Question

An electrical energy storage system is based on a 30 W PEM fuel cell (mass 285 g, efficiency based on HHV of 50%) and a number of metal hydride hydrogen storage canisters each capable of storing up to 1.2 wt% hydrogen with an uncharged mass of 134 g (NB 100% includes mass of canister plus hydrogen here). At what minimum total electrical energy delivery capacity would this system have a system gravimetric energy density advantage over a battery bank based on a number of lithium polymer batteries, each weighing 88 g and rated at 1800 mAh with a nominal voltage of 7.4 V? Assume an 80% depth of discharge for each battery and a 12% drop in voltage, linear with usage, during discharge. Consider just whole numbers of MH canisters and batteries. At the gravimetric energy density crossover point, how many MH canisters would the hydrogen fuel system employ and how many batteries (rounded to the nearest whole numbers)?

Answer 1