The design of products, processes, and systems with explicit consideration of potential environmental impacts. Green engineering does not set environmental objectives as higher priorities than other design requirements, but it does require them to be considered explicitly. Like traditional engineering, green engineering solutions must meet design constraints as well as optimize objectives. Unlike traditional engineering, this new approach looks beyond performance and costs and adds the objective of minimizing environmental impacts (see illustration). This design balance is difficult, and many environmentally focused products have failed in the marketplace because of poor performance, higher safety risks, or unreasonable costs.
Engineering applies math and science to design products, processes, and systems. In its best forms, engineering improves the quality of life across society by providing clean water, healthy and abundant food, efficient transportation, comfortable housing, access to information, and advanced medical treatments. Engineering advances since the Industrial Revolution have led to tremendous increases in life expectancy, economic growth, and quality of life. This progress, however, has come with external costs in the form of increasing environmental damage and depletion of resources. See also: Wastewater reuse; Water supply engineering
Many of the environmental issues resulting from engineering practice are not obvious to citizens and consumers because they happen behind the factory doors, often in distant lands, and they may accumulate slowly over time. These issues include depletion of resources and the degradation of ecosystems, as well as increasing concentrations of hazardous substances in the air, soil, and water. As both populations and individual incomes increase globally, engineering plays a critical role in providing the products, processes, and systems demanded by consumers to improve their lives. However, as the scale of engineering grows around the world, so do the environmental impacts. Scientists continue to research and document problems such as deforestation, bleaching of coral reefs, ozone depletion, increased atmospheric carbon dioxide, mineral and fossil-fuel depletion, soil loss and degradation, and water pollution. Given Earth’s finite supplies of fuels and materials, and the increasing consumption of resources, it is clear that we cannot continue on this path and sustain our lifestyles indefinitely. Green engineering offers one potential path to a more sustainable society. See also: Coral bleaching; Deforestation; Hazardous waste; Hazardous waste engineering; Soil degradation; Stratospheric ozone; Water pollution
No single definition captures all of the complexities of sustainability. At a minimum, sustainability must address the so‐called triple bottom line, which includes the environment, society, and economics. As ecosystems are damaged, human quality of life and the costs to maintain it are affected. While sustainability is an abstract concept, green engineering is more practical and tangible. It can be thought of as a set of technical concepts and skills that people can use to design products at manageable costs that are better for society and the environment.
Green engineering quantifies environmental impacts to help make more sustainable design choices. Without quantifiable metrics, it is difficult to determine which choices are best from an environmental perspective. For example, one can claim that electric vehicles have lower carbon dioxide emissions than gasoline vehicles, but a quantified comparison of gasoline vehicle emissions to the emissions from electricity‐generating power plants must be done to know which vehicle has less air emissions.
Furthermore, environmental impacts must be considered over the entire life cycle; otherwise, improvements in one part of the life cycle can lead to larger problems in another area. Nuclear fuel for electricity generation produces relatively low carbon emissions, but the trade-off is radioactive and toxic waste materials that must be managed for centuries in the disposal phase. In the past, engineers often made environmental decisions rather loosely, based on intuition rather than life-cycle assessments. This often led to poor decisions from an environmental perspective, because the complexity of the full life cycle cannot be judged accurately without detailed analysis. Green engineers minimize environmental damage by understanding the details of the different life-cycle phases and making choices that reduce environmental impacts without sacrificing other critical constraints. See also: Nuclear fuel cycle
In this sense, green engineering is a form of systems engineering, because there are complex interactions among all the components in the life cycle. For example, while engineers have traditionally been trained to focus on performance when selecting a specific material for an application, it is important that they understand that this choice affects the entire system. Materials extraction and manufacturing varies by material, as do the options for their end of life. Systems engineering is inherently interdisciplinary, because expertise from different fields is required to understand and assess each phase of the life cycle. With this systems perspective in mind, it is easy to understand that green engineering is most effectively employed early in the design phase, because changes there can have cumulative benefits across all life-cycle phases. See also: Systems engineering
Extraction is the start of the life cycle and is dictated by raw material and chemical selections. Minerals, metals, and fossil fuels are mined and then refined with various levels of environmental degradation. Supplies of these materials are finite, so society’s use depletes them. Other materials, including wood, plant fibers and chemicals, food, and other biomass, are renewable in the sense that they can be regenerated. However, these processes are sustainable only if we regenerate the resources faster than we remove them, and if we don’t damage the ecosystems that support them. Green engineers strive to select materials that are abundant and can be extracted with less energy and ecosystem damage. See also: Forest timber resources; Mining; Oil and gas well drilling; Renewable resources
Manufacturing takes the extracted raw materials and transforms them into useful products with specific functions and advanced properties. Like extraction, manufacturing requires electricity, heat, and various indirect chemicals, such as solvents, which are emitted intentionally below federal limits or unintentionally by accidents. Regular events, such as earthquakes, hurricanes, oil spills, chemical leaks, manufacturing accidents, or illegal emissions, continue to affect workers, the public, and the environment locally, regionally, and globally. Green engineers strive to select less toxic chemicals, develop processes that use less energy and materials, or design more safeguards into manufacturing processes that require the use of hazardous substances. See also: Green chemistry; Sustainable materials and green chemistry
End of life
End of life or disposal is the last phase of the life cycle. All products must go somewhere at the end of their useful lives. The most common options are landfills, incineration, and recycling, and each has trade-offs. Landfills produce methane from anaerobic digestion of organic matter, but this strong greenhouse gas can be captured and used as a fuel. Incineration recovers some of the embodied energy from materials that would otherwise be landfilled, but the environmental trade-off is carbon dioxide and other atmospheric pollutants from combustion. Recycling saves large amounts of embodied energy and ecosystem disruption because the extraction phase is avoided completely, but energy is still required for the transportation and recycling processes. Green engineers strive to reduce all forms of waste and to develop closed‐loop recycling processes. See also: Air pollution; Recycling technology; Waste-to-energy
Transportation is required to move raw materials, finished products, and waste through the entire system. Emerging electric vehicle systems are more energy efficient, but the emissions for these systems depend on the fuel sources that generate the electricity. Moreover, a transition to electric vehicles requires a major conversion of infrastructure for the electrical grid. See also: Electric vehicle; Transportation engineering
Life-cycle assessment (LCA) is a quantitative technique to quantify the environmental impacts of products, processes, and systems. It is a key green engineering tool that allows quantitative comparison of products across the life cycle to help with sustainability decisions. There are other useful environmental assessment tools, but LCA covers the broadest set of environmental impacts and is the most detailed. Energy Star® rates products based only on energy use. Similarly, carbon and water footprints are one‐dimensional ratings based on carbon dioxide emissions and water use, respectively. The Leadership in Energy and Environmental Design (LEED) rating system measures a number of key environmental impacts for building design, construction, and operation. See also: Architectural engineering; Buildings; Civil engineering; Life-cycle analysis of civil structures; Resilient building design
Engineering has the potential to either make the world more sustainable or to contribute to the growing environmental problems. Green engineering is a conscious design effort to analyze environmental impacts across the full life cycle to make products, processes, and systems that are better for all aspects of the triple bottom line. Adding environmental constraints makes the job of engineers more difficult, but also provides a path to sustainable quality of life for societies in the long run.