le, it is a thermodynamic limit (e.g. a lower bound for consumption or work or heat or an upper bound for work or heat production) that must be ensured in a technology. We limit ourselves here to ‘static limits’, that is, those in steady systems. This Chapter also introduces the reader to certain controls called ‘Carnot variables’. The practical and industrial systems of interest include thermal and chemical generators of mechanical energy (engines) and the apparatus or devices in which this energy is consumed (refrigerators, heat pumps, and separators). In principle, irreversible thermodynamics is a general field suitable to evaluate such limits for processes occurring in finite time and in systems of a finite size. However, theoretical irreversible thermodynamics seldom attacks systems with explicit work flux (power) (see De Groot and Mazur, 1984 for a basic description). For the purpose of energy systems analyses, irreversible thermodynamics is most often applied in a discrete rather than a continuum form, which means that thermal fields are seldom attacked. Yet, the continua are not excluded in treatments of energy problems (Orlov and Berry, 1990, 1991a,b, 1992). A typical irreversible thermodynamics analysis of an energy system refers to a topological structure that belongs in the thermal networks (diverse units connected by appropriate links). Such structures could in principle be treated by network thermodynamics (NT; Peusner, 1986), a general field that transfers meanings and tools of electric circuit theory to macrosystems described by discrete models.