Entropy-guided redesign of a compact R290 heat-pump evaporator under variable operation

Compact heat-pump evaporators must preserve thermal duty under variable operation while balancing heat-transfer enhancement against refrigerant-side pressure losses. Conventional fixed-geometry optimization gives limited insight into how the least-irreversible reference changes with compressor speed and air-to-water temperature lift. This study develops an Imperfection Design Curve (IDC)-based second-law framework for interpreting thermodynamic redesign directions of an R290 finned-tube evaporator under average-climate Energy-related Products conditions. A manufacturer-calibrated Dymola/Modelica baseline model, supported by 51 proprietary steady-state test-bench points, supplied local thermodynamic states. These states were post-processed to identify two-phase and superheated regions, separate heat-transfer and flow-friction entropy generation, and normalize each operating condition against its state-dependent minimum-entropy reference. Two effectiveness regimes were identified. At higher effectiveness, compressor-speed-induced increases in refrigerant mass flow caused flow-friction irreversibility to dominate. The reference area shifted across the operating envelope, and the resulting IDC indicated an area-reduction direction. A parameter-screening-guided numerical candidate reduced the total evaporator heat-transfer area from 1.58 to 1.11 m 2 , a 29.6% reduction, while satisfying the required simulated thermal duty across all assessed operating points. Therefore, the present study should be interpreted as a theoretical and numerical demonstration of the IDC methodological framework rather than a practical design guide or an experimentally validated replacement geometry.