Submersibles: Batteries ppsx

Submersibles: 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 cho ice 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 an d 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 requirements to buoyancy control are relaxed as sma ll 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 suc h 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 cate gories: • those inside a pressure hul l operating at normal pressure; • those that are electrically insulated from the sea, but working at ambient pr essure (‘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 alumin um 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 367 . Submersibles: Batteries Ø Hasvold, FFI (Norwegian Defence Research Establishment), Kjeller, Norway &. 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. the at- mosphere and the transport of heat. The volume is sealed; thus, gas evolution from the batteries must be 367