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Forum Index : Windmills : Wind Turbine Wattage Ratings
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nrgcycle Newbie  Joined: 09/08/2013 Location: United StatesPosts: 4 |
How does one rate a particular wind turbine? Is there a standard wind speed to use? Or an *optimum* wind velocity to use for the determination, where that wind velocity can be likely to be different for each wind turbine design? For example, the retail unit at http://www.frys.com/product/6367571?site=sr:SEARCH:MAIN_RSLT _PG says 600 watts. Yeah well maybe for a given wind velocity x. And the value of x? I want to rate the one at https://www.youtube.com/user/NRGcycle/videos . Do I need to first find a wind velocity at which it performs best for any given generator, assuming the wheel/blades are the same (swapped out, attached to different generators)? Thanks, Gary H http://NRGcycle.com/ |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
I think the question should be "How SHOULD one rate a particular wind turbine?" The problem is most of the cheaper wind turbines on the market are overrated, which leads to many unhappy customers and gives the industry a bad reputation. Wind turbines will have a maximum power they can make before their built in control system takes over, or they fail, and this power figure is often used as the rated power of the machine, and no mention of the wind speed as you discovered. So the average customer buys a cheap windmill rated at 1000 watts, only to discover it never makes anywhere near that power in its lifetime. Ideally you need to see a power versus wind speed specification sheet or graph for the wind turbine. A turbine rated for 1000 watts may need to see wind speeds over 50kmh to reach that power, but the average wind speed for many of us is less than 15kmh, so that turbine would probably make less than 100 watts on a average day. If you know your average wind speeds for your area, then you can use the turbines power versus wind speed specifications to see how much power it will really make for you. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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solarwind Regular Member  Joined: 03/02/2008 Location: South AfricaPosts: 51 |
From my side also, thanks GIZMO! This means that on a homemade machine, one will have to measure wind speed AND tubine output over a period and record all data, then plot it to really see what's going on! I shall also have to do this in the near future when I put up my turbine. Regards! You don't have success until you've tried it! |
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| nrgcycle Newbie Joined: 09/08/2013 Location: United StatesPosts: 4 |
I would add that it's even so bad that in the equation for calculating wattage for a given sweep area, efficiency and wind density (think altitude perhaps) folks are most often also mis-using the term "velocity" (and I repeated the mistake above, and then you used "speed" properly) because velocity is the value of a *vector* representing two things, speed *and* direction combined. With wind turbines, wind direction doesn't usually matter (or at least is never included in the equations presented) so they should use the term "speed" instead. Overall, what to do? Your 50 kmh or 50 kph (or kilometers per hour) would be about 31 mph (as one who likes to try to think like a scientist yet many of the cars in this nation don't show those values <sigh>). Your 15 kmh (15 kph or 9.3 mph) is close to what I once calculated as the average wind speed across the U.S (9.2. mph). Likely varies. I want a standard wattage rating system in place that everyone adhere's to, and switches over to. Suggestions for rating the wattage output of wind turbines: a) Wattage at a particular wind speed (such as 15 kph) at sea level between a given range of barometric pressures (to both sides of average). Downside: Some companies will incur a cost hauling units to sea level to measure and then grappling with timing/weather. b) Maximum wattage as measured with only one condition, *up to a particular wind speed limit* (such as 50 kph). Any company is welcome to measure output in any location (sea level not necessary) in whatever conditions they consider to be most advantageous to them (could even be in a wind tunnel) so long as they do not exceed 50kph/31mph wind speeds. c) Some sort of average wattage that specifies whether measured or theoretical, or both? d) <anyone else?> Right now, anyone entering the wind turbine market is going to be faced with the ethical dilemma of creating false impressions with the wattage ratings like most everyone else is doing, or being realistic and truthful while hoping people will understand, and possibly taking that lack of sales hit for lack of knowledge among people. I invite you to take a look: http://www.youtube.com/user/NRGcycle/videos (the quick preview is only 41 seconds) When I talk about my bicycle wheel wind turbines, the first question from people, logically, in various forms, is, so what's the output? I've been answering them with: Depends on the wind speed, and that isn't very satisfying to them (or me either). I've been telling people: Maybe 25 watts at 25 mph (40 kph) if 25% efficient. Their response is usually that 25 mph is rare. While higher wind speeds produce more impressive wattage numbers, since wind speed in the equation is cubed (an example at http://www.find-solar.org/?page=solar-calculations down the page for example), they're right that 25 mph doesn't happen that often, at least in Seattle. (Maybe common on Kauai. Oddly I only saw three wind turbines there.) Maybe I'll resort to replying to people with both figures, average and something higher, 15 kph and something like 50 kph, once having measured those. Actually, please plug that into (d) above. How about that, wind industry, listen up, save your future esteem by becoming honestly realistic now. Wind turbine wattage ratings on the box in the store presented to the public should be two values, both average (based on some standard agreed upon average wind speed value to use) along with what most sellers are currently using, what they like to call Max. So how do they typically measure the maximum? Or maybe it is usually theoretical, donno. There's a lot to consider, braking, blade lengths, tip speed, rotational forces that materials can withstand before flying apart, etc. How then to measure efficiency of a wind turbine to be able to plug that into the equation? If I'm not mistaken it would be something like: Wind Power in Watts = .5 * air density (1.225 at sea level) in kg/m3 * swept area in square meters * wind speed in meters per second cubed * percent efficiency Thanks, Gary H http://NRGcycle.com/ |
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ChrisOlson Regular Member Joined: 19/01/2010 Location: United StatesPosts: 60 |
Like most newbies to wind power, I think you're too hung up on watts. It's kWh that gets the work done, not instantaneous watts. You can have the wind blow hard in a storm for a 1/2 hour and the turbine produces 1,000 watts. Then the wind doesn't blow the rest of the day. So you say, "oh yes - it is a 1,000 watt turbine." So what, exactly, does that mean? It produced .5 kWh for the day. Or you can have just a gentle breeze that blows all day, and the turbine produces 2.4 kWh in a day but it only produces 100 watts. Which is the more meaningful relationship? -- Chris off-grid in Northern Wisconsin, USA |
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| nrgcycle Newbie Joined: 09/08/2013 Location: United StatesPosts: 4 |
Thanks for your input Mr. Olsen and for clarifying the important point about consistent wind vs peak so succinctly. You're right about kilowatt-hours instead of any particular instantaneous watts. When someone asks you for the power rating of your own wind turbine(s), what's your answer? People ask me--every person does--what can it do? First and second questions are always input/output, like, what input is needed from me to get one of those on my roof and what's the output? Output should be in terms they can relate to. I'm only half joking now in bringing up 'standard laptop units', how many standard laptops can a particular wind turbine power while watching video in a 15 kph wind? Have to define a "standard". If I can say my laptop is 1.1 standard laptop units, that could be something normal bipeds can relate to. Thinking out loud. And yet, seriously, I designed and built my own wind turbine from scratch so now how am I supposed to rate it? Is there currently ANY STANDARD (?) or guideline whatsoever in the wind turbine industry for rating wind turbines? Is the current practice basically just printing peak on the box even if that's at hurricane force winds? Surely we're not in THAT BAD of shape as people on the planet. Again I suggest that we can initiate a standard wind turbine rating system. Now I'm suggesting in part we can possibly call it a 'nominal average' by determining power at 15 kph (which should be average wind speed across the U.S. I think to the nearest whole number and possibly even the world average). |
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Don B Senior Member  Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Hi nrgcycle, Your question is a little like the one "how long is a piece of string"? The amount of power that you can potentially harvest from a given stream of wind at a given speed is a function of the diameter or area of the turbine, and its efficiency. The efficiency is, in the ultimate, limited to about 60% of the available wind energy (see Betz limit) but, in reality, you are doing well if you can achieve even half of this. It is also important to realize that wind velocity is not uniform from ground level up. At actual ground level, the wind speed is closer to zero. At any height, the actual wind speed is influenced by what is upstream of it. If there are buildings, bushes, or tall trees, the incoming wind is turbulent, and it is not until you get some significant height above any obstacles upstream that you get a more "pure" or uniform velocity wind stream. To help visualise this, have a look at fast flowing river, and the evident velocity differences and whorls and eddies from bank, to mid-stream It is probably only in a wind tunnel that you can obtain the sort of consistent and uniform wind velocity that would make meaningful comparisons possible. Regards Don B |
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Madness Guru  Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
I used to live on the edge of a mountain range there was lots of wind. My wind generator was on a 24 metre tower well above any trees within 60 KM on the eastern side where most wind came from. However as the wind was coming up a slope with some large trees I never got good clear air. The generator could be putting out heaps of power for a 20 seconds then next it would do a complete 360 degree turn. There are so many variables within the generator and the site where you plan to put the generator. Mounting a generator on a vehicle and driving at various speed on a calm day would be good way to plot how the generator will perform at a given wind speed. This would then apply to a perfect site if there is such a thing. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Only realistic way to rate a wind turbine is to plot output power versus wind speed. Its a bit mickey mouse, but I have seen small turbines fitted to the roof of a car or trailer, and output watts at various road speeds measured on a still day. Or you can set up an anemometer and a wattmeter and log the results over a long period, and plot delivered watts, versus metres per second, or Kmh curve. The other end of the problem is judging a potential wind site. All sorts of things to consider such as prevailing wind direction, local topography, and proximity of significant local objects such as trees and large buldings. You then purchase a machine that will provide the required power under the prevailing wind speeds at that site. Peak power, or rated power really means nothing. At the end of the day you need a machine suitable for a SPECIFIC purpose. Cheers, Tony. |
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| Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Tony makes a good point when he says that you can get an approximation of clean air by setting a turbine up on a vehicle and driving it at varying speeds in a large cleared area on a calm day. To ensure that you account for any air movement, two runs need to be done at the same speed, but in opposite directions, and the results averaged. The turbine also needs to be mounted as high above the vehicle and as far forward as possible so that the vehicle's "bow wave" has minimal effect on the test. Note also that car speedometers can be inaccurate, so you are best off using GPS speed. One other problem in trying to set a standard is that the electrical load on the turbine will affect the Watts output. If the load is a battery, its state of charge at the time will determine the charging voltage, and hence current, and hence the Watts output. For a standard to be meaningful, the electrical load needs to be consistent for all tests. This brings up other problems relating to the characteristics of the turbine and its alternator. The maximum theoretical output of a turbine follows the wind speed in a fixed ratio, known as the tip speed ratio. If the wind speed doubles, then the turbine speed needs to double also to achieve maximum output. The amount of power in the wind follows the cube of the wind speed. This says that, if the wind speed doubles, then the available power increases eight times. The voltage output of, say, a typical permanent magnet alternator varies in proportion to its rotational speed. If the speed doubles, then the voltage output will double. If the load on an alternator is a resistor, then, if the output voltage doubles, the current through the resistor will also double, and the Watts delivered into the resistor will be four times higher. Note though that limitations in the alternator may mean that it can't deliver four times the power. Note also that, with double the wind speed, you have eight times the power available in the wind, but only four times the power output possible from the alternator, at best. If the load is a battery, then it will tend to hold down the delivered voltage to something just above its rated voltage, and this, in turn, may well limit the speed that the turbine can spin up to, further limiting the potential output to something less than four times. All of this says that trying to establish a simple standard of Watts per mph (or whatever) of wind speed as a goodness factor for any particular turbine needs to be heavily qualified as to the clean air and load conditions at the time if it is to be meaningful, and provide any basis for comparison. Regards Don B |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
I may be wrong but I thought if you double the wind speed the power available is squared? There are only 10 types of people in the world: those who understand binary, and those who don't. |
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