Environmental Engineering Reference
In-Depth Information
Figure 2.1.
Energy density in terms of mass and volume of different materials and fuels.
Hydrogen has a very high energy density per kilogram of mass, but being a gas, even at
700 bar of pressure its volume is large and it has a low energy density by volume.
Batteries have low energy density both by volume and mass. Liquid fuels offer the best
compromise.
Oil: The Slippery Slope to Fossil Fuel Dependency
When we think of oil fields, there's a tendency to imagine viscous underground lakes.
However, the word 'petroleum', from the Greek
petra
(rock) and
elaia
(oil), betrays its
true origin: as oily rock. Because this rock is buried deep within the Earth and is subject to
enormous pressure, once we drill into the rock, the liquid is naturally pushed to the surface.
Oil is a complex solution of different hydrocarbons; long molecules made of a scaffold
of carbon atoms to which hydrogen atoms are bound (see
Figure 2.3
). The longer the
molecule, the heavier and more viscous the substance. Substances with shorter molecules,
such as methane, butane and propane, are gaseous at normal temperature and pressure (see
Figure 2.2
)
.
Figure 2.2.
A 3-D model of methane and propane molecules, the two main components
of natural gas and liquefied petroleum gas (LPG). They are composed of carbon and
hydrogen atoms (hydrocarbons). Molecules with 1 to 4 atoms of carbon in each molecule
are, respectively, called methane, ethane, propane and butane.
Figure 2.3.
A 2-D model of octane, a molecule common in petroleum. It has 8 carbon
atoms in a chain attached to hydrogen atoms. Of all hydrocarbons, octane makes the best
motor fuel. Therefore, any motor fuel that burns as well as pure octane is termed 100
octane. Most fuels are between 85 and 90 octane.
