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

W. Bergholz GEE2 Spring 2012 1

09_wind energy

Wind PowerWind Power– Explain the special features of wind power– Compare to solar cells


09_wind energySpecial features of wind power 1: AvailabilitySpecial 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


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Special features of wind power 2: CostSpecial 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)


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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


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– 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


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Special features of wind power 2a: installed capacitySpecial 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


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Special features of wind power 2a: installed capacitySpecial features of wind power 2a: installed capacityUPDATE to 2008UPDATE to 2008


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installed capacity as of 2010:installed capacity as of 2010:

http://www.gwec.net/fileadmin/documents/PressReleases/PR_2010/Annex%20stats%20PR%202009.pdf


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Special features of wind power 3: total Special features of wind power 3: total theoretical capacitytheoretical 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?


007_ Solar cell and wind energy

Technical details: conversion efficiencyTechnical 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)


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Technical details: which energy is converted?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)


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Technical details: which energy is Technical details: which energy is converted?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


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Technical details: power is proportional Technical details: power is proportional to vto v33 ! !

– Simplify the equation:

– So: Power is

proportional to the air density d proportional to diameter squared D2

Proportional to the velocity cubed v3


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Based on this result:Based on this result:

How should we build our windmills?


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How should we build our windmills?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.


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How much of this power How much of this power can actually be used?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


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Power output as a Power output as a function of tip speed of function of tip speed of the generatorthe generator

– See Sarma

– The power conversion is at maximum for a certain tip speed (rotation rate)


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Power output as a Power output as a function of tip speed of function of tip speed of the generatorthe 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


09_wind energyTherefore, 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?


09_wind energyAnswer 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


09_wind energyAnswer 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


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Win Drive


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Win Drive


09_wind energyMap of the US with the wind energy potential: (taken

from a wind energy web site)


09_wind energyMap of the US with the wind energy potential: (taken

from a wind energy web site)



09_wind energyPhotos of wind generators near Vienna (taken by Anca Dragan 2006)

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

Documents

wind energy wind power

wind energy available

solar power wind

wind parameter

wind energy installed

wind energy technical

solar power little wind

wind bad weather