Multi-Criteria Refrigerant Screening and Performance Optimization Comparison of a Basic and Composition-Adjustable Dual Cooling Source System

Abstract

Climate variability and staff mobility have exacerbated the vulnerability of the indoor environment, which calls for the air-conditioning units can accommodate the active regulation demand of the changing heat and humidity loads. The identification of the refrigerant constitutes an essential sector of the air-conditioning system design. A composition-adjustable dual cooling source (CADCS) chiller has been proposed utilizing the characteristics of inefficient condensation of the non-azeotropic mixture. Separated flows of the working fluid with flexible mass fraction and flow rate respectively treat the waste heat and moisture. Multi-criteria screening mechanism for the working fluid is established from the aspect of the application field, environmental impact, cycle architecture adaptability, and liquid separation potential, which pre-selects R32/isobutane, R32/R1234ze(E), R32/R1234yf, propane/isobutane and propane/R1234ze(E) as the candidates. Thermodynamic performance optimization is conducted on the CADCS cycle and basic dual cooling source (BDCS) cycle under design and off-design conditions. Propane/isobutane outperforms all the candidates at a mass fraction of 0.5:0.5 characterized by obvious temperature glide and close to temperature change of the heat transfer medium. The maximum coefficient of performance (COP) reaches at 5.2 for the BDCS cycle and 4.42 during separation at vapor quality 0.436 for the CADCS cycle. The exergy loss of the evaporator is reduced by 17.1%-73.7% while the exergy efficiency is improved by 3%-59.7% due to temperature match refinement in the CADCS cycle. The refrigerant charge in the CADCS cycle can be saved by 5.54%-15%. CADCS cycle can dynamically address the heat load variation employing active composition adjustment. The accumulating waste heat demands a lower vapor quality during separation, while moisture load barely exerts an effect. Moreover, the effect of the subcooling degree on the cycle performance is analyzed and suggested at 1°C. The CADCS cycle constitutes a promising prospect of dynamic treatment for thermal and humidity load in the air-conditioning room due to its flexible function and improved heat transfer.

Keywords: Composition adjustable, Multi-criteria screening, Performance optimization, Zeotropic mixture, Dual cooling source