First, everything I state in this article is my opinion,nothing more. I do not agree with everything that I read about alternative energy, and I certainly do not expect everyone to agree with me. I welcome friendly debate. Got a question? At the end of this article is a place for comments. If you have a question just put it there. If I can find you an answer, I will put it with your question, Thanks.
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I will try to first do an overview of systems in general, and then a more in depth discussion of a system that I have operating. Now, even if your planning on spending your hard-earned cash on a complete system, you stand to gain a wealth of info and knowledge by trying your hand at scratch-building a small system. Even if it is just a simple one with one or two batteries and a light or two.
If your thinking of a small-scale wind-energy system you have lots of options. There is no end to the amount of companies that are willing to sell you a system that is almost right for you. The problem is that the average persons electric bill can not begin to justify the cost of a grid-tie wind system. Two years ago I had a wind site assessment done on my property, after which I was given some suggestions for a few systems that might work for me. Comparing the system costs against the savings on my electric bill, any system that I might have chosen would have been an antique before it had payed for itself. Now, I'm not claiming that this means all the systems are bad, but at my location,given an average wind speed between 12 and 13 MPH, there would simply not be enough energy produced to justify the cost. Remember that the companies that are selling the units have probably not been out to your site to make an assessment.
If you are considering any type of wind system, even just an experiment with one, you really need to know at least a little about average wind speeds at your site. By the way,when I had my assessment done, most of the cost was picked up by a grant from Focus on Energy, so look them up. You can also find information about your area on-line.
Now, lets talk about the generators themselves. Thirty years ago I had tried my hand at wind generators just like a lot of people, using automotive alternators and generators. A few sort of worked,but sort-of is not what we need. The problem with using automotive parts is that to produce power, they are engineered to operate at speeds considerably higher than your system will run. This means that we need to put together a system that can produce usable power at a lower speed. And a lower RPM unit is kinder to itself, with less wear. And unless you have NASA building your rotor, less RPM means less vibration.
I will start at the rotor, and try to work my way through the system. If anyone has ever had their car battery jumped, you may have noticed that when the cables are connected between the cars, you can hear the motor of the running car drop a little in RPM. That is because the alternator on the running car is having to work harder. It takes just about one horse to produce 750 watts. So, the horse power, which is derived from RPM and torque, is one factor in setting the limit of output. I have tried to carve a few sets of blades over the years. I currently have a small wind system on my cabin that had blades that I put an awful amount of time into. And although they looked nice, I was surprised at the increase in performance when I replaced them with a fiberglass set that I purchased on EBAY. You are aware that a longer lever can increase power. The diameter of the rotor on a system is no different. But, everything in this world is a system of trade-offs. As moving parts increase in size, balance becomes more critical. And remember, you are putting together a machine that will be high in the air, meaning that if it lets go, fast moving pieces can cover a wide area.
But, enough rambling. Back to producing power. Before we get into losses of the system, we need to cover more about those blades. First, the area of the circle that the blades cover will set how much power is available. The formula for the area of a circle is (3.14 X Radius ^2). So if yo have a small system with blades that make a circle 6 feet in diameter, the formula would look like this: 3.14 X3^2, or 3.14 X 3 X 3.= 28.26 square feet. Now ,because the area is derived by multiplying the radius^2, by 3.14, you will find that even a slight increase in the diameter of the rotor has a proportionally larger increase in the total area. If our system has blades that make a circle seven feet in diameter, our formula looks like this:3.14 X 3.5 X3.5 =38.465 square feet. So an increase in total length of app. 16% produced an increase in total area of app. 10.20 square feet ,or an increase of app. 36%. But the output of power would increase considerably more than 36%. Now, ever notice one of those farm windmills with all those blades? More area means more starting torque to get that heavy pump jack going. But it simply would not have the rpm you need. Gear it up? Even though it has more starting torque,power losses from the gearing would mean that it would make less electrical power than a three-blade system of the same diameter. In working with home brew systems, it would be pretty hard to come up with an exact formula for square feet equals a certain amount of Watts, but as a general rule, from what I have seen, If your system has a diameter of 7 feet, you might expect wind speeds in the range of 12 to 15 MPH to produce around 250-300 watts. Now,if you have built one or two smaller systems and are looking to break that 1000 watt barrier, you will need at least a 10-foot diameter rotor and a little more wind. And by the way, if your shopping for a system, and they claim 1000 watts with an 8-foot rotor, better check at what wind speed they are talking. Unless you live in a wind tunnel, your concern needs to be your average wind speed. A system with claims of really high output is likely rated at high wind speeds. If that is your average, great, buy it. But a slight increase in wind speed means power increases by the cube. It also means that at slightly lower wind speeds, power drops by the cube.
What the heck is single-phase, three-phase or DC about?
Hey, good question,I'm glad you asked. When current flows in a conductor, what happens is that electrons flow along the conductor. In DC current, the flow is continuous in one direction. In AC current, the direction of the flow rapidly changes. You may see on an incandescent light bulb 60 hertz. This is the standard in North America for the rate of change. ( In Europe most AC equipment operates at 50 Hertz. )There would be no point in going into depth about about electron flow here, but there are two points that I would like to mention. If you work on your car wiring,(DC) you may notice that the wiring is usually the stranded type. In your house wiring, (AC), you more often see solid wire. This is because of the difference in the way the electrons flow. In DC current, the electrons tend to flow along the surface of the conductor. In AC current, they tend to flow through the core of the conductor. Now a # 12 stranded wire would actually have more surface area than a #12 solid wire. These are things to keep in mind if you are out there scrounging project parts like I do. When sizing your wire, pay attention to the type. OK, I know that this won't be an exact comparison, but if you lay out enough garden hose, the rate of water flow decreases. The same is true with electric current. On long runs resistance will cause a voltage drop. This becomes a larger problem with DC current, which is more susceptible to line losses than AC current. So I prefer to run AC whenever possible.
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You may have seen images like the ones above. This is how an oscilloscope would display AC current. In figure A it is the image of single phase current. The horizontal line represents zero volts. voltage makes an arc of 180 degrees (1/2 of a circle), then the polarity switches and the voltage climbs again. The points of the arc furthest from the horizontal line represents peak voltage. Our standard household current goes through the entire 360 degrees 60 times a second. (60 Hertz). But this means that the voltage is also dropping to zero 60 times a second.
Now look back at figure B. In three-phase current there are three over-lapping phases. When one of the phases has completed 120 degrees of rotation, the next phase starts. This means that the current never drops completely to zero. Because three-phase is more efficient than single-phase, you can typically run one size smaller wire for a motor on three-phase than you can for a motor on single phase. You can click on the images if you need to see them better.
I had always wanted a cabin with a wind generator. Now anyone looking at the picture can see that the generator is way too close to the trees. But I work with what I have. The generator is sitting to the South of the trees, on a rise overlooking a small field. I get the best output from winds out of the East, and good output from East to around the bottom of a compass app 160 degrees. The generator project was put together over a period of about a year with lots of scrounging for cheap parts. The entire project including everything in the cabin and a couple of lift rentals came in at about $750.00.
The generator itself is based on the axial-flux designs that can be found at otherpower.com (Lots of info there!). By based on I mean that I saw what was being done there and then tried to figure out how I could get the end result using what I have on hand. The actual framework of the generator itself is quite a bit heavier than it needed to be, but my good friend Mark Gass has a shop and he had some 4-inch square steel tubing that I used. (Also thanks to Mark for allowing me to use his shop and welder ). For the rotating hub I purchased a trailer repair axle and hub from Fleet Farm. I feel that this makes a better choice than a car hub. If you needed bearings or anything they are readily available, and cheap. (Did I mention that I was trying to be cheap on this project?). For a tower, I placed an ad in the local paper that I needed a used antenna tower and would be willing to take one down. With everyone using cable and satellite these days I had plenty to choose from, including some for free if I would take them down. And a note on used towers would be that you should be aware that when metal sections have been together outside in the weather for a number of years they may not want to be taken apart. One tower had a row of pine trees near it. We cut one leg off near the ground and let the other two bend hinge-like and the tower layed into the trees with no damage. Then we slid it to the ground to work on it. Because an antenna tower would not normally see the wind loads produced by a windmill, you have to guy wire.
Power the the cabin. Because I am running a three-phase system I needed a three conductor cable to run from the tower to my controller. A neighbor had a barn fire a few years ago and he had a roll of heavy three conductor cable that the fire department had taken down. Bigger than I really needed, but that's way better than too small, and at a good price.
Inside the cabin I needed an enclosure to house the inverter that will change the three-phase AC into DC so that I can charge batteries. I was lucky enough to find a really nice electrical enclosure. On the front panel I mounted two meters. One is DC volts, showing the battery status, and an amp meter that lets me see what the generator is putting out. Because the enclosure is large enough, I also added an inverter inside the enclosure to change the DC to 120 AC for power tools ect. At first I had installed one 12 volt light with a pull-chain by the door and some 120 volt lights that drew power from the inverter. But the inverter draws a little power just to maintain itself. (Internal fans,ect). So I made all the lighting 12 volt and added a battery disconnect switch on the panel cover to tun on the outlets that are wired from the inverter.
Inside the panel I mounted two aluminum bars on insulators. On each bar I mounted three diodes. One bar has forward biased diodes and the other bar has reversed biased diodes. I mounted three more insulated bars in the middle that are connected to the incoming three-phase power. The large red component is the DC to 120 AC inverter. I am now planning a smaller, more compact panel using rectifiers instead of diodes. When I get it finished I will test it on my system, and hope to sell them.
I had mentioned earlier that the framework of the generator was built heavier than needed. This can be seen in the following pictures.
I had some help on this project. This is a picture of Mike Ramos. He was living with my wife and I while he was attending Northcentral Tech in the welding program. Sadly, a short time after this picture was taken,he was killed when he rolled his pickup truck on a foggy night. He is missed.
The powerful magnets will be epoxied to the disks, then the disk will be formed up and covered with fiberglass. The placement of the magnets, while not a hard job, can be the part of the project where you can get in trouble. The magnets are 1/2 inch thick and 1 by 2 inch in size. They are polarized with the North and South poles on the 1 by 2 inch sides. They need to be placed on the disks with alternating North, then South poles facing, up evenly spaced. An area that can be confusing is that the magnets need to be located relative to the bolt circle in the disks so that when the two disks are facing each other the magnets will be straight across from each other with a North pole magnet facing a South pole magnet. When the system is put together, the two disks will be rotating together with the windings, or stator, which does not move, sandwiched between them.
