Combined supercritical CO2 Brayton cycle and Organic Rankine Cycle for exhaust heat recovery

Abstract

In order to reduce energy consumption and related CO2 emissions, waste heat recovery is considered a viable opportunity in several economic sectors, with particular attention on industry and transportation. Among different proposed technologies, thermodynamic cycles using suitable organic working fluids seem to be promising options, and the possibility of combining two different cycles improves the final recovered energy. In this paper, a combination of Brayton and Rankine cycles is proposed: the upper cycle has carbon dioxide as the working fluid, in supercritical phase (sCO2), while the bottomed Rankine section is performed by an organic fluid (ORC). This combined unit is applied to recover the exhaust energy of the flue gases of an internal combustion engine (ICE) for the transportation sector. The sCO2 Brayton cycle is directly facing the exhaust gases, and it should dispose a certain amount of energy at lower pressure, which can be furtherly recovered by the ORC-unit. A specific mathematical model has been developed, which makes use of experimental data of the engine to assess a realistic final recoverable energy. The model is able to evaluate the performance of each subsection of the recovery, highlighting the interactions and possible trade-offs between them. Hence, the combined system can be optimized from a global point-of-view, identifying the most influencing operating parameters and also considering a regeneration stage in the ORC unit.