Exergy and the Dissipative Structure

As an ecosystem is nonisolated, the entropy changes during a time interval, dt, can be decomposed into the entropy flux due to exchanges with the environment, and the entropy production due to the irreversible processes inside the system such as diffusion, heat conduction, and chemical reactions. It can also be expressed by use of exergy:

where de Ex/dt represents the exergy input to the system and di Ex/dt is the exergy consumed (is negative) by the system for maintenance, etc. e is used to indicate an external source and i to indicate the internal exergy change.

Equation [11] shows among other things that systems can only maintain a nonequilibrium steady state by compensating the internal exergy consumption with a positive exergy influx (de Ex/dt> 0). Such an influx induces order into the system. In ecosystems, the ultimate exergy influx comes from solar radiation, and the order induced is, for example, biochemical molecular order. If de Ex > — d; Ex (the exergy consumption in the system), the system has surplus exergy input, which may be utilized to construct further order in the system, or as Prigogine calls it, dis-sipative structure. The system will thereby move further away from thermodynamic equilibrium. Evolution shows that this situation has been valid for the ecosphere on a long-term basis. In spring and summer, ecosystems are in the typical situation that de Ex exceeds —di Ex. If de Ex <—di Ex, the system cannot maintain the order already achieved, but will move closer to the thermodynamic equilibrium, that is, it will lose order. This may be the situation for ecosystems during fall and winter or due to environmental disturbances.

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