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MIT 2 611 - Air Independent Propulsion

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⋅ ⋅ Air Independent Propulsion This is an ever moving technology. These notes represent an overview but mayFor non-nuclear submarines, submersibles and unmanned vehicles; not represent the latestAUV, UUV, torpedoes parameters.Torpedo propulsion was originally stored high pressure air.** It evolved to heated air at the turn of the century using kerosene, alcohol or Otto fuel. Current torpedoes employ electrical storage or lithium sulphur hexafluoride systems. (Sulphur hexafloridegas is sprayed over a block of lithium which generates heat. As is well known typical submarine propulsion uses a storage battery with engine recharging. In all the stored systems, the challenge is storage of the oxygen component, ** The initiative behind the self-propelled torpedo was provided by an Austrian frigate captain Giovanni Luppi. After some unsuccesful attempts to propel a charge laden boat with a springdriven clockwork. In 1864 he turned to Robert Whitehead (1823-1905), then technical manager in an Italian factory to design an improved version. The result was a torpedo in October 1886: length 3.35 m, diameter 25.5 cm, weight 136 kg. Propulsion was provided by 20 to 25 kg of compressed air, driving a reciprocating engine with a high and low pressure cylinder. Taken from "Swedish Torpedo 100 Years; 1876 - 1976. Secondary Batteries Lead acid-discharges 2 - 1.8 V per cell solid solid liquid => solid liquid charges 2.1 - 2.6 V per cell electrolyte H2SO4 Pb + Pb O2 + ⋅ ⋅O4 = 2Pb⋅S O4 + 2H2⋅O energy density 67 lb/kW*hr⋅ 2H2⋅S ⋅ ⋅ 2Pb⋅ 2Pb⋅ lbf kgfcheck 67 = 30.391(2 + 8) O⋅ 2S kW hr⋅ kW hr⋅ ⋅ 4H= (8 ⋅⋅ + 2) O2S 4H 1Whr⋅ = 14.925 lbf lbf67⋅ ⋅ Silver - Zinc kW hr discharges 1.1 - 0.8 V per cell lbf kgf charges 1.6 - 2.0 V per cell 20kW hr⋅= 9.072 kW hr ⋅ electrolyte KOH energy density 20 lb/kW*hr 1 ⋅ Whr= 50 lbf lbf20⋅ kW hr⋅ problem (both cells): hydrogen release in charging. New developments: NiCd, Li rechargeable Fuel Cell originally developed by Roger Bacon. H2 and O2 are supplied to special electrodes with various electrolytes. KOH in the alkaline cells, proton exchange membranes (PEM) and high temperature carbonate in the molten carbonate cells, solid oxides in other cells. Energy conversion is relatively high ~ 60% ⋅ ⋅H 2H2 + 2electons figure later overall reaction H2 + 1 ⋅O2 = H2⋅O complete H2 + 2O = ⋅ ⋅O 2 1 H2⋅O ⋅⋅O2 + = 2electrons + 2 O⋅H 2 theoretical voltage: 1.23 V, practical voltage ~ 0.8 V 12/11/2006 1maximum power at w_dotmax = − ⋅ − − ⋅ = − = ΔG G = Gibbs_functionconstant T1 m_dot h1 T1 s1 (h2 T1s2) G1 G2 h1 − h2 = heating_value_of_fuel ΔG = 0.825_to_ 0.95depending on T1 and state of H2O liquid or vapor with internal⋅ hhv losses ( ~ 60% conversion) H2 consumption: 0.111 lbf = 0.05 kgf kW hr⋅ kW hr⋅ O2 consumption: 0.889 lbf = 0.403 kgf kW hr⋅ kW hr⋅ lbf kgf1.0 = 0.454reactants kW hr⋅ kW hr⋅ the volume is important and depends on the storage method: as cryogenic liquids: O2 sp_gr = 1.14 71 lbf = 1.137 × 103 kgf 3 3ft m H2 sp_gr = 0.064 4.0 lbf = 64.074 kgf 3 3ft m Other methods of storage include: high pressure gas, hydrides (driven out by heat and pressure reduction) or as liquid fuel which has to be reformed. A summary of ypes of fuel cells from Fuel Cell Handbook (Sixth Edition) DOE/NETL-2002/1179 By EG&G Technical Services, Inc. Science Applications International Corporation Under Contract No. DE-AM26-99FT40575 U.S. Department of Energy Office of Fossil Energy National Energy Technology Laboratory P.O. Box 880 Morgantown, West Virginia 26507-0880 November 2002 12/11/2006 2A brief description of various electrolyte cells of interest follows. A detailed description of these fuel cells may be found in Sections 3 through 7. Polymer Electrolyte Fuel Cell (PEFC): The electrolyte in this fuel cell is an ion exchange membrane (fluorinated sulfonic acid polymer or other similar polymer) that is an excellent proton conductor. The only liquid in this fuel cell is water; thus, corrosion problems are minimal. Water management in the membrane is critical for efficient performance; the fuel cell must operate under conditions where the byproduct water does not evaporate faster than it is produced because the membrane must be hydrated. Because of the limitation on the operating temperature imposed by the polymer, usually less than 120°C, and because of problems with water balance, a H2-rich fuel is used. Higher catalyst loading (Pt in most cases) than that used in PAFCs is required for both the anode and cathode. Because CO “poisons” the catalyst, the fuel may contain no CO. Alkaline Fuel Cell (AFC): The electrolyte in this fuel cell is concentrated (85 wt%) KOH in fuel cells operated at high temperature (~250°C), or less concentrated (35-50 wt%) KOH for lower temperature (<120°C) operation. The electrolyte is retained in a matrix (usually asbestos), and a wide range of electrocatalysts can be used (e.g., Ni, Ag, metal oxides, spinels, and noble metals). The fuel supply is limited to non-reactive constituents except for hydrogen. CO is a poison, and CO2 will react with the KOH to form K2CO3, thus altering the electrolyte. Even the small amount of CO2 in air is detrimental to the alkaline cell. Phosphoric Acid Fuel Cell (PAFC): Phosphoric acid concentrated to 100% is used for the electrolyte in this fuel cell, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor, and CO poisoning of the Pt electrocatalyst in the anode becomes severe. The relative stability of concentrated phosphoric acid is high compared to other common acids; consequently the PAFC is capable of operating at the high end of the acid temperature range (100 to 220°C). In addition, the use of concentrated acid (100%) minimizes the water vapor pressure so water management in the cell is not difficult. The matrix universally used to retain the acid is silicon carbide (1), and the electrocatalyst in both the anode and cathode is Pt. Molten Carbonate Fuel Cell (MCFC): The electrolyte in this fuel cell is usually a combination of alkali carbonates, which is retained in a ceramic matrix of LiAlO2. The fuel cell operates at 600 to 700°C where the alkali carbonates form a highly conductive molten salt, with carbonate ions providing ionic conduction. At the high operating temperatures in MCFCs, Ni (anode) and nickel oxide (cathode) are adequate to


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