water wheel question

[komatsu200]

I have a sight with low head but a good flow rate. I dont know the exact flow rate but the available head is only about 5 feet. I know that water wheels are not very efficient but in my situation it is my most easily built and cheapest option. I am not looking for phenominal efficiency. My question is that if an over shot wheel has to be under 5 feet in diameter for the water to go over it and a breast shot wheel can be around 10 feet in diameter since the water only has to come in close to the axis then will the leverage produced by the longer spokes of the breast shot wheel give me more power than the shorter overshot wheel?

[Nando]

Non are right non are wrong.

You need FIRST to measure the water volume then the head and the length of pipe or canal, THEN the proper analysis to determine what is best.

Nando

[Ungrounded Lightning Rod]

Actually water wheels can be very efficient.  (Not that efficiency matters when the "fuel" is free, provided there's enough that you have adequate power after all the conversion steps.)

Water wheels also have the advantage of running 24/7 (even if not necessarily 365), so a modest amount of power adds up and you mostly need storage to flatten your load rather than cover for days of no supply, as with sun or wind.

Overshots are easier to make efficient than undershots, but the poncelet undershot design has the same potential efficiency as overshots and similar realized efficiency (70% vs. 75-80%).  Modern center-wheels can do 75% also.

Get the numbers on your flow and head and describe your river geometry to us.

[Countryboy]

What are you wanting to do with the power?  Make electricity, or do mechanical work?

A 10 foot breast shot wheel will have higher torque, and lower RPMs than a 5 foot overshot with the same head and flow.

If you want to make electricity, you may have to gear these wheels up considerably, killing efficiency.

Personally, I think a Banki/Mitchell turbine would be your cheapest and simplest option, and they are fairly efficient with a 5 foot head.

[spinningmagnets]

You are correct. A 10' breastshot will have much more torque than a 5' overshot. But be aware that the bigger wheel will spin much slower, and have about 3 times the volume and weight of materials, also requiring bigger bearings.

You can also construct a flume to elevate the water feed to an overshot by a couple feet (of course depending on the shape of your property terrain)

You can increase the torque of the overshot by making it wider, so you have many options, depending on the torque/RPM's that would be useful to your application.

Its usually a given assumption that because of the cost of materials, it is desireable to make a small high-RPM alternator. However, if high RPM's are not a viable option in your situation, its the magnet speed that does the trick. There is a post somewhere here from "Cameroon" where a water wheel turned a very large diameter/low-RPM alternator. Although the shaft RPM was relatively slow, the rim speed was high, bearing in mind that the load resistance (charging a battery) also has more leverage to push back against the alternator shaft.

You may find it useful to run AC from your water-wheel/alt to the battery, as AC doesnt degrade as much as DC over a distance. Then put the AC/DC rectifyer by the battery, which should be near the building the elctricity is going to be used in, to shorten the distance the DC travels.

Whatever you do, please post some pics and a write up of how it turned out. Good luck!

[RP]

Bear in mind that AC degrades the same with distance but with AC you have the option to   wind your generator for a higher voltage and send that over distance and then use a transformer to lower the voltage at the destination.

That WILL reduce your losses since you'll have a lower current flowing in the wire and current is what cuases losses in the wire run.  If you run your line voltage at 2X your battery voltage, you'll cut the amperage in the run by 1/2 which will cut your losses by 4.  I^2*R is the formula for loss to heat in a wire run.

Transformers are pretty efficient when used near their design frequency so it can be well worth the trouble.

[komatsu200]

Here is some more info on what I have. available head is 5 feet and flowrate is 40gpm. This is running through 400' of 4" pvc pipe. I have already built a 7'diameter by 24" wide breast shot wheel with 36 buckets that will hold approximately 1 gallon of water each. I have thought about trying to use a car alternator and I know it will require a lot of gearing to get the rpm needed but i would really like to go this route if at all possible. If not then I could build my own brake disk alternator. Im sure everyone is wondering why i would want a water wheel as opposed to a turbine of some sort. I chose the water wheel because I dont want to buy a manufactured turbine and I dont have access to metal working tools to build one myself. I do however have access to woodworking tools and since I have my own sawmill I have plenty of lumber to build with. Plus the water wheel is big and makes noise.  

[komatsu200]

Correction on that last post I only have 18 buckets on the wheel. I set it up today and did some testing and got about 25 rpm but I have several holes in my pipe and think that this will speed up once I get everything sealed up and straightened out. I dont know how to measure the torque at the axle but I know it has enough to keep me from stopping it by hand.    

[DBuller]

Greetings,

   Please take a look at my site for som intesting ideas. On my "Contact" page you will find many sites dealing with waterwheels. Many unique configurations.

  Good Luck!

http://www.wildwaterpower.com/contactus.html

-Dennis