An Assessment of the
Botanical Gardens Waterwheel Project

Assessment requires planning to ensure a good engineering design that also is attractive. A prototype was constructed to assess the operation and esthetics of the unit. Here is a PowerPoint presentation on the project. This prototype was modified slightly to become the final version.

Existing Dam/Pond Structure

The dam's impoundment has a ~10 inch butterfly valve that can be used to drain the water down for maintenance. While the pond was drained, we measured the width of usual water flow at 96 inches. The location of the drain handwheel is midway across and about 20 inches down from the lip of the spillway. The pond has a concrete floor, but there is a minor subterranean leak beneath the west wall, and water flows in below the spillway on the west side. This leak tends to clog after some time of receiving sandy water.

Water Flow and Hydrology

The entering stream has approximately a 6 to 12 inch clearance below existing walkways. If these are topped with more concrete to cover broken areas, additional clearance will exist, but the water level should not be raised. There is a probability of debris being washed into the pool and impeding water flow. A small trash gate at the pool entrance or upstream would help keep submerged trash out of the pool and would be more-readily cleaned.

Available Power and Energy Calculations

On March 10, 2004 at 1130, Mr. Bill Battin and I measured water flow over the dam. There had been no rain for perhaps a week, so the flow is likely a minimum. The source is essentially drainage from the residential areas to the west of Country Club Road. The large pond drain valve was opened slightly, so these flows were possibly 5% lower than usual.

Using a 6.25-inch-diameter bucket, we measured 2 quarts in 3.5 seconds in a low-flow area and about 2.1 seconds for a high flow area. These are equivalent to one gallon in 7 seconds and one gallon in 4.2 seconds. The flow rates are thus 8.6 gallons per minute and 14.3 gallons per minute, respectively. The average of these rates is 11.4 gallons per minute in a 6.25 inch width. The full spillway is 84 inches, so the estimated flow is 11.4 * 84/6.25 = 153.8 gallons per minute (20.6 cubic feet) in dry weather (some seven days since the last rainfall).

If the head or height of the water is 1.5 feet, the power is 62.42lb/ft³ x 20.6 ft³/minute x 1.5 ft/33000 = 0.058 hp. Assuming a wooden breastshot water wheel is about 70% efficient, the extracted power = 0.058 hp x 70% x 746 watts/hp = 30.5 watts. [Actual tends to be 3W to 20W.]

Over a year, if this flow were constant, the energy extracted would be 30.5W x 8766 hr/year = 267 kWh/year. The value of this electricity would be 267 kWh x $0.07 = $26.7. If the entire installation cost $400, the time to payoff would be $400/ $26.7/yr = 14.9 years. However, the purpose is an educational facility rather than cheap power.

The 24 inch width of the wheel may be reduced if the spillway width is restricted to force the flow through a narrower opening. In flood conditions, water would flow over the added height to avoid overflow of the side walls of the dam. We selected 24 inches for the width of the wheel, less than half the full spillway width, thus allowing a bypass for trash.

Water Wheel Design

The available space appears to allow a 24-inch radius wheel. A smaller diameter wheel would turn faster. A concrete pad was poured to support a new aluminum frame with treated lumber to support the generator box. The wheel shaft is one-inch stainless steel.

The new bearings have plastic pillow blocks. The bearings must support the weight of the contained water, the shaft, and the wheel. The shaft extends out of the bearing at the west side where a "V"-belt pulley can be be attached. This pulley will occasionally be used for power take-off for various projects. The west end of the wheel has a turned V-belt groove at 23.5 inches diameter to drive the generator.

A future equipment pylon will have embedded brass bolts protruding for a mounting board to be attached. Student projects can be mounted to a board with holes that are predrilled (and tested) to match the pylon bolts. After testing elsewhere, the project may be brought to the gardens for testing.

The wheel will be of the "breastshot" design where the water strikes the wheel on the side nearest the dam and about 75% of the way up. The water does not fall very far (perhaps 16 inches to the existing lower water level. An overshot wheel is not practical for such low levels. In drought, the water barely extends out from the spillway and can miss the wheel. Adding a short flume would help, but we are compromising with the nature of the gardens.

The wheel was constructed of treated fence board cut-offs for water resistance. This surface weathered to a pleasing dark color with some five years immersion. Attaching hardware is primarily corrosion resistant stainless steel.

Recommended Total Design

Spillway modifications

The spillway will be evaluated with increased blockage to force all water to the west side when the depth is about 1/2-inch. Paver blocks will temporarily be used to raise the east side of the spillway by some 1.25 inches to assess this approach. In high flow conditions, the water will rise to flow over the top of the sandbags, preferably not over the pool walls. This will leave the water entering the waterwheel about 2 feet wide to match the water wheel interior width.

Wheel Design

The prototype wheel was built as 2 ft wide by 2 ft in diameter. Six angled buckets are used. We still are using that version.

Support Pedestal

The area below the spillway was protected by diverting the water to the east side. The pad is about 43 inches by 34 inches with some 2 inches to 15 inches depth.

Main Shaft and Power Takeoff

The stainless steel shaft is 1 inch diameter by 40 inches long. The wheel power to the generator is by a half-inch V-belt over 104 inches long. The belt rides in a groove in the 24 inch diameter, plywood end plate of the wheel.

Generator Power Calculations

With no load and ~1 inch water over the spillway, the wheel turns 10 turns in 12.6 seconds, or about 50 rpm.

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Estimated Materials, Labor, and Other Costs

~$26 + ~$60 so far; [webpage update needed]

Misc.

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