Critical Review Exergy: Its Potential and Limitations in Environmental Science and Technology JO DEWULF,* ,† HERMAN VAN LANGENHOVE, † BART MUYS, ‡ STIJN BRUERS, ‡ BHAVIK R. BAKSHI, § GEOFFREY F. GRUBB, § D. M. PAULUS, 4 AND ENRICO SCIUBBA ⊥ Ghent University, Ghent, Belgium, Katholieke Universiteit Leuven, Leuven, Belgium, The Ohio State University, Columbus, Ohio, USA, Wasco, Milwaukee, Wisconsin, USA, and University of Roma La Sapienza, Rome, Italy Received July 12, 2007. Revised manuscript received December 18, 2007. Accepted December 18, 2007. New technologies, either renewables-based or not, are confronted with both economic and technical constraints. Their development takes advantage of considering the basic laws of economics and thermodynamics. With respect to the latter, the exergy concept pops up. Although its fundamentals, that is, the Second Law of Thermodynamics, were already established in the 1800s, it is only in the last years that the exergy concept has gained a more widespread interest in process analysis, typically employed to identify inefficiencies. However, exergy analysis today is implemented far beyond technical analysis; it is also employed in environmental, (thermo)economic, and even sustainability analysis of industrial systems. Because natural ecosystems are also subjected to the basic laws of thermodynamics, it is another subject of exergy analysis. After an introduction on the concept itself, this review focuses on the potential and limitations of the exergy concept in ( 1) ecosystem analysis, utilized to describe maximum storage and maximum dissipation of energy flows ( 2); industrial system analysis: from single process analysis to complete process chain analysis ( 3); (thermo)economic analysis, with extended exergy accounting; and ( 4) environmental impact assessment throughout the whole life cycle with quantification of the resource intake and emission effects. Apart from technical system analysis, it proves that exergy as a tool in environmental impact analysis may be the most mature field of application, particularly with respect to resource and efficiency accounting, one of the major challenges in the development of sustainable technology. Far less mature is the exergy analysis of natural ecosystems and the coupling with economic analysis, where a lively debate is presently going on about the actual merits of an exergy- based approach. 1. Introduction to the Exergy Concept In science classes everywhere, students learn “Energy can neither be created nor destroyed. It just changes forms”. This scientific fact about what science calls energy is not what people experience in their everyday lives. What is typically called “energy” comes in a myriad of tangible forms for which people and businesses pay money — energy in the form of gasoline, electricity, natural gas, etc. Although the scientific energy is conserved, this other energy, “useful energy” or “marketplace energy” is not. The word energy as used by science and the word energy as used in everyday life carry two distinct meanings. The first refers to an abstract additive, conserved property that is tremendously useful in modeling. The second refers to exergy, which quantifies the ability to cause change, and this is certainly not conserved. These basic thermodynamic considerations date back from the 1800s; see Supporting Information S1 for a historical overview. As will be shown in this review, marketplace or useful energy has its own value to science and technology. By definition, the exergy (Ex) of a system or resource is the maximum amount of useful work that can be obtained from this system or resource when it is brought to equilibrium with the surroundings through reversible processes in which the system is allowed to interact only with the environment. The environment used in the calculations must be chosen properly, for example as the so-called “dead state”. The exergy concept was originally derived by Gibbs as a special case of Gibbs’s available energy. He referred to this as “the available energy of the body and medium” when the body is sur- rounded by a “medium at constant temperature and pres- sure”, see Supporting Information S1 and S2. In practice, transformation of resources through a process results in work, heat, and/or products, byproducts, and wastes that embody part of the intake exergy. The final exergy embodied in the delivered work, heat, primary and secondary products, and waste is not equal to the initial exergy content of the resources: the difference is dissipated through irreversible entropy generation. In fact Gouy and Stodola independently showed that the absolute value of this loss of exergy (Exloss ) is equal to the entropy production (S gen ) multiplied with the tem- perature of the surroundings (T 0 ): Ex loss ) T 0 S gen (Figure 1). The exergy or work potential of a system (or resource) is usually split up into four contributions: potential exergy due to its position in a given body force field (gravitational, magnetic, etc.), kinetic exergy due to its velocity with respect to a fixed reference frame, physical exergy due to its pressure (P) and temperature (T) being different from the surroundings P0 and T 0 , and chemical exergy due to its composition being different from the surroundings. Systems without kinetic, * Corresponding author phone: ++32 9 264 59 49; fax: ++32 9 264 62 43; e-mail: jo.dewulf@ugent.be. † Ghent University. ‡ Katholieke Universiteit Leuven. § The Ohio State University. 4 Wasco. ⊥ University of Roma La Sapienza. 10.1021/es071719a CCC: $40.75  2008 American Chemical Society VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2221 Published on Web 02/27/2008