(271c) Computer-Aided Synthesis and Design of Working Fluid Mixtures for Organic Rankine Cycles

Organic Rankine Cycle (ORC) systems are receiving increasing attention world-wide as they enable power generation from widely available resources of low thermal content (e.g. industrial waste heat streams, geothermal fields etc.). Their operation is based on the vaporization of a single or blended working fluid to drive a turbine, hence there are two major characteristics affecting the thermodynamic and economic cycle performance for a given heat source, namely a) the physical and chemical features of the working fluid and b) the design and operating system configuration. ORCs commonly utilize single working fluids to facilitate heat extraction, although reports [1] indicate that mixtures enable a significant increase of the ORC thermodynamic performance, while decreasing the produced electricity cost and improving the overall environmental and safety system features.

Despite the enormous importance of both single and blended working fluids for ORCs, their selection is based largely on trial-and-error approaches, applied in empirically compiled repositories containing conventional options. Such a small set is extremely limiting in view of the vast number of molecules/mixtures that could be considered as candidate ORC fluids, hampering opportunities for innovation. The authors have been the first to address such limitations through the use of a Computer-Aided Molecular Design (CAMD) method applied in the design and selection of single ORC working fluids [2, 3]. The considerable economic, operating, environmental and safety advantages of the designed fluids compared to conventional choices constitute a significant motivation for the development and implementation of a CAMD-based method for the design of ORC mixtures. The challenges associated with such an effort are due to the need to simultaneously determine the optimum mixture composition (chemical structure of all participating working fluids) and concentration (amount of each fluid in the mixture). While CAMD-based mixture design has yet to be considered in ORC research, it has only been reported for few other applications involving solvent-based separations and refrigeration, leaving significant opportunities for improvements. 

This work presents a method for the simultaneous determination of the composition and concentration of binary ORC working fluid mixtures using CAMD-based optimization, implemented in two interacting stages. The first stage aims to explore and identify the highest possible economic, operating, environmental and safety performance limits of a wide set of mixtures in an ORC system. This is approached by searching for chemically feasible molecular structures only for one of the two components of a binary mixture, while emulating the mixture behaviour of the remaining component within a much wider structural design space by removing the chemical feasibility constraints. The identification of multiple optimum mixture candidates is accomplished through a multi-objective formulation of the CAMD-optimization problem, treating multiple ORC performance measures simultaneously and resulting in a comprehensive Pareto front revealing useful structural and property trade-offs among mixture components. The second stage serves to determine the optimum and chemically feasible structure of the second component for each one of the molecules already obtained in the first stage, together with the optimum mixture composition. In this stage, the mixture performance limits identified in the first stage are used as a reference point to efficiently avoid sub-optimal choices. The proposed approach avoids the need for immediate targeting of a single mixture with optimum features which often requires enormous computational effort. Instead, the effort is reduced as the user is allowed to review, interpret and analyse rich intermediate insights prior to exploiting meaningful conclusions between design stages. Optimum solutions are identified in a Pareto sense, enabling the exploitation of the often conflicting design objectives.

The merits of the proposed approach are illustrated through a case study on ORC systems. The considered design indices employed in CAMD reflect important ORC performance measures such as thermodynamic efficiency and exergy through an ORC mathematical model utilized in the course of CAMD-optimization. Important mixture properties such as flammability, toxicity and azeotropic mixture behaviour are also considered.

[1] Angelino, G., Di Paliano, P. C., Multicomponent working fluids for organic rankine cycles (ORCs), Energy 1998; 23(6): 449.

[2] Papadopoulos A.I., Stijepovic M and Linke P., On the systematic design and selection of optimal working fluids for Organic Rankine Cycles, Applied Thermal Engineering 2010, 30, 760.

[3] Stijepovic M.Z., Linke P., Papadopoulos A.I., Grujic A.S., On the role of working fluid properties in Organic Rankine Cycle performance, Applied Thermal Engineering 2012, 36, 406-413