Thermodynamic performance assessment of SOFC-RC-KC system for multiple waste heat recovery

Highlights: • New energy system of solid oxide fuel cell-Rankine cycle-Kalina cycle is proposed. • Rankine cycle as top cycle is combined with Kalina cycle as bottom cycle. • Parameter variation and structure improvement are simultaneously optimized. • Stack temperature, pre-reformer temperature, and recycling ratio both affect power generation. • The power generation obtained by simultaneous optimization is increased by 8.4%. Solid oxide fuel cell (SOFC) has attracted increasing attention as an alternative to conventional cogeneration systems, owing to its higher energy conversion efficiency. However, its high-temperature operating characteristics pose an enormous challenge for efficient waste heat recovery. To address this issue, a novel distributed energy system SOFC-RC-KC is proposed by combining Rankine Cycle (RC) as top cycle with Kalina Cycle (KC) as bottom cycle, in a bid to recover multiple waste heat of SOFC. In developing the system, an enhanced Duran-Grossmann optimization model (D-G model) is established in a sequential/simultaneous manner, aiming at maximizing power generation, by combining genetic algorithm to calculate the maximum output power of SOFC system. The D-G model is used to constrain the thermal utility of the SOFC-RC-KC system. Under this condition, genetic algorithm is used to optimize the parameters of SOFC, RC, and KC to maximize the system power generation. In sequential optimization, the operating conditions of SOFC system are optimized under the constraint that SOFC system does not need thermal utility. After obtaining these operating conditions, the RC-KC power generation is optimized. In simultaneous optimization, the thermal utility of the SOFC-RC-KC system is taken as the restriction condition, the power generation of the SOFC-RC-KC system is taken as the optimization objective, and SOFC and RC-KC parameters are optimized simultaneously. The results indicate that the power generation of RC, KC, and SOFC obtained by simultaneous optimization are 170.1 kW, 59.39 kW, and 145.0 kW, respectively, which roughly surpass the corresponding ones of RC, KC, and SOFC (146.32 kW, 52.02 kW, and 146.9 kW) obtained by sequential optimization. Finally, sensitivity analysis is performed to investigate the effects of key parameters (including component stack temperature, pre-reforming reactor temperature, and recycle ratio) on thermodynamic performances.