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Otherpower.com
Dan Bartmann and Dan Fink
Copyright 2008 by Otherpower.com
Wind is the most difficult energy resource to harvest
Solar, hydro, and fossil fuel sources give smooth and steady energy, so efficient generators for them are easy to design.
The wind varies wildly and unpredictably, with an exponential power curve.
Power available in the wind (in watts) =
½ x air density x swept area x wind speed³
(kg/m³, m², m/s)
Copyright 2008 by Otherpower.com
The objective: Slow down the wind, and transfer this energy into a spinning shaft
Wind quantity and quality are both important
Most winds come to us at low speeds
Tall towers are essential—you wouldn't mount a solar panel in the shade!
Turbulence is also a problem
Copyright 2008 by Otherpower.com
HAWT vs. VAWT1kw HAWT
1kw VAWT
½ of a VAWTs rotor is moving in the wrong direction
VAWT blades are also ½ as efficient as HAWTs
Therefore VAWTs must be 4 times as large as HAWTs to make the same energy
VAWTs experience severe cyclical stresses every rotation
VAWTs cannot be yawed out of the wind to control them
VAWTs are not more quiet than HAWTs, nor are they safer for birds
VAWTs do NOT perform better near the ground, on turbulent sites, or in low winds, than HAWTs—despite the advertising hype
Copyright 2008 by Otherpower.com
Comparison of wind turbine rotor design efficiency and Tip Speed Ratios
No wind turbine power coefficient (cP) can exceed the Betz Limit of 59.26%
Drag-based turbines are very inefficient, suitable only for grinding grain or pumping water
High-solidity rotors have a very high drag component, and are also not suitable for producing electricity
3-blade, low drag, lift-based turbines operating at TSRs of 5-7 are the most efficient, and are easiest and cheapest to build.
Graph source: NREL
Before you try to think “outside the box,” find out what's IN the box and why.
Copyright 2008 by Otherpower.com
Objective: produce electricity
Savonious: Mostly drag based VAWT with very low RPM
Waterpumper: Lift-based but high solidity and high drag give low RPM
The lower the shaft RPM, the heavier and more expensive the alternator must be
Gearing to increase RPM is undesirable due to friction, which hurts low wind performance
Using electromagnets to produce magnetic flux hurts low-wind performance
Ferrous coil cores increase magnetic flux, but hurt low-wind performance by cogging
In winds of 7-28 mph that we want to capture, turbine RPM of 80-500 is ideal for TSR=5-7 and therefore maximum cP
POOR POOR
FANTASY
There are many reasons this is a graphic and not an actual photo!
Copyright 2008 by Otherpower.com
Wind turbine alternators No effective commercial or surplus
products exist:
Vehicle alternators need 1000 RPM plus; too fast
Servo motors are wired wrong: very inefficient, with high possibility of burnout
Induction motors and surplus gasoline generators need constant RPM
The only effective solution:Build your own low-RPM alternator.
Just like what all legitimate wind turbine companies do!
NO
NO
Copyright 2008 by Otherpower.com
Axial flux air gap alternator Low RPM, specifically designed for
individual system voltage
Permanent magnets eliminate parasitic losses from electromagnets
Large mass of very powerful magnetic material makes up for large air gap and no ferrous coil cores
Start-up cogging losses are eliminated
Easy to build using standard vehicle parts and/or flat steel sheets, even in remote areas
Common in commercial wind turbine designs
Copyright 2008 by Otherpower.com
Furling Systems Wind turbines must cope
with an exponential increase in power from the wind to survive
Alternators react to power input on a mostly linear curve
Matching the turbine rotor to the alternator is critical
Reducing the swept area is the best way to regulate small wind turbine output
The most reliable, elegant, easy to fabricate, and effective solution is the furling tail
Copyright 2008 by Otherpower.com
Renewable energy systems Hybrid solar/wind systems
produce the most energy, in kwh per month, at most locations
Rated output is overrated
You cannot compare solar and wind systems by “rated output”
Solar PV rated output assumes full sun, fairly common
Wind rated output assumes 28 mph wind speeds, very rare
The critical figure is ENERGY production in watt-hours, not instantaneous POWER output in watts. Instantaneous power output does not matter much—what matters is for how long you gained or used it.
Copyright 2008 by Otherpower.com
Local Manufacture and Training
Local manufacture is possible, these photos from Bluefields, Nicaragua, Blue Energy Group NGO
Skills and equipment needed include welding, metalworking, electrical wiring, resin casting, wood carving
Copyright 2008 by Otherpower.com
Frame Fabrication Flat steel parts can be cut
with torch or hacksaw
Strong welding is absolutely essential
Standard trailer bearings are available worldwide
Copyright 2008 by Otherpower.com
Stator Fabrication
The stator must be a solid resin casting
Copyright 2008 by Otherpower.com
Magnet Rotor Fabrication
Again, a strong casting is needed
Copyright 2008 by Otherpower.com
Blade Carving Wood--the original
carbon-graphite composite!
The main advantage of fiberglass is in mass production
Copyright 2008 by Otherpower.com
Towers Towers must be both
tall and sturdy Turbines must fly 30
feet above anything within 300 feet in any direction
Tower cost and time to build it is at least equal to that of the turbine itself
Climb or tilt up—depends on your skill set and training
Copyright 2008 by Otherpower.com
Regular Maintenance A regular, yearly maintenance schedule
is essential with ALL wind turbines, not just homebrew ones.
Maintenance training is critical with inexperienced turbine owners
Climate affects the maintenance schedule of turbines and towers tremendously
Attention to detail is critical!
The “EVENT CASCADE” is the start of almost all wind turbine failures.
For example, a $5 rectifier fails from a heat sink that's too small. The turbine runs single phase, and the stator overheats. The stator melts and swells, causing magnets to rub it. Friction heating brings the magnets past their Curie temperature. The magnets no longer adhere to the rotors, and fly out, hitting the blades. You are left with a pile of junk!
Copyright 2008 by Otherpower.com
Homebrew wind turbine books
Windpower Workshop by Hugh Piggott
Axial Flux Wind Turbine Plans by Hugh Piggott
Homebrew Wind Power by us
Copyright 2008 by Otherpower.com
Wind turbine seminars Hands-on experience for
educators is essential
Develop your own curriculum to suit the ultimate location, no matter how remote
Custom tailor the curriculum to the skill level of your students
Copyright 2008 by Otherpower.com
Wind energy should be:
- Cost effective- Reliable- Silent- Easy to live with- Fun!
Copyright 2008 by Otherpower.com