Green Engineering Principle #5

Output-Pulled Versus Input-Pushed

Products, processes, and systems should be "output pulled" rather than "input pushed" through the use of energy and materials.

Contributed by Michael A. Matthews, Professor of Chemical Engineering, Associate Dean for Research and Graduate Education, College of Engineering and Computing, University of South Carolina. Fellow of the American Chemical Society.

There is a familiar saying, attributed to Abraham Maslow (1966), that, to a man with a hammer, everything looks like a nail. The chemical and energy industries historically have been driven by inexpensive and abundant raw materials (the hammer): natural gas for fuels and chemicals, and coal for power plants. Very recently (Kaskey, 2013), improvements in drilling technologies have led to an increase in supply and decrease in cost of natural gas, encouraging both the fuels and chemicals industries to expand utilization of this essential clean resource for both power and chemical products (the nails).

The importance of inexpensive raw materials to the chemical enterprise cannot be discounted. Material availability, combined with the economies of large-scale facilities, have brought enormous value to the quality of life. Generic feedstocks and large facilities, however, will inevitably generate some waste. Principle 5 suggests that the availability of hammers is not the only consideration; rather, one must consider the real and immediate need for items to be nailed. The need should pull the act of production, rather than the ease and cost of production driving the need.

Three-dimensional printing (3D) is a most recent example of this principle in action. A diversity of products have been produced, ranging from automotive parts to a bronchial trachea for an infant. In 3D printing, the desired product is produced with layer-by-layer deposition of the appropriate resin. Biomedical applications, such as the artificial trachea, are an outstanding example of “output-pulled” manufacturing . The exact, unique dimensions for the individual patient are determined by a variety of medical imaging technologies. A digital model is then produced, and the model drives layer-by-layer printing of the product, with no wasted raw material.

Electrical power production is another area where Principle 5 is beginning to be implemented. Most power today is produced in centralized, large-scale plants, including hydroelectric facilities. Distributed generation is the use of small-scale converters to produce power as needed for a localized facility, such as a home or a single building. The generator is chosen to suit the availability of local resources, such as a solar cell array in sunny areas, or small wind turbines in windy areas. In the future, as the technology of biomass conversion increases, there is the prospect of building small power plants or even solid oxide fuel cells to convert biomass waste to electrical power, on-demand, to satisfy local demands. Where excess power can be produced locally, it can be fed back to the grid. Ideally this will reduce the need for new, large-scale power plants.

Chemical engineering education emphasizes the economies of scale of large plants. However, there are many examples of small-scale, specialty production facilities in existence, and the examples above illustrate additional opportunities. Future education should include the realization that sometimes the output-pulled approach to design and manufacturing will be best. Moving to a greener chemical and materials manufacturing enterprise calls for deeper understanding and the ability to balance and evaluate all principles of green chemistry and engineering.


Abraham H. Maslow (1966). The Psychology of Science. p. 15.

Jack Kaskey, Business Week, Chemical Companies Rush to the U.S. Thanks to Cheap Natural Gas. July 25, 2013. Accessed at

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