ICOPE-15-1188 Optimized preliminary design method for mini-ORC power plants : an integral approach

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The development of small-scale Organic Rankine Cycle systems is currently pursued, especially for automotive applications. The design of such systems is very challenging because the small capacity and constraints on weight and volume imply designs which are far from the design envelope of conventional ORC power plants. Achieving an optimal design by integrating all systems components is very difficult if performed with the conventional approach, which tackles all the design sub-problems in sequence. This work presents, therefore, a new methodology whereby the thermodynamic cycle and the components preliminary design are simultaneously optimized. To this purpose, a modeling code has been developed that couples a routine for the thermodynamic cycle calculation with a 1D tool for the preliminary sizing of plate heat exchangers and a 0D model for the evaluation of the turbine performance. The case study considered to test such a methodology is the design of an ORC unit for waste heat recovery on board of long-haul trucks. The results show that, unlike the sequential approach, the integrated methodology reveals the relations between the design variables and the constraints, notably those concerning the turbine technical feasibility and the exhaust gas pressure drop. Thanks to this information, the designer can identify the trade-offs among the different design objectives and evaluate the sensitivity of the system performance with respect to key design variables, e.g., minimum temperature difference in the exchangers and evaporating pressure. For the specific test case, the optimal solution corresponds to a waste heat recovery system featuring at the design point (Diesel engine power output of 100 kW) a mechanical power close to 3 kW, evaporating pressure of 1.1 MPa, and a 94 krpm turbine with an isentropic efficiency of almost 80%. Although applied to the design of a mini-ORC system, this integrated design procedure is applicable to any conventional or novel ORC turbogenerator.

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