Design questions - battery charging vs. resistive heating

[Dave B]

I plan only to heat water with my next 18' genny. I am currently running a 12' single phase with laminants for the same purpose, this works fine for pre-heating but obviously requires good wind for hours on end to effectively heat any useful amount of water. With the 18' I want to furl at about 200 rpm at possibly 20-25 mph wind and shoot for 2-2.5 KW. Most here are charging batteries - with that in mind maybe the experienced out there could suggest any significant design changes they would make to the typical dual rotor 3 phase alternators (and control) for resistive heating exclusively. This will cause some moans and groans but I have to ask if anyone would consider a dual rotor single phase AC set up ? My current set-up is this with a simple step controller switching (50A solid state relays) the wild AC to the heating elements at different RPM. No DC (except for the control circuit) and very simple. Yes, I know not as efficient as 3 phase and vibration may be an issue but again I'm looking for suggestions and ideas on the current dual rotor design for resistive heating exclusively and not as a dump load. I look forward to your comments, thanks Dave B.  

[Flux]

To get good results with heating you need to operate on the peak of the power curve.

Most of the big machines here are working well into stall at the top end. 200rpm is rather slow for the full load speed with a typical tsr. If you want to run with lower tsr then you will need a bigger alternator to gain full benefit.

If you are happy with about 3 kW you can probably get that at 200 rpm but you must limit the speed by furl, not drag it down with the load.

You can ignore the lower winds so you can wind for a cut in about 12mph, that will reduce resistance and push efficiency up.

I assume you want single phase to cut down on the number of static switches in the control. Yes you can use single phase but expect to use a bigger diameter magnet rotor with more magnets for the same output. If you build it like a battleship it should hold together. High speed single phase machines work well enough, the severe vibration probably has something to do with the inertia at the operating speed so go for thick discs. I won't advise you to use single phase, but I am not going to say it won't work.

If I was doing this I think I would use a 3 phase mixed bridge with 3 diodes and 3 thyristors and run the load on dc, but as far as I know there is no commercially available 3 phase firing circuit that works with variable frequency so it would be a project.

I have had a quick look at the figures using your 18ft prop on DanB's 20 ft alternator wound for twice cut in speed and it looks as though you can get to 4kW at near 80% efficiency with a speed near 200 rpm. That was 3 phase, I have no data to base a single phase result on.

Flux

[SmoggyTurnip]

I have built a 3 phase, dual rotor alternator for heating.

One thing I think you should is keep the voltage high.

My system puts out about 1 volt per RPM (phase voltage).

With this I only lose about 3% of the power in a 750 ft transmission line.

My blades are 18ft 8" diameter and I have had them as high as 215 rpm so far.

I am thinking I might let them go as high as 285 rpm.

What do you think Flux? (about furling at 285 rpm).

.

The sooner you start the longer it takes.

[Flux]

285 just exceeds the 80M/s that seems a good standard to aim for for nice quiet results but it should be ok. You may need to think about blade leading edge protection and it starts to be worth worrying about blade tip shape to keep things quiet.

I agree about the high voltage but it does become a lot more demanding on a completely waterproof stator casting. Do you really mean phase voltage that would give a line voltage of 1.7 times 285V. Ok if you keep water out, may be awkward for switching devices.

I would have kept to 240V heaters.

Flux

[Dave B]

Thanks for the suggestions. I already have 2 16" x 1/2" rotors with 20  1x2x 3/4" thick grade 40 neos mounted, maybe this changes some ideas for the design as I will be using these for sure. Sounds like higher voltage is the way to go and I had thought switching 120 v 1500 watt elements like I currently do would work well. We are currently having 40-50+ mph gusts in the North East and being my 12' is set up a little hot being slow to furl I really want the 18' to run slow and easy. I plan to work out a cable arrangement also to manually furl it. At 85' it takes a bit of time to lower it and there isn't always time to think about doing that so it will be a nice bit of security to be able to  manually furl it without bringing it down. I still like keeping things simple and I'll be testing the idea of single phase also. Comments are always welcome, thank you.  Dave B.  

[WXYZCIENCE]

about blade tip shape to keep things quiet.


Flux, can you please explain how this shape would be? Is the 80 M/sec a standard for a good tip design?


Joseph

[Flux]

I don't claim to be an expert on high speed blade design but certainly at higher tip speeds things can become very noisy. There is nothing very magic about the 80M/s tip speed but noise does start to become more of an issue if you go above it.

With tip speeds less than 7 and things running towards stall noise doesn't seem to be much of an issue.

I suspect that tip speed alone is not the only issue, low tsr blades can be as noisy at runaway as higher tsr well loaded.

For the lower speeds just choping the ends of the blades off square seems as good as any, but with higher speeds attention to the tips does help and probably one vital factor is to keep the trailing edges of the blades sharp.

The best information I have seen on blade tip shape is in a Sandia labs PDF

Sand 2002-3101  Oct 2002 that someone here gave a reference to.

It is rather complicated to explain so perhaps someone can turn up that reference.

The basic idea is a radius on the leading edge of the tip and a square trailing edge and the whole thing is brought to a fine edge along the tip.

Surface finish has a considerable effect on noise and I am sure the actual profile must also be an issue.

Flux

[scottsAI]

http://infoserve.library.sandia.gov/sand_doc/2002/023101.pdf

This the one?

[tecker]

This probably won't fit your plan but I've noticed that higher voltages with good power factors are seen in long coil legs with radical end turns.

[Ungrounded Lightning Rod]

The basic idea is a radius on the leading edge of the tip and a square trailing edge and the whole thing is brought to a fine edge along the tip.

As I understand it:

The wind tries to run around the end of the blade due to the pressure difference between the front and back sides.  Eventually it succeeds (creating a "tip vortex" of spinning air).

If it succeeds near the blade tip (the center of the tip vortex is near the tip) it reduces the pressure difference between the front and back near the tip, and thus depowers the end of the blade.  This is bad. The end of the blade represents a LOT of your swept area, so depowering it costs you a bunch of output.  Also:  the wind making the turn and being deflected by the backside turns some of that lost power into noise.

Letting the tip be round makes it easy for the wind to run around it.

Making the tip sharp causes the wind leaving the front side to be jetted off for a distance before it can turn around.  (The tip vortex is pushed out a bit.)  By the time it gets turned around the blade is gone.  So the pressure difference is maintained and the tip vortex spins away merrily downwind.  That's why you want the end sharp.  (This can be interpreted as virtually making the blade longer:  Not only does the end remain powered, but the tip vortex's interaction with the incoming air can actually collect power from the flow a small distance beyond the end of the blade, increasing the effective swept area further.)

Rounding the corner makes the transition from the leading edge smooth.  Discontinuities create vortex peel, drag, and noise.

(The leading edge is rounded so it splits the air cleanly, leaving the airflow attached to both the front and the back of the blade, at a range of angles-of-attack.  Sharp would be somewhat more efficient - but only for a particular angle of attack.  The blade would stall at other angles, with the airflow detaching from one side or ther other, losing far more than was gained.  The trailing edge, on the other hand, is sharp so the two attached airflows reconnect cleanly, going in the desired direction, without creating a vacuum behind the join that would drag on the blade.)

I'm not sure about the significance of the angle between the flat portion of the sharp edge of the tip and the radius of the blade - whether it's better to have the trailing end farther out, closer in, or at right angles to the radius.  I presume you want it to end just before it would be caught by the returning air from the tip vortex.  I note that Sandia designed it to be about three degrees off from a right angle with the radius, farther out at the trailing edge, so the sharp edges of the tip and the blade's trailing edge made a sharp 87 degree corner.  (They also started the sharp part of the tip within the rounded front corner, a bit behind the 45 degree point, rather than where the corner rounding transitioned to the straight part of the edge.  And they curved the sharp edge in the upwind-downwind direction to track the midline of the airfoil shape.)

[scottsAI]

Hello Dave B,

Wondering why your so focused on heat?

Looking at your back posts I see the interest not the why?

For less money and less risk Solar heat vs wind?

Unless way way up north and have lots of wind, then the interest in low winds debunks this?

18. footer 200RPM at 25mph is TSR = 5.1 power is 7280watts (0.36 eff)
    
200. RPM at 20mph TSR = 6.5 power 3727watts
     
10. mph power is 465watts.
    
5. mph power is 58watts.
   
   

Suggestion on single vs 3 phase.

Use 3 phase, engage each phase at different voltage levels.

Start with phase A connected only, as the voltage goes up by 1.25 add phase B, then 1.25 more phase C. This covers the first 2x in RPM change. 5 to 10, then A ... another to 20 then anything beyond. This would be covering 6 to 7 stepped loads.

The three phases do not have to be loaded the same.

I do not know how many steps you have planned for the load, but the closer the load matches the wind power the more power you can get. Over loading the gen based on the wind speed will slow down the blades until the load matches the available power.

Under loading the reverse occurs, blades speedup until matched, up to 2x speed is possible, or called an unloaded blade. I mention the RPM profile of the blade, get too far off the curve the efficiency may drop.

Have fun,

Scott.

[Dave B]

Hi Scott,

  It's a hobby and to me much more exciting and challenging than hot water solar. Solar may be later down the road. Switching 3 seperate phases sounds like 3 single phase machines, I guess I don't see the benifit as compared to the single phase machine always at max. output for a given RPM. and stepping the load. I have even had good results with a single load connected direct. Yes, it's always a compromise  but I prefer to be a "do-er" to learn from like many on this board. I am always open to ideas and if you or anyone out there has switched 3 phase independently for heating as you mention I would be very interested in hearing of your results. I could learn much from the working circuit drawing besides as I know only enough electronics to be dangerous. Thank you for your idea.  Dave B.        

[scottsAI]

Hello Dave B,

Then I can't help you. I have not built a wind gen recently, like 20-30 years.

Myself am a voyeur, I tell people how things work and how to build.

Automotive electrical engineer supplier by trade, very dangerous on mechanical.

I loose interest after the design, time for the next thing. This is frustrating

3 phase, several ways to look at this.

-benefits of higher pole count, the extra phases are like two more poles.

-Redundant, one wire opens, still have a single phase machine.

-Ever decide to charge battery, much better with 3 phase.

Last two are my favorite, like redundant and flexibility.

Are you using any wind power for your house electrical?

to me much more exciting and challenging than hot water solar.

Solar heat is very challenging; low cost, long term reliability and pass codes.

Not sure I have seen a design that meets above. Solar hot water cost around $1500 DIY. Payback is decades long. I went with gas on demand heater for $600. (like very much). Gas reduction was noticed immediately, payback would have been long. Demand unit cost 2x tanked, should last more than 3-4x times. So from that point of view was a good buy. Solar at $1500 with no gas bill, 12years 7% payoff. Michigan must have back up so system will cost more and require gas or electric, this is all assuming it will last 12 years. Payoff of $2100 at 7% of $15/mo = 25yr a rather long payoff. Looking over the numbers I do not see how solar or wind is cost effective unless DIY, or off grid or down south.

Things look better when home heat and hot water are combined as a system, same backup can work for both!

Have fun with your hobby!

Scott.

[SmoggyTurnip]

For me solar vs wind is easy to decide.  In January when I need the most heat I have average of 2 hrs per day of usefull sunlight.  In the summer I have lots of sun but I don't need heat.  Have lots of wind in the winter when I need it most and less in the summer.

.

[Dave B]

Same situation here in the North East for generating heat. More wind than sun in the Winter and just opposite in the Summer. Besides being more of a challange and certainly more exciting wind power here for heat just makes more sense. Dave B.