A Comparison between Cascaded and Single-stage ORC Systems Taken from the Component Perspective

orc2019 Tracking Number 113

Compared to single-stage ORC systems, cascaded ORC systems could have benefits for relatively high-temperature waste-heat recovery applications, which include the potential for higher expander isentropic efficiencies owing to lower expansion ratios, the removal of sub-atmospheric condensation pressures and the possibility to utilise two-phase expansion. Previous investigations suggest that cascaded systems could produce up to 5% more power than an equivalent single-stage system. The aim of this paper is to compare the different systems in terms of exergy destruction within the system and the heat-transfer area requirements. Firstly, the exergy analysis reveals that cascaded systems reduce the total exergy destruction related to the expansion process, but this is offset by the exergy destruction within the additional heat exchange process. However, cascaded cycles also lead to less exergy destruction within the heat-addition process. To assess the heat-transfer area requirements, a discretised double-pipe heat-exchanger model is developed for the condenser and intermediate heat exchanger that transfers heat from the topping cycle into the bottoming cycle, whilst a discretised finned-tube cross-flow heat-exchanger model is developed for the evaporator. The geometry of each heat exchanger is optimised to minimise the heat-transfer area subject to imposed pressure drop constraints. The results reveal that cascaded cycles require larger heat-transfer areas, which is due to the additional heat-transfer process and reduced temperature differences within the evaporator. Ultimately, the best performing cascaded cycles, which produce 4.0% and 5.9% more power than their single-stage counterparts, require 22.7% and 23.2% more heat-transfer area. Future investigations should investigate how this trade-off impacts economic performance.