Thermodynamic analysis of ammonia–water power/chilling cogeneration cycle with low-grade waste heat

An ammonia–water absorption cycle for power and chilling output cogeneration from mid/low-grade waste heat was analyzed and optimized, which is a modified Kalina cycle adding an evaporator and a subcooler to realize the chilling effect. The cycle achieves higher efficiency by generating chilling output from proper internal recuperation process without consumption of additional heat resource and by realizing heat transfer with suitable ammonia concentrations for variable phase change processes to match both heat source and cooling water. Analysis of the impact of key parameters for the system on the thermal and exergy efficiencies was carried out. The results show that there are matching basic and work concentration pairs for a higher efficiency. The smaller circulation multiple and greater chilling fraction are favorable to the efficiencies but restricted respectively by heat transfer constraint of recuperator and the demand. The calculation example with the turbine inlet parameters set at 195 °C/2.736 MPa and the cooling water inlet temperature set at 25 °C with chilling fraction of 0.5 shows that the thermal efficiency and exergy efficiency reach up to 16.4% and 48.3%, about 24.24% and 8.16% higher than those of an ammonia–water power cycle under identical condition. - Highlights: •A modified Kalina cycle is proposed for cogeneration from low-grade waste heat. •A subcooler, throttle and an evaporator are set to complete cooling sub-process. •The adjustable concentrations make the system with higher efficiency. •The chilling fraction can be set to fulfill various demands for power or refrigeration. •Thermal and exergy efficiency of combined cycle can reach up to 16.4% and 48.3%