SOLID Learning Project
The intent of SOLID Learning is to integrate rapid prototyping and direct digital manufacturing technologies into the educational setting to provide educators and students with resources available in a downloadable format that can be created directly into a physical form using whatever type of 3D printing systems schools have available.
- Introduction
- Lessons
- Object Files
- Self-Replication in SOLID Learning
- Teacher Input
- Why the MakerBot?
Wind Turbines
Converting kinetic energy into electrical energy. Kinetic energy of wind turns the blades which drives a generator that converts the kinetic energy to electromagnetic energy.The current is used to do work, while voltage (the resulting force after overcoming resistance) is the energy required to drive the flow of the current.
A Turbine
The basic concept is simple. A framework or support column holds up a large wheel or turbine blades that looks like a fan or airplane propeller. The angled blades catch passing winds and deflect some of the wind's energy, which pushes the blades around, turning a central shaft. The shaft can be connected via gears or belts to machinery on the ground or in a nearby structure. Alternatively, the shaft can be connected directly to a generator that produces electricity, which is then transmitted.Blades
Blades of a windmill spin because of two principles:- Newton's Third Law—For every action there is an equal and opposite reaction. So when the wind hits the blade, the blade is pushed. If the blade is at a certain angle, the wind is deflected at an opposite angle, which pushes the blades
away from the deflected wind. You can see this in action with the flat blade. If you push the blade it will will move in the direction away from your finger.
- The Bernoulli Effect—Faster moving air has lower pressure.
Wind turbines are cambered so that the air molecules moving around the blade travel faster on the downwind side than on the upwind side. This shape is like a teardrop. The downwind side is curved, while the upwind side is almost flat.
Air moves faster on the curved, downwind side of the blade so there is less pressure on this side of the blade. The difference in pressure on the other side of the blade causes the blade to be lifted toward the curve of the airfoil.
Experiment:
- Take two pieces of paper
- Fold them in half
- Unfold them and place them together so that the folds you made line up but the creases should be on the outside
- Blow between the papers
- Did it match your expectations
The speed of the air is higher between the two pieces of paper than outside the papers. Higher velocity leads to a lower pressure between the sheets.
How blades capture wind power
Wind turbine blades work by generating lift with their shape. The more curved side generates low air pressures while high pressure air pushes on the other side of the airfoil. The net result is a lift force perpendicular to the direction of flow of the air.
Lift and Drag Vectors from WE Handbook- 2- Aerodynamics and Loads
The lift force increases as the blade is turned to present itself at a greater angle to the wind. This is called the angle of attack. At large angles of attack the blade stalls and the lift decreases again. When it comes to generating the maximum lift, there is an optimum angle of attack.
Another force also exists. This is called drag. This force is parallel to the wind flow, and also increases with angle of attack. If the airfoil shape is good, the lift force is much bigger than the drag. But at very high angles of attack, the drag will increase dramatically. When the angle is slightly less than the maximum lift angle, the blade will reach its maximum lift/drag ratio. The best operating point will be between these two angles.
The blade's own movement through the air means that the wind is blowing from a different angle. This is called apparent wind. The apparent wind is stronger than the true wind but its angle is less favorable: it rotates the angles of the lift and drag to reduce the effect of lift force pulling the blade round and increases the effect of drag slowing it down. So, to maintain a good angle of attack, your blade must be turned further from the true wind angle.
The closer to the tip of the blade you get, the faster the blade is moving through the air and the greater the apparent wind angle is. Therefore the blade needs to be turned further at the tips than at the root. Your blade must be built with a twist along its length. Typically the twist is around 10-20° from root to tip.
In general the best lift/drag characteristics are obtained by a blade that is fairly thin.
Length: The blade length determines how much wind power can be captured.
Aerodynamic Section: The blades have an aerodynamic profile in their cross section to create lift and rotate the turbine.
Planform Shape: The planform shape gets narrower towards the tip of the blade to maintain a constant slowing effect across the swept area. This ensures that none of the air leaves the turbine too slowly (causing turbulence), yet none is allowed to pass through too fast (which would represent wasted energy).
Airfoil Thickness: The thickness increases towards the root to take the structural loads, in particular the bending moments. If loads weren't important then the section thickness/chord ratio would be about 10-15% along the whole length.
Blade Twist: To maintain optimum angle of attack of the blade section to the wind, it must be twisted along its length.
Blade Number and Rotational Speed: Typically three blades.
Pitch Control: Because the wind power varies so greatly (with the cube of wind speed), the turbine must be able to generate power in light winds and withstand the loads in much stronger winds. Therefore, above the optimum wind speed, the blades are typically pitched either into the wind (feathering) or away from the wind (active stall) to reduce the generated power and regulate the loads.
