A water wheel is a time tested way to convert flowing water into usable, renewable energy. In times past that energy was used directly to power mills and factories. Today a water wheel can spin a generator, converting water power to electrical current. Regardless of the size of your project, basic strategies will help ensure a successful result.
The amount of power generated by a water wheel is directly related to the level at which water is directed to the wheel minus the level at which water leaves the wheel. This drop, called "head," and the quantity of water carried by the wheel as it turns are key factors in determining potential energy production. Available head is also critical in determining the optimum diameter of a wheel. Unless your site allows you to take advantage of a natural waterfall, you will have to alter natural flow to provide a manmade cascade or flume to drive the wheel. Install a debris boom and/or screen upstream to prevent large debris from reaching the wheel.
Homemade wheels have been made from large squirrel cage fans, wooden cable spools with paddles added, steel drums with paddles welded on, or built from scratch using wood, screws, bolts, glue and varnish. Commercial wheels incorporating various materials and paddle designs are also available. If you build your own wheel, improve efficiency by designing the paddles with a curved or "J" shape so that they catch and hold more water. Maximise efficiency and minimise debris-related problems by positioning the wheel so that the water falls over it, spinning it in the direction of flow. This is called an "overshot " wheel. Alternatives include less efficient "breastshot" and "undershot" wheels driven by water flow striking the wheel at or below its midpoint, spinning it in the opposite direction.
Generator Selection and Gearing
Expect your water wheel to spin at 5 to 25rpm under load. Use pulleys and belts (least efficient), sprockets and chains (better), or a gear box (better still) to achieve a spin rate compatible with your generator. Look for a generator designed to produce optimum output at relatively low speeds. For small projects, a motor salvaged from an ancient computer disk or tape drive can be used as a low-speed generator. Automotive alternators are designed to work at much higher speeds, and require more complex gearing. For larger projects, consider a wind turbine generator designed for low speed, high output operation.
Basic Electrical Controls
Convert fluctuating power produced by your water wheel into a more usable form by using it to charge one or more batteries. Controls can be as simple as a diode that prevents stored energy from flowing back through the generator/motor, thus discharging the batteries. If there is any possibility of overcharging the batteries, you'll need a charge controller. Use an inverter to convert the battery's DC power to household AC current as needed. More elaborate controls can be added, but these provide the basics.