Low-grade heat utilization for combined cycle power plants using organic rankine cycle

This paper presents a comprehensive thermodynamic analysis of trinary power plants, focusing on the efficiency of Organic Rankine Cycle (ORC) configurations and regenerative heating methods. Despite advancements in nuclear and renewable energy, fossil fuels still dominate electricity generation, necessitating improved efficiency in existing power plants. The study reveals that low-pressure mixing-type heaters provide higher efficiency compared to surface heaters, with net efficiencies of 0.099%, 0.227%, and 0.425% at deaerator pressures of 0.12, 0.3, and 0.7 MPa, respectively. The analysis highlights the impact of feedwater temperature on the thermal efficiency of steam turbine units (STUs), noting that while optimal feedwater temperatures enhance efficiency, they can reduce STU capacity. The study identifies configurations for regenerative heating that optimize exhaust gas temperatures, facilitating additional electricity production through a low-boiling working fluid in the ORC. The findings indicate that R245fa refrigerant is optimal for ORC without recuperative heater, achieving maximum net power at a feedwater temperature of 115°C. For ORC with a recuperative heater, R236ea is preferred for temperatures between 115°C and 154.5°C, while R245fa is optimal for higher temperatures. The results also demonstrate that trinary power plants with recuperators achieve greater efficiency and net capacity compared to double-circuit systems, with notable improvements in thermal efficiency attributed to effective regeneration schemes. This research underscores the potential for optimizing existing domestic power units to enhance their efficiency and performance without significant financial or technical burden, thereby contributing to more sustainable energy generation.