The world faces significant challenges as population and consumption continue to grow while nonrenewable fossil fuels and other raw materials are depleted at ever increasing rates. Green Engineering is a technical approach to address these issues using engineering design and analysis.
The scope of Green Engineering depends upon one's perspective and discipline, but it is broadly defined as minimizing environmental impacts across all life cycle phases in the design and engineering of products, processes, and systems.1,2 It is important to realize that Green Engineering is only one possible approach to addressing the larger issue of sustainability which includes environmental, economic, and social aspects.
Green Engineering is necessarily interdisciplinary, and therefore, is best considered as a set of concepts which can be applied across engineering disciplines. That said, the discipline of Materials Science and Engineering is a focus area since the design of essentially everything one can imagine requires materials. As such, materials scientists and engineers have a responsibility to design using materials which not only meet design requirements, but also consider the environmental, health, and societal impacts across the life cycle.
The life cycle of a product (you can typically interchange "product" with "process" or "system" throughout this discussion) is a key concept for Green Engineering materials considerations. The life cycle consists of all the inputs and outputs required from extraction to manufacturing to use, and finally to disposal.
For any given product, raw materials are extracted from the earth resulting in depletion of these materials as well as waste, ecosystem disruption, and energy use. Raw materials are then manufactured (refined, purified, mixed, formed, alloyed, chemically reacted, shaped, etc.) to transform them into materials which have desired properties and performance. In this step, energy and chemicals are the primary inputs with solid, liquid, and gaseous outputs to the ground, water, or atmosphere. The use of many products requires energy or fuel as well as additional chemicals or materials for maintenance. Finally, at the end of a product's useful life it is disposed typically by landfill burial or incineration. In some cases, products might be reused, composted, or recycled. Each of these disposal methods carries potential environmental impacts. Transportation also needs to be accounted for within and between life cycle phases.
Life Cycle Assessment (LCA) is a powerful analytical tool with many uses in Green Engineering. This formal discipline has specific methodologies and international standards (ISO 14040: 2006 and ISO 14044: 2006)3 to objectively quantify the environmental impacts of all inputs and outputs for a product. LCA can be used to compare environmental impacts of different products or different life cycle phases of a given product. It is important to understand that while the analysis of inputs and outputs as well as their environmental impacts can be quantified objectively using current scientific understand, decisions based on these results necessarily depend on the weighting of various environmental categories and are therefore subjective.
As shown schematically in Figure 1, there are typically inputs and outputs for each of the life cycle phases. To compare the environmental impact between products or between phases for a given product, the inputs and outputs must be quantified in terms of environmental impacts. For complex products, there may be hundreds to thousands of inputs and outputs. As such, the boundaries of an LCA must be carefully chosen to balance complexity with accuracy.
Various software packages and databases allow for the complex analysis required for a thorough LCA.4,5 Materials scientists can take advantage of such information, even if not conducting an LCA, to obtain data regarding the inputs, outputs, and embodied energy for most commonly used engineering materials. Materials specific design software also can provide useful data such as materials' embodied energy, reserves, recyclability, or toxicity for considering specific environmental impacts without conducting a full LCA.6
The benefits to society of implementing green engineering concepts specifically to materials science are significant and include health benefits, improved environmental quality, and cost reductions. Explicitly considering the environment as an initial design constraint along with economic and performance metrics is critical; many significant environmental impacts that would be difficult to remediate can be minimized if considered early. Moreover, applying Green Engineering concepts early in the design stage provide benefits which compound throughout the life cycle. For example, using less mass in a product due to better design will require less raw material extraction, less material manufacturing, less material transport, and less material disposal. In this example which is relevant to almost all products, a seemingly small reduction in material mass can have a significantly larger life cycle environmental benefit especially for high volume products.
Green engineering concepts for materials should also be applied outside of the design stage in any of the life cycle phases. Examples of this include substituting materials with similar performance but lower embodied energy, larger natural reserves, less energy- or chemical-intensive extraction or manufacturing, lower toxicity, less maintenance, better recyclability, or longer durability. Opportunities to incorporate such thinking are all around us using both current technology and innovative new thinking.7,8
The transition of standard engineering practices and education to include concepts of Green Engineering for Materials Science and Engineering will not happen overnight. Given the potential benefits, however, it is important for materials professionals to start this transition now rather than later. The process starts by asking some basic questions whenever materials are considered for a product:
1) From where and how is the material physically extracted from the environment?
2) What are the energy and chemicals requirements to manufacture the material?
3) What are the inputs/outputs for the use/maintenance of the material over its life cycle?
4) What is the ultimate fate of the material when the product is disposed?
The answers to these basic questions are generally not simple and should lead one to carefully consider the environmental impacts of materials choices.
References and Further Reading
1. P. Anastas and J. Zimmerman, "Design Through the 12 Principles of Green Engineering", Environmental Science & Technology, 37(5), 2003, pp. 94A - 101A.
2. United States Environmental Protection Agency (EPA)
3. International Organization for Standardization (ISO)
4. Pré Consultants, www.pre.nl/simapro/
5. PE International, www.gabi-software.com/
6. CES EduPack, Granta Design Limited, http://www.grantadesign.com/education/index.htm
7. P. Hawken, A. Lovins, and L. H. Lovins, Natural Capitalism - Creating the Next Industrial Revolution, Back Bay Book, 2000.
8. W. McDonough and M. Braungardt, Cradle to Cradle - Remaking the Way We Make Things, North Point Press, 2002.
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