Optimization of Supercritical CO2 Cycle Combined With ORC for Waste Heat Recovery

Waste heat recovery is a broadly considered opportunity for efficiency improvement in several energy consumption sectors, intending to reduce energy consumption and related carbon dioxide emissions to the atmosphere. The attention of research activities is focused on transportation and residential sectors, where the possible recovery is characterized by low enthalpy, but with a wider potential market. Therefore, the maximization of recovery is one of the principal aims of this option, and different kinds of technologies have been proposed in this regard. Thermodynamic cycles, which exploit the waste heat considering it as the upper thermal source, seem to be a promising option, and the possibility to combine two different cycles can increase the thermal power harvested. In this paper, a combination of a supercritical CO2 Brayton cycle with an ORC-based unit has been proposed to recover waste heat from the exhaust gases of an internal combustion engine for the transportation sector. Using CO2 as working fluid is under investigation in literature, for its low Global Warming Potential and its suitable thermodynamic characteristics in dense phase (just above the critical point). An Organic Rankine Cycle (ORC), then, has been bottomed to the CO2 section, to further recover thermal energy and convert it into mechanical useful work. Indeed, the CO2 cycle must have a lower temperature cold sink, where thermal power can be furtherly recovered. The introduction of this second stage of recovery interacts with the upper one, modifying the overall optimization parameters. Hence, this work aims to find the maximization of the recovery from a global point-of-view, identifying possible trade-offs happenings between the two recovery sections. Minimum sCO2 pressure, stack exhaust temperature, and the possibility to have a regeneration stage have been considered as optimizing parameters. Finally, the optimized system has been applied to a specific mission profile of a commercial vehicle, in order to evaluate the recovery potential during a realistic engine working points sequence. A recovery higher than 4% in every mission considered has been achieved, with values up to 7% in motorway and long-hauling conditions.