Performance improvement of CO2 air conditioner by integrating photonic radiative cooler as sub-cooler or/and roof envelope

Passive radiative cooling, which creates a direct in-situ cooling effect, along with a natural refrigerant-based vapor compression system, could be a potential alternative for building air conditioning to reduce the heat island effect and global warming. Hence, the radiative cooler is integrated with transcritical CO2 air conditioner as a sub-cooler or/and roof envelope. Three configurations of radiative cooler integrated CO2 system (Case 1: radiative cooler as roof envelope only, Case 2: radiative cooler as sub-cooler only and Case 3: radiative cooler as sandwiched roof envelope and sub-cooler) are proposed and simulated for tropical Indian summer climate. Energy, exergy and economic analyses along with gas cooler pressure optimization are performed. The Monte Carlo method is used to calculate annual energy savings for all three cases. Results show that case 3 exceeds the performance of other cases. Case 3 also reduces the variations of optimum gas cooler pressure and cooling load, leading to the smooth operation of the CO2 air conditioner. COP increment of 33.8% and exergy efficiency improvement of 29.1% are observed for case 3. A simple payback period of 5.3 years is worked out for case 3. Annual energy saving per unit area for case 3 is found comparable to solar photovoltaic integration.