Improving the economy-of-scale of small organic rankine cycle systems through appropriate working fluid selection

Abstract

Organic Rankine cycles (ORC) are becoming a major research area within the field of sustainable energy systems. However, a major challenge facing the widespread implementation of small and mini-scale ORC systems is the economy-of-scale. To overcome this challenge requires single components that can be manufactured in large volumes and then implemented into a wide variety of different applications where the heat source conditions may vary. The aim of this paper is to investigate whether working fluid selection can improve the current economy-of-scale by enabling the same system components to be used in multiple ORC systems. This is done through coupling analysis and optimisation of the energy process, with a performance map for a small-scale ORC radial turbine. The performance map, obtained using CFD, is adapted to account for additional loss mechanisms not accounted for in the original CFD simulation before being non-dimensionalised using a modified similitude theory developed for subsonic ORC turbines. The updated performance map is then implemented into a thermodynamic model, enabling the construction of a single performance contour that displays the range of heat source conditions that can be accommodated by the existing turbine whilst using a particular working fluid. Constructing this performance map for a range of working fluids, this paper demonstrates that through selecting a suitable working fluid, the same turbine can efficiently utilise heat sources between 360 and 400K, with mass flow rates ranging between 0.5 and 2.75kg/s respectively. This corresponds to using the same turbine in ORC applications where the heat available ranges between 50 and 380kWth, with the resulting net power produced by the ORC system ranging between 2 and 30kW. Further investigations also suggest that the same pump could also be used; however, the heat exchanger area scales directly with increasing heat input. Overall, this paper demonstrates that through the optimal selection of the working fluid, the same turbomachinery components (i.e. pump and turbine) can be used in multiple ORC systems. This offers an opportunity to improve the current economy-of-scale of small ORC systems, ultimately leading to more economical systems for the utilisation of low temperature sustainable heat sources.