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Thermo-economic comparison of single and dual loop ORC for industrial waste heat recovery

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Waste heat recovery (WHR) is widely recognized as a promising technology to improve the degree of sustainability in thermal processes: it improves fuel utilization, producing an eco-friendly electrical power from a material flow that would otherwise be discharged into the environment. Industrial interest in Organic Rankine Cycle (ORC) technology is growing since it allows for the production of " free " electricity out of wasted thermal energy. The optimization of the system architecture is a key aspect in the minimization of the investment cost. Several investigations available in literature demonstrate that the specific cost of a competitive ORC for industrial applications should not exceed 2000 €/kW. Different optimization techniques have been proposed to properly select the ORC design parameters. Among those, non-linear programming algorithms can be conveniently applied to this specific field. This paper compares two different ORC architectures. The first one consists in a regenerated ORC with a non-flammable working fluid, the second one being a dual ORC cycle in which two ORCs operating on different working fluids are stacked on top of each other. The objective is to understand to what extent the latter configuration offer better performance. The thermodynamic model of the ORC system developed using the software MATLAB® is presented. Using nonlinear programming techniques of the open source software NOMAD, the cycle operative parameters have been optimized to maximize the power exploitation. Hence, a thermo-economic analysis is carried out to investigate over the possible improvements of the configurations proposed. This study underlines the need for an optimization approach in which the thermodynamic parameters are selected with the objective to lower the ORC system specific cost.
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... In the second case, the ORC has the role of recovering the residual CO 2 heat. Hence, the CO 2 cycle recovers energy from the upper thermal source, releasing heat at a lower temperature, which, in turn, acts as an upper thermal source for an ORC unit; i.e., the supercritical CO 2 cycle is placed between the thermal source to be recovered and the ORC The specific WHR from the exhaust gas in the ICE has been the subject of a wide range of recent studies exploiting several aspects concerning the fluid choice [29,30], operating conditions [31,32], technology of the components [33], interferences with the engine [34,35] and the vehicle [36,37], control needs [38], and cost and weight of the recovery unit [39,40]. Unfortunately, despite the invested effort, overall net efficiencies greater than 4% are difficult to achieve. ...
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