Types of Life Cycle Energy Inputs
In this study, we track several types of energy flows through each fuel life cycle. For clarity, each of
these energy flows is defined below.
· Total Primary Energy. All raw materials extracted from the environment can contain7
energy. In
estimating the total primary energy inputs to each fuel’s life cycle, we consider the cumulative energy
content of all resources extracted from the environment.
· Feedstock Energy. Energy contained in raw materials that end up directly in the final fuel product is
termed “feedstock energy.” For biodiesel production, feedstock energy includes the energy contained
in the soybean oil and methanol feedstocks that are converted to biodiesel. Likewise, the petroleum
directly converted to diesel in a refinery contains primary energy that is considered a feedstock
energy input for petroleum diesel. Feedstock energy is a subset of the primary energy inputs.
· Process Energy. The second major subset of primary energy is “process energy.” This is limited to
energy inputs in the life cycle exclusive of the energy contained in the feedstock (as defined in the
previous bullet). It is the energy contained in raw materials extracted from the environment that does
not contribute to the energy of the fuel product itself, but is needed in the processing of feedstock
energy into its final fuel product form. Process energy consists primarily of coal, natural gas,
uranium, and hydroelectric power sources consumed directly or indirectly in the fuel’s life cycle.
· Fossil Energy. Because we are concerned about the renewable nature of biodiesel, we also track the
primary energy that comes from fossil sources specifically (coal, oil, and natural gas). All three of
the previously defined energy flows can be categorized as fossil or nonfossil energy.
· Fuel Product Energy. The energy contained in the final fuel product, which is available to do work in
an engine, is what we refer to as the “fuel product energy”. All other things being equal, fuel product
energy is a function of the energy density of each fuel.