Systematic Assessment of Working Fluid Mixtures for Absorption Refrigeration Based on Process and Sustainability Indicators

Abstract

Absorption Refrigeration (ABR) has been widely considered as an environmentally benign cooling process. ABR employs a working fluid (WF) mixture to extract heat for cooling achieved through a series of phase changes within different equipment. Organic fluids are promising WF candidates as they avoid disadvantages of other types of fluids, such as crystallization, toxicity etc. Current studies use very few selection criteria (e.g. COP), while they completely omit the structure and operation of the rectifier after the generator and its impact on ABR performance. Only very few fluids are considered in each study, while sustainability-related fluid indicators are never considered. Our aim is to assess the performance of numerous organic WF using a systematic approach considering multiple different indicators pertaining to operating ABR performance and fluid sustainability, while employing a properly modelled rectifier in a single effect ABR. The latter includes the number of separation stages and the distillate purity as decision parameters together with the mixture composition and mass flowrates of its components, in an optimum screening formulation. A multi-criteria assessment method identifies a Pareto front that highlights performance trade-offs among important indicators. The latter include the ABR COP and exergertic efficiency, the number of stripping stages, the mass flowrates of the mixture components, the cycle high pressure and the stripping distillate-to-feed ratio. Sustainability indicators include toxicity, flammability, biodegradation probability ozone depletion and global warming potentials. The proposed approach is illustrated for 36 mixtures containing HCFC and HFC refrigerants with various absorbents and the results are compared with NH3/H2O, which is a commercially used ABR fluid. It is observed that R32-DMF (dimethylformamide) exhibits good performance trade-offs.