Assessment and working fluid comparison of steam Rankine cycle -Organic Rankine cycle combined system for severe cold territories

In this work, organic Rankine cycle (ORC) is employed to expand the available temperature region of the conventional steam Rankine cycle (SRC) in severe cold territories. The feasibility of the proposed steam Rankine cycle -Organic Rankine cycle combined system is investigated and verified. Working fluids are compared and screened, the matching scheme of different working fluids suitable for different working conditions and objects (thermodynamic or economic performance) are obtained. The results show that the net power increment (NPI) of the combined system increases with the decrease of the condenser outlet temperature (CT). The NPI of ORC system can be effectively improved by adding a regenerator. For wet working fluids, when the evaporating pressure of ORC system is close to the critical pressure of working fluid, there exists an optimal superheat degree with maximum NPI of the system. The combined system using ammonia and R600 as working fluid has the largest NPI values. When the CT of SRC is higher than 90 °C, the Rankine cycle using ammonia as working fluid has better performance. When the CT of SRC is less than 90 °C, the regenerative ORC system using R600 as working fluid has a better performance.