Methodologies developed for the analysis, improvement and optimization of energy systems must take into account not only energy use (exergy consumption) and financial resources expended (economics), but the scarcity both present and future of all resources used as well as any pollution and degradation of the environment which may occur. Along these lines, parallel works which take some or all of the above factors into account have recently been published by Frangopoulos (1991, 1992) and von Spakovsky (1991, 1992). What follows is Part I of a series of two papers. They represent an attempt to combine certain aspects of these works into a global methodology