Making the case for cascaded organic Rankine cycles for waste-heat recovery

The design of single-stage organic Rankine cycle (ORC) systems can be challenging owing to large volumetric expansion ratios and sub-atmospheric condensation pressures. Cascaded systems could lead to more efficient expansion processes, higher condensation pressures, whilst introducing the possibility of two-phase expansion to enhance performance. The aim of this paper is to compare single-stage ORC systems to a novel two-phase cascaded system that combines a two-phase expansion topping cycle and a single-phase bottoming cycle for waste-heat recovery applications. Thermodynamic cycle models are integrated with variable efficiency expander models and discretised heat-exchanger sizing models, and single- and multi-objective optimisation studies are completed for three heat-source temperatures (473, 523 and 573 K). The results indicate the relative performance improvement of cascaded systems increases as the heat-source temperature and relative heat-sink size increase, and could increase power output and first-law thermal efficiency by up to 11.1% and 9.5% respectively. The multi-objective optimisation reveals that for a fixed total heat-transfer area the cascaded systems produce approximately 3.6% and 10.5% more power than the single-stage systems for the 523 and 573 K cases respectively with a heat-sink mass-flow rate of 1 kg/s. This increases to 11.7% and 14.5% for heat-sink mass-flow rate of 4 kg/s.