Introduction Submersibles are a large and nonhomogeneous group of vehicles that range from small unmanned autonomous vehicles such as Gavia and REMUS that can be carried by one or two me
Trang 1Submersibles: Batteries
Ø Hasvold,FFI (Norwegian Defence Research Establishment), Kjeller, Norway
& 2009 Elsevier B.V All rights reserved.
Introduction
Submersibles are a large and nonhomogeneous group of
vehicles that range from small unmanned autonomous
vehicles such as Gavia and REMUS that can be carried
by one or two men, to medium-large unmanned
sub-mersibles such as torpedoes, survey vehicles such as
HUGIN and Bluefin, larger manned deep-diving
ve-hicles such as the US Alvin and the Russian MIR up to
naval diesel-electric submarines With a few exceptions,
all these use electrochemical power sources for
pro-pulsion The rate of discharge is very different, however,
being C/50 or less for a survey type of an autonomous
underwater vehicle (AUV) to 5C or more for an electric
torpedo
Submersible Physics
With the exception of torpedoes that can easily use
dy-namic lift because of their high speed and gliders,
sub-mersibles are neutrally buoyant; their weight equals the
weight of the water displaced Thus, the average density
of a submersible equals the density of the displaced water
The weight of the pressure hull is a function of its shape
and the choice of construction materials and increases
with the design depth As a consequence, at some design
depth, the empty weight of the pressure hull will be so
large that the amount of electronics and energy
(bat-teries) the hull can carry will no longer be sufficient for
the intended mission Spherical pressure hulls give the
best volume-to-weight ratio for a given design depth, but
even with spherical pressure hulls, at some design depth,
the weight of the pressure hull equals the weight of the
displaced water At present, the conventional pressure
hull design limits the diving depth for manned
sub-mersibles to B6000 m At this depth, the ambient
pres-sure is 600 atm (60 MPa) To go deeper, one must add
sufficient amounts of a material that is lighter than water
and nearly incompressible to achieve neutral buoyancy
This is similar to the zeppeliners or blimps that use
hydrogen or helium for buoyancy The bathyscaphe
Trieste is an example of this design, using a large volume
of petrol for buoyancy
More recent deep-diving submersibles use hollow
glass spheres for buoyancy Glass has exceptional
com-pressive strength It is used either as ‘syntactic foam’ that
is composed of small (o0.1 mm diameter) hollow glass
spheres suspended in a polymer matrix or in the form of
large, thick-walled spheres (Benthos Inc., USA) Large spheres in the form of two hemispheres may also be used
as containers Other restraints to the design of slow-moving submersibles are that in order to be stable, the center of gravity (G) must be below the center of buoy-ancy (B) and the line GB vertical The battery is a sig-nificant part of the vehicle mass and a movable battery has been used as part of the vehicle control system in some small underwater vehicles such as gliders Trim considerations are of special importance for power sources changing mass or volume during discharge When comparing power sources, the associated trim tanks should be included in the volume of the power source A second consideration for deep-diving sub-mersibles is that the compressibility of the vehicle and of the seawater should be similar If not, a change in buoyancy with depth will take place
For submersibles that are always in constant move-ment such as survey AUVs, the requiremove-ments to buoyancy control are relaxed as small changes in buoyancy or trim are mitigated by the hydroplanes at the expense of a small increase in vehicle drag For high-speed sub-mersibles such as torpedoes, an excess weight of 25% of the displacement or more is quite common
Power sources for submersibles can be classified into four different categories:
• those inside a pressure hull operating at normal pressure;
• those that are electrically insulated from the sea, but working at ambient pressure (‘pressure balanced’,
‘pressure tolerant’);
• those using the seawater as electrolyte; and
• those using the seawater as oxidant and electrolyte
In addition, there has been some research on oxygen extraction from seawater in order to operate a fuel cell (FC) (fueled with aluminum or hydrogen)
Shallow-Diving Submersibles
The preferred concept for the design of shallow-diving submersibles, such as diesel-electric submarines and swimmer delivery vehicles (SDVs), is to put the batteries inside the pressure hull It allows for the use of primary and secondary batteries of conventional design, but with two additional considerations: composition of the at-mosphere and the transport of heat The volume is sealed; thus, gas evolution from the batteries must be
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