Preliminary System Design and Off-Design Analysis for a Gas Turbine and ORC Combined Cycle

Organic Rankine cycle (ORC) is a more efficient way to recover exhaust heat from medium- and small-scale gas turbines than conventional steam power cycles. Plate-fin heat exchangers (PFHEs) were applied as the evaporator and recuperators because gas-liquid and gas-gas heat transfers occurred. This study focused on the preliminary system design and off-design analysis of the gas turbine (GT)-ORC combined cycle. Therefore, optimum-rated parameters for the ORC and geometric parameters for the PFHEs were designed first. Based on the established off-design model and constant pressure control approach, off-design simulations were done and results showed that the combined cycle efficiency decreased monotonically with the decline of GT load. However, the ORC still had good performance and played a more important role in the combined cycle under low-load conditions. Moreover, sensitivity simulations concluded that the ambient temperature impacted the combined cycle performance mainly by the gas turbine under high-load conditions and the ORC under low-load conditions. In addition, the ORC had better performance with higher ambient temperature. This model can provide reference parameters for detailed design of the GT-ORC combined cycle and predict its off-design performance under variable conditions.