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#Post#: 257--------------------------------------------------
Vertical Axis Wind Turbine Quiet Design
DIR By: AGelbert
Date: November 4, 2013, 1:42 pm
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What Makes a Great Wind Turbine for Distributed Applications?
Jamie Schlinkmann, Inerjy
November 04, 2013
When investing considerable resources in a distributed wind
energy project, doesn’t it make sense to at least evaluate the
minimum criteria?
Let’s start with this list:
Good Value
Quiet
Attractive
Durable
Good Value
There are three initial factors to consider in terms of value:
(1) installed cost; (2) estimated annual power production; and
(3) confidence in the estimated annual power production. As a
gut-check on a turbine supplier’s output estimate, a very good
aerodynamic lift-based turbine will be rated between 300-400
W/m² and a drag-based turbine’s rating shall be less than 200
W/m² of swept area. Any claims outside of these norms should be
thoroughly investigated. Other factors like operating costs and
social concerns are more subtle and require deeper exploration.
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Quiet
Who wants their quiet enjoyment of the outdoors disturbed by a
relentless noisy machine? In order for distributed wind to gain
broad acceptance, this reality must be addressed. A wind turbine
design, as in any machine, is a balancing act. Every decision is
a tradeoff and every designer a chef looking for that winning
flavor. So far, most successful distributed-scale turbines have
mimicked their utility-scale counterparts as 3-bladed
horizontal-axis wind turbines (HAWTs). A key tradeoff in this
scenario is noise vs. storm wind resistance.
In a storm, a HAWT’s blades must bear the forces of the wind
from any direction. Storm resistance being a key design driver,
minimizing loads on the machine in high winds is important. It
is this requirement that drives a designer to want to utilize
thin blades. Here something called tip speed ratio comes into
play. A turbine’s tip speed ratio is the optimal velocity of its
blade relative to the wind speed. The higher the tip speed
ratio, the thinner the blades can be and still produce optimal
power. Designers of traditional HAWT’s normally hone in on a tip
speed ratio of about eight, which equates to blade speeds of 200
mph in a 25-mph wind. This is the point at which storm wind
blade forces and noise levels are ‘tolerable’. Is tolerable good
enough for your environment?
The key to quiet is lowering blade speeds without increasing
storm wind loads. Is there a way to do this without sacrificing
efficiency of the turbine? Yes, but it does require thinking
outside the box. By combining full pitch control with a straight
bladed VAWT, aka Pitch Controlled H-VAWT (PCH-VAWT), blades can
be feathered into the wind. If blades are appropriately
positioned, the rotor can even point itself into the wind like a
weathervane. When appropriately implemented, this approach can
have a much higher factor of safety than the locked rotor
approach of a traditional HAWT. When each mph of wind speed
results in thousands of pounds of structural forces, the added
margin is meaningful. With straight (non-twisted) blades that
can more effectively shed wind load, these blades can grow in
sectional area to reduce the optimal tip speed ratio. At a tip
speed ratio of three, for example, blades are traveling about 75
mph in that same 25-mph wind. This can be a quiet turbine!
Attractive
Unlike solar panels, where architects, project planners, and
owners typically try to hide on rooftops and within landscape,
wind turbines are big and tall and must be seen if used. If you
are thinking about mounting something to the roof of your
structure and ‘hiding’ it in your architecture, you will have
some challenges to overcome. The physics of incorporating a high
wind load dynamic machine into a dwelling or other structure not
designed for the task is potentially dangerous. Solutions
typically sacrifice power output to minimize impact on
structural integrity and occupant safety.
So if it’s going to be big, what makes it attractive? Beauty is
normally in the eye of the beholder, but keep in mind customers,
neighbors, and the general public will be the eyes beholding
your new turbine. It needs to be aesthetically pleasing.
Durable
Operating costs are directly impacted by turbine durability and
design again rules the day in a durability match. Turbines have
several systems that must be able to withstand the mechanical,
electrical, and environmental stresses to which they are
exposed. Structural systems like the blades, tower, frame, hub,
etc.; mechanical systems like drivetrains and bearings;
electromechanical systems like the generator and pitch/yaw
control drives; and control systems are all in this category.
Every system must also be environmentally durable for its own
operating environment. Material selections and interfaces should
therefore be appropriate.
Structural systems are designed around modeled (software
simulated) load cases. For traditional HAWT designs, software
packages are commercially available to generate this load data.
One such software, ADAMS, uses an aerodynamic library originally
developed by NREL called AeroDyn. The designer must run the
software for different cases and use the load outputs to check
the structural design. FEA, or finite element analysis, is often
used to analyze the structural elements to insure they will
withstand the loads.
Problem: composites like the materials used in traditional blade
construction don’t simulate well by this method. Material
variations, manufacturing process inconsistencies, and software
limitations all contribute to inaccuracies in simulated results.
These are key reasons why Airbus has major structural issues
with the wings of its new A380 and Boeing’s 787 is years behind
schedule. It also is why most of the major utility-scale turbine
manufacturers are having blade failures.
Lightning can also cause structural failure in a composite
blade. IEC 61400-24 defines blade lightning protection systems
for utility-scale turbines, but even with these elaborate
systems lightning damage remains the leading cause of blade
failure. The primary reason for this damage is that while the
blade has conductors to discharge the energy through the
machine, there is localized heating within the composites
themselves. Often, the heating is enough to cause moisture
within the blades to vaporize and the resulting increase in
pressure to delaminate materials. Perhaps the most dangerous
attribute of blade lightning damage is that it can go unnoticed
until the weakened structure has a fatigue failure days or weeks
later.
One way around these problems is to make blades out of materials
that are more predictable, and are more resistant to lightning,
like metals. With traditional HAWT blade geometries — having
twisted airfoils and variable chords — this is certainly
impractical, but what about the straight airfoils on the
PCH-VAWT turbine? In this machine the blades can be constructed
more like a typical aluminum airplane wing with spar sections,
ribs, and sheet skin. Very well-established and consistent
manufacturing processes exist and FEA simulations are accurate
and validated. The biggest drawback is the need for a unique
mathematical model to generate simulated machine loads.
Drivetrains are critical elements of a wind turbine. The best
drivetrain is no drivetrain, or direct-drive. Any components or
systems between the mechanical energy takers (blades) and the
mechanical to electrical energy converters (generators) are
opportunities for failure. For this reason major utility-scale
turbine manufacturers are widely adopting direct-drive
topologies in their latest machines.
Variable speed synchronous permanent magnet generators are the
state-of-the-art, and further coreless designs are the best of
the best. A coreless PM generator has no steel laminations, thus
eliminating a potential corrosion failure point and the
associated eddy current and hysteresis losses that occur in the
iron of these laminations. Also eliminated is the attraction
force between stator and rotor, a design element requiring
considerable mechanics and friction to deal with. The drawback
is the need for more magnet material.
Pitch and yaw drives are often necessary hardware elements to
meet turbine design objectives. VAWT machines have the advantage
of not needing yaw control, as it would be the same axis as the
main hub rotation. Similar to the drivetrain scenario, the fewer
parts the better for these systems. Direct drive is the best and
most reliable way to perform these motions, although the vast
majority of HAWT’s still have gears on these actuators.
Controllers provide another opportunity for balance. A good
turbine control system should have enough reach to thoroughly
monitor and track the condition of the machine. The most
technically relevant comparison of a state-of-the-art control
system today is an automobile. With distributed nodes all
performing relatively simple functions and communicating on a
CANbus interface, reliability, redundancy, and safety can be
insured. A modern wind turbine should at a minimum have a
controller that monitors vibration, bearing temperature,
generator coil temperatures, and pitch control actuator
performance as well as overall turbine output performance. A
comprehensive lightning protection system is also critical to a
machine controller’s durability.
The lack of any one of these sub-systems could allow a
catastrophic circumstance to occur. I believe this to be even
more important in the case of a distributed generation machine
that might be near people than a utility-scale machine that is
more likely in the middle of a field or ocean.
Conclusion
The same factors that make a durable turbine will also make a
low-maintenance one. A great wind turbine can bring a facility
owner many benefits. Clean and cost-effective power, local jobs,
a sense of pride, and an icon of commitment to protecting
natural resources are some. Along with the benefits comes a
responsibility to invest wisely.
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HTML http://www.renewableenergyworld.com/rea/news/article/2013/11/what-makes-a-great-wind-turbine-for-distributed-applications
Comments
A. G. Gelbert
November 4, 2013
Excellent and informative article! It's a great example of how
innovation will make renewable distributed energy the norm and
centralized renewable energy the main source of energy storage
for varying grid demands.
This statement, "The key to quiet is lowering blade speeds
without increasing storm wind loads. Is there a way to do this
without sacrificing efficiency of the turbine? Yes, but it does
require thinking outside the box. " shows a firm grasp of our
present blade technology.
However, I hasten to point out that during storm wind
conditions, blade efficiency is a non issue and wind loading is
THE issue you are addressing to prevent structural damage.
As a former pilot of light twin aircraft, I can testify to the
varying sound levels of a much more rapidly rotating set of
blades (aircraft propellers with a controllable pitch
mechanism). A "feathered" propeller is a state when, to avoid
wind resistance on the aircraft with an engine failure in
flight, the pitch of the blades are adjusted to position the
leading edge directly into the wind. In that position, the
propeller does not turn (windmill) even though the aircraft is
flying at 100 mph or greater. The other extreme of prop pitch is
the "flat" pitch position. This is nearly perpendicular to the
relative wind and provides maximum power (AND NOISE) for landing
and take off. The Less the angle of the prop to the relative
wind (cruise prop settings), the quieter the prop.
That said, a VAWT incorporating some of that whale bump low
drag, high efficiency blade design might also be used to lower
sound levels. The military have done a lot of work in keeping
helicopter blades as quiet as possible. There must be some
government research available to wind generator designers on
blade sound level dampening without sacrificing efficiency.
Good luck to your enterprise.
HTML http://thehalloffame.wikidot.com/agelbert
AGELBERT NOTE: The following videos were NOT posted above or at
the Renewable Energy World Original article and comments. . I
provide them here for clarification purposes to those unfamiliar
with blade action in different wind conditions. Enjoy! ;D
HTML http://www.youtube.com/watch?v=SemZcahGd6M&feature=player_embedded<br
/>
HTML http://www.youtube.com/watch?v=uTOJEP8jIGA&feature=player_embedded<br
/>
Variable controllable pitch mechanism
HTML http://www.youtube.com/watch?v=U15Z7k9a_nE&feature=player_embedded<br
/>
C130 controllable pitch mechanism
HTML http://www.youtube.com/watch?v=tIAfmY42siI&feature=player_embedded<br
/>
[b]More on controllable pitch props - They are called "constant
speed" propellers there is an automatic mechanism that attempts
to maintain the prop rotation speed CONSTANT for a given manual
prop setting. This video makes all this clear. You can see how
this technology is both well established (since the 1930s) and
useful for wind turbine applications.
Anumakonda Jagadeesh
November 4, 2013
Good article. A Good Wind Turbine for distributive power should
operate in reasonable wind velocities as durability and
dependability are major criterion.
Dr.A.Jagadeesh Nellore(AP),India
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