IF GRAIN MUST be dragged to market on an oxcart, how far can it go before the oxen eat up all the cargo? This, in brief, is the problem faced by any transportation system in which the vehicle must carry its own fuel. The key value is the density of energy, expressed with respect to either mass or volume.

The era of large steam-powered ocean liners began during the latter half of the 19th century, when wood was still the world’s dominant fuel. But no liners fired their boilers with wood: There would have been too little space left for passengers and cargo. Soft wood, such as spruce or pine, packs less than 10 megajoules per liter, whereas bituminous coal has 2.5 times as much energy by volume and at least twice as much by mass. By comparison, gasoline has 34 MJ/L and diesel about 38 MJ/L.

But in a world that aspires to leave behind all fuels (except hydrogen or maybe ammonia) and to electrify everything, the preferred measure of stored energy density is watt-hours per liter. By this metric, air-dried wood contains about 3,500 Wh/L, good steam coal around 6,500, gasoline 9,600, aviation kerosene 10,300, and natural gas (methane) merely 9.7—less than 1/1,000 the density of kerosene.

How do batteries compare with the fuels they are to displace?

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