Exergoeconomic optimization of a novel cascade Kalina/Kalina cycle using geothermal heat source and LNG cold energy recovery

In recent studies, cascade power generation systems are introduced as promising methodologies in power plants for recovering the waste heat of topping cycle, resulting in low waste heat rejection from the heat exchangers of the single-stage power plant. Due to the importance of this topic, a cascade Kalina cycle (CKC) power plant is proposed using geothermal energy as heat source and liquefied natural gas (LNG) as thermal heat sink. The proposed system is integrated from a high pressure KC (HPKC) as topping cycle and a low pressure KC (LPKC) as bottoming cycle. Energy, exergy and exergoeconomic analysis of the proposed system are conducted to give more detail information for designers. In addition, single- and multi-objective optimizations of the proposed system are performed, using genetic algorithm (GA) method. It is demonstrated that the optimum net power, thermal efficiency, exergy efficiency and total sum unit cost of product of the proposed system can be obtained 9044 kW, 29.87%, 43.19% and 127.8 $/GJ, respectively. Exergy analysis results demonstrated that among all components, the second heat exchanger accounts for the biggest exergy destruction rate. To obtain a better understanding from characteristics of the proposed system, a comprehensive parametric study is conducted showing that the thermal efficiency of system can be optimized based on the geothermal inlet temperature as well as the basic ammonia concentrations of HPKC and LPKC. In addition, it is found that a higher thermal efficiency can be obtained by increasing vapor generator pinch point temperature difference and turbines expansion ratio or by decreasing the second heat exchanger's temperature. Moreover, it is concluded that the exergy efficiency can be increased by increasing the first and second turbines' expansion ratios, geothermal inlet temperature, and basic ammonia concentration of HPKC or by decreasing the second heat exchanger's temperature, third turbine's expansion ratio and vapor generator pinch point temperature difference. In addition, the total sum unit cost of product of system can be decreased by increasing of the geothermal inlet temperature, second heat exchanger temperature, basic ammonia concentration of HPKC and basic ammonia concentration of LPKC or by decreasing of the vapor generator pinch point temperature difference and turbines' expansion ratios.