Page 202 - Design for Environment A Guide to Sustainable Product Development

Analysis Methods for Design Decisions 181

comparison of proposed designs based on an approximate bill of materials and
can display a variety of life-cycle indicators, ranging from a simple carbon or
water footprint to a comprehensive ecosystem goods and services consumption
profile [17].
Exergy Analysis
The latest advance in LCA involves modeling the material and energy
flows in complex systems based on the laws of thermodynamics.
Exergy is defined as the available work that can be extracted from a
material; for example, the exergy content of a fuel is essentially its
heat content [18]. More generally, exergy tends to be correlated with
material scarcity and purity, since it measures the difference of a mate-
rial from its surroundings. Bhavik Bakshi and his colleagues at The
Ohio State University have shown that all of the factors of industrial
production—energy, materials, land, air, water, wind, tides, and even
human resources can be represented in terms of exergy flows. There-
fore, exergy can be used as a universal indicator to measure eco-
efficiency and sustainability in industrial-ecological systems [19].
This method has the unique capability to quantify the contributions
of most ecosystem services and is particularly useful for analyzing
new technologies when detailed process-level data are nonexistent. It
is also useful for aggregation of environmental impacts, since it cor-
rectly accounts for the differences in quality among various resources
(e.g., energy from sunlight is much lower in quality than electrical
energy).
An example of an ethanol life-cycle assessment that incorporates
exergy analysis is shown in Figure 9.3. This study uses a hybrid
methodology, combining a detailed process model of corn ethanol
production with the above-described Eco-LCA™ model of the U.S.
economy (see below) to represent commodity flows from outside
the process boundaries. Based on this approach, Figure 9.4 shows
the results of a comparative life-cycle study of biofuels in terms of
two sustainability metrics—renewability (percentage from renewable
sources) and return on energy (mega joules delivered per megajoule
required over the life cycle). This analysis indicates that the sustain-
ability of fuel derived from municipal solid waste is far superior to
corn ethanol, which requires energy-intensive harvesting. Gasoline
has the highest return on energy, although it is not renewable [20].
Predictive Simulation
The methods described above are useful for assessing the perfor-
mance of a design with respect to specific environmental indicators.
However, at some point in the new product development process,
there is a need to consider the trade-offs between environmental fac-
tors and other important objectives—cost, quality, manufacturability,
reliability, and so forth. If environmental performance were inde-
pendent of these other factors, they could be analyzed separately.