Very misleading to describe this as a source of energy and also highly misleading to dwell on the amount of aluminium ore in the earth's crust as if that's an immediate source of energy. To convert all the world's aluminium ore deposits into the metal would probably consume all the earth's energy sources many times over. The aluminium metal in the process described is consumed and has been previously refined into the metal using huge amounts of electrical energy as is the norm for aluminium production. Converting the waste aluminium oxide back into aluminium uses about as much electrical energy again. This energy would come from conventional CO2 producing fossil fuelled power stations. Or from a nuclear smelter like the Wylfa one presently irradiating Anglesey. Or it could come from the new hydroelectric smelters that are presently destroying irreplaceable glacial wilderness on Iceland.

Also gallium is a rare element, never found at more than 50ppm concentrations in various minerals and ores. Because of this it costs around 700$/kg. This scarcity would be likely to make the price soar if cars started carrying around several kgs of it so the Al/Ga 'fuel' slabs would have to have a hefty deposit.