Major Energy Metrics

A variety of metrics are used to quantify the consumption of primary energy in the production of biofuels (Murphy et al. 2011b; Cleveland 2010; Chwalowski 1996). In contrast to the process energy used directly in a given process, primary energy represents the total of all energy resources consumed to produce the materials and energy used in the process. For example, operating a 40 W light bulb for 1 h would use 40 W-hours of process energy (electricity), while the total primary fuels consumed to generate and transmit that required electricity would be on the order of 108 W-hours (assuming a primary energy factor of 2.7) with wide variability due to specific geographic location, technologies, and above all the specific mix of primary fuels used to generate the electricity.

Several principle energy metrics are defined and summarized in Table 18.1. One key measure considered for biofuels is the net energy balance (NEB) also

Table 18.1 Key energy metrics



Formal definition

Net energy balance


Energyoutput — Energyinput

Energy return on investment


Energy Output Energy Input

Net energy ratio


Energy Output Energy Input

Fossil energy ratio


Energy Output Fossil Energy Input

Energy breeding factor


Energy Output

Y Nonrenewable Energy Input

known as net energy value (NEV). NEB is defined as the difference between the energy in the biofuel product (and coproducts) and the total primary energy required to produce the fuel. A positive net energy balance is one criterion for sustainable fuel product. The energy return on investment (EROI) and net energy ratio (NER) represent ratios of the total energy in the biofuel product and its coproducts to the total primary energy required to produce them. NER values greater than one correspond to a positive energy balance.

A variation of energy return on investment, fossil energy ratio (FER) considers only the consumption of nonrenewable fossil fuel resources and is defined as the ratio of the sum of energy content of the products over the primary fossil fuel inputs consumed during production. This metric considers only the nonrenewable fossil fuel components of primary energy and serves to evaluate how much of fuel product is generated per unit investment of fossil fuels. FER values greater than 1 are net fossil energy positive, reflecting more energy in the products than the fossil energy consumed during production. Thus, FER is a measure of the renewability of the fuel. The energy breeding factor, an analogous metric, represents the energetic return per unit of nonrenewable energy consumed.

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