We know that for windmills to operate there must be wind, but how do they work? Actually there are two types of windmills — the horizontal axis windmills and the vertical axis windmills. The horizontal axis windmills have a horizontal rotor much like the classic Dutch four-arm windmill. The horizontal axis windmills primarily rely on lift from the wind. As stated in Bernoulli’s Principle, “a fluid will travel from an area of higher pressure to an area of lower pressure.” It also states, “as the velocity of a fluid increases, its density decreases.” Based upon this principle, horizontal axis windmill blades have been designed much like the wings of an airplane, with a curved top. This design increases the velocity of the air on top of the blade thus decreasing its density and causing the air on the bottom of the blade to go towards the top … creating lift. The blades are angled on the axis as to utilize the lift in the rotation. The blades on modern wind turbines are designed for maximum lift and minimal drag.
There are many types of windmills, such as: the tower mill, sock mill, sail windmill, water pump, spring mill, multi-blade, Darrieus, savonis, cyclo-turbine, and the classic four-arm windmill. All of the above windmills have their advantages. Some windmills, like the sail windmill, are relatively slow moving, have a low tip speed ratio and are not very energy efficient compared to the cyclo-turbine, but are much cheaper and money is the great equalizer.
There have been many improvements to the windmill over the years. Windmills have been equipped with air breaks, to control speed in strong winds. Some vertical axis windmills have even been equipped with hinged blades to avoid the stresses at high wind speeds. Some windmills, like the cyclo-turbine, have been equipped with a vane that senses wind direction and causes the rotor to rotate into the wind. Wind turbine generators have been equipped with gearboxes to control [shaft] speeds. Wind turbines have also been equipped with generators which convert shaft power into electrical power. Many of the sails on windmills have also been replaced with propeller-like airfoils. Some windmills can also stall in the wind to control wind speed. But above all of these improvements, the most important improvement to the windmill was made in 1745 when the fantail was invented. The fantail automatically rotates the sails into the wind.
Another necessity for a windmill is the tower. There are many types of towers. Some towers have guy wire to support them and others don’t. The towers without guy wires are called freestanding towers. Something to take into consideration about a tower is that it must support the weight of the windmill along with the weight of the tower. Towers are also subject to drag. Scientists estimate that, by the 21st Century, ten percent of the world’s electricity will come from windmills.
If the efficiency of a wind turbine is increased, then more power can be generated thus decreasing the need for expensive power generators that cause pollution. This would also reduce the cost of power for the common people. The wind is literally there for the taking and doesn’t cost any money. Power can be generated and stored by a wind turbine with little or no pollution. If the efficiency of the common wind turbine is improved and widespread, the common people can cut back on their power costs immensely.
Ever since the Seventh Century people have been utilizing the wind to make their lives easier. The whole concept of windmills originated in Persia. The Persians originally used the wind to irrigate farm land, crush grain and milling. This is probably where the term windmill came from. Since the widespread use of windmills in Europe, during the Twelfth Century, some areas such as the Netherlands have prospered from creating vast wind farms. The first windmills, however, were not very reliable or energy efficient. Only half the sail rotation was utilized. They were usually slow and had a low tip speed ratio but were useful for torque.
Since its creation, man has constantly tried to improve the windmill. As a result, over the years, the number of blades on windmills has decreased. Most modern windmills have 5-6 blades while past windmills have had 4~8 blades. Past windmill also had to be manually directed into the wind, while modern windmills can be automatically turned into the wind. The sail design and materials used to create them have also changed over the years.
In most cases the altitude of the rotor is directly proportional to its efficiency. As a matter of fact, a modern wind turbine should be at least twenty feet above and three hundred feet away from an obstruction, though it is even more ideal for it to be thirty feet above and five hundred feet away from any obstruction. Different locations have various wind speeds. Some places, such as the British Isles, have few inhabitants because of high wind speeds, yet they are ideal for wind generation. Did you know that the world’s largest wind farm is located in California, and the total wind power generated there exceeds 1,400 megawatts of electricity? (A typical nuclear power plant generates 1,000 megawatts.)
SPECIFICATIONS OF THE WIND TURBINE
Angle 45 °
Angle b/w blades 60°
Hole Diameter 2.54cm
Total Height of the assembly 198cm
Center Distance of Pulley 33.5cm
• It is a renewable source of energy.
• Wind power systems are non-polluting so it has no adverse influence on the environment.
• Wind energy systems avoid fuel provision and transport.
• On a small scale up to a few kilowatt system is less costly.
• On a large scale costs can be competitive conventional electricity and lower costs could be achieved by mass production.
• They are always facing the wind – no need for steering into the wind.
• Have greater surface area for energy capture -can be many times greater.
• Are more efficient in gusty winds – already facing the gust.
• Can be installed in more locations – on roofs, along highways, in parking lots.
• Can be scaled more easily – from mill watts to megawatts.
• Can have low maintenance downtime – mechanisms at or near ground level.
• Produce less noise – low speed means less noise
• The rotor can take wind from every direction.
Disadvantages of vertical axis wind turbines
1) Some disadvantages that the VAWTs possess are that they have a tendency to stall under gusty winds. VAWTs have very low starting torque, as well as dynamic stability problems. The VAWTs are sensitive to off-design conditions and have a low installation height limiting to operation to lower wind speed environments.
2) The blades of a VAWT are prone to fatigue as the blade spins around the central axis. The vertically oriented blades used in early models twisted and bent as they rotated in the wind. This caused the blades to flex and crack. Over time the blades broke apart and sometimes leading to catastrophic failure.
3) Research programmers’ (in 2011) have sought to overcome the inefficiencies associated with VAWTs by reconfiguration of turbine placement within wind farms. It is thought that, despite the lower wind-speed environment at low elevations, “the scaling of the physical forces involved predicts that [VAWT] wind.
Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.