Cooling Efficiency in Thermal Systems: Metrics, Drivers, and Pathways to Ultra-Low Energy Cooling

Abstract

Cooling services—space conditioning, industrial refrigeration, and process thermal management—are expanding rapidly with urbanization, rising ambient temperatures, and escalating internal heat densities. This review synthesizes definitions and metrics of cooling efficiency, classifies technological families, analyzes part-load behavior, and maps the roles of climate, humidity, controls, refrigerants, and heat rejection on seasonal performance. We unify performance assessment across rating schemes (COP, EER, SEER, SCOP, IPLV) and discuss end-use context, including comfort, dehumidification, and ventilation penalties. We highlight pathways to high performance—variable-speed compression, advanced heat exchangers, low-GWP refrigerants, hybrid evaporative cycles, sorption and thermally driven systems, free cooling, and system-level optimization—alongside emerging digital enablers such as model-predictive control and fault detection. Original figures illustrate COP degradation with temperature, part-load gains, climate sensitivity, and building energy distributions. The review culminates with a roadmap emphasizing integrated design, verified seasonal metrics, and grid-interactive operation to minimize life-cycle energy and emissions.