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Unit 2: Wind Energy Resource, Wind Turbines & Wind Farms. EE361. Contents. Advantages Types and their comparisons Selection & design considerations Assessment on energy yield Operation principle and its associated generators/converters Integration into the power grid
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Contents • Advantages • Types and their comparisons • Selection & design considerations • Assessment on energy yield • Operation principle and its associated generators/converters • Integration into the power grid • Environmental impacts
Wind Atlas of western Europe
Wind Atlas of western Europe – offshore Source: Risø National Laboratory, Roskilde, Denmark.
Advantages of wind power • Renewable and “free” • Almost the lowest cost/kW among all popular renewable energy sources (may be not as low as hydropower) • Wind turbines take up less space than the average power station. • Great resource to generate energy in remote locations, such as mountain communities/ islands and remote countryside.
Disadvantages of wind power • Energy output is very location dependent • Unpredictable and intermittent, sometimes very fluctuating • Construction can be very expensive and costly to surrounding wildlife during the build process • Noise pollution, visual impacts & ecology impacts is a major problem during operation • Still expensive as compared with fossil fuel based power station, specially in low speed areas
Source: BWEA – British Wind Energy Association (for on shore wind farms)
Reasons of falling cost of wind energy • The turbines themselves are getting cheaper as technology improves and the components can be made more economically. • The productivity of these newer designs is also better, so more electricity is produced from more cost-effective turbines. • There is also a trend towards larger machines. This reduces infrastructure costs, as fewer turbines are needed for the same output. • The cost of financing is also falling as lenders gain confidence in the technology.
Cost comparison (for new installations) • Source: Pure Energy Systems Wiki
Types of wind turbines • On-shore/off shore • Horizontal/vertical axis • Upwind/down wind • Fixed blade, adjustable blades • Number of blades, direct drive/indirect drive, etc.
Off shore wind farm • Piles (1) • Aerodynamically shaped blades (2) • Nacelle (3) • Subsea cables (4) • Offshore transformer (5) • A substation on land (6)
Classical modern 3-blades wind turbine • Horizontal axis • Number of blades : 3 • Greater aesthetic appeal
Source: The Wind Turbine Company • Classical modern 2-blades wind turbine • Horizontal axis • Number of blades : 2 • Higher running speed, but noisier, appearing ‘jerky’ when running • Advantage of the lighter weight structural benefits
Features include its elegant silhouette, low cost, ease of use and extremely quiet operation due to the single blade design. • Source: Power House Wind Limited. • Single blade wind turbine • Horizontal axis • Number of blades : 1
Pumping wind turbine • Horizontal axis • Number of blades : ~ 20 • Suitable for small installation only
Effect of higher number of blades • Turbines with larger numbers of smaller blades operate at a lower Reynolds number and so are less efficient. • Small turbines with 4 or more blades suffer further losses as each blade operates partly in the wake of the other blades. • Also, the cost of the turbine usually increases with the number of blades.
Material used in blades • One of the strongest construction materials available (in 2006) is graphite-fibre in epoxy, but it is very expensive and only used by some manufactures for special load-bearing parts of the rotor blades. • Modern rotor blades (up to 126 m diameter) are made of lightweight protruded glass-reinforced plastic • Smaller ones also from aluminum, or sometimes laminated wood.
Various vertical axis wind turbines VAWT A helical twisted VAWT Source: Quiet Revolution Ltd. 30 m Darrieus wind turbine in the Magdalen Islands, Quebec, Canada
Vertical rotor wind turbine (hydrogen University in Apulia, Italy) • Source: Ropatec Ltd. • Star vertical wind turbine (McDonald a Brema, Italy ) • Source: Ropatec Ltd.
Advantages of VAWT • Intercept wind in all directions, they don’t have to actively orient themselves to the direct • Generator located nearer the ground, easier to maintain the moving parts • ion of the wind • A massive tower structure is less frequently used, lower bearing mounted near the ground • May be built at locations where taller structures are prohibited.
Disadvantages of VAWT • Uses guy-wires to hold it in place puts stress on the bottom bearing . Guy wires attached to the top bearing increase downward thrust in wind gusts. Solving this problem requires a superstructure to hold a top bearing. • The stress in each blade due to wind loading changes sign twice during each revolution as the apparent wind direction moves through 360 degrees. This reversal of the stress increases the likelihood of blade failure by fatigue. • Having rotors located close to the ground where wind speeds are lower due to the ground's surface drag
A=D2/4 D Power from the Wind • A wind turbine of cross-section A, intercepting a wind front travelling at a speed u0 and density will produce power given by where CP is an efficiency factor termed power coefficient which varies with speed for individual machines • Doubling A may yield twice the amount of power, doubling the wind speed would produce eight times the power potential • This fact has an important bearing when investigating locations for a wind turbine. A location having 10% more wind resource than a second location, would yield a 33% increase in potential wind resource, i.e., 1.1 1.1 1.1 = 1.33
Power from the Wind • The maximum rated power capacity of a wind turbine is given for a specified: • - wind speed of 11-12 m/s • - power production of 300 W/m2 of cross-section • - power coefficients between 35% and 45% • However, even at a good wind site, annual average power production is expected to be only between 25% and 33% of the rated capacity • Originally, machines were expected to last for at least 15 to 20 years and their cost was about US$500 to US$1000 per kW rated capacity. However, the first generation of wind turbines that were installed in 1980’s, have only recently began to reach the end of their useful life • Since these early wind projects were, more often than not, in prime wind resource locations, they are being dismantled and replaced with a smaller number of larger, newer turbines – given rise to a concept termed REPOWERING
u0 m=A1u0 A1 Energy Extraction – Linear Momentum • In the unperturbed state, a column of wind upstream of the turbine, with cross-sectional area A1, has kinetic energy passing per unit time of • This is the power in the wind, P0, at the unperturbed wind speed u0 and an air density . m is the air mass • Air density varies with height and meteorological conditions. A typical value for is 1.2255 kg-m3, at sea level, but an empirical expression due to C. Biber, popular in wind farm design, uses temperature and altitude: • where T is the absolute temperature in degrees Kelvin, is the altitude in meters, C0=273.15, C1=10 km and C2=1.2255 kg-m3
energy extracted by actuator disk air stream lines u0>u1>u2 Energy Extraction – Linear momentum • Wind speed increases with height, is affected by local topography, and varies with time following rather complex patterns • To simplify matters let us consider constant velocity air stream lines passing through and by the turbine, with the rotor treated as an “actuator disk” where there is a change in pressure as energy is extracted • Such a situation is illustrated in the figure opposite, Betz model of expanding air stream; where as a result in the change in pressure there is a decrease in the linear momentum of the wind • Area A1 is the rotor swept area and areas A0 and A2 enclose the stream of constant air mass, at different pressures, passing through A1
Energy Extraction – Linear momentum • In Betz model, the following idealized conditions are assumed: • A0 is positioned in the oncoming wind front unaffected by the turbine and A2 at the position of minimum wind speed before the wind front reforms down-wind • Perturbation to the smooth laminar air flow due to the angular momentum exerted by the blades, are neglected even though they are very significant • The force or thrust on the turbine is the reduction on momentum per unit time from the air mass flow rate: • The power extracted by the turbine is: • According to the linear momentum theory: