Geometry optimization of plate heat exchangers as absorbers in compact absorption refrigeration systems using H2O/ionic liquids

Highlights: • A PHE absorber with H₂O/[DMIM][DMP] is modeled considering solution pressure drop. • Effect of three geometrical parameters on absorber and cycle performance is studied. • H₂O/[DMIM][DMP] can provide higher COPs than H₂O/LiBr under specific geometries. • Trade-off between absorber volume and cycle COP is analyzed in detail. • Geometries optimization strategies are provided for a compact and efficient cycle. H₂O/LiBr absorption refrigeration is promising in renewable energy utilization but suffers from large size and crystallization. Therefore, geometry optimization of plate heat exchanger (PHE) absorbers plays a vital role in increasing the compactness whilst maintaining a high efficiency. It is the first time that an absorption refrigeration cycle using a PHE absorber with H₂O/[DMIM][DMP] was modeled considering solution pressure drop and heat transfer, taking the conventional H₂O/LiBr as a baseline. Results showed that solution pressure drop in absorber had a more pronounced effect on the cycle performance than that in desorber. Parametric studies indicated that the required absorber length, overall heat transfer coefficient, and pressure drop all decreased with the increase of the three PHE geometries (channel width, channel height, and plate number), which caused the absorber volume and cycle coefficient of performance (COP) to increase accordingly. H₂O/[DMIM][DMP] could perform comparably to H₂O/LiBr with an absorber temperature difference below 3 °C and an absorber volume over 0.00015 m³. The Pareto diagram of H₂O/[DMIM][DMP] under an absorber temperature difference of 5 °C showed that the cycle COP increased from 0.700 to 0.724 with the absorber volume being increased by 16.6%, but increasing the COP from 0.724 to 0.748 required a volume increase of 71.46%. Meanwhile, the Pareto frontier formed by optimal designs presented the trade-off between absorber compactness and cycle efficiency, which needs careful balance at the design stage. To achieve a targeted COP of 0.748 on the Pareto frontier, the optimal geometry set had the smallest volume of 0.001 m³, with 0.05 m width, 0.01 m height, 0.0816 m length, and 24 plates. This study was expected to provide guidance for geometries optimization of PHE absorbers by balancing the absorber volume and cycle COP towards compact and efficient absorption refrigeration.