Underwater Power Generation?

An anonymous reader writes: "We keep seeing stories here about tidal power, and that's cool, but I don't see how it can be done without a column rising all the way to the surface. So here are the ideas I've got right now." Read on for some interesting thoughts on the subject...

"Keep in mind that the device will probably be housed in a length of 4-inch PVC or ABS pipe, and it needs about 0.5 ma at 1.5 volts:

• Surge power. Put a couple of funnels back-to-back with a CPU cooling fan-sized turbine and generator in the middle, and run the output through a rectifier and capacitor. But how reliable will those moving parts be after years underwater?
• Self-winding watch concept. Float the thing tethered to the bottom and install some sort of pendulum inside with a magnet on it, moving through a coil. The moving parts are protected, but will it be enough power?
• Yank the chain. Again, tether it, but use the varying tension on the tether to drive a dynamo of some sort. Not sure how this would work.
• Magnetohydrodynamic generator. Like the surge power thing, but using the flow of cunductive seawater through a magnetic field to generate a current. I have no idea how much power this would generate, if any, or how to deal with ion accumulation at the electrodes.
• Nukes. Anyone got a spare radioisotope thermoelectric generator? Any idea how many smoke detectors I'd need to cannibalize to get enough Americium-241?

The generator need not fit inside the 4-inch cache tube, but it shouldn't be huge, either. It needs to be practical to build, and not terribly expensive. Above all it's got to be reliable and enduring. Any ideas?"

Comments

Piezo electric rods...

[Perdo]

piezo electric rods placed along the coast to capture the energy of wave motion and tidal effects. Unstable airfoils could be placed on top to "play" the rod even during relativly calm periods. In vast mumbers, They would provide two benifits: Act as bariers to coastline erosion because they would absorb the energy of coastal wave action and The generation of electricity of course.

Channels

[Deanasc]

The currents through some channels are well known and steady. Put underwater "windmills" in the channel and sit back and collect the "juice". This is similar to sailboats that tow small electric generators or put magnets on the propeller shaft to trickle charge the batteries.

6 watt hours per year.

[hamjudo]

I must have missed something, check my numbers.
.0005 amps * 1.5 volts * 24 hours/day * 365 days/year is 6.13 watt hours per year.

If your circuit could tolerate voltage droop, a single D-cell would run it for a few years. But voltage droop is probably annoying, so put in a few D-cells and a voltage regulator.

Alkaline batteries aren't rated to last more than 8 years or so, so use lithium batteries if you need something good for decades.

We need to know how many years this should work to give you more precise advise.

I can think of lots of ways to produce 0.75 milliwatts, just none of them will be more reliable than a lithium battery.

Re:You aren't giving us very much info...

[Nyarly]

If he cannot build a watertight battery compartment, what chance in heck does he have of building a generator?

Watertight I've no doubt of. Survive ocean submersion for the decade or more he seems to want it for is more difficult. Maybe cast the whole thing in silicone and without air pockets. Frankly, he'll be putting it in an environment that most plastics and metals fair poorly in, and probably changing it's ambient pressure constantly.

Further, comparing the difficulty of a battery compartment to the difficulty of a generator is disingenuous, I think. What I'm cautioning against is not the difficulty (which I think is about the same, and made quite arduous by the proposed lifetime and environment); I'm concerned about the consequences of an engineering failure.

If almost any of the generation ideas he proposes goes south, worst case is that the device stops working. If a battery compartment leaks, the battery leaks alkalines or lithium compounds into the surrounding seawater.

That's my two cents.

I agree with those that say use batteries...

[cr0sh]

Coupled with resin or silicone potting (as others have suggested), it seems the best, most reliable, and cheapest way to go.

But, there is another option, though it probably won't last as long - think seawater and different types of metal...

That's right - akin to a "spud/lemon - battery" - if such a thing could power a small watch, then it could be scaled up a bit to power an LED...