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Skriven 2005-05-20 09:00:00 av TOM WALKER (1:123/140)
Ärende: AIP and Submarines    2/3
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*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. There is little
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