Design of the Iskra AT5-1 Wind Turbine Dr Mike Wastling

Dr Mike Wastling
Technical Director
Iskra Wind Turbines
bringing wind energy to life
Design of the Iskra AT5-1 Wind Turbine

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
bringing wind energy to life
Contents

inging wind energy to life
Small Wind in context
• Micro Wind: Battery charging, boats, signs
• Small Wind (2m to 20m diameter?)
• Large Wind - Utility scale electricity generation

Characteristics
• Owner of turbine uses the
electricity (or reduces
consumption)
• Can be grid connected or standalone
• Typical generation shown in table
(typical household consumption
4000 kWhrs per year)
bringing wind energy to life
Small Wind
Diameter
2m
5m
10m
Typical
rated
power
1kW
5kW
20kW
Annual
Energy
Yield
(kWhrs)
1000 –
2000
6000 –
12000
24000 -
48000

• Generate electricity
bringing wind energy to life
Small Wind Applications
– 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, but
higher value. Remote homes, remote communities,
telecommunications.
• Other applications
– Water heating
– Ground source heat pumps
– Water pumping

inging wind energy to life
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

inging wind energy to life
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.
25m 50m
N

• Primary consideration
bringing wind energy to life
Energy Yield
Design considerations
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

inging wind energy to life
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!

• Energy yield and thrust load
are roughly proportional to
swept area (D 2 )
• 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 D 3 .
(Neglects self-weight, which
becomes more important for
large D)
bringing wind energy to life
Wind turbine scaling
Thrust
Blade and
tower are
beams

• Theoretically,
small turbines can
use much less
material in their
manufacture
relative to their
energy yield.
• Not there yet!
bringing wind energy to life
Mass of turbine per MWhr per year
(kg/MWh/yr)
Wind turbine scaling
Necessarily crude control
and protection systems,
manufacturing restrictions,
low parts count
30
Iskra AT5-1 5.5m
diameter, typical
8.7MWhrs/yr, 0.3 tonnes
5.5
Turbine diameter (m)
Sophisticated
control and
protection systems
Theoretical line from
simple scaling law
Vestas v90, 90m,
typical
3500MWhr/year,
111tonnes
90

Power (kW)
Time (hrs), Energy
available (kWhrs)
500
400
300
200
100
0
1
2000
1500
1000
500
0
5
1
6
9
Power available to wind turbine
bringing wind energy to life
13
17
21
25
29
wind speed
33
37
41
Energy available to wind turbine
wind speed (m/s)
11
16
21
26
31
36
41
Design Challenges
Power (W)
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.

Power available to turbine is
proportional to wind speed 3
Maximum thrust proportional
to wind speed 2
Limiting rotor speed is almost
as effective as parking the
rotor
bringing wind energy to life
Protection against excessive loading
Thrust (N)
x 104 Importance
4.5
4
3.5
3
2.5
2
1.5
1
0.5
of over speed protection
Freewheeling
-15 degrees pitch, 250rpm
parked
0
0 10 20 30
wind speed (m/s)
40 50 60

Do nothing –
make it strong!
Micro wind
Crude systems to minimise
parts count
bringing wind energy to life
Furling, or tilting rotor
How to shed surplus power?
Passive blade pitch control
Electrical or mechanical brake
Active pitch control
Wind speed monitoring and
automatic shutdown
Mechanical brake backup
Large wind
Sophisticated systems, to
minimise loads and save
weight. Can bear maintenance
costs

• 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
bringing wind energy to life
Over speed protection - furling
rotor
Preloaded hinge against stop
Yaw axis
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!
bringing wind energy to life
Passive pitch regulation
Efficient below
rated power
Minimise
fluctuations in
load in high winds

Fixed configuration
Air-X
bringing wind energy to life
Over speed protection - examples
Furling
Bergey XL-1
Passive pitch
Iskra AT5-1

• The Company
• Overview of the turbine
• The pitch system
• The generator
bringing wind energy to life
The Iskra AT5-1

inging wind energy to life
Iskra - The Company
• Concepts 1998
• Company founded Feb 1999
• DTI support 2000
• Prototype 2001
• 1st customer 2004
• Significant investor 2005
• 50 th turbine 2006
• VC investment 2007
• 100 th turbine imminent

inging wind energy to life
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.

inging wind energy to life
Overview

Feature
•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
bringing wind energy to life
Means that:
Energy capture not
compromised
Pitch system
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
Centrifugal
Aero torque
Loads balanced
Pitch axis
Loads
distributed

Feature
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
bringing wind energy to life
Means that:
Generator
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
Air cored
stator
Magnets
Structural hub

inging wind energy to life
Summary
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
The company
Target markets
Design drivers
Key technology