He, J., Johnston, B., Dhar, D., and Lohmeyer, L., "R744 Parallel Compression Cycle for Automotive Climate Control," SAE Technical Paper 2017-01-0175, 2017, doi:10.4271/2017-01-0175.
The natural refrigerant, R744 (CO2), remains a viable solution to replace the high GWP refrigerant R134a which is to be phased out in light-duty vehicles in EU and US market. In this study, thermodynamic analysis is performed on a R744 parallel compression system to evaluate its potential in automotive climate control. The model adopts a correlation of isentropic efficiency as a function of compression ratio based on a prototype R744 MAC compressor and accounts for the operating limits defined in the latest DIN specifications. Optimization is run over typical MAC operating conditions which covers both transcritical and subcritical domain. Comparing to the conventional single compression cycle, effectiveness of parallel compression is found most pronounced in low evaporating temperature and high ambient conditions, with up to 21% increase in COP and 5.3 bar reduction in discharge pressure observed over the considered parametric range. Correlations of the optimal discharge pressure and intermediate pressure are developed as a function of evaporating temperature and gas cooler exit temperature using linear regression, which facilitate utilization of dynamic control towards optimal operating efficiency. Finally, various compressor designs are discussed for possible implementation of parallel compression in a R744 MAC or heat pump system. Consistent with the findings in commercial refrigeration, parallel compression holds promise from performance perspective to promote use of R744 technology in warm regions for automotive climate control.