Astounding amounts of money is being spent (primarily by governments such as USA and Germany and Spain) in installing very large numbers of these devices, nearly all of which are three-bladed rigid-rotor (medium sized) and two-bladed teeter-rotor (very large sized) HAWT designs. In certain areas, very impressive amounts of electricity are being generated. There are two very clear concerns that no one seems yet interested in even considering!
No one seems to have told those lawmakers or taxpayers that transporting electricity long distances is not very efficient! This has been known for a hundred years, and designers knew even then that there was value in sending the electricity at the highest possible voltage, to reduce the Current needed to transfer a specific amount of electricity. THAT is why there are High Tension Lines everywhere! Most of them operate at just over 100,000 volts. Really long lines are often operated at around 500,000 volts.
Even then, the wires pretty are much like the wires inside your kitchen toaster when carrying extreme amounts of electricity. In fact, the standard design rule is to design everything so that around 90% of the electricity put in at one end of a 60-mile long stretch of high-tension lines comes out the other end. Ten percent of the energy put in is therefore wasted, as heat, along that 60 miles. This pretty much explains why virtually all electric power plants are build within around 60 miles of the center of a major city!
But consider a SECOND 60-mile stretch (for a total of 120 miles). We only had 90% get through the first section, and ten percent of the remainder will get wasted in the second section (or 9% of the original). We can also think of this as being (9/10) that gets through a section raised to the second power (because of two sections) as being the amount that actually gets through. Well, if we are talking about trying to carry electricity for over 2,000 miles, it turns out that very little actual benefit will really be obtained! If we say 1,980 miles, that is 33 of those 60 mile segments. That means that the amount that would actually get to Southern California would be (0.9)+33. This turns out to only be 3.09% of the electricity created by the North Dakota windmills would ever actually be able to arrive in Southern California. The other 96.91% would have been lost along the way as heat by the hot wires!
So what is the likely effect if we take really massive amounts of power out of winds? The effect slows down those winds to around 2/3 their original speed. Is that a bad thing? NO ONE KNOWS because NO ONE HAS EVER EVEN INVESTIGATED IT!
Yes, with the rather limited energy we now extract from winds, there probably are few serious side effects. But we are getting far less than 1% of the needed electricity from wind now. What will be the environmental, weather effect in the future when we have 20 times as many or 100 times as many of those giant wind farms in operation? No one knows. I have a suspicion that it will "surprise" the leaders, as they tell the public, we never even knew that there could be this problem! Just like New Orleans leaders all seemed totally surprised that the levees broke during Katrina. Duh!!! Is THIS the level of Design and Engineering and Planning we have sunk to? Only think about the consequences for the coming week, and assume that no future problems could ever arise?
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. In fact, it can ONLY be stored as Direct Current (DC), as in batteries, and Alternating Current (AC), as in your house, cannot be stored at all. 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) WECS 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. The AC can then be used in the house, or even sent back into the Power Grid, for potential profit! This is an aspect that caused many people to spend the $7,000 plus installation cost back in the 1980s for small WECS systems. Sadly, as explained below, virtually none of those people ever received more than a dollar grand total for providing power to the Power Grid! The customers had been seriously misled, or, as I see it, lied to.
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.
In recent years, our government has invested enormous amounts of (taxpayer) money in "windfarms" where large numbers of amazingly large windmills look like fields of daisies! Sadly, each of those installations I have visited have had only a few actually facing the wind and rotating as intended! Some are usually just stationary, and a few are usually pointed in wrong directions! For this and other reasons discussed below, it seems like a somewhat foolish investment of taxpayer money, although politicians want to "look good" with such projects because it seems like they are really caring about our future.
The efficiency of farm windmills is never above 30%, 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 old farm windmills have flat blades and not airfoils.
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 explains the usage of Propeller-style turbines on those wind farms.
There are two varieties of the propeller-type which have been used, two-blade and three-blade. Nearly all modern wind farms use three-blade, rigid-rotor for medium sized turbines and two-blade, teeter-rotor for the very largest turbines. It does not seem appropriate here to discuss the relative design advantages of each.
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, and to survive serious storms.
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 VAWT mechanisms. Nearly all of the others involve advanced airfoil shapes and complicated structures. The economy and simplicity of a Savonius Rotor cannot be matched.
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 32%, over a fairly wide range of wind speeds.
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 can be extremely high, around 60%. This is actually higher than the theoretical maximum efficiency of any fixed airfoil design (because of all the tweaking of airfoil orientation angles)! Unfortunately, the great complexity and cost 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.
Undisturbed wind contains power from kinetic energy (energy flux) equal to:
E = 0.5 * r * V3
* p * R2.
Note that this is a simple application of the kinetic energy definition. It is an equation that Rankine derived long ago. 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. (r is the density of air.)
The analysis of momentum by Rankine produced the following equation for the axial torque applied to a turbine:
T = 2 * p * R2 * r * V2 * a * (1 - a)
r is the air density or 0.00237 lbf * sec2/ft4.
For a ten-foot diameter (R=5) farm windmill in a 60-mph wind (88 ft/sec), this total Thrust calculates to be a maximum of 775 pounds, quite a horizontal load on the turbine rotor for the tower to have to withstand. (This calculation is based on an "ideal" efficiency where a = 1/3, something that is very difficult to approach for a farm-style windmill.
In case you're curious, using that kinetic energy content equation presented above, we can see that 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 really strong winds can knock buildings down!
A 10-mph wind has far less power in it, around 3.7 ft-lb/sec, or about 1/150 horsepower per square foot. For reference sake, a horsepower is 746 watts, so this 10 mph air has around 5 watts of power in it. (There are few locations that have more than around 10 mph average windspeeds near the ground). Notice that only around 1/3 of this power can actually be converted to rotary motion (the ideal value of a in the Rankine momentum equation above), so we are only talking about 1.7 watts of (mechanical motion) output for each square foot of wind area blocked. This explains why farm windmills were always quite large!
A ten-foot diameter farm windmill intercepts a maximum of 78 square feet of wind area, so that (10 mph) wind initially contained about 0.534 horsepower in it. At its maximum efficiency of 30%, the farm windmill could capture around 0.16 horsepower, a sufficient amount for pumping water. The 0.16 horsepower actually collected is around 120 watts. That is not really enough to seriously consider trying to make electricity! Such a (fairly large, ten-foot diameter on top of a fairly large tower) windmill might be able to provide a reasonably consistent 50 watts of electricity (when the wind blew), not even enough to light a single modern home light bulb! Of the 120 watts, there are many losses that cannot be avoided. There is gearing it up to a high enough speed to drive an alternator, which has a lot of frictional gear or belt losses, and then the alternator is only around 80% efficient at best, and then the battery has inefficiencies, which is why the 50 watts is actually somewhat optimistic.
You probably know of people who announce that they will be Energy-Independent because they will make 2,000 watts or 5,000 watts of electricity from wind. Can you realize how large the turbine has to be for that level of production? Salespeople sometimes SAY such extravagant things, because they know in enormously increases the enthusiasm of the customer, especially regarding spending $7,000 or far more, as the salesperson sees a very attractive Commission for himself IF a sale gets made!)
A crude Savonius Rotor made from two halves of an old 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. This can seem somewhat discouraging! But any salesperson for such products or systems will say spectacularly different things!
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. But they make clear that ACTUAL performance can never be very close to these theoretical numbers.
A farm windmill or a Darrieus Rotor only reduces the (average) wind velocity by a maximum of around 8% (16% total, including the wake slowing), and this accounts for their 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. As noted above, the extremely huge new propeller turbines on wind farms can have efficiencies around 45% (when the wind is blowing, of course). A wind farm might cost $100 million to build and install, and it might realistically produce a total of 10 Megawatts of power when there is wind. IF the wind would keep going for all 8800 hours that are in a year, that would mean that it could provide 88 million kilowatts per year. At current large-quantity electricity prices, that would mean around $5 million each year. Of course, that would be gross income, having to cover all the employees of the site, and all materials and repairs. If such a wind-farm could generate even $1 million in net profits, it would not even come close to covering the Interest on the $100 million that was invested, much less EVER pay for itself. You might note that I do not see much cause to be a huge fan of large-scale wind-farms. However, SMALL-scale setups to provide electricity for an individual house, seems like a wonderful idea! Especially if the total cost for the equipment can be kept below around $20,000! (Your annual electric costs are probably around $1,000. If interest rates on an investment of $20,000 is no higher than 5%, that is also $1,000, which means that such a system would at least hold its own! If the system cost' LESS than $20,000, it might even have a chance of eventually paying for itself!
This is the general logic behind this presentation. Simple, traditional farm windmills or Savonius rotors cannot make enough electricity to even be worth the trouble of building (personal opinion). But with some fairly simple improvements, their performance can be improved to several times as much output, and a simple and inexpensive (maybe under $500) system might actually be able to provide noticeable amounts of electricity.
One spectacular problem that occurred many times in the 1980s and early 1990s was where the designer had not taken into account the design factors of forced vibrations, and specifically the Strouhal number calculations! As a result of such Engineering blunders, when the wind would be at a certain speed to cause the turbine to rotate at a specific speed, that speed would be a resonant frequency of the tower structure! The tower would start violently shaking and destroy itself. This problem has been well known for at least 60 years, as many early tall smokestacks would destroy themselves in surprisingly moderate speed winds, and you may have seen the popular movie of the Tacoma Narrows Bridge which vibrated and twisted itself into destruction around 1940.
Another related problem also occurred many times in large windmill installations. Each time a blade would pass behind or in front of the tower structure, there was a brief loss of thrust and great change of local forces on that blade. Some large windmills wound up having the rotors wildly vibrating, and at the time, no one seemed to even know why! Same thing, a simple application of the Strouhal number could have identified potentially dangerous rotation rates, and simple stiffening of the tower easily always solved that sort of problem. If the wind would ever be 400 mph, there may then be a vibration problem, but the entire system would probably have fallen over first in such ridiculous winds.
There are a number of other serious Design / Engineering issues in the truly huge devices now being made. Some are so large that the lengths of the rotor blades are as long as a football field, around 300 feet! They also have considerable weight. When such massive and huge objects are expected to reliably rotate for years on end, many complicating surprises often show up. One of the more interesting is that the rotors are now so huge that the weather / windspeed for one portion of the rotating motion is different than for a different area! This can cause immense mechanical stresses on the structure of the blades, the rotor shaft and bearings and the tower. Since no one has ever built anything of that size that is intended to move and survive intense storms, designers are often at a disadvantage!
There are other really serious problems that often apply to the home/commercial-sized propeller-style units. When those many people in the 1980s were paying small fortunes for their high-tech windmills, not only were they led to believe that they would make 2,000 or 5,000 watts for their own use, they were sold on the concept where they were going to be able to sell huge amounts of electricity back to the Utility Companies, a sort of revenge! Unfortunately, the designers of the systems had massive lack of understanding of what they were trying to design and sell!
One of the most severe problems was / is that the electrical generators commonly used are poorly suited for this application. Nearly all of the home/commercial-sized windmills sold in the 1980s and 1990s used either synchronous or induction generators, because the cost of such types of generators is moderate enough to be able to sell the systems! However, synchronous generators are great IF the spin rate is exactly controlled and constant. When the spin rate changes, such generators create very large harmonic voltages in the Power Grid!
One popular supposed solution to this over the past 20 years has been to use a control system that included a synchronous inverter to convert created DC voltage into AC. This control system generally is of very reasonable cost, and fairly simple, and it works fine for an individual home, but when it tries to feed into a power grid, have power quality and harmonic injection problems, and there are resulting inductive LOADS on the Power Grid, and generally wind up DRAWING MORE inductive (volt-ampere) power FROM the grid than the watts they are (resistively) putting into it! Utility companies also did NOT like the fact that their power lines were then having undesirable and destructive resonant voltages in them. As a result, Utility companies today insist on only accepting certain types of WECS systems for providing energy to their Power Grid lines. Nearly no home-sized systems qualified.
The other kind of generator, the induction generator, is generally more expensive, and it can tolerate a SMALL range of frequency variations but it had its own set of problems. The point here being: You should probably not really count on your local Utility Company agreeing to pay you for excess wind electricity that you want to put onto their Grid. When they agree to such an arrangement, they now usually insist on extra electronic controls that are quite expensive, probably so much so that you would never sell enough excess electricity to ever pay for that extra equipment. Better to simply think about providing electricity for your own house, and maybe a very dear neighbor!
One consistent problem that seems to show up in all of the larger-sized WECS installations has to do with the vibrations and stresses in the equipment. Some is due to wind gusts and turbulences, while others are due to the vibrations of the rotating turbine and other components that have to be able to move. Over time of operation of such systems, it has been sadly common that various fasteners become loosened from the vibration and stresses. Depending on which fasteners come loose (and usually fall out) the range of the bad things that then happen is pretty broad.
If the entire cost of an installation is added up, including the turbine, the tower, labor, the electronics and control system, and all other costs, it is likely that around $2,000 will need to be spent for each Nameplate kilowatt. So for a system that would have the maximum capability of 5 kw, this is around $10,000 of investment. However, the wind does not always blow at the high speed needed for maximum performance! In fact, when all conditions are considered, a Capacity Factor defines what average performance can be expected. For a turbine that is mounted on a 50-meter (160-foot) tall tower, in a consistently windy location, the capacity factor can be around 20% to 25%. This means that the $10,000 investment just discussed could realistically be expected to produce an actual average of just over one kilowatt of electricity. You could only use a toaster (1.5 kw) when it was especially windy and you were using no other electricity! At the current common 10 cents per kwh, in a year of 8700 hours, around 9,000 kWh of electricity could be expected to be created, worth around $900. If there were no need for maintenance or repair, that would mean that it would take around 11 years to pay back the original investment, but actually at least twice that long to also make up for the interest expense on that borrowed money.
Regarding the government-financed projects, they all have tremendous drawbacks that no one seems to notice! Above, we noted Southern California's heavy investing in wind farms in North Dakota. Assuming they manage to generate a million watts of electricity from all those giant propeller-type wind turbines. They would then have to use inverters and transformers to convert it to high voltage alternating current to be sent along high-tension powerlines. (There is loss in doing these conversions.) Instead of 1,000 kW actually created by the windmills, only around 20 kW would actually arrive, nearly enough for one city block of homes! Definitely NOT worth the millions of dollars of investment in such a silly project! And, in even attempting it, the other 980,000 watts of electricity put into the wires would simply heat up the atmosphere, one of the worst possible side-effects!
This also should make clear why I tend to focus on small-scale LOCAL electricity production, for a single household!
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, these devices are best suited for small-scale mechanisms, not being suitable to scaling up to giant versions, 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.
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.
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.
Begin with the same cylindrical shroud described above, but add an intake chute to the front, and possibly an exit chute to the rear. The front of the shroud would therefore 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 frontal area of the Savonius/shroud, virtually twice as much air, and therefore nearly twice as much available energy (doubling the performance, and actually better).
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.)
This Acoustic Impedance essentially means that any possible turbulence is virtually eliminated, and the pressure and velocity of the air is changed very smoothly and evenly along the exponential horn.
A number of people have shown interest in this concept, but are not familiar with an exponential horn. And, without drawings, I guess it is hard to visualize this intake chute. The following description is meant to help!
Again, begin with a cylindrical shroud just slightly larger than the width and height of the Savonius rotor itself, just big enough so nothing rubs when either rotates. And slightly less than half of the front surface of that shroud is removed. When it is operating, only the inside of one of the rotor cups would be visible through that opening. Now, extend the OUTER wall STRAIGHT (flat) ahead, say 6 feet, with the full height of the shroud. That piece is therefore rectangular, right?
Next, there is another rectangular piece, also with the full height of the shroud, and maybe also 6 feet in the other dimension. This piece should not be too thick, because it has to be curved, flexed. It will also attach to the cylindrical shroud, but at the edge of the opening that is near the middle of the front, initially also sticking straight forward.
Now, make another rectangular piece exactly the same as the FIRST, flat outer wall. Mount it on the opposite side of the cylindrical shroud from where the first piece was attached, and it also points forward. There is NO hole in the shroud anywhere near this piece! Its function is only to allow air passing nearby to not be disturbed by the irregular shape of the exterior of the structure.
The assembly should now have three vertical panels sticking forward from the cylindrical shroud. Now, the middle one of these is now bent, bowed, so that it's outermost edge meets the frontmost edge of the third piece described. They would be attached together, at least temporarily. There should now be an empty space trapped in front of the non-open side, and there should be a funnel-shaped opening into the rotor itself.
A top and bottom (flat) surfaces are then added to enclose those two spaces and to add structural strength to the three surfaces.
Now that you understand the five pieces that are necessary, you can pre-plan building it. The middle vertical panel must be curved in a specific exponential shape. It is easiest to create the correct shape by first attaching a 2x10 to the floor and roof pieces. One edge of these 2x10s is cut to follow a curves shape. Once those backing plates are mounted to the top and bottom surfaces, the inner surface can be very easily and smoothly bent against them in getting its front edge to meet the other front edge.
A simple way to do this is to draw out on the floor and roof panels, the curve that will be necessary. For an intake chute that is to extend three feet in front of a six-foot diameter shroud, the outer side walls would be the (3 + 3) 6 foot dimension mentioned above. The initial width of the chute would be the 6 feet of the full shroud width and it would narrow down to slightly less than half of that, slightly less than 3 feet wide. The following chart could be used to draw the curved line:
| Dist.Forward | Width inches | Width as a decimal |
|---|---|---|
| 0" | 35.0" | 1.000 |
| 3" | 37.2" | 1.062 |
| 6" | 39.4" | 1.127 |
| 9" | 41.9" | 1.197 |
| 12" | 44.5" | 1.271 |
| 15" | 47.25" | 1.350 |
| 18" | 50.2" | 1.433 |
| 21" | 53.25" | 1.522 |
| 24" | 56.5" | 1.616 |
| 27" | 60.0" | 1.716 |
| 30" | 63.75" | 1.822 |
| 33" | 67.75" | 1.935 |
| 36" | 72.0" | 2.054 |
The last column is straight from a standard exponential chart from any math book. The middle column is simply 35" times that number, and represents the distance from the OUTER edge of the chute.
The top line indicates that the chute width would be 35" as it releases the air into the rotor. The bottom line indicates that it is 72" wide at the very front, so all the air that would have hit the Savonius is now re-directed to go through the rotor.
This chart could be used for any size Savonius rotor, and any length of intake extension. Just start with the width of the opening that will feed the Savonius cups (taken here as 35", slightly less than half the 72" width of the actual rotor to provide for the center vertical axle shaft). Multiply the value in the last column by that measurement to get each width value.
As to other lengths of the intake extension, consider this: In principle, the intake chute could be made 1" in length! But then, the curved inner wall would essentially be flat against the wind. It would technically be an exponential horn, but the air wouldn't have enough time to adjust for its tapering shape.
On the other hand, consider a 50-foot long intake extension. The incoming air would have plenty of time to move sideways the couple feet to adjust for the tapering shape, and it would have extremely high overall efficiency. However, the outer shroud, including this intake extension, must be able to rotate to face the wind! Imagine the tail structure that would be necessary to swing such a huge intake horn into a strong wind! And imagine that it would knock down anything or anyone that was in the way!
As long as the intake extension is at least as long as the width of the final output, as in the example above (both 3 feet), the air generally has adequate time to adapt and the efficiency will be greatly improved. However, a six-foot long extension on the example system above would increase performance efficiency by several percent. It may or may not be worth considering! In general, our attitude is that, if a few percent more output power is a serious concern, just make the Savonius rotor a couple inches bigger in diameter, or taller, or both. In any case, the actual effect of any length of exponential horn intake can be calculated, although it is fairly messy to do. Our experience is that an extension length equal to the half-width of the Savonius rotor seems to provide about as much of improvement as is practical without having a really long extension swinging about! An intake chute extension length equal to the whole-width of the Savonius rotor seems to give around 2% or 3% higher overall efficiency and significantly better performance.
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.)
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.
This design, too, can benefit from several different levels of improvement.
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.
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 pressure 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.
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.
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( http://mb-soft.com/public/othersci.html )
C Johnson, Physicist, Physics Degree from Univ of Chicago