Beyond COP: process operability analysis reveals constraint-dependent refrigerant rankings in vapor compression chillers

The global phase-down of high-GWP hydrofluorocarbons, mandated by the Kigali Amendment and the European F-Gas Regulation, requires methodologies for evaluating replacement refrigerants beyond single-point performance metrics. Conventional studies compare refrigerants at fixed design conditions, reporting a single COP value that cannot capture how the achievable operating envelope changes with refrigerant choice or under which conditions performance rankings invert. This work introduces process operability analysis (POA) — a framework never previously applied to refrigeration systems — as a context-dependent tool for refrigerant selection. Four refrigerants spanning the GWP spectrum (R-134a, R-1234yf, R-32, and R-410A) are compared across 180,000 steady-state simulations of a vapor compression chiller performed in the open-source process simulator DWSIM, and the Operability Index (OI) is computed rigorously, as the Lebesgue measure of the intersection between the achievable and the desired output sets divided by that of the desired set, under engineering-grounded constraint scenarios. The refrigerants separate into two non-overlapping capacity clusters governed by operating pressure level, so that refrigerant choice determines not just system efficiency but the class of applications served. More critically, the operability ranking is not absolute: the optimal refrigerant depends on whether condenser cooling water capacity, chilled water demand, or thermodynamic efficiency is the binding limitation. Under cooling water scarcity the ranking inverts — R-1234yf (GWP = 1) outperforms R-134a (GWP = 1430) and R-410A (GWP = 2088) outperforms R-32 (GWP = 675) — whereas under a high cooling load it reverts to R-134a and R-32. Embedding the index in a multi-objective selection step, which trades operability against efficiency and GWP under the F-Gas ceilings, shows that the efficiency-optimal and the operability-optimal fluid genuinely diverge once cooling water becomes scarce, and exposes a regulatory gap: among the four fluids studied, no high-capacity option satisfies a GWP limit of 150. A parametric sweep of the condensing temperature confirms that the water-scarce inversion persists across the entire seasonal range. These findings transform refrigerant selection from a single-metric exercise into a constraint-aware, installation-specific decision framework aligned with the ongoing regulatory transition.