Page 225 - Materials Chemistry, Second Edition

EXERGY ANALYSIS AND ITS CONNECTION TO LIFE CYCLE ASSESSMENT 211
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while the highest ODP (1.37 x 10~ g Rll-eq) is observed for a plant capacity of
62,500 kg H 2 /day and a 10 year plant lifetime.
The ExLCA results can be observed in Figure 8.7, which gives all exergy effi-
ciencies and exergy destructions. The primary contributor of the life cycle irre-
versibility of nuclear-based hydrogen production is fuel (uranium) processing,
for which the observed exergy efficiency is 26.7% and exergy destruction is
2916.3 MJ. Nuclear plant operation exhibits a lower exergy destruction (673.8
MJ) and a greater exergy efficiency (36.4%) than fuel processing. The hydrogen
plant has the highest exergy efficiency (93.2%) and lowest destruction (8.6 MJ).
The significance of heat recovery in the cycle, which increases the exergy
efficiency of the hydrogen plant, can be observed in terms of acidification and
global warming potentials in Figure 8.12. It is seen in that figure that GWP can
be reduced to as low as 5.4 g C0 2-eq per MJ exergy of hydrogen if an exergy
efficiency of 98% is obtained. Also, AP can also be reduced from an initial value
of 0.041 to 0.027 g S0 2-eq per MJ exergy of hydrogen, if the exergy efficiency is
increased from 67% to 98%.

8.6 Conclusions

Exergetic life cycle assessment is described in this chapter, as is exergy and
the manner in which it can enhance life cycle assessment. Relations between
exergy and environmental impact are also discussed, to better demonstrate
the potential usefulness of exergy in addressing energy-related environmental
problems through its incorporation into LCA. Like LCA, ExLCA is an analyti-
cal tool to identify, quantify and decrease the overall environmental impact of
a system, process or a product, but the approaches of the two methods differ.
The differences of ExLCA from LCA are highlighted, the extension of LCA to
ExLCA is explained and the advantages of ExLCA are identified. ExLCA is
shown to be a useful method to address the irreversibilities associated with the
life cycle of a system in order to reduce the environmental impacts. The main
advantage of ExLCA over LCA is its incorporation of thermodynamic analysis
throughout the life cycle of a process or system.
The application and benefits of exergetic life cycle assessment are dem-
onstrated through a case study involving an environmental assessment of
nuclear-based hydrogen production via thermochemical water decomposi-
tion using the Cu-Cl cycle. Environmental impacts are quantified using LCA
and ExLCA, and exergy efficiencies and gaseous emissions are evaluated for
all process steps, including uranium processing, nuclear plant operation and
hydrogen production. LCA results are presented in four categories: acidifica-
tion potential, eutrophication potential, global warming potential and ozone
depletion potential. The ExLCA results indicate that the greatest irreversibility
is caused by uranium processing, and that the effect of plant lifetime on envi-
ronmental impact per per 1 MJ exergy of hydrogen production diminishes at
large scale production capacities.