Energy Saving in Multistage Chilled Water Cooling System with Hybrid Compressor and Thermal Energy Storage Technology: Focus on Compressor Isentropic Efficiency

Abstract

In a refrigeration system, the compressor accounts for 50–70% of the energy consumption. Improving the compressor’s operating efficiency is a key factor for energy savings. This study focuses on the compressor’s isentropic efficiency and optimizes a large refrigeration system in the food industry through compressor selection, cooling capacity management, and energy-saving technology integration. The multi-cooler system contains a hybrid compressor that combines variable speed and variable displacement types. The speed and displacement are adjusted based on the compressor characteristics to keep both values in the optimal range. The variable capacity compressor prioritizes full load operation, while the variable speed compressor is used for wide-range power regulation. Cooling capacity is controlled by optimizing the primary chilled water flow rate and secondary return water temperature. These adjustments maintain the chiller's Part Load Rate (PLR) within the optimum Coefficient of Performance (COP) range, achieving two-stage optimization at both device and system levels. In addition, thermal energy storage (TES) technology is integrated to achieve “load shifting,” reducing the operating time of the three chillers from 550 minutes to 93 minutes and avoiding frequent on/off cycling. This allows the variable capacity compressor to avoid low-load operating conditions throughout the day while maintaining the capacity regulation capability of the variable speed compressor, improving energy efficiency in part-load conditions. In a real milk production system, the proposed optimization achieves energy savings of 32.2–33.3% and reduces the difference in peak-valley energy consumption by 44.8–47.4%. This study provides a new approach to energy-efficient and robust operation of large refrigeration systems with hybrid compressors.