Wind energy technologies use the energy in the wind as the source for generating electricity, charging batteries, pumping water, grinding grain, and so on. Most wind energy technologies can be used as stand-alone applications, connected to a utility power grid. For utility-scale sources of wind energy, a large number of turbines are usually built close together to form a wind farm that provides grid power.
Wind Energy Systems
Several electricity providers use wind farms to supply power to their customers. Stand-alone turbines are typically used for water pumping or communications.
However, home owners and farmers in windy areas can also use small wind systems to generate electricity. Wind energy can be produced anywhere in the world where the wind blows with a strong and consistent force.
Windier locations produce more energy, which lowers the cost of producing electricity. The regional resource maps and wind data can be found from the Solar and Wind Energy Resource Assessment (SWERA). SWERA provides high-quality information in suitable formats on renewable energy resources for countries and regions around the world. It also provides the tools needed to apply these data in ways that facilitate renewable energy policies and investments.
The two critical factors are wind speed and the quality of wind. The most suitable sites for wind turbines are turbulence-free locations since turbulence makes wind turbines less efficient and affects overall stability of the turbines. Wind turbulence is influenced by the surface of the earth. Depending on the roughness of the terrain, the wind can be more or less turbulent.
Wind power can be categorized into seven classes, as shown in Table 1, according to the wind speed (m/s) and wind power density (W/m2 ). Each wind power class corresponds to two power densities. For example, the wind power class 3 represents the wind power density in the range between 150 and 200 W/m2 .
Although all wind turbines operate on similar principles, there are several varieties. These include horizontal axis turbines and vertical axis turbines. One can view the online video on how a wind turbine works.
Horizontal Axis Turbines
Horizontal axis turbines are the most common turbine configuration. They consist of a tall tower, at top which sits a fan-like rotor that faces into or away from the wind, a generator, a controller, and other components. Most horizontal axis turbines have two or three blades.
Horizontal axis turbines sit high at the top of the towers to take advantage of the stronger and less turbulent wind at 100 ft (30 m) or more above ground. Each blade acts like an airplane wing, so when wind blows, a pocket of low-pressure air forms on the downwind side of the blade.
The low-pressure air pocket then pulls the blade toward it, which causes the rotor to turn. This is called lift. The force of the lift is actually much stronger than the wind’s force against the front side of the blade, which is called drag. The combination of lift and drag causes the rotor to spin like a propeller, and the turning shaft spins a generator to make electricity.
Vertical Axis Turbines
Vertical axis turbines are of two types: Savonius and Darrieus. Neither type is in wide use. The Darrieus turbine was invented in France in the 1920s. Often described as looking like an eggbeater, it has vertical blades that rotate into and out of the wind. Using aerodynamic lift, it can capture more energy than drag devices. The Giromill and cycloturbine are variants on the Darrieus turbine.
The Savonius turbine is S-shaped if viewed from above. This drag type turbine turns relatively slowly but yields a high torque. It is useful for grinding grain, pumping water, and many other tasks, but its slow rotational speeds are not good for generating electricity. Windmills are still used for a variety of purposes. Windmills have more blades than modern wind turbines, and they rely on drag to rotate the blades.
The turbine power is cubic power of the velocity. Betz’s law gives the theoretical maximum power that can be extracted from the wind. A higher wind speed gives a higher extracted energy. The speed va entering the turbine blade is higher than the speed vb leaving the turbine. That is, there are two speeds as shown in Figure 1: one before the wind approaches the front of the turbine (va) and the other behind the turbine (vb).
The output power Pout from the wind can be expressed as
The available input power to the turbine is
Therefore, the turbine efficiency becomes
The condition for maximum efficiency can be found by setting dηt/dt = 0, which can be found from Eq. (3) as given by
which gives vb/va = 1/3
which, after substituting into Eq. (3), gives the maximum turbine efficiency as ηt(max) ≈ 59.3%
Therefore, according to Betz’s law, the maximum theoretical extracted wind power is 59.3% of the total available power.
However, the efficiency of the practical wind turbines is slightly lower. Air density of the wind δ is a function of air pressure, temperature, humidity, elevation, and gravitational acceleration, and ρ can be approximately determined from
Where pat = the standard atmospheric pressure at sea level (101.325 Pa or N/m2 );
T = the air temperature;
Cg = the specific gas constant for air (287 Ws/kg K);
g = the gravitational acceleration (9.8 m/s2 );
h = the elevation of the wind above the sea level (m).
Substituting these values, Eq. (6) gives a nonlinear relation as given by
Therefore, if the temperature decreases, air is denser. Also, air is less dense at high altitudes. For the same velocity, wind with higher air density (heavier air) possesses more kinetic energy.
Speed and Pitch Control
The linear velocity of the blade, as shown in Figure 2, is known as the tip velocity vtip. The wind turbine is normally designed to run faster than wind speed va to allow the turbine to generate electricity at low wind speeds.
The ratio of the tip velocity vtip to the wind speed va is known as the tip speed ratio (TSR).
TSR = vtip/va (Equation 7)
While the turbine blades are rotating, most of the wind passes through open areas between the rotor blades if TSR is too small. If TSR is too large, the fast-moving blades would block the wind flow.
The moving rotor blades passing through the wind creates a stir of air turbulence and there must be enough time to damp out the turbulence. If the TSR is high, the next blade may arrive to the area swept by the earlier blade before the turbulence is damped out. Therefore, the effectiveness of the blades are reduced, reducing the turbine affiance.
The tip velocity vtip (m/s) is a function of the rotating speed of the blade ω (rad/s) and the length of the blade r (m) and is given by
vtip = ωr = 2πnr (Equation 8)
Where n is the number of revolutions the blade makes in 1s.
Substituting vtip in Eq. (7) gives
TSR = 2πnr/va (Equation 9)
Turbine blades are aerodynamically optimized to capture the maximum power from the wind in normal operation with a wind speed in the range of about 3 to 15 m/s.
In order to avoid damage to the turbine at a high wind speed of approximately 15 to 25 m/s, an aerodynamic power control of the turbine is required. The most commonly used methods are pitch and stall controls.
In the stall control method, the blades of the turbine are designed in such a way that if the wind speed exceeds the rated wind speed of about 15 m/s, air turbulence is generated on the blade surface such that it is not facing the wind.
The stall control is normally used in small-to-medium-size wind turbines.
Pitch control is normally used for large wind turbines. During normal operating conditions with the wind speed in the range from 3 to 15 m/s, the pitch angle is set at its optimal value to capture the maximum power from the wind. When the wind speed becomes higher than the rated value, the blade is turned out of the wind direction to reduce the captured power.
The blades are turned in their longitudinal axis by changing the pitch angle through a hydraulic or electromechanical device. The device is normally located in the rotor hub attached to a gear system at the base of each blade. As a result, the power captured by the turbine is kept close to the rated value of the turbine.
The value of TSR can be adjusted by changing the pitch angle of the blades. At light wind conditions, the pitch angle is set to increase the TSR. At high wind speeds, the pitch angle is adjusted to reduce the TSR and maintain the rotor speed of the generator within its design limits.
In some systems, the TSR can be reduced to almost zero to lock the blades at excessive wind conditions. Wind turbines with variable TSR can operate for wider ranges of wind speeds.
The power curve relates the mechanical power of the turbine to the wind speed and it defines the power characteristics of a wind turbine. The power curve of a wind turbine is specified and guaranteed by the manufacturer. Power curves are made by a series of measurements for one turbine with different non-turbulent wind speeds. A typical power curve is shown in Figure 16.20.
The power curve can be divided into three regions: cut-in, nominal, and cut-out.
The “cut-in” shows the minimum wind speed needed to start the turbine (which depends on turbine design) and to generate output power. Usually it is 3 m/s for smaller turbines and 5–6 m/s for larger ones.
The wind turbine starts to capture power at the cut-in wind speed. The power captured by the blades is a cubic function of wind speed until the wind speed reaches its rated value.
As the wind speed increases beyond the rated speed, aerodynamic power control of blades is required to keep the power at the rated value either by stall or pitch control.
The “cut-out” wind speed represents the speed point where the turbine should stop rotating due to the potential damage. The wind turbine should stop generating power and be shut down when the speed is higher than the cut-out wind speed.
The theoretical curve, as shown in Figure 3, is illustrated with an abrupt transition from the cubic characteristic to the constant power operation at higher speeds. But in practical turbines, the transition is smoother.
The control of a variable-speed wind turbine below the rated wind speed is achieved by controlling the generator. The capture of the wind power can be maximized at different wind speeds by adjusting the turbine speed in such a way that the optimal tip speed ratio is maintained.
For a given wind speed, each power curve has a maximum power point at which the tip speed ratio is optimum. To obtain the maximum available power from the wind at different wind speeds, the turbine speed must be adjusted to ensure its operation at all the MPPs.
The trajectory of MPPs represents a power curve, which can be described by
Pmax ∝ ωm3 (Equation 10)
Since the mechanical power captured by the turbine is related to the turbine torques by Pmax = Tmax ωm, the turbine mechanical torque becomes
Tmax ∝ ωm2 (Equation 11)
The relationships among the mechanical power, speed, and torque of a wind turbine can be used to determine the optimal speed or torque reference to control the generator and achieve the MPP operation.
Several control schemes have been developed to perform the maximum power point tracking (MPPT).
Wind Energy Systems
The main elements of a wind energy system, as shown in Figure 4, are turbine, gearbox, and power electronics converters. Transformers are often needed to connect to the utility grid. The output voltage of the generators is converted to a rated utility voltage at a rated frequency through one or more stages of power electronics converters.
There are various types of electrical machines that are used in wind turbines. There is no clear criterion for choosing a particular machine to work as a wind generator. The wind generator can be chosen based on the installed power, site of the turbine, load type, and simplicity of control.
The commonly used types of generators in wind turbine applications are brushless dc (BLDC), permanent-magnet synchronous generators (PMSGs), squirrel-cage induction generators (SCIG), and synchronous generators (SG).
Squirrel-cage induction (SCIG) or BLDC generators are generally used for small wind turbines in household applications. Doubly fed induction generators (DFIGs) are usually used for megawattsize turbines. Synchronous machines and permanent-magnet synchronous machines (PMSMs) are the other machines that are used for various wind turbine applications.
The rotor of a large three-blade wind turbine usually operates in a speed range from 6 to 20 rpm. This is much slower than a standard four- or six-pole wind generator with a rated speed of 1500 or 1000 rpm for a 50-Hz stator frequency and 1800 or 1200 rpm for a 60-Hz stator frequency.
Therefore, a gearbox is normally required to match the speed difference between the turbine and generator such that the generator can deliver its rated power at the rated wind speed. The speed of the turbine rotor is generally lower than the speed of the generator. The gear ratio is designed to match the high-speed generator with the low-speed turbine blades. The gearbox ratio GR can be determined from
Where Ng and Nt = the generator and turbine rated speeds in rpm;
s = the rated slip;
fs = the rated stator frequency in Hz;
p = the number of pole pairs of the generator.
The rated slip is usually less than 1% for large induction generators and zero for synchronous generators.
The wind turbine gearboxes normally have multiple gear ratios to match the turbine rotor to the generator.
The MPPT control can be achieved by
- maximum power control,
- optimum torque control, and
- optimum tip speed control.
A simplified block diagram for the control of maximum generated power is shown in Figure 5.
The power versus wind speed curve is generally provided by the manufacturer. The output of the wind speed sensor vw is used to generate the reference power Pm* , which is compared with the output power of the generator to produce the gate control signals of the power converters.
Under steady-state conditions, the mechanical power Pm of the generator will be equal to its reference power Pm* . Assuming that the power losses of the gearbox and drive train are neglected, the mechanical power of the generator Pm is equal to the mechanical power PM produced by the turbine.
Since the turbine mechanical torque TM is a quadratic function of the turbine speed ωM, as shown in Eq. (16.47), the maximum power can also be achieved with optimal torque control.
Assuming that the mechanical power losses of the gearbox and drive train are neglected, the turbine mechanical torque TM and speed ωM can be easily converted for a given gear ratio to the generator mechanical torque Tm and speed ωM, respectively. A simplified block diagram for the control of optimal-generated torque is shown in Figure 5b.
The generator speed ωm is used to compute the desired torque reference Tm* , which is compared to the output torque of the generator Tm to produce the gate control signals of the power converters. Under steady-state conditions, the mechanical torque Tm of the generator will be equal to its reference torque Tm* .