Concerns over the depleting fossil energy sources, mounting regulatory compliance and the resulting push towards environmentally conscious process design in the last decade are forcing designers to consider reduced environmental impact as one of the product design objectives. With greater appreciation of such challenges, systems engineering has gradually broadened its analysis boundary from the equipment and manufacturing scale to the life cycle or value chain scale. Selecting the system boundary by expansion from the manufacturing to the value chain scale is appealing for environmentally conscious decision making, however the approach still relies on selecting the most relevant processes of the value chain. Such selection, as is the case with traditional life cycle assessment (LCA) is often quite arbitrary, subjective, and vulnerable to convenient drawings of the boundaries. On the other hand, expanding the system boundaries to include all interactions is computationally intractable and time consuming. In reality, data and models are available at multiple spatial scales ranging from individual equipment and processes, to the supply and demand chains, to the economy and ecosystems. The outstanding challenge here is the development and adoption of a broader multiscale and multiobjective approach that systematically accounts for all the conflicting design objectives as opposed to the narrowly focused cost-benefit analysis.

This work presents a novel multiscale and multiobjective optimization oriented approach to utilize the available information at multiple scales for gaining insights into the trade-off between ecological and economic aspects of manufacturing processes thus yielding a comprehensive scenario on which process alternatives can be evaluated. Economic aspects are accounted via traditional cost analysis. Ecological aspects are considered via exergy analysis of the inputs at each scale, and depend on the selected processes. Exergy or available energy accounts for the first and second laws and is the ultimate limiting resource since it is lost in all processes. This multiscale approach represents inputs and outputs in terms of cumulative exergy consumption (CEC). The scales considered in this work are the individual equipment or process manufacturing, value chain, economy, and ecosystem scales. The finest equipment or manufacturing scale corresponds to a traditional exergy analysis [1]. The trade-off between ecological and economic aspects is usually quite large at this scale because the narrow manufacturing boundary ignores the environmental impact of other processes in the life cycle that incur an economic cost such as capital equipment. The next coarser scale is at the value chain scale and corresponds to an exergetic life cycle analysis. The boundary is further expanded to include activity in the entire economy by combining exergy analysis with economic input-output analysis [2]. Finally, contributions from ecosystem goods and services are included at the coarsest ecosystem scale [3].

The utility of the proposed multiscale and multiobjective approach and its benefits over existing methods will be illustrated using the case studies of a heat exchanger and chemical product design. Trade-off between the economic and ecological objectives will be represented via a series of pareto optimal surfaces at various scales, thus avoiding arbitrary combinations until the final stages of decision making. The implications of the proposed approach in guiding ecologically and economically conscious methods and heuristics for broader engineering design will be described. Current work is in progress to explore the use of stochastic modeling for incorporating uncertainty information in the proposed methodology.

References

1. J. Szargut, D. R. Morris, and F. R. Steward. Exergy analysis of thermal, chemical and metallurgical processes. Hemisphere Pubs., New York, 1988.

2. N. U. Ukidwe and B. R. Bakshi. Industrial and ecological cumulative exergy consumption of the united states via the 1997 input-output benchmark model. Energy, 32(9):1560–1592, 2007.

3. J. L. Hau and B. R. Bakshi. 2004. Expanding exergy analysis to account for ecosystem products and services. Environmental Science and Technology. 38(13): 3768-3777, 2004.