Archimedes screw turbine control for off-grid generators

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bobc
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Archimedes screw turbine control for off-grid generators

Post by bobc » Nov 16, 2016 6:53 am

Thought this might be of general interest; I've been looking at a potential micro hydro project where an archimedes screw would directly drive the generator (e.g. one from a broken petrol generator), and how it could be controlled. The petrol engine speed was governed by a vane in its cooling fan airflow directly actuating the throttle. So we need a similar control for the screw turbine.
The screw is 1m diameter and 4m long - with 30degrees incline this can output about 5kW at 1 rev/s.
The main control problem is the delay from the control input (water flow control into the screw) to the output torque changing; it takes 4 seconds for water to travel through the screw.
A safety issue is that if control is lost the screw could spin faster and faster until it breaks - this sort of rules out electrical controls. So I'm looking at a centrifugal governor. For simplicity's sake I can do proportional only.
The key to making it work is to have a disturbingly large flywheel at the generator. I can't see another way to supply a sudden demand for 5kW for 4s while the screw fills up along its length....
I simulated the system in XL - looks as though it works OK. I get decent performance with a 1kgm^2 flywheel on the genny
screwperf.JPG
(227.25 KiB) Downloaded 10 times
The sim starts with 4kW output, dropping to 200W halfway through
the first graph is speed (supposed to be 1 rev/s) - so we're seeing +/- 3% or so speed. Increasing the controller gain reduces this but it soon becomes unstable. Gain can only be increased if the flywheel is made bigger.
The graph on the right is "how full are the segments" within the screw - max 69litres (0.069 m^3), min is zero
the graph below is screw output torque in Nm
I'm itching to get on with this - should be a proper giggle!

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Re: Archimedes screw turbine control for off-grid generators

Post by Punx0r » Nov 16, 2016 8:42 am

I think the answer is that something so large, heavy and slow will never provide fractional-second response to large step changes in electrical demand.

Tweaking the speed to maintain frequency lock at a steady load should be feasible, and providing a safety feature to prevent gross over-speed should be easy (centrifugal governer/brake as used on steam engines or lifts(elevators)?).

If you were to use a DC generator instead of an alternator then speed regulation wouldn't matter, it'd be like a wind or conventional water turbine. DC would also allow use of capacitors or a battery to smooth load demand while the screw changes speed.

Is the whole thing better than a conventional turbine with a 4m head? Manufacturing this screw sounds challenging.

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Re: Archimedes screw turbine control for off-grid generators

Post by bobc » Nov 16, 2016 2:05 pm

The old archimedes screw has had a resurge in popularity with riverside micro-hydro in recent years for many reasons, it's efficient, easy to make and install, interesting/educational for passers by & doesn't make passing fish into sushi. Loads of stuff on the web; most suppliers advertise "water to wire" efficiencies around 80% over a wide range of heads & flows. I became interested when I saw a small installation in a nearby river - a 2metre screw with 4metres of head pulls 100kW out of the river dane with payback time around 7.5yrs.
You do have to be quite careful sizing these things (typical ludicrous officialdom...) pull out too much power & you don't get the good feed in tarriff & payback goes back to 20years....

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Re: Archimedes screw turbine control for off-grid generators

Post by bobc » Nov 16, 2016 2:14 pm

Been looking at "what does a 1kgm^2 flywheel look like". A 500mm disc in 25mm steel gets there, weighs about 26kg. FWIW the screw above weighs around 525kg and has moment of inertia about 80kgm^2. When geared up x50 that's 0.03kgm^2; that's 1/30 the inertia of the little steel disc.....
flywheel1kgm2.jpg
laser cut flywheel 1 kgm^2
flywheel1kgm2.jpg (37.78 KiB) Viewed 1930 times
intakesim.jpg
5kW archimedes screw turbine
intakesim.jpg (21.71 KiB) Viewed 1930 times
The turbine is fabricated around a 20inch central tube byt welding on laser cut 2mm sections (quite a lot of them)

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Re: Archimedes screw turbine control for off-grid generators

Post by craneplaneguy » Nov 16, 2016 11:42 pm

My pelton wheel micro hydro uses an electric heater ( more or less, designed for this purpose and sized correctly, it looks like a heater when working, glowing red and putting out heat) as an overspeed control. It is controlled by a load diverter (MorningStar). So far so good, we had a 4 hr grid outage the other week, and the turbine didn't blow up from overspeeding, so I assume it worked as it should!

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Re: Archimedes screw turbine control for off-grid generators

Post by bobc » Nov 17, 2016 5:28 am

Hi CPG,
good to hear from someone who's "been there" and "done that" (and relieved to hear I'm not the only one worried about overspeed...). Your system is grid- tied isn't it? My understanding of pelton wheels is that it would double its design speed when unloaded (?) whereas the ultimate speed of the screw is less easy to predict; I suppose there's an ultimate speed of water running down a slopey channel, the design choices regarding screw pitch and slope would determine that..... somehow..... google here I come....

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Re: Archimedes screw turbine control for off-grid generators

Post by bobc » Nov 17, 2016 3:33 pm

Used an on-line calculator for water running in a rectangular channel to estimate "water to screw shaft" efficiency for my turbine
screweff.JPG
hydro efficiency vs water depth in screw segments
screweff.JPG (25.69 KiB) Viewed 1901 times
Some shaky assumptions in the derivation of that, but it looks plausible (& better than I feared)
Being a fixed speed operation, I have to underfill the screw segments to reduce power (using regulator described above) so the x axis is "depth of water in the bottom of the screw"
Also note, losses in bearings, gearbox and electrics are not included
PS - same analysis predicts max possible overspeed of 2x which is not so bad.....

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Re: Archimedes screw turbine control for off-grid generators

Post by craneplaneguy » Nov 18, 2016 10:30 am

I have never tested how fast it would spin up unloaded, but it was made clear to me by Harris Hydro (my supplier of choice) that a dump load was needed. Yes, I'm grid tied, so when the grid goes down I absolutely need a reliable way to keep the thing under load. It all works as it should, during one of our very infrequent grid outages, I managed to run down to the turbine's power shack (1100' from the house) and sure enough the heater grid was doing it's thing, and the pelton wheel RPM's seemed about normal. If I had to guess..... I'd say it'd probably survive unloaded, if not for too long, but for sure better to have a way to keep it loaded!

One big concern I'd have with the screw design, and I'm sure you have considered it, is keeping debris out of it. Intakes are a big part of any hydro setup, and keeping that intake maintenance free is important, you don't want to have clean it every day. Thus, the Hydro Screen type of self cleaning intakes. All I do to mine is once or twice a year take the screen off the box (2 bolts, quick and easy) and then I pressure wash the back side, where in warm months and low flow, I get moss build ups that slow the flow.

I have about another 2 or 3 weeks to go, then decreasing temps will force me to shut the system down for the winter. No problem, as I have over 12,000 KWH stashed in my utilities system as a credit. Point being, with the way our grid tie system works, this is a more practical approach then attempting the operate it all winter.

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Re: Archimedes screw turbine control for off-grid generators

Post by kc8adu » Nov 20, 2016 3:47 pm

i would also use a pm generator head.
much easier to regulate and a dc-dc converter is easier and safer than a flywheel and governor.
the alternator wants to be run at 3600 rpm to get its rated voltage and frequency.
and the dc genrerator is easy to prevent runaway.just switch in a diversion load in your water heater.
you buffer it with a battery bank to handle heavy demand.
almost forgot.
one could detect utility failure and make the system disconnect and run the loads from the inverter.
make it do double duty.

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Re: Archimedes screw turbine control for off-grid generators

Post by bobc » Nov 23, 2016 9:16 am

Regarding intake screening, I'll have to wait and see. One of the big "plus points" about the archie screw is that big lumps of stuff just pass straight through, they don't need fine screening like anything with a nozzle (pelton) or complex guide vanes (francis) would. But if I think about what's likely to be floating down a river, logs/branches, such 'rod' shaped stuff could poke through a smallish mesh & still gum up the works (a 3" diameter branch might do some serious damage to the screw input when the screw contains a ton of water...)

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Re: Archimedes screw turbine control for off-grid generators

Post by whereswally606 » Nov 24, 2016 9:30 am

Hey Bob, where do you intend to use this. Do you have some land near a river or is this for someone you know that does. Looks interesting though, have subscribed.
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Re: Archimedes screw turbine control for off-grid generators

Post by bobc » Nov 24, 2016 12:50 pm

Hey Andy,
couple of possible trial locations, it's for a 3rd world initiative at the end of the day, but I'm keen to try it out & gain confidence in design ruggedness and build costs. That's why I'm aiming for deliberately low tech, off grid & no reliance on "western" hi tech... I'd expect it to be totally built and maintained locally.

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Re: Archimedes screw turbine control for off-grid generators

Post by whereswally606 » Nov 24, 2016 1:21 pm

That's awesome and has the benefit of less daft regulations to navigate.
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Re: Archimedes screw turbine control for off-grid generators

Post by bobc » Dec 14, 2016 3:22 pm

I visited a local "commercial" micro hydro archie setup today, on the river Dane near Holmes chapel. It's just 10 miles or so from home. They're pulling 90kW from a 14foot head - the installation is reasonably quiet and compact, just a smallish hut. It's next to the weir so you can barely hear the screw for the noise from the waterfall. They are clearly taking just a small fraction of the flow at the moment - but it has been pretty rainy locally recently.
Somebody told me that the river Dane had more hydro energy schemes along its length than any other river in UK in the 1800s - a proper industrial ribbon development. There are certainly still weir after weir after weir when you walk along.

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Re: Archimedes screw turbine control for off-grid generators

Post by craneplaneguy » Dec 15, 2016 9:34 am

That's cool, I've also toured several commercial hydro plants. Simply being able to talk the lingo and show an interest got me access, to where the public usually didn't get to go.

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Re: Archimedes screw turbine control for off-grid generators

Post by bobc » Jan 01, 2017 3:45 pm

Still looking for a practical solution for the bearing at the bottom end of the screw.
Problem is that that bearing is, to all intents and purposes, in the river.
I asked a manufacturer of marine "cutless" bearings but it's not spinning fast enough for one of them.
So looking at ceramic and stainless ball bearings now - anybody got any suggestions?
I was thinking, have a conventional top bearing (maybe even simply use the gearbox bearings) to take radial and thrust loading, it's out of the water and accessible for maintenance.
so the lower bearing only has a static radial load - easy job... :)
Also top shaft has to be meaty enough to take the turbine torque (about 75mm dia). Bottom shaft only needs to support 1/2 the weight of the screw, less than a ton

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Re: Archimedes screw turbine control for off-grid generators

Post by amberwolf » Jan 02, 2017 12:34 am

What kind of water will it be immersed in? If it's full of minerals, is there a risk of buildup in the bearings, or reaction with those minerals?

My uneducated guess would be that the ceramics would last a lot longer under the likely conditions...but would deposits or grit / etc be more likely to damage the ceramics (get between a bearing and race and cause a fracture)?

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Re: Archimedes screw turbine control for off-grid generators

Post by bobc » Apr 22, 2017 4:57 pm

Beautiful day today - spring has sprung, bright sun - I went for a ramble to look at a potential test site on Sabden Brook. OK but a bit light on flow and head - could rig up a small one I guess.
The setting was SO nice, looked like no-one had been there in years, small herd of wild deer appeared at one point! (to put that in perspective, I grew up within 2 miles and NEVER saw a single wild deer.... ever). It has been a dry month so the site might have a kilowatt in it normally.
Anyway - it's on a friend's land so it's all doable, he's got any amount of agricultural & earthmoving gear & is into renewables (2 decent size windmills & some solar going in) - on reflection it might be the best option going for a test setup.
AW - nice river water, 1 small village upstream with no industry, I've fished for trout in there (long long ago). Catchment area is gritstone (sandstone) and peat so on the acid side of neutral pH

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