natural refrigerant, light commercial, efficiency, comparison, design
Light commercial refrigeration units exist for a large variety of different applications and include beverage coolers, ice machines, chilled drink dispensers, and merchandizers and storage units for food items. Among the natural refrigerants available, only propane (R290) and carbon dioxide (CO2, R744) are realistic candidates to replace currently used HFC refrigerants. While many of the thermodynamic and transport properties of R290 are favorable over R744, propane is listed as an ASHRAE A3 refrigerant and therefore bears an inherent flammability risk even when the refrigerant charge is limited to 150 g or less as required by applicable safety standards. While it seems possible to design and implement safe, low-charge hydrocarbon refrigeration systems, some manufacturers prefer solutions that completely eliminate the flammability risk and therefore focus on R744, which is a non-flammable, non-toxic, natural refrigerant listed as ASHRAE A1. The challenges encountered with R744 are mostly due to lower performance at elevated ambient temperatures requiring a transcritical cycle, which often makes R744 systems more expensive for designs that are aimed at providing comparable cooling capacity and energy efficiency. Therefore, the component and system design challenges encountered with the two fluids are very different, which drives design solutions in very different directions. This study elaborates on the different fluid-specific challenges that are inherent to the each of the two refrigerants and demonstrates the resulting consequences in terms of system and component design. Since the cooling target is the same in both cases, the pros and cons of R290 and R744 can be fairly compared and meaningful conclusions can be drawn.