Comparison of SOFC and PEM Fuel Cell Hybrid Power Management Strategies for Mobile Robots

Mobile robots for surveillance applications require quiet, high-endurance power sources for their long mission run times. This need cannot be met by batteries alone due to their low energy density or with combustion engines due to noise. A hybrid system combining a high power Lithium ion battery with a high energy density fuel cell (FC) system is ideal for this application. A propane powered 200W solid oxide fuel cell (SOFC) developed by Adaptive Materials Inc. (AMI) is an example of one such system. This fuel cell should be operated at full power to obtain maximum system efficiency or turned off to conserve fuel. This operating strategy with quantized power levels (either on or off) must be paired with a battery to provide the energy to re-start the fuel cell and power the vehicle during turndown periods. We compare the hybridized SOFC system to one with an open cathode Proton Exchange Membrane (PEM) Fuel Cell paired with a battery that allows the PEM FC to load-follow or load-level. The application to military tracked robots for two distinct profiles (realistic driving and scouting) is considered as an example and highlights the need for concurrent power source sizing and power splitting.