Comparing the Performances of an Ionic Liquid-Based Absorption Cycle and an Organic Rankine Cycle for Power Generation from Low-Grade Waste Heat

The Organic Rankine Cycle (ORC) has been employed over the past half-century to generate power using lower temperature (lower-grade) heat sources than the typical steam temperatures in the common water-based Rankine cycle. While often advertised as requiring only “waste” heat, most commercial ORC units require heat sources well in excess of 100 °C, while extensive amounts of waste heat near 100°C are often vented to the atmosphere by industry. Absorption power cycles (APCs) have been proposed to overcome these deficiencies of the ORC, especially when using low-grade waste heat sources at temperatures of 100 °C or lower. In this work, the performance of an APC using low-grade waste heat and a mixture of the ionic liquid (IL) 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIm][Tf2N]) with the common fluorinated refrigerant R-134a was compared against an ORC using R-134a and operating at similar conditions. Both cycles were modeled in Aspen Plus using two thermodynamic models, the Peng-Robinson equation of state with parameters fitted to vapor-liquid equilibrium data of our working fluid mixtures, and the COSMO-SAC purely predictive model. Our results show that the IL-based APC has higher efficiencies than the ORC, with the gap increasing as the temperature of the waste heat sources is lowered. The effects of the cycle operational temperatures on the efficiency and flow rates of working fluid will be presented and discussed.