Energy Storage Capacity

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

There is a lot of discussion at present of what we can expect from improved batteries, or ultracapacitors, or flywheels, or hydrogen, as possible media for storing the energy used in vehicles. We’ve all grown up in a society that powers its cars, trucks, motorcycles, ships, trains, and aircraft with liquid hydrocarbon fuels derived from crude oil. Not only is the supply of crude oil finite, but mathematical climate models and the melting of polar ice suggest that carbon dioxide created by combustion of such fuels may be causing global warming by acting as a heat-reflective atmospheric blanket, retaining solar heat that would otherwise have been radiated into space (hence “the greenhouse effect”).

What If We Powered Airliners With Firewood?

New forms of storage would allow carbon-free energy derived from non-combustion sources such as wind and solar to be compactly stored on, and used to propel, our vehicles. Just this morning I found a table of the energy storage density (in megajoules per kilogram) of many present-day and possible future media. It appeared as an article in a 2009 issue of the “Bulletin of the Atomic Scientists.”

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One megajoule (MJ) equals approximately 278 Watt-hours, or 0.373 horsepower-hours. One kilogram (kg) is 2.204 pounds. Sharpen your pencil and you’ll see that a gallon of crude oil stores about 48 horsepower-hours of energy.

Storage Medium	Energy in MJ/kg	As a % of That of Crude Oil
Crude oil	47.0	100%
Natural gas	55.0	117%
Firewood	14.9	32%—theoretical
Firewood	10.4	22%—in practice
Coal	20.0–35.0	43–75%
Jet-A aircraft fuel	43.0	92%
Nuclear reactor fuel	800,000	1,700,000% —1% burn-up (energy rich!)
Lead-acid battery	0.1	0.2%—in practice
Lead-acid battery	0.7	1.4%—theoretical
Lithium-ion battery	2.0	4%—theoretical, carbon anode
Lithium-ion battery	3.0	6%—theoretical, silicon anode
Lithium-ion battery	0.5	1.1%—actual
Hydrogen-scandium	5.0	10.6%—note: scandium is extremely rare
Zinc-air battery	5.3	11.3%—theoretical
Zinc-air battery	1.3	2.7%—present level
Aluminum	32.0	68%—burned in air @ 100% efficiency
Lithium	43.0	92%—burned in air @ 100% efficiency

The article signed off by noting that hydrocarbon fuel will remain at least 25 times more dense an energy storage medium than even ideal future lithium-ion batteries. Because a Boeing 787 burns about 12,000–13,000 pounds of Jet-A per hour in cruise, energy storage for a 10-hour flight in a version powered by battery-driven electric motors would weigh more than 3 million pounds (12,500 x 10 x 25 = 3,125,000), or more than ten times the 285,000-pound empty weight of the aircraft. For this reason new proposals are being made to power future aircraft by using clean wind/solar/other electricity to drive chemical reactions producing synthetic liquid fuels.

Some other interesting figures I found are that hydrogen, compressed to 700 bar (10,000 psi) gives 6 MJ per liter, as compared with gasoline (at atmospheric pressure) with 34 MJ per liter. Liquefying hydrogen makes it “smaller” but considerably increases the price of production and requires excellent insulation to slow its boil-off. It is proposed instead to contain hydrogen by letting it diffuse into titanium chips, as few of us would choose to travel next to a container of hydrogen at 10,000 psi.

Toward the end of World War II it was fairly common in fuel-starved Europe and Asia to power civilian and a few military cars and trucks with the combustible gas given off by dry firewood heated in a sealed container. Plant trees as you go and one day you may reach carbon neutrality!

How does the story end? Will all political persuasions agree on one binding world government that carries out a solution satisfactory to all? Will Mr. Gore bring us another movie that converts us all to his opinions on global warming and what is to be done about it? Or will we muddle along as always?

Live long and learn more.

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