New Age Submarines: Run Silent, Run Deep

With sensors and submarine detection methods becoming more sophisticated and advanced, a submarine's primary advantage—its ability to operate undetected and unobserved—has been vitiated because it needs to surface frequently for recharging batteries. Even coming up to snorkel depth while evading human visual capabilities is now well within the cognisant ability of advanced ‘eyes in the sky’.

The dream of all submariners ever since submarines were first invented, of staying submerged for an unlimited period of time with an Air Independent Propulsion System (AIP), was first realised when nuclear-powered submarines were built. But because nuclear propulsion was such an expensive dream, only a very few navies could afford it. For the others, equipped with conventional diesel submarines, this dream is only now being realised thanks to the Fuel Cell System. 

Earlier efforts at AIP systems were focussed upon developing mechanical solutions – either closed-cycle diesel engines, where the exhaust gases were processed and recycled or where external combustion engines, burning Ethanol and liquid Oxygen, were used to generate electricity, with the exhaust gases being cooled and condensed before release into the sea, to minimise emission signatures. Compared to Fuel Cells, these mechanical methods are relatively inefficient, noisy, and dangerous and have relatively easily detectable signatures despite all efforts.

Fuel cells are energy converters that transform chemical energy into electric current without generating noise, combustion, or emissions. Distilled water is the only by-product, providing the lowest possible signature. Other than nuclear power, Fuel Cells (FC) are the only practical solution to providing Air-Independent Propulsion to submarines when they have to operate submerged for a considerable length of time. 

The FC developed for submarines by Howaldtswerke-Deutsche Werft AG (HDW), Siemens, Ferrostaal AG, and Ingenieurkontor Lübeck in close cooperation with Germany’s Federal Office for Military Technology and Procurement (BWB) works on an operating principle that is the reverse of water electrolysis. In this system, an energy carrier (Hydrogen or Methanol) and an oxidation agent (liquid Oxygen – LOx) are combined to produce water while emitting electricity. This electric energy is fed to the submarine’s main switchboard. The amount of waste heat and resulting IR signatures is low thanks to very high efficiency. 

In Class 214 submarines, which incorporate the FC-based AIP, the propulsion system plant is a hybrid system that allows the submarine to perform missions in an ultra-silent mode for extended periods of as long as three weeks while permanently submerged and independent of outer air. In contrast, normal diesel-electric submarines without the FC AIP can operate underwater in a totally submerged mode for only a day or two.

The heart of the FC system is the Siemens polymer electrolyte membrane (PEM) fuel cell. It contains a solid polymer electrolyte, which conducts Hydrogen ions to the cathode, where they react with Oxygen. The fuel cells are assembled in rows and form one complete module within a pressure-tight housing. The PEM Fuel Cell operates at low temperatures (less than 80°C) with an efficiency of about 65 per cent. The Class 214 is equipped with a 120kW module.

Hydrogen is stored in metal hydride cylinders outside the submarine's pressure hull section. The metal hydride's lattice structure enables it to absorb hydrogen atoms, which can be released by heat application and conducted as an energy carrier to the fuel cell. Heat is supplied from the waste heat produced by the FC process. The hydride contains no free-flowing gas, and the amount of Hydrogen released depends on the amount of thermal energy introduced.

The Oxygen is carried on board in liquid form in an insulated tank mounted inside the pressure hull. The tank has an integrated evaporator that uses waste heat from the fuel cell operation. This Oxygen supply system also covers the crew’s requirements for breathing air.

Fuel Cell plants do not create any requirements for weight compensation since reaction water is kept on board for weight balance purposes. It may also be used for the crew’s needs.

With no emissions, mechanical noise, or IR signature, the Fuel Cell can provide continuous power while the submarine is trying to evade detection by running silently, deep underwater, or just lying ‘doggo’ waiting for searchers to pass overhead. 

AIP submarines could be a formidable threat when operating in coastal waters, marginal ice zones, maritime straits and other global ‘choke points’. In addition, there is a near certainty that new underwater weapons will help to equalise the performance disparity between AIP vessels and nuclear-powered submarines. They could be termed the ‘poor man’s nuclear subs’ of the future.