A study of the optimal control approach for a Kalina cycle system using a radial-inflow turbine with variable nozzles at off-design conditions

Highlights: • A novel control approach for Kalina cycle is proposed at off-design conditions. • An ammonia-water radial-turbine using variable nozzles is specially designed. • Novel control approach is compared with two traditional control approaches. • Net power, exergy utilization efficiency and thermal efficiency are evaluated. -- Abstract: To maximize the exergy utilization efficiency of a Kalina cycle system with an ammonia-water radial turbine at off-design conditions, this study proposes a novel control approach named optimal control approach. An off-design model for ammonia-water radial-inflow turbine using variable nozzles is constructed. The optimal control approach is realized by changing the outlet angle of the radial-inflow turbine nozzle and the turbine inlet pressure. Design parameters of heat source (waste hot water) are 130 °C and 10 kg/s. To find out the performance advantage for the novel control approach, it is compared with two traditional control approaches. The results show that the proposed novel control approach presents the highest exergy utilization efficiency and net power at off-design conditions. The net power ratio of the optimal control approach to the traditional sliding pressure control approach reaches 111.22% as the heat source mass flow rate declines to 5 kg/s. Compared with the traditional approach for constant pressure control (changing turbine nozzle outlet angle), the optimal control approach has a potential to produce 3.11% more net power. The exergy utilization efficiency is generally declined with the elevated heat source mass flow rate at the approach for optimal control, while the system thermal efficiency is slowly increased.