Subj : AIP and Submarines 2/3 To : ALL From : TOM WALKER Date : Fri May 20 2005 10:00 am >>> Continued from previous message *Closed-cycle Diesel Engines Typically, a closed-cycle diesel (CCD) install- ation incorporates a standard diesel engine that can be operated in its conventional mode on the surface or while snorkeling. Underwater, however, it runs on an artificial atmosphere synthesized from stored oxygen, an inert gas (generally argon), and recycled exhaust products. The engine exhaust - largely carbon dioxide, nitrogen, and water vapor - is cooled, scrubbed, and separated into its constituents, with the argon recycled back to the intake manifold. The remaining exhaust gas is mixed with seawater and discharged overboard. Generally, the required oxygen is stored in liquid form - LOX - in cryogenic tanks. CCD systems have been developed by a number of firms in Germany, Britain, the Netherlands, and a few other countries. However, except for a 300-horsepower demonstration system refitted onto the German Navy's ex-U 1 in 1993, no modern CCD systems have entered naval service. England's Marconi Marine recently acquired CCD pioneer Carlton Deep Sea Systems and is marketing a CCD retrofit package for existing conventional submarines, such as South Korea's nine Type 209s. Although one key advantage of CCD systems is their relatively easy backfit into existing submarine engineering plants, there have been no takers. Despite the additional supply complication of needing regular replenishment of cryogenic oxygen and inert gas, there are logistics advantages in retaining standard diesel engines and using normal diesel fuel. *Closed-cycle Steam Turbines The only steam-turbine AIP under active investigation is the French MESMA system (Module d'Energie Sous-Marin Autonome). This is essentially a conventional Rankine-cycle turbo-alternator powered by steam generated from the combustion of ethanol (grain alcohol) and stored oxygen at a pressure of 60 atmospheres. This pressure-firing allows exhaust carbon dioxide to be expelled overboard at any depth without an exhaust compressor. Basically, the MESMA approach is a derivative of French nuclear-propulsion experience using non-nuclear steam generation. Although MESMA can provide higher output power than the other alternatives, its inherent efficiency is the lowest of the four AIP candidates, and its rate of oxygen consumption is correspondingly higher. The first full-scale undersea application will be in Pakistan's three new Agosta 90B submarines, which will each be fitted with a 200 kilowatt MESMA system for increasing submerged endurance by a factor of three to five at a speed of 4 knots. The first installation is expected to be completed in 2001. *Stirling-cycle Engines In the Stirling cycle, heat from an outside source is transferred to an enclosed quantity of working fluid - generally an inert gas - and drives it through a repeating sequence of thermodynamic changes. By expanding the gas against a piston and then drawing it into a separate cooling chamber for subsequent compression, the heat from external combustion can be converted to mechanical work and then, in turn, to electricity. Like MESMA, this approach has an advantage over internal combustion systems, such as the CCD, in that the combustion processes can be kept separate from those that actually convert heat to mechanical work. This provides significant flexibility in dealing with exhaust products and controlling acoustic radiation. The Stirling-cycle engine forms the basis of the first AIP system to enter naval service in recent times. The Swedish builders, Kockums Naval Systems, tested a prototype plant at sea in 1989, and today, three Swedish Gotland-class boats are each fitted with two adjunct, 75 kilowatt Stirling-cycle propulsion units that burn liquid oxygen and diesel fuel to generate electricity for either propulsion or charging batteries within a conventional diesel-electric plant. The resulting underwater endurance of the 1,500-ton boats is reported to be up to 14 days at five knots, but significant burst speeds are possible when the batteries are topped up. *Fuel Cells In simplest terms, a fuel cell is an electrochemical conversion device that combines hydrogen and oxygen to produce water, electricity, and heat. Fuel cells are already seeing a number of promising applications in the space and automotive industries, and many authorities believe that fuel cells offer the best potential for developing more capable AIP systems in the future. There are several alternative configurations, but for submarine propulsion, so-called "Polymer Electrolyte Membrane" (PEM) fuel cells have attracted the most attention because of their low operating temperatures (80ø Centigrade) and relatively little waste heat. In a PEM device, pressurized hydrogen gas (H2) enters the cell on the Diagram of a Fuel Cell; Caption follows. In a typical fuel cell, gaseous hydrogen and oxygen are combined catalytically to produce water, heat, and useful electricity. Already successful in the U.S. space program, fuel cells are seeing increasing use as submarine power sources. *Principle of Operation There is basicly two reactions. Anode side, where a platinum catalyst decomposes each pair of molecules into four H+ ions and four free electrons. The electrons depart the anode into the external circuit - the load - as an electric current. Meanwhile, on the Cathode side, each oxygen molecule (O2) is catalytically dissociated into separate atoms, using the electrons flowing back from the external circuit to complete their outer electron "shells." The polymer membrane that separates anode and cathode is impervious to electrons, but allows the positively-charged H+ ions to migrate through the cell toward the negatively charged cathode, where they combine with the oxygen atoms to form water. Thus, the overall reaction can be represented as 2H2 + O2 => 2H2O, and a major advantage of the fuel-cell approach is that the only "exhaust" product is pure water. Since a single fuel cell generates only about 0.7 volts DC (direct current), groups of cells are "stacked" together in series to produce a larger and more useful output. The stacks can also be arrayed in parallel to increase the amount of current available. The greatest challenge for fuel-cell AIP systems lies in storing the reactants. Although oxygen can be handled with relative safety as LOX, storing hydrogen onboard as a liquid or high-pressure gas is very dangerous. One solution is to carry the hydrogen in metal hydride accumulators, at low pressure and ambient sea temperature. (A metal hydride is a solid compound of hydrogen and metallic alloy, in which individual hydrogen atoms occupy interstitial positions in the host metal's crystalline lattice. By manipulating temperature and pressure, hydrogen gas can be absorbed or released at will.) Another, less efficient, approach is to generate gaseous hydrogen from a stored liquid hydrocarbon such as diesel fuel, kerosene, or methanol. This requires an auxiliary device called a "reformer," in which a mixture of hydrocarbon and water is vaporized and superheated under pressure to yield a mixture of hydrogen and carbon dioxide. Several manufacturers are currently offering fuel cell systems for submarine AIP. Prominent among these is the German Siemens firm, which is collaborating with Howaldtswerke Deutsche Werft (HDW) and Italy's Fincantieri to supply fuel cell installations for the forthcoming 1,840-ton German and Italian U 212-class submarines. These will consist of nine PEM fuel-cell modules each nominally rated at 34 kilowatts, to yield a total of approximately 300 kilowatts (400 horsepower). With metal-hydride hydrogen storage, the system is predicted to yield 14 days submerged endurance and the ability to run up to eight knots on the fuel cells alone. Siemens is working on a next-generation PEM module rated at 120 kilowatts, and two of these will be incorporated into HDW's 1,860-ton U 214 boats, planned as export successors to the U 212 series. Other nations, such as Russia and Canada - the latter with significant under-ice requirements - are also considering fuel-cell modules for either new construction or for upgrading older boats. Other key advantages here are both higher efficiency and lower specific stored-oxygen consumption than the other alternatives. *An AIP Perspective Although it is a remarkable tribute to Hellmuth Walter's engineering genius that he fielded a fully functional - if troublesome - 5,000-horsepower AIP system in 1945, the maximum power output of current AIP installations is typically on the order of 400 horsepower (300 kilowatts). In comparison, the conventional diesel-electric plant of the U 212 class described above is rated at over 3,000 horsepower, and a typical nuclear submarine propulsion plant produces over 20,000. Since the power required to propel a submerged body varies with the cube of its velocity, it should be apparent that at least for the near future, AIP will be valuable primarily as a low-speed, long-endurance adjunct to the under- water performance of conventional submarines. 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