Comparative numerical assessment of a hybrid air-liquid cooling unit: impact of different refrigerants and climatic conditions

This thesis evaluates the performance of an air–liquid hybrid cooling unit developed by Vertiv S.r.l. for hyperscale data center applications. The operational principles of the system are analyzed across all operating modes, with particular emphasis on performance variations under different boundary conditions. The flexibility of the hybrid architecture enables a direct comparison between air-based and liquid-based cooling solutions for data center waste heat rejection. The unit operation is numerically simulated for different climate zones in order to assess annual energy consumption trends and to highlight the impact of climate conditions, demonstrating the relevance of the Pump Refrigerant Economizer cycle in cold climates. Furthermore, a qualitative analysis of efficiency variations under partial-load conditions is conducted, including the evaluation of alternative refrigerants with respect to the design fluid. All steady-state operating points and energy performance calculations are carried out using the com pany’s proprietary rating software. Results highlight the energy savings introduced by free-cooling cycle in cold climate zones and the positive effects of the adoption of liquid cooling technology on the efficiency of the system. R454B is proposed as a substituting refrigerant to replace R410A and align with specific regulations.