Wind Turbines. Figure 1. Wind farm (by BC Hydro)

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1 Wind Turbines Figure 1. Wind farm (by BC Hydro) Purpose Observe the operation of a wind turbine at different wind speeds Contextualize the size of an industrial wind turbine Introduction and Theory Humans have gathered energy from the wind using sails and windmills for centuries. With the shift to alternative energies, the wind turbine is accordingly one of the fastest growing technologies, particularly in Europe, where wind supplies more than 10% of the total electricity in Denmark, Portugal, Spain and Ireland. A wind turbine converts some of the mechanical energy of the moving air in the wind into electrical energy. To consider the amount of energy coming into the turbine, it is useful to picture the amount of air that passes through the turbine (Figure 2) Wind Turbines - 1 Saved: 6/5/12, printed: 6/5/12

2 v A l Figure 2. The volume of air passing through a wind turbine For a given wind speed, v, the volume of air passing through the blades in time t is V = Al = Avt. The maximum energy that the wind turbine can get from this wind is its kinetic energy: 1 K = mv = ( ρ V ) v = ( ρavt) v = ρav t where ρ is the density of the air, and A is the area of the wind turbine. K 3 So the maximum available power is 1 P Av 3 P = = ρ. The power per unit area, = 1 ρ v, is related t 2 A 2 to the wind speed to the third power, making it very important to select sites with high wind speed. Nonetheless, there are other limitations, particularly at high and low wind speeds that we will explore in this lab. In the lab we will see how the power output of a toy wind turbine changes with the wind speed in the wind tunnel. The wind speed inside the wind tunnel is easily measurable with the Dwyer manometer. According to fluid dynamics, an increase in the speed of incompressible fluid results in a decrease in the pressure, and the Dwyer manometer measures this pressure difference by the height of a water tube. According to Bernoulli s equation, the quantities are related by 1 2 ρ airv = P0 P = ρ 2 where ρ air = kg/m 3 and ρ water = kg/m 3 at the room temperature. We can calculate the wind speed v from the Dwyer height reading h using Eq. (1). water ghlllllllllllllll (1) 1124 Wind Turbines - 2 Saved: 6/5/12, printed: 6/5/12

3 Name: Partner(s): Desk #: 1124 section: Date: Wind Turbines In this lab, groups will take turns to work on the wind tunnel to do Part I. When not on the wind tunnel, groups will work on Part II. Part I The power output of a model wind turbine We will measure the power output of a toy wind turbine at different wind speeds. Materials Computer with Logger Pro, voltage probe, current probe, connecting wires, toy motor with blades, and wind tunnel. Procedure 1. Calculate the wind speed for each Dwyer height in the data table below, using Eq. (1). 2. Install the toy motor in the wind tunnel. We use it as a generator converting mechanical energy to electricity. Connect the output of the generator in series with a current probe (to channel 2 of the interface box) and a decade resistor box, set to 10 Ω. Then connect the voltage probe (channel 1 of the interface box) in parallel to the resistor box. Open file 1124 Wind Turbine.cmbl. The program will measure the voltage and the current outputs and calculate the power. 3. Start the wind tunnel and gradually increase the speed so that the manometer reads 0.6 inches. Then decrease the speed to each height in the data table. At each height, collect data and record the average (mean) power output over 5 seconds. Dwyer Height h (inches) Wind Speed v (m/s) Power Output P (W) Wind Turbines - 3 Saved: 6/5/12, printed: 6/5/12

4 4. Sketch the power output of the model wind turbine. List any qualitative similarities and differences to the power curve you look up in Part II. Output Power Wind speed Similarities: Differences: You can try to plot P-v, P-v 2, P-v 3 using Excel to see which graph is most linear Wind Turbines - 4 Saved: 6/5/12, printed: 6/5/12

5 Part II Industrial size wind turbine We will look up online information related to the wind turbines used at the 102-MW Bear Mountain Wind Park near Dawson Creek in North-eastern BC. 1. The first wind farm in BC is the Bear Mountain Wind Park, consisting of 34 Enercon E82 E3 turbines with rated power of 3 MW each. Look up a specification sheet for this model of wind turbine. Record the site you found below. (site: ) 2. Find the following data: Rotor Diameter: Maximum Rotational Speed: Cut-out Wind Speed: 3. To get a physical sense of the size of the rotor, use the trundle wheel to pace out the rotor diameter. How long does it take for you to cover that distance by walking? 4. At the maximum rotational speed, how fast is the tip of the blade moving? (remember to use radius and not diameter) What speed that you know is similar to this? 5. Why do you think there is a cut-out wind speed? 1124 Wind Turbines - 5 Saved: 6/5/12, printed: 6/5/12

6 6. Record the power output for different wind speeds. Work out the amount of available mechanical power at each wind speed and calculate the fraction of the input power converted to electrical power. Wind Speed (m/s) Power Output (W) Power Input (W) 1 Fraction of Power Converted For what reason do you think the fraction of the converted power is low at low wind speed? 8. For what reason do you think the fraction of the converted power is low at high wind speed? 9. What was the maximum fraction of power converted? 10. Note that even in ideal circumstances (no friction, no rotor inertia, etc.) it is still not possible to extract all the mechanical energy out of the wind, because then the wind would just sit behind the turbine and block any more incoming air to pass through the turbine. The maximum possible fraction is about 0.59, known as the Betz limit. How does the maximum fraction of converted power (from the table above) compare to the Betz limit? 1124 Wind Turbines - 6 Saved: 6/5/12, printed: 6/5/12