Optimal allocation of heat exchangers in a Supercritical carbon dioxide power cycle for waste heat recovery

Highlights: • The net power of the sCO₂ cycle for waste heat recovery increases with a preheater. • The optimal allocation of heat exchangers is existed to generate maximum net power. • The CO₂ split ratio has an optimal point to maximize the net power. • The CO₂ split ratio and turbine inlet temperature are linear at the maximum power. -- Abstract: Supercritical carbon dioxide (sCO₂) power cycles have attracted attention because of their high efficiency and flexibility in various temperature heat sources including low-temperature waste heat applications. In waste heat applications, the size of the heat exchangers is an important issue because of the trade-off between performance and installation cost. For the optimal design of an sCO₂ cycle for waste heat recovery, a thermodynamic model for the basic cycle and the preheating cycle was constructed. The heat exchangers were then modeled by a finite volume analysis under the fixed total UA value, with the equivalent conductance representing the size of the heat exchanger. The net power and thermal efficiency of the cycle were calculated. The results of the optimization confirmed that the application of the preheater improves the performance of the basic cycle, and an optimum point of the split ratio exists. From the simulation, with an increase in the turbine inlet temperature (TIT), the thermal efficiency improves, but the net power does not always increase. Instead, a close linearity between the optimum CO₂ split ratio (ϕ) and the turbine inlet temperature was found at the maximum net power, even under different turbine inlet pressures and total UA values. From these results, the configuration of heat exchangers for waste heat applications can be planned appropriately to operate at the maximum net power.