ABSTRACT
So far, all our contributions and attention have been focused on the thermodynamic ef ciency of processes. Theory provided us with thermodynamic tools with which we could establish the difference between the real and the minimum amounts of work required and identify this difference as lost work. We could also show the way to keep these losses to a minimum, given the various constraints to the process: thermodynamic optimization. But operating processes ef ciently is not enough; an emerging requirement is that our technology also be sustainable. Most people have an intuitive notion of what sustainability is, but our examples will show that there is a need for substantiation and, if possible, for quanti cation of this concept. Biology shows us that nature most probably provides us with what is, in our opinion, an elementary and perfect example of sustainability. Similarly, a simple economic analysis points to the essentials of what a sustainable economy is and what this implies for operating our industry. One conclusion seems to be that technology needs to learn some essential lessons from nature. Industry, mainly based on nonrenewable resources, should be transformed into one based on renewable resources. In more general terms, a society, driven by material energy sources with, as a consequence, material emissions, should be transformed into one driven by immaterial sources of energy such as radiation, wind, geothermal energy, and so on, resulting in nonmaterial emissions.
