Design and thermo-economic optimisation of small-scale bottoming ORC systems coupled to biomass CHP gasification cycles

Optimisat ion of a small - scale bottoming organic Rankine c ycle (ORC) engine is carried out for a combined biomass - gasifier - CHP system, drawing heat from the syn gas conditioning unit of the gasif ier and the exhaust gas of the internal combustion (IC) engine . The optimisation considers different working fluids and selection of a positive - displacement expander. Single - and two - stage screw expanders and single - stage reci procating - piston expanders are modelled in order to capture the variation in their performance at a range of design points. Double - pipe heat exchangers are employed for both evaporator and condenser, le ading to a low - cost but bulky design. The system is op timised first for maximum electrical power output, and second for minimum specific investment cost (SIC) . Cost correlations are used for each of the principal ORC components. The optimal design for minimum SIC is found to be a two - stage screw expander with ethanol as the working fluid, which produces a 13 .6 % increase in the electrical power output relative to the system without an ORC. The investment attractiveness of the whole system with and without the bottoming ORC is assessed and the system is found to be profitable for avoided electricity costs above 150 $/MWh el and biomass costs of 50 $/t , with the ORC making the system more attractive in all cases studied. Discounted payback periods range from 1 2 years at 150 $/MWh el to 3. 5 years at 250 $/MWh el for the system with ORC