Exploring optimal working fluids and cycle architectures for organic rankine cycle systems using advanced computer-aided molecular design methodologies

The combination of computer-aided molecular design (CAMD) with an organic Rankine cycle (ORC) power-system model presents a powerful methodology that facilitates an integrated approach to simultaneous working-fluid design and power-system thermodynamic or thermoeconomic optimisation. Existing CAMD-ORC models have been focussed on simple subcritical, non-recuperated ORC systems. The current work introduces partially evaporated or trilateral cycles, recuperated cycles and working-fluid mixtures into the ORC power-system model, which to the best knowledge of the authors has not been previously attempted. A necessary feature of a CAMD-ORC model is the use of a mixed-integer non-linear programming (MINLP) optimiser to simultaneously optimise integer workingfluid variables and continuous thermodynamic cycle and economic variables. In this paper, this feature is exploited by introducing binary optimisation variables to describe the cycle layout, thus enabling the cycle architecture to be optimised alongside the working fluid and system conditions. After describing the models for the alternative cycles, the optimisation problem is completed for a defined heat source, considering hydrocarbon working fluids. Two specific case studies are considered, in which the power output from the ORC system is maximised. These differ in the treatment of the minimum heat-source outlet temperature, which is unconstrained in the first case study, but constrained in the second. This is done to replicate scenarios such as a combined heat and power (CHP) plant, or applications where condensation of the waste-heat stream must be avoided. In both cases it is found that a working-fluid mixture can perform better than a pure working fluid. Furthermore, it is found that partially-evaporated and recuperated cycles are optimal for the unconstrained and constrained case studies respectively.