Fuel Cells in Small AUV’s
The focus of this project is to evaluate the possibility of using fuel cell systems for electrical powering of AUVs. Fuel cells convert a fuel and an oxidant, in most applications hydrogen and oxygen gas, directly into electrical power at high efficiency only forming water. Larger fuel cell systems have started to be used for combined heat and power generation, and for the propulsion of vehicles. Smaller fuel cell systems have reached some maturity and can today be found as a complement for batteries in some portable electronics and for backup purposes. However, using smaller fuel cells for underwater applications still involves some major challenges.
Hydrogen gas stored in a pressure vessel or as a hydride is likely the most suitable fuel for this application. The hydrogen gas can easily be converted in the fuel cell but has the disadvantage of low energy density and specific energy when stored. However, a liquid fuel alternative like methanol would make the rest of the system much more complicated, especially in a closed compartment where formed carbon dioxide would need to be separated and released. Air is normally used to supply oxygen to fuel cells, except in space and under water applications where the use of pure oxygen gas has advantages. Thus, the most suitable way for AUVs to bring oxygen gas needs to be evaluated. A potential complication that needs to be investigated is the operation of the fuel cell at higher ambient pressures, which may occur at higher depths. A further possibility would then be to actually use the fuel cell for adjusting the gas pressure inside the AUV.
The energy efficiency of the system will not only depend on the fuel cell, the power electronics as well as the electrical motor for propulsion must be well matched. The electrical power will besides propulsion also be used for communication as well as other electronic equipment on-board. The operations of these are many times intermittent, and to reduce the peak-power demand of the fuel cell system hybridisation of the power system by means of a rechargeable battery may prove to be efficient, and needs to be evaluated.
Contact: Göran Lindbergh (KTH)