The three axes of refrigerant goodness: theoretical COP, second-law efficiency, and volumetric capacity
By DigiEntropy Engineering · 2026-03-21 · 13 min read
Picking a refrigerant by COP alone gives you one of three answers that actually matter. This post unpacks the theoretical COP from a vapor-compression cycle, the Carnot-based second-law efficiency, and the volumetric refrigerating capacity that drives compressor displacement — and grounds each in actual polynomial COP from Bitzer compressors.
About this post — Authored by an AI assistant using DigiEntropy's polynomial corpus, the universal compressor predictor, and the project's chart-generation tooling. Charts are produced by Python scripts that read the same database the live site queries; tables and formulas are pulled from the same engineering modules. Findings reflect the project's current dataset and methodology — send corrections or deeper questions to admin@digientropy.com. The question every refrigeration engineer hits Picking a refrigerant is rarely about COP alone. R290 (propane) sits near the top on most thermodynamic charts, yet it loses to R744 (CO₂) whenever the design constraint is "smallest possible compressor for this duty." R134a beats both on raw COP at warm condensing temperatures, but its molecules are too heavy to fit a heat-pump booster rack. R1233zd(E) tops every theoretical ranking and is used in roughly zero supermarket cabinets, because its evaporator pressure is below atmospheric and the safety case is a nightmare. The conventional approach — "compare the average COP of available compressor models" — answers one of those questions, badly. It mixes refrigerant physics with whichever compressors the vendors happened to publish polynomials for, in a single number. To choose well, you need to separate the refrigerant's thermodynamic limit from what compressors actually deliver against that limit and from how much swept volume you'll need to deliver the duty. DigiEntropy's Efficiency 360 wizard (Step 1, Choose Refrigerant) now exposes all three. For every refrigerant in our database — 135+ entries spanning natural, HFC, HFO, and HFO/HFC blends — it computes three independent metrics live from CoolProp at the band you picked: - COP Ideal — the theoretical COP from an ideal vapor-compression cycle (refrigerant only, no compressor model). - % Carnot — second-law efficiency: how close the refrigerant gets to the thermodynamic ceiling. - VRC — volumetric refrigerating capacity, the driver of compressor displacement. Together with the polynomial-derived COP (already in the table — call it COP Actual for this post) and the regulatory axes (GWP, ASHRAE class, F-Gas timeline), you get a five-axis picture of every refrigerant in 30 seconds. This post unpacks each new metric, shows what the rankings reveal, and grounds the theory in actual COP from Bitzer compressors. Axis 1: theoretical COP — what the refrigerant can do alone The theoretical COP comes from the textbook four-state vapor-compression cycle: 1. State 1 — saturated vapor leaves the evaporator and superheats to…