The
lower end of the TRL range in this case is not associated with technical
challenges but with policies towards data sharing and mining. In order
to push big data analytics to full maturity, the industry has to unlock
and allow disclosure of their data. What we have seen so far, is still a
level of industrial development which is in its infancy. The potential for
the application of big data analytics in areas such as resource, turbine
design, operations and maintenance and forecasting could be so much
greater than at present particularly if attitudes to open data access can
change. The pace of TRL development of this area could be considered
to be average, but it will become important in the coming years. The
technology to process data already exists and it is an integration of
developed technologies that is required. However, the wind power in-
dustry is not yet mature enough to properly proceed with the devel-
opment of this area, since commercial competition discourages data
sharing. There are at least two EU projects related to data sharing,
namely: the New European Wind Atlas project that is being developed
[145], and the MARINET network aimed at accelerating the develop-
ment of marine renewable energy [152]. These are just examples of
publicly-funded initiatives where data sharing could be beneficial to the
development of big data analytics.
5. Conclusions
In this paper, future emerging technologies in wind energy have
been identified by a number of European experts. The range of tech-
nologies identified come primarily from academic research, with many
concepts being the subject of early stage development by university
spin-out and start-up companies. A few technologies have been actively
Table 1
Technology readiness level and power scale assessments for the different FETs identified.
Technology TRL (2017) and TRL trend Power (2017) Scaling up target
Airborne wind energy TRL=3-5
Trend: Slow
100 kW nominal
power
∼ MW
Multiple drone systems (MD AWES) TRL = 2-3
Trend: Slow
N. a. Up to ∼10 MW
Autonomous take-off and landing systems TRL = 2-3
Trend: Slow
N. a. N. a.
Offshore Floating wind concepts TRL = 4-9
Trend: Slow
6 MW 10 MW
Floating hybrid energy platforms: a) wind-wave and b) wind-wind systems TRL = 1–5 a): 1–5; b): 3–4
Trend: Slow
100 kW 10 MW
Smart rotors: a) Bend Twist Coupling; b) Segmented Ultra Morphing Rotor; c) Vortex generator;
d) Blades with movable parts; e) Circulation control systems; f) Active control systems
TRL =2–7 a): 5–7; b): 4; c): 5–6;
d): 2–4; e): 2–4; f): 2–3
Trend: Average/Fast
100 kW ÷
12 MW
∼10–50 MW
Wind induced energy harvesting from aeroelastic phenomena: a) flutter and galloping-based
devices; b) vortex induced vibrations-based devices
TRL = 2–4 a):2–3; b): 3–4)
Trend: Slow
∼W (2W/m) ∼ kW
Wind turbine with tip-rotors TRL = 1-2
Trend: Slow
N.a. N.a.
Unconventional power transmission for wind turbine rotors TRL = 1-7
Trend: Slow
7 MW ∼100 MW
Multi-rotor system (MRS) wind turbines TRL = 2-6
Trend: Fast
900 kW ≥20 MW
Diffuser augmented wind turbine TRL = 5-6
Trend: Fast
10–100 kW ≤1 MW
Future supporting technologies: a) Alternative fixed bottom support structures; b) Self-rising
towers; c) New materials for towers and support structures
TRL = 2–8 a) 2–8
b) 2-5
c) 2-8
Trend: Average
N.a. a) 30 MW
b) 30 MW
c) N.a.
Modular HVDC generator TRL =3
Trend N.a.
45 kW N.a.
High-fidelity multi-scale integrated models for complex wind flow TRL = 3
Trend: Slow
N.a. N.a.
Knowledge from wind energy databases TRL = 3
Trend: Average
N.a. N.a.
Innovative blade manufacturing techniques and materials TRL ≤3
Trend: Slow/Average
N.a. N.a.
Note: N.a.: Not applicable or Not available.
S. Watson, et al. Renewable and Sustainable Energy Reviews 113 (2019) 109270
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investigated by larger industrial organisations. New generation con-
cepts have been reviewed, the challenges assessed and their future
potential highlighted. Both large-scale and small-scale generation
technology have been considered. A number of underpinning technol-
ogies has been assessed such as the support structures and new ways to
manufacture blades to enable future cost-effective scaling up of wind
turbines. The role and current status of knowledge enhancement has
also been highlighted, in particular, the requirement for integrated
multi-scale models which are able to capture the full range of interac-
tions between the environment and the turbine as well as the need to
find ways to process, understand and extract useful knowledge from the
very large volumes of data produced by the current and future wind
energy industry.
From this review it is clear that there are a number of cross-cutting
challenges that can be identified. In some cases, there is the need to
develop materials which are cheap to manufacture, durable and light-
weight. This is a particular challenge for the soft-wing airborne wind
energy devices, fabric blades and small-scale energy harvesting devices.
The development of advanced control strategies is a significant chal-
lenge in several cases including the floating devices, airborne wind
energy generators, multi-rotor devices and smart rotors. Advanced
control is likely to be a key enabler to scaling of the size of several of the
devices reviewed. This needs to be complemented by a better under-
standing of the unsteady aerodynamics which results from the complex
interaction between the turbulent inflow and the wind power gen-
erator. An improved understanding of the aeroelastic and fluid-struc-
ture interaction behaviour will be needed to realise the development of
the floating and alternative support structures. Some of the technolo-
gies will need more fundamental proof of concept studies and scale
model experiments before development can proceed, e.g. tip-rotor de-
vices.
A comprehensive assessment of the technology readiness level of
these technologies and their development trend has been performed,
together with their typical power and scaling up potential. These
quantitative assessments are summarised in Table 1.
It should be noted that this review reflects the state of knowledge at
the present time, based largely on a set of more relevant technologies
some of which have undergone testing and some of which are still very
much at the concept stage. There may be other as yet unknown or
undeveloped concepts which could radically change the economics of
wind energy.
The authors of this review highlighted that emerging wind power
technologies need more fundamental research to overcome still limited
knowledge in several research areas such as airborne wind energy,
offshore floating wind, multi-rotor systems, new support structures and
high-fidelity modelling of complex wind inflows.
The wind energy industry has faced the challenge of how to scale up
generation technology in order to reduce costs. It is foreseen that
scaling up will continue to be a challenge but that there are a number of
innovative technologies which could make this possible. However,
there must be a combined push using both public and private funding to
make this happen. In order to do so, fundamental research (funded
mainly with public contribution) on emerging technologies needs to be
correctly targeted and should not overlap with areas of interest of ad-
vanced industrial research, as industrial development may outpace
fundamental research faster than anticipated.
Appendix A. Technology Readiness Levels (TRLs)
TRL #1: Basic principles observed.
TRL #2: Technology concept formulated.
TRL #3: Experimental proof of concept.
TRL #4: Technology validated in lab.
TRL #5: Technology validated in relevant environment.
TRL #6: Technology demonstrated in relevant environment.
TRL #7: System prototype demonstration in operational
environment.
TRL #8: System complete and qualified.
TRL #9: Actual system proven in operational environment.
More detailed guidance on how the TRL scale could be applied in
the wind power sector can be found at: https://publications.europa.eu/
en/publication-detail/-/publication/1da3324e-e6d0-11e7-9749-
01aa75ed71a1/language-en/format-PDF/source-61073523.
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