Windmill – What is it? How do They Work?
Wind power has been around for centuries and humans have been taking advantage of it for almost as long. Windmills actually date all the way back to the eighth century. The most popular and noticeable design was that of the Dutch. A few are still found around today. Back then, a windmill was used to grind grain into flour, among other uses. Today, wind energy is making a huge comeback. The way it is harnessed is very similar. However, the wooden, four blade windmills have been replaced with towering, metal, three blade wind turbines. These are capable of converting the wind into usable energy for anyone and anything.
Even though that the most popular design is that of the Dutch windmill, they actually pre-date the Dutch. Of course, the exact date of the very first windmill design is unknown. They may have been used in China as long ago as 2000 years, however, the earliest record in China is 1219 A.D. The very first recorded design in history originated in Persia around 500-900 A.D. It was originally made to pump water and was later adopted to grind grain. The first design was very inefficient. It was made using vertical sails bound by lightweight wood and attached to a vertical shaft by horizontal struts. That design was known as panemore.
After the crusades, the design found its way to Europe. The earliest documented designs date back to 1270 A.D. These had a horizontal axis instead of vertical ones like the panemore. The reasoning behind this design might have come from the fact that European windmills were probably patterned after the water wheel design which had the horizontal axis. Of course, the water wheel was in Europe well before the windmill. The other reason was that the horizontal axis actually made the windmill more efficient. The way this design looked was it had four blades on a central post. A cog and a ring gear that would convert the horizontal motion of the central shaft to a vertical motion for the grindstone or wheel, depending on what the windmill’s used for.
This technology improved immensely over the years. The most innovation came from the Dutch and the English. The most important improvement was the creation of the tower mill. This design allows for the windmill’s blades to be adjusted to the wind as needed and the main body remains fixed in one place. The Dutch introduce tower mills with multiple levels inside. This way, the people who worked the mill were able to sleep and essentially live. The English made a number of automatic controls that made them even more efficient. During the industrial revolution, Europe adopted the windmill to power a great number of different things. Some of these new applications include saw mills, processing spices, dyes, and also tobacco.
America perfected the windmill design in the 19th century. The Halladay windmill, made in 1864 by the Aermotor and Dempster designs, had over six million made between 1850 and 1970. They are also still in use today.
How Do Windmills Work
How the old windmills work is a bit different from how wind turbines work today. However, both were very good at harnessing the energy of the wind, for their times.
Water Pump Windmill Process
- Wind turns the blades atop the windmill.
- As the blades turn, they turn a set of gears called the motor.
- The motor pulls a pump rod up and down.
- The pump rod controls a piston in a cylinder pump in the well. The piston contains one or more valves.
- As the piston goes down, the valve opens to allow the piston to pass through a water column held by another lower valve.
- When the piston goes back up, the valve closes to stop the water from flowing backward as the piston pulls the column up the pipe.
- At the same time, the lower valve opens to allow water to come into the vacuum made by the upward motion of the piston. This is the new water column.
- The cycle repeats, again and again, working the water up the pipe until it flows into a tank.
Modern Windmill Process
Modern wind turbines act a bit differently than their ancestors. These turbines generate electricity and distribute it where the need is.
- Wind blows toward the turbine’s rotor blades.
- The blades spin around, capturing some of the kinetic energy from the wind. This turns the central drive shaft supporting them.
- Most modern turbines can move at the hub so they meet the wind at the best angle (pitch) for harvesting energy. On large turbines, small electric motors or hydraulic rams move the blades back and forth under precise control. With smaller ones, the pitch control is usually natural. However, many turbines do have fixed rotors and no pitch control at all.
- Inside the nacelle, which is the main body of the turbine, behind the blades is the gear box. This converts the low-speed rotation of the drive shaft to high-speed rotations, fast enough to drive the generator efficiently. The low speed is usually around 16 rpm while the high speed is 1600 rpm.
- The generator takes the kinetic energy from the spinning of the drive shaft and turns it into electrical energy. When running at maximum capacity, a standard turbine generator will produce 2 million watts of power at 700 volts.
- Anemometers, which are automatic speed measuring devices, along with wind vanes on the back of the turbine, provide wind measurements like power and direction.
- As said in #3, the top of the turbine can rotate by a yaw motor, which is between the main body and the tower. This is done using the measurements from the anemometers so the turbine can catch the direct wind and captures the most energy it can. If the weather is bad and the wind is too strong, there are brakes that can stop the rotors from turning. Strong winds can damage the blades and the turbine as a whole.
- The electricity created by the generator flows through a cable which runs down the inside of the tower.
- A transformer then converts the electricity from the turbine to around 50 times higher voltage so it can be transmitted efficiently to the power grid, or nearby buildings or communities. If the power is going to the grid, a nearby sub station converts it to a higher voltage of 130,000 volts or more. These substations service multiple turbines in the same area.
Modern Wind Windmills
Today’s windmills look and work much different than the classic Dutch windmills. However, the function of these windmills is essentially the same. To harness the energy from the wind. Of course, there is a lot more that goes into getting that energy now than there was. And we get more out of wind power as well.
Wind turbines sit 100 feet or more above the ground. This gives the blades a better advantage of catching faster winds at higher altitudes. The movement of the blades of the turbines also is very unique. When the wind blows, a pocket of low pressure air forms on one side of the blade. This low pressure air pocket pulls the blade (instead of the common misconception of wind pushing the blades.) This is called lift. The power of lift is more strong than the wind’s force against the blade, which is called drag.
Types of Turbines
There two types of windmill turbines. Both of these incorporate the axis style of the original designs, respectively. Most of the turbines use the horizontal axis design. This being a propeller style design with blades that rotate around a horizontal axis. These designs can either be upwind style or downwind. Upwind is when the wind hits the blades before the tower. Downwind is when the wind hits the tower before the blades. Upwind style windmills include the yaw drive and motor which turns the nacelle to keep the turbine facing the wind.
The second type uses a vertical axis. Though they have not really hit the utility scale market to the same degree as the horizontal axis turbines, there are a few companies that manufacture them. Vertical axis style turbines fall into two styles. The first is a drag type, also known as Savonius. These turbines usually have rotors with solid vanes that rotate on a vertical axis. The second design is a lift type or Darrieus. These turbines have a tall, vertical airfoil style. Basically look like giant eggbeaters. The Windspire is a lift type turbine that is undergoing independent testing.
The blade design of today’s wind turbines is extremely different than the classic Dutch windmills. This is because they act differently. The blade design is like that of airplane wings. The rear side is much more curved than the front. When talking about a wing, the air moves much faster over the top of the wing which reduces the pressure and creates the lift needed for the airplane to actually fly. The new design is exactly for that reason. As the wind begins to blow and pass over the blade, the air behind the blade starts traveling faster than that in the front. This is why the blades are pulled rather than pushed. This design is extremely aerodynamic and allows the blades to create their own headwind. This increases the lift and causes the blades to spin faster and faster until they reach their maximum velocity.
Why Three Blades
One of the most notable design changes from the Dutch windmills is the reduction from the iconic four blades to only three blades. This reason is that of efficiency. Most people will probably think that more blades mean that the turbine will be able to catch more wind and create more electricity. This is not true. The more blades there are, the higher the torque there will be. Torque is the force that creates the rotation. The higher the torque means a slower rotation speed. This is because the increase of drag caused by wind flow resistance. However, the turbines need to operate at very high speeds to generate electricity and actually do not need a lot of torque. This means that the fewer blades there are, the better the system will be for generating power.
One, No. Two, No. Three, Yes
With that information, one bladed turbines are the most aerodynamically efficient design, in theory. Unfortunately, it is not very stable because the one blade would constantly unbalance the turbine. The same goes for a two blade design. This design actually causes a wobbling phenomenon like that of a gyroscopic precession.
Turbines have to be able to turn to face the wind. Said before as yawing. With the two blades set up in a vertical design, there is very little resistance to the yawing motion. However, when they in the standard horizontal design, they have a lot of resistance to the yawing motion because they sit further away from the axis. This makes the yawing motion stop and start twice per rotation and creates a lot of stress on the turbine itself.
Therefore, three blades create a perfect balance between efficiency and aerodynamics. When one blade is in the horizontal position, the other two blades assists to lower the resistance to the yaw force. Hence, high rotation speed and low stress.
Wind Power Pros and Cons
Since wind power has been harnessed by windmills for centuries, it only makes sense that we continue to develop and improve on the technology to take advantage of it. In Ireland, there are 11 wind farms that are operational. Their combined effort generates 68 megawatts of electricity. That is enough to power over 44,000 homes.
There are a lot of advantages of using wind power as a renewable energy source. The biggest advantage is the fact that there is basically no water or air pollution as it requires no fuel to create the electricity. Farmers who have windmills on their property can receive compensation from any electricity generated. The biggest issue today is money. Fortunately, wind farms are fairly cheap to build as the materials are readily available, unlike solar panels which require some exotic materials.
However, the disadvantages of windmills are basically the exact same issues that people have had, probably for centuries. There needs to be wind for windmills to work. No wind equals no power. Surely, the owners of older windmills had to deal with the same problems, but it was probably a much more significant problem for them. Another problem has to do with our sense of aesthetic. Wind turbines can cause “visual” pollution as they are huge and white and can obstruct the skyline. The last issue is that these wind farms require a lot of land as there needs to be a few windmills for the electricity generation to really count.