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Niall McMahon © 2013
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This lecture considers a basic design process as well as some of the technologies that make up a wind turbine. As before, we focus on the small-wind technologies.
Small wind turbines are used for remote power, from yachts and exploration rigs to isolated homes, for homes and farms, remote and grid connected. Larger, medium-scale machines, are used to power commercial enterprises or farms, where the power requirement is higher. Large wind turbines are used, usually, as part of a wind farm.
Read around (e.g. Wind Energy Basics by Paul Gipe), study other manufacturers' machines, read online, trade magazines and academic journals (EXAMPLES).
Safety, cost/performance and social acceptance are all key design considerations.
Use basic equations, rules of thumb and engineering handbooks to estimate the principal loads and magnitudes associated with the proposed design. The design application determines the annual load requirement, the design wind resource, the rotor size and the design generator capacity. Arriving at the design generator and rotor size is an iterative process.
Basic Example for a Direct Drive, Permanent Magnet Generator Small Wind Turbine Application: > Annual Energy Requirement > Likely Resource at Hub Height (Wind Speed Distribution) | \|/ Solution: > What size rotor will capture this energy at the imagined target site over a year? > Taking efficiencies into account, what generator capacity will we need at the average wind speed? > At a rated wind speed of 10 m/s, what is the nominal generator capacity? > Using generator manufacturers' manuals, find match to rotor RPM/TSR and nominal capacity.
Loads acting on a wind turbine can be classed as static (weight), steady (thrust), cyclic (vibrations resulting from imbalances, for example), impulsive (tower shadow effects on blades), stochastic or apparently random (turbulence) and transient (starting and stopping), among others.
The rotor size has the biggest impact on performance and calculating its coefficient of performance, Cp, is the biggest initial concern of wind turbine designers.
Most components have an associated efficiency. The rotor has a peak theoretical efficiency, the Betz limit, of about 59%. Generators often have efficiencies of 85% or so. Gearboxes usually have high efficiencies, 95% or more.
The cost of energy (COE) associated with a particular design captures the cost of the machine relative to the pay-back anticipated. The turbine cost includes installation and financing charges (interest). The pay-back is a function of the likely energy production over its lifetime at a particular site; this depends, of course, on the resource available as well as the turbine's power curve.
Based on these preliminary considerations, refine the design and repeat the process until the preliminary design is promising. Conduct the process for different configurations.
Taking the most promising result(s), manufacturers will use more advanced analysis tools to refine the designs. These tools include blade element/momentum (BEM) codes and other rotor performance analysis codes, possibly computational fluid dynamics (CFD) codes, drivetrain dynamic analysis codes, finite element codes (to assess component strengths), among others. More next semester. In parallel, usually, experimental testing is conducted on components. A combination of experiment and theoretical design is always required. Theoretical predictions need to describe reality, i.e. results must be valid. The aim of theoretical analysis is to reduce the need for expensive experiments.
The design is modified until the manufacturer is confident that it will perform as expected.
Somewhere close to this point, a prototype machine will be built and tested. Unforeseen peculiarities will be caught at this stage. In parallel, design for manufacture, i.e. volume production, will begin.
Designing for manufacture means, simply, to arrive at a design that is relatively easy to make and to design and prepare the infrastructure necessary for production.
Wind turbine prototypes. Horizontal Axis Wind Turbine (HAWT) Family. Wind turbine prototypes. Vertical Axis Wind Turbine (VAWT) Families; Savonius (left) and Darrieus (right). Modified from a public domain image.
This depends on many things. As with all design, some decisions are determined by the application, e.g. what is appropriate for a large machine will not always be appropriate for a small machine, and some involve making a choice between options that come with a range of advantages and disadvantages. Such is life.
For small wind turbines, downwind designs eliminate the need for tail fins or active yaw mechanisms: an advantage. A disadvantage, on the other hand, is the effect of the periodic forces associated with the tower shadow. These can lead to early fatigue failures. Variable speed machines allow operation at the peak coefficient of performance in all wind speeds to rated. At the same time, variable speed machines require power electronics for synchronising to the grid. Nowadays, however, such electronic systems are relatively cheap and robust. This was not always the case. Permanent magnet machines are relatively simple in construction. Unfortunately, permanent magnets are rare and can be expensive. Additionally, they are dangerous during assembly.
The rotor size will be determined by the application. The design might call for a variable or fixed pitch rotor. Very small machines are often fixed pitch to reduce complexity and cost. Variable pitch systems generally require complex hub mechanisms and are, consequently, more expensive. Variable pitch machines allow better power and overspeed control.
The number of blades does not affect the rating of a wind turbine: the swept area is all that counts. At the same time, there are some differences between one, two, three and multi-bladed machines. In terms of performance, there are small efficiency differences, due largely to energy losses associated with blade tips. Other points to note include:
Gaia-Wind 133-11kw turbine hub with its teeter mechanism clearly visible.
Stiff blade materials help to reduce resonance and other vibrational effects.
Gaia-Wind 133-11kw drivetrain: the low-speed shaft from the two-blade rotor enters from the right; a gearbox increases the rate of rotation; moving left, the high-speed shaft from the gearbox is attached to a disc brake (high speed, low torque); a coupling connects the high-speed shaft to the generator.
The gearbox is the most frequent failure point in a large wind turbine. Disc brakes are often placed on the high-speed shaft where torques are lower. The downside is that the gearbox must handle the large rotor torques during braking. There is a shift towards gearbox-free, variable speed, direct drive machines. These most often use permanent magnet synchronous generators.
Wind turbine braking systems must be fail-safe and incorporate redundancy, i.e. a single failure cannot render the machine unsafe. At least two independent systems must able to bring the rotor to a halt (or close to a halt). At least one of these systems must operate at the high torque (rotor) side of the gearbox, if one is used. Braking systems include: pitch control, low-speed shaft disc brake, high-speed shaft disc brake, electrodynamic braking, aerodynamic brakes (aero-brakes, just like those used in an aircraft), among others. See the IEC design standards 61400-1 and 61400-2.
See also Lecture 5.
Gaia-Wind 133-11kw fixed-speed asynchronous (induction) generator.
The rotor of an Alxion 300STK2M 2.5 kW permanent magnet synchronous generator. The permanent magnets are clearly visible.
The rotor and stator of an Alxion 300STK2M 2.5 kW permanent magnet synchronous generator. The system has an overall diameter of 300 mm.
Composites are commonly used as blade materials. Composites are materials formed by combining two quite dissimilar materials with complentary properties, e.g. strength and stiffness. They usually take the form of a fibre of some sort suspended in a plastic binder. Fibers can be distributed randomly or knitted into cloths.
Blade materials include:
Blade manufacturing methods include:
The ease of manufacture often more important than optimum aerodynamic or structural design.
Other materials include carbon steels for towers and structural components as well as high-strength steels for drive-shafts. Irons and aluminium can be used for some components. In permanent magnet systems, alloys of rare earth metals such as neodymium are typical. Wiring is almost always copper. Concrete is used to anchor towers.
Please see here.
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