Instead of being stored in batteries, the electricity made by solar panels can be used to split water into oxygen and hydrogen molecules, the latter of which can be used as a fuel with high energy density.

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Viruses actually need population density as fuel.

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Chu conceded that oil is a unique fuel, pairing high energy density with ease of transport.

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But for anything much bigger, the energy density of aviation fuel still looks to be unbeatable.

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Proponents point to the enormous energy density of the fuel (the amount of energy contained per unit).

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Every 12 months or so GM develops a new generation of fuel cells, and recent improvements in power density and part streamlining are impressive.

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This amount of power density far exceeds anything else done with microbial fuel cells.

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Tourism is creating other problems for the local environment, too, including increased population density, over-dependence on wood as a fuel and construction source and the overgrazing and cutting of mountain slopes to feed livestock.

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