Case studies in computer-aided molecular design (CAMD) of low- and medium-grade waste-heat recovery ORC systems

Organic Rankine cycle (ORC) engines are suitable for the conver sion of low - grade h eat into useful power. While numerous substances are available as ORC working - fluid candidates, computer - aided molecular design (CAMD) techniques allow the rigorous selection of an optimal working fluid during system optimisation. The aim of this present s tudy is to extend an existing CAMD - ORC framework [1,2] by incorporating , in addition to thermodynamic performance objectives, economic objectives when determining the optimal system design, while maintaining the facility of selecting optimal working fluids . The SAFT - γ Mie equation of state is used to predict the thermodynamic properties of the working fluids (here, hydrocarbons) that are relevant to the system s ’ economic appraisals and critical/transport properties are estimated using empirical group - contri bution methods . System investment costs are estimated with equipment cost correlations for the key system components, and the stochastic NSGA - II solver is used for system optimisation. From a set of NLP optimisations, it is concluded that the optimal molec ular size of the working fluid is linked to the heat - source temperature. The optimal specific investment co st (SIC) values were £10,120/kW and £4,040/kW when using heat - source in let temperatures of 150 °C and 250 °C (representative of low - and medium - grade heat) respectively, and the corresponding optimal working fluids were propane, 2 - butane and 2 - heptene