REW WIND POWER 02 / 05 / 05
Thinking bigger
By Eize de Vries
Are there limits to turbine size?
Recent progress in wind technology –
especially the pace of upscaling – can
only be described as breathtaking. The
world’s first commercial 4.5 MW prototype
was created in 2002 and two 5 MW
prototypes in 2004. Now industry insiders
are talking about next-generation offshore
turbine giants of 7.5 to 12 MW with rotor
diameters of up to 200 metres. But how
realistic are these plans? Is bigger better,
and are there limits to wind turbine
upscaling? Eize de Vries reports.
Ten years ago, the largest wind turbine by output on
the market was the 800 kW Nordex N52, with other
manufacturers offering 600 kW machines. Last year,
the average capacity of wind turbines in Germany rose
to 1696 kW, which compares with an average of just
246 kW back in 19941.Most commercial turbines are
now fitted with rotors ranging in size from 61 to 90 metres,
compared with the 37–44 metres of the mid-nineties.
For many in the wind industry, continuing this
upscaling of wind turbine capacity and rotor size
seems therefore something to be taken almost for
granted. Yet will a continuous stream of clever
innovations fuelled by scientific and technological
progress remain a key driving force that could push
upscaling towards perhaps 12 MW – and beyond?
GE's 1.5 MW turbine is one of the first genuine
utility-type megawatt class turbines to be sold
in large numbers – over 2500 units had been
sold by the end of 2004 - GE ENERGY
And will the current highly successful evolutionary
development strategy prove the best recipe for the
future, or should we expect a radically different design
approach?
Could innovation push wind turbine upscaling
towards 12 MW and beyond?
All the successful megawatt-class wind technology
developments to date – including the latest multimegawatt turbines of 3–5 MW – can be described as
fruits of rather conventional evolutionary design efforts.
In other words, they have been mainly variable-speed,
pitch-control machines with upwind-type, three-blade
rotors, either geardriven, as most are, or direct-drive,
such as Enercon, Jeumont, ScanWind, Vensys, Zephyros
When REW examined the prospects for growth in
wind turbine size back in 1999, the industry had
doubts about whether megawatt turbines would
take off in North America, while the cost benefits
of megawatt turbines remained unclear.
units. Multibrid turbines – a third design which occupies the middle ground between direct drive and
‘conventional’ multi-stage, gear-driven systems – are considered promising yet are still new
developments with a limited track record.
SHIFTING PERSPECTIVES
Ever since the emergence of the first-generation of modern commercial wind turbines in the mid-1970s
experts have repeatedly, and for various reasons, predicted definite limits to turbine size. During the
mid-1980s analysts were wondering whether turbines as large as 250 kW (diameter 23–30 metres)
could be built economically. Less than 10 years later similar doubts were raised about the emerging
500–600 kW (diameter 34–44 metres) class, not to mention ‘daring’ plans for commercial 1.5 MW
turbines!
RADICAL DESIGN: 10 MW ICORASS
Early this year a research group led by Dutch rotor blade maker Polymarin Composites completed a prefeasibility study for an innovative 10 MW offshore wind turbine project called ICORASS, which stands for
Integral Composite Offshore Rotor Active Speed Stall control. The other two Dutch partners involved are
energy research centre ECN and the TU Delft DUWIND wind institute. ICORASS aims to develop a new
‘robust’ down-wind offshore turbine with an output of about 10 MW range, with virtually no need for
maintenance. Polymarin Director and ICORASS spokesperson Bart Roorda explains that realizing this
ambitious aim requires the number of key components and joints within each component to be minimized, and
needs a higher degree of systems integration than common today: ‘Current offshore designs mostly originate
from land turbines, with a focus on control and monitoring. Our aim is optimal robustness in order to achieve a
much higher level of reliability and availability.’
The three partners envisage manufacturing the 160-metre diameter
two-blade rotor with integrated rotor in one piece, from advanced
composite material, without any seams or other additional joints. Says
Roorda: ‘We have had experience with “one-shot” type rotor blade
manufacture since the 1980s and intend to use this technology in
combination with advanced thin-wall composite materials. Even the
manufacture of a 200-metre rotor is, from a technical point of view, no
problem for Polymarin. The rotor main bearing and direct drive
generator will to a large degree be integrated into the rotor hub.’
Other key features are a preference for power control at wind farm
level and the revival of stall power control (fixed blade angle), which is
Artist’s impression
known for simplicity and robustness. However, this time it is to be
of ICAROSS 10 MW - ICAROSS
applied in a combination with ‘active rotor speed’ technology. The latter
involves using the permanent magnet ring generator in an added capacity as an electronic brake. This will keep
power output constant from nominal power-up to the wind speed at which ‘cut-out’ occurs. Jeumont Industrie of
France applied a similar system in a small number of 750 kW J48 direct drive turbines until production ceased
recently. German engineering consultancy aerodyn Energiestysteme initially also considered rotor speed
control during the 5 MW Multibrid’s concept design stage. However, the designers later made a switch to active
pitch control for the M5000 prototype.
Four ICORASS patent applications include the integrated hollow rotor-hub composite construction, an elastic
rotor-hub interface, active speed power control and the integral generator/hub construction.
The next step, according to Roorda, is the formation of a consortium with sufficient financial leverage to shift
into phase two. This will start with pre-design and has to produce a prototype by 2007. The envisaged longterm goal is a 10% global offshore market share, or erecting 6 GW of the 60 GW planned by 2020.
Considering the possibility of a 1.5 MW ‘super class’, those first-hour critics argued that the huge
dimensions would limit the number of suitable potential locations. Many of these arguments focused
not so much on technological challenges but on aesthetics, landscape integration and transport
logistics.
A former Dutch importer of Danish wind turbines stressed the point in 1997 with a computer
visualization of a 1.5 MW turbine positioned in a rural landscape. He had earlier erected two smaller
600 kW turbines at this location. His conclusion, which turned out to be wrong, was that the total Dutch
market for these new landscape-dominating 1.5 MW class machines would not exceed five to 10 units.
What this example demonstrates, above all, is a shifting perspective of size over time. Today these
very machines – initially perceived as giants – have already started to make way for even bigger
turbines. Those arguments also fuelled a fierce discussion about the need to develop four different
1.5 MW prototypes in Europe. However, it did not stop German firms Enercon and the former Tacke
(now GE Energy), and Denmark’s former Nordtank (later NEG Micon, now Vestas) and Vestas.
Interestingly, each of the four 1.5 MW prototypes they developed featured a different choice of drive
train and operational system, and rotor sizes between 60 and 66 metres, as Table 1 shows.
By the end of 1996 all these prototypes were operational, a wind industry achievement that saw
surprisingly few serious design flaws and few operational teething problems. That is remarkable in
itself, considering the numerous uncertainties that were faced and the huge leap in upscaling that was
involved. After all, these turbines came from the 250–300 kW designs that were state-of-the-art
technology back in 1992–3.
TABLE 1. Main specifications of four 1.5 MW pioneer prototypes
Make and model
Rotor diameter
(metres)
Drive system
Operation
Enercon E-66
66
Direct-drive
Pitch-controlled
variable speed
Tacke TW 1.5
65
Gear-driven
Pitch-controlled
variable speeda
Nordtank NTK
1500/60
60
Gear-driven
Fixed-speed ‘classic-stall’
limitationb
Vestas V63
63
Gear-driven
Pitch-controlled
‘OptiSlip’c
Sources: wind industry, 1996–2005
a
Doubly fed induction generator
b
Fixed blade angle setting
c
Semi-variable speed (nominal speed plus 10% temporary speed variation)
GROWTH AND INNOVATION
Since Enercon successfully launched the 500 kW E-40 turbine series in 1992, directdrive has
developed into a mature wind technology and has remained the almost exclusive domain of the
German market leader. This is in spite of several attempts by competitors to enter the market with
novel concepts for units of output 600 kW to 3 MW. Enercon now boasts a direct-drive track record of
over 7400 turbines ranging between 30 kW and 4.5 MW. By contrast, the closest competitor, the
former Lagerwey of the Netherlands, erected only about 175 of its 750 kW direct-drive model before it
went bankrupt in 2003. The 2 MW Zephyros Z72, again developed under the Lagerwey umbrella,
leaves a track record of one prototype only in 2002. Late in 2004, Zephyros – then short of capital –
tried to sell two non-exclusive licences to GE Energy and Harakosan of Japan. However, GE pulled
out for undisclosed reasons, and Zephyros became insolvent shortly afterwards. Early in 2005
Harakosan became the new owner of the renamed Harakosan-Zephyros BV.
The largest operational direct-drive wind turbine prototype, apart from the 4.5 MW Enercon E-112, is
the Norwegian 3 MW Scanwind DL 3000 (2003), which is fitted with a permanent-magnet Siemens
generator.
The doubly fed induction generator first introduced by the former Tacke in the TW 1.5 took the wind
industry by storm, largely because during the mid-nineties power electronics was very costly. With a
doubly fed generator only 25–30% of the power has to be fed through a frequency converter. The
largest doubly fed generator today is the gear-driven REpower 5M.
Some turbine suppliers recently made a switch to gear-driven systems
which have either synchronous generators, such as GE Energy’s 2.X
series, or asynchronous squirrelcage generators, such as Bonus
Energy’s 2.3 MW and 3.6 MW VS.Both types of generator operate with a
so-called full converter (100% power passage).
The Nordtank NTK 1500/60 matured into the 2 MW NEG Micon NM
72/2000, fitted with a new active-stall type rotor in 2000–2001 and aimed
at both demanding onshore and offshore locations.However, the product
had to make room for a larger 2.75 MW pitch-controlled variable-speed
series machine. The once dominating combination of fixed-speed and
‘classic’ stall limitation is now retreating rapidly from the market and has
even disappeared completely from models rated at 1.5 MW and above.
After Bonus stopped doing so in December, Siemens Wind Power is the
only company still successfully marketing a fixed-speed active-stall
turbine, its CombiStall 2.3 MW machine, which has pitchable blades.
Active stall itself is characterized by weather-independent power-output
control and superior grid behaviour than classic stall. Neither technology
violates GE’s variable speed US patent, which is an advantage for
marketing.
A 1996 advertisement for the
1.5 MW Nordtank NTK
1500/60, an elegant and
beautifully crafted design by
Jakob Jensen of Denmark.
The heavy turbine was
apparently expensive to
manufacture and did not go
into production.
Vestas upscaled its V63 prototype into a 1.65 MW V66 series turbine,
which has a diameter of 66 metres. This lightweight design in turn served
as a model for the pitchcontrolled variable speed 2 MW V80 successor,
of which 1200–1500 units are in operation, according to company sources. Vestas switched to variable
speed operation in 2000 but continues to sell mainly 1.8 MW OptiSlip type V80 turbines in North
America, as the latter technology does not violate GE’s variable-speed US patent.Other
manufacturers,such as Gamesa and Fuhrländer, offer a comparable technology called Rotor Current
Control, developed and made by Weier of Germany.
The 1.5 MW Tacke (now GE) Windtechnik TW 1.5 and Enercon E-66 concepts evolved into what can
be called the first-generation, utility-type turbines. In other words, costeffective mature turbines that
can still be transported easily which are typically made in large series for a range of wind markets
and climates. By March GE had installed about 3000 of its 1.5 MW units worldwide, and Enercon over
2400 E-66 series turbines, which are rated at 1.5–2 MW.
Several new entrants swiftly joined the initially small 1.5 MW manufacturing base. Turbines rated at
2 MW-plus are now in turn rapidly replacing them. Rotor sizes of these larger machines range between
70 and 94 metres, while 70–72 metres was state-of-the-art blade technology as recently as 2000–1.
The combination of variable speed and active rotor blade pitch control has become the trend,
especially for wind systems from 1.5 MW and up.
MULTI-MEGAWATT
The success of this rapid progress in wind technology may have been largely evolutionary but several
recent key innovations have emerged, such as the lightweight Vestas V90-3MW and the Multibrid
family.Also, Enercon of Germany has come up with a groundbreaking blade design with a Cp, or rotor
power efficiency, of 0.56. Cp is a measure of the fraction of the total power available which the blades
are able to convert. Its theoretical maximum of 0.593 is known as the Lanchester-Betz limit. Enercon
claims its innovation can boost aerodynamic efficiency by about 12% towards the theoretical
limit.Enercon’s spokesperson for the Benelux countries Bernhard Fink says that innovations like this
type make feasible a cut in the cost per kilowatt hour of 5–10% every two years. ‘As an example, our
new 2 MW E-70 produces annually 10–12% more energy compared to the previous 2 MW E-66
model, while manufacturing costs have only risen between 5%–7%,’ he says.
According to a Vestas company spokesperson, this year will see the
manufacture of 500–600 V90 3 MW units, including 60 turbines for
two UK offshore plants. This puts the world market leader ahead of
the competition in the prestigious multimegawatt class capacity-wise,
both onshore and offshore. Also new is a 100 metre rotor for the V90
platform. It has increased the model’s rotor swept area by 23%, a
clear sign that blade development is keeping pace with capacity
upscaling and sometimes advances even faster.
In May 2002 GE Energy’s forerunner Enron Wind erected a 3.6 MW
GE 3.6 Offshore prototype of diameter 104 metres in Spain. In 2003
a 3.2 MW GE 3.2 land prototype of diameter 104 metres followed in
France, along with seven more 3.6 MW turbines for the Arklow Bank
offshore project in the Irish Sea.
Enercon built the world’s first commercial 4.5 MW prototype in
August 2002 near Magdeburg, Germany: the E-112, which has a
diameter of 112.8 metres. An additional four of these direct-drive
machines with an enlarged rotor (diameter 114 metres) were
operational at the end of last year, one in the water near Emden,
Germany. Another three-five units are envisaged for this year.
With its modified blade design,
Enercon’s 2 MW E-70 produces
10–12% more energy annually
than the 2 MW E-66 model
ENERCON
Former NEG Micon of Denmark, now integrated into Vestas, followed in October 2003 with a 4.2 MW
prototype of diameter 110 metres. An upscaled 4.5 MW prototype fitted with new rotor blades is
planned for late this year, the renamed V120 – 4.5 MW of diameter 120 metres. Its top head mass
(THM) is a remarkably low 210 tonnes.
In 2004 two small German companies each erected a 5 MW prototype, the REpower 5M, of diameter
126 metres, and the Multibrid M5000, of diameter 116 metres. Both plan additional land prototypes for
2005. REpower announced2 that it will erect the first two 5M turbines offshore during 2006. The
machines will operate in the 44-metre deep waters off Scotland’s east coast for oil company Talisman.
Late last year, Bonus Energy (now Siemens) erected a 3.6 MW prototype of diameter 107 metres,
intended for upscaling to 5 MW. A total of 17 prototypes with capacities of 3.6 MW or more were
operational at the end of 2004, delivered by six manufacturers.
TABLE 2. Upscaling – examples of existing design concepts
Make and type
Capacity
(MW)
Rotor diameter
(metres)
Bonus 1 MW
1.0
54
Base
Bonus 1.3 MW
1.3
62
Upscaling
Enercon E-66
1.5
66
Base
Enercon E-66
1.8
70
First upscaling
Enercon E-66
2.0
70
Second upscaling
Enercon E-70 E4
2.0
71
Third upscalinga
REpower MD 70/77
1.5
70/77
Base
REpower MM 70/82
2.0
70/82
Upscaling
a
New rotor blade design
Source: Wind industry, 1996–2005
Model
MULTI-ROTOR SYSTEMS – THE FINAL UPSCALING?
Multi-rotor turbines have for nearly a century challenged the imagination of engineers and inventors. The
technology offers the prospect of achieving much higher capacities per foundation compared with conventional
single-rotor types by making use of multiple state-of-the-art series-produced devices. For example, a system
with five 4 MW units makes a 20 MW multi-rotor system. However, a well-known problem of multi-rotor systems
is the more complex system dynamics. This is due to several factors, including the fact that the combined THM
of individual units is not concentrated in one spot as is the case with the nacelle of a single-rotor turbine.
One of the most famous multi-rotor pioneers was German engineer and manufacturer Hermann Honnef (1877–
1961), who started work in 1931 on beautifully detailed large-capacity systems. One of his most famous designs
comprises a 250-metre high lattice type tower fitted with three double-rotors arranged with a 120-metre
diameter front rotor and two 160-metres diameter rotor at the rear. A striking design feature was that the
complete rotor assembly could pivot into a safe horizontal position during stormy weather. Also unique at the
time was the application of ring generators. Honnef never succeeded in building a complete multi-rotor turbine
due to the outbreak of the Second World War, but he manufactured parts of the 150-metre high tower.3
During the mid-1980s a joint venture of Dutch companies Multiwind
and the former Lagerwey erected the 300 kW Quadro, comprising four
75 kW two-blade Lagerwey 15/75 turbines. After some teething
problems the installation performed well for about 15 years at the
Maasvlakte industrial area near the port of Rotterdam. More recently
German wind pioneer Professor Friedrich Klinger, also known for the
Genesys 600 and Vensys 62 projects, developed a 2.4 MW multi-rotor
system comprising four 600 kW turbines. Lattice wind turbine tower
specialist SeeBa of Germany developed a 4.5 MW system with three
1.5 MW turbines, and Multiwind of the Netherlands proposes a
patented 6 MW system comprising three 2 MW units. The current Artist’s impression of Octopus 12.5 MW
status of all these initiatives is unknown but none of the three systems
OCTOPUS WIND TECHNOLOGY
has yet been erected.
Piet Eikelenboom is director of Rotterdam-based Octopus Wind Technology, which plans to develop a 12.5 MW
five-rotor system comprised of 2.5 MW units. He is starting with a smaller system that will be gradually upscaled
in time. Eikelenboom comments: ‘Our new multi-rotor system in its ultimate form will consist of a lattice-type
tower up to about 150–200 metres in heght with a freely-hinging horizontal beam for rotor attachment. The outer
four turbine units with coning blades hinge under load, and the fifth is positioned right above the tower.’ For the
jawing action Octopus Wind has developed a novel system for which a patent is pending. It abolishes the need
for a costly jawing assembly, which typically consists of a slewing ring with internal or external gearing and
multiple jaw motor drives. Eikelenboom sees big opportunities for his system, despite the less positive
experiences of multi-rotor pioneer competitors in the past. He gives his reasons as rising oil prices and
increasing demand for oil-derived products: ‘Our approach will lead to far fewer problems as we require from
manufacturing [industry] mainly standard components. Small cheap components lead in turn to less complex
transport logistics.’ These factors combined will make the design in his view suitable for application in remote
onshore locations and offshore sites. He is unable to disclose at the moment when a first prototype is likely to
be operational.
UPSCALING STRATEGIES
Depending on manufacturer preferences and risk perception strategies the testing phase of the current
largest prototypes, 4–5 MW, is likely to last two to five years, probably involving several pre-series
turbines. Initial series manufacture is often followed by an upscaling of capacity or rotor size or both
within a few years of production, as Table 2 shows, the idea being to utilize unused ‘built-in’ reserves
as a means of reducing costs of energy in €/kWh per 20 years, as shows.
Typically, when power rating goes up, rotor diameter increases too. Maintaining an optimum ratio
between rated power and rotor swept area is essential, but the optimal value depends to a large extent
on average wind speed at hub height. Also, large turbines usually sit on high towers. One implication
of increasing rotor diameters is increased aerodynamic noise. Rotor speed typically has to come down
to curb these emissions. Table 3 shows the relationship between rated capacity, rotor speed and rotor
torque when a hypothetical wind turbine is scaled up from 500 kW to 1.5 MW to 4.5 MW. Its rated
power would increase by a factor 9, its rotor
speed drops to about 25% of the initial value,
and rotor torque increases by a factor 36!
According to the square cube law (SQL), a
component with dimensions 1x1x1 when
geometrically enlarged by a factor of 2 sees
its volume and mass increase by a factor of
eight. This means that when upscaling a
turbine, the system’s specific mass has to
increase more rapidly. So larger ‘input’
torque loads for upscaled gear-driven wind
turbines mean higher loads in practice and
the need for larger diameter shafts, stronger
LEFT Repower’s 5M is the biggest turbine in the world JAN
gears and bearings, and a more heavy
OELKER, REPOWER SYSTEMS AG
gearbox housing, rotor hub and machine bed
RIGHT Vestas’ V90 has a new rotor measuring 100 metres,
frame. However, some recent Danish wind
increasing rotor swept area by 23%, a clear sign that rotor
turbines, especially the lightweight 3 MW
blade development is keeping pace with the upscaling of
Vestas V90 and the proposed 4.5 MW Vestas
capacity - VESTAS
V120, seem to contradict this requirement.
For example, the THM of the V90 is almost identical to that of the 2 MW V80.Behind this technological
achievement are, according to Vestas, a radical redesign of the nacelle, integration of the hub
bedplate directly into the gearbox, elimination of the traditional main shaft, the use of new lightweight
materials for the rotor blades, and a number of additional load-reducing measures.
The 5 MW Multibrid M5000 is an example of a design in which an integrated drive train could
substantially reduce mass.
Meanwhile, REpower has adopted the policy of increasing the mass of the main components – such
as the gearbox and main bearings – of all its turbine models by 20% to increase its systems’ lifetime
reliability.
TABLE 3. Some implications of wind turbine upscaling
Rated capacity (kW)
500
1500
4500
1
3
9
Rotor speed (rpm)
40
20
10
Relative rotor speed
1
0.5
0.25
Rotor torque (Nm)
-
-
-
Relative rotor torque
1
6
36
Relative rated capacity
a
b
a
Rounded rotor speed figures in the example indicate the range but do not necessarily reflect actual
product figures.
Actual rotor torque figures not relevant for the comparison. Source: Example from Aloys Wobben
(Enercon), 1999–2000.
b
In upscaled direct-drive turbines, the reduced rotor speed requires the use of a larger-diameter ring
generator to maintain an acceptable generator pole speed, which in turn becomes increasingly heavy.
With a 12-metre diameter ring generator and a THM of about 500 tonnes, the E-112 is by far the
heaviest of the four prototype turbines rated at above 4 MW.
COOLING DOWN
Turbine upscaling also makes component manufacturing and supply more difficult, a challenge that is
a recurring issue for each generation of large turbines. Initially there are only a few qualified
component suppliers available, but this number typically expands when experience and series volume,
which is the same as market potential, both go up. For example, the controlled cooling down period for
large, complex and heavy castings for 4.5–5 MW machines, such as the frame and rotor hub, requires
up to six weeks. This means in practice that only six or seven castings can be made from one mould
each year. Engaging in large-scale series production therefore requires huge investment in additional
moulds. Also crucial, say industry sources, are far-reaching partnerships with casting firms for sharing
risk and developing skills. On the investment side, precision machining capacity has to expanding
substantially.
Producing complex 25–40 tonne castings for multimegawatt
turbines in sufficient quantities requires capable manufacturers that
can do the job at an acceptable price, deliver uniform quality and
turn out the required numbers. So far, all manufacturers have found
that the number of European foundries capable of meeting these
requirements is small.
According to the experts at aerodyn who developed the 5 MW
Multibrid M5000, it is not just a matter of sheer size. The main
issues in their view are the taxing demands placed on the quality of
texture, for example, and the presence of casting defects as size
increases. For other components, too, Europe’s manufacturing
industry has run up against its limits. In the case of the M5000,
components that proved critical – apart from the huge castings –
were large-diameter internal geared rings for the gearbox, and
bearing rings. Of significance here are the maximum width to which
a ring can be machined and the maximum hardening that can be
achieved. These factors determine the maximum diameter of a
component.
The Multibrid M5000 – clever
technology with a high degree of
systems integration
JÖRG WEUSTHOFF/MULTIBRID
On the other hand, LM Glasfiber marketing manager Steen Broust Nielsen says that his firm has the
know-how and capability to produce rotor blades of length 70 metres, to give a diameter of
150 metres. If required, he says, it could even create 80 metre length blades to give a diameter of
170 m): ‘In our analysis, LM has not come across any longterm limits to rotor blade size,’ says Nielsen.
‘We foresee a continuing trend towards larger offshore turbines for two key reasons. First, the future
German offshore market has been estimated at 30 GW, and the majority of these proposed wind farms
are projected in deep water relatively far from the coast. Second, in German deep-water offshore
projects the cost of infrastructure [foundations, cables] represents 40–60% of total project costs. Both
factors favour the application of large turbines, or in other words to maximize the installed capacity per
foundation.’
Broust Nielsen also predicts an increased application of adaptive technologies in wind turbines and
rotor blades, such as condition-monitoring systems aimed at reducing loads. These, in turn, allow
manufacturers to build lighter rotor blades.
LEAD
Finally, while there might still be considerable bottlenecks to the next major upscaling 7.5–12 MW,
putting faith in the rise of a huge offshore market is less a matter of debate. All four current large
prototypes – those rated at 4 MW plus – have been developed in a high-risk environment without, as
yet, a viable market from which to recover the huge investments involved. A key question is therefore
which parties will take the lead in a renewed offshore multi-megawatt race, and who will be prepared
to pay for the huge initial investments. It is likely, say experts, that an upscaling of the current largest
prototypes will be a more logical and much less risky first step. This could push capacities well into the
6 MW range with rotor sizes of between 130 and 150 metres. Rumoured major upscaling of Enercon’s
E-112 into the 6 MW class, to feature larger rotor blades with the new profiles, would for instance be
the extension of a well tried evolutionary product-optimizing strategy.
From the point of view of wind technology preferences, conventional gear-driven systems still dictate
the choice of primary manufacturer. Direct-drive, by contrast, has proven to be a successful wind
technology, but apparently with considerable constraints for new entrants. And does Multibrid
technology, despite its promise, really have the potential to become a proven cost-effective and
reliable alternative to existing technologies? Is there a chance of success for new offshore concepts,
such as the 10 MW down-wind ICORASS or the even larger multi-rotor concepts as envisaged by
Dutch inventor Piet Eikelenboom? Or will it be up to huge global players such as GE Energy or
Siemens Wind Power or the two together to provide the necessary leverage that will lead the wind
industry in to the next phase of offshore wind technology development?
A final, largely unanswered, question is whether the continuous push for cheaper kilowatt-hours from
offshore wind plants can be maintained as upscaling continues. In other words, should the industry
start developing substantially bigger wind turbines now before the first generation of 4–5 MW giants
have established a track record and actual costs are still to be determined?
Eize de Vries is wind technology correspondent for Renewable Energy World.
e-mail: [email protected]
NOTES
1. Erneuerbare Energien (issue 2/2005)
2. Erneuerbare Energien (issue 3/2005)
3. Wilhelm van Heys, J. (1947) Wind und Windkraftanlagen. Georg Siemens Verlagsbuchhandlung,
Berlin, Bielefeld