Session: 06-13 Innovations in Bottoming Cycles and Components

Paper Number: 153224

Optimal Cycle and Turbine Design for MW-Scale Waste Heat Recovery ORC With Partial Evaporation 

The industrial sector has considerable potential for the recovery of waste heat, with the EU alone discharging approximately 920 terawatt hours (TWh) of heat per year, primarily below 400 °C. The key energy-intensive industries include iron, steel, chemicals, paper, food, glass, and cement. It is estimated that this untapped energy could meet approximately 5% of the EU's electricity demand, which equates to 150 TWh/year for the EU as a whole and 15 TWh/year for Italy.

The Organic Rankine Cycle (ORC) technology has the potential to enhance process efficiency by converting low-quality heat into electricity, particularly effective for temperatures below 300 °C. Recent studies have concentrated on partial evaporation ORCs (PE-ORCs) operating with wet-to-dry expansion. Such configuration has the potential to enhance conversion efficiency by optimising the matching of heat sources with organic fluids. It has been estimated that PE-ORCs can achieve a notable increase in exergy efficiency, with improvements ranging from 14% to 29% in comparison to traditional subcritical ORCs.

The development of PE-ORC technology faces a number of challenges. These include the selection of an optimal working fluid and cycle conditions, as well as the design of a non-conventional two-phase turbine. For larger industrial applications (MW scale), turboexpanders are required to utilise higher heat-source temperatures. However, challenges exist due to the lack of proper turbine designs that can efficiently handle two-phase mixtures without incurring additional losses or reliability issues.

In this context, the objective of this research is to investigate a novel ORC system that employs wet-to-dry expansion through the use of a radial turbine. The optimisation of the thermodynamic system will be achieved through the development of a calculation model designed to identify the optimal working fluids and cycle conditions, which will depend on the characteristics of the waste heat source. The calculation routine comprises three principal sections, devised to fulfill the requisite tasks: the fluids pre-screening, which identifies a list of potential working fluids for evaluation; the thermodynamic model of the PE-ORC; and an optimiser based on a genetic algorithm. For each fluid candidate, the optimiser seeks to identify the optimal working conditions, particularly the vaporisation pressure and turbine inlet vapor quality, that maximise the ORC net power output.

The proposed optimisation procedure will be tested using three relevant case studies, which represent real waste heat recovery applications. The results assign the optimal design parameters for each case study, ensuring the best performance in terms of waste heat valorisation. The increase in power output for two-phase configurations compared to single-phase systems will also be evaluated. Since the innovative two-phase turbine is the only non-standard component, a detailed design is conducted for the most promising case to verify the feasibility of the proposed technology. Based on the operating conditions identified during the thermodynamic optimization, a preliminary design is obtained through the optimization of a validated mean-line method. These estimates are then used to design the radial stator vanes and centripetal rotor. Given that the entire wet-to-dry expansion occurs in the stator, special attention is dedicated to its design, as the presence of two-phase flows prevents the application of conventional design methods (e.g., the method of characteristics). To address this challenge, a surrogate-based shape optimization combined with two-phase CFD simulations is employed to design the radial stator vanes. Finally, full three-dimensional CFD simulations of the radial-inflow turbine are carried out and provide reliable estimates of the turbine overall performance, confirming both the feasibility of the technology and its performance advantage over conventional single-phase ORC systems.

Presenting Author: Riccardo Gioia Politecnico di Milano

Presenting Author Biography: Riccardo Gioia obtained his Master of Science at Politecnico di Milano (Italy) in 2023, developing in his thesis a Method of Characteristics to design supersonic nozzles of diverse shapes and operating with nonideal flows. He is currently PhD Candidate at the Department of Energy of the same University, working on theoretical, numerical and experimental development of two-phase turbines for Organic Rankine Cycle applications.

Authors:

Riccardo Gioia Politecnico di Milano
Saverio Ottaviano Università di Bologna
Alessandro Romei Politecnico di Milano
Antonio Peretto Università di Bologna
Lisa Branchini Università di Bologna
Andrea Spinelli Politecnico di Milano