The Ratio of Eco Exergy to Emergy Flow

The basis of emergy analysis is the conversion of all the inputs to a process, both in form of energies or materials, into the same function (emergy) by using a conversion factor called transformity. Transformity is the intensive correspondent of emergy; it is not a state function, and it is dimensionless, albeit being usually expressed in solar emjoules per joule (sejJ~ ), reflecting the fact that the numerator is formed by a sum of solar emergies (therefore of a consistent quality), while the denominator is exergy of whatever form (that of the product of the process).

We can view emergy as the work that the biosphere has to do, in order to maintain a system far from equilibrium or in order to reproduce an item once it has been used. If natural selection has been given time to operate, the higher the emergy flux necessary to sustain a system or a process, the higher is their hierarchical level and the usefulness that can be expected from them (maximum empower principle). This is often not sufficient when dealing with shorter runs and with systems involving relations between humans and natural systems.

Among emergy-related indices, the empower (emergy flow) density is particularly interesting from an ecological viewpoint: it is the emergy flow per unit time and unit area, and it is a measure of the spatial and temporal concentration of emergy flow within a system. A high value of this index can signify a high stress on the environment, due to large quantities of inputs converging to the system, or a situation where space is becoming a limiting factor for further development of the system.

Emergy (and empower) is a donor-referenced concept rather than a receiver-referenced one. It is therefore necessary to compare it (or better its 'flow', empower) to a function of the state of the system that considers the information in the system, including the difference in size and quality of the components. Eco-exergy is a perfect candidate for this task. The eco-exergy to empower ratio is a holistic indicator that presents the state of the system (as eco-exergy) per unit input (as emergy). Therefore, the eco-exergy/empower ratio can be regarded as the efficiency of an ecosystem, even though it is not dimensionless, as efficiency usually is, since it has the dimension of time. Svirezhev has found this fact normal, since this concept resembles that of a 'relaxation' time, that is, the time necessary to recover from disturbances, so the eco-exergy to empower ratio should be related with concepts such as resilience and resistance of an ecosystem. This parameter indicates the quantity of external input necessary to maintain a structure far from equilibrium. The higher its value, the higher the efficiency of the system in transforming the available direct and indirect solar energy into organization within the system; if the eco-exergy/empower ratio tends to increase (apart from oscillations due to normal biological cycles), it means that natural selection is making the system follow a thermodynamic path that will bring the system to a higher organizational level.

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