Annual Operation Energy Efficiency Benchmarking of Chilled Water Plants: A Systematic Review of Global Cases (2014–2025)

Improving the energy efficiency of chilled water (CHW) plants has become a critical pathway toward reducing energy consumption in buildings. Gaining a thorough and up-to-date understanding of current CHW system performance is essential for informing standard revisions, guiding retrofit strategies, and assessing operational effectiveness. Yet, existing research often remains fragmented, with a predominant focus on isolated cases under limited conditions, lacking broader synthesis. In response, this study conducts a systematic review of annual operational energy efficiency in CHW plants spanning from 2014 to 2025, drawing upon 124 publications encompassing 229 individual cases. Through multi-dimensional analysis—including case characteristics, energy efficiency metrics, and rating outcomes—the study further examines optimized scenarios to identify key factors driving performance improvements. The research reveals the following results: (1) Optimization efforts led to an average efficiency gain of 18.87% (median energy efficiency ratio (EERao) increased from 4.61 to 5.48), though 34.41% and 59.04% of cases still failed to meet top-tier efficiency levels defined by U.S. and Chinese standards, respectively. (2) Climatic region and nominal cooling capacity (NCC) are significant determinants of system performance and should be explicitly integrated into future evaluation frameworks. (3) Systems with lower initial efficiency showed greater improvement potential (71.13% vs. 9.71%), while combined strategies involving equipment and control upgrades outperformed control-only approaches (35.38% vs. 11.60%). Additionally, model-based and model-free control techniques yielded comparable results (11.71% vs. 10.19%). These insights offer a valuable foundation for cross-case benchmarking and point to several priorities for future research and policy development.