Turbine tips

• Fans - $10-$20 box fans work great, but the more you spend the more you get! Hair dryers will not work!
• Materials - Tinker-Toys, k'nex, legos, scrap wood, PVC pipes and fittings, popsicle sticks, poster board, fabric, building "junk", generators, balsa wood etc...
• Tools - hammers, drill, saw, craft knifes, glue guns‚ multimeters‚ wire strippers, solder, soldering iron, wire‚ safety goggles.

Other neat testing materials (not required):

• Handheld wind speed meter (Kestrel or InSpeed)
• Computers for graphing, simulations, etc.
• Small hand-held tachometer to measure blade rotational speeds.
• Camera for photos

You will need some wind to test your turbine. You can use a fan or you can go outside. Both have their advantages and disadvantages.

Inside Using Fans?

Fans are great for testing turbines inside. Some things to note about fan wind that reduce the efficiency.

• Highly Turbulent & Rotational Wind - Turbine Blades may spin better in one direction rather than another
• Highly Variable Wind Speed - Wind speed is about 10-13 MPH on high for a $20 circular fan. Wind speeds near the middle will be much different than the edges.
• Limited Diameter - Blades bigger than the fan will not "catch" more wind; they will just add drag and slow down the blades.

If you want to see the performance of lots of different fans, download our Fan Performance Cheat Sheet. Basically the more expensive the fan, the faster the wind will move.

Want to see the ultimate indoor wind source? check out the KidWind Challenge Wind Tunnel!

Testing Area

Safely set up your testing area like the picture below. It is important to clear this area of debris and materials.

It is important to wear safety goggles when testing blades. NEVER make blades out of metal or any sharp edged material, as these could cause injury while spinning fast.

Going Outside?

While you can use your wind turbine outside, you must make sure that you face the turbine into the wind. This is because if your turbine is not designed to YAW (rotate), wind will hit the blades from the sides, causing stress and inefficiency.

How to Clean Up Wind?

Want "professional wind"? You can try to build a simple wind tunnel. Lots of plans can be found online (search term:classroom wind tunnel). One simple way to make more laminar‚ smooth, straightened‚ flow is to build a honeycomb in front of your fan using milk cartons, 2" PVC pipe or some other material. While it will straighten your wind it will also slow it down.

Most real wind turbines use one of three types of towers:

• Lattice Towers - This tower design uses a criss-crossing steel framework. These are very strong, self-supporting towers. They can be quite expensive due to the amount of steel they require.
• Guyed Towers - These towers have a single pole or lattice structure supported by "guy wires" which anchor the tower to the ground. Another tower type commonly used for small wind turbines, Tilt-up Towers, also have guy wires and fall under this category.
• Monopole Towers - A single, freestanding pole. Very strong, and very expensive. These require a crane to erect, and are used for large wind turbines. Monopole towers are also considered the most visually pleasing.

Creating Your Own Tower

You can make a tower for your wind turbine out of practically anything. Check out our plans on how to make a great tower using PVC pipe, but don't limit yourself! We have seen some great towers made from wood, cardboard tubes, Tinker-Toys, plastic, etc. Try experimenting with different designs! Which type of tower seems strongest? Why do you think certain wind turbines use the type of towers they use?

The only rules for making your tower is that it must have a firm base resting securely on the ground. It must also be tall enough so that your blades will not hit the ground.

If your turbine has a gear or pulley system, you will need to have some kind of platform or housing on top of your tower to hold the gear/pulley box.

Other things you'll want to think about: does your tower sway when the wind moves fast, or are your blades are out of balance? In really fast winds with large blades, you should have a weight on the base. Otherwise the turbine will tip over. This is why large turbines have huge foundations.

A generator is a device that converts mechanical energy into electrical energy. In 1831, Michael Faraday discovered that while a magnet is moving inside a coil of wire, an electrical "voltage" is produced between the ends of that wire. This discovery, known as Faraday’s law, demonstrated that a relationship exists between electricity and magnetism.

A typical generator uses powerful magnets and many coils of copper wire. Faraday’s law states that a magnet moving within a coil of wire has the potential to cause electrons to flow in a circuit. It is also possible to move the coil of wire within a magnetic field to generate electricity. Most generators consist of a rotating shaft, to which coils or magnets are attached. In the case of a wind turbine, rotating blades designed to catch wind are fixed to this shaft. Using generators, we are able to capture mechanical energy and convert it to electrical energy, powering the electrical devices we use every day.

As you start to build your turbine, you will need to decide if you want to build an AC generator or use a DC generator.

DC Motors/Generators

The generator in many KidWind kits is actually a DC motor that spins using the energy in the wind. There are DC generators all over your house and school. We have spent a great deal of time looking for good small DC generators for a small turbine. You could compare our DC Generators to generators harvested from old household electronics, or to store-bought generators. Old VCRs, electrical toys and CD players are good places to start finding DC motors and gearboxes. LEGO motors are awesome! They crank out some great voltage, which can light lots of LED bulbs.

AC Generators

You could also use or build a simple AC generator in your turnine. KidWind has a kit called the GenPack that will allow you to explore AC generation by building your own generator. There are other companies that sell small AC generator kits as well. Build one, then try to use it as the main generator in your turbine.

When you start to build your turbine, you will have to make a decision on whether you not you want to make your turbine direct drive or use some kind of gear advantage to increase the RPM on the generator. Many residential scale turbines do not have gearboxes, and many utility scale turbines do have gearboxes, but that is starting to change.

While building a gearbox or a pulley drive can be challenging, it can greatly increase your power output. It is easier to build a pulley drive because you do not have to line things up as precisely. Gearboxes have less friction, but getting the teeth to line up and work well at high RPM can be difficult.

Pulleys or gears can give your wind turbine a mechanical advantage. This means that they multiply the mechanical force of the turning blades. This is done by using pulleys with different diameters or gears with different numbers of teeth. When the larger gear makes one full revolution, the smaller gear has to spin faster to keep up. Increasing the speed of rotation through the use of gears or pulleys is an example of a mechanical advantage.

Pulley/Gear Ratios

The gear ratio of your system will influence the power and torque you can produce with your turbine. "Gear ratio" is the relationship between the number of teeth on two gears that are meshed. When you are using a pulley system, this ratio is the relationship between the circumferences of the two pulleys you have connected.

Example: You have a geared system for your wind turbine. Your hub is connected to a driveshaft with a gear on it that has 40 teeth. That gear is meshed with a gear that has 12 teeth. The 12-tooth gear is connected to your motor (generator). The gear ratio of this system would be 40 divided by 12‚ or 3 1/3. That means that every time your hub and blades do a full rotation, the motor driveshaft does 3 1/3 rotations.

A pulley system works much the same way. If the circumference of the large pulley is 3 times that of the small pulley, the small pulley (attached to your generator) will rotate 3 times for every 1 time the large pulley (attached to your hub) rotates.

One thing to note: we find that on small box fans it can be hard to drive gear ratios of more than 10:1. With more wind and better blades you can drive larger gearboxes. At the KidWind Challenge, when we use our large wind tunnel, we have seen students with 100:1 gearboxes. Your small fan at home cannot do this!

Problems Building Gears & Pulleys

Lining up the gears or pulleys is very important. If you are using a pulley system, the two pulleys will not have to be lined up precisely. This is because the drive belt (rubber band) connecting them can run at a slight diagonal. If the drive belt is not lined up well, it may have a tendency to slip off one of the pulleys. While a pulley system can increase your voltage a great deal, you may find it difficult to get enough amperage for high-torque demands‚ such as water pumping. This is due to the belt slipping on the pulleys.

With a geared system, lining up the gears is extremely important. The gears need to mesh together just right, so take time to find a setup that works. You'll need to make sure that the driveshaft and pulley system is secure and will not move too much when the wind blows. If it does, your drive train won’t work.

Pulleys

• Advantages: High RPM of the generator, very high voltage, easy to assemble and line up
• Disadvantages: Bands will slip in high torque, low Amperage, rubber bands can slip off

Gears

• Advantages: High RPM of the generator , High Voltage ‚More Torque, Generates High Amperage
• Disadvantages: Difficult to assemble and line up the gears, Less voltage than pulley

The blades on modern turbines "capture" the wind and use it to rotate the drive shaft of a generator. As we have mentioned, the spinning shaft of the generator spins wires near magnets to generate electricity. How well you design and orient your blades can greatly impact how fast these blades spin and how much power your turbine produces.

As you build your turbine you will need to perform experiments with blades to see which ones work best on your turbine. These experiments can be simple or very complicated, depending on how deeply you want to explore. Some blade variables you can test include:

Length, Shape, Number, Materials, Pitch, Weight

One thing we note at many challenges is that students do not spend enough time working to make very efficient blades. The teams that spend time and make very good blades often win the competitions.

Making Great Turbine Blades

To make blades, carve or cut different shapes and sizes out of a variety of materials (balsa wood, cardboard, felt, fabric). Tape or hot glue them to the dowels. Students have made blades out of styrofoam bowls, pie pans, and paper and plastic cups. Anything you find around the house or classroom can be made into blades as long as it is stiff and kind of light!

MAJOR Caution!! Never make blades out of metal or any sharp edged material because they could cause injury during testing. Blades on these small turbines tend to spin very fast (300-600 RPM) and can easily cut people if they have sharp edges.

Sloppy, poorly made blades will never make enough electricity. It takes time and thought to make good blades. One thing you must always think about when making turbine blades is: “How much drag are my blades encountering?” Sure, your blades are probably catching the wind and helping to spin the hub and motor driveshaft, but could they be spinning faster with more torque? If they are adding drag, your whole system will slow down. In most cases, low RPM means less power output. The faster the blades spin, the more power you make!

Quick tips on improving blades

• Shorten blades: Many times, students make very long blades, thinking bigger is better. While that is sometimes true, beginners have a very hard time making long blades without adding drag. Try shortening them a few centimeters.
• Change the pitch: Often, students will set the angle of the blades to around 45° the first time they use the turbine. Try making the blades more perpendicular to the wind flow. Pitch dramatically affects power output. Play with it a bit and see what happens.
• Not Spinning? If you have your blades attached, and they are not spinning, check the pitch of the blades. Are your blades oriented in the same direction? Are they flat? Are the blades hitting the tower?
• Use fewer blades: To reduce drag, try using 2, 3, or 4 blades.
• Use lighter material: To reduce the weight of the blades, use less material or lighter material.
• Use stiffer material: If your blades are bending in the wind or deflecting when the wind hits, you need to find a stiffer material.
• Smooth surfaces: Smoother blade surfaces experience less drag. A blade with lots of tape and rough edges will have more drag.
• Get more wind: Make sure you are using a decently sized box or room fan, one with a diameter of at least 14”–18”.
• Blades vs. fan: Are your blades bigger than your fan? This could be a problem, as the tips of your blades are not catching any wind and are just adding drag.
• Blade shape: Are the blade tips thin and narrow or wide and heavy? The tips travel much faster than the roots. Wide tips add drag.

Advanced blades

Two major forces act on wind turbine blades as they rotate: lift and drag. These forces are in constant competition. When you are optimizing wind turbine blades, try to maximize lift force but minimize drag force.

Wind turbine blades are airfoil shaped, much like airplane wings. This airfoil shape is designed to generate lift and minimize turbulence. Lift is primarily produced as a result of the angle-of-attack of the blade. This angle creates a deflection force on the upwind side and a vacuum force on the downwind side of a wind turbine blade. The deflected air causes a reaction force that pushes the blade.

Turbine blades are tapered more at their tips and are also twisted slightly. Because of this twisted pitch, they have a greater angle-of-attack near their root where rotational velocity is slowest. Velocity is higher at the tip of the blade, so the angle-of-attack there is smaller. Turbine blades are designed in this manner to optimize the balance between lift and drag at all points on the blade.

Most electrical generating wind turbines use two or three blades. This configuration allows them to capture the most power with the least wind resistance. Using the fewest number of blades possible also reduces cost. The actual angle and taper of the blades depends on the anticipated wind speeds at the turbine’s location.

KidWind blades

Flat blades create a great deal of torque, and therefore work well for weight-lifting experiments. Airfoil blades have less drag and can generate more power. You can make more sophisticated blades by giving them twisted pitch and an airfoil shape.

Ideas for constructing advanced blades

• Bend card stock into an airfoil shape. Glue a dowel inside the blade. Tape bent card stock to a flat piece of corrugated plastic or balsa wood to produce an airfoil shape.
• Take a block of foam and form it into an airfoil shape. Try to incorporate both a taper and a twist into the design.
• Carve and sand a piece of soft wood into an airfoil.
• Cut blades out of some form of cylinder. Try a cardboard tube, a paper or plastic cup, etc.
• Soak card stock in water for a few minutes. Form it into the desired shape and clamp or tape it in place until it dries and holds that shape.

There are few ways we can examine turbine performance

• Power Output
• Energy Output
• Turbine Efficiency
• Tip Speed Ratio

In this section we offer you some guidance and tools to help you understand the performance of your small wind turbine.

Power and Energy Output

Before you can calculate your power and energy, you need to understand how to use your multimeter and what voltage, current and resistance are.

Using a multimeter, you can quantify the voltage and/or current your turbine is producing. Learning how to accurately measure the voltage and current for a range of situations will help you compare data when testing blades, comparing gearing, or changing any other variables on small turbines. You will also need this information if you want to calculate the power your turbine produces.

We have a great video series on understanding your multimeter. Watch all the episodes and you will be an expert!

What is voltage?

Voltage (measured in volts), is also called "potential difference" or "electromotive force" (EMF) and is a measure of the amount of “potential energy” available to make electricity flow in a circuit. It is the electric "pressure" causing the current to flow.

What is current?

Electric current is a measure of the rate at which electric charge (electrons) are flowing through a circuit. It is given in the unit of amperes ("amps"). Smaller amounts of current are often stated in “milliAmps” (mA). A mA is 1/1000 of an amp.

We always speak of electric current flowing through a single point in a circuit, whereas we refer to voltage existing across or between two points in a circuit.

What is resistance?

Electrical resistance is the opposition to the flow of electricity. Measured in “ohms,” it reflects how much electric “pressure” (voltage) is required to push a given amount of current through a device or part of an electric circuit.

If it takes a lot of voltage for current to flow through the device, its resistance is high. If current flows easily, even with a small voltage applied, its resistance low.

How to Properly Measure Voltage

Attach the wires from the generator to the multimeter. Polarity is not relevant at this point.

To check the voltage, select DC volt (V) and set the number to 20. Place your turbine out in the wind or in front of a fan and let it spin. It is normal for the voltage readings to fluctuate. Voltage output is often unsteady because of the inconsistent nature of the wind or unbalanced blades.

Voltage is related to how fast the DC generator is spinning. The faster it spins, the higher the voltage. When there is no load on the generator, it has little resistance and can spin very fast.

You can measure voltage with no load, but it is more realistic to place a resistor in the circuit and measure the voltage across the resistor. We commonly use 10, 30, 50 or 100 ohm resistors when measuring voltage on KidWind Turbines.

How to Properly Measure Current (Link to videos)

To calculate your turbine's power output, you will need to measure current as well. To collect amperage data, you will need to place a load, preferably a resistor, in series with the multimeter so that the generator is forced to do some work.

When measuring current, you are monitoring how many electrons are being pushed through the wire by the turbine. We measure current from our turbine in milliAmperes. Recall that 1A =1000mA

Not sure how to do all this electrical stuff? We have some great videos online that will show you how to use and understand a multimeter.

How to Calculate Power Output

If you can measure voltage across a resistor you now have enough information to calculate power output.

Power = voltage × current

We have taught you how to measure voltage and current but it can be hard to measure current and voltage simultaneously. You need two multimeters and lots of clip cords. In certain situations, we can make this easier, using Ohm's law.

If we know the voltage that our turbine is producing and the size of the resistor in the circuit, we can determine our power output by only recording the voltage using the equation below.

Power = Voltage²/Resistance (ohms)

We also have an online calculator that helps you do this as well. Check out the KidWind Turbine Performance Calculator!

Advanced: Calculating Energy Output

Knowing how powerful your turbine is can be helpful but many times at KidWind Challenges we will be calculating how much ENERGY your turbine produces over a 60 second trial. Knowing the difference between POWER and ENERGY is very important and you may be asked this at a Kidwind Challenge.

A small wind turbine with a low power output in a windy place produces lots of energy.

A large wind turbine with high power output in a not-so-windy place will produce little energy.

When planning the location of a wind turbine, the amount of energy it will actually produce is more important than its maximum power output.

Energy is the ability to do work. We need energy to do many things in our daily life: heat our homes, move our cars, light our rooms, and enable our bodies to move. Life would not be possible without energy.

The primary unit of energy is the joule (J). It is a quantity defined as the work required to move an object 1 meter against a force of 1 Newton. This is about the energy required to lift a 12 oz soda can 1 foot straight up.

Power is the rate at which energy is used and is measured in watts (w), which is 1 joule transferred every second, or J/s.

Power and energy are similar to speed and distance. Velocity multiplied by time gives the total distance traveled. Power multiplied by time gives the total energy used or produced. Power is a RATE -- Energy is a QUANTITY! Remember this!!

To measure the energy output we use a much smaller unit for measurement: millijoules, or 1/1000 of a joule.

While you can use software and data collectors like Vernier to calculate how much energy your turbine produces in one minute you can also just use your multimeter and a stopwatch.

If you measure and record the voltage across the resistor at 5 second intervals, you can calculate how much energy your turbine produced.

Check out the KidWind Turbine Performance Calculator and use our super computer to calculate your energy output!

Power in the Wind and Turbine Efficiency

If a large truck or a 250 lb linebacker was moving toward you at a high speed, you would move out of the way, right? Why do you move? You move because in your mind you know that this moving object has a great deal of energy as a result of its mass and its motion. And you do not want to be on the receiving end of that energy. Just as those large moving objects have energy, so does the wind. Wind is the movement of air from one place on Earth to another. That’s the motion part.

What is air? Air is a mixture of gas molecules. It turns out that if you get lots of them (and we mean lots of them) together in a gang and they start moving quickly, they will give you a serious push. Just think about hurricanes, tornadoes, or a very windy day!

Why aren’t we scared of light winds, but we will stay inside during a hurricane or wind storm? The velocity of those gangs of gas molecules have a dramatic impact on whether or not we will be able to remain standing on our feet. In fact, in just a 20 mph gust, you can feel those gas molecules pushing you around.

Humans have been taking advantage of the energy in the wind for ages. Sailboats, ancient windmills, and their newer cousin the electrical wind turbine, have all captured this energy with varying degrees of effectiveness. They all use a device to “catch” the wind, such as a sail, or blade. Sailboats use wind energy to propel them through the water. Windmills use this energy to turn a rod or shaft.

A simple equation for the power in the wind is described below. This equation describes the power found in a column of wind of a specific size moving at a particular velocity.

• P = 1/2 * rho * (Pi r²) v3
• P = power in the wind (watts)
• rho = density of the air (kg/m³)
• r = radius of your swept area (m)
• v = wind velocity (m/s)
• Pi = 3.14

From this formula you can see that the size of your turbine and the velocity of the wind are very strong drivers when it comes to power production. If we increase the velocity of the wind or the area of our blades, we increase power output.

The density of the air has some impact as well. Cold air is more dense than warm air, so you can capture more energy in colder climates.

This formula tells you the available power in the wind. Your turbine cannot capture all of this power. If you built the best turbine ever you could only extract 59% of this power due to the Betz Limit.

What is the Betz limit?

Wind turbines are limited on how much power they can capture from the wind. These limits can be caused by generator efficiencies, blade design, friction in the drive train, but most importantly wind flow through and around the wind turbine. A wind turbine cannot capture 100% of the power in the wind, because that would mean that the wind would have to be stopped completely. For a turbine to work properly, some wind has to move out the back of the wind turbine and keep the blade spinning. Albert Betz calculated that a perfect turbine could only extract 59.3% of power in the wind stream, and we now call this number the Betz limit.

What is my Turbines Efficiency?

To determine the efficiency of your small turbine you need to know the following:

• How much power your turbine is generating (We can calculate this).
• How much available power is in the column of wind hitting your turbine (we can calculate this if we know the wind speed and the rotor diameter)

We have an online calculator that will determine your turbine efficiency. The hardest variable to get a number on is the wind speed hitting your turbine. We have a cheat sheet that allows you to estimate the wind speed of the fan you are using. Otherwise you can buy some wind meters. Or there are a number of simple wind speed meters you can try to make.

Tip Speed Ratio

One last performance parameter of your wind turbine is the Tip Speed Ratio (TSR).

Tip Speed Ratio (TSR) is a ratio of how fast the tips of your turbine blades are moving relative to the wind hitting the turbine.

Example: If the wind hitting your turbine was traveling at 5 m/s and your blade tips were moving at 5 m/s you would have at TSR of 1.

What is an optimal Tip Speed Ratio? That depends on a number of factors such as rotor diameter, number of blades, blade pitch, RPM needed by the generator, and the oncoming wind speed. Typically higher TSRs are better for generators that require high rpms--but the wind speed characteristics at your particular site will make a big difference.

What is happening? If the rotor of the wind turbine turns too slowly, most of the wind will pass undisturbed through the gap between the rotor blades. If the rotor turns too quickly, the blurring blades will appear like a solid wall to the wind.

When a turbine blade passes through the air it leaves turbulence in its wake. If the next blade on the spinning rotor arrives at this point while the air is still turbulent, it will not be able to extract power efficiently from the wind. However if the rotor spun a little more slowly the air hitting each turbine blade would no longer be turbulent.

If you know the diameter of your wind turbine rotor, the velocity of the wind and your RPM you can calculate TSR. Generally the higher your TSR the more electricity you are going to be able to generate on a three bladed turbine. But this has some limits.

Check out the KidWind Turbine Performance Calculator and calculate the TSR of your wind turbine! Do the experiment at a variety of wind speeds and see what happens!