Direct methanol fuel cells are a thing. It may not fit your definition of efficient but that is the technology I think should be pursued. There are multiple bi

Direct methanol fuel cells are a thing. It may not fit your definition of efficient but that is the technology I think should be pursued. There are multiple biological, chemical and electrochemical pathways to produce methanol. That means that there could be an economic way to produce it nearly everywhere. Another interesting technology is redox flow batteries. The fluid itself is charged. Fluid storage can be sized to the charge/use requirements. Or you can haul in "charged" fluid. But since the fluid is not consumed, discharged fluid would need to be taken away. Making hauling is less efficient.