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Chapter 8 focuses on rotor blade technology, covering design, materials, manufacture, and testing. The role of fibre-reinforced composites is discussed, examining their superior mechanical and manufacturing properties. Their property of anisotropy enables composites to be tailored to match the direction of principal stresses in the most material-efficient way. Blade structural design is illustrated using bending theory for a cantilever beam, with stress and strain equations developed for a composite structure. The importance of section thickness and cross-sectional geometry is illustrated using the SERI/NREL blade profiles. An overview of blade attachment methods considers adhesive bonded root studs, T-bolts, and fibre-embedded studs that are integrated during the blade-moulding process. Most large blades are nowadays manufactured by vacuum resin infusion moulding (VRIM), and the chapter includes a description of this technique. There is a section on wood-laminate blades, which are still used in some applications, and comments on blade balancing and testing. The chapter concludes with a review of blade weight and technology trends based on some historic commmercial blade designs.
This chapter summarises the key aerodynamic theory of horizontal-axis wind turbine rotors. The actuator disc concept leads to the relationships between induced velocity, axial thrust, and power extraction. The theory is extended to multiple streamtubes, which, combined with 2D wing theory, establish the basis of blade-element momentum (BEM) theory. A straightforward mathematical treatment of BEM theory is included, with an iterative procedure suitable for coding. Measurements from a full-scale rotor illustrate the applicability of BEM theory but also its fundamental limitations: the latter are described, and measures are outlined to compensate for them in practical BEM codes. Simple relationships are given for the axial and tangential load distributions on an optimal HAWT blade. The structure of the rotor wake is described, leading into a description of vortex-wake theory, which provides a more physically realistic description of the airflow. Vortex wake codes are described in non-mathematical terms. The chapter includes wake measurements from full-scale wind turbines and small models. Vorticity maps from the latter verify the underlying mathematical model of a helical vortex wake.
The final chapter is a largely non-technical overview of economic and political aspects of wind energy policy. The cost of wind energy is assessed in terms of Levelised Cost of Energy (LCoE), with equations given in full and simplified forms. Using a large database, historic installed costs for UK wind both onshore and offshore are given, from the earliest projects to the present day. The observed trends are discussed. Operational and balancing costs are outlined, the latter reflecting the intermittency of wind power. LCoE estimates are made for a range of installed costs and output capacity factors at typical discount rates and compared with current generation prices. The chapter considers the economics of onsite generation with the example of a private business using wind energy to offset demand; the energy displacement and export statistics are extrapolated to compare with a national scenario for 100% renewable electricity generation. The topic of ownership is introduced and examined in the context of the UK’s first community-owned windfarm. The chapter concludes with a brief review of UK renewable energy policy, which originated with legislation to protect the nuclear power industry.
Chapter 7 considers structural loading and response of horizontal-axis machines, with some theoretical background and illustrative measurements from different wind turbine types. The chapter begins with a recap of the dynamics of a single degree of freedom system, leading into a discussion of multi-DOF systems and modal analysis. The cyclic loads affecting a wind turbine structure are described, including wind shear, tower shadow, and rotationally sampled turbulence. The concepts of stochastic and deterministic loading are explained and the principle of aerodynamic damping illustrated. Qualitative descriptions are given of gyroscopic, centrifugal, and electromechanical loading. The phenomenon of blade edgewise stall vibration is explained, with discussion of mechanical damper solutions. The last part of the chapter draws on an early experimental campaign in which the dynamic loading on a full-scale wind turbine was measured and compared with the results of software simulation. Results from the same trials also demonstrate the difference in rotor thrust loading arising from positive and negative pitch control. The chapter concludes with a brief summary of fatigue prediction methods.
Chapter 6 considers wind turbine control, including supervisory control, power limiting, starting and stopping, electrical power quality, and sector management. The importance of accurate yaw control is discussed in terms of energy capture and cyclic loading, and an active yaw system is illustrated. The main focus of the chapter is real-time power control, and the chapter builds on the aerodynamic and electrical concepts covered previously in Chapters 3–5. The differences between stall and pitch regulation are explained, in the latter case in the context of both constant and variable-speed operation. Power measurements from constant-speed and variable-speed pitch controlled machines illustrate the superior accuracy of the latter. Control block diagrams are given for both methods, with qualitative explanation of the principles. The procedure for starting and stopping different wind turbine types is explained, and the advantages of pitch control in this context are illustrated. The chapter includes a short description of sector management, a control strategy based on external factors such as wind speed and direction, and used for noise reduction, shadow flicker prevention, or fatigue mitigation.
Chapter 5 deals with electrical issues and is broadly divided in two. The first half explains the operating principles of the several different types of generator found on wind turbines and their influence on dynamics and electrical power quality. Generator types are illustrated schematically and their characteristics explained using simple physical principles. Geared and gearless (direct drive) generators are discussed, and there is a brief historical review of generator developments. The second half of the chapter deals with electrical networks and further examines the issue of power quality. The importance of reactive power and how modern generators can manipulate it to aid voltage stability are explained; the role of external devices such as Statcoms, SVCs, and pre-insertion resistors is also discussed in this context. Measurements from a MW-scale wind turbine illustrate voltage control via reactive power management over a period of several days. The challenge of low grid strength is illustrated with a practical example of a small wind farm development on a rural network with low fault level. The chapter concludes with a brief discussion of wind turbine lightning protection.
This chapter contains a broad overview of the technical and environmental issues to be addressed in the construction of onshore wind energy projects. The former include ecological considerations, including birds and mammals; the requirements of typical pre-construction ornithological surveys are described with an example. Public safety and acceptance is discussed in the context of catastrophic damage to wind turbines, visual impact, shadow flicker, and noise nuisance. In the last case, equations and simple rules for noise assessment are given in the context of typical planning guidelines. Sound power levels for a range of commercial wind turbines are compared, and empirical relationships are given relating noise to rated output, rotor size, and tip speed. Risks to aviation are discussed, covering aircraft collision and interference to radar systems, including both primary and secondary surveillance radars. The concept of ‘stealthy’ wind turbine blades is discussed and described in outline. Other siting criteria include avoidance of RF and microwave communications beams and television interference. Rules are given to avoid interference, while minimising required separation distances.
Chapter 9, on siting and installation, considers some of the key steps leading to the successful installation of a wind energy project, whether a single machine or large array. A section on resource assessment considers site wind measurements, the IEC Wind Classification system, and the measure-correlate-predict (MCP) procedure for establishing long-term characteristics at a prospective site. Array interactions are described in terms of energy loss and increased turbulence: empirical models are given for predicting both effects, and wake influence is illustrated with field measurements from large and small arrays. The civil engineering aspects of project construction are examined, with description of different foundation types; simple rules are given for conventional gravity base design, with illustrations. The construction and environmental advantages of rock anchor foundations are described, and some examples are given. Transport, access, and crane operations are discussed. The use of winch erection is illustrated with the example of a 50kW machine. The chapter concludes with a short summary of the necessary electrical infrastructure between a wind turbine and the external grid network.
Chapter 4 extends the aerodynamic discussions of the preceding chapter to show how the rotor net loads (power, thrust, and torque) are developed. The dimensionless power coefficient (Cp) curve is introduced, and the relationship between rotor tip speed ratio and optimum solidity is explained. The variation of thrust loading with wind speed on an ideal pitch-controlled rotor is explained from simple theory and illustrated with measurements from a full-scale wind turbine. Equations governing the chord and twist distributions for an optimised blade are given and discussed in the context of some historic blade types, with illustrations. Rotor aerodynamic control is explained with reference to fixed-pitch stall regulation and variable blade pitch in both positive and negative senses. The influence of blade number is examined, with discussion of the advantages and disadvantages of one-, two-, and three-bladed wind turbines. The chapter concludes with a brief overview of alternative aerodynamic control devices, including tip vanes and ailerons, and the downwind rotor configuration (with examples).
This chapter is an overview of wind power meterorology at a relatively simple level without too much mathematical complexity. The origins of the wind are explained in the action of solar thermal radiation on the atmosphere, and the equation is given for the geostrophic wind at the top of the earth’s boundary layer. The role of the boundary layer in creating wind shear and turbulence near the earth’s surface is explained, and appropriate engineering equations are given to allow wind speed and turbulence to be estimated. Surface roughness and its relationship to turbulence and shear are explained. Experimental measurements are used to illustrate shear and turbulence for a range of different terrain types. The time and space dependency of wind speeds is also illustrated with site measurements, showing the long-term dependability of annual wind speeds, through the more variable monthly averages, to short-term turbulent variation. Gust factor is explained and illustrated as a function of turbulence intensity. The chapter includes high-resolution wind measurements taken during a storm in the Scottish Outer Hebrides, illustrating the extreme levels of turbulence arising in complex terrain.
The introductory chapter is a brief recap of the history and origins of wind power, from windmills in ancient times to today’s multi-megawatt turbines. Energy security has arguably been the historic driver for wind power, and it was a primary source of mechanical power until the advent of the Industrial Revolution, when it was superceded by coal and oil. The first electricity-generating wind turbines were built in the late nineteenth centry, and the technology was pursued most vigorously in Denmark, a country with limited energy reserves: the role of this country in creating the modern wind turbine is described. The worldwide energy crisis of the 1970s brought wind power into the frame internationally, and the pivotal role of legislation under President Carter in expanding the market for wind energy in the US and elsewhere is outlined. Since then, the rationale for wind power has expanded to include climate change, and the technology has grown exponentially in terms of global installation of wind power and the physical size of wind turbines. The chapter concludes by introducing some of the technological steps that have enabled this process, which are detailed in subsequent chapters.
In the multi-disciplinary field of wind energy, students and professionals can often be uncomfortable outside their own specialist areas. This essential textbook explains the key aspects of wind turbine technology and its application in a single readable text. Covering a broad range of multi-disciplinary topics, including everything from aerodynamics through to electrical and control theory, to structures, planning, economics, and policy, this reference is an excellent toolkit for undergraduate students, postgraduate students, and professionals in the field of wind energy. Key concepts, including more challenging ones such as rotational sampling of turbulence, vortex wake structures, and reactive power management, are explained using clear language and simplifying illustrations including experimental graphs, photos, and line drawings.
This paper investigates the interaction between reality and imagination in the architectural design process. It engages with four inter-related inquiries. First, the interplay between reality and imagination in the architectural design process in student design-build live projects. Second, the interplay of reality and imagination for different agents in the architectural design process. Third, how the work of John Hejduk (1929-2000) enables a reappraisal of conceptions of reality and imagination in architectural design. Fourth, we address a live project for The Story Museum in Oxford, UK – a physical architectural space concerned with imaginary spaces – that suggests how an understanding of reality and imagination might be deepened in the architectural design process.
The text reappraises the interplay of reality and imagination in architectural design as a cognitive process. There are two aims: to reassess empirical responses and received wisdom about what is real and what is imagined in architectural design; and to reassess the perception of differences between imagination and reality occurring across education and practice. ‘Thing Theory’ is proposed as a conceptual framework which allows us to improve our understanding of how architectural designs emerge, are transformed in the designer's mind, how architects communicate them to others and how they are understood and shared by others. Reference is also made to interviews with prominent architects. The term ‘thing’ was repeatedly used by these architects to describe moments in the design process when a break from reality had occurred. In these moments, subject and object seemed at their most intertwined. This moment is strong because it is so flexible in responding to change and managing complexity. It is also weak because it is a time when the designer tends to neglect vital everyday constraints such as occupation and ethics. However, this is a key moment because it allows possible future realities to emerge.