Index number 50 /Technology

AIP system for submarines

Land-based AIP unit developed by SENER and TKMS at TKMS’s shipyard in Kiel (Germany).

Land-based AIP unit developed by SENER and TKMS at TKMS’s shipyard in Kiel (Germany).

Air-independent propulsion (AIP) represents a technological break-through for conventional submarines. In addition to the diesel-run energy generator that requires exterior air for combustion of the conventional submarine, those with an AIP system also use energy sources that do not require surface air, such as fuel cells. This allows them to be submerged without being detected for much longer than diesel electrical submarines, which have to emerge to the surface and operate with snorkel when their diving range given by their batteries on board is over.

The fuel cell-based AIP, which is currently being used with successful results, allows the submarine to generate the energy it needs from hydrogen and oxygen. However, hydrogen generation system still needs to be improved, as it is stored in very bulky, heavy metal hydride cylinders which have a big impact on crucial factors such as the shipped weight and the space occupied inside the submarine.

To solve this problem, and given that the general market trend is to build ever larger submarines, SENER has partnered with ThyssenKrupp Marine Systems (TKMS), the European leader in submarine construction, to develop an AIP based on the methanol reforming process, which allows the hydrogen needed to feed the fuel cell to be produced on board.


The development of the AIP system for submarines has been made possible thanks to the multidisciplinary structure of SENER, which is made up of experts from very diverse fields. Specifically, the team working on this project has included engineers from the Aerospace Division for integrating the fuel cell; Process, Instrumentation, Control, Electrical, Boiler and Mechanical engineers; experts in Shock and Acoustics and Piping; technicians specializing in SENER’s FORAN marine construction system and other professionals. Between them, they have developed a system with excellent commercial potential in the submarines market, and one of SENER’s most noteworthy innovations of recent years: the CO2 dissolver patent. Through this project, SENER has acquired the capacity to act as prime contractor for the entire AIP system alongside the major shipyards.

SENER’s involvement in the new AIP system includes four main subsystems:

CO2 compression unit
Its purpose is to release the exhaust gases from the reforming unit into the sea when the submarine reaches depths of over 200-250 m. Because the reforming unit operates at 25 barg, the difference in pressure between the exhaust gases and the exterior conditions below this depth has to be closed, by means of a compression phase. At lesser depths, the gases can be released directly into the sea (by means of the dissolution unit) from the reforming unit, avoiding the compressor and saving energy. This unit consists of an expansion tank and a wet CO2 compressor.

High-pressure oxygen supply unit
This is used to adapt the condition of the low-pressure liquid oxygen available on board in a self-contained tank, based on the requirements of the high-pressure methanol reforming unit, and to then supply it (at high pressure) uninterruptedly to this unit.

Supply and management of the methanol storage unit and reactive water
This subsystem has the same purpose as its predecessor: firstly, it adapts and supplies the methanol required by the reforming unit and stored in the in-built tanks of the boat, and secondly, it manages and stores the pure water produced by the fuel cell, as part of the submarine’s load compensation.

CO2 dissolution unit
The biggest technological challenge for SENER was to develop a dissolution unit for the exhaust gases that result from the process.

SENER has designed a system that releases these gases outside the submarine, while ensuring that the boat’s acoustic signature is not detectable at a certain distance. To achieve this, SENER has succeeded in dissolving these gases (approximately 98% CO2 and 2% O2) in sea water, by suctioning directly from the submarine at the same pressure as the depth at which the vessel is located. In the process’s toughest conditions, SENER achieves a maximum bubble size of 200 μm at a depth of 18 m for warm waters, thereby contributing towards the submarine’s stealth discretion.

The CO2 dissolution unit is an innovation that is in the process of being patented in Europe, as there is currently no similar product on the market.


Land-based AIP unit developed by SENER and TKMS at TKMS’s shipyard in Kiel (Germany). Land-based AIP unit developed by SENER and TKMS at TKMS’s shipyard in Kiel (Germany).

Copyright SENER 2018