Thermoeconomic optimisation of small-scale organic Rankine cycle systems based on screw vs. piston expander maps in waste heat recovery applications

The high costof organic Rankine cycle (ORC) systems is a keybarrier to their implementation in waste heat recovery (WHR) applications. In particular, the choice ofexpansion device has a significant influence on this cost, strongly affecting the economic viability of an installation. In this work, numerical simulations and optimisation strategies are used to compare the performance and profitability of small-scale ORC systems using reciprocating-piston or single/two-stage screw expanders whenrecoveringheatfrom the exhaust gases ofa 185-kWinternal combustion engineoperating in baseload mode. The study goes beyond previous work by directly comparing these small-scaleexpanders fora broad range of working fluids, and by exploring the sensitivity of project viability to key parameters such as electricity price and onsite heat demand. For the piston expander, a lumped-massmodel and optimisation based on artificial neural networks are used to generate performance maps, while performance and cost correlations from the literature are used for the screw expanders. The thermodynamic analysis shows that two-stage screw expanders typically deliver more power than either single-stage screw or piston expandersdue to their higher conversion efficiency at the required pressure ratios. The best fluids areacetone and ethanol, as these provide a compromise between the exergy losses in the condenser and in the evaporatorin this application. The maximum net power output isfound to be 17.7kW, from an ORC engine operating withacetone anda two-stage screw expander. On the other hand, the thermo economic optimisation shows that reciprocating-piston expandersshow a potential for lower specific costs, and since such an expandert echnology is not mature, especially at these scales, this finding motivates further consideration of this component. A minimum specific investment cost of 1630€/kW is observed for an ORC engine with a piston expander, again with acetone as the working fluid. This system, optimised for minimum cost, also gives the shortest payback time of4 years at an avoided electricity cost of 0.13€/kWh. Finally, financial appraisalsshowahigh sensitivityof the investment profitability to the value of produced electricity and to the heat-demand intensity.The comparison of these expansion devices,from both thermodynamic and economic perspectives,is of interestto the research community, developers and potential users ofnext-generation, small-scale ORC systemsfor WHR