 # Piezoelectric Power Yield

I am looking at building an ROV and was wondering whether I could use a Piezoelectric material to generate some energy. I was wondering how I would calculate the watts output by a piezoelectric material as a function of the pressure (so I can see how much energy it generates according to how deep the ROV is).

Since you're dealing with static pressure (assuming the ROV isn't bobbing up and down at many times per second) the output will be DC and an uneducated guess will be somewhere in the region of milliwatts at best. The available force will be a direct function of depth (pressure) and cross sectional area of the transducer. Depth pressure increases by approximately 1 bar per 10 metres increase in depth.

Yes I know a spark igniter produces thousands of volts but the instantaneous force applied on these is many 10s of newtons.

If it's useful power you require then use a battery (stronger, cheaper, simpler and more reliable)

Problem with batteries is they run out after a while. I'm not keen on this except as a backup power supply to execute in case of onboard power generation failure.

The depth calculator I use is: http://www.calctool.org/CALC/other/games/depth_press kPa to N/m^2: http://www.unitconversion.org/pressure/newtons-per-square-meter-to-kilopascals-conversion.html

At a depth of 50 m the pressure should be 603916 N/m^2. Does this mean that given the power to get milliwatts of energy is (for arguments sale) 50N of force, the power output with 60 kN of force is quite a bit more? I'm not really sure I have been unable to find a reliable calculation method.

Also I want to add this is simply a brain exercise for me, I am not actually going to build an ROV at the moment.

I can't prove it to you mathematically but trying to extract enough power from a piezo element to power a ROV is a lost cause. Even if that wasn't the case a piezo needs a change of mechanical force acting on it for it to generate a electrical response. A steady pressure no matter how high does not generate a continuous voltage. A piezo element is fundamentally an AC device, not a DC device.

Lefty

That makes sense, I did consider that it might be a change in pressure that generated the energy rather than just the pressure itself, but wikipedia wasn't really clear. Having said that, let's say the ROV was 1000m down where the pressure is 10153100 N/m^2, if I go down just 1m the pressure becomes 10163200 N/m^2 making the pressure difference 10100 N/m^2, at depths such as these does it become feasible if the ROV bobs up and down by a metre every so often?

The voltage generated appears to be from a nearly instantaneous change in force, not the slow change caused by bobbing up and down a small amount.

How many square meters of material were you planning on using?

For the purposes of this brain exercise, 1 m^2.

This is all getting rather silly !

Wikipedia etc contains lots of useful information on the principles of piezo voltage generation

The voltage generated by a piezo crystal bears no relationship to its cross section. If you want more volts you must "stack" elements

A piezo element is a "charge" type device in that whilst it is capable of producing voltage, its impedance is so high that it effectively cannot produce current.

Whilst it is only practicable when considered as sensing a varying force, the frequency of this may be extremely low, typically less than 0.5hz

Whilst this all might well be a personal exercise of the grey matter, you have to temper dreaming up conceptual ideas with realism. If you think about it, an ROV powered by the depth of water that it's sailing in is effectively a perpetual motion machine.

Apart from that, where does the Arduino aspect fit in.