Design Considerations
From Thermal-FluidsPedia
There are two types of wind turbines, horizontal and vertical. In horizontal turbines, the axis of the rotor is parallel to the ground, whereas in vertical turbines, the axis of the rotor is perpendicular to the ground. Horizontal turbines are usually of a propeller type, while vertical turbines have C-shaped blades and look like eggbeaters. These turbines (called Darrieus machines) are made with 2-3 blades and are designed to capture wind from any direction; they have the advantage that the heavy components (the gearbox, generator, and controllers) can be placed on the ground where wind is weaker. Because these turbines are mounted close to the ground, wind speeds are relatively low so they are generally less efficient than horizontal-axis machines. As a result, most modern wind turbines are of the horizontal type.
Turbines can also be designed such that either the front side (upwind) or rear side (downwind) of the rotor faces the wind. Most commercial turbines use an upwind design. The major advantage of upwind turbines is that there are no wake losses. The primary advantage of downwind machines is that the blades can be made from cheaper and more flexible materials because there is no danger of them hitting the tower. The main disadvantage of downwind turbines is that wind speed suddenly drops behind the tower, so there is additional wind shade where the blade passes behind the tower which causes vibration of the rotor and results in more fatigue.
How many blades are optimal? Theoretically a wind turbine needs only one blade, but most have two and a few have three or more. The greater the number of blades, the more stable the turbines are, but also the heavier and more expensive. Multi-bladed rotors used in older designs are less efficient, but provide more torque and have therefore been used traditionally for pumping water even at low wind speeds. Two- and three-bladed rotors are cheaper, lighter, and run at faster speeds; however the increased tip speed can also make them noisier. They also have moderate starting torque which makes their operation difficult at very low wind speeds. One especially important factor in two-bladed wind turbines is wind shear. Because air has a greater velocity further from the ground, wind puts greater force on the top blade than the bottom blade. To correct this, wind turbines are equipped with a teetering mechanism, which equalizes the forces by allowing the blades to tilt slightly around their central pin. In some other designs, blades have a fixed pitch so instead of pivoting at the hub, these turbines flex as wind speed picks up. To guard against over-speeding, each blade has a small tip brake that at high speeds tips into the wind, stalling the turbine. Figure 1 compares performance of various rotor designs at different wind speeds.
Question: Windmills traditionally used for pumping water have small solid rotors with many blades. What is the main advantage of these designs?
Answer: The many blades assure operation at very slow wind speeds, allowing continuous operation all year. These windmills are, however, very inefficient at high speeds and must be shut down to prevent damage.
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Size
The size of wind turbines varies greatly and, depending on application, they produce power from a few watts to many megawatts. Micro turbines produce power in the range of 20-500 watts and are used in such applications as battery chargers and recreational vehicles. Turbines generating less than 50 kW are considered small, those producing between 50 kW and 1 MW are medium-sized, and turbines producing above 1 MW are large (Figure 2). Small turbines are used in applications such as pumping water, whereas medium and large wind turbines are used for producing electricity.
For larger metropolitan areas, either a large number of small turbines or a few large turbines must be installed. There are advantages and disadvantages to each option. The choice of using small or large rotors and generators depends on the application and on the distribution of wind energy throughout the year. For the same total capacity, larger turbines occupy less space and are cheaper to install. For example, a 400-kW turbine costs considerably less than four 100-kW turbines. The cost of delivering electricity is also lower for larger turbines than smaller ones. In addition, larger turbines can utilize the energy contained in high-speed winds and have greater efficiency. The drawback is that they cannot produce power at low speeds. The main advantage of small turbines is that they can produce continuous power for most of the year, as they require only slow to moderate wind speeds to operate. However, much of the energy in high-speed wind is wasted.
Taller towers are generally better than shorter towers, as wind velocity rapidly decreases near the ground. As shown earlier, the total energy in any wind stream is proportional to the cube of the wind speed, so turbines installed on taller towers could, in principle, be much more efficient. The cost of tower construction, however, could be much higher and may not justify the extra gain in efficiency. As a rule of thumb, tower heights are roughly equal to rotor diameter and cost one-fifth of the total price of the turbine. Stand-alone turbines are usually more economical, less complicated, and require less maintenance. They are used mainly in remote areas and small cities where local electrical grids are not able to support the power generation from larger wind turbines.
Power Control
To prevent failure, the system must have safety features that control sudden increases in power. Modern wind turbines are equipped with mechanisms that automatically adjust the rotor speed as wind speed and direction change. This can be done by controlling the pitch, controlling the angle of yaw, or initiating a stall.
The best way to control the power is by changing the blades’ pitch. Pitch is the angle that the airfoil chord makes with the rotor plane of rotation (also called the angle of incidence). With variable pitch designs, bearings are inserted between the blades and rotor hub. When the wind speed becomes too great, rotor blades are turned along their longitudinal axis and slightly out of the wind to allow some wind to pass by without increasing lift. At low wind speeds, blades are turned into the wind. The advantage of a pitch-controlled scheme is that relatively constant rotor speeds can be maintained within a large range of wind speeds.
Another method of controlling rotor speed is to turn the rotor out of the wind by adjusting its yaw angle and tilting it either into or away from the direction of the wind. To optimize efficiency, turbine blades must face the wind at all times (they must be in a plane perpendicular to the direction of the wind). A large wheel called a yaw bearing turns the nacelle with the rotor into the wind. At very high speeds, the rotor is intentionally turned away from the direction of the trailing wind, reducing the total volume of air passing through the rotor and reducing the power. Since yawing bends the rotor, turbines running with yaw- or tilt-control experience large fatigue loads.
In stall-controlled wind turbines, blades are designed to ensure stall when wind velocity reaches a set value. The blades are shaped such that excess wind speed creates sufficient turbulence on the backside and the rotor stalls. The basic advantage of stall control is that the rotors remain fixed to the hub, which removes many of the complex control-mechanisms necessary in pitch and yaw control methods. Most large wind turbines use this approach.
Generator
In order to generate electricity, the wind turbine must be coupled with a generator. Most generators are directly connected to the grid and produce power at a nearly constant frequency; they are therefore constrained to operate within a narrow range of wind speeds. Sudden gusts of wind can speed up the rotation, so the drive train and tower must absorb the extra torque. Normally, generators produce 690 V, three phase alternating currents at frequencies of 60 (for the US) or 50 (for most other places in the world) hertz. The current is amplified through transformers in order to increase the voltage to the 10-30 kV used in most local transmission lines.
In indirect grid connections, wind turbines operate independently of the generator and the system operates in a variable speed mode determined instantaneously by wind speed. This setup has the advantage of being able to run a generator at all wind speeds. The cost is, however, considerably higher. In this case, voltage from the generator must be modified to match that of the neighboring electrical power grid. This must be done in several steps. First, the variable frequency alternating current must be converted into a constant frequency alternating current. Next, the alternating current must be converted to direct current using a rectifier. Finally, using a DC-AC inverter, the direct current must be converted to an alternating current at a frequency matching the grid line. Some filtering may also be required to eliminate unwanted frequencies, thus producing clean electric power with the desired frequency.
Power Train
The power generated by the wind turbine must be transferred to a generator. If the generator and turbine rotor are connected directly, then the rotor must turn with the same rotational speed as the generator. For a 50 Hz generator, this translates to 3,000 rpm. For a large diameter rotor, the tip speed will be extremely high, reaching several times the speed of sound. This causes an extreme torsional load and an excessive amount of noise which is not acceptable in populated areas. Rotational speed of the generator can be lowered in several ways.
The first option is to use a slow-moving AC generator with a large number of poles. For example, if the number of magnets in the stator of a synchronized generator is doubled, the magnetic field must rotate only half a revolution before it changes direction and the generator will run at 25 Hz (or 1,500 rpm). In theory, we can keep increasing the number of poles until the generator revolves at the same speed as the wind turbine rotor. The more practical approach is to use a gearbox. A gearbox is a device that converts slow-speed, high-torque power from the turbine into high-speed, low-torque power required to run the generator. An alternative approach is to use direct-drive generators that can operate at the rotational speed of the rotor, thus eliminating the need for a gearbox.
Hybrid Systems
According to World Bank estimates, as many as two billion people, 40% of the world’s population, live in villages that are not tied to a utility grid. For these villages, a hybrid of energy sources including wind, solar, and diesel-powered generators would be most suitable. In the United States, a typical stand-alone hybrid system involving photovoltaic cells and wind is advantageous over either system by itself because it takes advantage of longer and brighter sunlight in summers when wind speeds are lower. In winters, the opposite is true; winds are strong when there is less sunlight. During periods of peak demand or when neither wind nor sunlight is sufficiently available, an auxiliary diesel can produce the additional energy required. In either case, to assure power is available at nights or during periods of low winds, a battery storage system is needed (Figure 3a).
When grid connection is available, it is possible to sell extra power to the utility companies and buy back electricity when demand exceeds the capacity of the system. In this case, there is no need to store the electricity in batteries or have an auxiliary diesel power generating plant (See Figure 3b).
Safety
To make wind energy economically viable, wind turbines must endure a wide range of hazardous conditions and have a long lifetime. Modern designs deploy various sensors that protect turbines, gearboxes, and generators against excess vibration, overheating, and high speed wind gusts.
Depending on wind speed and direction, the wind turbine produces fluctuating torque and varying forces on the blades, causing the rotor and tower to swing back and forth. The blades are made to be flexible and are able to vibrate. The frequency of oscillation depends on the height of the tower and on the material and weight of the rotor and nacelle. If the rotor spins at a synchronized speed with other vibrational frequencies, the oscillation can amplify and the tower could sway out of control. Wind turbines are often equipped with a controller system which starts the turbine when wind reaches a certain velocity (around 3-6 m/s) and shuts the machine off when wind velocity exceeds 20-30 m/s. Most wind turbines are designed to have a maximum output at wind speeds around 15 m/s. Higher wind speeds are rare, and lower wind speeds cannot produce sufficient power.
Noise can also be a problem. There are two sources of noise: mechanical and aerodynamic. Major sources of mechanical noise are the gearbox, drive shaft, and turbine blades. The primary source of aerodynamic noise is the air flow from the trailing edges of the blades.
One of the interesting features of electric motors and generators is that they require some electrical load to operate. If they overheat or a load is removed, the rotors will accelerate out of control and the devices will fail. To avoid overheating, most generators are equipped with either air or water cooling systems. To protect them from overspeeding and accidental disconnection from electrical grids, the common practice is aerodynamic braking, in which rotor blades are rotated 90 degrees along their longitudinal axis. Once the danger is over, a built-in hydraulic system returns the blades to their original orientation.
In addition to the control strategies described above, large turbines are equipped with complex, computer-operated control systems. These monitor environmental parameters (such as wind speed, direction, atmospheric temperature, and pressure), operating parameters (rotational speed of the rotor, torque, and the yaw angle), and a large number of devices such as pumps, actuators, and valves.
Siting
One of the important considerations in the design of any wind power generation station is its location. In general, wind generators must be installed in areas with an open view and follow the altitude contours of the prevailing winds. Accessibility to the site is also important. The site must be chosen so that the wind is mostly clear of obstacles, such as trees and tall buildings. Pattern and distances between wind turbines must be chosen so that the wake of one turbine does not interfere with the operation of adjacent turbines. The practical guideline is that turbines facing the prevailing wind direction should be spaced between 5 and 9 rotor diameters apart; turbines facing perpendicularly to prevailing winds should be from 3 to 5 rotor diameters apart. (1) Hilltops have the added advantage that they can pick up drafts, and the wind speeds are generally higher. Valleys are also suitable because tunnel effects result in higher wind speeds in valleys than those found in open spaces. When wind energy is chosen to service a small community or a local region, a site must be selected near a 10-30 kV power line; otherwise, the additional cost of extending the power grid can be prohibitive.
Because of their appearance, noise, and their potential adverse effects on the price of surrounding properties, wind farms can be objectionable to nearby communities. Modern planning procedures and sensitive site selection can help to minimize visual impact. Turbines can be obscured from view by planting trees or constructing other, similar screens. Some consider large wind turbines more aesthetically pleasing than smaller ones because they generally have lower rotational velocities. The selection of certain turbine colors, structures, and layouts can also help to minimize intrusiveness. Urban architects and city planners need to study these effects and design sites to match the local landscape or provide interesting additions to the nearby structures (Figures 4 and 5). (2)
Offshore Wind Farms
Wind turbines do not always need to be placed on land and, if they are installed offshore, may actually benefit from generally cooler and smoother lake and sea surfaces. Furthermore, because noise is not as much of a concern as with onshore facilities, turbines can be designed to operate at higher rotational speeds. As a result, a 20% greater efficiency is achieved at offshore wind farms (Figure 6). The main disadvantages of offshore wind farms are potential interferences with shipping routes and the additional cost of undersea cabling needed to connect the generator to the main electrical grid.
References
(1) Danish Wind Industry Association, (http://www.windpower.org).
(2) Palmer, A., “Towers of Power,” Popular Science, Vol 259, Iss 6, Dec 2001.
(3) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Further Reading
Gipe P., Wind Energy Basics, —A comprehensive guide to modern small wind technology. AWEA (http://www.awea.org).
Elliott, D. et al., Wind Energy Resource Atlas of the United States, by American Wind Energy Association (http://rredc.nrel.gov/wind/pubs/atlas).
Khennas, S., Small wind systems for rural energy services, London: ITDG Pub., 2003.
Solar Energy, Direct Science Elsevier Publishing Company, the official journal of the International Solar Energy Society ®, is devoted to the science and technology of solar energy applications, and includes the indirect uses such as wind energy and biomass.
Home Power Magazine—bimonthly magazine for farm and home wind turbines (http://www.homepower.com).
External Links
Energy Efficiency and Renewable Energy Clearinghouse (http://www.eren.doe.gov).
National Wind Technology Center, National Renewable Energy Laboratory (http://www.nrel.gov/wind).
Energy Efficiency and Renewable Energy: Wind Energy Technologies, US DOE (http://www.eere.energy.gov/windandhydro/wind_technologies.html).
American Wind Energy Association (http://www.awea.org).