Introduction to the Concept of Exergy - for a Better Understanding of Low-Temperature-Heating and High-Temperature-Cooling Systems

Chapter I describes the characteristics of a thermodynamic concept, exergy, in association with building heating and cooling systems. Exergy is the concept that explicitly indicates 'what is consumed'. All systems, not only engineering systems but also biological systems including the human body, work feeding on exergy, consuming its portion and thereby generating the corresponding entropy and disposing of the generated entropy into their environment. The whole process is called 'exergy-entropy process'. The features of 'warm' exergy and 'cool' exergy and also radiant exergy are outlined. General characteristics of exergy-entropy process of passive systems, which would be a prerequisite to realize low exergy systems, are discussed together with the exergy-entropy process of the global environmental system. Chapter II introduces the various forms of exergy and the mathematical formulations used to evaluate them. The exergy balance on an open steady state system, which is much more relevant to thermodynamic analysis of energy systems, is also described, as well as the different exergetic efficiency factors introduced in the thermodynamic analysis of energy systems. Next, an exergy analysis example is outlined through an air-conditioning application. Air-conditioning applications are widely used in heating and cooling of buildings. Chapter III introduces an example of exergy calculation for space heating systems. The issues to have a better understanding of low-exergy systems for heating and cooling are raised. It is suggested that a prerequisite for low exergy systems would be rational passive design of building envelope systems.