Type 208
In the 1980s the West German navy planned a major construction effort for the renewal of its submarine force. The Navy wanted to replace its current "Type 205" and "Type 206" submarines with the "Type 208", a class with an air-independent propulsion system (probably a fuel cell). These units, which the West Germans were developing with the Norwegians, involved a major technological advance and could not be produced by the time the existing force reached retirement age. As of 1982 plans called for completion of 9 between 1991-4. Therefore the navy planned to upgrade 13 of its "Type 206" boats to "Type 206A" between 1988-91. The 6 "Type 205" boats would reach retirement age in the 1980s, and the navy considered filling the gap between their retirement and the availability of the "Type 208" by building some "Type 210" units. Such an action would also help sell the "Type 210" to Norway, for whom it was designed.
Conventional diesel-electric submarines must surface periodically to recharge their batteries by using generators driven by air breathing diesel engines. During this time, submarines are most vulnerable to detection. Air independent propulsion (AIP) systems were developed to generate electrical power while the submarine is submerged. Such systems provide power for recharging the batteries, for propulsion and for the submarine's other electrical equipment requirements, while the submarine is submerged.
In the 1980s there were about six competing AIP technologies, at various stages of development. Heat engine systems include closed-cycle diesel engines, Stirling engines, closed-cycle gas turbines, and Rankine engines powered by hydrocarbon fuel (French MESMA system) or a small nuclear reactor (Canadian AMPS system). Fuel cells and semi-cells were the only non-heat engine technologies under development, other than development of high energy density batteries such as lithium aluminium iron sulphide (LAIS).
Fuel cells, one of the leading AIP contenders, are electrochemical energy converters that enable the chemical energy of a stored fuel and an oxidant to be converted directly to electricity. Ramifications of the installation of fuel cell AIP technology into submarines are generally more complex than for other AIP systems which are able to use diesel fuel (though generally of a higher purity than currently used in air breathing diesel enginesin submarines). The fuel cell AIP system must be considered as a whole, and the optimum choice of fuel cell and reactant storage and processing depend on the particular application.
Fuel cells directly convert the chemical energy of the reactants, a fuel and an oxidant, into direct current electricity. Fuel cells will continue to operate for as long as theexternally stored reactants are supplied. The fuel cell system includes the fuel cell stacks and control systems, the stored fuel and oxidant, storage vessels, associated pipework and reactant processing systems, and exhaust product handling system. Commonly used reactants include air and oxygen as the oxidant and pure hydrogen or hydrogen derived from catalytically reformed hydrocarbons or cracked ammonia as the fuel.
In 1987 fuel cell AIP plant was installed on a German Type 205 submarine, and underwent nine months of successful sea trials. Following decomissioning in 1992, closed cycle diesel AIP plant was installed prior to two months successful sea trials in early 1993.
Siemens in Germany had been active in the development of the low temperature (80°C) alkaline H2/02 fuel cell since the mid-1960s. A feature of Siemens alkaline fuel cells was the use of cheaper electrode materials (Raney nickel anode, doped silvercathode) instead of platinum electrocatalysts. The fuel cell produced a current density of 400 mA/cm2 at 0.8V cell potential. A 60 cell 6 kW fuel cell module was constructed, producing 48 V at 125 A, with an efficiency (based on the lower heating value of hydrogen) of 61-63% at rated load and a maximum efficiency of 71-72% at 20% load factor. This proved to be areliable system with over 20,000 h of accumulated module operation in a sixteen module 96 kW land based demonstration system. This was then installed on the German Navy Type 205 submarine U1. The nine month sea trial in 1988-89 clearly demonstrated that this fuel cell propulsion system was modular and flexible with a high reliability, conferring operational and tactical advantages on submarine operation. Siemens then began developing a solid polymer electrolyte fuel cell system which promised better performance than that used in the U1.
Safety is very important in the confines of a submarine, where the effects of fire or explosion are potentially catastrophic. Safe storage and handling of the reactants and byproducts should minimize risk to personnel and the submarine itself. The AIP system including reactant storage and any reactant processing sub-system must also be resistant to shock, particularly for storage of cryogenic fluids such as liquid oxygen (LOX). There is some accumulated experience with the safe usage of LOX insubmarines, with the Swedish Navy having safely operated inboard LOX tanks on the Nacken for over five years, and German experience with an external LOX tank on the Type 205 Ul in fuel cell and closed cycle diesel trials.
Germany planned to employ an extensive defensive barrier throughout the eastern, central and western Baltic utilizing hit and run and defense-in-depth tactics. Submarine operations, to include offensive minlng in the eastern Baltic, would be the first echelon in this barrier of attrition. The small nonmagnetic Type 206 diesel submarine which the Germans built specifically for Baltic operations was ideally suited to this role. With its excellent minelaying and torpedo capability coupled with its silent operating capability on station, these units would exact a precious toll in pact shipping.
As of 1985 German plans were to have the Type 206 units remain in service through the end of the century, with twelve of the eighteen units undergoing retrofit to De delivered during 1988-91 to fill the capability gap created by the postponement of the Type 208 submarine program until after the year 2000. The type 208 class was originally scheduled for delivery in the 1990's but was postponed due to technical and financial reasons. This submarine was planned to nave a conventzonal propulsion system insepenaent of the outside air.
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