Underwater Generator Design for Autonomic Underwater Vehicle

OK, go ahead. Give us what you have so far. No sense in starting over since you must have done some research, right?

I wonder how air will be supplied to the engine. If there is an engine, why bother with batteries at all?

I'll make an EE comment. You haven't mentioned your battery size, or your vehicle's speed/power capability, so we can't comment with specificity. Let's assume you've picked an optimum impeller/generator combination. That leaves the charger. We all have, or should have, considerable respect for the trouble that Lithium secondary batteries can get us into, with fires, explosions and the like. But having just finished the section about Li-Ion battery charging for the 3rd edition of our book, I can comment about that. In our writeup we have a long list of dos and don'ts and advice about how to design a safe charger. It can be a bit intimidating. So my first piece of advice might be not to design a charger at all.

There are many well-designed Li-Ion single-cell battery chargers that expect to work from a +5 volt source, so if you use one of these, your task is reduced to creating 5V from a varying input voltage. I'm not sure what power level you're considering, but I'll just make a stab, and suggest you read the datasheet for Linear Technology's LTC1735 Buck converter IC, which can work from 5 to 36-volt inputs to efficiently give you a 5V output. The LTC3785 Buck-Boost converter can work from 2.7 to 10-volt inputs, if you have a lower-voltage generator. Both of these parts use external mosfet switches, which means you can select FETs sized to work with any power level you may need.

If you're determined to design your own complete charger, you could wait for our book to come out (just kidding), you could educate yourself by studying the datasheets of these ICs: BQ24070, LTC4070, DS2762, BQ29410, BQ76PL102, STC3100, LTC6801, LTC6802, LTC4070, and LTC3534. And a few application notes: Linear Technology's AN68, TI's slyt319, and Maxim's AN123 and AN138.

Thank you Hill. Apparently, you are an expert in this field. My idea is using the underwater current to generate electricity say, from 5V to 24V. Which can be stored in Li-Ion battery. Using the electricity to drive a micro-motor reduction gear to actuate a piston to slowly compress a fixed amount of air to smaller volume or extend to bigger volume. The required reduction gear can produce 20 in.lb torque. That is it!!

I guess what we're trying to understand about your setup is the need for a generator to charge your battery. I mean, if your AUV has a motor, why not take from the power source of the motor to charge your battery? In fact, mightn't that power source be a battery? You did mention being attached under another vessel, maybe that's what we need to know more about.

There's also the interesting possibility of a Slocum glider type of AUV. Developed about 10 years ago, these power themselves through the water by changing their buoyancy and gliding up or down, toward their destination.

http://en.wikipedia.org/wiki/Underwater_glider

http://www.webbresearch.com/

http://www.i-cool.org/?p=197

So, yes, they could perhaps benefit from an impellor-generator to grab energy from their gliding. Perhaps one like this? :-) http://en.wikipedia.org/wiki/File:Francis_Runner_grandcoulee.jpg

Hi Hill: Yes, I like to test a towed AUV by a tow boat for underwater object detection use. It has small high pressure tank provide buoyancy control(up ad down) for the AUV and small servo motor control rudder for yaw ! I seldom examine the AUV and like to make it as much self sufficient as it can. All control in its gears, or motion control power provided by Li-ion battery. I am thinking using the water current to drive the tail motor generator to recharge battery, it is a logical solution to use all possible energy I can use. I do not have experience in charging system design!! Your proposal using buoyancy to fly up and down to charge battery is in my design scenario. Another question :: the same generator can also be motor to propel AUV also right??

This whole project is one of the most ill-described and confused I have ever seen. It will be amusing to see what, if anything, results.