Innovation in Wind Turbine Design

To Adele and Rose

Foreword

Those of us who have been active in the wind energy industry for the past few decades have been lucky. We have been involved in an industry that is technically fascinating, commercially exciting and thoroughly worthwhile. We have seen turbines increase in diameter from 10 to 120 m and in power from 10 to 10 000 kW – what a fantastic journey!

The size of the turbines is the most obvious characteristic because it can be so clearly seen – wind turbines are now by far the biggest rotating machines in the world. Less visible is the ingenuity of the designs. Looking back a couple of decades, there were many ‘whacky’ ideas that were seriously contemplated and even offered commercially and some of those whacky ideas have become conventional. Superficially, the latest generation of turbines may all look the same, but underneath the nacelle and inside the blades there are many fascinating differences. For a long time, the mantra of the wind turbine industry has been ‘bigger and bigger’, but now it has moved to ‘better and better’ and this change marks a change in the areas of innovation.

Peter Jamieson is one of the clearest thinkers in the industry and I am delighted and honoured to have worked with him for almost 20 years. He is a real blue sky thinker unimpeded by convention and driven by a strong sense of rigour. Innovation in wind turbine design is what Peter has been doing for the past 30 years and it is about time he wrote a book about it. I fully supported Peter's idea that he should put his professional thoughts on record and now he has done so.

Anyone interested in the technical aspects of both the past developments and the exciting future of wind turbines should read this book carefully and be inspired. This is no arid technical text or history – this is real intellectual capital and, of course, innovation.

Andrew Garrad

Preface

This book is about innovation in wind turbine design – more specifically about the evaluation of innovation – assessing whether a new concept or system will lead to improved design-enhancing performance or reducing cost. In the course of a working life in wind energy that began in 1980, the author's work has increasingly been, at the request of commercial clients and sometimes public authorities, to evaluate innovative systems providing reports which may or may not encourage further investment or development. In some cases, the clients are private inventors with a cherished idea. Other cases include small companies strategically developing innovative technologies, major industrial companies looking for an entry to the wind turbine market or major established wind turbine companies looking to their next-generation technology.

There is substantial conservatism in the wind industry as in most others and largely for the best possible reasons. Products need to be thoroughly proved and sound, whereas change is generally risky and expensive even when there is significant promise of future benefit. To some extent, change has been enforced by the demands year by year for larger wind turbines and components. There has been convergence in the preferred mainstream design routes but, as new players and new nations enter the wind business, there is also a proliferation of wind technology ideas and demand for new designs. The expansion of wind energy worldwide has such impetus that this book could be filled with nothing but a catalogue of different innovative designs and components.

It may initially seem strange that as much of the book is devoted to technology background as to discussion of specific innovative concepts. However, innovation is not a matter of generating whacky concepts as an entertainment for bored engineers. The core justification for innovation is that it improves technology, solving problems rather than creating them. To achieve that, it is crucial that the underlying requirements of the technology are well understood and that innovation is directed in areas where it will produce most reward. Hence is the emphasis on general technology background. Within that background some long-established theory is revisited (actuator disc and blade element momentum) but with some new equations developed.

Among much else, this second edition contains predictable updates regarding new larger turbines and new systems plus expanded sections on the ever-growing offshore applications and the developing interest in airborne wind power. There is also new content relating to presentation of basic theory, a fuller evaluation of many issues concerning ducted rotors, various new top-level analyses of the low-induction rotor concept, flow relativity (relating to driving rotors through still air as a means of performance measurement) and kite performance, for example.

Innovative ideas by definition break the mould. They often require new analytical tools or new developments of existing ones and, in general, fresh thinking. They do not lend themselves to a systematised, routine approach in evaluation. Evaluating innovation is an active process like design itself, always in evolution with no final methodology. On the other hand, there are basic principles and some degree of structure can be introduced to the evaluation process.

In tackling these issues, a gap was apparent – between broad concepts and detailed design. This is territory where brainstorming and then parametric analyses are needed, when pure judgement is too limited but when heavyweight calculation is time consuming, expensive and cannot be focused on with any certainty in the right direction. This why ‘detailed design’ is not much addressed. It is the subject of another book. This one concerns building bridges and developing tools to evaluate innovative concepts to the point where investment in detailed design can be justified. Innovation in wind energy expresses the idealism of the designer to further a sustainable technology that is kind to the planet.

Peter Jamieson

Acknowledgement

My professional life in wind technology began in 1980 in the employment of James Howden and Company of Glasgow and I very much appreciate many colleagues who shared these early days of discovery. Howden regrettably withdrew from turbine manufacture in 1988, but by then my addiction to wind was beyond remedy.

In those days I much admired a growing wind energy consultancy, Garrad Hassan and Partners. I was delighted to join them in 1991 and, as it happened, founded their Scottish office. I felt that it would be great to have a working environment among such talented people and that I would have a continuing challenge to be worthy of them.

In particular, I would very much like to thank Andrew Garrad and Dave Quarton for encouragement, practical support and great tolerance over 4 years in the preparation of the first edition. At the end of 2013 I retired from Garrad Hassan, by then part of DNV GL. Commencing in 2009, I was employed part time in the Centre for Doctoral Training in Wind Energy in the University of Strathclyde and enjoy working with great teams of staff and students who, now numbering over 40, are studying wind and marine topics at PhD level. I am much indebted to Bill Leithead, director of the centre, especially for many valuable brainstorming sessions on wind technology over the years.

I have to say special thanks to the late Woody Stoddard, who was an inspiring friend and enormously supportive, especially considering the few times we met.

Considering the very many times I have imposed on his good nature, I have equally to thank Mike Graham for his freely given help in so many projects and as an excellent, unofficial aerodynamics tutor. Much thanks also to Henrik Stiesdal, who, as an extremely busy man at the technical helm of a large wind turbine manufacturing company, found time to contribute a chapter to this book.

My warm thanks also go to very many other work colleagues and associates who, knowingly or otherwise, have made valuable contributions to this book. Among them are:

Albert Su, Alena Bach, Alexander Ovchinnikov, Andrew Latham, Anne Telfer, Ben Hendriks, Bob Thresher, Carlos Simao Ferreira, Chai Toren, Charles Gamble, Chris Hornzee-Jones, Chris Kirby, Christine Sams, David Banks, David Milborrow, David Sharpe, Ed Spooner, Emil Moroz, Ervin Bossanyi, Fabio Spinato, Fatma Murray, Iain Dinwoodie, Jan Rens, Geir Moe, Georg Böhmeke, Gerard van Bussel, Herman Snel, Irina Dyukova, Jamie Taylor, Jega Jegatheeson, Jim Platts, John Armstrong, Kamila Nieradzinska, Kerri Hart, Leong Teh, Lindert Blonk, Lois Connell, Lutz Witthohn, Magnus Kristbergsson, Marcia Heronemus, Mark Hancock, Martin Hansen, Masaaki Shibata, Mauro Villanueva-Monzón, Mike Anderson, Mike Smith, Nathalie Rousseau, Nick Jenkins, Nils Gislason, Patrick Rainey, Paul Gardner, Paul Gipe, Paul Newton, Paul Veers, Peter Dalhoff, Peter Musgrove, Peter Stuart, Rob Rawlinson-Smith, Roger Haines, Roland Schmehl, Roland Stoer, Ross Walker, Ross Wilson, Ruud van Rooij, Sandy Butterfield, Seamus Garvey, Stephen Salter, Steve Gilkes, Stuart Calverley, Tim Camp, Takis Chaviaropoulos, Theo Holtom, Tomas Blodau, Trevor Nash, Uli Goeltenbott, Unsal Hassan, Uwe Paulsen, Varan Sureshan, Vidar Holmöy, Win Rampen, Wouter Haans, Yuji Ohya.

Peter Jamieson

Introduction

0.1 Why Innovation?

Fuel crises, concerns about global environmental threats and the urgent needs for energy in expanding new economies of the former third world have all contributed to an ever-increasing growth of renewable energy technologies. Presently, wind energy is the most mature and cost-effective of these.

While other more diverse applications are discussed, this book keeps the main focus on wind energy converters that produce electricity. This is primarily because the greater part of the author's experience is with such systems. However, in a more objective defence of that stance, it may be observed that by far the largest impact of wind technology on the world's energy supply presently comes from systems generating into electrical networks.

Innovation is about new ideas, and some quite unusual designs are evaluated in this book. Why give attention to such designs which may not be in the mainstream? Exploring alternative concepts not only deepens understanding of why the mainstream options are preferred but also suggests where they should be challenged by alternatives that have significant promise. In any case, ideas are grist to the mill of technological progress and those which fail in one embodiment may well later be adapted and successfully reincarnated.

As is discussed shortly, the generation of power from the wind presents unique challenges. Unlike cars and houses, for example, energy is a commodity which has utilitarian value only. No one prefers a particular petrol because it has a nicer colour. The wealthy may indulge in gold or gold-plated bathroom taps, but no one can purchase gold-plated electricity. Energy must meet generally stringent specifications of quality in order to be useful (voltage and frequency levels particularly in the case of electricity). Once it does, the main requirement is that it is dependably available and as cheap as possible.

The end purpose of innovation in wind turbine design is to improve the technology. Usually, this means reducing the cost of energy and this is the general basis of evaluating innovation in this book. However, even this simply stated goal is not always the final criterion. In some instances, for example, the objective is to maximise energy return from an available area of land. Sometimes capital cost has a predominant influence. The bottom line is that any technology must be tailored accurately to an engineering design specification that may include environmental, market, cost and performance issues.

The detailed design of a wind turbine system is not a minor or inexpensive task. By the time an innovative design is the subject of a detailed design study, although it may yet be some way from market, it has already received significant investment and has passed preliminary tests as to the potential worth of the new concepts.

Thus, there is an intermediate stage between first exposure of a concept up to the stage of securing investment in a prototype when the concepts are examined and various levels of design are undertaken. Usually, a search for fatal flaws or obvious major shortcomings is the first stage. The design may be feasible but will have much more engineering content than its competitors and it is therefore unlikely to be cost-effective. More typically, there is no clear initial basis for rejecting the new concepts and a second level of appraisal is required. A systematic method is needed to review qualitatively, and where possible quantitatively, how the design compares to existing technology and for what reason(s) it may have merit. At this stage, detailed, expensive and time-consuming analyses are precluded, but there is a great need for parametric evaluations and simplified analyses that can shed light on the potential of the new concepts.

This book is very much about these preliminary evaluation stages, how simple insightful methods can provide guidance at a point where the value of the innovation is too uncertain to justify immediate substantial investment or detailed design.

0.2 The Challenge of Wind

According to Murray [1], the earliest written reference to windmills is of the fifth century BC. Windmills (although probably only then existing as children's toys) are listed, among other things, as something a devout Buddhist would have nothing to do with! The aerodynamic rotor concept is evidently ancient.

To generate electricity (by no means the only use for a wind turbine but certainly a major one under present consideration), requires the connection of such a rotor to an electric generator. Electric motor/generator technology began in Faraday's discoveries in the mid-nineteenth century. About 70 years ago and preceding the modern wind industry, the average household in the United States contained about 40 electric motors. The electric motor/generator is therefore not ancient but has been in mass production for a long time in recent history. What then is difficult about the marriage of rotor and generator into successful and economic power generating systems? The challenge of modern wind technology lies in two areas, the specification of an electricity-generating wind turbine and the variability of the wind.

0.3 The Specification of a Modern Wind Turbine

Traditional ‘Dutch’ windmills (Figure 0.1) have proliferated to the extent of 100 000 over Europe in their heyday. Some have survived 400–600 years, the oldest still operating in the United Kingdom being the post mill at Outwood, Surrey built in 1628. A short account of the history of early traditional wind technology in Eggleston and Stoddard [2] shows that they exhibit considerable practical engineering skill and empirical aerodynamic knowledge in their design and interesting innovations such as variable solidity blades (spilling the air through slats that can open or close) that have not surfaced in modern wind turbine design. However, these machines were always attended, were controlled manually for the most part, were integrated parts of the community and were designed for frequent replacement of certain components, and efficiency was of little consequence.

Figure 0.1 Jill post mill at Clayton Sussex.

Reproduced with permission of Paul Barber.

In contrast, to generate electricity cost-effectively is the specification of a modern power-generating wind turbine. To meet economic targets, it is unthinkable for the wind turbine to be permanently attended, and unacceptable for it to be much maintained. Yet, each unit is a self-contained mini-power station, requiring to output electricity of standard frequency and voltage into a grid system. Cost-effectiveness is overriding, but the efficiency of individual units cannot be sacrificed lightly. Energy is a prime value; whereas the lifetime costs comprise many components, each one of which has a lesser impact on cost of energy. Also, the total land area requirements per unit output will increase as efficiency drops.

It should be clear that wind technology embraces what is loosely called ‘high-tech’ and ‘low-tech’ engineering. The microprocessor plays a vital role in achieving self-monitoring unmanned installations. There is in fact nothing particularly simple about any kind of system for generating quality electricity. Diesel generators are familiar but not simple, and have a long history of development.

Thus, it is by no means enough to build something ‘simple and rugged’ that will survive any storm. Instead, the wind turbine must be value engineered very carefully to generate cheap electricity with adequate reliability. This is the first reason why the technology is challenging.

0.4 The Variability of the Wind

The greatest gust on record was on 12 April 1934 at the peak of Mount Washington in the Northern Appalachians [3]. ‘On record’ is a revealing phrase as anemometers have usually failed in the most extreme conditions. At 103 m/s, a person exposing 0.5 m² of frontal area would have experienced a force equivalent to about 1/3 of a tonne weight. In terms of annual mean wind speed, the windiest place in the world [3] is on the edge of Antarctica, on a mountain margin of East Adelie land. At 18 m/s annual mean wind speed, the available wind energy is about 200 times that of a typical European wind site. These are of course extreme examples and there are no plans to erect wind turbines on either site.

Nevertheless, it underlines that there is enormous variation in wind conditions. This applies both on a worldwide basis but also in very local terms. In the rolling hills of the Altamont Pass area of California, where many wind farms were sited in the 1980s, there are large differences in wind resource (100%, say, in energy terms) between locations no more than a few hundred metres apart. Wind turbines are situated right at the bottom of the earth's boundary layer. Their aerofoils generally travel much more slowly than aircraft or helicopter rotors, and the effect of wind turbulence is much more consequential for design. The crux of this is that it is hard to refine a design for such potentially variable conditions, and yet uneconomic to design a wind turbine fit to survive anywhere. Standardisation is much desired to cheapen production, but is in conflict with best economics at specific local sites. Designs often need to have adaptive features to accommodate larger rotors, uprated generators or additional structural reinforcement as necessary.

Anemometry studies, both to determine suitable sites and for the micro-siting of machines within a chosen area, are not academic exercises. Because of the sensitivity of wind energy to wind speed and wind speed to short- and longer term climatic patterns, the developer who is casual about wind resource estimates is playing a game of roulette on profit margins. Thus, the variability of the wind is the second major reason wind turbine design is challenging.

0.5 Early Electricity-Generating Wind Turbines

Rather presciently, from a twenty-first century viewpoint, Sir William Thomson (later to become Lord Kelvin) suggested in his address to the British Association meeting in York in 1881 that, as fossil fuel resources were consumed and become more expensive, wind power might be used to generate electricity. Professor James Blyth, of Anderson's college, Glasgow (later to become Strathclyde University) was thus inspired to build and test in 1887 the first windmill to be used for electricity generation. Power stored in a battery was used to light up to 10, 8 candlepower, 25 V, incandescent lamps in Blyth's cottage.

There is no photographic record of Blyth's first electricity-generating wind turbine of 1887, a horizontal-axis, American-style multi-bladed rotor. A little later in 1891, he developed a vertical-axis wind turbine which provided lighting for his holiday home at Marykirk, a small village in Scotland about 45 km from the city of Dundee. The diameter was about 10 m and the ‘blades’, 8 semi-cylindrical boxes, as Blyth called them, are each about 1.8 m wide and 1.8 m high. Soon afterwards he had this design engineered more professionally (Figure 0.2) and sold a small number of these, the first ‘commercial’ electricity generating wind turbines in the world.

Figure 0.2 Blyth windmill commercial prototype.

Reproduced with permission of the Andersonian Library, University of Strathclyde.

Blyth was succeeded by the American, Charles Brush who in 1888 used a large multi-bladed windmill to illuminate his Cleveland mansion. The rotor had 144 blades and a diameter of 17 m and with the tower weighed about 36 tonnes. At full load, the dynamo would then turn at 500 rpm and give an output of 12 kW. The early stand-alone electricity-generating windmills had significant problems with highly variable input, affecting the reliability of the accumulators. The Dane, Poul la Cour built his first windmill at Askov (in Jutland, about 40 km east of Esbjerg) in 1891 with a diameter of 11.6 m and four sails each 2 m wide. In 1891, la Cour invented the ‘kratostata’ to smooth out the power fluctuations that result from the turbulence in the wind. This was a mechanical device allowing some slip in a belt transmission alleviating rapid changes in load from turbulent wind variations, and it appreciably helped the problems with batteries. However, at the end of the nineteenth century, there was neither sufficient technology development nor a suitable market context for wind-generated electricity to progress further and become cost-effective.

0.6 Commercial Wind Technology

In the twentieth century, wind technology headed towards mainstream power generation beyond the water pumping and milling applications that had been exploited for several thousand years. The Gedser wind turbine is often credited as the seminal design of the modern wind industry. With assistance from Marshall Plan post war funding, a 200 kW, 24 m diameter, three-bladed wind turbine was installed during 1956–1957 on the island of Gedser in the south east of Denmark. This machine operated from 1958 to 1967 with about 20% capacity factor.¹

In the early 1960s, Professor Ulrich Hütter developed high tip speed designs, which had a significant influence on wind turbine research in Germany and the United States.

In the early 1980s, many issues of rotor blade technology were investigated. Steel rotors were tried but rejected as too heavy, and aluminium as too uncertain in the context of fatigue endurance. Wood was a logical natural material designed by evolution for high fatigue bending strength-to-weight ratio. The problem of moisture stabilisation of wood was resolved in the wood-epoxy system developed by Gougeon Brothers in the United States. This system has since been employed in a number of small and large wind turbines (e.g. the former NEG-Micon NM82). Wood-epoxy blade technology was much further developed in the United Kingdom, latterly by Taywood Aerolaminates who were assimilated by NEG-Micon and then in turn by Vestas. The blade manufacturing industry was, however, dominated by fibreglass polyester construction which evolved from a boat building background, became thoroughly consolidated in Denmark in the 1980s and has since evolved into more sophisticated glass composite technologies using higher quality fibres (sometimes with carbon reinforcement), and more advanced manufacturing methods such as resin infusion.

During the 1980s, some megawatt-scale prototypes had appeared and this history is well documented by Spera [4] and Hau [5]. In general, these wind turbines had short lives and, in some cases, fatal flaws in design or manufacture. Valuable research information was gained; yet, in many respects, these designs followed technology routes rather disconnected from the emerging commercial wind turbine market. In contrast to this, in Denmark during the 1970s and 1980s, a gradual development of wind technology had occurred. This was a result of public pressures to develop renewables and to avoid nuclear energy combined with a lack of indigenous conventional energy sources. Wind turbine design development proceeded with incremental improvement of designs which were being maintained in commercial use and with gradual increase in scale into ratings of a few 100 kW. And, a much more successful technology resulted.

Just as the first-generation commercial Danish designs were emerging in the early 1980s, a combination of state and federal, energy and investment tax credits had stimulated a rapidly expanding market for wind in California. Over the period 1980–1995, about 1700 MW of wind capacity was installed, more than half after 1985 when the tax credits had reduced to about 15%.

Tax credits fuelled the indiscriminate overpopulation of various areas of California (San Gorgonio, Tehachapi and Altamont Pass) with wind turbines many of which were ill-designed and functioned poorly. It was the birthplace and graveyard of much more or less casual innovation. This created a poor image for the wind industry, which took time to remedy. However, the tax credits created a major export market for European, especially Danish, wind turbine manufacturers who had relatively cost-effective, tried and tested hardware available. The technically successful operation of the later, better designed wind turbines in California did much to establish the foundation on which the modern wind industry has been built.²

0.7 Basis of Wind Technology Evaluation

A summary of some of the key issues in the evaluation of new wind technology is presented here. These topics are addressed in more detail in Chapters 9 and 10.

0.7.1 Standard Design as Baseline

The most straightforward way to evaluate new technology is to set it alongside existing state-of-the-art technology and conduct a side-by-side comparison. This is particularly effective in the case of isolated components which are innovative and different in themselves from the standard solution but have little direct impact on the rest of the system. It is then reasonable to assume that all other components in the system have the same costs as in the standard design and conduct a cost of energy analysis in which only the new component is differentiated. Other innovations may be much more challenging. For example, a new rotor concept can have wide-ranging implications for system loads. In that case, one approach is to tailor certain key loads to be within the same level as the standard design and therefore to have no impact on the components designed by those loads. Another more challenging route is to develop analyses where the impact of loads on component cost is considered. The development of a baseline standard, state-of-the-art design will be seen as a key element in most of the evaluations of innovative technology.

0.7.2 Basis of Technological Advantage

If an innovative system is feasible in principle, the next obvious question is why is it better than anything that precedes it? Does it offer performance gains or cost reduction, does it enhance reliability? In the first instance, it is not a matter of assessing the level of merit or the realism of the claim so much as confirming that there is a core reason being offered why the system may have merit.

0.7.3 Security of Claimed Power Performance

Especially with radically new system designs, there may be a question mark over the likely level of performance. The evaluation of this is clearly critical for the system economics and a number of evaluations go no further than consideration of whether the proposed system has a sufficiently good power performance coefficient. This is particularly the case in systems that sacrifice performance for simplicity. Sometimes the illusion of a very simple and cheap system will persuade an uncritical inventor that an idea is very promising when, in fact, the system in its essential concepts sacrifices so much energy that the considerable capital cost savings that it may achieve are not enough to justify the concept and the cost of energy is high.

0.7.4 Impact of Proposed Innovation

Where can successful innovation make the greatest impact?

This is addressed by looking at the relative costs of components in a wind turbine system and any impacts they have on system productivity through efficiency or reliability. Innovation is disruptive and needs to offer sufficient benefit to be worth the trouble. The capital cost of a yaw system of a large horizontal axis wind turbine is typically around 3% or 4% of total wind turbine capital cost. About half to two-thirds of this cost is in the yaw bearing. This major component is generally not dispensable and so it is clear that no yaw system solution, however innovative, can make large capital cost savings in relation to wind turbine capital cost. On the other hand, if a new yaw system has improved reliability, its total value in terms of impact in cost of energy is enhanced.

0.8 Competitive Status of Wind Technology

After many years of battling to reduce costs, assailed by critics about the extent to which wind was subsidised, as if other energy supplies had not been, a breakthrough picture is emerging. The following article [6] by Paul Gardner, Global Segment Leader, Energy Storage at DNV GL, summarises the situation very well:

In November, the UK Government published an updated version of its Electricity Generation Costs analysis. This is a rigorous assessment of Levelised Cost of Energy (LCOE) for a very wide range of generation options … a major benefit of this kind of study is that it compares competing technologies on the same basis, or at least on mutually consistent assumptions.

…—the report has a section specifically highlighting… enormous reductions from the costs forecast in the previous issue (2013), for large-scale PV, onshore wind, and offshore wind.

For projects commissioning in 2020, the cheapest options available at significant scale all have similar costs: H-class combined cycle gas turbines (CCGTs) at 78 EUR/MWh, onshore wind (projects larger than 5 MW) at 74 EUR/MWh, and large-scale ground-mounted solar at 79 EUR/MWh. By 2025, wind and PV are the clear winners at 72 and 74 EUR/MWh; CCGT costs increase to 96 EUR/MWh due to higher assumed gas and carbon costs.

A good result for renewables. In fact, by 2025 even large hydro, small building-mounted PV, and near-shore wind will be competitive with large CCGTs. However, on closer inspection of the figures there's a more important and perhaps surprising conclusion. A common criticism of costings of the variable renewables (wind, PV, and others) is that they don't include the costs for ‘backup’ generation to cover demand when needed. In the UK the worst case is an extended period of anticyclonic weather in winter, resulting in days or weeks of very low winds, low temperatures, and high electricity demand. This criticism is justified, though the assumption that every wind or PV project should be ‘charged’ the capital cost of fossil generation of the same capacity is overly simplistic. However, the UK figures show that even with this overly simplistic assumption, wind and PV still win.

How? Well, the cost forecasts include fixed and variable costs. For CCGTs in 2025, the fuel, carbon, and variable O&M costs alone total 86 EUR/MWh. This is significantly more expensive than the total costs for wind and PV. So, wind and PV projects could indeed afford to pay for the costs of CCGTs, to be treated as ‘firm’, and would still be the cheapest generation option available at scale. Or in other words, a CCGT operating in 2025 as ‘baseload’ will find it cheaper to buy the output of wind or PV projects, whenever available, in preference to buying and burning gas.

This marks the next stage in cost-competitiveness of renewables. First there is ‘retail parity’, where behind-the-meter wind or PV beats the retail price of electricity to residential, commercial or industrial consumers. Then there is ‘wholesale parity’, where renewable costs compete on wholesale or spot markets. And now on the horizon we can see ‘fuel parity’, where renewables become cheaper than just the fuel (and carbon) costs of fossil generation.

In fact, the ‘horizon’ is not that far off: interpolating the UK figures for 2020 and 2025 shows that fuel parity is forecast for 2023. That is only 6 years from now. Companies currently developing potential new CCGT projects will no doubt be factoring this into their calculations.

References

1 Murray, H.J.R.A. (1913) A History of Chess, Oxford University Press, London; (1985) Benjamin Press, Northampton, MA. ISBN: 0-936317-01-9.

2 Eggleston, D.M. and Stoddard, F.S. (1987) Wind Turbine Engineering Design, Kluwer Academic Publishers. ISBN: 13: 9780442221959.

3 Watson, L. (1984) Heavens Breath: A Natural History of the Wind, Hodder General Publishing Division. ISBN: 0340430982 (0-340-43098-2).

4 Spera, D.A. (ed.) (2009) Wind Turbine Technology: Fundamental Concepts in Wind Turbine Engineering, 2nd edn, ASME Press, New York.

5 Hau, E. (2006) Wind Turbines: Fundamentals, Technologies, Application, Economics, Springer-Verlag. ISBN: 13 978-3-540-24240-6

6 Gardner, P. https://www.linkedin.com/pulse/renewables-track-beat-fuel-parity-combined-cycle-gas-turbines-paul?trk=v-feed&lipi=urn%3Ali%3Apage%3Ad_flagship3_feed%3BsXZmYFRfaHIrsHuo%2FxkBiQ%3D%3D (viewed February 2017).

¹ A historical review of wind technology (also written by the author) similar to the text up to this point may be found in the EWEA publication, Wind Energy the Facts.

² Capacity factor is the ratio of energy output produced over a period to that which would be produced if the system operated always at its full nameplate rating over the same period. Providing the wind turbine is reliable, capacity factor in the context of wind energy is primarily a measure of how good the site wind conditions may be.

Part I

Design Background

Chapter 1

Rotor Aerodynamic Theory

1.1 Introduction

Theoretical background in energy extraction generalities and, more specifically, rotor aerodynamics of horizontal axis wind turbines (HAWTs) is developed in this chapter. Some prior knowledge of fluid dynamics in general and as applied to the analysis of wind turbine systems is assumed, in particular basic expressions for energy in a fluid flow, Bernoulli's equation, definitions of lift and drag, some appreciation of stall as an aerodynamic phenomenon and blade element momentum (BEM) theory in its conventional form as applied to HAWTs. Nevertheless, some of this basic knowledge is also reviewed, more or less from first principles. The aim is to express particular insights that will assist the further discussion of issues in optimisation of rotor design and also aid evaluation of various types of innovative systems, for example, those that exploit flow concentration.

Why focus much at all on theory in a book about innovative technology? Theory is often buried in more or less opaque computer code, which may generate loads of information that engineers can use in design. However, as is amplified in the following chapters, theory is in itself:

Food for innovation and suggestive of methods of performance enhancement or alternative concepts;

A basis for understanding what is possible and providing an overview appraisal of innovative concepts;

A source of analytic relationships that can guide early design at a stage where many key parameters remain to be determined and there are too many options to subject each to detailed evaluation.

Prior to discussions of actuator disc theory and the BEM theory that has underpinned most practical engineering calculations for rotor aerodynamic design and determination of wind turbine loads, some discussion of aerodynamic lift is presented. This is intended particularly to highlight a few specific insights which can guide design and evaluation of wind energy systems. In general, a much more detailed understanding of basic aerodynamics is required in wind turbine design. This must cover a wide range of topics, 2D and 3D flow effects in relation to aerofoil performance, stall behaviour, aeroelastic behaviour, unsteady effects including stall hysteresis and induction lag, determination of suitable aerofoil data for wide ranges in angle of attack, and so on. References [1–10] are a sample from extensive published work covering some of these issues.

1.2 Aerodynamic Lift

The earliest wind turbines tended to use the more obvious drag forces [11] experienced by anyone exposed to wind on a windy day, and use of the potentially more powerful lift forces was almost accidental. Exploitation of the aerodynamic lift force is at the heart of efficient modern wind turbines, but surprisingly the explanation of lift has been quite contentious. Before entering that territory, consider first Bernoulli's equation which is derived in many standard sources on fluid mechanics. Ignoring gravitational, thermal and other energy sources and considering only pressure and kinetic energy, this equation becomes: c01-math-001 , where p is static pressure in a fluid element moving with a velocity of magnitude U, ρ is fluid density and p0 is the total pressure which, in the absence of energy extraction, is constant along any streamline in the flow field.

Bernoulli's equation is essentially an energy equation expressed dimensionally in units of pressure and can be viewed as conservation of energy per unit volume of the fluid. In that connection, pressure can be regarded as the source potential energy (per unit volume) that drives fluid flow. This interpretation is discussed subsequently and is seen to be crucial to a clear understanding of how a wind turbine rotor works.

Returning to the issue of aerodynamic lift, one view of the explanation of the lift force has been that the fluid, should it have a longer path to traverse on one side of an aerofoil, will travel faster in order to meet the fluid flowing past the other side at the trailing edge of the aerofoil. With increase in velocity, the associated static pressure in that region will reduce in consequence of Bernoulli's equation. The pressure deficit on the side of the plate with the longer flow path is then considered the source of the lift force.

There are various problems with this as an explanation of the lift force. Firstly, a thin plate set at an angle in a uniform flow field will generate significant lift when, considering its shape, there is negligible difference between the upper surface and lower surface paths. Secondly, if an aerofoil with a shape with a noticeably longer flow path on one side is considered and the assumption that the flow on each side will traverse the length of the aerofoil in equal times (something that in itself can be challenged) is made, the difference in static pressure calculated on the basis of the implied velocities on each side of the aerofoil will be found quite insufficient to account for the observed lift force.

An apparently authenticated story relates to the efforts of the famous physicist Albert Einstein in aerofoil development. Einstein's effort, inspired by the path-length-related concept, was a miserable failure¹ and he later commented ‘That is what can happen to a man who thinks a lot but reads little.’

Considering the basic definition of lift as the force created on an object at right angles to the incident flow, it is evident that such a force, like all forces according to Newton's Second Law, will be associated with a rate of change of momentum in that direction. Thus, the magnitude of the lift force will, in principle, be unambiguously determined by integrating all the components of momentum in the flow field normal to the incident flow that result from the object causing deviation of the flow.

Whilst this explanation is pure and fundamental, it does not immediately shed light on why lift forces can be so large.

The explanation relating to Bernoulli's equation has some relevance here. Where flow is accelerated around a curved surface, the reduction in static pressure assists in maintaining attachment of the flow and contributes to large suction forces. As nature proverbially abhors a vacuum, strong suction on the boundary layer near a curved surface will induce a large deviation in the general fluid flow some distance from the surface, thereby giving a large overall change in fluid momentum and producing a strong lift force. Aerofoil design is very much about the extent to which such forces can be sustained as the curvature is increased and more severe changes of flow direction are attempted in order to increase lift.

An associated consequence of the Bernoulli equation is the so-called Coanda effect. Aerofoils with elliptical section were developed and used on the X-wing plane/helicopter design [12]. Such aerofoils will have only moderate lifting capability attributable to their shape alone. However, the discharge of a thin jet of air tangential to the surface near the trailing edge will attract the general flow to the jet and cause a much larger deviation in flow direction and consequently much enhanced lift.

The ‘attraction’ of the jet to the surface arises as the jet brings increased momentum into the boundary layer where the jet flow is next to the body surface. This overcomes the natural tendency of the (reduced momentum) boundary layer to separate under the adverse (rising) streamwise pressure gradient due to the aerofoil curvature. Due to the large curvatures involved, there is a noticeable pressure change across the jet, which can be calculated from the mass flow rate in the jet and the radius of curvature of the flow. The jet tries to entrain any fluid between itself and the wall (very efficiently because it is normally turbulent) and this entrainment keeps it attached to the wall. Then, because the streamlines are now curved, the wall pressure falls below the external ambient value. In fact, in the absence of external flow incident on the aerofoil, such a jet will almost completely encircle the aerofoil.

This phenomenon is often called the Coanda effect in recognition of Henri-Marie Coanda, who discovered it apparently through rather hazardous personal experience.² Controlling lift on an aerofoil section by blowing a jet tangential to the surface is often referred to as circulation control. It is a form of boundary layer control which has been considered for regulation of loads and control or performance enhancement of wind turbine blades [13].

Lift is intimately related to vorticity [14]. Associated with this is the Magnus effect, whereby a rotating cylinder (or sphere) can generate lift. This affects the flight of balls in many sports, has been employed in the form of the Flettner rotor [15] to power ships and has been exploited in at least two innovative wind turbine designs [16, 17]. Wikipedia [18] is quite informative on lift, vorticity and the Magnus effect and also provides a commentary on some popular incomplete views such as have been discussed. Finally, in the context of wind turbine systems, lift may also be involved in the performance of wind devices that have been casually categorised as ‘drag’