An Assessment of the Botanical Gardens Waterwheel Project
An Assessment of the 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 ~12 inch butterfly valve that is 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.
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 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 water wheel is about 70% efficient, the extracted power = 0.058 hp x 70% x 746 watts/hp = 30.5 watts.
Over a year if the 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 = $18.70. If the entire installation cost $400, the time to payoff would be $400/ $18.70/yr = 21.2 years. However, the purpose is an educational facility rather than cheap power.
The 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, about half the full spillway width.
Water Wheel Design
The available space appears to allow a 24-inch radius wheel. Square pedestal bearing supports can be placed at opposite ends of the spillway just outside the water flow. A 3/4-inch galvanized steel pipe would make a good support shaft. The bearings must support the weight of the contained water, the shaft, and the wheel. The shaft must extend out of the bearing at the west side where a "V"-belt pulley will be attached. This pulley will occasionally be used for power take-off for various projects.
An 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 25 inches to the existing lower water level. An overshot wheel is not practical for such low levels.
The wheel would likely be constructed of redwood, cypress, or cedar for water resistance. The prototype is made of cedar fence boards. This surface will weather to a pleasing color with time. Attaching hardware will require corrosion resistant hardware.
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 to 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.
Support Pedestal
Treated 2x4 lumber was driven ~4.5 ft into the streambed for anchors. A permanent installation might use poured concrete posts.
Main Shaft and Power Takeoff
A 3/4 inch galvanized steel pipe threaded at both ends is used. Bearings are PVC pipe on the steel pipe with marine grease for lubrication.
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.
[webpage update needed]
Estimated Materials, Labor, and Other Costs
~$26; [webpage update needed]
Misc.
[webpage update needed]
Helpful Links
Prony Brake horsepower: http://www.buckleyoldengineshow.org/HorsePower/horsepower.htm
Questions to be answered
What is the drainage area that feeds the garden stream?
(Est. 1 square mile)
What amount of flow occurs in the garden stream one hour after a one-inch rainfall?
(Est. 200 cfm)
What is the accumulated flow as a function of time during a year?
(Est. low in Nov-Apr; High in May-Sep)
What data are available from the Crane Creek gage station at Babcock St?
USGS Station for St. Johns River Water Management District
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Conventional/Hydro/assessment.htm updated 041015