Sea Technology

NOV 2017

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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26 st / November 2017 www.sea-technology.com Long ago it was realized that pure aluminum undergoes an energetically favorable disassocia- tion reaction with water; H 2 O reacts with pure alu- minum to provide pure H 2 and a benign byprod- uct. The byproduct depends on the temperature of the reaction but all are a mix of aluminum, hydro- gen and oxygen atoms and are found in the natural ore bauxite. Conveniently, the liquid can be very "dirty" and still perform well, as long as there is H 2 O—tests with seawater operated just fine. This reaction has been studied intensively for decades in many countries. The problem historically has been that aluminum quickly forms a very tough oxide layer when exposed to air. This prevents further oxidation and is what enables the widespread use of aluminum in so many other ways. However, if the oxide layer can be penetrated or pre- vented from forming, then a very dense form of energy stor- age can be developed. To be a useful process, it needs to be consistent, cheap and reliable to produce. Over the past several years, GA has developed an alloy of aluminum with several trace elements and an industrial production process to enable its use as an energy store. Testing of this material has shown that it can produce up to four times more pure hydrogen than liquid hydrogen, after accounting for yield, system piping, etc. One aspect of this reaction that must be considered is that the byproduct material is about nine times larger by volume than the aluminum alloy fuel. Thus, the system must either be designed to carry this byproduct until refueling, or can flush it to sea. A system that uses seawater for the reactant and flushes the byproduct to sea is most energy dense and may reach 10 times the energy density of lithium-ion batteries. nearly ended the lives of those aboard Apollo 13 in 1970. There are different types of fuel cells, and one that has gained traction for undersea use is the Proton Exchange Membrane (PEM) fuel cell, which is fed pure oxygen and hy- drogen. This type of fuel cell has several advantageous fea- tures for undersea use: relatively low temperature operation, high efficiency, high power and demonstrated performance. However, the materials and construction cost is compara- tively high. In many undersea applications the price is worth the performance. Supplying oxygen and hydrogen to such a fuel cell can be done in numerous ways. Historically, the gasses were generated by standard industrial means, then bottled into pressurized cylinders up to 10,000 psi. To get some ad- ditional capacity, sometimes the oxygen and/or hydrogen are kept as liquids at low temperatures. This requires bulky thermal insulation schemes. Both pressurized gas and liquid storage are conceptually straightforward but have boil-off issues; practically speaking, there is a finite lifetime to the storage since there is inevitably some small amount of leak- age. (Left) Hydrogen storage chart. (Right) ALPS fuel cell materials.

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