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Practical Wind Generated Electricity
By Carl Johnson, from Index of Public Service Pages.
Several technologies have
existed for a number of years that use the power that is present in wind
movements to create electric power. In general, the wind movement causes some
sort of rotary motion.
In virtually all practical devices, this rotation occurs at a rather low
rate. Most devices therefore use some sort of gear train to get a rotation rate
that is fast enough for electrical alternators.
Electricity is somewhat of an inconvenient commodity. It is extremely
difficult to STORE in any substantial quantity. Wind is also somewhat
inconvenient, because it is not constant or controllable. The consequence of
these two facts is that the need (or demand) for electricity seldom matches the
supply available from a wind generator. Alternating current electricity (what
supplies all of our homes and businesses) cannot be stored at all. These facts
have caused an almost universal reliance on a direct current (DC) system.
A limited amount of direct current CAN be stored, as in automotive batteries.
Since almost all common appliances only operate on alternating current (AC),
it is therefore generally necessary to use an Inverter to convert the
direct current into alternating current.
Therefore, a wind generating system will necessarily include: (1) a mechanism
to convert wind power to rotary motion; (2) a gear train or the equivalent; (3)
an alternator or generator; (4) a number of automotive type (or better)
batteries; and an Inverter. Each of these components can be of various designs,
but the function must be as described.
Mechanism
There are two main categories of mechanisms.
Wind-Axis Turbines
This category includes all mechanisms where the axis
of rotation must be oriented to face directly into the wind.
- An extremely early version of this mechanism was the Dutch-style windmill,
which was used primarily for milling grain.
- An early style of this mechanism was the American windmill that was on most all
farms in the early part of the Twentieth Century. It was a rather simple (and
somewhat inefficient) design, but it was only intended to pump water up a
well. Its extremes of performance were of little importance, as were its low
efficiency in converting the energy in the wind to useful work.
The efficiency of farm windmills is never above 15%, and that is only at a
fairly narrow wind velocity range, with efficiency dropping off rapidly for
both faster and slower windspeeds. This variety is technically called a
slow-tip-speed wind-axis turbine. Most actual windmills have flat blades and
not airfoils.
- An improved version is called the multi-blade wind-axis turbine. This
version uses airfoils instead of flat surfaces, which improves its maximum
efficiency to around 40%. Otherwise, the operation is relatively similar to
farm windmills.
- The third common version of wind-axis turbine is the Propeller-style.
These tend to be too expensive for small residential installations, and they
are almost universally used on giant towers on wind farms.
The Propeller style is what is called a high-tip-speed wind-axis turbine.
Because of the high tip speed, the theoretical efficiency can be higher,
around 45%. This higher efficiency probably explains the usage of
Propeller-style turbines on those wind farms.
On these large, expensive systems, often the individual blades are
rotatable, like on a helicopter rotor. These variable pitch blades can be
tilted to capture more or less wind energy, in order to try to maintain fairly
constant rotational speed.
Cross-Wind-Axis Turbines
This category includes wind turbines where the
axis direction is at a right angle to the wind direction. In practical terms,
this virtually always involves a vertical axis. There is a wonderful bonus from
this situation. It works identically well, no matter what direction the wind
comes from! So no provision for aiming the mechanism is necessary.
- An early style is generally called a Savonius Rotor. The spinning part of
a weatherman's windspeed device (anemometer) is a Savonius Rotor. Sideways
mounted cups catch the wind and cause the vertical shaft to spin. A Savonius
Rotor has an advantage over the farm windmill in that it does not have to be
pointed into the wind. It works equally well with wind from any direction.
Savonius Rotor has a maximum efficiency, around 30%.
That efficiency does not drop off as rapidly as most other designs (only the
propeller style has a wider range of windspeeds for high efficiency). In
addition, the Savonius Rotor has tremendous starting torque where most other
designs have very little torque at low rotational velocity.
This design is technically called a low-tip-speed (or slow speed)
cross-wind-axis turbine. No airfoil shape is involved, which is part of the
explanation for the very low efficiency.
However, the Savonius design is by far the simplest of these various
mechanisms. Nearly all of the others involve advanced airfoil shapes and
complicated structures. The economy and simplicity of a Savonius Rotor cannot
be matched.
- A very sophisticated cross-wind-axis turbine is the Darrieus Rotor. This
design looks something like an egg-beater, with usually either two or three
curved airfoils.
This design is technically called a high-tip-speed cross-wind-axis turbine.
The airfoils and the high airfoil velocities allows this style to have
efficiencies as high as about 35%, over a fairly wide range of wind speeds.
Another, Technological Design
There is another design that does not fit
in either of our main categories, called the Cyclogiro. This design vaguely
resembles the Darrieus Rotor but the operation is extremely different. The
Cyclogiro spins like a Darrieus, but its airfoils are vertical and straight
instead of curved. Those airfoils are individually controllable as to pitch
orientation.
A Cyclogiro is direction-sensitive and must be configured for a specific wind
direction. In operation, the individual rotor airfoils continuously vary in
pitch, to maximize the effect at some points in the orbit and to minimize the
wind drag in other places. Because of all this sophisticated technology, the
efficiency of the Cyclogiro is extremely high, around 60%. This is actually
higher than the theoretical maximum efficiency of any fixed airfoil design!
Unfortunately, the great complexity of the control systems and mechanisms have
tended to make Cyclogiros have minimal application.
In addition, physically large units have shown evidence of sometimes becoming
unbalanced, and a number of them have destroyed themselves as a result, even
causing some accidental deaths. There might be some possibility of controlling
the rotor blade pitch by a computer program, but this design still seems to have
stability problems.
Actual Functionality
Any moving material carries kinetic energy and
momentum. The basic laws of kinematics allow an easy analysis of a first
approximation of performance. Essentially, any wind-power mechanism captures
energy by slowing down the speed of the wind involved.
Undisturbed wind contains power from kinetic energy (energy flux) equal
to:
E = 0.5 * (rho) * V3 * (pi) * R2.
Note that this is a simple application of the kinetic energy definition. Also
note that the power is dependent on the THIRD power of V, the wind speed. A
20-mph wind has about 8 times as much power as a 10-mph wind, and a 40-mph wind
has about 64 times as much power. (rho) is the density of air.)
In case you're curious, a 60-mph wind (88 feet/second) has:
E = 0.5 *
(0.00237) * (883) * 12
or
E = 810 ft-lb/sec, about
1.5 horsepower per square foot of wind area!
You can probably see why strong winds knock buildings down!
A 10-mph wind has far less power in it, around 4 ft-lb/sec, or about 1/150
horsepower per square foot. A ten-foot diameter farm windmill intercepts about
78 square feet of wind area, so that (10 mph) wind initially contained about 0.5
horsepower in it. At its maximum efficiency of 30%, the farm windmill could
capture around 0.15 horsepower, a sufficient amount for pumping water. Not
really enough to seriously consider trying to make electricity! Such a windmill
might be able to provide a reasonably consistent 50 watts of electricity, not
even enough to light a single light bulb!
A crude Savonius Rotor made from two halves of a 55-gallon drum would
intercept about 10 square feet of wind, and its 14% efficiency would get about
0.01 horsepower from that 10 mph wind. If such a device was used to drive an
automobile alternator, only around 3 watts of reliable power would likely be
created.
Rankine first showed that simple analysis of energy and momentum establishes
that the MAXIMUM theoretical efficiency of any wind turbine is 4 * a *
(1-a)2, where 'a' is the fractional reduction in wind speed (called
the interference factor) from the original free flow to the location at the
plane of the turbine blade. This suggests a theoretical maximum at a = 0.3333,
where the efficiency would be 59.3%. If the free wind velocity was reduced by
one third at the plane of the turbine blade (and reduced by another third
immediately behind it, the theoretical maximum efficiency could be had.
In practical terms, there are swirls or turbulences in the wake that have not
been accounted for, and there are radial pressure gradients (centrifugal
effects) that are also not accounted for, in this simplistic analysis. More
thorough equations exist that better account for these matters, which are beyond
the scope of this article, and they fairly accurately represent the performance
of the various turbine technologies.
A farm windmill or a Darrieus Rotor only reduces the wind velocity by a
maximum of around 8% (16% total, including the wake slowing), and this accounts
for the maximum 30% efficiency. For a Savonius Rotor, the reduction in net wind
speed is around 3.5% (7% total) maximum for its 14% maximum efficiency.
Improvements
In general, very little research is being done on
wind-power capturing techniques. What IS being done is financed by the
government and the big power companies, and is virtually exclusively associated
with the two known methods of highest efficiency, the Propeller-style and the
Cyclogiro. Even the Cyclogiro has been getting little attention due to a number
of catastrophic failures of equipment due to stability problems. These two
technologies are both best suited to large-scale mechanisms, which explains the
government and power company interest as related to significant electric power
generation.
I prefer to concentrate here on potential improvements to some of the
low-expense designs, specifically the farm windmill and the Savonius Rotor. In
both cases, there are seemingly obvious ways to greatly improve their low
performance efficiency, and I have been surprised at not seeing the following
improvements regularly used.
In both cases, the devices are best suited for small-scale mechanisms, and
therefore the government and power companies seem to have little interest in
refining them. However, improved versions of either of these small-scale
technologies make electric power generation for an individual home very
realistic in many areas where there are reasonable winds. This is the thrust of
these suggestions for improvements.
Improved Savonius Rotor
(I invented these two improvements in March,
1998.)
Two obvious levels of improvement seem possible. Both somewhat defeat the
non-directionality advantage of the Savonius, but they greatly improve
efficiency to a point at or above other technologies.
Improvement 1 - Rotatable Shroud
The greatest source of inefficiency of
a Savonius is the need to push the back side of one 'cup' INTO the wind while
the other 'cup' is catching the wind, creating useful power. This is essentially
like a waterwheel that has water randomly coming down at it from all points.
Such a waterwheel would still work, but much of the productive action of the
filled 'cups' would be wasted in pushing the upward returning 'cups' up against
the down-flowing water there. A waterwheel is far more efficient by selectively
having the water ONLY hit the downward moving 'cups'. This is the concept of
this improvement.
A large enclosing cylindrical 'shroud' surrounds the whole Savonius Rotor,
and this shroud is mounted on bearings on the vertical shaft so it can
independently rotate around the Rotor. It is only slightly larger than the
Savonius Rotor, so there is relatively little clearance between the inside of
the shroud and the moving outer edges of the Savonius rotor cups.
This shroud has a 'tailfin' to always orient it in a specific way to the
wind. This is very much like the idea of a weathervane, which therefore always
points directly into the wind.
Roughly half of the 'front' of the cylindrical shroud is cut away, exposing
the catching 'cups', while the remaining half of the 'front' of the shroud
blocks the wind from hitting the back side of the returning 'cups'. At least
half of the rear side of the shroud is also cut away, to allow the air to leave
after it has given up its power to the Savonius Rotor.
This simple improvement more than doubles the net efficiency of a Savonius
Rotor, essentially to the level of other technologies. It also tremendously
increases the starting torque.
Improvement 2 - A Better Shroud
The shroud described above greatly
improves the performance of a Savonius Rotor, but there are two problem areas
that would remain. Half of the area that is presented to the wind is blocked off
by the presence of the shroud covering the forward-moving 'cups' (so the energy
in that air is not captured), and a LOT of turbulence occurs in the wake of that
non-aerodynamic configuration. This leads to the second level of improvement.
The front of the shroud would be extended several feet forward. This forward
section would have a funneling effect, where the very front of the shroud would
extend the full width of the entire Savonius/shroud width, but the airpath would
taper to about half of that width where it entered the original shroud opening
to feed pressurized air to the productive 'cups'. Basically, this now captures
the air over the entire area of the Savonius/shroud, virtually twice as much air
and therefore nearly twice as much available energy.
Appropriate aerodynamics is important here, both inside the air funnel and
exterior to it, to reduce turbulence losses in both areas. For example, the
intake tunnel should ideally not be a constant taper but it should have the
shape of an exponential horn, to better match the acoustic impedance of the
Rotor intake with that of the ambient surroundings. (Loudspeaker engineering
concepts are used here, for the exponential horn design.)
The rear of the shroud would also be extended several feet (rearward). The
exiting funnel shape is more complicated here, but an exponential horn shape is
pretty close to the best shape to match the existing acoustic impedances.
This second improvement also greatly improves the net efficiency of a
Savonius Rotor. With well-designed and engineered exponential horns for acoustic
impedance matching, this improved Savonius Rotor version can produce enough
power to supply a substantial portion of a household's electrical needs. Because
of the intake horn, the 10-mph wind is traveling at 20-mph as it gets to the
rotor cups, so it contains eight times as much power per square foot, in
accordance with the equations above.
Even a 55-gallon-drum sized Savonius can realistically create around 100
watts of relatively reliable electricity. Bigger ones obviously could provide
even more. However, with the extreme universal availability of surplus 55-gallon
drums, it seems logical to just make ten of these assemblies to be able to
provide a consistent Kilowatt of electricity.
Regarding bigger ones: A simple Savonius Rotor with two six-foot-high by
three-foot-wide cups would intercept about 36 square feet of wind area. With
that 10 mph wind speed of our previous examples, that represents more than 0.9
horsepower of initial wind energy. After losses for the Savonius mechanism,
friction in bearings, pulleys/gears and losses in the alternator, this might
realistically
create around 400 watts of reliable electricity.
Such a Savonius and (fiberglass? plastic? aluminum?) shroud could be
constructed quickly, easily and inexpensively, and could be hooked up to drive
an automotive alternator hooked to some car batteries. At very low initial
expense, a reasonably consistent source of electricity for (some) house lighting
could be provided!
(I still prefer the 55-gallon drum approach, especially for remote locations.
In case future repair/maintenance is needed, replacement drum halves can
probably be found.)
A Later Revision!
In late 2002, I have discovered that this version of a
Savonius and this shroud can both be substantially simplified, and it even
improves the performance!
Since the airflow now is exclusively on one side of the rotor, the shroud
actually only needs to surround that one half. A front quadrant should probably
still remain to shield the forward-moving portions from being directly in the
wind.
In addition to this, if the shroud is reasonably close fitting around the
rotor, a cup-shape for the rotor blades is really not necessary. Once the air
has entered into the intake funnel, it really has no choice but to push the
rotor blades through. The latest version resembles the rotating doors on big
department stores. A person (or air) cannot get by without pushing the door
(blade). As to the number of these new flat blades, I am still experimenting.
Two seems like a bad idea, because if the blades happened to be aligned with the
wind, it would never rotate. Three would always rotate, but construction and
static and dynamic balancing are harder. Four seems to be a currently attractive
number. The air path is always blocked by either one or two blades, so it would
always self-start, and that structure is naturally symmetric so static and
dynamic balancing should not be too difficult.
One reason why this actually improves the overall performance has to do with
the exiting air. The cup shapes have a tendency to keep air in the bottom, so
some of the air is somewhat sucked over to the returning side, causing
substantial turbulence. Flat blades, instead, do not have such a sucking action,
and so the air is freer to continue to travel relatively straight. There is
still turbulence generated, but far less than with the cup-shaped blades.
Improved Farm Windmill
(I invented these improvements, as a 'Duct-Axial
Wind Generator' in 1975.)
This design, too, can benefit from several different levels of improvement.
Improvement 1 - Another Shroud!
The greatest efficiency loss of a farm
windmill is due to air that hits the blades doesn't all have time enough to
continue on through the slots between the vanes, and much of it 'spills'
radially outward to go past the turbine structure and be wasted.
This improvement essentially places the farm windmill turbine inside of a
large (horizontal) cylindrical shroud. Technically, this is called a 'ducted
turbine'.
A normal farm windmill has its turbine upwind of the tower it is mounted on.
There are a variety of stability reasons why it seems better to have the actual
turbine wheel rearward of that axis with a shroud surrounding it. The shroud is
basically a tube that is collecting and enclosing wind (and wind pressure). Once
air is within the shroud, it has no choice but to eventually pass through the
turbine wheel in order to escape. In the process of this, we are developing a
'stagnation pressure' inside the shroud just forward of the turbine wheel. This
modification somewhat changes the concept from being a 'momentum' capturing
device to one that uses this dynamic pressure differential to rotate the turbine
wheel.
The presence of the shroud slightly increases the maximum efficiency, but it
also greatly widens the windspeed range for high efficiency operation. The
result is a much greater actual average power output during the natural
variations in wind speeds.
Improvement 2 - A Better Shroud
As with the Savonius, aerodynamic design
considerations can GREATLY improve acoustic impedance matches for both intake
and exhaust. Again, versions of exponential horns are very close to ideal
choices.
Improvement 3 - Avoidance of Self-Destruction!
A ducted turbine design
is SO efficient at developing the desired pressure gradients, that this can
represent a problem. In extremely strong winds, a LOT of pressure can develop
inside the shroud, pressing against the turbine wheel structure. Using previous
examples, if our shrouded farm-style windmill is ten-feet in diameter, in a 60
mph wind, there is about 9.2 pounds per square foot of force pressing against
the turbine structure, or 720 pounds of total (static) pressure pushing on it.
This represents around 115 horsepower present there. Such huge forces could bend
or break the mechanism.
A simple solution exists! Each of the (flat, not airfoil) blades of the
turbine wheel would be mounted to the turbine structure slightly differently.
Rather than being rigidly mounted (at a specific angle) to that structure, only
the front (leading) edge of each blade would actually be attached to the
structure, and that would be on a hinged mounting. Pressure from the wind
(inside the upstream shroud) would therefore act to 'feather' the blades and
greatly increase their tilt angle. A simple tensioning spring (like a screen
door spring) would normally hold each blade in its 'preferred angle' position.
Operation would be as follows: At low windspeeds, the blades would remain
very flat with the plane of the turbine wheel surface, very much resembling a
normal farm windmill. At higher windspeeds, the greater stagnation pressure in
front of the turbine wheel would cause each of the blades to stretch its spring
and feather back, basically opening up and allowing much of the air to pass
right through without further increasing the stagnation pressure. At extremely
high windspeeds, like our 60 mph example, the blades would be pressed back
completely 90° so the high speed wind could pass fairly freely through without
applying any significant presssure on or doing damage to the structure.
This improvement has an additional advantage of representing a sort of
automatic speed control for the turbine. With very low speed wind, the blades
are very closed and thus capture a substantial portion of the available energy.
As wind speeds would increase, the blades would automatically tend to open up,
to minimize the effect of over-spinning the turbine wheel. And, as mentioned, at
extremely high wind velocities, the blades would open up completely, which
essentially stops the system from trying to capture any significant portion of
that great wind energy.
There are several parameters that are pre-settable with this configuration.
Stop blocks would keep the individual blades from closing completely (due to the
spring action) and establishes the performance efficiency at low wind speeds.
The spring lengths and strengths can be chosen to allow the blades to open up at
any desired rate. The local wind history for an area, and the size and
construction of a specific turbine and shroud, would determine these choices,
and experimental trials would probably be necessary to establish the ideal
choices for a particular installation. For example, if a turbine structure was
made of light, thin materials, it might be more susceptible to damage from high
windspeeds, so weaker springs would be chosen, to allow the blades to feather in
lower soeed damaging winds.
Additional engineering and design is possible on each of these improvements.
They have all included various assumptions, in order to keep the mechanisms as
simple and low-expense as possible, while still giving high performance and
efficiency.
For example, in this last Improvement, there is actually an additional 'drag
coefficient' that exists in the calculations. Since our blades were normally
virtually flat facing the wind direction, we assumed a drag coefficient of
around 1.18 that applies to such situations. As the blades feather, this drag
coefficient drops to near zero, which explains the stoppage of useful power
production at very high windspeeds. At intermediate windspeeds, or with airfoil
shaped blades, that drag coefficient can be modified. For the purposes of these
inventions, it did not seem critically important to absolutely maximize the
ultimate performance at massive financial expense in construction. I am saying
this here in the event that some engineer wants to further refine these designs.
We have concentrated on the 'Mechanism' in this article. The other necessary
parts of the system, the gear train or pulleys, the alternator or generator, the
batteries, and the inverter, are all well-established technologies that are at
the point of being proven, economical items. Any of the 'improved' versions
could be arranged to spin an automobile alternator (which often includes a
voltage regulator inside it) to keep half a dozen generic car batteries charged.
Then, either low-voltage (12 volt) wiring and lights could be installed or a
standard inverter (12 volt DC to 110 volt AC) could be used to provide the
desired lighting or electricity. If you build the mechanism ($200 materials),
you get a rebuilt alternator ($50), you get six $30 new batteries ($180), and a
standard inverter ($100), you will only have spent around $500 to have a
wind-powered electric system. Depending on whether you could mount it on a
garage roof or a utility pole, or if you have to buy a strong tower, there's
another expense that could range from near zero to several thousand dollars. In
any case, there's the possibility that you could create an inexpensive
wind-powered electric system. Back in the 1980s, many people were generally
charged around $7,000 for wind-generating systems that did not perform much
better! Worse, they tended to include unique circuitry and unique parts, so if
anything broke or failed, the owner was up a creek! The approaches described
here all use very commonly materials. If the automotive alternator you use fails
some day, you would just find a different one!
Carl Johnson,
from Index of Public Service Pages.
© Copyright energysavingnow.com 2000.
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