Advanced French AIP proposal. “Simplified”
Layout of Diesel Fuel Processing Equipment (Diesel Fuel Autothempermal Reformer – SOFC). Layout
and description courtesy Bakst
Engineering.
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The
following is the section on AIP within an excellent article The Driving Factor
In The SEA 1000 Choice The Submarine Propulsion Chain by submarine expert Rex Patrick, from Sydney, Australia. This was on pages 40 to 44 of the October 2015 issue (Volume 41, Number 8) of the Asia-Pacific Defence Reporter (APDR). APDR is an excellent magazine provided by subscription.
It was written before the Australian
Government’s 26
April 2016 SEA 1000 decision in favour of the French DCNS Shortfin – but the
article's comments on AIP still appear accurate.
Significantly the
article is also published on the Siemens' website
here.
Pete
has bolded for emphasis some words in the following AIP section and also added footnotes [1] and [2]. The footnotes indicate that the AIP section is still current
and accurate. Japan and Lithium-ion Batteries (LIBs) are also discussed towards
the end.
ARTICLE
AIR
INDEPENDENT PROPULSION
The primary purpose of an AIP system is to convert stored reactant energy into electrical energy for the submarine’s main battery and to do so independently of the surface atmosphere. It provides little benefit during transits but is invaluable when operating slowly within an operational area.
The primary purpose of an AIP system is to convert stored reactant energy into electrical energy for the submarine’s main battery and to do so independently of the surface atmosphere. It provides little benefit during transits but is invaluable when operating slowly within an operational area.
TKMS will almost certainly offer
Australia a reformer/ FC solution. Being a large submarine, it will demand two
reformers and four 120 kW FCs.
The
first element of TKMS’ AIP solution
is a Methanol reformer that extracts hydrogen from methanol and feeds it
directly into the FC. Methanol is selected because of its worldwide
availability, high hydrogen content, low reforming temperature (250°C),
reformation ease and high reforming efficiency (80 to 90%). LOX is also used in
the reforming process. Sub-system waste is pressurised CO2 which can be
discharged to sea down to full diving depth. The reformer is packaged in an
enclosure with its own special ventilation system for cooling.
Each
reformer is capable of producing enough hydrogen to supply two fully loaded 120
kW cells. It removes the need to store hydrogen on board, which is problematic
from a supply availability and refuelling complexity perspective, and is also
difficult on 2000+ tonne submarines because of the weight of the hydrogen’s
metal hydride storage bottles.
The
reformer has a two to three hour start-up time. Operationally, the idea is to
start it up in the patrol area in block periods where AIP can be exploited,
potentially for weeks on end, dependant on the amount of reactant stored on
board.
The
reformer has been in development since 1995 and a test site has been in
operation for a decade, with FCs connected to it since 2010. A reformer
suitably packaged for installation on board submarines is currently undergoing
set-to-work in Kiel. Whilst the reformer has not been fielded on a submarine
yet it is at the test bed state and therefore it attracts a low SEA 1000
project risk label.
Moving
to the FC, TKMS will offer the
second generation Siemens 120 kW Polymer Electrolytic Membrane (PEM) FC. The
PEM FC works by feeding standard industry-grade LOX and high purity hydrogen
into the cell which generate electricity in response. It does this silently and
at a low temperature (80°C). It is different to a battery in that it stores no
charge; it simply generates electrical energy so long as the reactants are fed
into the cell. The cell is extremely (fuel burn) efficient at between 50 and
70%. Its ‘waste’ outputs are potable water, which is fed into holding tanks,
and (1%) oxygen, which is fed into the submarine’s atmosphere to assist in
maintaining breathable air during prolonged AIP dived periods.
The
FC has been under development by Siemens
since the early eighties. It was first trialled on a German Type 205 test
submarine in 1988 and then contracted for supply into the German and Italian
Type 212 program. The first production FC went to sea in 2002 and it is now a
very mature system at sea on 24 submarines, meaning it is a minimal project risk
component of the German SEA 1000 solution.
The
reformer/FC system meets all of the fundamental requirements of an AIP system;
high efficiency, silent, low magnetic signature, light and compact, generates
no pollution or heat, reliable, relatively easy to maintain and requiring no
additional operating personnel.
It is
interesting that the German Defence Department funded TKMS starting in 2007 to conduct a methanol vs diesel reformer
comparison, because the diesel reformer approach would negate the need for
storage of an additional fuel, methanol, on board the submarine. TKMS built a small 10 kW diesel
reformer to support the study. The study conclusions were instructive. The
diesel reformer was less efficient because diesel has a hydrogen to carbon
ratio of only two to one, whereas methanol has a hydrogen to carbon ratio of
four to one. The diesel reformer also needs to run at around 850 degrees which
implies heat inefficiency as compared methanol. The higher temperature also
means a longer start-up time than the methanol reformer. Finally, unless the
diesel carried by the submarine is sulphur free, and standard diesel is not,
the required sulphur purifier at the reformer output would likely take up
considerable space (as big as the reformer itself). The idea was abandoned.
Public domain information shows DCNS [now called Naval Group] have abandoned their MESMA AIP solution on the Pakistani Agosta 90s and will use a diesel reformer/FC solution on the Shortfin Barracuda. [No supporting detail seen to date.]
Further on the French FC, it appears as though two options are on the table; a PEM or Solid Oxide FC (‘SOFC’) type. If a SOFC is chosen, noting they offer good energy conversion efficiency, long life and operating cost advantages, other drawbacks need to be addressed. Most of these drawbacks relate to the high 600 to 1000°C operating temperature which brings hot exhaust issues and brittleness related shock resistance problems.
Further on the French FC, it appears as though two options are on the table; a PEM or Solid Oxide FC (‘SOFC’) type. If a SOFC is chosen, noting they offer good energy conversion efficiency, long life and operating cost advantages, other drawbacks need to be addressed. Most of these drawbacks relate to the high 600 to 1000°C operating temperature which brings hot exhaust issues and brittleness related shock resistance problems.
Novelty,
complexity and uncertainty put this solution’s inclusion in the French package as high risk. Even if
the technical challenges of the diesel reformer and FC are solved, the enemy of
the DCNS development will be
schedule. DCNS are believed to have
started their reformer/FC work back around 2006/7 and announced it as a future
solution in 2008 as part of their SMX 24 concept design. [1] It is instructive that the Germans have developed and perfected
their reformer/FC solution over four decades. It is also worthy of note that
the Spanish have had issues with
their S-80 submarine ethanol reformer/FC solution and have announced that the
first S-80 will now be fitted-for-but-not-with AIP.[2]
The Japanese will not offer up an AIP
solution, rather fill any potential AIP space with additional Li-Ion batteries. It is believed this
decision stems from their experience with the inefficiency of the Swedish origin Stirling AIP solution. All things considered with respect to
reported Stirling engine maintenance overheads and the lack of differential
between the Stirling energy density and the Li-Ion energy density, the decision
is likely valid.
However, the energy-density differential
between the DCNS and TKMS FC and the Japanese Li-Ion’s is large, giving the
Japanese solution a poorer indiscretion ratio than the Europeans’ FC approach. It is known that the Japanese originally approached TKMS about adopting their FC AIP solution,
but the adoption of the German technology was problematic for two reasons;
firstly, the reformer necessary for the larger Soryu submarine was not mature at the time and TKMS/Siemens were not inclined to transfer knowledge of what they
considered to be the ‘crown jewels’ of their submarine program. Whilst Li-Ion
may have advantages in transit situations, which is why the French and Germans have Li-Ion as part of their solutions, the FC provides the
advantage where it really counts; in the operational area. Whilst an all Li-Ion
Japanese solution may have
advantages with respect to transiting, it means little if the boat is then sunk
upon arrival in its assigned patrol area.
FOOTNOTES
[1] The possibility of French DCNS
progress is recorded in a DCNS
article of 13 October 2016 for Euronaval 2016: “FC2G AIP – Fuel Cell:
Second-generation Air-Independent Propulsion. DCNS has developed the 2nd
generation AIP system using fuel-cell technology. FC2G AIP provides the best
possible dive autonomy in total safety and easy support.”
[2] Spain's problems are covered in IHS Jane’s article
of 24 January 2017, which indicates: “Spain's first S 80-class submarine
will not be fitted out with [AIP] as development of the system will not be ready
in time, according to the admiral in charge of
Maritime Action (Almart)...He also said he was not sure which of
the four new boats would be the first to be fitted with the AIP system.”
