Ultra low rpm HAWT Wind Turbine~5 KW @ 115rpm!

[Uaf Usman]

I recently spotted a wind turbine development project for VAWT at a local university, where they have designed a 5KW Alternator at only 115 rpm! The voltage output is 250VDC, after rectification. The alternator seems to be superbly designed and weights around 175 lbs. My system voltage is 48V but I may change that to match up with this generator or step it down or rewind it for lower voltage. I don't see that a major issue.

Since my motive was to come up with a ultra-low turning HAWT turbine with very little noise, I am investigating if this alternator can solve my quest.

I would like to know if a 3-blade conventional 5KW turbine rotor from another commercial wind turbine manufacturer would be of any good match for this alternator.

But I notice that most of the other 5KW Battery charging models on the Market have rotor diameters in the region of 5-6.5 meters (16-21 footers) and are rated at least 250 rpm or more, in most case around 350 rpm!

I can go for a larger rotor diameter, if that helps, by putting on blade-extenders or modifying the original 5KW rotor. The alternator is said to be good up to 220 rpm where it would produce around 9 KW or more.

If it can be done without problems, what about the efficiency of the resulting turbine?

Matching Generator to a rotor is the key issue here, but still my weakest point. Any advice on this issue would be appreciated.

Thanks for reading,

Usman.

[Mark Ebert]

Do you have a link to the project or any pics?

Must be a beast.What diameter and how many poles?

Thanks,

Mark :cool:

[Rob Beckers]

Congratulations Uaf!
That's quite a score. A good permanent magnet 5 kW slow-speed alternator goes for many thousands of dollars new. The low RPM at 5 kW would make for a beautifully quiet wind turbine. There are several posts on angle-of-attack and tip-speed-ratio, you may want to take a look at those to get some idea of what the blades need to do (use the 'search' option). Also check out Hugh Piggott's site on wind turbine theory (http://users.aber.ac.uk/iri/WIND/TECH/WPcourse/index.html). You could carve your own blades out of laminated wood. They'd be large, at around 2.5 - 3 meters each, but should be doable if you are a bit handy. Stew is the blade guru around these parts, get in touch with him for some pointers on profiles and such.

It would be a shame to rewind that alternator. At 48V and 5kW you'd have to either use monstrously thick wires, or put the turbine right next to the batteries to keep the line losses in check. Do you have to do battery charging? For grid-tie you could use either a WindyBoy or an Aurora MPPT inverter. For battery charging you could still use those, and backfeed an Outback inverter (they call it "AC coupling"). There's also an MPPT battery charger, made by Magnetek/Power-One, named the MWI-5000-48 (I have a refurbished one for sale if you're interested). The MWI does work as advertised, but they are expensive, not UL listed, and have their quirks. AC-coupling would be a better way if you have that choice.

-RoB-

[Uaf Usman]

Thanks Rob,

The problem is the difficulty in imagining the rotor at such a low rpm. And then the fact that it would cut in (with the battery) at such a low-rpm or wind speed increasing possibility of stalling it in low winds when the rotor hasn't yet build up any momentum. I would presume that it would cut in at less than 30 rpm! So, it has to be very carefully designed.

I am investigating if a conventional rotor from a 5KW turbine, say from Eoltec machine would be any good? Or a similar blade in three bladed configuration.
I suppose not!

From my limited understanding on blade design, I would presume that the cord would have to be really wide and the root part has to be wider than usual to enable more torque than conventional blades. But again, that’s just a rough guess.


Secondly, regarding charging the batteries from it's 220VAC output, I would like to get more information on your 'AC-Coupling" technique, as you are right Magnetek are fairly hard to deal with and the price is high, especially if my loads are all DC and I do not require such a heavy inverter. Please further specify on that issue.

You wrote: "Stew is the blade guru around these parts, get in touch with him for some pointers on profiles and such". Who is this?

I am continuing investigating in this subject and would post my results as and if at all they come up.

Thanks.

[Rob Beckers]

"Stew" is Stewart Corman. Do a search and you'll find what he has posted.

As to AC-coupling; this used to be an undocumented feature of Outback inverters. After Outback was informed of this, and more people tested it, they embraced it officially and now actively advertise it. Do a little reading on the Outback site and you'll find out. What it does is back-feed the batteries from the house-side of the inverter. So a regular grid-tie inverter on a wind turbine (or PV modules inverter) is hooked up to the house wiring, and this will cause excess energy to go towards charging the batteries (through the Outback inverter). The downside is that there's no charge control, so it requires the addition of a diversion controller on the batteries to avoid overcharging.

I believe you're right that blades for 115 RPM @ 5kW would have to be a bit wider than usual. That means taking blades from an existing wind turbine is probably not going to work too well (they're expensive too, if you're talking about commercial wind turbine blades). Running oversized blades would also allow for a relatively 'normal' TSR (and normal blade proportions) while keeping RPM down. The downside of that is cost, and having to deal with more excess energy at high wind speeds that needs to be shed.

Do you know what the voltage limits are for the stator of the alternator? You don't have to extract 5kW @ 115 RPM. An MPPT controller would load up the alternator according to the profile you program into it. So you could extract 5kW @ 175 RPM for example. Alternator voltage is roughly proportional with RPM, so it could run faster than 115 RPM and still fall well within the voltage window of a WindyBoy or Aurora inverter. For direct battery charging you could do the same thing and use a transformer (ie. run it at beyond 115 RPM for max. power, takes some careful matching though), though with a transformer you'll probably need to use a relays/contactor to avoid loading up the turbine when it's stationary or at very low RPM, that would cause the blades to stall and increase cut-in.

As to Magnetek/Power-One inverters in general; they're not really all that hard to deal with, and it is a well-known inverter brand. The Aurora line is great, just that their MWI-5000 has IMO room for improvement (it works well for battery charging but the other parts of the box leave something to be desired).

-RoB-

[Rob Beckers]

Just a couple more thoughts that came up: There's no problem with stalling at low RPM. Rotors stall at low RPM because they were designed to run at a higher RPM, so their angle-of-attack at low RPM is beyond stall angle. A rotor that's designed to run at 115 RPM will have the proper AOA, and will cut in proportionally lower (if those blades are wide enough). Think of those farm water pumping mills with many blades: They run at very low RPM, and TSRs of less than 2 (or so), all at relatively small diameters for the RPM. It's all in the design, not that low RPM has to stall by definition.

You asked why manufacturers don't run their turbines at lower RPM if that's not an issue: The main reason is that the alternator gets very, very, very expensive at lower RPMs. That 5kW of yours has to be a beast of an alternator, with a small fortune in copper in there, to deliver that power at 115 RPM. The original owner must have paid a pretty penny for it!

There's much to say for low RPM turbines (to a point, there are practical limits to get good performance). They are quiet, there's less wear (more reliable). But they are also very expensive to manufacture and not competitive in the market place.

-RoB-

[Laurie Forbes]

I recently spotted a wind turbine development project for VAWT at a local university, where they have designed a 5KW Alternator at only 115 rpm! The voltage output is 250VDC, after rectification. The alternator seems to be superbly designed and weights around 175 lbs.

Anywhere to access more info on this alternator??

[Stewart Corman]

Usman,
Just read this thread and you are probably making the turbine rotor design much more complicated than it should be.

I have posted an xls "windpower" for basic rotor diameter, power, TSR, and rpm:

474

I have highlighted in red, specs that are easily customised.
If you look at the 18 footer at TSR=4.5 ..you are good to 30mph @ 210rpm!
If you aren't using a self limiting blade design, then you need mechanical furling set around 20mph.
Max Power column is total power of the wind ...presume 35% max efficiency to determine generator load.
My problem is speeding up the 18' rotor with gearing by 16x to get my servo generator spinning at 1500rpm=230v

If you read thru my previous related post on this subject:

http://www.greenpowertalk.org/showthread.php?t=4693

you will note that fixed cord width, non twist ala Bergey, Jacobs, Dave B on Otherpower:

http://www.fieldlines.com/story/2008/5/25/22230/7133

...work as good as anything in a fixed pitch design ...don't make things more complicated then they need to be ....BTW ...Dave B's buddy Dave Moller sells his blades fairly inexpensively if being shipped to USA:
http://royalfabrication.com/index.htm

just remember ..bigger diameter = slower rpm, lower TSR = slower rpm

Stew Corman from sunny Endicott

[Uaf Usman]

Thanks Stew,

The links were helpful, especially the one related to constant cord. That's a different subject and might divert the focus of this thread. So, I am going to initiate a new thread "Constant cord Vs. Tapered Cord", since I feel I have something to contribute on this subject, learned from my past experience with composites and European blade designers.

Back to our main subject here, I may repeat that I 'coincidently' spotted a project undertaken by a local energy institute for development of a low-cost VAWT (H-rotor) and they worked on a direct-drive system for a 5KW turbine.
It was designed for around 110 rpm. They had a smaller version, 48V-1KW (rectified) for battery charging, and that is around 210 rpm only! There were several of those built and salvaged, and I am now working on to take one of each i.e. 48VDC-1KW & 220VAC-5KW.

I foresee a great opportunity with these slow turning Alternators as being ultra-low speed, they could offer several advantages if I could configure those to be used in a low-rpm/low-noise HAWT design.

I am now working on designing the rotor for this alternator in a battery-charging situation. I have two challenges here:
1-The voltage: since the alternator is 220VAC Vs. the battery bank is 48VDC.
2-The Rotor: designing or modifying an existing 18-20footer rotor.

Regarding the voltage issue, I suppose I have only two available solutions here:
A-Using a step-down transformer: But this is going to add substantially to the conversion losses.
B-Use an electronic platform like Aurora/Power-One/Magnetek MPPT control, as Rob suggested: But that’s going to be very expensive, especially if I haven't got any use of the inverter, as most of my loads are 48VDC. So, I would be using the Magnetek MWI only as a charge controller.

Here I need help to find another equivalent charge controller (only) such as the MX-60, but for wind turbine application, which could take in the higher voltage and step it down/MPPT it to the 48V Battery-Voltage, but nothing seems to be available in the market offering this. Any other options?

I wouldn't go into each & every bit of what the design process involved here but rather outline the primary efficiency objectives: i.e. highest possible efficiency, keeping in mind the following major challenges:

i-The "battery cut-in" rpm of the rotor gets too low here i.e. 20-30 rpm would get it cut-in and it may stall prematurely due to not sufficient momentum in the rotor. (I presume that this is also solve able be if I were to use the MWI?)

ii-The commercially available rotors are all 250-400rpm for 5KW ratings, so, I am afraid that minor modifications may not help much and a new rotor may have to be designed, with increased torque. So, any advise, ideas, recommendations…..etc that could lead me to an efficient slow-speed rotor?

I am not worried about the over-speeding here since there is external system to protect the turbine under high wind conditions.

I am attaching pictures and design of the 1000W/48V Alternator. I am working on getting more information and pictures of the 5000W/220VAC Alt. but that may be 2-3 weeks away.

Hopefully someone on this forum could assist.

(Please note that: for ease, I have outlined the areas needing assistance in Bold Red).

Thanks, Usman.

[Stewart Corman]

Without going line by line, I have a quick question ...are you locked into battery charging?

You are rightly concerned about using stepdown transformer with associated losses, but the mere fact that charging battery banks inherently have 30% losses ( we have documentation on this) ...you are behind the eight ball. Off grid PV solar requires battery banks so you would have no other option there.

Almost all of the newer commercial designs are coming out with generator options for direct resistance heating or grid tie.

As far as
Here I need help to find another equivalent charge controller (only) such as the MX-60, but for wind turbine application, which could take in the higher voltage and step it down/MPPT it to the 48V Battery-Voltage, but nothing seems to be available in the market offering this. Any other options?There has been an offline communication about a proposed "controller" PWM scheme using a Darlington pair driver which could give you reduced voltage, but I am not sure how the interaction with batteries would fit in as compared to direct resistance heating ...there is no "cut-in" for heating...Mark would probably help you here or at least tell you what options are available.

The commercially available rotors are all 250-400rpm for 5KW ratings, so, I am afraid that minor modifications may not help much and a new rotor may have to be designed, with increased torque. So, any advise, ideas, recommendations…..etc that could lead me to an efficient slow-speed rotor?Hugh's tutorial says it all ....the pitch angle of the blades and number of blades and their solidity, determines the TSR ...using standard blades w/6" cord, such as the ones provided by Royal, you could make a rotor with 5 blades and pitch angle of 12 degrees rather than 10 degrees (you can custom order) and you can cut the rotation by reducing TSR from 7 to 3.5, so the "250-400rpm" becomes 125->200 rpm ...Clarkson Univ did exactly this on a Bergey XL-1.

You simply pick the operating point you need for the generator you have.
The Dave B 18" "WinCharger" was running at TSR=5 with pitch = 5 degrees with 3 blades (GOE222 almost identical to the newest Bergey blade profile!)

477

the "Cd/Cl" in above chart is the inverse of L/D, so pick TSR=3.5 and blades = 5 with L/D =100 and you are in the ballpark ..this part is NOT rocket science


Stew Corman from sunny Endicott