Design of the Iskra AT5-1 Wind Turbinehemh1/climate/wastling.pdfbringing wind energy to life Small wind turbines •Small wind turbines in context •Design considerations •Design

bringing wind energy to life

Design of the Iskra AT5-1 Wind Turbine

Dr Mike Wastling

Technical Director

Iskra Wind Turbines


Small wind turbines

•Small wind turbines in context

•Design considerations

•Design constraints

•How the turbine configuration is likely to vary with the size of the turbine

The Iskra AT5-1

•Overview of Iskra Wind Turbines

•Target markets

•Design drivers

•Key technology

–Overspeed protection

–Integrated design

Contents


Small Wind in context

• Micro Wind: Battery charging, boats, signs

• Small Wind (2m to 20m diameter?)

• Large Wind - Utility scale electricity generation


Small Wind

Characteristics

• Owner of turbine uses the electricity (or reduces consumption)

• Can be grid connected or stand-alone

• Typical generation shown in table (typical household consumption 4000 kWhrs per year)

24000 -

48000

20kW10m

6000 –

12000

5kW5m

1000 –

2000

1kW2m

Annual Energy Yield (kWhrs)

Typical rated power

Diameter


Small Wind Applications

• Generate electricity

– Grid connected – reduce or eliminate electricity consumption and sell surplus. Homes, schools, farms, community centres. The value of the displaced electricity consumption is at the electricity ‘buy’ price.

– Stand alone – use instead of mains electricity – needs battery bank and probably diesel / Photo Voltaic backup. More expensive, buthigher value. Remote homes, remote communities, telecommunications.

• Other applications

– Water heating

– Ground source heat pumps

– Water pumping


Worldwide

• There are over two billion people in remote rural communities

without electrical power

• Enormous potential for the establishment of clean energy

sources for health clinics, water pumping, desalination,

refrigeration of medicines, telecommunications, battery charging as well as heating and lighting.

• Avoid punitive imported fuel costs


Small Wind Turbine Applications - Constraints

• Needs open spaces with clear wind (Rural not Urban). Houses can reduce wind speeds for about 20 x height

• Noise: require about 50m to owners house and 100m to neighbour.

• Shadow flicker: require about 50m between turbine at windows West, through North to East.

N25m 50m


Design considerations

• Primary consideration

Energy Yield

Cost turbine + Cost Installation + Cost Maintenance + Cost making sales

These costs fairly insensitive to size of turbine

Note: Efficiency of energy conversion is not a primary

consideration


Other considerations

• Aesthetics and noise considerations pretty much dictate need for slow tip speed, and use of 3-blades. (Despite cost

penalty).

• In the UK, grid connection is more straightforward for less

than 10kW rated power.

• Ability of customer to pay!


Wind turbine scaling

• Energy yield and thrust load are roughly proportional to swept area (D2)

• Treating the blades and tower as simple beams, and scaling all dimensions in proportion to D, you find that stress remains about the same, but mass changes with D3.

(Neglects self-weight, which becomes more important for large D)

Thrust

Blade and

tower are

beams


Wind turbine scaling

• Theoretically, small turbines can use much less material in their manufacture relative to their energy yield.

• Not there yet!

Turbine diameter (m)

Ma

ss o

f tu

rbin

e p

er

MW

hr

pe

r ye

ar

(kg

/MW

h/y

r)

Theoretical line from simple scaling law

Vestas v90, 90m,

typical

3500MWhr/year,

111tonnes

Iskra AT5-1 5.5m

diameter, typical

8.7MWhrs/yr, 0.3 tonnes

30

Sophisticated

control and

protection systems

Necessarily crude control

and protection systems,

manufacturing restrictions,

low parts count

905.5


Design Challenges

Power available to wind turbine

0

100

200

300

400

500

1 5 9 13 17 21 25 29 33 37 41

wind speed

Po

we

r (k

W)

Power (W)

Energy available to wind turbine

0

500

1000

1500

2000

1 6 11 16 21 26 31 36 41

wind speed (m/s)

Tim

e (

hrs

), E

ne

rgy

ava

ila

ble

(kW

hrs

)

Time at wind speed

hours/year

Energy per year

(kWhr/yr)

Power available to the wind turbine increases with wind

speed cubed.

The energy available to the

wind turbine is virtually all below 20m/s (Example

based on 7m/s annual mean

wind speed)

Challenge is to be efficient at

low wind speeds, and shed power and loads in strong

winds. Solution must be totally reliable.


Protection against excessive loading

0 10 20 30 40 50 600

0.5

1

1.5

2

2.5

3

3.5

4

4.5x 10

4 Importance of over speed protection

wind speed (m/s)

Thru

st (

N)

parked

-15 degrees pitch, 250rpm

Free-wheeling

Power available to turbine is

proportional to wind speed3

Maximum thrust proportional

to wind speed2

Limiting rotor speed is almost

as effective as parking the rotor


How to shed surplus power?

Large windMicro wind

Active pitch control

Wind speed monitoring and

automatic shutdown

Mechanical brake backup

Do nothing –

make it strong!Passive blade pitch control

Furling, or tilting rotor

Sophisticated systems, to

minimise loads and save

weight. Can bear maintenance

costs

Crude systems to minimise

parts count

Electrical or mechanical brake


Over speed protection - furling

• Furling:– Likely to be misaligned

in light winds (noisy, reduced energy)

– Will be misaligned in strong winds (noisy, high cyclic loads, rapid yawing loads)

– Since thrust loading is linked to power divided by wind speed, furling speed depends on wind speed. Wind turbine can still over speed in light winds, when off-load

Preloaded hinge against stop

Yaw axis

rotor

Tail vane


• Use centrifugal load to pitch blades to reduce efficiency

• Need preloaded spring or equivalent

• Fixed pitch at all times below rated power

• Aligned with wind direction at all times

• Since pitch system has low inertia, so response is rapid

• Easier to get it wrong!

Passive pitch regulation

Efficient below

rated power

Minimise

fluctuations in

load in high winds


Over speed protection - examples

Fixed configuration

Air-XFurling

Bergey XL-1

Passive pitch

Iskra AT5-1


The Iskra AT5-1

• The Company

• Overview of the turbine

• The pitch system

• The generator


Iskra - The Company

• Concepts 1998

• Company founded Feb 1999

• DTI support 2000

• Prototype 2001

• 1st customer 2004

• Significant investor 2005

• 50th turbine 2006

• VC investment 2007

• 100th turbine imminent


The Iskra AT5-1 - overview

Key design features

1. Passive pitch control for over-speed protection in high winds and efficient operation in light winds.

2. Efficient aerodynamic blade shape, designed to maximise the ratio of energy yield to loads (i.e. low cost of energy), and to achieve low noise.

3. Integrated direct-drive permanent magnet generator, for low component count and reduced mass/cost.

4. A sleek and slender design for low visual impact.


Overview


Pitch system

Loadsdistributed

Loads balanced

Centrifugal

Aero torque

Energy capture not

compromised

Effective off-load and high

wind speed rpm regulation

Maximum over speed can

be comparable to normal

running speed

Rotor is always mass and

aerodynamically balanced

Makes it much easier to

integrate hub/generator

Minimise cyclic aerodynamic

noise

Minimise cyclic loads

•Blade normally fixed at optimum

pitch

•Centrifugal loads pitch to stall to

shed surplus power and regulate

speed

•Aerodynamic torque delays

power shedding in the presence

of generator reaction torque

•Blades are all linked in pitch

•Distributed/balanced loads

minimise hub deflections

•Rotor is always aligned with the

wind

Means that:Feature

Pitch axis


Generator

Structural hub

Air cored

stator

Magnets

High value from extra energy compared to extra cost of magnets and copper.

Easy to cool

Easy to achieve large diameter, so much reduced need for stiff structure/bearings

Low component count/cost

Reduce eddy current losses, and no need for laminated rotor

Effective cooling

High efficiency, purpose built direct drive generator

Air cored – no magnetic attraction between stator and rotor

Single hub plate for structure and flux

Innovative arrangement of coils*

*patented

Means that:Feature


Summary

Small wind turbines in contextDesign considerationsDesign constraintsHow the turbine configuration is likely to vary with the size of the turbine

The Iskra AT5-1The companyTarget marketsDesign driversKey technology

Design of the Iskra AT5-1 Wind Turbinehemh1/climate/wastling.pdfbringing wind energy to life Small wind turbines •Small wind turbines in context •Design considerations •Design

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