Modeling CO2 System Refrigeration Loads

The hydrofluorocarbon phasedown mandated by the Kigali Amendment in the Montreal Protocol and California’s aggressive approach to reduce HFC emissions 40% below 2013 levels by 2030 has made CO2 look appealing as a future-proof alternative to other refrigerants.  But current energy modeling methods struggle to accurately simulate the fast-changing dynamic conditions of a transcritical (TC) CO2 system without significant real-world input data being added manually. This project aims to update and refine an existing Openstudio grocery store analysis tool built on the Energy Plus modeling framework and to improve modeling methodologies using collected data at two grocery store locations to accurately estimate real world transcritical CO2 refrigeration loads. The goal is to support utility programs by building open-source models for TC CO2 refrigeration systems that are flexible, transparent, and efficient in both new and existing stores that will provide a streamlined and structured approach for CA IOUs to claim savings on these projects.  The analysis of the data showed that the average case live load was usually only 35% of the design load.  Because the system was oversized, the compressors operated less efficiently, and the suction pressure dropped below 420 PSI.  Resulting in additional defrost cycles to be required due to case icing. The collected data was used along with the custom refrigeration schedules for the sites to bring simulated results within approximately 10% of the actual metered energy consumption.  The use of the simulation tool will allow engineering firms, consultants, and California utilities to perform whole building energy modeling for grocery stores installing TC CO2 systems and being able to right size the equipment to optimize operations while remaining an open-source tool.