Power

Introductory information

There are essentially two types of power sources, primary which is not rechargeable and secondary which is the rechargeable versions.

Here are two tables comparing the energy densities of different types of batteries:

System	Specific Energy	Density	No of discharge cycles
silver zinc	130 wh/kg	240 wh/l	40-50
lead acid	30	65	100
nickel cadmium	30	75	unknown
lithium polymer	130	300	600
lithium ion	slightly lower	250	1000

Advantages of lithium polymer:

It is a solid state battery source and therefore can operate at any ambient pressure. Consequently it does not have to be enclosed in a pressure hull, which reduces mass of the final auv design. It has a high cell voltage of 3.8vdc, and a battery life of over 5 years, ideal for our self-sustaining underwater habitat.

Advantages of lithium ion

Lithium ion can be used for over six years and can be charged over 1000 times. It is also hermetically sealed, and can be recharged in the AUV without opening pressure hull.

Disadvantages of li+

Lithium ion batteries offer significantly increased energy density over NiCd and NiMh technologies. Their self-discharge rates are low and thier weight is very low for a given amount of energy compared to other technologies. However, the volume may be as high as NiMH for a given amount of energy. Li+ is also power limited and high currents are not usually possible with this technology. Lithium batteries have also to be carefully charged to avoid fires and burning because the heating effect during charging can ignite the Li+. This is because as Li+ charges it heats up and the internal resistance goes down drawing more current thus creating a potential runaway condition. The solution is utilization of well-designed chargers that provide careful current limiting. Also, with Li+ hydrogen can be produced which is potentially explosive.

Conclusion on Optimal Power Source

The vehicles should initially use silver zinc which is what most current technologies employ. As the lithium technology is improved they can utilize Li polymer technology (as opposed to Li+ technology.) Ultimately, within the next 8 years, fuel cells will replace even the sophisticated lithium technology as they have a very high energy density.

The power equation

et = (p + p0 + pv)r/3600v
Where et = energy carried in kwh
pv = vehicle equipment power
p = propulsion power in watts
r= survey range in km
p0= survey sensor power in watts
v=velocity in meters per second (usually about 2 ms-1 for most purposes)

thrust power = thrust* V = o.5*pv^3A*coefficient of drag
thrust = drag if it is moving with uniform
velocity= 0.5*p*v^2+A+coefficient of drag

Resources

1. Lithium Ion Battery Photo: http://www.evac.ca/photoweb/Batteries/pages/..%5C..%5CBatteries%5Cimages%5Cfull%5CSaft%20Lithium-Ion%20battery.jpg