Environmental Engineering Reference
In-Depth Information
vehicle) would provide a better perspective. The functional unit should be measurable, meaningful
to the intended audience, and relevant to data collection (Keoleian and Spitzley 2006).
11.1.3 m EtricS
LCA provides metrics that can be used to measure progress toward environmental sustainability
(Keoleian and Spitzley 2006). Metrics are established in the goal and scope definition phase of an
LCA. In this chapter, the net energy ratio (NER) is often used in biomass energy life-cycle studies
and is defined as the ratio of output energy (e.g., delivered energy such as transport fuel energy or
electricity) to input energy (total nonrenewable energy invested). The NER is a unitless intensity
factor that gives an idea of overall system efficiency. However, the NER gives no indication of crop-
land productivity (Liska and Cassman 2008).
Net energy value (NEV) is another commonly used energy metric in biofuel life-cycle studies.
An example formula used to calculate this metric, taken from a study presented later in this chapter
(Farrell et al. 2006), is shown in Equations 11.1-11.3.
NEV = Output Energy - Input Energy
(11.1)
Input Energy = (Agricultural Energy/Net Yield) + Biorefinery Energy
(11.2)
Output Energy = Fuel Energy + Co-product Energy
(11.3)
Agricultural Energy is the farm input, machinery, and labor energy required per hectare of culti-
vated cropland. It is calculated in units of megajoules (MJ) of input energy per hectare and includes the
following upstream and on-site energy types: embedded energy in seed, fertilizer, and farm machin-
ery; transport energy of farm inputs; farm input packaging energy; and on-site farm energy consump-
tion. The Net Yield is in units of liters of ethanol fuel per hectare. Agricultural Energy divided by Net
Yield is in units of MJ input energy per liter of ethanol. This is added to Biorefinery Energy , which
considers transport of corn to the refinery, refinery energy consumption, energy embedded in refinery
water, and equipment manufacturing (capital) energy—all per liter of ethanol delivered.
Critics of net energy metrics point out that they do not account for the quality of different energy
inputs (Dale 2007). For example, 1 MJ of coal is not equivalent to 1 MJ of petroleum in terms of
energy density or delivery requirements. Energy metrics that are more relevant to policy could be
in terms of petroleum displacement or fuel production per area of land used. However, no policy
decision should be based solely on the performance of a system in terms of a single metric. For
example, Farrell et al. 2006 determined that the life-cycle petroleum requirements of average corn
grain ethanol are less than those of cellulosic ethanol (note that technology for cellulosic ethanol is
in an earlier stage than corn ethanol). However, according to results from the same model, cellulosic
ethanol has a much higher life-cycle net energy value.
11.1.4 a llocation m EthodS
As biofuels are converted from biomass to refined liquid fuels through industrial processes, usable
or otherwise marketable co-products are generated. These co-products required energy inputs and
created impacts. Allocation is the assignment of responsibility for positive and negative impacts to
processes, products, and co-products. Allocation methods vary; different studies have used mass,
market value, energy content, equivalent product displacement, and service time as bases.
Search WWH ::




Custom Search