Tong W. Wind Power Generation and Wind Turbine Design
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Design and Development of Megawatt Wind Turbines 195
lev lev lev lev lev lev
= An(Rev Ops Int prin Tax + PTC Dwnpymt) + OEM−−−− −TV
(2 )
where An = NPV factor = ((1 + DR)Term - 1)/(DR*(1 + DR)Term); Rev
lev
=
f (Term, DR, Initial PPA Rate, PPA Escalation, Merchant Index Point, Initial
Merchant Rate, Merchant Escalation, Initial Green Tariff Rate, Green Tariff Esca-
lation, Green Tariff Term, Annual Energy Production, VAT %, Takeout Cost, VAT
Method, Capital Subsidy Type, Capital Subsidy Cap, Capital Subsidy %); Tax
lev
=
f (Term, DR, Rev, Ops, Int, Depreciation Type, Depreciation Straight Line Term,
Depreciation Declining Balance %, Carry Forward Loss Duration, Duration Cor-
porate Tax Start, Duration Corporate Tax Duration, AMT Rate, Tax Rate); PTC
lev
=
f (Term, DR, Initial PTC Rate, PTC Infl ation Factor, PTC Escalation, PTC Infl ation
Reference, PTC Rate Adjustment, Annual Energy Production); OEM = f (Turbine
Cost, Margin %, # of Turbines) [48].
2.2 The systems view
Common parameters to study:
WPP size range – 50 MW, 100 MW, 200 MW or more •
WT net power rating in relation to WPP size and markets •
Land-constrained versus MW-constrained markets •
– Land-constrained – Fixed amount of land, not limited by MW that can be
accepted by the local grid
– MW-constrained – “Unlimited” land, fi xed MW connection
IEC TC – Turbine designed to meet specifi c fuel (wind) attributes •
A systems view is required to ensure that the fi nal outcome is as expected.
Focusing on individual components and managing interfaces is not enough. A
MW WT system is complex, and it takes viewing it from different perspectives in
an iterative way in order to arrive at an answer that is truly a systems solution. The
successful turbine designer will need to circle back many times, zooming in and
out from system level to component level and back again.
2.3 Renewables, competitors and traditional fossil-based energy production
A WT designer must understand the competition and continually focus on TV and
CoE. Assumptions change quickly, and re-assessment must be provisioned in the
schedule and crosschecked often. In addition to modern wind power, what are the
alternatives for producing electricity? It is important to keep the excitement for
investing in new WT designs alive and builds commitment from all team members
across an organization. This is not easy, as there is always fear of the unknown, and
for many organizations there is a deep entrenchment in the old way of doing things.
As with any new product development, the revenue potential will be the stron-
gest motivation for an OEM. Companies will invest in technology that will bring
the best returns to their stockholders. Large WTs have become very competitive
196 Wind Power Generation and Wind Turbine Design
with traditional forms of electricity production. Figure 6 is an example showing
the current fi nancial merits for a range of mainstream electric power generation
technologies [ 15 ].
There will not be pressure to produce new and better WT designs if the economics
are unfavourable. When fossil fuel prices are low, most customers will not be moti-
vated to purchase renewables unless there are incentives, usually resulting from gov-
ernment policy. Whether it is a WT or a traditional fossil fuel-based electricity option,
using value analysis is the key to guiding an organization’s decision to proceed.
2.4 Critical to quality (CTQ) attributes
The fi rst step of a new turbine design is to identify the critical few characteristics
that make all the others seem trivial. These provide a common set of goals and
clarify the critical attributes that will help focus the design across a wide range of
operational environments.
Entry into the 21st century coincided with the maturity of large-scale WT man-
ufacturing capability. Prior to this, WT designers and manufacturers were rela-
tively small start-ups by entrepreneurs and environmental enthusiasts. For the
most part, these start-ups did not fully understand the importance of value analysis
and risk mitigation. This has all changed with the entry of industry participants
striving for new WT designs that have the lowest capital expense, the lowest O&M
costs, and the highest availability.
3 The product design process
Truly great new products follow a disciplined design process. This is especially
true for large rotating equipment such as a modern MW-size WT. A large collec-
tion of people and skills must be brought together in an environment where the
people can exchange information and build on each other's ideas. The primary
Figure 6: Generating alternatives − North America 20 Yr LCoE, $c/kWh.
Design and Development of Megawatt Wind Turbines 197
elements of a successful new WT design include strong support from stakeholders,
fi nancial backing to see the project through, and an adaptable framework for the
teams to work together and get things done. This ensures world-class results that
are fully optimized for the different system level and component views.
3.1 Establishing the need
In order to begin a design for a new turbine, studies must be performed to fi nd the
optimum concepts and characteristics for a proposed market. The process starts
with the defi nition of customer needs, and to this end the following are defi ned:
Market target segment; e.g. regions, customer types, WT design TC •
Market settings; e.g. prevailing fi nance terms, government policies, incentive •
and tax schemes, local supplier content requirements
Company internal goals; e.g. amount of build-to-print versus spec, unit volume •
scenario’s, sourcing strategies including supplier base, strategic partners
A list of technologies ready (or will be in time) for product implementation •
Target internal design metrics for concept selection and optimization •
Range of turbine ratings, rotor diameters and HHs to scan for optimum •
confi gurations
Target metrics relative to other products including the competition. •
At a minimum, the last item should include the CoE and the specifi c TV (STV) of
WPPs that assume different numbers of turbines. The CoE (c$/kWh) can be deter-
mined for each year of the turbine’s life or expressed as levelized CoE (LCoE), and
the specifi c TV ($/kW) is the customer NPV plus OEM margin normalized by plant
rating. Market and customer needs outline an optimum solution for a turbine design.
3.2 The business case
What does a company want to be known and remembered for? Are better returns
for stakeholder investments available? Are the risks worth the time and effort?
These and many more questions form the foundation of business growth and vital-
ity. There are a great number of opinions throughout an organization, and usually a
large number of factors are continually changing, making consensus complicated.
A new WT design may be judged too costly or an organization may answer these
questions with alternative profi t strategies, but increasingly the economics of MW
WTs and WPPs provide strong profi tability. More important than today’s profi t
expectations are actions that promote sustainability. Achieving a balance among
all of these factors should be the ultimate goal of the WT designer. Tax benefi ts/
government incentives?
3.3 Tollgates
Tollgates are the orderly process for developing new products and technolo-
gies. They are applicable for component development as well as the higher-level
198 Wind Power Generation and Wind Turbine Design
system. TG’s include the all-important cross-functional and business level interaction
that ensures a new product development effort remains focused and relevant. Effec-
tive tollgate reviews cut across department and business unit boundaries and integrate
all skills and requirements throughout all phases of the WT development program.
The following example tollgate structure can be used for new product and
component technology programs:
TG1: Investigation and justifi cation of a business opportunity •
TG2: Product options identifi ed and cross-functional buy-in secured •
TG3: Conceptual design •
TG4: Preliminary design •
TG5: Detailed design •
TG6: Factory test or validation + pre-series production •
TG7: Validation or redesign + product introduction •
TG8: Customer feedback, fi eld experience and resolution •
Some notes on TGs:
TGs 1 1. − 3 require an overall technical platform leader for a new turbine product
design that’s not a part of the customer-facing engineering department; e.g.
advanced technology or conceptual design department separate from product
engineering. The hand-off to product engineering is upon completion of TG3.
This does not mean that product and component engineers are left out of 2.
the early TGs; on the contrary, their buy-in and contributions as part of the
consensus-based decision process is the key, but they do not lead or overwhelm
the early processes of value analysis and developing the business case for a
potential new turbine design.
Be fl exible when applying any TG process for specifi c turbine design programs. 3.
Do not be afraid to combine some TGs or opt for a best effort design through
to component and even full prototypes with a strategy to learn and provision
for a cycle of optimization prior to committing drawings to pre-series or serial
production.
Limit the stakeholders or fi nal decision makers to one key leader represent-4.
ing each of the major departments; e.g. marketing, product line management,
engineering, sourcing, manufacturing and services.
A strong Chief Engineering Offi ce is the conscience of the technical com-5.
munity and becomes increasingly important as the TGs progress through the
design phases [53].
Do not be afraid to partner with suppliers early under non-disclosure agreements 6.
(NDA) to leverage the best and the brightest early in the process.
The spirit and intent of the foregoing elements are in complete alignment with 7.
concurrent engineering (a.k.a. simultaneous engineering), an approach in which
all phases of the product development cycle operate at the same time. Product
and process are coordinated to achieve optimal matching of cost, quality and
delivery requirements. There are a large number of resources readily available
on this topic such as Curran et al. [ 16 ] and CERA [ 17 ].
Design and Development of Megawatt Wind Turbines 199
While some OEMs have demonstrated faster WT product development cycles
than others, it generally takes 4 − 5 years to progress from TG1 through TG7 for
new designs. Concurrent engineering techniques will compress time in the product
development and business cycles in general. Every OEM has basic cycles that
govern the way that documentation is processed, decisions are made and parts are
manufactured. Another major factor that affects this timing is the level of invest-
ment a company makes in basic research and funding stability from year to year.
At one extreme is a “Manhattan” type project where experts and resources are
assembled for a specifi c purpose, while the other extreme is a corporate research
laboratory that may not have much coordination in terms of a specifi c vision or
product development program. What is recommended is a blend of both worlds
and the ability for the OEM to adjust the approach as needed without having to
recreate either realm from scratch.
3.4 Structuring the team
The best chance for success is an organization that includes all points of view
from the beginning. It is important to choose a new platform leader who is well
seasoned and has the respect of the organization. This is the single most impor-
tant position on the conceptual design engineering team, particularly for executing
TGs 1 − 3. Although the platform leader works closely across the entire organiza-
tion, there is particular focus in the early TGs between the product line leadership
and the conceptual design engineering team. A general outline for a new turbine
organization is:
Marketing and product line management – commercial departments 1.
Conceptual design engineering – leads for TGs 1 2. − 3
Platform leader as member of conceptual design engineering (TGs 1 3. − 3)
Advance manufacturing, sourcing, service engineering 4.
Systems engineering and component engineering – system engineering leads 5.
for TGs 4 − 8
Platform leader as member of systems engineering (TGs 4 6. − 8)
Validation department, project management 7.
Fleet engineering 8.
Services, aftermarket opportunities 9.
3.5 Product requirements and product specifi cation
The product requirements document (PRD) specifi es the commercial and
product requirements that are to be achieved by the new product development
program. The development of the PRD starts during the business opportunity
investigation phase (TG1), continues during the identify product options phase
(TG2), and gets refi ned and frozen during the concept design phase (TG3).
Different from the product specifi cation document (PSD) at TG3, the PRD
200 Wind Power Generation and Wind Turbine Design
specifi es what the product should do, not how technical details are chosen to
answer the requirements.
The PSD is created during the conceptual design (TG3) to collect all of the
detailed technical specifi cations for components and their aggregation into an
overall system that satisfi es the PRD. The PSD establishes the roadmap for com-
pleting the design, ensures everything that can possibly be considered is included,
and builds the framework for measuring progress towards completion. It is
important to note that once the PSD is established, changes are accepted only if
the entire team (including Conceptual design engineering) are brought back
together for discussion and approval. The document includes outline models,
component technology selections and value assessments. The PSD is one of the
key hand-offs from conceptual design engineering as the systems engineering
team embarks on the preliminary design phase of the new product development
program (TG4).
3.6 Launching the product
Obtain and document buy-in from all the stakeholders. Initial commitment for
funding and resources are needed for the best possible start. Long-term commit-
ment for a sustained level of funding is the bedrock of the best technical orga-
nizations. Regardless, if specifi c development programs are carried through or
replaced with emergent business opportunities, the best talent that is fully engaged
in an environment of creativity cannot be maintained if too much energy is focused
on continually developing funding opportunities.
3.6.1 Re-assessing on a regular basis
Most if not all of the conditions and assumptions that had initially supported a
decision to proceed on a new MW WT design will change over time. The impor-
tance for having some level of ongoing technical competitive surveillance cannot
be emphasized enough. Plan regular stand-downs across the functional organiza-
tions to absorb, vet, and act on emergent information that changes any of the previ-
ous conditions and assumptions. This is the key to know when to change course, or
even terminate a particular MW WT program.
3.7 Design defi nition: conceptual → preliminary → detailed
Conceptual design (TG1 − 3) permits the rapid exchange of ideas and proposals for
how to physically achieve the goals laid out for a business proposition. Concep-
tual design is physics-based, realistic and timely. Successful conceptual design is
dependent on the creation and maintenance of a computer-based environment for
MW WT and WPP value analysis.
Preliminary design (TG4) builds on the foundation provided by the concep-
tual design. Conceptual design transitions to preliminary design when all of
the stakeholders agree that the current and estimated future market conditions
Design and Development of Megawatt Wind Turbines 201
and assumptions, together with the best conceptual design, continue to meet
the program CTQs and the business goals. Preliminary design’s primary thrust
is to test all of the conceptual assertions with real world suppliers and advance
manufacturing teams.
Detailed design (TG5) builds on the foundation provided by the preliminary
design. Preliminary design transitions to detailed design when all of the stake-
holders agree that the latest and estimated future market conditions and
assumptions, together with the best preliminary design, continue to meet the
program CTQs and the business goals. Detailed design carries through every
aspect of the new wind plant design including analyses, proofs and modelling.
Concurrently, sourcing, suppliers, and manufacturing are key players in fi nal-
izing component drawings. Detailed design is not complete until the fi rst qual-
ifi cations for all the parts and components are substantially fi nished and the
fi rst prototype turbine is successfully running. This approach permits learned
improvements to be put into up-revised drawings and specifi cations prior to
fi nal release for serial production.
3.7.1 Design parameters for value and CTQ attributes
Market specifi cs that are provided by the OEM’s Marketing Department include
opportunity size, permitting limits and timing. The conceptual design team:
Develops turbine design parameters fl owing down from CTQs and vets them 1.
across the team.
Evaluates WPP alternatives for a range of turbine sizes and ratings. 2.
Facilitates iterative improvement cycles to refi ne the turbine specifi cations. 3.
Evaluates a range of design boundaries for best NPV and LCOE. 4.
Documents all results, including internal design records for advance studies or 5.
a PSD for a new product, as applicable.
Results are also compared with competitors to see where machines may be
improved. Develop your own strategy of future improvements that could be intro-
duced for your design at a later time. Setting design targets is part science and part
market savvy. Regardless of the initial values set at the beginning of a design cam-
paign, be ready to modify specifi c ones with an eye on maintaining the overall
optimum design – this includes re-assessing the market profi tability profi le. The
following sections provide a starting point for new MW WT design targets based
on the industry study set trends.
3.7.2 Technology curves
The industry study set introduced earlier in this chapter includes all of the various
technologies of today such as single-bearing, double-bearing, geared, direct drive
(DD), and so on [51]. Larger turbines in the 3 − 6 MW ranges generally have archi-
tecture that departs from the traditional architecture of early 1 − 3 MW turbines.
Architecture changes with size results in a natural adjustment to the industry trend
202 Wind Power Generation and Wind Turbine Design
when projecting forward to the 7 − 10 MW ranges. This projection is probably
on the conservative side of what can be expected, assuming similar technology
progress over the last decade projected to larger machines of the next decade. It
is also reasonable and highly desirable to beat this trending with an acceleration
of breakthrough technology. This scenario is becoming more likely due to the
massive increase in public and venture capital funding that is beginning to be
injected into the wind industry, not to mention the recent exponential response by
academia to shift research to the renewables sector.
Noise emission is one of the fi rst considerations when setting out to design a
new turbine. Blade tip speed is considered one of the primary drivers of noise
with sensitivity proportional to the tip speed raised to the 5th power [ 10 ]. Fig-
ure 7 shows the range of blade tip speeds for today’s industry study set as a func-
tion of turbine power rating. The curve fi t shown is not very good, in part because
of the wide range of blade lengths for machines with the same generator rating,
but the correlation is better than other types of fi ts. Its character is also consistent
with other related parameters such as the main shaft rotational speed. Tip speeds
of 70 − 85 m/s are being used, with the higher end of the range tending to be for
larger machines. The trend for this data suggests that tip speeds much beyond
90 m/s are unlikely for machines in the 7 − 10 MW sizes. Other noise reducing
considerations would need to be developed if advantages for even higher tip
speeds proved desirable.
As shown in Fig. 8 , there is a much better correlation for values of rated rotor
speed with turbine size. Today’s machines have rated rotor speeds in the low 20s
to the high teens, but speeds are not projected to fall below about 10 RPM for
7 − 10 MW sizes. This is also consistent with the practical limit of torque that
increases with lower speeds.
Figure 7 : Blade tip speed for the industry study set.
Design and Development of Megawatt Wind Turbines 203
Figure 8 : Rated rotor speed for the industry study set.
To best illustrate the range of MW WT design parameters and how they infl u-
ence technology choices for turbines and WPPs using value analysis methodology,
a 10-turbine analysis group was prepared by aggregating the trends (e.g. P / A , rotor
diameter, rated rotor speed, etc.) from the industry study set. Table 1 contains the
summary listing for this value analysis group. Many of the results shown in the
remaining sections of this chapter are based on this uniform group of basic
MW WT design parameters.
3.7.3 Infl uence coeffi cients and functional relationships
The design wind conditions and market settings are fi rst entered into the value
analysis tool and sweeps of turbine design cases yield trends for the output per-
formance and fi nancial merit parameters. These trends, and their corresponding
sensitivities to variation, are next used to assess the relative merits for compo-
nent technology decisions. This process is repeated for combinations of compo-
nent technologies to arrive at conceptual design proposals that are further vetted
by all the various groups within the organization. Once a number of these studies
have been performed, a pattern of infl uence coeffi cients (ICs) and functional
relationships result. These provide a convenient guideline for the WT design TC
and market conditions of prime interest, and can be quickly applied to assess
emergent component technologies without the need to go through the rigorous
value analysis calculation. However, use these parameters with caution, always
keeping in mind the assumptions that were used to generate them. If the main
assumptions are different enough, revert back to full sweeps using the value
analysis tool. Over time, this process of developing “rule of thumb” ICs will
emerge again and again, with better appreciation across the organization as more
people gain experience using them.
204 Wind Power Generation and Wind Turbine Design
Table 1: 10-Turbine analysis group inputs for the value analysis program.
P/A = net rating*
1e6/((rotor
dia^2)*0.7854) Given
Rotor dia =
59.354*(net
rating^0.47)
RPM = 22.781*
(net
rating^–0.3595)
Tip Spd =
rotor dia
*PI()* RPM/60
HH = rotor
dia /(0.225*
(rotor
dia^0.3464))
Twr base dia =
HH/18.60 **
P/A (W/m^2)
Net
rating (MW)
Rotor dia
(m) RPM
Tip Spd
(m/s)
HH
(m)
Twr Base OD
(m)
361.4 1.00 59.35 22.78 70.80 64.11 3.45
376.8 2.00 82.21 17.76 76.43 79.33 4.26
386.0 3.00 99.47 15.35 79.94 89.85 4.83
392.8 4.00 113.87 13.84 82.52 98.15 5.28
398.1 5.00 126.46 12.77 84.58 105.11 5.65
402.4 6.00 137.78 11.96 86.30 111.17 5.98
406.2 7.00 148.13 11.32 87.78 116.56 6.27
409.4 8.00 157.73 10.79 89.09 121.44 6.53
412.3 9.00 166.70 10.34 90.25 125.92 6.77
415.0 10.00 175.17 9.96 91.31 130.06 6.99
**18.60 is taken as the standard tower height aspect ratio = GE –1.5sle 80m HH/4.3m base dia.