Author Topic: Wind Turbine Efficiency  (Read 5057 times)

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SparWeb

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Wind Turbine Efficiency
« on: November 16, 2007, 11:54:06 PM »
In recent threads, I've noticed some funny assumptions about the term "efficiency".  It is not as high as many people believe.  This has confused some members, and led others to believe that it is more complicated than it really is.  By the way, I hate it when other members get up on their soapbox and start making announcements, but even some very experienced members are making misleading statements.  So... I'm politely submitting this and hope it will help.


[blackboard=ON]


I want to quickly go through an efficiency calculation to show that it really isn't difficult, and with a few measurements, you can figure out everything you need on your own.  If more of us would do so, it would put many ideas about "improvements" and "losses" in context.


Let's take, for example, an alternator built by Dan B, who was kind enough to test it and publish the results: http://www.fieldlines.com/story/2007/2/13/14052/0763


I will summarize the test data for Alternator 1:

Resistance per phase = 3 Ohms (assumed it's star connected)

Cut-in speed = 140 RPM

Battery Voltage = 48V

Charging Voltage = 56V (approx)

Power Output 750Watts @ 300 RPM


Now for the math:


Charging current = 750W / 56V = 13.4 Amps

Stator resistance loss = [3/4] * (13.4A)^2 * 3 Ohm = 404 Watts

Rectifiers: (assume 0.7V drop across each) = 2 * 0.7V * 13.4A = 19 Watts


(friction in trailer hubs is not negligible)

Hub: (assume 12 inch-pounds torque) = (1 ft-Lb)(300RPM)/ 5252 * 746 = 43 Watts


Total power that must be put into the alternator:

  750W + 404W + 19W + 43W = 1216 Watts


The generator, at this point, is 62% efficient.  Looking at the power curves that DanB worked out, this is in the middle of the working range.  I'm sure it would work well in a stronger wind, and it will also produce power in lighter winds.  If you care, for some reason, about efficiency, then take heart that it will be more efficient at slower speeds.  But, it is also putting out less power, so it doesn't matter.


Now I want to go farther, because it's so easy:


Assume that the prop on this generator will have a 30% power coefficient.  The Betz limit is 59%, so not that bad.  In that case, the incoming wind power must be:


   1216 / 0.30 = 4053 Watts.


Is this realistic?  At sea level, with a 12 foot diameter prop, this much power is available in a wind speed of:


   cube-root [ 4053W / (rho/2)(pi/4)(12ft)^2 ] = 19.2 mph


If you live in or near the mountains like I do, the air is thinner, so you need 20 mph.  Overall, it sounds okay, but there's only one step left to tie it all together:


   TSR = Radius * RPM / Windspeed = (6ft)(300RPM)/(19.2mph) = 6.4


Sounds great; many wind turbine blades are made with a TSR of 6 or 7, and here is an alternator well suited to them.


[blackboard=OFF]


How can the efficiency of a system be improved?

Making better blades that have a 35% power coefficient will increase the power output by 5% in the same winds.  Whittling down the resistance and friction losses by 15% (somehow) will only improve power output by 2%.


What if Dan's system ran at 24Volts, or worse, 12Volts?  The power output would be seriously reduced.  Efficiency would also plummet.  If you want to have a more "efficient" system, then kick its voltage up an increment.


My working wind turbine is a motor conversion.  Conversions suffer from eddy current losses in the stator, which sucks an extra 50 watts out of my system that can't be turned into useful power.  That's unfortunate, but the thing is rugged and won't cook its epoxy.  I'm currently running at 12 Volts, myself, but when I plug in the new batteries, I will be working with 24Volts (maybe 48V in the future).


The bottom line: make a good alternator - and make GREAT blades, and you will be happy with your windmill!  All the other little things are small in comparison, when you've built a good system.


By the way, I skipped showing most unit conversionss, I hope nobody minds. If you work it out for yourself, watch out for the units!  Explanation of the stator resistance loss calculation can be also provided later, if it isn't easy to figure out.  It is a little over-simplified.

« Last Edit: November 16, 2007, 11:54:06 PM by (unknown) »
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
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vawtman

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Re: Wind Turbine Efficiency
« Reply #1 on: November 16, 2007, 05:39:15 PM »
Stephen

 I think your taking this to extremes that really arent that big a deal in the overall scheme.

 Which is HAVE FUN!


 Your"bottom line"says it all.


 Mark

« Last Edit: November 16, 2007, 05:39:15 PM by vawtman »

commanda

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Re: Wind Turbine Efficiency
« Reply #2 on: November 16, 2007, 06:28:10 PM »
You also left out cable losses, another good argument for running a higher voltage system.


Amanda

« Last Edit: November 16, 2007, 06:28:10 PM by commanda »

Countryboy

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Re: Wind Turbine Efficiency
« Reply #3 on: November 16, 2007, 07:01:31 PM »
You keep using the word assume.


If we assume variables to be anything we want, we can arrive at any answer we desire too.


We need to remember what happens when we assume anything.

« Last Edit: November 16, 2007, 07:01:31 PM by Countryboy »

jimjjnn

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Re: Wind Turbine Efficiency
« Reply #4 on: November 16, 2007, 07:11:06 PM »
Then you have added losses to any system such as Charge controller,Battery, and Inverter losses which are directly or indirectly related.Other losses could be thrown in also, such as Wall Warts and others if you don't have them in switchable outlets.
« Last Edit: November 16, 2007, 07:11:06 PM by jimjjnn »

Kevin L

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Re: Wind Turbine Efficiency
« Reply #5 on: November 16, 2007, 07:59:24 PM »
Cease Fire
« Last Edit: November 16, 2007, 07:59:24 PM by Kevin L »

Flux

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Re: Wind Turbine Efficiency
« Reply #6 on: November 17, 2007, 01:44:31 AM »
Interesting thoughts.


Agreed the blades have to be the starting point but unless you attack the big problem it won't help much. As things are normally done you can't keep the blades on the peak of the power curve, you can only hit it at one point. If you make the alternator more efficient then you drag the blade tsr down and the power input from your superior blades falls way off the peak of the curve and you still perform badly.


Solve the power tracking problem and you have the potential to double the output in the higher winds. You can keep the alternator efficiency up as high as you like and gain the extra output without excessive heat and you can actually realise the potential of your more efficient blades without dragging them down to a performance worse than miserable ones.


Whatever you do the bottom line always comes to the same thing, the most effective way to increase power capture is to increase swept area.


I am playing with a6ft machine at present that does track the power curve fairly well, I don't have precise measurements at present and may not have for some time, but it cuts in about 6mph and does 750W at something like 25mph. It is dramatically better than a previous 6ft machine loaded direct. I have done not done anything special with the prop, no doubt that offers even more scope for improvement, but rather than spend ages making perfect blades I would still do far better by increasing it to 8ft. No messing with blades, losses, matching or anything else can give the same improvement as a bit of extra swept area.


Flux

« Last Edit: November 17, 2007, 01:44:31 AM by Flux »

thefinis

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Re: Wind Turbine Efficiency
« Reply #7 on: November 17, 2007, 05:35:43 AM »
I have to say that system efficiency is one of the least understood areas of RE but that is partly due to so many influences/losses between the original capture point and the end use point. I will not even get into the fact that the beginning and end points are variable and open to debate.


Whatever you do the bottom line always comes to the same thing, the most effective way to increase power capture is to increase swept area.


I have to agree with Flux on this as long as it is built within normal ranges. The wind blows free and usually materials are more or less a one time expense. Wind systems really should be built as good/efficient as is economically feasible. In practice it seems to me it is cheaper and easier to build a turbine a little bigger than to spend the extra time and money on trying to maximize efficiency. For most home units they are a one time build and get'r done and just make it a little bigger seem to work out easiest. If a person was making a bunch of units like DanB or is putting out an example/blueprint for others to follow then it needs to be refined and done to best practices ie best practical efficiency and use of materials.


There is a time and a place for all sides of this issue. In the end safety first then a project must be built to suit the builders needs, resources and skills.


Finis

« Last Edit: November 17, 2007, 05:35:43 AM by thefinis »

tecker

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Re: Wind Turbine Efficiency
« Reply #8 on: November 17, 2007, 07:07:05 AM »
 I putting together some tests now . The amount of power it takes to bring up batteries is more of a feeling to me now as I try to put a 15 amp  30 min charge into both banks before I leave each morning from an Ebike and take three batteries with me in the bed of my truck to charge on the road to augment the solar and other devices  (pulse charging the lowest bank when I return) .It truly is a lot of work for regular usage. I think it would be good to maximize the efficiency of that task and hope to do some serious improvements soon .
« Last Edit: November 17, 2007, 07:07:05 AM by tecker »

SparWeb

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Re: Wind Turbine Efficiency
« Reply #9 on: November 17, 2007, 08:36:11 PM »
By golly, that last idea is intriguing!

I drive about 2 hours per day, about 1 hour of it flat-out highway driving.  A small pickup, or even a solid car (with ventilation in the trunk) could carry a few lead-acid cells along for the ride every day, and be plugged into the house system when I'm at home.  I'd have a set of full batteries every evening.  Put a big receptacle on the bumper, like you find on big airplanes.  A switch on the dashboard, to switch on and off the charging of the spare batteries, and a few meters to keep an eye on their status in the back...  The fuel economy might take a penalty; it would take some work to figure out if the cost of gas would balance the saving on the electric bill.  Is that roughly what you're doing, tecker?
« Last Edit: November 17, 2007, 08:36:11 PM by SparWeb »
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
www.sparweb.ca

wdyasq

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Re: Wind Turbine Efficiency
« Reply #10 on: November 17, 2007, 10:06:53 PM »
To charge batteries while engine is running some have used relays hooked to accessories  part of the ignition. When engine is running the batteries are parallel to car batteries. when car is 'dead', batteries are disconnected.


This avoids a voltage drop the use of diodes will cause but still protects the primary batteries.


Ron

« Last Edit: November 17, 2007, 10:06:53 PM by wdyasq »
"I like the Honey, but kill the bees"

tecker

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Re: Wind Turbine Efficiency
« Reply #11 on: November 18, 2007, 12:22:35 AM »
I makes a ready emergency backup I've only had One chance to use it but it was better than siphoning Gas.
« Last Edit: November 18, 2007, 12:22:35 AM by tecker »

tecker

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Re: Wind Turbine Efficiency
« Reply #12 on: November 18, 2007, 01:54:33 AM »
I don't see any change in fuel they come up fairly fast I don't take them down very far with a 1 sec pulser .I do a lot of driving the weight has only been a problem a few times . I've been planning a horizontal Vawt for the back . Really this  started as a test for a hydrogen cell I have the cell now ( not installed yet) and the increase in current of a Genset with acceleration  should be just what the doctor ordered . There's a ratio there Acceleration = current increase . 10 to 30 amps on the Hwy will make alot of Gas . Here's a Picture of the Cell  . Alot of planning yet I hate screwing around with this stuff on the Highway .I ordered an Accel kit I'm lookin forward to adding that  unit .Any Input would be great  






« Last Edit: November 18, 2007, 01:54:33 AM by tecker »

tecker

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Re: Wind Turbine Efficiency
« Reply #13 on: November 18, 2007, 02:29:03 AM »
A few more thoughts here as this does kinda require some improved getset controling Ie improved efficentcy .


With the cell only working  as the  genset is charging  it shuts down when the truck is at a stand still and the engine uses gas pressure before engine shutdown . Indicator light and bypass ( the cell make a great tourch)

 This cell works in the 12 to 18 volt range

 Distillation is another problem It works well with filtered well water No lye . I think with the clearences in the cell the dab of minerals ( mainly iron ) might help .That remains to be seen .

 

« Last Edit: November 18, 2007, 02:29:03 AM by tecker »

bob golding

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Re: Wind Turbine Efficiency
« Reply #14 on: November 18, 2007, 09:10:56 AM »
hi ,

what you want for doing this is a changeover switch from a boat. they make them esecially for just this job. the idea is that you run your radio, radar, and lights ect off the secondary  battery so you dont flatten your main battery which is reserved for engine starting and nav lights. have a look on ebay under boat electrics. thaks for the equations, always useful to have some maths to check things out from. still a bit confused about how you calulate the power curve with so many changing conditions. the wind speed is never constant so it will always be a guessimate at best.


cheers

bob golding

« Last Edit: November 18, 2007, 09:10:56 AM by bob golding »
if i cant fix it i can fix it so it cant be fixed.

DanB

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Re: Wind Turbine Efficiency
« Reply #15 on: November 18, 2007, 05:34:40 PM »
good posting sparweb.... and good information for people to consider.  a couple comments.


Let's take, for example, an alternator built by Dan B, who was kind enough to test it and publish the results: http://www.fieldlines.com/story/2007/2/13/14052/0763


'Now for the math:


Charging current = 750W / 56V = 13.4 Amps

Stator resistance loss = [3/4] * (13.4A)^2 * 3 Ohm = 404 Watts'


Like Flux said recently - we can probably assume the resistance to be even higher in an AC situation so it's likely worse than that.


'Rectifiers: (assume 0.7V drop across each) = 2 * 0.7V * 13.4A = 19 Watts'


Yes - that makes sense - and this is more of a problem at lower voltage.


'(friction in trailer hubs is not negligible)'

Hub: (assume 12 inch-pounds torque) = (1 ft-Lb)(300RPM)/ 5252 * 746 = 43 Watts'


I think it is damned near negligible (especially if you don't use a seal).  I can put 1/4 pound of weight 4" from the center of the alternator and it turns the alternator.  Much much less than 1 foot pound of torque required to turn the alternator.  My guess is it's more like 5 - 10 Watts there.  


Total power that must be put into the alternator:

  750W + 404W + 19W + 43W = 1216 Watts


The generator, at this point, is 62% efficient.  Looking at the power curves that DanB worked out, this is in the middle of the working range.'


I would say that for that alternator, 750W output is about 'rated' - occasional peaks will be higher but its no good for more tha 750W without burning out.


 ' I'm sure it would work well in a stronger wind, and it will also produce power in lighter winds. '


Its much muchmore efficient in higher winds.


' If you care, for some reason, about efficiency, then take heart that it will be more efficient at slower speeds.  But, it is also putting out less power, so it doesn't matter.'


Most of the energy from a wind power system (especially a battery charging system) comes in at lower winds when the machine is actually quite efficient.


'How can the efficiency of a system be improved?

Making better blades that have a 35% power coefficient will increase the power output by 5% in the same winds.'


Right - not really a big deal in my mind....  at some point it becomes easier to build a larger machine than it does to split hairs over a tiny improvement in blade efficiency with fancy airfoils etc...  There is always a compromise to be made somewhere, I suppose everyone finds their own.


 ' Whittling down the resistance and friction losses by 15% (somehow) will only improve power output by 2%.'


Yes - and unless you some sort of MPPT system or other power electronics involved - the machine is matched to the load by resistance and you must have a certain amount of inefficiency in order to keep the blades out of stall.  I can build a much more efficient alternator - but to keep it working well with the blades, I have to add inefficiency somewhere else in the line  - or else... I've have a very efficient alternator and the blades would be forced to such a low speed they'd be quite inefficient.


'What if Dan's system ran at 24Volts, or worse, 12Volts?  The power output would be seriously reduced.'


Not really  - slightly (assuming we wind a stator that's appropriate for the voltage).  If you scale up the line appropriately to deal with the greater current of a lower voltage system then all things should be equal except for rectifier losses.  at 12V - rectifier losses are about 10% and there's no getting around it.


'  Efficiency would also plummet. '


Not if you wind the stator correctly.


' If you want to have a more "efficient" system, then kick its voltage up an increment.'


Or use larger conductors.  I hate 12V systems - it becomes impractical in my mind to have much more than a 10' diameter turbine at 12V.  But - you can do it and the efficiency isn't that much worse.  


'My working wind turbine is a motor conversion.  Conversions suffer from eddy current losses in the stator, which sucks an extra 50 watts out of my system that can't be turned into useful power.  That's unfortunate, but the thing is rugged and won't cook its epoxy.'


If you build an efficient enough machine and furl at the right time 'cooking the epoxy' is not a concern.  All different kinds of wind turbines burn out -axial flux machines with potted stators do have more trouble getting rid of heat but again - it can work fine if you do things right.  Stators burn out when people build alternators too small/too inefficient and it's not just an axial flux problem.  You see it more with axial flux machines around here because that's what most people have built...  I've seen plenty of commercial wind turbines burnout and if there were hundreds of 'motor conversions' out there you'd see them burning out all the time too.  Not because the basic idea behind the alternator is a problem - just because somebody didn't get something right... bad match of alternator to blades - bad furling system.... burned out rectifier... lots of things cause it.  We've made over 50 wind turbines and I've seen about 5 burnouts.  Some were due to bad design - one was due to 'user error' (running heaters directly with out good control) - one was due to a burned out rectifier so it ran single phase/burned out.


'  I'm currently running at 12 Volts, myself, but when I plug in the new batteries, I will be working with 24Volts (maybe 48V in the future).'


Good plan!~


'The bottom line: make a good alternator - and make GREAT blades, and you will be happy with your windmill!  All the other little things are small in comparison, when you've built a good system.'


Agreed.  But don't get to thinking that the most efficient alternator is the best thing - there are compromised to make all around this stuff.  Lately I've been trying to build more efficient machines that tend to stall blades and then tuning the system with resistance down stream.  Doing so doesn't gain anything for efficiency but it pretty much solves the problem of a hot stator... at the cost of a bit more magnetic material.


Good posting and it is fun to think about efficiency - with simple systems that are matched to the load with resistance you can have very good efficiency in low winds but forget about it high winds...  it's impossible.  It's also fairly unimportant - again, most energy comes in low winds.  If you can do OK there then odds are in higher winds you have more power than you want/need anyhow.  At least that's my experience.

« Last Edit: November 18, 2007, 05:34:40 PM by DanB »
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jeffbirkle

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Re: Wind Turbine Efficiency
« Reply #16 on: November 18, 2007, 09:04:54 PM »
About blade efficiency even with only small % gain.If one was to have time and a full machine shop at there disposal. Would it be best to have variable pitch blades with an airfoil design on the back side and slight concave on the front??? also would there be any twist and or taper to the blades ????

Thanx jeff
« Last Edit: November 18, 2007, 09:04:54 PM by jeffbirkle »

Flux

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Re: Wind Turbine Efficiency
« Reply #17 on: November 19, 2007, 06:03:32 AM »
Several issues here.


Variable pitch is the best method of control beyond the normal working range.


To extract the most from a set of blades they should track rotational speed with wind speed over the working range so you should not need to change pitch in the operating region. If your alternator is not capable of tracking the cube law then you should gain something by increasing the pitch as the wind speed increases ( assuming a powerful alternator, not one running away). You can't properly change tsr by pitch change alone, you should also increase solidity so it is not a complete fix but should at least help.

The biggest virtue may be that you can actually choose the best angle even if your calculations are wrong.


What blade profile you use is more likely to be determined several factors and arbitrarily choosing something with a concave driving face because you fancy it may not be wise. A high lift profile will need a different setting angle but may have no other advantage.


You would do better to choose a profile based on similar Reynolds number than something that flies well on a high speed plane. Probably better to look at man powered flight. Even then if you go for something exotic you need to be sure that you can reproduce it and that you can maintain it with erosion and damage that takes place in the real world. The more sophisticated the profile the worse it is affected by departure from perfection.


Variable pitch has its virtues but only for those with skill and facilities, its not something that ever works when poorly implemented


Flux

« Last Edit: November 19, 2007, 06:03:32 AM by Flux »

BigBreaker

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Re: Wind Turbine Efficiency
« Reply #18 on: November 19, 2007, 07:47:55 AM »
I am really interested in your power tracking project.  Once the power electronics are in place other means of increasing efficiency become available.  The most obvious is increasing the alternator's voltage.  If the charging current can be moderated at high speed, the early cut-in doesn't hurt high speed performance.  Lots of efficiency there - just have a look at the stator's resistive losses.  Also letting the RPM track up with wind speed helps the blades efficiency.  It's pretty hard to approach Bentz limit with stalled blades.
« Last Edit: November 19, 2007, 07:47:55 AM by BigBreaker »

SparWeb

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Re: Wind Turbine Efficiency
« Reply #19 on: November 20, 2007, 03:30:41 PM »
On retrospect, it does look like a "brain-dump"...  I have lamented, at times, how the big picture is lost when discussing minute details, and that previous thread got me going.


I'll admit that I knowingly made some simplifications - again, it's too easy to get bogged down in minute details and lose the big picture.  I made a similar estimate of my own system's wins and losses, and I included such things as the resistance in the cable, plus the trio of trimming resistors that I have in series.  They do the same thing you describe, Dan.  But it all makes the discussion complicated.


My last point also reflected what I find the most fun - carving blades.  I love working with wood!


Overall, I wanted to provide a posting where the whole system is "roughly" sized up together.  With so many members wanting to figure out how to line up the numbers, I'm surprised at how rarely a summary like this is done for anyone.  I think people are afraid to showing their calculations in public.


Life's better when you shake hands with Mr. Math, and be friends.  Try it.  :^)

« Last Edit: November 20, 2007, 03:30:41 PM by SparWeb »
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
www.sparweb.ca

DanB

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Re: Wind Turbine Efficiency
« Reply #20 on: November 21, 2007, 08:47:20 AM »
I basically agree Steven...

I use math when I can and when I know how ;-)

Trouble is - really calculating the efficiency of a system gets very complicated and the equipment required is not something most folks have.  I guess the joker in the deck for most folks is calculating wind speed... a 10% error in that can throw everything way off.


Good posting though - it is good for folks to realize all the issues here.  I wish I had equipment to get really good wind speed data.

« Last Edit: November 21, 2007, 08:47:20 AM by DanB »
If I ever figure out what's in the box then maybe I can think outside of it.

kitestrings

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Re: Wind Turbine Efficiency
« Reply #21 on: November 21, 2007, 07:52:05 PM »
SparWeb,


Great post.  I'd add one other consideration.  [Increasing] rotor diameter will fetch more power, but increasing the tower height will also get you increased performance at relatively low incremental cost.  My two cents.


Regards,


-kitestrings

« Last Edit: November 21, 2007, 07:52:05 PM by kitestrings »

Dr Robert

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Re: Efficiency is good; cost / watt is better.
« Reply #22 on: December 11, 2007, 08:33:18 PM »
Efficiency is nice, but I think the ultimate criterion is the cost/watt.  


IF it costs $10/ watt to get a 98% efficient machine,I'd much rather have a $2 / watt machine that only gave me 50% efficiency and get two of them!.


Remember the initial cost of electrical generation from the commercial sources.


Coal fired generators cost $1 / watt.

Big Wind HAWTs   originally cost $4 watt and now run about $1.60 a watt.

Gas fired Electrical generators cost $4 / watt.

Dammed up Water power generators run between $2 and $4 / watt.

P/Vs cost $5 / watt + another $5 / watt to install.

Outbacker Inverters to the grid + extras cost around $1 / watt.


Most of the hobby class HAWTs seem to be in the $1 / watt class, plus provide a great learning experience and friendship.  I am working on a VAWT using Rare earth magnets from obsolete low density Hard drives destined for the landfill (free) that so far seems to be in the 10 cents
watt class!

« Last Edit: December 11, 2007, 08:33:18 PM by Dr Robert »