Page 200 - Materials Chemistry, Second Edition

186 LIFE CYCLE ASSESSMENT HANDBOOK

awareness by providing 'greener' products and using 'greener' processes. The
negative environmental impacts associated with products and processes have
become a significant concern, leading many companies to investigate ways to
reduce their impacts on the environment (Dincer and Rosen, 2007). Life cycle
assessment (LCA), which is a valuable tool for such activities, is essentially
a cradle to grave analysis for investigating and reducing the environmental
impacts of a system or process or product.
Exergy is a thermodynamic quantity that is used in assessing and improving
the efficiency of processes and systems, as well as their environmental impacts.
However, the concept of exergy has only begun to be introduced into the LCA
approach (Cornelissen, 1997; Dincer and Rosen, 2007). Exergy is defined as
the maximum amount of work which can be produced by a system or a flow
of matter or energy as it comes to equilibrium with a reference environment.
Exergy is a measure the potential of a system or flow to cause change as a
consequence of not being completely in stable equilibrium with a reference
environment (Rosen and Dincer, 2001). Unlike the energy, exergy is not subject
to a conservation law (except for ideal, or reversible, processes). The first law
of thermodynamics states that energy is conserved, i.e., cannot be destroyed.
Exergy, however, is not conserved, and is consumed or destroyed due to irre-
versibilities in any real process. The exergy consumption during a process is
proportional to the entropy created due its irreversibilities. Exergy is utilized
in assessments via the tool exergy analysis. In evaluating exergy and applying
exergy analysis, it is necessary to define a reference environment. This is com-
monly done by specifying its temperature, pressure and chemical composition.
The fundamental patterns and forces affecting changes in the environment
may be revealed by an understanding of the relations between exergy and the
environment. Therefore, integrating exergy concepts into LCA is important for
identifying and understanding the underlying reasons for many environmen-
tal impacts. There are various relationships between exergy and environmental
impact, including order destruction and chaos creation, resource degradation
and waste exergy emissions (Rosen and Dincer, 1997). These relations apply over
the life cycle of a process or product, supporting the inclusion of exergy in LCA.
Exergy losses, particularly due to the use of non-renewable energy forms,
need to be reduced to make societal activities and development more sustain-
able. Reducing the depletion of exergy resources and emissions of waste exergy
to the environment can improve the sustainability of people, industry and
nations. Although LCA can assist in achieving this objective, extending LCA with
exergy considerations can provide a complementary tool that reveals additional
insights. Exergetic life-cycle assessment (ExLCA) identifies the exergy utiliza-
tion and destruction during the life cycle of a system or product. Overall exergy
utilization and destruction cannot be properly assessed by examining only oper-
ation, but must consider all life stages from resource extraction to disposal.
In this chapter, comprehensive descriptions are provided of the link-
ages between exergy analysis and LCA, the rationale for ExLCA, the ExLCA
approach and methodology, applications of ExLCA and the advantages of
ExLCA over LCA. To illustrate ExLCA and its differences from LCA, a case