Tong W. Wind Power Generation and Wind Turbine Design
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Design and Development of Megawatt Wind Turbines 245
Figure 37: Example of a circular raft type foundation under construction [40].
Figure 38: Tubular tower foundation after pour and backfi ll – tower ready [41].
246 Wind Power Generation and Wind Turbine Design
foundation mass is about 2.48 times the total mass for the WT equipment above
the tower base fl ange; i.e. the OEM’s typical “scope of supply.”
The term “monolithic” foundation (e.g. gravity foundation) refers to a single
base for a WT. As shown in Fig. 40 , this single base can be of a number of forms
Figure 39: Foundation mass – 10-turbine analysis group.
Figure 40: Example foundation sizes relative to tower heights.
Design and Development of Megawatt Wind Turbines 247
that include a monopile or raft confi guration. Figure 40 relates the relative size of
the required monolithic foundation dimensions relative to their respective towers
for some representative turbines from the 10-turbine analysis group. Turbines in
excess of 5 − 7 MW require excessive monopile depth or raft outer diameter, thus
further supporting the thesis for multiple smaller foundations and multi-legged
towers for larger turbines.
4.7.3 Offshore foundation
A signifi cant amount of research is underway for offshore foundations, and many
designers are attempting to leverage experience with offshore foundations in the
oil and gas industry. Currently, two types of offshore foundations have been used
with success in the wind industry – gravity and monopile. The gravity foundation
was more common in the past, but it is more expensive and has a much larger
footprint. More common today is the monopile foundation, which has a very small
footprint and can be used for somewhat deeper water installations (up to about
30 m water depth).
As turbines become larger and near-shore locations become less available, the
industry may need to look towards installations in deeper water. Many different
types of foundations are being explored for these applications, such as different
types of trusses, multiple legs and even fl oating foundations stabilized by ballasts
and cable anchored to the seabed or lake bottom.
5 S pecial considerations in MW WT design
5.1 Continuously circling back to value engineering
The proportion of how total project cost is divided amongst the turbine, BOP,
developer, and transportation costs vary as turbine rating is increased. Table 4
shows the breakdown in total project cost for a 100 MW WPP with fl at terrain using
a nominal technology MMW WT. Multiple trends in the proportions of project cost
can be observed as the turbine rating is varied.
Table 4: Major cost breakdown for an onshore WPP.
Wind Power Plant “all-in” cost fraction (total = 100%)
Rated Net Power Turbine BOP Developer Transportation
[MW] [%] [%] [%] [%]
2.0 63.6% 25.0% 7.3% 4.1%
4.0 66.3% 22.5% 6.8% 4.4%
6.0 67.8% 20.9% 6.3% 5.0%
8.0 68.7% 20.4% 5.9% 4.9%
10.0 68.6% 20.5% 5.8% 5.2%
100MWWPP; Flat terrain w/easy access & good geotechnical; Nom technology MMWWT price;
10% BOP cont. inclusive. Number of loads determined by weight (no dim considerations) Larger
components assumed capable of multiple loads.
248 Wind Power Generation and Wind Turbine Design
A trade-off between the turbine and BOP cost is apparent as rating is
increased. A site using a larger number of relatively inexpensive ($/kW) small
turbines will have a lower turbine and higher BOP project cost proportion than a
site using fewer expensive large turbines. Many BOP costs, such as roads and
cabling, are more dependent on the total land area that the plant occupies than the
rating of the turbines used. The larger turbine therefore has a BOP cost advan-
tage on a MW-constrained site due to the reduced number of turbines required
for a specifi ed block of power. These factors reduce the specifi c cost ($/kW) of
BOP as turbine ratings increase. Turbine cost behaviour further drives this
proportionality trend as the specifi c cost of a turbine increases with rating ( Fig. 41 ).
Figure 41 : Additional detail – onshore WPP cost breakdown.
Design and Development of Megawatt Wind Turbines 249
See Wiser et al. [ 42 ] for additional insight into U.S. wind power installation,
cost, and performance trends.
The developer expenses proportion of total project costs drops as turbine rating is
increased. Many developer costs, such as legal fees and title insurance, are not depen-
dent on the machine selected. The total cost of these expenses are mainly dependent
on the plant rating and not turbine rating. For a baseline or constant WPP rating, the
total installed cost of a plant increases with turbine rating, so a reduced proportion of
project costs for the developer are observed as turbine rating is increased.
The proportion of transportation cost increases slightly with rating, assuming
technologies and designs for larger machines account for reasonable shipping
limitations. The same savings found with BOP costs (because of reduced numbers
of turbines for a fi xed WPP size) are not realized in transportation. This is because
transportation cost increases rapidly with rating due to the limited availability of
136 tonnes (150 ton) capacity 18-axle trailers in the United States. The 82-tonne
(90 ton) capacity 13-axle trailers needed to ship the components of turbines with
ratings less than approximately 2 − 3 MW are more readily available. Shipping a
component 1000 miles on an 18-axle trailer is approximately fi ve times the cost of
using a 13-axle trailer. Additional costs are further observed for larger turbines
because their components must be partially disassembled to allow transportation
even on the large capacity 18-axle trailers [57].
5.2 Intellectual property (IP)
One of the most important activities for any technical organization is the ability to
create and document IP as a natural part of their everyday engineering activities.
For the wind industry today, obtaining patent protection for innovation must be a
top priority. The industry is still young, and there is plenty of open IP landscape.
Once new designs and methods are devised, these investments must be protected
as they can provide additional income from licensing and sales.
Larger OEMs start the process of evaluating invention potential using an invention
disclosure letter (IDL) process that contains the following elements:
A brief explanation of the invention. 1.
Description of how the invention works (being very specifi c and including fi gures 2.
and images).
Description of the problem that is solved by the invention. 3.
Descriptions of any prior attempts at solving the problem and how others 4.
may have tried to address the problem before.
Relating the technical and commercial advantages for the invention. 5.
Explanation of how someone could design around the invention. 6.
5.3 Permitting and perceptions
Know your markets – local ordinances typically specify max turbine rotor dia-
meters, overall heights, and max noise emissions. These need to be accounted for
new designs.
250 Wind Power Generation and Wind Turbine Design
5.4 Codes and standards
Compliance with industry and equipment design regulations is one of the most over-
looked, yet crucial details to ensuring widespread acceptance and trouble-free shipping
and installation of new WTs. This is particularly true for personnel health and safety
requirements that need to be considered from conceptual through detailed design
[ 43 , 44 ]. Det Norske Veritas (DNV) and Risø National Laboratory, Copenhagen, pro-
vide a good starting point in listing several relevant codes and standards for large WT
design [ 45 ]. Since this is a rapidly developing and ever changing arena, it will likely
pay for itself several times over to engage the services of a conformity assessment
service prior to formally launching a new WT development program.
5.5 Third party certifi cation
Unless adopting a strategy of self-certifi cation, involving a third party conformity
assessment vendor to provide certifi cation guidance during new turbine prelimi-
nary and detailed design phases is critical to avoiding a large number of costly
issues later. Some banks and insurance companies may require WT certifi cation to
specifi c standards (e.g. GL [ 23 ]) as a condition for customers securing their ser-
vices. No matter the fi nal path taken, third parties do provide their services under
non-disclosure and offer a unique perspective for assessing design choices and
leveraging a wide range of turbine confi guration experience.
5.6 Markets, fi nance structures and policy
One should not underestimate the infl uence of fi nancing options and government
policy on MW turbine design. Some of the most potent infl uence parameters in
value analysis of new WT systems are the target market settings. A machine and
overall WPP that does well for some markets can be clearly disadvantaged in oth-
ers. Strong guidance from the OEM’s Marketing and Product Line Management
(PLM) teams are crucial for engineering to account for the range of scenarios and
sensitivities in the value analyses. An important corollary for this activity is that
feedback for effective policy structures can be developed during the normal course
of evaluations, and used by the OEM to proactively infl uence adoption of the best
policies across the various market segments.
6 MW WT development techniques
Regardless of whether a turbine is large or small, the path to a successful new machine
starts with conceptual design and value analysis. The value analysis includes a thor-
ough understanding of where the market and competitor machines are positioned,
and where any of the players can go in the future based on the technologies known
or believed will become available. The path will not likely be a sweeping big step –
rather, it is almost always the result of a number of iterations from an initial design.
The trick is to develop the incremental technologies in the most economical way, and
with as little exposure (i.e. lowest cost) to the customer as possible.
Design and Development of Megawatt Wind Turbines 251
6.1 Validation background
Testing is a mandatory activity for any product development program. Validation
is more than just collecting data for a new machine – it is about thorough col-
lection and analytical reduction of specifi c data to make measurable changes to
optimize a design.
6.1.1 Technology roadmaps
Maintaining technology roadmaps that identify pathways and timing are a key part
of ensuring continuous improvement. Roadmaps should be maintained across the
various systems as well as the individual components. This supports continuous
improvement by showing how far the technology has come and providing a vision
for the future of what technology needs to become.
6.1.2 Jugular experiments
Early on, new ideas are best demonstrated in their most basic setup – why spend
more than is needed or mask understanding in too complex of a test apparatus?
A “jugular experiment” is the fi rst demonstration of the feasibility of a new techno-
logy that provides a proof of concept under the simplest conditions. While further
testing may be necessary to determine whether an idea will be further developed
or integrated into a new product, jugular experiments move the decision process
along in the most cost-effective manner possible.
6.1.3 Technology demonstrators
When major component design or material changes are proposed – the best way
to mitigate risk are limited trials of these components as incremental changes to
existing machines. Demonstrate the technology and understand the design space
before committing to serial production.
6.1.4 Prototypes
Product demonstration is the fi nal step ensuring that the full system effects are
accounted in the turbine design. A number of turbine prototype sites should be
chosen to gather operating data for a range of environmental conditions. A small
group of turbines (i.e. pre-series or limited production) undertaken after a period
of successful prototype testing helps establish reliability and availability statistics,
as well as power plant interaction effects.
6.2 Product validation techniques
Various techniques are used to validate products, and products must be validated
at every level. Techniques that can be used include:
Analytical experiments •
Jugular tests •
Sub-scale models •
Full-scale prototypes •
252 Wind Power Generation and Wind Turbine Design
Products should be validated at the system level and at the component level. The
systems to components and back to systems approach should be used, as it
supports continuous cycles of improvement.
6.2.1 System level validation
Systems level validation is the key activity that closes the loop between design and
fi eld performance and facilitates the process of continuous improvement for WT
equipment. Validation also enables realistic noise and power curve specifi cation for
customer documents and project pro formas. In addition, turbine reliability improve-
ments and optimized structural designs result from measuring load conditions and
responses identifi ed from validation activity.
6.2.2 Component validation
Unexpected interaction of components is revealed during tests. Certifi cation
testing under environmental extremes permits observations not easily possible
in the fi eld. Components are the building blocks for the larger system – get the
components right, together with their interactions, and the system optimization
will follow.
6.2.3 Rotor blade static and fatigue testing
Rotor blade static and fatigue testing are used to validate design assumptions and
simulations used to predict the ultimate strength and 20-year life for the blade.
There are a number of wind blade test facilities around the world that can be used
to perform this type of testing, and plans for others have been announced to sup-
port the next generation of longer blades (i.e. >50 m in length). Of all the parts in
today’s modern turbines, the blades are perhaps the most fi ckle of all, requiring
not only structural and aerodynamic design execution, but also the most critical
of manufacturing and process control to ensure material and structural quality.
Provisioning the cost and time for proper validation for blades is crucial before
new blades are introduced into serial production.
7 Closure
Modern WTs are large complex structures that have achieved mainstream accep-
tance with rapid market growth and product standards development. The inex-
haustible wind is a great fuel for electricity production, even with the challenges
of turbulence, gusting, directional change and storm extremes. These impart the
highest fatigue loadings to any manmade machine, and require turbine designers
to carefully account for all effects to blades, hub, shaft, drivetrain, electricity gen-
eration system, support structure and the power plant system considerations that
include interaction with the grid.
Value engineering or value analysis is the cornerstone process for identifying
which innovations should be pursued and help the MW WT designer to focus on
what matters most to their customers. It guides the OEM and ensures successful
new products. The fundamental physics for the economic extraction of kinetic
Design and Development of Megawatt Wind Turbines 253
energy from wind results in rated net power densities on the order of 400 W/m
2
requires confi guring rotor systems such that worse case blade tip defl ections safely
stay clear of support structures, and requires rotor thrust-induced overturning
moments be accounted for every operational possibility and local geotechnical
condition. Today’s mainstream 1 − 3 MW WTs will give way to still larger turbines
with the introduction of more and more advanced materials and technologies.
Machines approaching 10 MW are within the realm of possibility. Successfully
exploiting offshore wind resources in part depends on these larger machines
becoming a reality, demands increased reliability, and the ability to install and
maintain these machines at a price comparable to onshore.
Deriving power from the inexhaustible wind – it is truly a great time for the
engineers that are taking up this challenge and for everyone striving to build a
sustainable future for our heirs.
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