Tong W. Wind Power Generation and Wind Turbine Design

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PART II

ESIGN OF MODERN WIND TURBINES

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CHAPTER 6

Design and development of megawatt

wind turbines

Lawrence D. Willey

General Electric Energy Wind, USA.

Electric power generation is the single most important factor in the prosperity of

modern man. Yet, increasing concerns over carbon emissions from burning f ossil

fuels have brought large-scale renewable energy technologies to the forefront of

technological development. Framework for the successful design and develop-

ment of large wind turbines (WTs) addresses the world’s power generation needs.

The motivation for this work, the broad framework for success, the best approach

for product design, various techniques and special considerations, and develop-

ment aspects are presented. Horizontal axis WTs operating inland or in near-shore

applications are speciﬁ cally addressed.

1 Introduction

Supplies of fossil fuels are inadequate to meet the growing need for more power

generation, which is driven by increasing demand for electricity [ 1 , 2 ]. The hori-

zontal axis wind turbine (HAWT) is one of the most economical forms of modern

power generation. HAWTs have numerous beneﬁ ts: they conserve dwindling fos-

sil fuel resources, reduce harmful emissions, and support a sustainable electric

energy infrastructure for posterity. Large wind turbines (WTs) are an engineering

challenge; they must endure some of the largest numbers of fatigue cycles for

structures while meeting size and cost constraints [49]. New WTs must maximize

reliability, availability, maintenance, and serviceability (RAMS) while having the

lowest cost of energy (CoE) and highest net present value (NPV) for a customer

and an original equipment manufacturer (OEM) [50]. One of the biggest motiva-

tions for accelerating technological development of WTs is in support for the U.S.

20% wind energy by 2030 initiative [ 3 ].

188 Wind Power Generation and Wind Turbine Design

1.1 All new turbine design

What design decisions will make a new multi-megawatt (MMW) WT successful in

today’s power generation market? How big should it be? What are the governing

parameters that drive its design and ensure that it operates without failure and for

the budgeted cost? These are just a few of the many questions at the beginning of

any new turbine design [54]. Providing the lowest CoE in the power generation

market is a challenge to the WT industry, but ever-improving designs are meeting

the challenge. MMW turbines often operate for over 20 years and are among some

of the largest manmade structures, especially from the perspective of controllable

moving parts. An all-new MW WT begins with conceptual design and follows a

rigorous technology building block and toll gate approach for component and sys-

tems development, and then undergoes a validation process involving prototypes

and pre-series projects prior to entering full production. Value analysis or value

engineering drives this process [4 − 6], where cost and ﬁ nancial return are continually

assessed to ensure success for the customer and the OEM.

1.1.1 Technology readiness levels (TRLs)

MW WT technology is advancing quickly, and concepts need to be character-

ized in a way that enables everyone to immediately understand the maturity of a

particular technology. The TRL system (used by NASA and U.S. Government

agencies) provides a common basis for assessing new concepts [ 7 ].

TRL-1: Basic principles observed and reported •

TRL-2: Technology concept and/or application formulated •

TRL-3: Analytical and experimental critical function and/or characteristic •

proof of concept

TRL-4: Component and/or breadboard validation in laboratory environment •

TRL-5: Component and/or breadboard validation in relevant environment •

TRL-6: System/sub-system model or prototype demonstration in a relevant •

environment (ground or space)

TRL-7: System prototype demonstration in a space environment •

TRL-8: Actual system completed and “ﬂ ight qualiﬁ ed” through test and •

demonstration (ground or space)

TRL-9: Actual system “ﬂ ight proven” through successful mission operations •

(space)

1.1.2 Technology proofs required (TRL-1, 2, 3, 4 and 5) − early phase

New turbine development cannot be undertaken effectively using unproven com-

ponent technologies. The best development strategy for new technologies involves

devoted internal research and development (IRD) programs. To mitigate risk, these

programs should be completed before embarking on a new product introduction.

New products released before proper validation and reﬁ nement may result in the

OEM having to mitigate problems later in the ﬁ eld, which can cost the OEM a factor

Design and Development of Megawatt Wind Turbines 189

of 10 or more relative to doing it correctly up front. A roadmap of technologies and

products relative to the marketplace and other manufacturers helps keep upfront

costs down and minimizes overall development cycles. Organizations need not

necessarily have their own IRD departments. There are examples of broad-based

collaborative research programs among industry, university and government par-

ticipants. Value analysis is critically important in understanding research priorities

and where to apply funding. Regular re-evaluation is required to ensure market

changes are reﬂ ected in re-direction or termination of programs.

1.1.3 Technologies in-hand (TRL-6, 7, 8 and 9) − late phase

Having a broad range of component technologies from wind or other sectors (that

are well understood and scalable) is an important aspect of new WT design. Even

with this, system integration readiness should not be underestimated. During the

product development cycle, early assumptions about the suitability of a component

technology may come under question, and new validation steps or even the invention

of new component technology may become necessary. Again, continuous and ever-

improving value analysis is the key activity for assessing technologies in-hand

and identifying new development opportunities. Carrying the value analysis cycles

throughout the development phases will help identify where additional technology

proofs are needed and will ultimately yield the very best product.

1.2 Incremental improvements to existing turbine designs

Once a product is introduced, later market entrants force competition and cause a

need for the original product to improve. Incremental product improvements can

be more tractable for some risk adverse organizations from the perspectives of

quick return on investment (ROI) funding allocation and minimizing investment.

There comes a point in every WT design when up-scaling component technol-

ogy, improving performance or further cost reduction are not possible, and a new

breakthrough design is required to begin the cycle anew [46]. Continual value

analysis provides the foresight needed to know which route to take and when, and

it helps build a comprehensive multi-generation technology plan (MGTP) that is

regularly reviewed and updated.

1.3 The state of technology and the industry

Thirty-ﬁ ve years into the modern WT era, less than half of this experience is in

today’s large utility scale machines. More than 27 GW of new wind capacity was

commissioned worldwide in 2008, a 36% increase over 2007. By the end of 2008,

global wind capacity grew nearly 29% reaching 121 − 123.5 GW with turbine and

wind power plant (WPP) investments worth about US$47.5B (€36.5B) [ 8 , 9 ]. The

state of WT design today is similar to the state of automobile design in the 1920s

and 1930s. Even the most advanced WTs are relatively immature when consid-

ering future advances for component integration, dry nanotechnology, quantum

wires and advanced digital controls [ 1 ]. There are many choices to be made today,

190 Wind Power Generation and Wind Turbine Design

and with tighter R&D budgets and a shortage of qualiﬁ ed wind engineers and

specialists in the industry, there is great opportunity for those who recognize this

view of the future today. The need for more electrical energy is driving installa-

tion of more wind energy. Rapidly increasing numbers of government, university

and industry collaborations are becoming involved with wind energy. All of these

groups are quickly recognizing the importance of value analysis to set R&D

priorities and guide innovation [55].

2 Motivation for developing megawatt-size WTs

Modern WTs have become very large structures that push the limits for structural

engineering and require the use of lightweight, low-cost materials ( Fig. 1 ) [52].

From ﬁ rst principles, the net power rating and size of a turbine grows with

swept area; i.e. rotor diameter raised to the second power. At the same time, the

amount of material (i.e. mass or cost) increases as the cube of the diameter. This is

known as the square-cube law [ 10 ]. Aside from improving energy capture by

accessing stronger winds at higher hub heights (HHs), the original motivation for

going larger in power rating and rotor size was to lower the CoE through econo-

mies of scale. Today’s most common turbine size is within the 2 − 3 MW rating, and

perhaps as high as 4 − 5 MW for some of the newest designs. Larger machines beyond

5 MW should become economically viable with further advances in material and

design technology, but a sudden change is unlikely over the next 5-year period.

To better understand where the large WT market is today in terms of size, an

“industry study set” of key design parameters for more than 150 utility scale tur-

bines is used to characterize trends and provide a basis for setting targets for new

WT designs [ 11 ]. This data is plotted in terms of rotor diameter and rated net

Figure 1: Megawatt-size WTs are large structures.

Design and Development of Megawatt Wind Turbines 191

power in Fig. 2 . Speciﬁ c rating is deﬁ ned as the rated net power of the turbine

divided by the rotor swept area ( P / A ) and is typically used to characterize the over-

all ability of a design to economically extract electrical energy from the wind. The

majority of today’s large WTs have a speciﬁ c rating in the range of 300 − 500 W/m

The general trend passes from speciﬁ c ratings of less than 400 W/m

for machines

below 4 − 5 MW sizes to greater than 400 W/m

for WTs above 4 − 5 MW sizes.

This result is due to plotting all the turbines within the study set without regard

for their particular International Electrotechnical Committee (IEC) type class

(TC). See IEC 61400-1 for an explanation of WT design TC [ 12 , 13 ]. In general,

turbines installed in the EU are typically designed to IEC TC3 and turbines for the

U.S. to TC2. Offshore turbines are typically designed to meet the requirements for

IEC TC1. Today’s largest machines are generally for offshore or IEC TC I applica-

tions. These TC1 data points pull the average trend line towards higher speciﬁ c

ratings with increasing rated net power.

To better illustrate this point, Fig. 3 shows the same data plotted by IEC TC,

together with the study group trend line. The TC data shows that today’s utility

scale turbines have speciﬁ c ratings independent of machine size; TC3 turbines

(7.5 m/s) have an average speciﬁ c rating of 327 W/m

, TC2 (8.5 m/s) of about

362 W/m

and TC1 (10 m/s) at 425 W/m

. The standard deviation for these results

is approximately 7%, 10% and 13% for TC3, TC2 and TC1, respectively.

The industry study set dimensions for 10 turbines along the mean characteris-

tic for the 150+ turbine data set are plotted in Fig. 4 . While it cannot be asserted

that turbines in the 5 − 10 MW range will end up having precisely these dimen-

sions, this does help to characterize a reasonable starting point. The tower height,

rotor diameter and rating are consistent with the natural progression of marketable

designs that have so far balanced higher electrical output per kg of material, cost

Figure 2 : Industry study set w.r.t. lines of constant speciﬁ c rating ( P / A ).

192 Wind Power Generation and Wind Turbine Design

of transport to the project site, installation and operations and maintenance (O&M)

costs. More importantly, this generalized set of turbines (herein after referred to as

the 10-turbine analysis group) provides a very useful datum for comparing value

analysis results with actual or derived mass and cost data.

2.1 Value analysis for wind

Based on extensive value analysis experience for a wide range of real project

economics, up-scaling today’s most popular turbine architectures and component

technologies beyond 4 − 5 MW results in unfavourable customer economics. The

very best value analysis procedures today are computerized in a format that allow

individual component owners to keep their models updated. These procedures

ensure that the modules maintain their functionality relative to each other and in

the overall roll-up to the WPP and electrical grid views.

2.1.1 The cost for value analysis

A rigorous value analysis program is an investment that an OEM must make to ensure

that every dollar spent on new WT technologies or products will result in satisﬁ ed

Figure 3 : Speciﬁ c rating ( P / A ), IEC TC and the industry study set.

Figure 4 : Turbine size geometry for the industry study set.

Design and Development of Megawatt Wind Turbines 193

customers and maximum shareholder value. Not provisioning for a rigorous ongoing

value analysis program is foolish. Experience shows that a value analysis program

with an annual budget of at least 1 − 1.5% of an organization’s overall engineering

budget will establish and maintain the system of continuous innovation and value

assessment needed for successful technology development and new product designs.

2.1.2 Concepts and deﬁ nitions

AEP – Annual energy production; kWh or MWh •

Turbine price – Cost of the WT equipment to the project developer •

BOP price – Cost of the balance of plant to the project developer; everything •

else needed to build a WPP beyond the WT equipment

Other capital costs – Financing terms and requirements •

Operational costs – O&M required to produce and deliver electricity •

CoE – The cost to produce 1 kWh (or 1 MWh) of electricity for a given time; •

e.g. for a given year

LCoE (levelized CoE) – Average CoE over the WPP life; e.g. 20 years •

Energy price – Power purchase agreement (PPA) – wholesale price paid to the •

WPP owner for the electricity produced

Taxes – Paid to local and federal government (in some instances may be an •

income to the WPP owner in the early years of the WPP life)

PTC – Production tax credit; any of a number of government policy incentives •

that provide income to the WPP owner. Typically a function of power production

and over a speciﬁ ed time schedule

OEM margin – The amount of proﬁ t required by the OEM to satisfy shareholders •

and provision for long-term sustainability of the business

Customer value –NPV for the customer •

OEM value – NPV for the OEM •

Total value (TV) – NPV for the customer and OEM combined •

A simpliﬁ ed illustration of the main contributors to value for both the customer

and OEM are shown in Fig. 5 . While hundreds of parameters are considered in a

Figure 5 : Main contributions determining CoE and TV.

194 Wind Power Generation and Wind Turbine Design

complete value analysis for a WT design and its use in an overall WPP, this simpli-

ﬁ ed diagram shows the main framework. As an example, lease payments to a land-

owner for the land the WTs are built upon may be accounted for in the operational

costs, and so on.

2.1.3 Value analysis outline

The most important aspect of new WT and WPP design is the calculation procedure

and the inclusion of all applicable business functions. This is a challenging

task that can only be realized if a considerable investment is made to establish

a digital environment to capture, maintain and continually update these inputs.

An example of a methodology that has been successful in new WT design is

shown below:

Input high level turbine parameters and design driving wind conditions 1.

Select the component technologies that make up the new WT design 2.

Select the WPP size and description governing installation including BOP and 3.

site construction characteristics

Select the territory that the WPP is to be analysed and check the default settings 4.

for the latest permitting requirements, ﬁ nancial terms and policy incentives.

Repeat steps (1) 5. − (4) if a range of design parameters are to be explored

Select any of a number of pre-deﬁ ned WTs from a library of existing products 6.

that you wish to compare the current results

Submit the overall inputs for analysis to compute a wide range of output that 7.

include value metrics such as CoE and TV relative to the pre-deﬁ ned turbines

The analysis method computes loads for the turbine, relates the loads to the

mass required for components to perform for their intended life, and then assigns

the cost required to procure the components and assemble the system. This is all

done in an environment that ensures that component engineers continually update

their physics-based transfer functions for loads and mass, sourcing and advance

manufacturing update their cost data, and ﬁ nance and marketing keep the project

funding and regional settings up to date.

2.1.4 Simpliﬁ ed equations for CoE and TV [ 14 ]

lev lev lev

CoE = (Ops Int prin Dwnpymt)/AEP++ +

(1 )

where Ops

lev

= f (Term, DR, VAT Recovery Interest, Property Tax, Landowner

Royalty, Utilities Cost, Insurance Cost, Interconnect Service Charge, Other

Royalty Performance Bond, Warranty, LTCSA, Machinery Breakdown Insur-

ance, Management Fee, Independent Engineer, Revenue Tax, O&M); Int

lev

f (Term, DR, Debt Type, Interest Rate, Takeout Cost, Debt %, Debt Term);

Prin

lev

= f (Term, DR, Interest, PTC Income, Takeout Cost, Debt Term); Dwnpymt =

f (Takeout Cost, Debt %, O&M Cost, O&M Reserve Months, # of Turbines,

Financing Fee %, Other Upfront Fees); AEP = Annual Energy Production, MWh

at the meter (billable)