Analytical theories of exergy and emergy for ecological research

Exergy and Emergy have been developed as two complementary goal functions. Quantitative applications of Exergy for higher organisms were made with traditional thermodynamic considerations. The concept of Emergy originally came from systems ecology. Exergy is a property of a system, measuring the maximum work that can be extracted from a system when it goes towards the thermodynamic equilibrium with a reference state, whereas Emergy is the energy required to make a service or product expressed in energy of one form. Exergy is a measure of the actual state, of the level of organization and of the information content, while the concept of Emergy contains the history, the time and all the different processed involved up to the present state of the system. Both of them have their respective characteristics on expressing the functions of ecosystems. They represent two complementary aspects of a system. Exergy is a useful, common measure of resource quality and quantity, applicable to both materials and energy. Therefore, Exergy can be used to measure and compare resource inputs and outputs,such as wastes and losses. Exergy is expected to increase as ecosystems mature and form the thermodynamic equilibrium. At the same time, Emergy is a powerful tool for determining energy flows through biological systems and for determining sustainable economic values in the field of ecological economics. Emergy is not only a measure of what have gone into a product, but also a measure of the useful contributions that can be expected from the product as a system self-organizes for maximum production. Accordingly, Emergy is the work that the biosphere has to do to maintain a system far from equilibrium. It is seams that emergy is a production side approach, where quality is measured by the cost of producing a flow or product, while Exergy is an end-use approach, where quality is measured by the usefulness to the receiver. The two approaches are very suitable for describing self-organizing systems such as ecosystems and even more complex systems such as compound eco-ecosystems. Exergy and Emergy accounting can provide insight into the environmental performance and sustainability of eco-ecological process and product. Moreover, the ratio of Exergy to the Emergy flow indicates the efficiency of an ecosystem in producing or maintaining its organization. Considered the variation in time of Exergy and Emergy, their ratio indicates the effect of the change of available inputs in the level of organization of the system under study. A great effort in research is presently made to integrate the two approaches, which also present many common points.