Modern power sources

Power sources are extremely important in underwater vehicle development, in particular for AUVs, UROVs and HROVs. Power source capacity limits the cruising range and mission style of underwater vehicles. Two main evaluation factors for power sources are the specific energy, energy per unit mass: Wh/kg, and the energy density, energy per unit volume: Wh/L. In vehicle design, not only the energy of the power source is considered, but also the maximum output power. In this section, modern power sources for intelligent underwater vehicles, secondary batteries and fuel cells in particular are described because these are rechargeable and are able to be run in a closed system in the underwater environment. a. Batteries

A number of kinds of secondary battery, lead-acid, silver-zinc, nickel-MH, lithium-ion and lithium-polymer batteries are utilized in AUV design. Lithium-ion and lithium-polymer batteries have the advantages of easy handling, higher energy density and longer cycle life as shown in Table 1. Davies and Moore (Davies 2007) have proposed the ratio of specific energy and energy density, D as an index for helping power source design.

If D is smaller than the density of seawater, approximately 1.03 kg/L, a battery system has positive buoyancy. Calculating the D of various batteries from table 1, it can be seen that lithium type batteries have a smaller density than other types. We thus focus on lithium-ion batteries and lithium-polymer batteries because they have good prospects for future use and development (Armand 2008). Both batteries use lithium metallic oxide in the cathode and carbon material in the anode. Lithium-ion batteries use lithium ions in an electrolyte inside the battery and these transfer between the cathode and the anode during charge or discharge. In contrast lithium-polymer batteries use a solid polymer composite. The advantages of Li-polymer over the lithium-ion design include lower cost of manufacturing and being more robust to physical damage.

D (kg/L) = energy density / specific energy

Battery Type

Specific energy, Wh/kg

Energy density, Wh/L

Cycle life

Lead-acid

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