Power Generation From Waste Heat: Ionic Liquid-Based Absorption Cycle Versus Organic Rankine Cycle

Aspen Plus (R) simulations using the Peng-Robinson (PR-EOS) and the COSMO-SAC models were performed to study absorption power cycles (APCs) using mixtures of R-134a with two ionic liquids, [C(2)C(1)im][Tf2N] or [C(6)C(1)im][Tf2N], and compared against an R-134a organic Rankine cycle (ORC) operating under similar conditions. The PR-EOS results were in agreement with calculations from a PR model fitted to the R-134a + IL experimental phase equilibrium data. The APCs have similar efficiencies and outperform the ORC by 3%-46%, with the largest differences observed when operating with lower grade (lowerT(H)) heating sources, lower quality cooling (higherT(L)), and lower subcooling in the pump inlet stream. The PR-EOS and the Conductor-like Screening Model Segment Activity Coefficient (COSMO-SAC) results follow similar trends, but numerical discrepancies are observed in the cycle efficiencies and stream flow rates and compositions due to differences in solubilities and enthalpy changes between both models, suggesting that improvements are needed to increase the accuracy of COSMO-SAC for these systems.