Defining Energy Efficiency
We report two types of energy efficiency. The first is the overall “life cycle energy efficiency”. The
second is what we refer to as the “fossil energy ratio”. Each elucidates a different aspect of the life cycle
energy balance for the fuels studied.
The calculation of the life cycle energy efficiency is simply the ratio of fuel product energy to total
primary energy:
Life Cycle Energy Efficiency = Fuel Product Energy/Total Primary Energy
It is a measure of the amount of energy that goes into a fuel cycle, which actually ends up in the fuel
product. This efficiency accounts for losses of feedstock energy and additional process energy needed to
make the fuel.
The fossil energy ratio tells us something about the degree to which a given fuel is or is not renewable. It
is defined simply as the ratio of the final fuel product energy to the amount of fossil energy required to
make the fuel:
Fossil Energy Ratio = Fuel Energy/Fossil Energy Inputs
If the fossil energy ratio has a value of zero, then a fuel is not only completely nonrenewable, but it
provides no useable fuel product energy as a result of the fossil energy consumed to make the fuel. If the
fossil energy ratio is equal to 1, then this fuel is still nonrenewable. A fossil energy ratio of one means
that no loss of energy occurs in the process of converting the fossil energy to a useable fuel. For fossil
energy ratios greater than 1, the fuel actually begins to provide a leveraging of the fossil energy required
to make the fuel available for transportation. As a fuel approaches being “completely” renewable, its
fossil energy ratio approaches “infinity.” In other words, a completely renewable fuel has no
requirements for fossil energy.
From a policy perspective, these are important considerations. Policymakers want to understand the
extent to which a fuel increases the renewability of our energy supply. Another implication of the fossil
energy ratio is the question of climate change. Higher fossil energy ratios imply lower net CO2
emissions. This is a secondary aspect of the ratio, as we are explicitly estimating total CO2 emissions
from each fuel’s life cycle. Nevertheless, the fossil energy ratio serves as a check on our calculation of
CO2 life cycle flows (since the two should be correlated).