Feature 5
designers, is LOX. However, a liquefaction the volume taken up by the fuel cells providing an immediate leak indicator. Th e
plant would need to be carried in addition to and electrolysers a reversible PEM FC/ O bottles are positioned alongside the HPA
2
the storage tanks and, as they are bulky and electrolyser system was selected (henceforth bottles within the ballast tanks, the volume
maintenance intensive, compressed oxygen referred to as RFC). It is understood that of which would require a corresponding
was preferred. Oxygen bottles similar to this technology is still immature but can be increase. On operations the RFCs produce
existing UK SSN high-pressure air (HPA) expected to develop rapidly over the next few 3tonnes of water a day, equating to 43litres/
bottles of approximately 3.2m
3
volume and years. Th e majority of this fuel cell’s depth man/day for the 70-man crew (minimum
500kg mass were used for calculations. is taken up by the retaining plate structure, required 23litres/man/day). At emergency
In order to guarantee reactor safety there the size of which is largely independent of power levels the RFCs still produce 18litres/
must be an immediate notice back-up the number of cells stacked up. Th erefore, a man/day, which is suffi cient for drinking
power source in the event of a reactor scram, linear relationship cannot be used in sizing a and cooking. In electrolyser mode the RFCs
capable of providing suffi cient power until larger fuel-cell stack from this example. As a produce 477m
3
of hydrogen and 238m
3
of
the reactor can be restarted or a suitable result the (admittedly large) assumption was oxygen per hour which, taking into account
port reached. Current SSNs divide this task made that an appropriate device need be no continued crew oxygen consumption, results
between a battery and diesel generators. Th e larger than a dedicated PEM FC with each in a three-day recharge.
battery is used for immediate power and 120kW fuel-cell stack assumed to be the size Th e total volume of the fuel-cell system
lasts long enough for the majority of scram of a Siemens BZM120 (as fi tted to the U214 (RFCs, H cylinders, O bottles, O
2 2 2
events. In serious cases where longer term class), namely 500litres and 900kg each. compressors, RO plants, BOP and a 100%
power is required the diesel generators are Any size increase associated with the access envelope) is 285m
3
. This could
used but these can only be started once the reversible nature of the fuel cell is assumed either be distributed around the hull (an
submarine is near the surface and has access to be off set by continuing improvements in option that was preferred owing to the
to air for the diesels. Broaching the surface PEM FC power densities. BOP volume is increased survivability and electricity/
may well compromise a mission and not be assumed to be equal to the fuel-cell stack. water-generation capabilities of the fuel
possible at all under ice. In order to release hydrogen from the metal cells in survival situations) or inserted as a
Were fuel cells used as the back-up hydride heat must be applied and vice dedicated 5.6m plug. Th e mass is 313tonnes,
power source, a small battery would still be versa; therefore the charging/discharging roughly 10% of the total displacement.
required in order to provide instantaneous mechanism requires a heating/cooling Although exact fi gures are diffi cult to obtain,
power following a scram before the fuel-cell system. Both electrolysers and pressurised very approximate calculations show that this
system can be started. Th is battery would water reactors require demineralised water is approximately half the volume and three
also be used to provide a high-speed and can therefore share the same water times the mass of the now-redundant diesel
sprint capability when the reactor is shut tanks. A single reverse osmosis plant such generators and diesel tanks.
down. In addition, sufficient hydrogen as the Derwent RO4/2 is suffi cient to meet
and oxygen must be carried to ensure that the electrolyser water demand. Th is would Safety analysis
there is always power available following a have to be fi tted in addition to normal water Th ere are three areas that hydrogen could
scram. If supplies were limited to a single production needs, otherwise water for be released from: the hydride cylinder water
charge of the onboard tanks, as per the other uses would be limited during the two tanks (HCWTs), the RFC compartments
U212/4 class, then reactor safety could be days required to replace the hydrogen and and the transfer pipework between the two.
compromised following a series of multiple oxygen used during operations. Additional Th e metal hydride cylinders are located
scrams. Th erefore, it is necessary to provide electrolysers would be required to produce low down within the pressure hull.
onboard recharging facilities for the oxygen oxygen for crew consumption during SSN Should a leak develop in a metal hydride
and hydrogen consisting of electrolysers mode with a proportion of the compressed cylinder the hydrogen would expand out
and compressors as required. Th is has the oxygen used for the same purpose when in of the leak site, thus cooling down the
added advantage of making the submarine fuel cell mode. surrounding hydride. As the hydrogen
independent of shore refuelling. Without The calculations were split into release process is endothermic the leak
diesel generators the submarine’s emergency operational and emergency modes. It would self-seal. Should a pipework fi tting
power comes from the fuel cells. Th erefore, was found that oxygen required for crew within a HCWT fail hydrogen would
a proportion of the hydrogen and oxygen breathing is negligible compared with that displace the water up the sounding tube.
must be set aside for emergencies in addition for fuel cells and has no effect on total Th e HCWTs temperature ranges from 4°C
to that used on operations. Th is must be numbers of oxygen cylinders required. A (lowest realistic sea temperature) to 40°C
suffi cient to provide 137kW for seven days total of 60 hydride cylinders and 20 oxygen (heated during discharge) and thus the
– that is, suffi cient power to achieve 4knots bottles are needed. In order to allow cooling water expands/contracts by ~1% over an
(37kW) and maintain an assumed absolute and heating of the metal hydride cylinders operating cycle. Assuming each HCWT
minimum of 50kW each for the reactor and (required for charging/discharging) and volume is 30% larger than the 30 cylinders
hotel service loads. to swift ly detect any leaks, the cylinders inside, each HCWT would hold 10.8m
3

A basic system sizing for the fuel-cell are located in seawater tanks fi tted with a water which would expand by 0.1m
3

plant was carried out. In order to minimise sounding tube connected to a detector thus during an operating cycle. Th e hydrogen
Warship Technology October 2008 47
WWT_Oct08_p42+43+44+46+47+50+51.indd Sec2:47T_Oct08_p42+43+44+46+47+50+51.indd Sec2:47 110/10/08 1:17:16 PM0/10/08 1:17:16 PM
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