A few thoughts from a trained engineer but a wind turbine Newbie

[BigBreaker]

Just for starters I want to reiterate that I am a wind turbine newbie and haven't made even ONE wind turbine myself.  Part of the reason is that I live in an apartment in New York City and they REALLY frown on setting up windmills.  I learned electrical engineering as an undergraduate at MIT and the mechanical/aerodynamics side in a few classes but mostly years as an hobbiest / RC airplane enthusiast and other sorts of projects.  My wife also happened to be a mechanical engineering major.  I have been lurking here for a while and really wanted to share some of my thoughts - for what it's worth.  I'd love nothing more than for them to help someone how can actually BUILD one of these things for their home.  Oh, just trivia, but electricity in NYC costs $0.19/kWh and last month my terrible A/Cs (mostly) used 1200 kWhs!  Yikes!

First on efficiency - perhaps I'll repost this in another section but you guys should really consider hydrocarbon coolants for all your refrigeration applicances like ice chests, refrigerators and A/Cs.  Hydrocarbons are about 30% more efficient than CFC/HCFCs, AKA freon.  Hydrocarbons are also nearly free and ozone friendly.  Sadly they are also somewhat explosive when mishandles but lots of DIYers use them safely.  Lots of great websites and discussion boards can help you with the practice, safety and legality of modifying existing products.

On stators and power electronics - I can't imagine anything but a high voltage machine being efficient.  The line losses on a 12v machine make me cry.  At least for me I'd want a very large turbine as well so the cost of copper becomes somewhat significant and lending an advantage to higher voltage.  I think there is a real advantage to putting in an automatic star/delta switch to help maximize the efficiency across the power curve.  I'd take it a step further and stack multiple but smaller dual rotor generators on the same drive shaft and use them in a disconnected/series/parallel fashion to maximize power.  Once you have the molds and jigs set up it is easy to crank out more generator units.  Smaller ones seem safer and more flexible.  I'd also like to cast the stator in two pieces so that I could experiment with different windings and air gaps without disassembling a magnetically dangerous dual rotor assembly.  I'd look at putting a microcontroller and plenty of relays in the key interconnects with temperature sensors, volt/amp meters and an RPM encoder to drive the logic.  After a few months of data collection you should be able to easily optimize the circuit configurations by rpm.

On tower construction - The tower costs a substantial fraction of the overall project and additional height can greatly improve a mill's performance.  A turbine's weight is the most direct factor in determining the required strength of the tower and its cost.  It therefore makes sense to think hard about how to lighten the turbine and decrease the loads that it will place on the tower.  In order to reduce the cost of the turbine many folks here like to use very heavy but cheap car parts like a brake assembly.  That may be penny wise but pound foolish if the heavy but inexpensive mill requires an expensive and overburdened tower.  I am thinking about "floating towers" with guy wires on pulleys that tie back to the tower base that would keep the forces balanced across the wires and the load centered under the mill.  Guy wires should be on short tight springs to help reduce peak tension.

Key forces in a wind turbine and how to withstand them with less material and reduce their transmission into the tower -   1. Letting the mill pitch upwards as well as yaw would eliminate a huge torque in the system.  As a mill tries to yaw towards the wind or furl a large up or down pitch is induced - precession.  The moment of inertia of a fast spinning mill is astonishing and fighting force that takes enormous amounts of structural material in the blades, hub, rotor, shaft and tower - so don't fight it and save lots of metal/fiberglass fatigue.  I thought about mounting the yaw pivot on a curved plate that could yaw up and down or installing a proper bearing below the CG of the mill and putting a small counter weight on an arm extending several feet below the pivot (to bring the CG of the mill/arm/counterweight unit  back below the pivot)  You need to limit downward pitch due to the tower and you want the furling direction to coinside with upward pitch (blade rotation direction determines that) 2.  Many of the mill designs I have seen you very short drive shafts from the blade hub to back of the rotor.  A longer drive shaft would be slightly heavier but would allow for a longer lever arm between the bearings.  You could then use a smaller bearing in the rear and allow the front bearing to float a bit on the pillow blocks using rubber grommets.  Furthermore the longer drive shaft could be somewhat narrower since it would carry lower radial torques through each linear segment and allow for multiple rotor/stator units on the same shaft.  3.  On the subject of rubber grommets.  Everything that can move a few mms should be on rubber blocks to allow that movement.  Stiff strucures transmit stresses at the speed of sound (in steel that is nearly instantaneous) and those stresses find the weakest, most plastic place to focus and deform.  Movement allows stresses to distribute in space and time.  Rubber can be replaced every few years but metal fatigue is permanent and progressive in a structure.

There is an online machine shop called eMachineShop that will create custom parts for you in small batches.  They have a CAD-like tool to help facilitate the process of "describing" each piece.  Perhaps we should start building a library of turbine parts so that people can "fill in" for what they can't find at the junk yard / auto parts store, etc.  If some parts were especially usefull we could order in greater quantities to reduce the marginal cost.  Sometimes having just the right part - even if it costs extra - is worth it to save weight or complexity.

Flow batteries are a really exciting storage technology that is starting to mature.  I suggest everyone google it!

Ok enough for now.  Thanks for listening.

Editors Note:

Please consider reposting this as a Diary entry it doesn't really fit any other section.

Editor

[DanB]

Welcome to the board!  Nice to see folks thinking about some of these problems.  I'll interject some comments/opinions below.

"First on efficiency - perhaps I'll repost this in another section but you guys should really consider hydrocarbon coolants for all your refrigeration applicances like ice chests, refrigerators and A/Cs.  Hydrocarbons are about 30% more efficient than CFC/HCFCs, AKA freon.  Hydrocarbons are also nearly free and ozone friendly.  Sadly they are also somewhat explosive when mishandles but lots of DIYers use them safely.  Lots of great websites and discussion boards can help you with the practice, safety and legality of modifying existing products."

I've always heard of used car dealers sticking propane into air conditioning systems... sounds rather scary but it is interesting!

"On stators and power electronics - I can't imagine anything but a high voltage machine being efficient.  The line losses on a 12v machine make me cry."

Most big systems are not 12V.  Smaller systems at 12V often use 12V appliances (car radios/cell phones - etc, stuff that runs directly off 12V.  What they lose in line loss they make up for in extra efficienccy sometimes.  Also - 12V systems usually cost less, and the inverters are cheap.  So long as they don't need to handle too much power they can be quite reasonably efficient.  Even a 10' wind turbine at some distance can be fine - keeping in mind that such a machine may be capable of 1KW at times, 90% of that power is coming in much lower winds at much lower power levels and overall efficiency is usually fine even if were losing half our power in really high winds.

  "At least for me I'd want a very large turbine as well so the cost of copper becomes somewhat significant and lending an advantage to higher voltage."

Yes, it makes sense if your designing a system from scratch to start out with higher voltage.  Lots of folks went 12V years ago... now they're kind of stuck with it.

  "I think there is a real advantage to putting in an automatic star/delta switch to help maximize the efficiency across the power curve. "

I suppose there are tradeoffs.  If I lived in a very windy place there may be some advantage to it.  The great majority of my power comes in lower winds and at those speeds were very efficient.  In high winds, I have more than I know what to do with anyhow - this seems the case with most folks that have wind turbines I think.  I don't believe that there's a great deal to be gained and I question if the added complexity/possibility of reliability problems is worth the tiny bit of gained efficiency in very high winds.  It is a matter of debate though - some feel differently and its fun to see folks working some of those problems out, even though I don't think I'd go there.

 "I'd take it a step further and stack multiple but smaller dual rotor generators on the same drive shaft and use them in a disconnected/series/parallel fashion to maximize power. "

Overall it would probably be a much heavier /expensive machine with lots more copper/magnets.

 "Once you have the molds and jigs set up it is easy to crank out more generator units.  Smaller ones seem safer and more flexible."

Our best use of materials by a long shot is to have lots of poles on a big diameter rotor.  It's nice to keep things simple as well.

  "I'd also like to cast the stator in two pieces so that I could experiment with different windings and air gaps without disassembling a magnetically dangerous dual rotor assembly.  I'd look at putting a microcontroller and plenty of relays in the key interconnects with temperature sensors, volt/amp meters and an RPM encoder to drive the logic.  After a few months of data collection you should be able to easily optimize the circuit configurations by rpm."

I can tell your an engineer ;-)

"On tower construction - The tower costs a substantial fraction of the overall project and additional height can greatly improve a mill's performance."

That's definitely true.  The tower is usually well over half the cost/time in setting one of these up.

  "A turbine's weight is the most direct factor in determining the required strength of the tower and its cost."

I don't think so.  Its the force of the wind against the machine in my experience that puts the tower under greatest load.

  "It therefore makes sense to think hard about how to lighten the turbine and decrease the loads that it will place on the tower.  In order to reduce the cost of the turbine many folks here like to use very heavy but cheap car parts like a brake assembly.  That may be penny wise but pound foolish if the heavy but inexpensive mill requires an expensive and overburdened tower. "

I think we can knock about 15 - 20 pounds off a machine if we avoid brake rotors and simply use flat steel.  But again - weight at the top of the tower is not really our problem, its the force of the wind.  There are pros and cons to a heavy alternator.  Car parts are handy - especially if one doesn't have the tools/resources to have steel disks cut/machined.  Tapered roller bearings are nice too.   Everyone has to work with 'available resources'.  

 "I am thinking about "floating towers" with guy wires on pulleys that tie back to the tower base that would keep the forces balanced across the wires and the load centered under the mill.  Guy wires should be on short tight springs to help reduce peak tension."

That may make sense though I think if we use proper stuff to begin with it can be kept fairly simple.

"Key forces in a wind turbine and how to withstand them with less material and reduce their transmission into the tower -   1. Letting the mill pitch upwards as well as yaw would eliminate a huge torque in the system."

Tipping up is a fairly common way to go for furling, although there is torque induced on the hub during that movement too.  Again - I think we can build it strong, and keep it simple.  I try not to spend too much time solving problems that don't exist to begin with.  During yaw there are great forces on the blade hub - especially a fast yaw.  It makes sense to not have the tail too big so the machines don't yaw too quickly.  

  "As a mill tries to yaw towards the wind or furl a large up or down pitch is induced - precession.  The moment of inertia of a fast spinning mill is astonishing and fighting force that takes enormous amounts of structural material in the blades, hub, rotor, shaft and tower - so don't fight it and save lots of metal/fiberglass fatigue."

But tipping up only solves half the problem - you cant tip it down.  The only fatigue forces during yaw are on the blade hubs - we need to make the blades and hubs strong enough - preferably out of materials that wont fatigue.

 "I thought about mounting the yaw pivot on a curved plate that could yaw up and down or installing a proper bearing below the CG of the mill and putting a small counter weight on an arm extending several feet below the pivot (to bring the CG of the mill/arm/counterweight unit  back below the pivot)  You need to limit downward pitch due to the tower and you want the furling direction to coinside with upward pitch (blade rotation direction determines that)"

We really dont need to worry about tipping up or down during the yaw I don't think.  Its an interesting Idea though.

[Flux]

Yes it is nice to see some thoughts from another point of view.

Dan you have covered most of the points I was going to make. As always there is no perfect solution, if you live in a remote part of the world your ideal will be very different from someone who has access to the latest technology.

12v has its limitations but many people get by quite well. Unless the mill is a long way away the transmission loss is not unreasonable. For very long distances a high voltage generator and transformers at the battery end may pay off but for shorter runs it is not going to be cheaper. If you want the ultimate a high voltage generator and buck converter is likely to be the most efficient but at the moment unless you can build it yourself it's not an option and if you are in an area prone to lightning it may not last long.

Star/ delta and tap changers also have their place and work fairly well with small machines, they are a bit rough with large ones and spend a lot of their time in the wrong state.

Multiple section generators are costly and suffer most of the snags of star delta. There is some point with induction generators where they only have friction loss when not excited. Except for air gap machines PMGs have iron loss that can not be got rid of. There would be some point in a small air gap machine in low winds and a larger iron cored one for high winds if it were not for the fact that the big machines iron loss may be greater than the small machines output.

A lot depends on your needs how far you trade simplicity for a small increase in output. In wind power virtually everything works backwards from normal generating practice, normally generators are designed for maximum efficiency at full load, that concept makes a poor wind generator, the only thing in our favour is the cube law for power, if it is efficient in low winds, there is so much power in higher winds that a drop in efficiency does not hurt much. Unless you use electronic converters you actually gain more power from the prop by keeping its speed up and accepting the lower alternator efficiency needed to do it.

I tend to agree with Dan's views on towers as well but there may be more room for thought there.

As technology improves we shall indeed see improvements in wind generators, some ideas will survive and others will fall by the wayside it all depends on what you want and can afford.

There can be little doubt about the superiority of pitch control but its complexity and cost has always limited its use. The most significant things that have improved wind power since I started playing are the silicon diode and neodymium magnets and even now an old Jacobs machine will hold its own without the benefit of either and it has pitch control, but could you build it at home with little facilities and could you build it at a competitive price commercially, I doubt it.

Flux

[richhagen]

Like you, I live in a big city (Chicago), and I can't really mount a large turbine.  I have built smaller ones though.  The main threat to the utility of the turbines built by people here is in my opinion, the reliability of the machine, not the efficiency as it is now in the common dual rotor machines here.  The challenge as I see it, is to build a machine that needs little more than annual maintenance to provide decades of service.  To that end, simpler is usually better as the more parts, the more parts to fail at some point.  Since most of the people build and maintain there own machines, it might not be such a big deal to hard wire the connections in the event of failure in a star/delta switch, or some other part, but if the parts arn't really necessary, then why include them in the design and the cost of the machine?  As someone stated in another post here, the turbines are designed to produce power at the prevailing wind speed, usually low, and simply to survive the gails that occacionally come.  This makes sense to me, as that if you have designed the system to keep the battery bank charged with an average wind speed of 10mph, then at 30mph, you would be producing so much power, that you would have little to do with it other than to dump it to heat anyway.  

I have thought that the weight would be a limiting factor to the size of machine that could be built, primarily because of the methods used by people to raise them.  A pickup can only pull up so much weight on a given tower height.  For that reason, saving some weight by mounting the magnets on spoked rings and other weight saving measures might be worth while.  Also, that might allow more robust, but heavier parts where they are needed.  I hope to build a big machine someday.  :-)  Rich

[BigBreaker]

Many thanks for all your responses.

Much of your feedback is an appeal to simplicity and I really can appreciate that.  Many of my attempted improvements revolve around pushing the power capabilities of a mill but keeping the weight in a catagory that a DIYer could handle.  Certainly if a 10 footer serves a person's needs than a there is no reason to complicate things.  If you are trying to hit 3-5kWs than it is a bit tougher nut to crack.  If storage technology improves, and I think flow batteries will cause this, than we may be able to cheaply store all the energy that a big mill can generate.

I also rethought the concept of multiple axial generators versus one larger generator.  The larger generator is moving the magnets faster at the same RPM since the lever arm is longer and that is a very useful feature for raising the voltage and power.  Basically to have three ways to a more powerful generator - more magnets, more wire or more speed.  Of those only speed is "free" and should be maximized by placing the magnets as far from the axis of rotation as possible.  Many thanks for that reminder - I did have a bit of a nag in my head about maximizing copper and magnet resources.

I think some folks misunderstood me when I talked about pitch.  I didn't mean blade pitch but having the axis of the mills rotation point up or down.  While it adds complexity I think it would help a mill to be able to point up/down AND left/right to keep those precession torques from stress everything from the blades down to the tower base.  Trust me that precession either gets to turn in the 90 degree direction or it transmits the force to the ground.  This of it like the two axis pivot on a telescope but with the center of gravity shifted below the pivot through the use of a suspended counter weight.  This keeps torque out of the tower and allows the guy wires to focus on the linear forces of wind pressure.  Blade pitch mechanism are very tough to do well.  Even I'd steer clear of pitch control.

[scoraigwind]

Welcome.  I agree with Dan and Flux as usual - they save me a lot of work answering this one.

"On stators and power electronics - I can't imagine anything but a high voltage machine being efficient. "

In the case of the stator itself the loss is the same under the smae power loading and the same speed regardless of the chosen voltage.  The losses come in the rectifier and the wiring downstream.  24 volts and 48 volts are coming in and I have done some 120 VDC systems.  I also use transformers but the high recitifier losses at 12 volts remain.

Simplicity usually beats efficiency in reality as a guarantee of maximum energy production.  Complex systems will fail and leave the efficiency near to zero.  so 12 volts with high line loss has some advantages because it is a simple way to get a good power speed curve to match the blades output.

" The line losses on a 12v machine make me cry.  At least for me I'd want a very large turbine as well so the cost of copper becomes somewhat significant and lending an advantage to higher voltage."

They come at a time when the wind is bountiful and batteries are not usually needing it so badly.  They often help blade efficiency, so the situation is not so bad for those who like things simple.

"  I think there is a real advantage to putting in an automatic star/delta switch to help maximize the efficiency across the power curve."

What I found was that it helps efficiency at the ends of the power curve whereas a lot of the best wind is in the middle.

"  I'd take it a step further and stack multiple but smaller dual rotor generators on the same drive shaft and use them in a disconnected/series/parallel fashion to maximize power.  "

If you do the maths on alternator design you will find that you get a LOT more by putting all the coils in the path of all the magnets and using them all the time.

"A turbine's weight is the most direct factor in determining the required strength of the tower and its cost. "

A heavy turbine runs more smoothly at the tower top and does not stress the tower anyhting like as much as the wind thrust will.

New ideas are always good and worth a try but don't try too many at once, is my advice.  Small wind turbines are very testing beasts and will try the patienceof a saint.  YOu need to team up with a few good technicians in a good windy spot and see what you can make work.

Keep us informed :-)

[ghurd]

Hi!

Welcome aboard!

But just some perspective.

""The line losses on a 12v machine make me cry...""

Just because a 12V machine is ABLE to make 1KW

does not mean it always will.

Even 5% of the time. Even 1% of the time?

Therefore a 5% increase in other parts costs

would be a better way to spend the money.

Usually.

And I can not put up a 25' tower or a 3' machine here.

I would love a 100W machine.

I could manage 10% bigger blades.

Should I just quit now or use #1/0 wire, monetarily speaking ?

My last mill cost about $18 "total".

Not that it is going to save any electric... or even pay for the

gas to go get the 'free' battery.

This has got to be a common concept question...

"My wife is fat and rides an excersise bike.

Can I hook a generator to the bike and have the electric company send me money?"

Think about it.

Someone 'not fat' makes about, what, 300W for an hour?

The inverter is $1000, plus parts, inspections, unions, etc...

Figure inefficiencies in. We'll all be long dead before even the inverter is paid for.

Just like everyone else...

Efficiency is great, as long as it doesn't cost me too much!

G-

[BigBreaker]

I appreciate your thoughts and definitely take the advice not to "try too many things at once" to heart.

A lot of my design concepts are centered on a large wind turbine connected to a ~30kWh+ capacity flow battery which would be used to power a remote but comfortable home/retreat.  The battery I would need to buy, but building the wind turbine would be fun.  I'm focused on efficiency because I anticipate being storage rich and power generation poor so there is some method to my madness.

I actually live on the twentieth floor of my apartment building and regularly have 30 mph winds whipping across my balcony due to some bizarre aerodynamics.  That could be the perfect test bed / torture test for turbine models.

[Zix]

Welcome to this fine group,I read your post with great interest and I must admit it all sounded good.These guys on here have the most knowledge of any one on the planet,wind turbines are really in their infancy with no real research done on anything except meggawatt units.

Do a lot of reading here and you will find a practical reason for all of the types of designs here mostly done thru trial and error.

Even if you do live in NY in a high rise you can build a small mill and let it teach you how to improve it.

Zix--(learning everyday)

[kitno455]

1. weight of gen is irrelevant, other than at raising and lowering time.
   
2. speed of mags is not 'free', as the lever arm that they present to the shaft also gets larger, no net change. 1hp == 1hp.
   
3. you CANNOT move blades down during precession, they will strike the tower or guys. you could move them up, but the kind of 'spherical' joint you end up with might need some serious spring/damper/counterweight help to avoid flopping in gusty conditions. i doubt its that big of a deal, as the precessive force is initally at a right angle to the wind normal, and as the frontal area diminishes, the precessive force moves in line, net force should stay pretty constant once furling starts. design for that, and you dont need the extra joint.
   
4. increasing length of guys by using a pulley system will only increase total elongation, by giving you more starting material. adding a spring helps, but why not just use the spring at the end of the shorter guy? besides, we have to mow under these towers, city boy
   
5. all the star/delta switches, and individual coil controls will double the cost of the machine. if a single one of them fails, and you lose a coil, guess what, your efficiency is now lower than if you had just left the thing alone, with a fixed config!
   
6. do not design for your highest output, design for your lowest. then you get usable power all the time, and if your efficiency is less than optimal at higher windspeeds, who cares, you are likely dumping the power anyway.
   
7. might want to re-think that 'no blade pitch' bit. you could do that with sandia style 20 degree blade flex as the force goes up. would require composite blades, but i think you are out of the wood ball park at 5kw. then some of your yawing issues go away (to be replaced with complex blade fatigue
   
   

allan

[windstuffnow]

  I think BigBreaker made some good points, as well as the replies....

  Just a quick comment on heavy turbines on a tower.   Every one seems to think the only load on the tower is from the wind pressing against the blades.  Well this is true but... what happens to the tower while the wind is pressing against it?  You have the tower weight itself, the turbine on top of it, and the wire tension all adding to the load.  When the wind blows against the turbine it tends to tighten the cables even more... where is this load going?

  Personally I like lite, low part, simple turbines.  For many reasons not just loading.  

  Take note I'm not trying be-little any ones building techniques, I find it quite amazing the unique ways people find to get one together... quite ingenious for the most part.  I simply wanted to make a point that sometimes we overlook the obvious that can lead to catastrophic failures in high winds.  It's only as strong as its weakest link!

.

[Chagrin]

Re-emphasize his "flow battery" comment -- they certainly are a cool technology. If they ever come to their potential they'll be the true answer to the "liquid fuel" problem.

[finnsawyer]

I too am a MIT graduate (in EE).  I also have some ideas about wind power which I've hinted at here, but since "the proof is in the pudding", as they say, I'm looking to someday build a prototype, if I live long enough.  I suggest you take your ideas and do the same.  Find somebody living in a rural area where you can build a prototype or mount your windmill on a vehicle for tests.  I'm sure you can find areas even in the New York City region where you can run the experiments.  The real issue with wind power is to get the most power out at least cost, particularly at low wind speeds (at high wind speeds small inefficiencies don't matter).  A system approach is needed involving blade aerodynamics, generator characteristics, transmission losses, electronics, and mechanical constraints.  As such, trade offs will have to be made starting with the size (power output) of the system.  In any case, have fun.

The devil is always in the details.

[kitno455]

no, actually, the 'proof is in the putting'. no food products are involved in that saying

allan

[Ungrounded Lightning Rod]

I also rethought the concept of multiple axial generators versus one larger generator.  The larger generator is moving the magnets faster at the same RPM since the lever arm is longer and that is a very useful feature for raising the voltage and power.  Basically to have three ways to a more powerful generator - more magnets, more wire or more speed.  Of those only speed is "free" and should be maximized by placing the magnets as far from the axis of rotation as possible.  Many thanks for that reminder - I did have a bit of a nag in my head about maximizing copper and magnet resources.

Right:  A given set of magnets, amount of copper, and size of air gap will generate twice as much power if constructed as one double-radius generator than as two of single-radius.

But that doesn't block you from playing with interconnects.  You can wind each coil in several sections and bring them out separately, then interconnect them to your heart's content.

And (as Hugh already pointed out) the voltage doesn't make any difference in the genny losses.  It's all in the transmission, rectification, and other processing.  If your tower is far enough from your storage you can save enough on the cost of the wiring to pay for the transformers, electronics, or whatever for doing downconversion on the far end.  If not, you're probably ahead to match your batteries and leave it at that.  You can save a few percent of generated power in the rectifiers by going with a higher voltage battery bank.  But if that means more expensive inverters and inability to use cheap automotive appliances straight off the batteries you just raised your capital costs.  You can get an extra ten percent by making your blades about five percent longer, so why bother?

Same with a lot of other tweaks.  When the "fuel" is free and the power is there for only the cost of making a machine to grab it and a system for storing and converting it, what do you optimize for?  I'd put "cost of the machine/system" high on the list, with "cost of maintainence", "time spend on maintainence", "mean time to failure", "time to repair", and "cost to repair" all in a crowd next to it.  B-)

Power generation tech is VERY simple and robust if you don't go after the last few percent - and in a small system with free "fuel" there's little payoff for the extra expense of the infrastructure to grab the small stuff.  So "efficiency" - beyond that you need to get the job done - is hardly even on the screen.

Engineering is all about understanding what the REAL requirements are, and not wasting time on bells and whistles at the cost of the meat and potatoes - or breaking the budget.

[TomW]

UGL;

Engineering is all about understanding what the REAL requirements are, and not wasting time on bells and whistles at the cost of the meat and potatoes - or breaking the budget.

Amen to that!

Another favorite of mine is [while achieving the same results] perfection is achieved when nothing more can be taken away and not when nothing more can be added. The last time I said this it apparently confused some folks. If I have to explain, they couldn't understand.

Cheers.

TomW

[BigBreaker]

Your point on pulling the coil leads separately was exactly where I started going after the inefficiency issue with multiple gen's was brought up.  Same idea but better use of magnets and wire.

Again I can't stress enough that I appreciate the value of simplicity and the robustness it can provide.  As structures get larger the weight and volume grows faster than the cross section of structural supports from pure scaling.  Cross section is the proportional element for strength.  Most people understand this in their gut, it's the reason why elephants have big, thick knees and insects do not.  I was trying to "run in front" of the problem by trying to engineer out some of the larger stresses in a big DIY mill so they look more like giraffes and less like elephants.  A lighter mill should also be a cheaper mill.

I also hear you about not trying for the last few %.  I agree and if you need extra power it probably makes since to go for a bigger mill than a more efficient smaller one - since the fuel is free!

PS I looked up the vertical vs horizontal stresses and you guys are totally right.  The wind force or horizontal force is about four times as strong as the vertical force.  Sounds like thick guy wires are key and gen weight doesn't matter that much.

[BigBreaker]

1.  You are totally right here
   
2.  Respectfully disagree - see the comment below.
   
3.  The blades can move down if the hub is a meaningful fraction of the prop diameter forward of the tower.  The guy wires attach below the blades anyhow.  Hugh's design has the hub right in front of the tower but it doesn't HAVE to be like that - it can be positioned farther forward and balanced against the weight of the tail.  You don't need a spherical joint - you can have two traditional pivots on top of one another but obviously it still is more complicated.  Precessional torque is a function of the spinning blade's moment of inertia (IE mass and speed) and the yaw rate which is always orthogonal to the axis of the moment.  It is significant anytime the mill yaws or furls.  I'll do a calculation at some point.  I think the tower can be a lot lighter if you let that torque go free.
   
4.  Very true on the mowing!  A different configuration would be more practical.  The additional length of a pulley system is partially offset by the lower tension in the wire (so you can sue a lighter one).  The pulley acts like part of a block and tackle to nearly halve the tension.
   
5.  It does complicate things and create additional failure points.  I'm an EE so cut me some slack  Power electronics are fun for meh!
   
6.  I am looking maximize efficency across the spectrum.  I did read up on wind strength distribution, cube law and average power maximization.  Low speed characteristics are very important and not to be marginalized in any design.  So I can agree with you here.
   
7.  Yea, I have looked into passive blade pitch "systems".  Basically letting the blades flex.  There are other pitch systems with more mechanics but still passive that might work.  Ugh - too complicated.
   

[Ungrounded Lightning Rod]

PS I looked up the vertical vs horizontal stresses and you guys are totally right.  The wind force or horizontal force is about four times as strong as the vertical force.  Sounds like thick guy wires are key and gen weight doesn't matter that much.

As for springs:  A loose guy wire IS a spring - and a guy wire can't be totally straight so it's necessarily "loose" for this purpose.  Part of it is the springyness of the material itself.   But most of it is the curvature of the guy.

Imagine a string stretched just taut (in zero gee, since you CAN'T get it straight otherwise).  Push the center of it to the side with a tiny bit of force.  It takes INFINITE tension to keep the string straight.

Do the vector diagram for the forces on the center point - two outward vectors along the directions of the halves of the string, one small one along the direction of the push.  They must be equal when you're not accellerating the midpoint.  So the two along the string MUST have an angle between them, and for small angles they must be much larger than the side-force vector.

Now tie a button to the middle of your string - to provide a small, constant, downward weight - and pull on the ends.  You'll notice that, as you straighten the string, it takes progressively more force as it gets straighter, until the force becomes very large and the string still has a kink at the button.  It's a non-linear spring, but a spring nontheless.

The same thing happens with a guy cable, but on a continuous basis.  The weight of the cable is "the tiny button" - a continuous bundle of them.  The curvature of the cable is the kink - again a continuous generalization of it.  When the tower swings away the guy gets straighter and the force rises drastically.  If it gets TOO straight the stretchyness of the cable starts to become significant, then to dominate (so it doesn't suddenly become VERY tight).  But you try to pick a cable strong enough that you don't get that far.

So the tower is kept centered by an array of "springs" consisting of just slightly-curved guy wires.  Non-linear as hell, but that's good too.  The restoring force increase becomes steeper as the deviation becomes larger, thus maintaining tight control on the location of the tower.  And the nonlinear spring "constant" avoids destructive combinations of resonances, by changing the resonant frequency of the tower deflection according to the amount of energy in the system.

Air friction against the tower and the guy wires is what damps the resulting nonlinear harmonic oscilator.

Again the KISS principle applies:  You don't need coil springs at the anchor points or the like.  Just get your tension right in your guy cables.

[finnsawyer]

It refers to golf?

[omweg]

in the past i was working in the recycling bussiness and have had some practical experiance with refilling refrigerators with propane. A big advantage is the less polution with freon and propane is much cheaper. I never knew that it was more efficient.

In holland they have done some testing with the propane refrigerators and the conclusion was not too dangerous, the amount of gas is hardly enought to open the refrigerator door after an explosion, and my own experience is a fast big lighter for a few minutes.  The amount of propane is 40% of the amount of freon so for a big fridge you are talking about 100 grams of propane.

After my opinion propane is a good alternative and a cheap way to give your refrigerator a seccond life.