Investigating the performance and cost effects of nanorefrigerants in a low-temperature ORC unit for waste heat recovery

The use of nanorefrigerants in refrigeration and heat pump units increases the cycle performance, with similar effects also applied in organic Rankine cycle (ORC) units, but with very few relevant studies. For this purpose, an existing thermodynamic model for the ORC simulation is extended to account for adding Al2O3 and Cu nanoparticles (NPs) in the refrigerant, introducing their effect in the energy balance and the heat transfer and pressure drop at the different parts of the cycle. Numerical analysis of an ORC with refrigerants R245fa and R1234ze(Z) has been then conducted, with a heat source in the range of 80-120 degrees C and a NP mass fraction of 0-10% wt., aiming at identifying the performance improvement. The results show that the ORC efficiency increases for higher NP mass fraction and especially with Al2O3. This is mostly attributed to the expansion process with the NPs acting as internal heat sources to the vapour organic fluid, whereas the heat transfer and pressure drop improvements are minor. Finally, the preliminary cost analysis showed that the use of NPs in an ORC brings a payback period of 11.7 years with Al2O3, but with high uncertainty as revealed by the sensitivity analysis. (C) 2020 Elsevier Ltd. All rights reserved.