Waterwheel Dimension

Gee, it sounds like you wasted your time and money in your classes. We don't do homework on this forum and we certainly don't do design projects. We can help you solve a problem when you get stuck part way through a problem, but we won't do the problem for you.

i don't ask you all to do the problem for me. I am a mechanical engineering student. but there is an electrical subject that we need to take amd this thing is not in syllabus. I do have the other data calculated. however, for the dimension of the waterwheel i got stucked. I have google around for the formula to get it. If you knows any formula just help me out by guiding me since you all are experienced people and might be some of you have done a giant waterwheel or turbine before. I don't ask for answer. I give the detail perhaps if there are other thing I haven't calculated that is necessary.

I was just asking for a guide from others to help me. I am not asking for answer....

OK that's a fair explanation of the added information you need. I'll fill in the pertinent electrical details I think you need.

First, a three phase, permanent magnet generator is an odd device your teacher chose to charge a battery. The three outputs of the generator must be combined to make a single input voltage source into the battery. Doing this will typically cause a 0.6V loss due to steering diodes voltage losses (the generator will actually have to produce a higher voltage) additionally to actually impart 500 watts of power into the battery you must exceed the battery's 12V and source resistance with your voltage out of your generator. This charging condition will constantly change as more charge gets packed into your battery so a constant power cannot be pumped into a battery. Also by using a permanent magnet generator instead of an electromagnet controlled alternator, the best feedback path for reliable control of these conditions has been lost. (This is why cars use alternators today instead of generators.) It's up to you to include this information in your answer because I don't think that your teacher is anticipating this kind of details or for you to try and work out these problems.

So what I believe that your teacher is expecting from your water wheel is a wheel that moves with this water flow that will spin the shaft fast enough to produce 12V when enough water gets slowed down by the paddle that 500 watts of power can be taken from the moving water. Now you've already calculated or were given the information that this permanent magnet generator will generate 0.06V/rpm. (This makes sense because a permanent magnet generator generates a voltage based on how fast it is turning. This is why a permanent magnet generator is used to measure velocity in test instruments.) Now remember that you cannot take all of the energy out of the flowing water because it will no longer flow.

I think I've clarified enough of what your teacher is asking on the electrical side of this problem so that you can move onto the mechanics.

hey you must be a teacher

i mean redfred must be a teacher if not he should do so

When I hear "River Turbine", I think of this sort of thing:

Image from Wikipedia

What is a "... floating waterwheel river turbine..."?

its actually a smart idea . if i interpret this correctly , rather than constructing concrete supports and raceways for a horizontal shaft water wheel , i think they are proposing to make floats to support the mass of the water wheel , shaft and bearings , make them long enough to counteract torque , and probably tether the floating assembly to a fixed object like a jetty , or perhaps use twin guy ropes tied to the shore each side of a river , with the power cable fastened to the guy rope

cheap and relocatable , " automatically adjusts " to water level ... lol

500 watts is not much to produce although an alternator and charge regulator would be easier to use than a Perm Magnet DC generator , rectifier and inverter

nothing is impo. some years ago I saw an old oil paintin of the danube crossing budapest and was full of some sort of pontoon boats with water wheels and they told me they where floating wheat mills using the flow of the river to grind wheat iaskmyself what type of energy would this generate any idea would kill my curiosity

I have never done a water wheel, but have admired several. Assuming an overshot wheel, I would expect the weight of the water in the buckets would have to generate enough torque to drive the generator to get the 500 watts. You will, of course, have to gear up the water wheel speed, which will increase the torque needed in the same ratio--assuming frictionless bearings, gears, etc. (at least that is what we assumed in college! not real world, but makes calculations easier.)

Yes, an overshot wheel will also gain from the dropping of water height. I assume that our future mechanical engineer will be able to take that into account or ignore it because the stated problem does not give a drop in height. I would assume that an undershot wheel was intended but accept an overshot that specifically pointed out the required height in water fall to meet design requirements.

what are you guys talking about ?

the OP stated floating water wheel which means undershot

To charge a 12V battery, the generator must put out somewhat more than 12V. This will tell you how fast the generator must turn. The waterwheel blades will slip somewhat, maybe going about 2/3 as fast as the water passing by. This will allow you to estimate the wheel's diameter.

[copied from duplicate thread]

In addition, you can compute the kinetic energy of area A x 2m of water per second. Guessing 50% efficiency, equate this to 1000 watts, and this will estimate the required blade area.

this is my calculation, however, i don't know how to get the radius and how to identify what is it width and height of blade. hope you can guide me.

it is in google docs

https://docs.google.com/open?id=1fS3Y_ZsD0nXjqvWU6yIWniJlA3nOSCZq4p_XuU1mJYVGPbv2tLfMVvvhqhxC

I think you've done well.

As it seems you know (or can see from your calculations) the power produced is (in theory) related only to the blade area (for a given water speed). A smaller radius wheel will have a higher rpm and lower torque. A larger radius wheel will have a lower rpm and higher torque.

In practice, the power produced will have something to do with the nature of the water's surface. If the waves are typically .1m high, then a paddle that is only .1m high will be in air much of the time, if mounted so that the top of the blade is at the average water surface. For your exercise, perhaps you can consider the water smooth.

So if you wanted to avoid gearing, you would need to find a radius that provides an rpm that produces 14 volts or so, right? (In practice, you'd need enough radius to ensure that the generator is not partly underwater.)

always build larger than you need because water flow can change

Experiment. Build a water wheel out of plywood, say with 4 blades, and try it. Measure the speed of water flow, torque on the wheel, and the rotational speed of the wheel when you apply a braking force thru a fish-weighing scale. Calculate. Nothing like experiment to get a feeling for the problem.

500 Watts is 2/3 of a Horsepower. Allow an efficiency of 40% for a start (probably wrong). The weight of water moving past your paddle per second is a measure of the energy available.

Have fun!

When you become an engineer, you may have peers to help you verify the validity of your ideas or you may work in a small firm as I presently do. When stumped or you just need to see how others solve a problem, the internet is your friend. I Googled "floating waterwheel" and this design came up. Us it to cross check your design.

Good luck.

You can start by thinking of a large wheel, perhaps 2 meters across. Then pick an area for each blade, say .125 m2. Calculate the force on this paddle as if it is a flat plate (1/2 rho V2...) Consider this force to be constant, and position the paddles around the rim to meet this assumption. Then you have a force, a moment arm (use the center of the paddle) and therefore a torque. You have an RPM (assume a 50% slip or loss, once the thing is loaded), based on water speed.

So with rpm and torque you have HP or Watts.

Fiddle with dimensions to get an ample value in watts -- say 1000.

Then figure the gearing required to get 14.5 volts or so. (This will need to be a large 12V battery to accept 40-some amps.) You don't want to exceed a loaded voltage of 14.5V, to avoid damaging the battery.

You could make the paddle wheel smaller in diameter but longer, axially. Calculate to see if a reasonable wheel size can eliminate the need for gearing. If you want this to actually work, you need to think about the nature of the water surface: smooth? rough?

You might also consider the propeller from an outboard motor. Apparently you only need about 250 rpm or so, and if you calculate pitch vs water speed, you may find that a prop will spin almost fast enough even without gearing. Maybe. Props are about 40% efficient.

You need to look at the paddle circumferential speed becuase the speed of the paddle decreases the effective water speed that is hitting the paddle. Therefore you have a juggling act to consider: If you use a large diameter paddlewheel, the wheel shaft speed will be slower, causing you to have to use high gear ratio (read low efficiency) gearing to get any decent rotational speed for your generator. A large diameter wheel will however, give you more torque for a given paddle area.

You may be able to Google an equation that describes the reaction on a flat plate set at right angles to water flow. It would be a better estimation than just using the (area of the plate) x (water mass/area pushing it) x (an arbitrary reduction factor)

Good luck!! You have an interesting problem.

Jon.