Thing I Learned Today: Waterwheels and Heat

by Brian Shourd on October 16, 2012

Tags: engineering

While I was at a local Chinese restaurant today, I saw a decorative waterwheel in a little fountain display in the corner. I was just sitting, waiting for my food, so it got me thinking about real waterwheels, and how one might best utilize one in one’s own home. You know, if one lived next to a river.

Idea 1: the most efficient use would be if there is something in your house that must be moved about regularly (say, a giant stone for grinding corn). The mechanical energy collected by the water wheel could be converted directly to motion via gears, pulleys, and the like. However, the only thing I could think of would be an elevator, which most people actually don’t have in their homes. Plus, all the time the elevator wasn’t running the energy would just be wasted.

Idea 2: hook it up to an electrical generator. Now, I don’t know how variable the speed of a water wheel actually is, but I guessed that it isn’t exactly clockwork. That throws a wrench into trying to generate AC directly, but you could always charge some giant batteries, and discharge them on everyday electric chores. Perfectly good (if boring) use of a water wheel.

Idea 3: use it to heat your house and/or your water. Every house needs heat and hot water, so this seems practical. However, I couldn’t think of a direct method for converting the mechanical energy of the wheel into heat. Friction didn’t seem like it would work. You could convert to electricity and then run an electric heater - but that’s not a direct conversion. I though of generating electricity to run electrolysis, generating hydrogen to use a fuel source, but that’s really kind of a silly Rube-Goldberg type of energy conversion, isn’t it? Rather poetic though - use water to generate electricity to turn water into hydrogen to burn, creating water and heat.

I wasn’t satisfied with any of my answers, so of course I looked it up. I was thinking all wrong. The trick isn’t converting the mechanical energy to heat, it’s collecting that heat in a useful way. By conservation of energy, all that mechanical energy is going to become something, and the default something that it becomes is heat through friction.

Then probably the easiest method would simply be to have the water wheel drive a rotating paddle in a closed water tank, thus effectively turning the tank into a water heater. Adding a valve to cycle in cool water if the tank gets too hot would help, though I’m not certain how much heat a water wheel could generate in reality. Some quick google searches indicate that a water wheel can provide all of the electricity necessary for an average household, so it seems that it could easily provide all the hot water for a household!