Energy, exergy, economic and exergoenvironmental analyses of transcritical CO2 cycle powered by single flash geothermal power plant

Abstract

Abstract The need for energy is increasing worldwide as the population has a continuous trend of increase. The restrictions on energy sources are becoming tougher as the authorities set these developed and developing countries. This leads to looking for other alternative energy sources to replace the conventional energy sources, leading to greenhouse emissions. Environmentally friendly energy sources (renewable energies), for example, geothermal, solar and wind, are viewed as clean and sustainable energy sources. Among these kinds of energy sources, geothermal energy is one of the best options because, like solar and wind energy sources, it does not depend on weather conditions. In this work, a single flash geothermal power plant is used to power a transcritical CO2 power plant is proposed. The energy and exergy analysis of the proposed combined power plant has been performed and the best possible operating mode of the power plant has been discussed. The effects of parameters such as separator pressure, CO2 condenser temperature and CO2 turbine inlet pressure and the pinch point on the energy efficiency, exergy efficiency and output power are determined and discussed. Our results indicate that the highest exergy destruction is in the CO2 vapor generator of 182.4 kW followed by the CO2 turbine of 106 kW, then the CO2 condenser of 82.81 kW and then the CO2 pump 58.76 kW. The lowest exergy destruction rates occur in the single flash geothermal power plant components where the separator has exactly zero exergy destruction rate. The results also show that the combined power plant produces more power and has better efficiencies (first law and second law) than the stand-alone geothermal power plant. Finally, Nelder–Mead simplex method is applied to determine the optimal parameters such as separator pressure, power output and pumps input power and second law efficiency. The results show that the power plant should be operated at a lower pinch temperature to reduce damage to the environment. As the condenser pressure increases, the environmental damage effectiveness coefficient decreases sharply until it reaches the minimum value of 1.2 to 1.7 MPa and then starts to increase. The trend of the impact of sports on environmental improvement is exactly the opposite of the trend of the effectiveness of environmental damage. Therefore, from an environmental point of view, it is recommended to operate the gas turbine at a high inlet pressure.