Energy Policy 53 (2013) 136–148
Contents lists available at SciVerse ScienceDirect
Energy Policy
journal homepage: www.elsevier.com/locate/enpol
Shifting towards offshore wind energy—Recent activity
and future development
J.K. Kaldellis n, M. Kapsali
Lab of Soft Energy Applications & Environmental Protection, TEI of Piraeus, P.O. Box 41046, Athens 12201, Greece
H I G H L I G H T S
c
c
c
c
An overview of the activity noted in the field of offshore wind energy is carried out.
Emphasis is given on the current status and future trends of the technology.
Wind energy production and availability issues are discussed.
Economic issues such as investment and energy production costs are also analysed.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 February 2012
Accepted 11 October 2012
Available online 21 November 2012
To date, most of the existing wind farms have been built on-land but during the last few years many
countries have also invested in offshore applications. The shift towards offshore wind project
developments has mainly been driven by European energy policies, especially in north-west countries.
In offshore sites the winds are stronger and steadier than on-land, making wind farms more productive
with higher capacity factors. On the other hand, although offshore wind energy is not in its infancy
period, most of the costs associated with its development are still much higher from onshore
counterparts; however some recent technological progress may have the potential to narrow this
gap in the years to come. In the present work, an overview of the activity noted in the field of offshore
wind energy is carried out, with emphasis being given on the current status and future trends of the
technology employed, examining at the same time energy production and availability issues as well as
economic considerations.
& 2012 Elsevier Ltd. All rights reserved.
Keywords:
Availability
Reliability
Levelized cost
1. Introduction
During the last 20 years, many countries all over the world
have invested in the wind power sector in view of facing the
rapidly increasing population and the limited fossil fuel resources
along with the adverse impacts of conventional power generation
on climate and human health. Wind energy is currently considered as an indigenous, competitive and sustainable way to
achieve future carbon reductions and renewable energy targets
but issues such as the scarcity of appropriate on-land installation
sites or public concerns related to noise, visual impact, impact on
birdlife and land use conflicts often block its future development
(Esteban et al., 2011; Kaldellis et al., 2012). As a result, a
substantial shift towards the vast offshore wind resources has
been made and an incipient market has emerged, i.e. offshore
wind power.
n
Corresponding author. Tel.: þ30 210 5381237; fax: þ 30 210 5381467.
E-mail address: jkald@teipir.gr (J.K. Kaldellis).
URL: http://www.sealab.gr (J.K. Kaldellis).
0301-4215/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.enpol.2012.10.032
Up to now, wind energy development has mainly taken place
onshore. Offshore wind power technology comprises a relatively
new challenge for the international wind industry with a demonstration history of around twenty years and about a ten-year
commercial history for large, utility-scale projects. In the end of
2011, worldwide wind power capacity reached 240 GW (WWEA,
2012), from which, 2% comprised offshore installations. The main
motivation for moving offshore, despite the low or even null
impact on humans and the opportunity of building wind farms in
coastal areas close to many population centres, stemmed from the
considerably higher and steadier wind speeds met in the open
sea, even exceeding 8 m/s at heights of 50 m. Compared to the
onshore counterpart, offshore wind energy has greater resource
potential, which generally increases with distance from the shore.
This fact results to considerably higher energy yield (Pryor and
Barthelmie, 2001), as the power output is theoretically a function
of the cube of the wind speed. However, the net gains due to the
higher specific offshore energy production are counterbalanced
by the higher capital, installation and maintenance costs and so
the economic prospects of offshore wind energy utilisation are
not necessarily better than the onshore ones.
J.K. Kaldellis, M. Kapsali / Energy Policy 53 (2013) 136–148
137
In the following years, relatively small offshore wind power
projects were installed in the United Kingdom, Denmark, the
Netherlands and Sweden, at distances of up to 7 km from the
coast and depths of up to 8 m. Multi-megawatt wind turbines
appeared later, along with the opportunity of experiencing deeper
waters in the sea. In 2000, the construction of the first large-scale
offshore wind farm of ‘‘Middelgrunden’’ with a total rated power
of 40 MW (20 wind turbines of 2 MW each) ended 2 km outside of
the harbour of Copenhagen in Denmark, where the seabed is
situated between 2.5 and 5 m under sea level. The demonstration
project of ‘‘Middelgrunden’’ in Denmark led the way for two
larger offshore wind power projects, i.e. Horns Rev I (160 MW) in
2002 and Nysted (165.2 MW) in 2003. However, the construction
costs of these projects were higher than anticipated, while some
unexpected failures occurred, resulting mainly from the turbines’
exposure to harsh wind and wave conditions. It was such drawbacks that held back development of the offshore wind power
market for a respectable time period (Fig. 1). Nevertheless, great
efforts made by manufacturers and developers in order to identify
and improve problems associated with this stage (Musial et al.,
2010) eventually led the way for a number of new commercial
offshore wind farm installations, all located in European waters.
The year 2010 was a record-breaking year for the European
offshore wind energy market. New installations accounted for
about 900 MW (Fig. 1) (which was about 10% of all new wind
power installations) (EWEA, 2011). As for the end of 2011, 235
new offshore wind turbines, with a total capacity of about
870 MW, were fully connected to the power grids of the UK,
Germany, Denmark and Portugal. In total, as for the end of 2011,
there were almost 1400 offshore wind turbines fully grid connected with a total capacity of about 3.8 GW (Fig. 1) comprising
53 wind farms spread over ten European countries.
As of February 2012, the Walney wind farm in the United
Kingdom is the largest offshore wind farm in the world
(367 MW), followed by the Thanet offshore wind project
(300 MW), in the UK. The London Array (630 MW) is the largest
project currently under construction which is also located in the
UK. In total, 18 new wind farms, totalling 5.3 GW are currently
under construction and 18 GW are fully-consented in twelve
European countries with Germany possessing almost 50% of the
total consented installations (EWEA, 2012). Once completed,
Europe’s offshore wind power capacity will reach 27 GW.
Up till now vast deployment has taken place in Northern
Europe, a situation expected to continue for the next few years as
well. Actually, more than 90% of the global offshore wind farms
are located in European waters. The leading markets are currently
As far as the technology employed is concerned, it should be
noted that the design of offshore wind power projects has been
based considerably on the long-term experience gained from
on-shore wind farms and from the oil and gas industry, while
the commercial wind turbines used, currently having capacity
ratings up to 5 MW, comprise adaptations from land-based counterparts. However, offshore wind turbine technology is evolving at a
fast pace and thus much larger machines are expected in the
foreseeable future, specifically constructed for offshore use, which
will likely benefit from economies of scale resulting in significant
cost reduction.
All the above issues are analysed in this work. More precisely,
the objective of the present study is to provide a short review of
the activity noted in the field of the offshore wind energy at a
global level, emphasising on global market issues, current status
and future trends of the technology employed, examining at the
same time energy production and availability issues as well as
economic considerations such as investment and associated costs
of electricity generation.
2. Global offshore wind energy activity
According to the existing literature, the first documented
theoretical concepts for installing wind turbines at sea were
developed in Germany in the early 1930s by Hermann Honnef.
Almost forty years later, off the coast of Massachusetts, Professor
William E. Heronemus introduced the idea of large floating wind
turbine platforms (Heronemus, 1972). None of these early visions
became reality however. The first offshore wind power test
facility was eventually set up in Sweden, twenty years later, in
1990. It was a single wind turbine of 220 kW rated power, located
at a distance of 250 m from the coast, supported on a tripod
structure anchored to the seabed about 7 m deep.
The first full-scale development of offshore wind power projects
was driven largely by commercial aspirations of the European wind
industry, considering oceans as a feasible solution to compensate
for the scarcity of onshore sites. The first commercial offshore wind
farm was commissioned in 1991 in Denmark, in shallow water
(2–6 m deep), 1.5–3 km north from the coast of the island of
Lolland, near the village of ‘‘Vindeby’’. This small wind farm, which
is still in operation, consists of eleven stall controlled wind turbines
of total rated power 4.95 MW (450 kW each) all being placed on
gravity-based foundations. The cost of construction for this project
is stated to be approximately 10 million Euros (SEAS-NVE, 2011).
1000
4000
3600
800
3200
700
2800
600
2400
500
2000
400
1600
300
1200
200
800
100
400
Fig. 1. Offshore wind farm installations in Europe.
2011
2009
2010
2008
2007
2006
2005
2004
2003
2002
2000
2001
1999
1998
1997
1996
1994
1995
1993
0
1992
0
1991
Annual Capacity (MW)
900
cumulative
Cumulative Capacity (MW)
new capacity
138
J.K. Kaldellis, M. Kapsali / Energy Policy 53 (2013) 136–148
2500
2093,7
Power (MW)
2000
1500
1000
857,3
500
26,3
25,2
25
2,3
2
Portugal
163,7
Norway
195
Japan
200,3
Ireland
233
Finland
246,8
Sweden
Belgium
Germany
China
Netherlands
Denmark
UK
0
Fig. 2. Global installed offshore wind power by country (end of 2011).
the UK, Denmark and the Netherlands with cumulative capacity
ratings of 2094 MW, 857 MW and 247 MW respectively (as for
the end of 2011), see Fig. 2. By 2020, offshore wind power
scenarios entail a quite ambitious development path with
75 GW installations worldwide, with significant contribution
expected from the United States and China (EWEA, 2007).
China, the world’s largest onshore wind power developer, with
a total of about 62 GW by the end of 2011, erected the first largescale commercial offshore wind farm (Donghai Bridge) outside
Europe in 2009, adding 63 MW by year-end for a project that
reached 102 MW upon completion in the early 2010. Thus,
although offshore wind power development in China has much
delayed, the year 2010 marked the start of transition for the local
offshore wind power sector from research and pilot projects to
operational wind farms. Today, China has about 230 MW (including
an intertidal project) of offshore wind power installations. According
to the Chinese Renewable Energy Industries Association (CREIA)
(CREIA, 2011), China is planning to exploit its vast offshore wind
resources (Da et al., 2011) by greatly expanding its offshore capacity
to 5 GW by 2015 and 30 GW by 2020, as a result of the country’s
commitment for 40–45% (from the base year 2005) (Zhang et al.,
2010) carbon emission reduction until 2020.
On the other hand, as for the end of 2011, there are no offshore
wind power projects operating off the United States, which is the
second world leader in land-based wind energy. The only
approved project, after a decade-long process, is to be located
off the coast of Massachusetts and is expected to comprise 130
3.6 MW wind turbines that will be operational in 2012. However,
U.S. offshore wind energy plans call for the deployment of 10 GW
of offshore wind generating capacity by 2020 and 54 GW by 2030
(U.S. Department of Energy, 2011).
Offshore wind power market is currently dominated by few
companies. On the demand side about ten companies or consortia
account for all the offshore capacity presently in operation. Dong
Energy (Denmark), Vattenfall (Sweden) and E.on (Germany) are
the leading operators, all being giant European utilities. On the
supply side, Siemens (formerly Bonus Energy A/S) and Vestas are
by far the leading wind turbine manufacturers worldwide in
terms of installed capacity. In Europe, their cumulative share
reaches 90% (Fig. 3). However, there are several other manufacturers that are now developing new offshore wind turbines’ types
which are close to commercial viability. For instance, both
Repower Systems and AREVA Multibrid installed commercial
turbines of 5 MW under a pilot project (comprising the deepest
Others: 6%
Repower: 5%
Vestas: 36%
Siemens: 53%
Fig. 3. Wind turbine manufacturers’ cumulative share up to 2011 in Europe.
large-scale operational offshore wind power project at that time,
with an average distance from the shore of about 53 km) named
‘‘Alpha Ventus’’ in Germany in 2009. Sinovel also entered the
market in 2009 with the SL3000/90, the first offshore wind
turbine manufactured in China and installed in the ‘‘Donghai
Bridge’’ project. More recently, General Electric re-entered the
offshore wind market with the announcement of its 4.1 MW
direct drive wind turbine, which is still under development
(GE Energy, 2011).
3. Technology challenges
3.1. Evolution of offshore wind farms’ main characteristics
Offshore, the size and capacity of wind turbines, as well as the
total rated power of wind farms follow the onshore increasing
trend and even more since there are fewer political limits. While
at land-based sites the size of wind turbines, in terms of height
and rotor diameter, is often restricted due to visual impacts, these
limits are not usually encountered in marine environments. Thus,
as it may be observed from Fig. 4, wind turbine capacity has been
increasing steadily every year since 1991. In the 90s, offshore
wind turbine rated power was well below 1 MW, while 2003 was
the first year of introducing commercial wind turbines above
2 MW. In 2005, one offshore wind farm went online using 3 MW
turbines, setting a new benchmark for the industry (EWEA, 2010).
Since then, offshore wind turbines installed generally in the range
between 2 and 5 MW although prototypes of power up to 7 MW
and even higher are currently tested, indicating the manufacturing trends concerning future wind turbines operating in maritime
environments. On top of that, wind farms’ total capacity has
increased as well. Before 2000, average wind farm size was below
J.K. Kaldellis, M. Kapsali / Energy Policy 53 (2013) 136–148
139
6
Bubble area represents power capacity of the wind farm
Turbine capacity (MW)
5
4
3
2
1
0
1990
1995
2000
2005
2010
Fig. 4. Size evolution of existing offshore wind farms and wind turbines (2010).
30
30
Average water depth (m)
Average distance from shore (km)
2010
2009
2008
2007
2006
2005
2004
2003
2002
2001
2000
0
1999
0
1998
5
1997
5
1996
10
1995
10
1994
15
1993
15
1992
20
Distance from shore (km)
25
20
1991
Water depth (m)
25
Fig. 5. Evolution of average distance from shore and water depth.
20 MW. Today, the experience has grown significantly so that
many countries are building large (average size of projects
exceeds 150 MW), utility-scale offshore wind farms or at least
have plans to do so.
Nevertheless, the vast majority of the existing large-scale
commercial projects still use shallow-water technology (located
at less than 30 m water depth) although the idea of going deeper
is gradually moving closer towards implementation. Actually, the
average water depth remains below 20 m (Fig. 5), excluding the
first full scale floating wind turbine (Hywind) which was installed
in 2009 off the Norwegian coast at a water depth of 220 m. On the
other hand, the average distance from shore ten years ago was
below 5 km, while today is close to 30 km—confirming that
offshore wind turbines are installed increasingly away from the
shores (Fig. 5).
So far, there is no standard type of support structure suitable
for all kinds of seabed conditions and depths. As mentioned
above, the majority of offshore wind power projects is currently
located in shallow water and employs fixed bottom structures
suitable for small (shallow) to moderate (transitional) depths.
More specifically, various support structures have been used up
till now, with the most common types being the monopile and the
gravity-based one (Fig. 6), both comprising fixed bottom structures mainly employed in shallow water depths. Fig. 7 summarises the main support structures for offshore wind turbines in
terms of maturity and water depth.
At this point it should be mentioned that during the last years
the floating concept (i.e. mounting a wind turbine’s tower on a
floating platform) has been introduced in order to eliminate as
much as possible the visual impact and take advantage of the
higher and steadier wind speeds found in deep waters. At deeper
water sites, the fixed bottom support structures are inapplicable
because as depth increases loading increases too and this requires
larger structural dimensions which are economically non-viable.
Nevertheless, floating wind turbine technology is still immature
(Fig. 7) and is associated with increased technical risk which
tends to drive costs upwards.
Up till now (end of 2011), four floating wind turbine concepts
have been installed (see for example Fig. 8), i.e.:
An 80 kW floating wind turbine was deployed 113 km off the
coast of Italy in 2007. It was then decommissioned at the end
of 2008 after completing a planned test year of gathering
operational data.
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J.K. Kaldellis, M. Kapsali / Energy Policy 53 (2013) 136–148
Gravity-based:
Jacket: 2%
21%
Gravity-based:
35%
Others: 2%
2000
2011
Monopiles:
65%
Monopiles:
75%
Mature
Fig. 6. Share of support structures’ types for the in operation wind farms. Based on data found in (EWEA, 2010, 2012)).
Monopiles
Gravity-based
Developed
Maturity
Jackets
Tripiles
Tripods
Developing
Floating concepts
Suction buckets
Shallow water
30m
Transitional water
60m
Deep water
Fig. 7. Main support structure technology in relation to water depth.
Fig. 8. Floating wind turbine prototype concepts which have already been installed (Left: SeaTwirl, Middle: Hywind, Right: WindFloat) (Seatwirl, 2012; Renewable energy
focus, 2012; Gotpowered, 2012).
The first large-capacity, 2.3 MW floating wind turbine
‘‘Hywind’’, became operational in the North Sea off the coast
of Norway in 2009 in 220 m water depth and is still operational (as of 2011). It is worth mentioning that this wind
turbine, in 2011, produced about 10.1 GWh which results in a
capacity factor of more than 50% (Statoil, 2012).
In October 2011, a WindFloat Prototype was installed 4 km
offshore a coast of Portugal in approximately 45 m depth. The
WindFloat was fitted with a Vestas V80 2 MW offshore wind
turbine.
Finally, SeaTwirl deployed a vertical axis (with blades above
the water and the generator below) floating grid connected
wind turbine off the coast of Sweden in 2011. It was tested and
recently de-commissioned.
It should be noted that, the world’s first full scale floating wind
turbine ‘‘Hywind’’ cost was around 400 million kroner (or around
54 million Euros) including construction and further development
(R&D) related to the specific wind turbine concept. Although this
cost may seem unreasonably high it must be underlined that the
specific project was the first of its kind and that its demonstrative
nature required considerable R&D efforts as well as monitoring of
its operating behaviour.
J.K. Kaldellis, M. Kapsali / Energy Policy 53 (2013) 136–148
3.2. Energy production and availability issues
As offshore wind turbines operate far from shores in harsh
environments, the need for high reliability and low operation and
maintenance (O&M) costs is higher than for on-land applications.
In fact, O&M costs may be up to three times higher than those of
land-based systems (Rademakers et al., 2003; Musial et al., 2010),
exceeding 20% of the overall cost of these projects during lifetime
(Blanco, 2009). One of the main causes for these high costs is the
rather frequent need for employing expensive transportation
means in order for the personnel to reach the wind farm and
perform the service work required (Bussel and van Zaaijer, 2001).
In this context, high operational reliability and adequate maintenance capability (mainly determined from accessibility restrictions) are two interrelated and critical issues for safeguarding the
long-term operation of an offshore wind farm, in both technical
and economic terms. For that reason, parameters such as minimisation of maintenance requirements as well as maximisation of
access capability are of great importance during the design
procedure of a project.
It is well known that the energy yield of a wind farm during a
specific time period (e.g. 8760 h) is estimated as a function of the
installation’s capacity factor ‘‘CF’’ (or utilisation factor of the local
wind potential) and the rated power of the wind turbines. The
technical availability, which is actually configured by the hours of
operation of a given wind turbine or wind farm by considering the
time period that the machine is kept out of operation due to e.g.
scheduled maintenance, unforeseeable events (e.g. lightning,
sudden fault of a machine) etc. (Herbert et al., 2010), may be a
determining factor to the CF of an installation. Obviously, many
days of downtime during a specific time period will reduce the
availability of the wind farm and therefore its energy production
and CF over that time period. However, the amount of energy loss
will depend on when the downtime occurs, e.g. the impact of the
availability on the energy performance of the plant will be much
higher when the local wind potential is high.
In general, actual capacity factors for onshore wind farms
oscillate across time and regions, with an average value being
between 20 and 30%. For instance, the average European value
between 2003 and 2007 has been recorded at about 21%. The
highest values have been recorded for Greece and the UK (i.e.
equal to 29.3% and 26%, respectively) (Boccard, 2009) due to the
existence of many low density population areas which benefit of
high wind speeds and enable the siting of wind farms.
On the other hand, one of the main advantages of offshore
wind power is that the wind turbines may have the ability to
demonstrate quite higher capacity factors than onshore counterparts (as a result of the higher mean power coefficient which is
usually met in offshore installations), typically ranging from 20%
to 40%. In this context, Fig. 9 depicts the annual capacity factor of
several representative offshore wind farms across Europe as
recorded for the year 2011. One may see that capacity factor
values, in some cases, even reached 50%, however, this is not the
rule since there are cases where the recorded capacity factor may
be quite low mainly as a result of the combination of extended
downtimes due to several system failures and the tough conditions usually met in marine environments.
As a result of the above, the critical role of the technical
availability over a period of time for the energy production of a
given wind turbine or an entire wind farm is reflected. At this
point, one should also note that technical availability of a wind
turbine depends, among others, on:
The technological status (experience gain effect throughout
the years) of the installation at the time it went online
(increasing experience in both production and operation issues
in the offshore sector suggests that the failure rate decreases
and the reliability increases respectively).
The technical availability changes (aging effect) during the
installation’s operational life.
The accessibility difficulties (accessibility effect) of the wind
farm under investigation. This parameter is, as aforementioned, of special interest for offshore wind parks, especially
during winter, due to bad weather conditions (high winds and
huge waves suspend the ship departure, thus preventing
maintenance and repair of the existing wind turbines).
Nowadays, contemporary land-based wind turbines and wind
farms reach availability levels of 98% or even more (Kaldellis, 2002,
2004; Harman et al., 2008) but, once these wind turbines are placed
offshore the accessibility may be significantly restricted, thus causing
a considerable impact to the availability of the wind farm and in turn
to the energy and economic performance of the whole project. This is
not always the case however; apart from the distance from the shore,
55
2009
2002
50
2009
2010
45
2009
30
2006
2007
Beatrice (UK)
2004
35
Barrow (UK)
2003
40
CF (%)
141
2008
2000
25
20
1991
2001
15
10
5
Rodsand 2
(Denmark)
Robin Rigg (UK)
Horns Rev 2
(Denmark)
Hywind (Norway)
Burbo Bank 1
(UK)
Scroby Sands
(UK)
Nysted
(Denmark)
Horns Rev 1
(Denmark)
Yttre Stengrund
(Sweden)
Middelgrunden
(Denmark)
Vindeby
(Denmark)
0
Fig. 9. Capacity factors for several offshore wind turbine projects for the year 2011 along with projects’ year of commissioning.
142
J.K. Kaldellis, M. Kapsali / Energy Policy 53 (2013) 136–148
the accessibility to a wind farm’s installation site depends also on
several other parameters such as local climate conditions and the
type and availability of the maintenance strategy adopted (the
limited size of some wind farms does not always justify the purchase
of a purpose built vessel so there may be significant delays if the
vessel is, for example, away for another assignment). Thus, there are
cases where the impact may be more or less significant than the
expected one.
A case with low recorded availability is North Hoyle offshore
wind farm, which is located in the UK, at an average distance from
the shore equal to 8 km (see also Table 1 where recorded
availability data for several wind farms are presented). As it is
mentioned in (BERR, 2005), the availability of this wind farm
during a one-year period (2004–2005) was recorded equal to 84%.
The most notable sources of unplanned maintenance and downtime have occurred due to termination of cable burial and rock
dumping activities as well as high-voltage cable and generator
faults. It is worth mentioning that the downtime recorded splits
to 66% owed to turbine failure, 12% to construction activities, 5%
to scheduled maintenance and 17% to site inaccessibility due to
harsh weather conditions. Another example with even lower
availability (67%) is the case of Barrow offshore wind farm (see
also Table 1), also located about 8 km far from shore, in the UK.
The total average availability of this project is quoted as 67% for
one-year period between July 2006 and June 2007. This low
availability is due to a number of wind turbine faults, mainly
generator bearings and rotor cable faults combined with low
access to the site because of high waves during that time period.
4. Economic considerations
4.1. Capital cost of investment
As stated earlier, in offshore sites the winds are stronger and
steadier than on-land, making wind farms more productive with
higher capacity factors. On the other hand however, as offshore
wind energy is a newcomer, most of costs associated with its
development are still much higher compared to onshore counterparts (Green and Vasilakos, 2011), although some recent technological progress in terms of more efficient production patterns
may have the potential to narrow this gap in the near future.
In general, for most of the existing projects, current turnkey (or
capital) offshore wind power costs are estimated to be within the
wide range of 1300–2500 h/kW while for onshore installations
they are significantly lower, i.e. approximately 1100–1500 h/kW
(Ecofys, 2011) for newly-established projects in Europe, while in
some emerging markets these costs may be lower due to cheaper
labour works, grid connection and upgrade issues etc. (Blanco,
2009). Capital costs of offshore wind power projects can be divided
into the following main categories:
Cost of wind turbines (e.g. blades, rotor, tower, condition
monitoring etc.).
Cost of electrical infrastructure (underwater cables for collec
tion of power and transmission to the grid, substations,
transformers etc.).
Cost of support structures.
Table 1
Availability of several offshore wind farms. Based on data from BERR (2009).
Capacity of
each turbine
(MW)
Recorded
Average
distance from availability
(%)
shore (km)
Comments
July 2006 –June 2007 2006
3
8
67
Kentish Flats—UK
January 2006–
December 2006
2005
3
9
87
Kentish Flats—UK
3
9
73.5
North Hoyle—UK
January 2007–
2005
December 2007
July 2004 –June 2005 2003
2
8
84
North Hoyle—UK
July 2005 –June 2006 2003
2
8
91.2
North Hoyle—UK
July 2006 –June 2007 2003
2
8
87.4
Scroby Sands—UK
January 2005–
December 2005
2004
2
3
84.2
Scroby Sands—UK
January 2006–
December 2006
2004
2
3
75.1
Scroby Sands—UK
January 2007–
December 2007
2004
2
3
83.8
Tunø
Knob—Denmark
January 1996–June
1998
1995
0.5
6
98.3
The low availability is due to a number of wind turbine faults,
mainly generator bearings and rotor cable faults combined
with low access to the site because of high waves
Availability was relatively low mainly due to faults on
generator bearings and rotor cables for the slip ring unit and
gearbox failures
Availability has been severely hampered during 2007 mainly
due to bearing failures in the planetary gear stage
Availability was mainly affected due to termination of cable
burial and rock dumping activities, high-voltage cable fault and
generator faults as well as site inaccessibility due to harsh
weather conditions
Availability was mainly affected due to extended outages due
to generator bearing faults and gearbox faults
Availability was mainly affected by gearbox bearing faults and
chipped teeth, resulting in gearbox replacements. It should be
noted that gearbox replacement was delayed by several
months as no specialist vessels were available. Other major
component failures include rotor cable faults, circuit breaker
issues etc.
Availability was mainly affected due almost to problems with
bearings in the gearbox. Furthermore, there were a total of 143
adverse weather days when access to the wind farm was
prevented
Availability was mainly affected due to gearbox bearings and
generator failures. It should also be noted that there were a
total of 76 adverse weather days when access to the wind farm
was prevented
Technical Availability only suffered during November due to a
high number of waiting on weather days which delayed
returning turbines to service following the generator
replacement work
Only one major problem was experienced on Tunø during this
time period: In one turbine, the hydraulic unit serving the pitch
mechanism and the brake was replaced, leaving the turbine
idle for almost 3 weeks. Weather conditions prevented a faster
response to the problem
Project
Period
Barrow—UK
Year
online
J.K. Kaldellis, M. Kapsali / Energy Policy 53 (2013) 136–148
143
monopiles
distance from shore: 12km
Thanet
2010
high rise pile cap
distance from shore: 11km
Donghai Bridge
tripods and jackets
distance from shore: 53km
Alpha Ventus
2009
monopiles
distance from shore: 30km
Horns Rev II
monopiles
distance from shore: 8km
Princess Amalia
2008
monopiles
distance from shore: 10km
Egmond aan Zee
2006
gravity-based
distance from shore: 23km
Nysted
2003
monopiles
distance from shore: 16km
Horns Rev I
2002
gravity-based
distance from shore: 3km
Middelgrunden
0
1
2
3
2000
4
5
million Euros/MW
Fig. 10. Indicative historical capital costs per MW of several offshore wind farms along with their year of commissioning.
80
offshore
Share of the Capital Cost (%)
70
onshore
60
50
40
30
20
10
0
Turbine ex works,
including logistics
and installation
Support structure
Electrical
infrastructure
Design,
development and
permits
Other
Fig. 11. Capital cost breakdown for offshore wind power projects in shallow water in comparison with onshore. Based on average data from (Blanco, 2009; EWEA, 2009b).
Cost of logistics and installation.
Development and engineering costs (e.g. licensing procedures,
permits, environmental impact assessment studies etc.).
Other miscellaneous costs which are not included in the above
and grid connection issues which are more expensive in case of
offshore wind power projects, so the percentage of the turbine’s
contribution to total capital cost is less (EEA, 2009). Specifically,
the above discrepancies are related with the following key issues:
categories.
Foundations in offshore sites are much more expensive
As one may easily obtain from Fig. 10, capital costs are not
standard, but they depend on a variety of factors such as distance
from the shore, water depth, foundation technology employed
etc., and thus can exceed by far the above 2.5 million h/MW
threshold for a distinct case. For example, for ‘‘Alpha Ventus’’
wind farm the capital cost per MW exceeded 4 million h mainly
due to the demonstrative nature of the project.
Fig. 11 presents data concerning main capital cost estimates
for offshore wind power projects located in shallow waters
( o30 m depth) in comparison with onshore. The estimates for
the offshore turbine’s contribution are about 45% to total capital
cost whereas the initial cost of a project on-land is dominated by
the cost of the turbine itself, which is approximately 2 times
higher ( 80%) (in absolute terms). These cost differences are
mainly attributed to the support structure, installation process
than on-land. For a conventional wind turbine installed
on-land, the share of the foundation is approximately 5–9%
(EWEA, 2009a) (see Fig. 11) while in an offshore site the
respective cost may be up to 30% of the initial capital invested
and even higher in case of very deep waters and unfavourable
soil conditions.
Construction and installation techniques adopted for offshore
projects are still immature and thus more expensive. In general,
at larger distances, installation costs increase because of the
greater travelling time needed for reaching the site. In addition,
weather conditions usually become worse, as distance from
shore increases, thus making installation of a project a quite
difficult task (Bilgili et al., 2011).
Offshore, electrical connection issues generate substantial
additional costs compared with onshore installations.
144
J.K. Kaldellis, M. Kapsali / Energy Policy 53 (2013) 136–148
Higher technical risks at offshore sites and siting complexity
which drive costs up.
As the distance of wind turbines from shore increases, installation and electrical infrastructure costs are driven upwards. The
construction of wind farms further from shore normally involves
higher depths which may require more complex installation
procedures (e.g. special vessels capable of carrying larger wind
turbines and components) and more expensive equipment (e.g.
support structures). Furthermore, greater distances, as aforementioned, result to an increase of the travelling time of the
construction vessels in the sea. In addition to that, one should
take into account a possible time-delay of the installation due to
harsher wind and wave conditions when moving deeper in the
sea. In this context, the additional time required for completing
the installation due to ‘‘weather downtime’’ may reach 20–30%
(EEA, 2009). Also, moving deeper implies a larger undersea
cabling system for connecting the wind farm with the electrical
grid and possibly the employment of an offshore substation, thus
affecting significantly the electrical infrastructure costs. Based on
official data, costs for offshore cables are between 500,000 and
1 million Euros per km (International Association of Engineering
Insurance, 2006).
4.2. Life-cycle cost
In addition to the higher capital investment costs, offshore
projects have higher levelized costs of energy from onshore wind
farms. Broadly speaking, the cost of generating energy for a wind
power project is the sum of initial investment and O&M costs
(split into the fixed maintenance cost and the variable one) over
the operational life of the project (e.g. 25 years), divided by the
total energy output of the project. Subsequently, the levelized
cost of electricity generation is estimated as a unit of currency
(life-cycle cost) per unit of energy produced (e.g. in b/kWh or
hcent/kWh).
Current levelized costs of energy of land-based projects vary
within the range of 4.5–8.7 hcent/kWh while for offshore installations respective costs are almost double, i.e. 6–11.1 hcent/kWh
(Blanco, 2009). It should be mentioned however, that offshore
costs are determined by a quite high degree of uncertainty,
mainly due to different policy measures and support mechanisms
adopted in several countries.
Estimated life-cycle cost (or total project cost during lifetime)
breakdown for a typical offshore wind power project is presented
in Fig. 12. As seen in the figure, costs are largely determined by a
range of parameters which extend far beyond the wind turbine
itself. Particularly, life-cycle cost is dominated by O&M costs
while logistics, installation, support structures and electrical
infrastructure also obtain a considerable part (almost 1/3 of the
total). It should be mentioned at this point that O&M costs are
substantially higher than those of land-based systems (two or
three times greater) (Rademakers et al., 2003), exceeding 20% of
the total levelized cost, while in some cases they can even reach
up to 30%, mainly due to difficult and expensive access to the site.
Concluding, it should be noted that due to the limited number
of offshore wind power projects currently being installed, accurate statistical trends of associated costs of development and
operation, as is the case of onshore counterparts, are difficult to
be extracted yet. Water depth, distance from the shore, foundations, grid connection issues, infrastructure required and O&M are
apparently determinant factors for the total energy cost during
lifetime. Nevertheless, offshore wind energy is still under evolution and requires special R&D efforts in terms of developing costefficient O&M strategies, high reliability, site access solutions,
innovative components and improved and fully integrated ‘‘wind
turbine-support structure’’ concepts.
5. Future expectations
Offshore wind energy development has shown a quite
unsteady progress since the beginning of its appearance but is
foreseen to expand significantly in the years to come and move
from the pioneering phase to a large-scale global deployment.
Offshore wind power experienced a record growth in 2010.
Total installed global offshore wind capacity, at the end of the
year, amounted to approximately 3 GW, out of which more than a
half was added in 2010 leading to an average growth rate which is
by far higher than the respective of the onshore wind energy
sector (WWEA, 2010). Currently (end of 2011), the total offshore
capacity is nearly 4 GW. As for the future expectations, despite
the world economic and financial crisis which definitely may have
an impact on the financing options available to investors, the
European Wind Energy Association has increased its 2020 target
to the challenging amount of 230 GW wind power capacity,
including 40 GW offshore wind. As for 2030, the wind industry
has also set the ambitious target of 400 GW of wind power
installations in Europe, out of which 150 GW to be located
offshore and produce more than 550 TWhe of electricity (EWEA,
2008; EWEA, 2009c). In fact, according to EWEA targets (EWEA,
2009c), for the forecasted annual wind power installations up to
the year 2030 as well as for capacity prices equal to 1250 h/kW for
onshore and 2400 h/kW for offshore (in 2005 constant prices),
investment in wind energy (both onshore and offshore) should
reach h23.5 billion in 2020 and h25 billion in 2030 (Fig. 13). The
decade up to 2030 is projected to be almost stable, almost h25
billion annually, with a gradually increasing share of investments
however going offshore.
Nevertheless, apart from the aforementioned ambitious targets and the undeniable progress met in the field of offshore wind
during the recent years, one should not disregard the fact that at
Other variable costs
11%
Over lifetime
32%
Wind turbine
29%
O&M costs
21%
Other capital costs
1%
Initial cost
68%
Project development and
permits
4% Logistics and installation
10%
Electrical infrastructure
11%
Support structure
13%
Fig. 12. Life-cycle cost breakdown for a typical offshore wind farm. Based on data from (Musial et al., 2010; U.S. Department of Energy, 2011).
J.K. Kaldellis, M. Kapsali / Energy Policy 53 (2013) 136–148
145
30
Offshore
Onshore
25
billion Euro
20
15
10
5
2030
2029
2028
2027
2026
2025
2024
2023
2022
2021
2020
2019
2018
2017
2016
2015
2014
2013
2012
2010
2011
0
Fig. 13. Projected annual wind energy investments up to 2030. Based on scenarios included in (EWEA, 2009c).
the moment, offshore wind farms represent only a very small
percentage of the global wind power capacity, in the order of 2%.
Despite the fact that the first project was built twenty years ago,
the offshore wind energy sector still remains under development
and thus exploration of prospects and technological trends is of
primary importance for determining its ability to compete with
onshore wind farms and more importantly with conventional
electricity generation.
In this context, evaluation of available offshore wind energy
potential and mapping of the most suitable regions is a big step
towards the promotion of the offshore concept. To this end, in
August 2008, the project ‘‘NORSEWInD’’ officially started (under
the EU’s 7th Framework Programme/duration: 2008–2012) aiming at producing high quality Wind Atlases for the North, Irish and
Baltic Seas (NORSEWInD, 2012), see also Table 2. Furthermore,
many interesting studies (e.g. de Castro et al., 2011; EEA, 2009; Lu
et al., 2009; deVries et al., 2007; Capps and Zender, 2010; Zerta
et al., 2008) have been implemented so far to evaluate the long
term technical potential of wind power, with most of them
concluding that the amount of the wind resource poses no limit
due to its extreme abundance. However, significant differences
among the estimated top limits in all these studies may be
noticed, e.g. the technical potential of wind power which is
examined by de Castro et al. (2011) is found one or two orders
of magnitude lower than other technical potentials estimated in
similar studies.
Relative to the evaluation of the European offshore wind
energy resources, the technically available as well as the constrained1 and the economically competitive2 wind energy potential are depicted in Fig. 14, for a time horizon up to 2030 (based
on the results of the European Environmental Agency Report
(EEA, 2009)). As one may see, the available offshore potential is
quite high, reaching 25,000 TWhe/year in 2020 (i.e. 7 times
greater than the projected electricity demand over that year
(approx. 4000TWhe) based on the ‘‘business as usual’’ and ‘‘EC
1
Environmental (Natura 2000 and other protected areas) and social constraints are taken into account. For offshore installations, limitations such as
shipping routes, military areas, oil and gas exploration and tourist zones are also
considered.
2
The wind resource potential that is considered as cost competitive in the
light of projected average energy costs in the future based on European Commission’s baseline scenario (EWEA, 2008).
Proposal with RES trading’’ scenarios (EC, 2008a, 2008b)),
although what is eventually estimated as economically feasible,
when taking into account projected average levelized energy
costs, is eventually much less, i.e. 2600 TWhe, covering
however–in rough numbers–almost 2/3 of the projected electricity demand. Accordingly, based on the corresponding results of
the same report for 2030, the economically competitive wind
resource potential could cover more than 80% of that time’s
expected EU electricity demand (i.e. approx. 4400 TWhe (EC,
2008a, 2008b)).
Apart from the excellent potential of the wind energy resource
in many maritime locations, however, offshore wind energy is
still faced with many unique technological challenges. Although
the technology has developed rapidly during the last years there
is a general view that further improvements can be expected in
terms of both energy performance and cost effectiveness, with
most R&D efforts currently concentrated in improving turbine
design and reliability and in developing the next generation of
offshore ‘‘wind turbine-support structure’’ concepts.
Today’s operational offshore wind turbines are adaptations
from land-based proven technologies and, as mentioned above,
have rated capacity up to 5 MW. Nevertheless, design variations
currently being pursued include first, as mentioned in section 3,
an increase of turbine capacities to tens of MWs (de Vries, 2009)
in order to maximise energy production and benefit from economies of scale and second, the adoption of advanced techniques in
view of increasing wind turbines’ reliability along with providing
faster, cheaper and more efficient maintenance. In this regard,
experience gained through ongoing or completed projects such as
the ‘‘UpWind’’ (funded under the EU’s 6th Framework Programme/duration: 2006–2011) and ‘‘HiPRWind’’ (started in
2010 as a part of the EU’s 7th Framework Programme) (Table 2)
is considered highly valuable. The main scope of the former was
the investigation of the limits of upscaling of very large wind
turbines (up to approximately 20 MW/250 m rotor diameter)
(Upwind, 2011), while the main aim of the latter is to unlock
vast new deepwater areas for wind farms by doing research on
very large, floating offshore wind turbines (HIPRWIND, 2011).
As regarding expected capacity factors, it is stated that by
2020, at the European level, average values for offshore wind
could reach the quite high percentage of 40–45% (Boccard, 2009;
EWEA, 2008). These high values are attributed to a variety of
concepts which have already been developed or are under
146
J.K. Kaldellis, M. Kapsali / Energy Policy 53 (2013) 136–148
Table 2
Offshore projects under the EU 7th Framework Programme.
Project
Acronym
Funds by the
FP7
Duration
Offshore Renewable Energy
Conversion Platforms—
Coordination action
ORECCA
1.6 Mh
03/2010–8/2011
(18 months)
Marine Renewable Integrated
Application Platform
MARINA
PLATFORM
8.7 Mh
Northern Seas Wind Index
Database
NORSEWIND
3.95Mh
Reliability focused research on RELIAWIND
optimising Wind Energy
systems design, operation
and maintenance: Tools,
proof of concepts, guidelines
& methodologies for a new
generation
Future Deep Sea Wind Turbine DEEPWIND
Technologies
High Power, High Reliability
Offshore Wind Technology
HIPRWIND
2030
4,400
2020
5.18 Mh
Scope
The objectives of this project are to create a framework for knowledge sharing
and to develop a research roadmap for activities in the context of offshore
renewable energy (RE). In particular, the project will stimulate collaboration in
research activities leading towards innovative, cost efficient and
environmentally benign offshore RE conversion platforms for wind, wave and
other ocean energy resources, for their combined use as well as for the
complementary uses
01/2010–06/2014 MARINA is a European project dedicated to bringing offshore renewable energy
(54 months)
applications closer to the market by creating new infrastructures for both
offshore wind and ocean energy converters. It addresses the need for creating a
cost-efficient technology development basis to kick-start growth of the nascent
European marine renewable energy (MRE) industry in the deep offshore
08/2008 - 08/2012 NORSEWInD is a programme designed to provide a wind resource map
(48 months)
covering the Baltic, Irish and North Sea areas. The project will acquire highly
accurate, cost effective, physical data using a combination of traditional
Meteorological masts, ground based remote sensing instruments (LiDAR &
SoDAR) and Satellite acquired SAR winds. The resultant wind map will be the
first stop for all potential developers in the regions being examined, and as such
represents an important step forward in quantifying the quality of the wind
resource available offshore
03/2008–03/2011 RELIAWIND consortium, for the first time in the European Wind Energy Sector,
(36 months)
and based on successful experiences from other sectors (e.g. aeronautics) will
jointly & scientifically study the impact of reliability, changing the paradigm of
how Wind Turbines are designed, operated and maintained. This will lead to a
new generation of offshore (and onshore) Wind energy Systems that will hit
the market in 2015
2.99 Mh
10/2010–8/2014
(46 months)
11.0 Mh
11/2010–10/2015
(59 months)
The objectives of this project for new wind turbines are: (i) explore the
technologies needed for development of a new and simple floating offshore
concept with a vertical axis rotor and a floating and rotating foundation, (ii)
develop calculation and design tools for development and evaluation of very
large wind turbines based on this concept and (iii) evaluate the overall concept
with floating offshore horizontal axis wind turbines
The aim of the HiPRwind project is to develop and test new solutions for very
large offshore wind turbines at an industrial scale. The project addresses critical
issues such as extreme reliability, remote maintenance and grid integration
with particular emphasis on floating wind turbines
Projected electricity demand
4,000
2030
3,400
2020
Economically competitive potential
2,600
2030
3,500
2020
Constrained potential
2,800
2030
30,000
2020
Technical potential
25,000
0
5000
10000
15000
20000
25000
30000
35000
40000
TWhe
Fig. 14. Projected technical, constrained and economically competitive potential for offshore wind energy development in 2020 and 2030.
development for increasing both wind turbines’ performance and
reliability of offshore commercial wind power plants. Indicative
examples in this field correspond to:
Adoption of advanced techniques and materials through an
The increase of wind turbines’ efficiency through the develop
ment of new systems with reduced maintenance requirements
(ReliaWind, 2012).
Implementation of advanced maintenance strategies.
integrated design of wind turbines to withstand the demanding
sea conditions and incorporation of Condition Monitoring Systems
(Amirat et al., 2009; OSMR, 2004), which can provide continuous
inspection of the state of the plant (Braam et al., 2003;
Wiggelinkhuizen et al., 2007; Wiggelinkhuizen et al., 2008).
Adoption of more efficient and newer drive-train concepts in
view of increasing wind turbines’ reliability.
J.K. Kaldellis, M. Kapsali / Energy Policy 53 (2013) 136–148
For the purpose of facilitating equipment transportation and
associated costs during installation process, the use of lightweight and cost-effective materials for the design of towers,
drive-trains and support structures or even bringing back the
idea of two-blade machines (Butterfield et al., 2005) (since visual
impact is unlikely to be an important consideration offshore)
could comprise feasible options. Furthermore, in view of improving
wind turbines’ reliability and providing cheaper installation
and maintenance, innovative ideas and strategies come to light.
An example is the adoption of innovative logistic concepts (e.g.
enhancing port infrastructure) and improvement of the wind
turbines’ ergonomy and accessibility either by means of increasing the number of the purpose-built vessels or by incorporating
special landing stages for helicopters. At this point the increasingly important role of the preventive maintenance with systems
and sensors that monitor the wind turbine’s state of operation
(e.g. Condition Monitoring Systems) should be underlined. In this
regard, ‘‘ReliaWind’’ project’s (funded under the EU’s 7th Framework Programme/duration: 2008–2011) main goal was to
develop a new generation of more efficient and reliable offshore
(and onshore) wind turbines, providing practical results to be
used in wind turbine design, operation and maintenance
(Table 2). Particularly, the project aimed to achieve better
efficiency for wind turbines, through the deployment of new
systems with reduced maintenance requirements and increased
availability (ReliaWind, 2012).
As already mentioned, up to now, offshore installations have
been predominantly limited to fixed bottom support structures
such as monopiles, gravity-based and jackets installed in shallow
water depths but as the technology advances, collective efforts
have also been made in developing deep water concepts, in order
to take advantage of the higher and steadier wind speeds. To this
end, a lot of research effort has been put on developing a feasible
floating concept (Breton and Moe, 2009; Lackner and Rotea, 2011)
with a number of possible offshore wind turbine platform configuration permutations in terms of available anchors, moorings,
buoyancy tanks and ballast options, being under investigation by
the offshore industry (Musial et al., 2006; Sway, 2012; Wayman
et al., 2006). In this context, it is expected that developing a new
wind turbine concept dedicated exclusively for offshore use in
deep waters will have the potential for improving cost efficiency
significantly. This is exactly the objective of ‘‘DeepWind’’ project
(started in 2010 as a part of the EU’s 7th Framework Programme)
which investigates a floating offshore wind turbine concept
consisting of a long vertical tube that rotates in the water, a
vertical axis rotor at the top, a bottom based generator and a
seabed fixing system at the bottom (Table 2). However, though
being simple, the technology behind the concept presents extensive challenges that need explicit research (DeepWind, 2012).
6. Conclusions
In the present work, a short overview of the activity noted in
the field of offshore wind energy is provided, highlighting worldwide technology developments and trends, installed capacities
and market issues, while emphasis is given on the most important
factors (costs and availability issues) which have so far delayed
the establishment of offshore wind farms as the next generation
of wind power.
Indeed, offshore wind turbine technology has been evolving at
a fast pace and a considerable number of projects has already
taken place, especially over the last few years, although there is
clear evidence that additional efforts are required. The opportunities for exploitation of offshore wind resources are generally
found abundant; however, the barriers and challenges are also
147
significant. Among the main reasons which drive this growth are
that the opportunities for wind development on-land become
increasingly limited, the existence of more consistent and higher
winds in offshore sites, the absence of obstacles such as mountains, buildings and trees in marine environments, the low or
even null impact on humans, the pressure to achieve renewable
energy targets and finally the enabling of building offshore wind
farms in coastal areas close to many population centres.
On the other hand, the most important drawback of offshore
wind energy is the high costs associated with its development.
In fact, costs are still much higher from onshore counterparts but
some recent technological progress in terms of more efficient
production patterns (increase of the size of wind turbines,
improvements in the design of the projects, increase of availability, incorporation of innovative O&M strategies etc.) may have
the potential to narrow this gap in the future. With stronger
winds and fewer conflicting issues than on-land, multi-MW
turbines in deeper water depths are likely to dominate the
offshore sector in the long run in order to maximise energy
production, capturing also economies of scale. In this context, the
considerably higher cost for deploying wind farms at offshore
sites is expected to change as more and more projects come
online and targeted R&D efforts continue to take place.
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