# Fire Safety Risk Analysis of Conventional Submarines
## 4 Fire Safety Risk Considerations--Technology Review
### Hydrogen Fuel Cell
#### 4.3.2 Functional Description  
Hydrogen fuel cells utilise an electrochemical reaction between hydrogen and oxygen to produce electricity as shown in Figure 6. This requires a constant supply of hydrogen and oxygen gas, the storage of which is a challenge for submarines. Most AIP technologies require high pressure liquid oxygen (LOX) to be stored on-board. Storage methods for hydrogen are varied however, ranging from metal hydride cylinders to carbon based fuels which require a reformer to extract and convert the stored hydrogen into a form that is usable by the fuel cell. The configuration shown in Figure 7 utilises metal hydride cylinders and a LOX tank. Waste heat from the fuel cell is used to release hydrogen gas from the storage cylinders and evaporate the LOX required to fuel the PEMFC. Water produced through the operation of the fuel cell is retained to maintain stability on the submarine as fuels are expended [55]. Although this configuration has proven feasible in a number of submarines, the storage efficiency in terms of the weight of metal hydride required to store sufficient hydrogen precludes its use on larger submarines greater than 2000 tonnes displacement [55]. This has driven the development of reformer technology which effectively involves the storage of hydrogen in liquid fuels such as methanol, ethanol and diesel and then extracting the hydrogen at the time of use within the fuel cell. Steam reformation of hydrocarbon gases and liquids is a popular hydrogen production method both for fuel cells and other applications. The designs and configurations are  
Figure 6: Diagram of a Hydrogen Fuel Cell [54]. Figure 7: PEMFC Configuration on Class 212 A and 214 Submarines [55]. widely varied, but generally consist of three main components [56]: (1) a burner or furnace; (2) a reactor, usually either a catalyst loaded microchannel heat exchanger; or an array of hydrogen permeable membranes with integrated catalyst; and (3) a gas purification system. A picture of a microchannel heat exchanger or micro-reactor is shown in Figure 8a. An integrated membrane reactor module is shown in Figure 8b. The reforming process varies depending on system design, however generally requires heating of the carbon based fuel or feedstock with steam and oxygen to produce mostly H\({}_{2}\) gas and CO\({}_{2}\). This requires the use of waste water from the fuel cell whilst LOX stores must feed the reformer as well as the fuel cell. Output gases from the reformer are then passed through a gas purification system which removes unwanted gases so that sufficiently pure hydrogen can be passed into the fuel cell [58]. To maintain equilibrium within the submarine, carbon dioxide gas must be vented to the open sea whilst seawater is taken on as ballast [55]. This requires an exhaust treatment system which ensures exhaust gases can be released underwater without compromising the acoustic or thermal signature of the submarine. The system consists of two key components: a CO\({}_{2}\) compressor for release of gas at extended depths and a gas dissolution unit which dissolves exhaust gases into seawater prior to release from the submarine [55]. A diagrammatic example of a fuel cell operating with a reformer is shown in Figure 9a. Depending on the feedstock chosen, varying amounts of LOX, fuel and compensatory ballast volume is required due to the chemical composition of the feedstock and the reactions necessary to obtain usable hydrogen. A comparison, across three different feedstocks, of the collective volume required to achieve an equivalent AIP capacity is shown in Figure 10.  
The exact volume of reformer feedstocks would be dependent on several factors, mainly the characteristics of the fuel cell such as size and efficiency as well as the amount of energy--and therefore hydrogen--desired to be stored by the system. Figure 10 is intended to show the relative difference between feedstock options in terms of total volume of fluids required to store an equivalent amount of hydrogen suitable to feed the fuel cell.