1. W. Bergholz GEE2 Spring 20121 09_wind energy Wind Power –Explain the special features of wind power –Compare to solar cells

2. W. Bergholz GEE2 Spring 20122 09_wind energy Special features of wind power 1: Availability –Not available all the time –Need to cope with wide range of mechanical forces (zero wind to storm) –Non-availability somewhat „orthogonal“ on solar power: Much wind  bad weather  less solar power Little wind  very often good weather  more solar power Wind also available at night and in winter! Small combined systems are available for off grid applications

3. W. Bergholz GEE2 Spring 20123 09_wind energy Special features of wind power 2: Cost –Typical capacity for a single unit is now 1000-2000 kW –Large units up to 7000kW or larger, particularly off-shore –Relatively high capital costs; low operating costs, largest amount for maintenance –Weak point: gearbox (fast torque changes, untypical compression of the gearbox in the axis direction  7 instead of projected 15 years lifetime, under improvement now)

4. W. Bergholz GEE2 Spring 20124 09_wind energy Special features of wind power 2: Cost Price Comparison from a recent Study. Levelized Costs: Wind: 4.8 cents per KWh (seems a bit optimistic, 6 - 7 appears more realistic) Offshore: 15 – 25 cents! Coal: 6.2 Photovoltaics: 12 – 20, going down all the time Advanced Gas Turbine: 4.6

5. W. Bergholz GEE2 Spring 20125 09_wind energy Special features of wind power 2: Cost –The cost will come down to 3 – 4 cents /KWh –Therefore, wind power is already competitive, provided There is enough wind at the site (20% utilization is considered good) There is no resistance against noise, „looks“, etc Under implementation: off-shore installations (since most of the good sites have been used) Note: Denmark generates already >14% of its electric power by wind! In Germany: most good sites already used, off-shore the main expansion

6. W. Bergholz GEE2 Spring 20126 09_wind energy Special features of wind power 2a: installed capacity Country/region MW Installed (2005) United States 1700 Denmark - 520 Germany - 330 United Kingdom 145 Netherlands - 132 Spain - 55 Greece - 35 Italy - 10 Other OECD - 70 India - 50 China - 25

7. W. Bergholz GEE2 Spring 20127 09_wind energy Special features of wind power 2a: installed capacity UPDATE to 2008

8. W. Bergholz GEE2 Spring 20128 09_wind energy installed capacity as of 2010: http://www.gwec.net/fileadmin/documents/PressReleases/PR_2010/Annex%20stats%20PR%202009.pdf

9. W. Bergholz GEE2 Spring 20129 09_wind energy Special features of wind power 3: total theoretical capacity –There is enough wind energy available –Assume: in Texas and North Dakota all good sites would be equipped with wind generators  enought capacity of the US!  For Europe: use special sites in Scotland, Ireland BUT: Transport of energy? (17 billion € need for off-shore integration into the grid!!) Storage of energy?

10. W. Bergholz GEE2 Spring 201210 007_ Solar cell and wind energy Technical details: conversion efficiency –Theoretical maximum: 59% (Si solar cells: 28%) –Note however: conversion efficiency comes at a price –Conversion efficiency is not the key figure, price for a kWh what needs to minimized –Typical efficiencies: 25 – 35% (old pre-1900 windmills: 5%) (source for this and most of the following graphs: Danish Wind industry association)

11. W. Bergholz GEE2 Spring 201211 09_wind energy Technical details: which energy is converted? –For a solar cell: the energy of the photons –For wind: the kinetic energy of the wind –Parameter to look for KE = kinetic energy v = wind speed m = mass of air at velocity v (source: Danish Wind industry association)

12. W. Bergholz GEE2 Spring 201212 09_wind energy Technical details: which energy is converted? –For a unit volume (1 cubic metre) of material, where d is the density, or mass per unit volume. (For air, d is about 1 kilogram per cubic metre.) –Volume of air per second through the blades of a vertical windmill is area A covered by the blades times the wind velocity v (D= diameter of rotor blades) –Power is the volume times the kinetic energy per second These 3 terms make up the mass of air through the wind generator per second

13. W. Bergholz GEE2 Spring 201213 09_wind energy Technical details: power is proportional to v 3 ! –Simplify the equation: –So: Power is  proportional to the air density d  proportional to diameter squared D 2  Proportional to the velocity cubed v 3

14. W. Bergholz GEE2 Spring 201214 09_wind energy Based on this result: How should we build our windmills?

15. W. Bergholz GEE2 Spring 201215 09_wind energy How should we build our windmills? –A large diameter D –Places with high wind speeds –High wind speed is much better than having a high probability for wind –If you can have both, that is of course much better! –A small effect, but it all counts: Effect of temperature and geographical height? Answer: low T and low height lead to higher density.

16. W. Bergholz GEE2 Spring 201216 09_wind energy How much of this power can actually be used? –Much less than the theoretical maximum – the figure is derived from typical actual statistics –High wind speeds cannot be used to maximum power since this would destroy the generator and the blades –Low wind speed is not worth it, it may actually consume more than it is worth due to system losses due to „overhead“ –typical cut-in speeds: 3 – 5 m/s –Typical cut-out speeds: 25m/s

17. W. Bergholz GEE2 Spring 201217 09_wind energy Power output as a function of tip speed of the generator –See Sarma –The power conversion is at maximum for a certain tip speed (rotation rate)

18. W. Bergholz GEE2 Spring 201218 09_wind energy Power output as a function of tip speed of the generator –See Sarma –The maximum varies with the the wind speed! –Therefore, the windgenerator needs to operate at different frequencies depending on the wind speed –That is bad ! Why ? Parameter: rotation rate of blades

19. W. Bergholz GEE2 Spring 201219 09_wind energy Therefore, the windgenerator needs to operate at different frequencies depending on the wind speed That is bad ! Why ? Answer: –The grid operates at 50 Hz (would correspond e.g. to 50 rotations / second) –If the rotation speed were kept constant one could adapt it by the construction of the generator so there would be a 50Hz output (e.g. by mechanical gears) –However: The turbine would almost never run at optimum speed How to solve this problem?

20. W. Bergholz GEE2 Spring 201220 09_wind energy Answer 1: Use Power Semiconductors! A simple system is: –Step 1: convert the variable frequency electric AC voltage to DC voltage –Step 2: chop the DC voltage to the desired AC at the desired frequency and get rid of high frequency components by filtering –Needed: robust semiconductor switches with almost no power loss  Thyristors  IGBTs Also needed: sophisticated control electronics to cut in /cut out the wind generator at min and max wind speed and turn it into the right direction so it is always fully into the wind

21. W. Bergholz GEE2 Spring 201221 09_wind energy Answer 2: a novel type of mechanical gear The Voith mechanical Engineering company has develop a new mechnical drive system: The WinDrive® Voith Turbo is at home wherever there is a need for maximum reliability, precise and dynamic control allied to low operating and maintenance costs. With the new WinDrive, Voith Turbo has developed an innovative, adjustable speed gearbox that converts variable input speeds from the wind rotor into constant output speeds for a synchronous generator. A synchronous mains connection is achieved without the need for frequency converters and transformers. Incorporating synchronous generators in the mid- voltage level allows network operators to set up stable and cost-efficient wind parks

22. W. Bergholz GEE2 Spring 201222 09_wind energy Win Drive

23. W. Bergholz GEE2 Spring 201223 09_wind energy Win Drive

24. W. Bergholz GEE2 Spring 201224 09_wind energy Map of the US with the wind energy potential: (taken from a wind energy web site)

25. W. Bergholz GEE2 Spring 201225 09_wind energy Map of the US with the wind energy potential: (taken from a wind energy web site)

26. W. Bergholz GEE2 Spring 201226

27. W. Bergholz GEE2 Spring 201227 09_wind energy Photos of wind generators near Vienna (taken by Anca Dragan 2006)