Gravimetric energy density as a transport constraint

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The quantity that decides which vehicles can be battery-electric and which cannot. Kerosene carries roughly forty-three megajoules of chemical energy per kilogram, about twelve kilowatt-hours; a complete lithium-ion battery pack carries something in the region of a fifth of a kilowatt-hour per kilogram at the pack level. That is a gravimetric ratio of order fifty to one. Two corrections apply and neither closes the gap. In favour of the battery, an electric powertrain converts stored energy to shaft work two to three times more efficiently than a gas turbine or a piston engine, so the useful-energy ratio is nearer fifteen or twenty to one. Against the battery, an aircraft burning fuel gets lighter as it flies — a long-haul jet can leave with more than a third of its take-off mass as fuel and land far lighter — while a battery aircraft carries its discharged mass to the destination, which worsens the range equation rather than merely shifting it. The consequence is a clean split. Where mass is a small fraction of the problem — cars, buses, urban delivery, short rail — electrification is straightforward and is happening. Where mass is the problem — long-haul flight above all — it is not a question of cheaper batteries but of a physical margin that incremental improvement does not cross. Short-range regional electric aircraft sit exactly at the boundary and are being flown; intercontinental ones are not near it. Not a place.

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