Performance improvement and optimizing an innovative dual-loop RC–ORC heat recovery system with nano-working fluids

The application of nanofluids to increase the heat transfer rate in heat exchangers of a proposed dual-loop RC–ORCheat recovery system (RC with water and ORC with R141b working fluids) is investigated here. The effects of adding nanoparticles into working fluids of a heat recovery system with two RC and ORC loops and its performance, has not been studied so far. Appropriate single-phase/two-phase heat transfer coefficients have been used for nanofluids in the system modeling by extracting their properties from available experimental data. Performance evaluation criterion (PEC) was also applied to assess and compare the performance of nanofluids in the proposed system. Optimization of the proposed system with exergy efficiency and payback period as objective functions for various nanofluids (working fluids mixed with four nanoparticles Al 2 O 3 , CuO, Cu, TiO 2 ) with various volume fraction (0.5, 1, and 2%) which satisfied PEC > 1 provided a Pareto front. The selected optimum point from Pareto front by TOPSIS method showed that the proposed dual loop RC–ORC system with water/Cu nanofluid (with volume percent of 2% for the RC loop) and 141b/Cu nanofluid (with a volume percent of 0.5% for the ORC loop) had the most suitable performance (higher thermal and exergy efficiencies and lower payback period). Under these conditions, the thermal efficiency, the exergy efficiency, the turbine power output and the annual net profit of the proposed RC–ORC system increased by (4.4%), (17.2%), (15.4%) and (15.6%) respectively.