Equation-oriented methods for optimizing Rankine cycles using radial inflow turbine

This paper presents methods for optimizing the design and the performance of Rankine cycles using radial inflow turbines. Both methods follow a novel equation-oriented approach and involve a single mathematical problem that is solved by an efficient gradient-based algorithm. The capabilities of the proposed methods were demonstrated through a case study for power generation from the batch-wise casting process at a representative ferroalloy plant. More specifically, the proposed methods were used to design and analyze three Rankine cycles with CO2 as the working fluid. The design optimization method converged in most cases to essentially the same solution regardless of the start values of the independent variables. The performance optimization method demonstrated that the control approaches with variable rotational turbine speed improved the turbine off-design efficiency over the control approaches with a constant rotational speed. Moreover, the control approaches with variable inlet guide vanes improved the thermodynamic performance of the cycle by facilitating operation at a higher pressure than the control approaches with a fixed geometry turbine. Considering the flexibility, robustness and the computational cost of the proposed methods, they can be regarded as a powerful tool for the preliminary design and performance prediction of Rankine cycles.