Advanced Underwater Power Systems for Autonomous Underwater Vehicles

31 Advanced underwater power systems Lieutenant Commander J G Hawley BSc PhD CEng MIMechE MIMarE RN Naval Architecture Ocean and Marine Engineering Department United States Naval Academy Annapolis Maryland USA S J Ashcroft BSc PhD CEng MInstE and M A Patrick BScTech PhD CEng MIChemE MInstE School of Engineering University of Exeter The defence commercial and scientiJ5c communities are all aware of the strategic importance that the oceans hold Future underwater operations are under evaluation using autonomous oceanranging submersibles of the unmanned variety Such vessels are commonly termed autonomous underwater vehicles AUVs and the search for suitable power systems thar are able to provide high reliability coupled with tong underwater durations has intensijied over the last decade This paper presents a review of those power systems that are under consideration design and development mainly for the AUV application No attempt is made by the authors to directly compare individual systems as this can only be eflectively undertaken once a welldefined vehicle and associated mission profile has been dejined 1 INTRODUCTION The underwater environment was until the early 1960s almost exclusively the preserve of navies and their sub marine fleets Although the military interest still domi nates the oceans are now recognized as a a means of providing opportunities for economic b one of the mechanisms that contribute to the earths A number of marine science programmes are current ly underway particularly in Europe Japan and the United States with the objective of developing leading edge technologies such that continuous exploration of the underwater environment can be made 1 Yet sur prisingly only 2 or 3 per cent of the oceans have been mapped or surveyed 2 One of the major obstacles now impeding the large scale exploration of the oceans is the lack of suitable underwater vehicles Deepdiving oceanranging sub mersibles are few in number and these are all of the manned type 3 The unmanned type can be subdivided by considering those that rely on a physical tether from a support station remotely operated vehicles ROVs and those that do not autonomous underwater vehicles AUVs The ROV has become the workhorse of the offshore hydrocarbon industry and as the name sug gests this type of vehicle is remotely controlled via a tether or umbilical cable from a support station The umbilical tether also acts as the power transmission cable The ROV has become well established for under water inspection observation and light work oper ations However their operational flexibility is restricted owing to the necessity of a tangible support station The AUV is capable of operating independently of a support station The underwater scientific 4 com mercial 2 and defence 5 communities have long been aware of the potential uses of such vehicles and devel opment work on prototypes commenced in the early 1960s AUVs offer the ability to gather large amounts of growth and climatic conditions The MS was received on I1 May 1993 and was accepted for publication on 3 December 1993 A01293 IMechE 1994 data at lower cost than manned research submersibles However such vehicles must be able to sense and inter pret the environment to locate and position themselves to avoid obstacles and to make realtime decisions regarding task information performance A comprehensive survey 6 of vehicle types has docu mented that over 62 different varieties of AUV have been proposed designed and developed by 10 countries during the past 30 years Of those listed a number remain in concept phase some have progressed no further than the drawing board while others either were or are prototype testbed vehicles The results of the survey revealed that for all present in service AUVs no reference has been made to other than secondary batteries as the underwater power source Such prototype vehicles have in the main been designed to test the operating philosophy of such vehi cles but only in conditions where mission endurance is limited Mission requirements that necessitate an enhanced underwater endurance capability require an onboard power system with superior performance in terms of reliability and energy storage density This paper is concerned with presenting a technical review of the underwater power systems that are cur rently in use and also those advanced power systems that are under design and development particularly for AUV applications 2 AUV POWER SYSTEMS CURRENTLY IN USE An analysis of the numbers of AUV that have been designed and developed is shown in Fig 1 according to the power system used 6 It is noted that the literature in many cases does not make reference to the type of power system However where the power system has been classified it has invariably been a secondary battery energy source although in a number of cases the specific type is not defined Present inuse AUVs are all powered in this manner The limitations of these battery systems are clearly illustrated by considering the American XP21 vehicle built by Allied Remote Tech nology of San Diego shown in Fig 2 The vehicle is powered by silverzinc batteries and the maximum underwater endurance is constrained to approximately 13 hours 7 Proc Instn Mech Engrs Vol 208 Downloaded from piasagepubcom at PENNSYLVANIA STATE UNIV on February 20 2016 38 J G HAWLEY S J ASHCROFT AND M A PATRICK NiCd 1 NK 19 Pbacid Leadacid NiCd Nickelcadmium AgZn Silverzinc NaS Sodiumsulphur SB Secondary battery NIK Notknown Fig 1 AUV power systems the figures refer to the number of AUVs that operate with that particular battery system Marconi Undersea Systems Limited MUSL have taken a sodiumsulphur electric vehicle secondary battery and developed a management system such that it can operate within an extorpedo testbed vehicle as shown in Fig 3 8 This particular battery system is claimed to give the vehicle 32 hours of continuous underwater operation at low speeds 9 The vehicle is initially intended for demonstrating underice survey work The advanced underwater power systems that are being considered or are under development for the AUV application will be discussed under the headings of electrochemical and heat engine systems 3 ADVANCED ELECTROCHEMICAL POWER SYSTEMS The advanced electrochemical systems being considered and developed for AUV applications include fuel cells semicells and primary batteries The literature has often been found to contain contradictory terminology and descriptions of these electrochemical systems In this paper the following definitions are used 1 Fuel cell A device that generates electrical energy via the reaction of a fuel and oxidant which are fed from an external source without consuming materials that form an integral part of its structure 2 Semicell A device that generates electrical energy via the reaction between a metal that is contained and consumed without its structure and an oxidant that is fed from an external source 3 Primary battery A device that generates electrical energy from a selfcontained chemical reaction within the cell structure A primary battery consumes the metal anode as part of the reaction and this can be replaced A primary battery is therefore mechani cally rechargeable while a secondary battery is elec trically rechargeable 31 Fuel cell systems Energy conversion in a fuel cell is from an electro chemical point of view the reverse of electrolysis The chemical energy of a fuel and an oxidant is converted into lowvoltage dc electricity via electrochemical reac tions similar to those associated with conventional sec ondary batteries However unlike these latter devices the fuel cell does not consume materials that form an integral part of its structure and can operate only as long as it is continuously fed with a suitable fuel and oxidant and the reaction products are removed In theory any fuel and oxidant can be used in a fuel cell but for almost all practical purposes the H0 cell is the only viable option for underwater applications The classification of a fuel cell is generally character ized by the electrolyte used There are five basic types Alkaline Phosphoric acid Molten carbonate Monolithic solid oxide Advanced proton exchange membrane APEM Fig 2 The XP21 AUV taken from reference 7 Sidescan Recovery Acoustic sonar beacon comms Fig 3 Marconi AUV courtesy of Marconi USL Part A Journal of Power and Energy IMechE 1994 Downloaded from piasagepubcom at PENNSYLVANIA STATE UNIV on February 20 2016 ADVANCED UNDERWATER POWER SYSTEMS 39 In many cases the fuel cell offers the potential to provide energy densities far in excess of any nonnuclear system The alkaline and the APEM fuel cells have attracted most attention for underwater applications For the alk aline cell this is due to its advanced state of develop ment good cell efficiency and moderate operating temperature The APEM fuel cell offers comparable operating characteristics and can be configured with few moving parts promising reliable and quiet operation However it is not as well developed as the alkaline variant The main operational problems encountered with the use of fuel cells in underwater vehicular applications are those associated with the methods adopted for on board storage of the gaseous fuel and to a lesser extent the oxidant To date cryogenic liquid storage has been the favoured method for carrying oxygen on board underwater vessels Hydrogen in liquid form is difficult to store because of its very low density and also the associated safety issues The use of hydrides adds extra weight to the vessel The potential to obtain the high energy storage densities that are attainable from fuel cells for underwater applications can only realistically be met by onboard reforming of a hydrogenrich com pound from which the fuel hydrogen is released 10 For both of the fuel cell types under consideration the development of a compact external reformer with elabo rate separation processes is crucial if the potential high energy densities are to be recoverable from such systems 32 Semicells The semicell is akin to a hybrid of a fuel cell and a conventional secondary battery As in a fuel cell an oxidant is pumped into the semicell but the fuel is sup plied in solid form within the cell as a metal anode Electrical power is produced by the reaction between the metal anode and the oxidant The anode usually made from lithium or aluminium is consumed during the energy conversion process by corrosive action re leasing heat and generating hydrogen The potential performance of the semicells is very similar to that of the fuel cells but the semicells offer many practical advantages because of a the relative ease of supplying and handling the fuel and oxidant and b their low operating temperatures typically 25 C 33 Primary batteries Primary batteries are either expendable oneshot devices or ones that require mechanical replacement of the active materials that are consumed during oper ation Until recently most primary batteries were only available with maximum power outputs of a few watts but this situation is rapidly changing In France Italy and the United States primary batteries are being developed for use in AUVs and torpedoes with power outputs of up to 600 kW and 100 kW h of stored energy capacity 11 The advanced semicells primary and secondary battery systems that are being developed particularly for AUV applications are highlighted in the next section r IMechE 1994 34 Advanced electrochemical AUV power system The largest single AUV development project underway is that which is currently being administered by the American Defense Advanced Research Project Agency DARPA for the United States Navy USN as part of the Naval Technology Programme on unmanned underwater vehicles The DARPANavy Technology programme on AUVs has concluded that by the year 2000 such vessels will provide important adjunct support to submarines and surface vessels The main mission oriented programme is the mine search system MSS This particular programme is intended to demonstrate autonomous mine detection and classi fication The overall energy storage target capacity of these vehicles is 3360 kW h at a modest power level of 10 kW giving a total mission endurance of 336 hours two weeks However the initial shortterm goal has been set at 1000 kW h Two prototype vehicles were delivered in 1990 and a schematic diagram of these vehicles which will be initially powered by silverzinc battery systems is shown in Fig 4 Preliminary sea trials with the prototype vehicles commenced in April 1992 and these trials are expected to be completed by mid1994 12 The silverzinc power system will not be used in the final vehicle since it does not meet the mission require ments but its use will enable the vehicle to be thor oughly tested especially the propulsion control navigation and sensor subsystems A number of different power systems have been con sidered for use with the DARPA AUV but it has been found that only advanced electrochemical power systems promise to be wholly compatible with these requirements The potential of the advanced electro chemical devices considered for this particular duty which requires 3360 kW h of energy to be stored in a hull section 44 inches in diameter and 112 inches in programmes Fig 4 DARPA AUV taken from reference l2 Proc Instn Mech Engrs Vol 208 Downloaded from piasagepubcom at PENNSYLVANIA STATE UNIV on February 20 2016 40 J G HAWLEY S J ASHCROFT AND M A PATRICK Proprietary hydride 1 Lithium borohydride s Sodium borohydride s Lithium hydride s I Fuel cell and procesor length Diesel I 0 Oxidant length Aluminium s 0 Fuel length Propane 1 0 Buoyancyiballast section Ethanol I Methanol I Hydrazine 1 Methane I Lithium s Ammonia 1 Liquid hydrogen I Methanol I 90 H 2 0 z Gaseous hydrogen g Methanol I 66 HzO Liisea water SC Li190 HO sc A190 HOz SC Li66 H20 SC Li thionyl chloride battery Silverzinc battery A166 H20 SC 0 50 100 150 200 250 300 350 400 450 500 Lengths of compartment inch Fig 5 DARPA electrochemical power system comparison Hatched shading indicates total energypower system compartment length taken from reference 13 length is illustrated in Fig 5 This figure shows the hull lengths that would be required for the energy systems under consideration to store 3360 kW h of energy within a fixed hull diameter of 44 inches 13 Figure 5 has been taken from reference 13 hence the use of imperial units This diameter of 44 inches has also been used by other AUV developers such that direct com parisons on energy storage can be made The two power systems that have been selected for advanced development are the aluminiumfuelled PEM fuel cell which is shown sixth down in Fig 5 and the aluminiumoxygen semicell shown as A190HO2 SC The length of the energy storage compartment that would be required by either a lithiumthionyl chloride primary battery or a silverzinc secondary battery to produce the target of 3360 kW h is also shown in Fig 5 for comparative assessments The UK AUV AUTOSUB Project 14 being admin istered by the Natural Environment Research Council NERC has as its primary aim the development of a number of different taskorientated vehicles for trans oceanic scientific studies The nominal longest under water endurance has been targeted at approximately seven days before refuelling and the initial prototype vehicle is 70 m long and 09 m in diameter As with the DARPAUS Navy AUV study advanced electrochemi cal power sources have been identified as the only viable energy source that could fulfil the mission requirements 15 4 HEAT ENGINE SYSTEMS The conversion of thermal energy into either mechani cal or mechanicalelectrical power is achieved by the Part A Journal of Power and Energy I192 0 use of dynamic energy convertors that is heat engines The main contenders for AUV applications include Stir ling engines gas turbines steam turbines and the non airbreathing diesel engine Apart from the diesel engine all the other heat engines have the capability to use any heat source Many of the power systems to be discussed can operate at any depth depending on the energy source used and they are usually referred to as depth independent however the submarine hulls are not depth independent Submarine hulls have a maximum diving depth and a collapse depth which for military vessels is highly classified information 41 Stirling engine systems The Stirling engine is an externally heated dynamic energy convertor that operates on a closed regenerative thermodynamic cycle In modern highperformance Stirling engines the entrapped internal working fluid is usually but not necessarily helium The power output of the engine is controlled by altering the internal working pressure of the system In practice the performance of a Stirling and a diesel system are very similar However the absence of a internal explosions and b the need for incylinder flow control valves of the IC engine type make the Stirling a quieter engine than its competitors These attributes coupled with the ability to use any heat source make the Stirling an ideal candidate for use in the underwater environment The first system to be developed installed and oper ated under water was undertaken by the Swedish company United Stirling now part of the Kockums Group Two 75 kW Stirling systems were retrofitted IMechE 1994 Downloaded from piasagepubcom at PENNSYLVANIA STATE UNIV on February 20 2016 ADVANCED UNDERWATER POWER SYSTEMS 41 LOX tank k Generator Fig 6 Kockums Stirling powered AUV energy subsection courtesy Kockums Marine AB into the 1200 tonne Nacken naval submarine of the Royal Swedish Navy to increase the underwater endur ance of the vessel by complementing its existing lead acid battery system 16 The engine at the heart of the system is the fourcylinder 275R engine which uses a helium internal working fluid pressurized to 70100 bar and produces 75 kW maximum shaft power Hydrocar bon fuel is burned in an atmosphere consisting of oxygen supplied from onboard LOX tanks and recir culated combustion gases in an external combustion chamber which can be pressurized to at least 30 bar This will also be the pressure of the ejected exhaust which means that it can be disposed overboard without assistance at depths of approximately 300 metres This then becomes the maximum diving depth of the sub mersible when it is operating with a Stirling system that uses pressurized exhaust gas discharge The Nacken after extensive trials was accepted into operational service in February 1990 Although exact performance details have not been made public it appears that the Swedish Stirling units have improved the submerged endurance of the Nacken by a factor of between 5 and 8 over that attainable with the sole use of leadacid bat teries The success of the Nacken has resulted in underwater Stirling engines being used on the new class of Royal Swedish submarine the A19 or Gotland Class 17 for refitting Uzushio class submarines of the Japan Mari time SelfDefence Force and for use on board the future class of Royal Australian Navy submarine the Collins class or Type 471 18 Since 1987 Kockums has been developing an AUV energy hull section comprising a LOX tank hydrocar bon fuel tank and associated control systems linked to the 495 four cylinders giving a total capacity of Sulphur hexafluoride 95 cm3 Stirling engine for incorporation into a 44 inch diameter hull Fig 6 This diameter was selected by Kockums in order to enable direct comparisons to be made with the stored energy performance of the DARPA vehicle Initially designed to produce a shaft output of 40 kW the 495 has been derated largely by reducing the inter nal helium pressure so that outputs of between 5 and 15 kW are obtained which are ideal for AUV applica tions The objective of the work was to produce a com plete energy subsystem using commercial LOX storage equipment with an energy capacity of 600 kW h which corresponds to 60 hours of operation at a continuous load of 10 kW The energy subsection is 160 inches in length approximately onethird more than the length of the DARPA vehicle Kockums claim that the energy density of its prototype hull section is 175 kW htonne compared with 116 kW htonne for an equivalent silverzinc hull section 19 Phase I of the programme in which a prototype was tested in the laboratory has been completed and phase 11 which has seen the engine integrated with LOX storage tanks and the fuel hull section was successfully completed early in 1991 20 Sea trials constitute phase I11 of the project and are expected to take place in 1994 Kockums Marine AB Malmo Sweden September 1992 personal communication 42 Gas turbine systems Gas turbines may be operated in open or closedcycle mode The closedcycle gas turbine is usually referred to as operating on the closed Brayton cycle CBC and is particularly advantageous for an underwater vehicle where airindependent operation is required Depth independent operation could also be achieved if the heat source was selfcontained as in a liquid metal reactor If hydrocarbon combustion was used as the heat source then operating depth would be limited Claimed advantages associated with gas turbines are low specific weight and volume low vibration levels and the ability to operate with any heat source Closedcycle gas turbine systems have been developed in the United States by the Garrett Fluid Systems Division of Allied Signal for space and terrestrial power and have recently been proposed for AUV applications 21 The AUV proposal utilizes the CCPS401 gas turbine which has a nominal rated power of 30 kW and uses argon as the working fluid The vehicle Fig 7 is intended for use as a mine layer The preferred heat source for the AUV application is a liquid metal combustion system known as the stored chemical energy propulsion system SCEPS In the SCEPS heat source lithium reacts with sulphur hexafluoride in a pottype combustion chamber The energy is transferred to the argon working fluid by a heat exchanger which is attached Combustor Lithium tank I CBC engine Fig 7 The AlliedSignal CBCAUV courtesy of AlliedSignal Limited IMechE 1994 Proc Instn Mech Engrs Vol 208 Downloaded from piasagepubcom at PENNSYLVANIA STATE UNIV on February 20 2016 42 J G HAWLEY S J ASHCROFT AND M A PATRICK directly to the gas turbine The predicted energy den sities of such systems are an order of magnitude higher than those available from existing battery systems 43 Steam turbine systems Steam turbine systems for AUV applications are being developed in France the MESMA system and are under investigation in the United Kingdom the WOLF system 431 The MESMA system MESMA is the French acronym for module denergie sous marine autonomous underwater energy module It is a Rankine power steam turbine generation system and it has been under development by Bertin et Cie France since 1982 22 A schematic diagram of the system is shown in Fig 8 The system consists of four main units a the primary circuit comprising a highpressure 60 b the secondary circuit a Rankine cycle using either a c fuel and oxidant supply system d a depthindependent exhaust gas liquefaction A highpressure jettype combustion chamber generates the heat required to raise steam in the primary circuit via the steam generator Oxygen in cryogenic form is the preferred oxidant The heat generation loop oper ates at 60 bar and the combustion chamber is cooled by recirculated combustion gas which in the main is carbon dioxide The uniqueness of the system lies in the onboard liquefaction and storage of the combustion products carbon dioxide and water The advantage of this method of exhaust gas management rather than direct discharge is that the displacement of the under water vehicle remains constant the operating system is depth independent and any associated noise generated with discharging the exhaust gases overboard is elimi nated bar combustion and heat generation loop steam turbine or organic fluid turbine system r Heat oroduction loo0 A MESMA module of 10 kW using toluene as the working fluid is under design and development with LInstitut Francais de Recherche pour 1Exploitation de la Mer IFREMER as the energy source for use on board a survey AUV of 6000 metres depth capability 23 432 The WOLF system A second steam generating based system being investi gated for AUV applications is the WOLF system of the British company Marconi Underwater Systems Limited WOLF is the acronym for water oxidant lithium fuel The system is based on the energy release from the lithiumwater reaction The basic reaction between water and lithium results in the formation of hydrogen and lithium hydroxide 24 Laboratory tests have proved the feasibility of such an energy source and a simplistic WOLF system for an AUV is shown in Fig 9 44 Nonairbreathing diesel engine technology Since its original conception in 1901 a number of differ ent forms of nonairbreathing diesel engine systems have been proposed designed and developed to various degrees of technological advancement The overall design of these individual nonairbreathing diesel systems has been influenced by the technique employed to remove and dispose of the surplus exhaust gas from the engine operating cycle Current developments which are at varying levels of operational maturity are under way in the United Kingdom 25 26 Holland 27 Germany 28 Italy 29 Japan 30 Canada 31 and China 32 44 I United Kingdom and AngloEuropean developments Research at the University of Newcastle upon Tyne has led to the development of a closedcycle system which can be completely autonomous and depth independent The system developed Fig 10 is termed the nitrocycle as the nitrogen is retained as the working fluid the CO being absorbed by nonregenerative potassium hydrox Ethanol Combustion chamber Condensor 2 Fig 8 The MESMA system taken from reference 22 Part A Journal of Power and Energy IMechE 1994 Downloaded from piasagepubcom at PENNSYLVANIA STATE UNIV on February 20 2016 ADVANCED UNDERWATER POWER SYSTEMS I Generator Fig 9 The WOLF system courtesy of Marconi USL ide KOH in a chemical absorption system 33 The concentration of oxygen is maintained at approximately atmospheric and the working fluid essentially becomes air Theoretically therefore the performance of the engine can match that during naturally aspirated running conditions A reduction in weight and space of the absorption unit was seen as a major area that would make the system more commercially attractive Subsequently the use of water as the CO absorbent was adopted in view of the infinite supply within the marine environment Newcastle were joined at this stage by Cosworth Engineering now Carlton Deep Sea Systems Limited in developing the Argodiesel system which uses sea water to absorb the mainly CO surplus exhaust gas 34 As water is a much poorer absorber of CO than KOH a tradeoff between system complexity and eff Control lines 43 ciency resulted in a water scrubber design which would not meet complete C02 absorption Subsequently rather than accept a degradation in engine performance due to recycled CO argon was injected in such pro portions as to maintain the ratio of specific heats for the gas mixture to a value comparable with that of air approximately 14 A schematic diagram of the Argo diesel is shown in Fig 11 The absorber was estimated to be typically 80 per cent effective in its capacity to remove CO The unique focal point of the Argodiesel is an ingenious water management system WMS which uses ambient pressure sea water to move expand ed sea water through the CO centrifugal absorber and discharge it overboard against the outboard pressure Only small differential pressure pumps are required for the circulation of water inside the submarine and the system becomes depth independent A comprehensive Fig 10 W IMechE 1994 I reservoir transducer f 1 The nitrocycle taken from reference 3311 Proc Instn Mech Engrs Vol208 Downloaded from piasagepubcom at PENNSYLVANIA STATE UNIV on February 20 2016 44 J G HAWLEY S J ASHCROFT AND M A PATRICK Pressure hull Fig 11 The ARGOdiesel system taken from reference 3511 explanation of the operation and control of the WMS has been detailed by Fowler 35 Cosworths have undertaken contracts for Thyssen Nordseewerke TNSW of Germany and Rotterdamse Droogdok Mij RDM of Holland to develop the Argo diesel for future military submarine applications 36 Both of these projects are in the process of retrofitting the system into existing submarines as proving boats The Thyssen diesel generator is 120 kW the RDM unit is rated at 150 kW Both systems are selfcontained within a pressure hull section which also includes the vacuum insulated evaporators for liquid oxygen storage The units operate at a simulated external sea pressure of 500 metres which suggests that this is prob ably the maximum operating diving depth of the sub mersibles Developments at the Royal Naval Engineering College in conjunction with the Jniversity of Exeter have concentrated on the design of a test facility that allows a fourcylinder 39 litre diesel engine to operate purely on an oxygenfcarbon dioxide mixture for AUV and midget submarine applications 25 The ability of an engine to operate on such an atmosphere while still retaining acceptable engine operating performance would result in a simpler and more attractive system An important advantage of this approach is the greater carbon dioxide partial pressure that can be tolerated in the recycle gas thus greatly reducing the duty and the size of the gasscrubbing unit Operation of the engine on a CO atmosphere consisting of 30 per cent 0 and 70 per cent CO both by volume when preheated to a temperature of 150C has been proved The overall effect on engine performance with this mixture under maximum torque conditions is to reduce the brake power by 2023 per cent and to increase the brake spe cific fuel consumption by 2328 per cent 442 Japanese developments Mitsui Engineering and Shipbuilding Corporation Limited in conjunction with the Institute of Industrial Science at the University of Tokyo have undertaken the design and development of a nonairbreathing diesel engine system that is similar in many respects to the nitrocycle developed at the University of Newcastle The application is as the power source for use on board an AUV The project is termed R1 and the objective is to construct an autonomous underwater freeswimming vehicle that can survey a wide area of ocean ridges by measuring water temperature and other characteristics in the vicinity of the sea floor 30 The general arrange ment of the Rl robot is shown in Fig 12 which uses a 7 kW diesel engine The R1 has a target depth of 3000 metres and so the move to this particular type of absorption system means that the operation of the power system is depth independent and the vehicle requires no buoyancy control as there is no weight change The vehicle is expected to be launched in 1994 5 CONCLUDING REMARKS A review of the literature has revealed that there are many candidate underwater power systems However Main propeller Lateral thruster Vertical thruster U Controt section t power section 1 I L 7 m I Fig 12 The R1 robot taken from reference 3011 Part A Journal of Power and Energy IMechE 1994 Downloaded from piasagepubcom at PENNSYLVANIA STATE UNIV on February 20 2016 ADVANCED UNDERWATER POWER SYSTEMS 45 the selection of one in preference to the others cannot be made unless a vehicle has been specified with a well defined mission profile Even at the vehicle specification stage other factors that will influence the choice of one power system over another will be cost available and supportable technology and tried and proven reliable prototypes at operating depthto name the more salient considerations From the technology aspect the advanced electro chemical energy convertors such as fuel cells semicells and primary batteries have demonstrated the potential to provide energy densities far in excess of those cur rently available from existing leadacid and silverzinc secondary battery systems However such systems are proving costly to develop especially within the space and weight limitations of small submersibles and AUVs Until they have attained technical maturity and demon strated reliable operation thermal systems will provide an inexpensive and shortterm alternative to the present secondary battery systems This view is demonstrated by the many heat engine development programmes that are under way particularly with respect to the nonair breathing diesel system This review paper on underwater power systems has taken the AUV as an example to highlight the wide range of options being considered for the next gener ation of advanced unmanned untethered vessels It is not possible to explain in a short discussion paper the detailed technicalities of the operation and problems likely to be encountered with such systems or the poli tics associated with the choice of one system over another However a comprehensive reference listing has been presented should the reader seek to undertake further investigation REFERENCES 1 Ocean technologpinternational programmes and markets 1989 DTI Publication London 2 Bryson L Ocean opportunitiesmanagement for the future J Underwater Technol Spring 1992181 1114 3 Busby F Manned submersiblesthe beat goes on Sea Tech nology December 1989 pp 1014 4 Babb R AUV navigation for underwater scientific surveys Sea Technology December 1990 pp 2532 5 Foxwell D Autonomous underwater vehiclesthe naval force multiplier Int Defence Rev February 199225 145150 6 Hawley J G Diesel engine operation on synthetic atmospheres for underwater applications PhD thesis University of Exeter 1993 7 Technical Literature Allied Remote Technology of San Diego Calif 199 1 8 Cockburn J Use of a sodiumsulphur battery system in an AUV test bed vehicle Oceanology International 92 Conference Brigh ton 1013 March 1992 9 Tonge A M An incremental 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1988 pp 593 607 Fowler A Closed cycle diesel propulsion systems Trans Inst Marine Engrs 1990102 129135 Corlett R Cosworth deep sea systemsand a new underwater power source Maritime Defence 1989149 297299 929058 pp 93105 lMechE 1994 Proc Instn Mech Engrs Vol 208 Downloaded from piasagepubcom at PENNSYLVANIA STATE UNIV on February 20 2016