Energy Density and the Carbon Economy

I read an interesting article in the Bulletin of Atomic Scientists regarding one of the fundamental limitations of pursuing energy storage alternatives to hydrocarbons: hydrocarbons are incredibly energy dense.

Evaluating various energy storage media in terms of megajoules per kilogram, author Kurt Zenz House describes the attraction of hydrocarbons for energy storage: they store large amounts of energy in a material that is stable at room temperature and easy to transport, store, and convert. One kilogram of crude oil contains 50 megajoules per kilogram. Natural gas and coal are in this ballpark at around 55 and 20-35 MJ/kg respectively.

Batteries pack virtually no energy wallop by comparison. Lead-acid batteries store about 0.1 MJ/kg, about the same amount of energy stored in half a teaspoon of crude (2 ml). Lithium-ion batteries can store about o.5 MJ/kg, with a theoretical limit of 2 MJ/kg that has been elusive to achieve.


Capacitors and superconductors have still yet to exceed a density of 1 MJ/kg and the theoretical limits of these strategies are shy of 10% of hydrocarbon's phenomenal capacity. House describes other fanciful strategies including toroid carbon nanotubes, zinc-air batteries, elemental aluminum, without suggesting any real hope these strategies could reach their theoretical limits or feasibly be implemented.

Hydrogen combustion releases triple the energy per kilogram as crude oil. The only problem: hydrogen is a gas at standard temperature and pressure, and pressurizing hydrogen to 700 bars yields only 6 MJ/l compared to 34 MJ/l for gasoline at 1 bar. Pressurizing hydrogen requires the baggage of a heavy cylinder for vehicle transport, and heavy pipelines for line transport.

House concludes that hydrocarbons will be with us for a long time until these limitations can be superseded. House's comments mislead in several ways.

While batteries have low energy density, they require very little overhead for the conversion of their potential energy to kinetic energy. Small motors driving wheels are far lighter than heavy engines, transmissions, and exhausts required in combustion systems. When the bulk of the energy released in the internal combustion engine is dissipated as heat or consumed in moving the weight of the vehicle itself rather than the occupants, we see that hydrocarbons have limitations, too.


Moreover, biodiesel offers a way to fix carbon dioxide from the atmosphere and reintroduce it into the hydrocarbon/combustion energy infrastructure. You can regard biodiesel fuel generation as a form of solar power, fixing solar energy, CO2 and water into organic compounds that can be burned. For biodiesel, hydrocarbons remain the energy storage medium, but fossil fuels aren't used.

The biggest problem with biodiesel is that current paradigms require freshwater and arable land, commodities that will become increasingly scarce before fossil fuels do. New paradigms of biodiesel employ saltwater based algae, no land or freshwater required.

Algae: some food for thought. Before Soylent Green was people, it was algae. Is there nothing Charlton Heston can't teach us?