Page 210 - Materials Chemistry, Second Edition

196 LIFE CYCLE ASSESSMENT HANDBOOK

• The impact assessment of ExLCA focuses on the determination
of the exergies of the flows, and the exergy destructions and
exergy efficiencies of the overall process and its subprocesses.
Determination of exergy contents of flows is often a combination
of utilizing data reported in the literature or by relevant organi-
zations and calculating data with exergy analysis (as noted in
section 8.3.2). A limited impact classification phase is included in
ExLCA.
• The improvement analysis in ExLCA is intended to reduce its
life cycle irreversibilities (Hermann, 2006). The summation of all
exergy destructions in the life cycle, which are calculated using
exergy analysis, identifies the life cycle irreversibility of the prod-
uct or the process.

Throughout ExLCA, the calculation of exergy values requires that the condi-
tions and composition of the reference environment be specified.

8.4.4 Applications of ExLCA

Relations between exergy and environmental impact have been reported and
applied by many researchers. Some of the more significant applications are
described here.
Ayres et ah (1998) argue in an article on exergy, waste accounting, and life-
cycle analysis that thermodynamics offers a means of accounting for both
resource inputs and waste outputs in a systematic and uniform way. They con-
clude that exergy is appropriate for general statistical use, both as a measure of
resource stocks and flows and as a measure of waste emissions and the poten-
tial for causing environmental harm.
Daniel and Rosen (2002) examine material emissions produced during 13
fuel cycles for automobiles, on mass and exergy bases. Chemical exergies of
fuel life cycle emissions are compared with the masses of fuel cycle emissions.
For the emissions data used, the chemical exergy results suggest that com-
pressed natural gas use in motor vehicles produces emissions that are the fur-
thest from equilibrium with the natural environment, relative to all other fuel
life cycle paths considered. It is also shown that diesel use in grid-independent
hybrid electric vehicles has the lowest chemical exergies of emissions for all 13
fuel-vehicle combinations considered, suggesting a lower potential for envi-
ronmental impact. It is concluded that the exergy methodology presented for
assessing the potential for environmental impact may help in the develop-
ment and design of transportation technologies that are more environmentally
benign than those presently used.
Neelis et ah (2004) analyze a several hydrogen production and hydrogen stor-
age systems for automotive applications using exergetic life cycle assessment.
Eight different fuel supply and use chains are analysed exergetically. Exergy
analysis is shown to provide considerable additional useful information com-
pared to conventional energy analysis based on the lower or higher heating