Looking for thoughts and opinions on water wheel design.

I have done alot of looking and reading on the web about water wheels. I have zero hands on knowledge/experience with them. Which lands me here, the first forum that I have found with very good info and real world people and feedback.

So here goes.....


I am looking to build a waterwheel to install below the dam that is located on my property. One of the three Weirs that I would be using to feed said waterwheel measures 12' wide and for 6 months out of the year has water 4-6" deep flowing over it constantly at aprox 3mph+. So I have a very good volume of water to to utilize.

The size and design of the waterwheel is where I am left scratching my head about which is a more efficient design.

My current thinking is to build an 8ft wheel that is a "hybrid" Pitchback wheel design, but encase the lower 1/4 of the wheel as most Breastshot wheels are to hold the water in the buckets until it exits the wheel in the 5-6 o'clock position. Thus using very close to 50% of the total surface area of the wheel at all times. (Pictures below) From my reading, Pitchback wheels usually have an efficiency of 80%+.




My other line of thinking is wheel diameter. Instead of building an 8ft Pitchback wheel, bulding a 14-16ft Breastshot wheel. I have not come across any good efficiency numbers for Breastshot wheels. Also, whould simply doubling the diameter of the wheel overcome the efficiency loss vs an 8ft pitchback at 80%+ efficiency?



With the volume and flow of water available, would a 1500-2000 Watt generator be a reasonable size for a waterwheel with either setup?

As you mention an 8' pitchback wheel, I assume you have 8' of head available. I think even if a breast wheel (as they were historically known) were to use as much of the water's energy as the pitchback wheel, the additional effort and expense of building a wheel twice as big in diameter and four times as massive would not be worthwhile. It would also rotate about half as fast in rpm, so would take that much more gearing up to spin a generator.

You mention flowing 3mph over the weir, which seems very slow. Do you have a weir that drops the water immediately after it, or is this just a cross section of a flowing channel?

Glenn Herbert wrote:As you mention an 8' pitchback wheel, I assume you have 8' of head available.

The workable drop from the top of the dam wall (bottom of Weir channel) is just over 10 feet (somewhere around 10'4"+/-) to the surface of the water at the bottom of the dam so I am figuring an 8 ft wheel is the most practical due to needing to build a Penstock and having some breathing room underneath for water drain. However, depending on final design of the wheel, I may be digging out an area and pouring an entirely new concrete foundation for the wheel. If I build a new concrete foundation, then a 10ft wheel is very doable with aprox 12" drop built into the Penstock over the 20ft distance from the current dam wall.

Glenn Herbert wrote:You mention flowing 3mph over the weir, which seems very slow. Do you have a weir that drops the water immediately after it, or is this just a cross section of a flowing channel?

3mph is a VERY vague guess. The speed is probably considerably higher. I just don't know how to measure the flow speed of the water as their are 3 water sources converging just below the dam so water is turbulent and 100 ft downstream the streambed is very uneven with rocks and such. I have ready about measuring using a float and timing it over a measured distance, but width and depth of the streambed would be needed according to what I read and those measurements very drastically over very short distance upstream or downstream.

Is there a kind of flow meter available that I could submerge or dunk into the water to measure water speed as their is to measure windspeed?  

The "Weir" that I mention is just a cross sectional area at the top of the dam. There are 3 of these sections separated by 24" tall divider walls.  Looks like           |______|______|______|    I figure utilizing one of these is the most practical and allows for the best regulation of water flow.


Thank you

This recent thread Waterwheel. Power expectations? has good information and a link to a weir calculator: WSU contracted rectangular weir calculator.

Glenn Herbert wrote:This recent thread Waterwheel. Power expectations? has good information and a link to a weir calculator: WSU contracted rectangular weir calculator.

Let's say then that a very conservative 1500 gallons per min is my working flow of water.


I am at a loss for the math of calculating the HP and thus the wattage capability of the wheel. Some calculations give me just over 3hp while others give me over 240hp.

1500 gallons per min, 8 ft wheel.

Aaron,

For the purposes of turning a generator, you might wish to increase the water velocity.  I don't know if this is possible in your setup, but I have heard of people making a little dam and then piping water downhill to the actual genset.  The purpose is to really get the pressure up high to get the water flowing fast enough to turn the genset a meaningful RPM.  At the end of the pipe, instead of being attached to a water wheel, the pipe is attached to a water turbine which then connects to the genset.

Again, I don't know if this is possible in you situation as it requires a fairly large drop in height, but it is one option.

Good luck,

Eric

B Beeson's post in the thread I linked to gives the power formula.
For you, 1500gpm x 8ft / 5.6 = 2142w. Apply efficiency & conversion factors, say 80% for a pitchback wheel, 50% for gearing up to generator speed. Flume losses as B mentioned are irrelevant, as that happens before you start the power calculation. This gives you something like 856 watts continuous output 24/7, which you would want to store in batteries for use as needed. I'm not an electrician or electrical engineer, but that sounds pretty good.

Eric Hanson wrote:Aaron,

For the purposes of turning a generator, you might wish to increase the water velocity.  I don't know if this is possible in your setup, but I have heard of people making a little dam and then piping water downhill to the actual genset.  The purpose is to really get the pressure up high to get the water flowing fast enough to turn the genset a meaningful RPM.  At the end of the pipe, instead of being attached to a water wheel, the pipe is attached to a water turbine which then connects to the genset.

Again, I don't know if this is possible in you situation as it requires a fairly large drop in height, but it is one option.

Good luck,

Eric

I have had the thought of a turbine, but am concentrating on a wheel instead.

Glenn Herbert wrote:B Beeson's post in the thread I linked to gives the power formula.
For you, 1500gpm x 8ft / 5.6 = 2142w. Apply efficiency & conversion factors, say 80% for a pitchback wheel, 50% for gearing up to generator speed. Flume losses as B mentioned are irrelevant, as that happens before you start the power calculation. This gives you something like 856 watts continuous output 24/7, which you would want to store in batteries for use as needed. I'm not an electrician or electrical engineer, but that sounds pretty good.

Thank you. After further measurements I should be able to adjust the number some.