Optimal ORC configuration for the combined production of heat and power utilizing solar energy and biomass

This paper aims to investigate the best configuration of an ORC plant for supplying power and useful heat to industrial processes, using solar energy and biomass. In particular, a solar plant based on linear Fresnel collectors integrated with a two-tank Thermal Energy Storage (TES) system and a biomass furnace is considered for the thermal energy production, while an ORC power plant is proposed for the combined production of electricity and useful heat (by recovering waste heat from the condenser). An optimization tool is implemented for the choice of the best ORC layout and cycle conditions with the objective of maximizing its exergy efficiency. System performance are evaluated for several working fluids and different heat demand temperatures by imposing an ORC inlet temperature of 250°C and a desired power output of 630 kW. The results show that the best working fluids belong to siloxanes (in particular the hexamethyldisiloxane), and linear alkanes (Pentane, Iso-Hexane etc.) characterized by high molecular complexity. The exergy efficiency is around 56-58%, but the optimal working fluid varies according to the condenser temperature. Finally, the yearly-based analysis of the hybrid biomass-solar CHP plant highlights the fundamental role of the biomass contribution (about 50% of the overall thermal energy input) for assuring the continuous daytime operation of the ORC unit under nominal conditions.