Formulation of the Principle of Energy Quality: Toward a Fourth Law of Thermodynamics

The formulation of general principles governing energy degradation and transformation remains an open problem in thermodynamics, particularly for irreversible and coupled processes. In this work, the Principle of Energy Quality is introduced as a unified thermodynamic formulation that characterizes how energy is transformed, degraded, and partially regenerated across physical, chemical, informational, and ecological systems. Within this framework, total energy is partitioned into two complementary components: exergy ( E ), the convertible fraction into useful work; anergy ( A ), the fraction degraded through entropy generation; and Onsager coupling potentials ( O ), which describe the redistribution and partial recovery of degraded energy through coupled irreversible processes. Building on classical exergy–anergy analysis and Onsager’s flux–force formalism, the principle establishes a direct link between entropy production and energy quality loss, while recognizing the structured and potentially regenerative character of degraded energy. The formulation is discussed within the modern enumeration of thermodynamic laws, which includes the zeroth law and acknowledges the historically non-canonical nature of their numbering. In this context, the proposed principle may be regarded as a step toward a fourth law of thermodynamics. The zeroth law defines the reference environment T 0 ; the first law enforcers energy conservation with explicit partitioning into useful and degraded components; the second law quantifies irreversibility; the third law bounds exergy as T→0 ; and the proposed fourth law generalizes efficiency and dissipation by identifying anergy as the fundamental measure of energy degradation and regenerative potential.