Thermodynamic analysis and optimization of an ORC hybrid geothermal-solar power plant

Energy and exergy analysis is conducted on a hybrid (solar-geothermal) organic Rankine cycle (ORC) power plant. The proposed system is designed to be installed in southern Tunisia. Energy and exergy balances are established for the different components of the power plant. A parametric study is conducted to analyze the effects of the main operating parameters on the hybrid power plant performance during two typical days in summer and winter. Energy and exergy efficiency as well as the influence coefficient are selected as performance criteria for the power plant. The results obtained show that the main source of irreversibility is the steam generator, followed by the air-cooled condenser and the turbine. The highest influence coefficients are obtained for the steam generator and the turbine. The lowest exergy efficiency is obtained for the steam generator. Results show that the volume flow rate and the inlet temperature of the heat transfer fluid affect the power plant performance. Linear regression analysis is applied to establish the relationship between these two operating parameters and the solar radiation in order to optimize the power plant performance. This optimization procedure permits the exergy efficiency to be improved by 10.35 and 6% in the winter and summer, respectively. The results also indicate that as the geothermal water temperature increases in the range of 55-75 degrees C, the overall exergy efficiency increases by about 4.35% in winter. In summer, the geothermal water temperature does not significantly affect the overall power plant exergy efficiency. The overall exergy efficiency increases from 41 to 53% when the condensation pressure decreases from 2 bar to 1 bar.