Water Hammer Electrical Generator

Plumbing has been around for a very long time, I have seen compelling evidence of domestic water routing dating back to at least the 1970's. The general concept is simple: water will move if there is a pressure gradient and there's nothing in the way. For example: take a bucket of water up a hill and pour it out; it will flow downhill with gravity the force providing the pressure gradient. But it's not plumbing yet, what you've made is a sort of labor-intensive small stream*. The key is having it move when you want it to, and not when you don't. This is best achieved via manipulation of the "nothing in the way" part of water flow. We achieve this with a group of technologies; valves, taps, faucets, that kind of thing. For this reason we don't have 1000's of liters a day of water rushing through every domestic abode. It would be noisy at the very least.

Being able to control water flow is all very good, you open a tap, water starts to flow. But now there is a continuous body of water stretching from your kitchen tap all the way back to the source - a water tower for example, and it's moving. When you shut off the tap, it stops moving. Water doesn't compress, something has to give. In extreme examples, you may have many tons of water moving at 10's of MPH... stopping it is like a truck crashing. It can cause catastrophic damage <link>. This is called "Water Hammer". Now, in a domestic setting we have smaller amounts of water moving slowly, but the energy is not negligible and can damage plumbing, or worse, be irritatingly noisy.

To mitigate domestic water hammer effects, water hammer arrestors may be fitted <link>. Often these are just a sealed container part-filled with air. As the water flow is switched off, the pressure spike forces water into the arrestor, compressing the gas.

Now, imagine that between the gas and the water is a piston, within which is a magnet. Around the outside of the piston containing pipe is a coil. Now, when the water is shut off, the piston moves up, absorbing the pressure spike, then back down as the system resets. This moves the magnet through the coil twice. We've made an AC generator. Add a bridge rectifier and a capacitor: we've made a DC power source.

How much energy are we dealing with? 10m of 15mm id pipe = ~ 2kg of water travelling at ~3m/s would equal something like 10J. Our coil will grab say, 20% of that. The rectifier/cap set up looses say another 50%. So around 1J per tap shut off event. Say 50 hammer events/day, that's an average of 100uW. Comfortably in the range of many IoT/remote sensor microcontroller devices. So, now we have free electrical energy to power a range of small electronics. Do with that what you will.

* Automated versions of these have been available for decades.