Feature 5 | SUBMARINE PROPULSION
event of a reactor scram and sprint speeds
Metal Hydride
whilst running on fuel cells. It is recognised
Water Production
Scenario Cylinders O Bottles
2
that this concept of operations is impossible
(litres/manday)
(H )
2
with today’s generation of reactors and
hence would require a new design. However,
Operations 42 14 43
next-generation naval nuclear power plants Emergencies 18 6 18
are already heading in the direction of low
Total 60 20 /
shut-down power and increased passive
safety, both of which are major requirements Metal hydride cylinder and O2 bottle numbers.
for such a stop/start operating concept.
Th e following specifi cations were chosen,
roughly equivalent to the Collins class SSK: are required to produce a total of 477kW. restricted space available within a submarine.
eff ective length 75m; diameter 8m; surface However, the average speed on operations Compressed and liquid hydrogen storage
area 1885m
2
; L/B ratio 9.375; block coeffi cient was assumed to be 6knots, which equates are considered unsuitable, the former is
0.72. Th ese fi gures were used to determine to 81kW eff ective power and a total average volumetrically inefficient and the latter
a power-speed curve for the submarine. power of 320kW; this fi gure was used for requires a significant plant to achieve
Using a seawater density of 1025kg/m
3
duration calculations. In the absence of a 70°K to liquefy the hydrogen, whilst both
and kinematic viscosity of 1 x 10
-6
m
2
/s the dedicated mission profi le it was assumed require significant containment vessels.
power-speed curve was calculated. Selecting that the submarine would be required to Th e practical alternatives are either metal
a maximum sustained operational speed of conduct fuel-cell operations for a period of hydride or fossil fuel reformation. Fossil
8knots (short-term sprints can be achieved up to seven days. fuel reformation produces carbon dioxide
using the battery, although it should be An IFEP plant was selected with a single that must be disposed of either through
noted that most fuel cells are capable of reactor driving two steam turbo-generators absorption or overboard discharge.
considerable short-term overload) results feeding onto a DC busbar, also supplied from Th e former requires dedicated storage
in an eff ective power of 187kW. Assuming four 120kW fuel-cell stacks providing a total space and the latter can increase the
the following effi ciencies: propeller 75%, of 480kW. No diesel generators are required, submarine’s signature. Therefore, metal
motor 95% and power electronics 95%, it with all auxiliary power coming from the hydride storage was considered the best
results in an installed propulsive-power fuel cells, if needed. All propulsion, ship, option; its two biggest disadvantages (cost
requirement from the fuel cells of 277kW. reactor, weapon and hotel service loads are and weight) are less relevant to a submarine
By comparison, the U214 class has 240kW supplied from the busbar with propulsion than for other applications. Submarine
of fuel cells installed. It is interesting to note being achieved with an appropriate electric designs tend to be volume rather than
that the air-breathing PEM FC developed motor mounted directly onto the shaft as on weight driven and often have ballast
for automobiles are rated in the range of contemporary SSKs. added to achieve the desired buoyancy
30-50kW. In order to minimise thermal signature, and stability conditions. So where the low
It was assumed that there is a 100kW hotel provide adequate start-up and response times gravimetric density of metal hydride storage
service load (obtained from similar size UCL and reduce onboard cooling requirements, a is a big disadvantage for most transport
Submarine Design Exercise concepts) and in low-temperature fuel cell would be required. applications, it is less so for a submarine.
the absence of any data for an appropriately Of these, PEM FCs provide the high power Th e metal hydride cylinders as used in the
sized nuclear power plant the reactor density required by submarines and U212/4 class are considered appropriate.
safety load was estimated as also equalling although a pure-oxygen breathing version Solid storage of hydrogen using metal
100kW (this fi gure is not representative of differs to the air-breathing types being hydrides or carbon nanotubes is an area of
current class SSNs). Th erefore, the fuel cells developed by the automotive industry they technology that is developing rapidly and
can still benefi t from technology transfer. signifi cant improvements in performance
Balance Of Plant (BOP) systems (thermal, can be expected in the next decade.
gas and water management systems, control Unlike the majority of fuel cells, which
Rated net power 0.6 W
circuitry) were assumed to occupy the use atmospheric oxygen and accept the
Rated net current 1 A same volume again as the fuel-cell stacks. consequential loss in efficiency for a
Air-breathing PEM FCs have an effi ciency ‘free’ fuel source, submarine fuel cells are
DC voltage range 1.65 V
range of 45-60% although the higher values designed to use a pure oxygen supply.
H production 10 ml/min
are projected only. A figure of 49% was Th is can either be stored as liquid oxygen
2
settled on for the PEM FC, although as pure (LOX) or as a compressed gas. LOX has a
O production 5 ml/min
2
oxygen increases effi ciency by approximately volumetric compression ratio of 860:1 and
20% a total system effi ciency of 69% was compressed oxygen of anywhere between
Volume 1.4x10-4 m
3
used for calculations. 200:1 and 300:1 according to the selected
Reversible PEMFC and electrolyser The chosen fuel storage system must storage pressure. As a result the obvious
specifi cations. be volumetrically efficient owing to the choice, and that chosen by the U212/4
46 Warship Technology October 2008
WWT_Oct08_p42+43+44+46+47+50+51.indd Sec2:46T_Oct08_p42+43+44+46+47+50+51.indd Sec2:46 110/10/08 1:17:16 PM0/10/08 1:17:16 PM
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