Introduction

The study of 'very complex systems' ('VLCS'), like the territorial system discussed here, requires a holistic approach to analyze the entire system and all of the 'external' and 'internal' interactions that characterize it. We maintain that the best way to approach such problems is to consider the VCLS as an 'extended' (in a sense to be specified later) thermodynamic system. The evaluation of the flows of matter and energy sustaining a territorial system and the knowledge of the transformations therein can be used to describe the rate of exploitation of the available natural resources. This kind of information could be an important support for both the internal and global policy planning and resources management.

In the last decade, a metaparadigm for the assessment of the state of the environment seems to have been universally accepted: environmental scientists, engineers, physicists, chemists, and biologists have the role of defining, troubleshooting, and calibrating a proper set of 'decision parameters', called 'ecological indicators' (Els in the following), that will be subsequently used by national and international agencies in their evaluations. It is noteworthy that, since the environmental committees of these agencies are usually composed by economists and sociologists who are not necessarily conversant with the physical sciences that originated the EI, the necessity arises of defining ecological indicators in such a way that they may convey at the same time some quantitative and qualitative information. This is not an easy task, and several EIs have been proposed that for one reason or another lack the necessary sharpness. This article is concerned with one of these EIs, 'exergy destruction', which is basically a measure of the irreversibility of natural and anthropogenic processes. As we shall try to argue in section titled 'Scope and function of an ecological indicator', a material balance, per se, cannot be a good EI: it provides a measure of the throughput of a certain sector of the society, or of the society as a whole, but it cannot properly measure the intrinsic quality of that throughput. For example, all 'material flow' methods cannot provide an answer to the question whether it is better to generate electricity by coal-fueled or nuclear power plants, because a 'mass-flow based' EI is unable to capture the diversity of the coal and uranium life cycles. In the same section we examine a broad class of methods based on an energy analysis (embodied energy, emergy), to conclude that they, too, are only marginally effective and even misleading in their descriptions of the inputs and outputs of a territorial or of an industrial system, because they cannot discern between 'low-quality' (heat) and 'high-quality' (mechanical work, electricity) energy flows. In the early 1980s G0ran Wall and others presented the first complete applications to territorial system based on exergy flows that, in contrast to energetic studies, explicitly included both first and second law of thermodynamics. With this approach it became possible to express any kind of system's input, either energetic or material, on a uniform thermodynamic scale, so that these inputs (and the corresponding outputs) could be 'normalized' and evaluated by attributing to each one of them an exact value of 'work content'. We shall though argue in section titled 'Exergy destruction as an ecological indicator?' that exergy destruction, which is indeed a direct measure of the irreversible entropy generation of a process, is not a completely acceptable EI, because it cannot properly account for toxicity-related chemical pollution phenomena.

Recently, an extension of classical exergy analysis called 'extended exergy analysis' (EEA) was presented by the author. This method seems appropriate to investigate industrial and territorial system alike, because it can compare the physical flows of energy and matter with nonenergetic quantities like capital, human labor, and environmental impact (quantified by necessary environmental remediation costs).

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