Energy, Entropy and Exergy

 Triade Nr. 130: Dincer, Ibrahim; Rosen, Marc A. (2013). Exergy Energy, Environment And Sustainable Development. Amsterdam, Boston, Heidelberg: Elsevier. S.2ff.

Energy

Energy comes in many forms. Thermodynamics play a key role in the analysis of processes, systems, and devices in which energy transfers and energy transformations occur. The implications of thermodynamics are far reaching and applications span the range of the human enterprise. Throughout our technological history, our ability to harness energy and use it for society’s needs has improved. The industrial revolution was fueled by the discovery of how to exploit energy on a large scale and how to convert heat into work. Nature allows the conversion of work completely into heat, but heat cannot be entirely converted into work, and doing so requires a device (e.g., a cyclic engine). Engines attempt to optimize the conversion of heat to work.

Entropy

The formulation of entropy is fundamental in the modern context for understanding thermodynamic aspects of self-organization and the evolution of order and life that we observe in nature. When a system is isolated, the entropy of a system continually increases due to irreversible processes and reaches the maximum possible value when the systems attains a state of thermodynamic equilibrium. In the state of equilibrium, all irreversible processes cease. When a system begins to exchange entropy with its surroundings, in general, it is driven away from the equilibrium state it reached when isolated, and entropy-producing irreversible processes begin. An exchange of entropy is associated with the exchange of heat and matter. When no accumulation of entropy within a system occurs, the entropy flowing out of the system is always larger than the entropy flowing in, the difference arising due to the entropy produced by irreversible processes within the system. As we will see in the following chapters, systems that exchange entropy with their surroundings do not simply increase the entropy of the surroundings, but may undergo dramatic spontaneous transformations to “self-organization.” Irreversible processes that produce entropy create these organized states. Such self-organized states range from convection patterns in fluids to organized life structures. Irreversible processes are the driving force that creates this order.

Exergy

A very important class of problems in engineering thermodynamics concerns systems or substances that can be modeled in equilibrium or stable equilibrium, but that are none in mutual stable equilibrium with the surroundings. For example, within the earth there are reserves of fuels that are not in mutual stable equilibrium with the atmosphere and the sea. The requirements of mutual chemical equilibrium are not met. Any system at a temperature above or below that of the environment is not in mutual stable equilibrium with the environment. In this case the requirements of mutual thermal equilibrium are not met. Any lack of mutual stable equilibrium between a system and the environment can be used to produce shaft work. With the SLT, the maximum work that can be produced can be determined. Exergy is a useful quantity that stems from the SLT and helps in analyzing energy and other systems and processes.