Supercritical $CO_2$ waste heat recovery system design of redundancy diesel engine for nuclear powered icebreaker application

Abstract

Nuclear powered icebreaker is possible to operate the ship faster or longer with limited fuel volume and maximize the torque of the propeller with higher power during icebreaking if a waste heat recovery of diesel engines is applied. In order to maximize the performance of the existing diesel power source in operating nuclear powered icebreakers, adding a waste heat recovery system for the redundancy power source is suggested for the first time in this thesis. In order to explore the possibility of the idea, a comparison of six supercritical carbon dioxide ($S-CO_2$) power cycle layouts recovering waste heat from a 17 MW redundancy diesel engine (RDE) was first presented. The diesel engine can supply two heat sources to the waste heat recovery system: one from exhaust gas and the other from scavenged air. The performance indicator of bottoming cycles was simply expressed by adopting the waste heat recovery index (WHRI). By utilizing this performance index, various bottoming cycle designs could be assessed with a unified framework especially for the waste heat recovery systems’ performance. This indicator matches to the concept of cycle net efficiency, except that it evaluates the performance replacing the cycle heat input with the maximum obtainable heat from the heat source for the bottoming cycle. Moreover, a sensitivity study of the cycles for different design parameters is performed, and the thermodynamic performances of the various cycles were evaluated. It was found that a partial heating cycle has relatively higher net produced work while enjoying the benefit of a simple layout and smaller number of components. This study also revealed that further waste heat can be recovered by adjusting the flow split merging point of the partial heating cycle. The main components, including turbomachinery, heat exchangers and pipes, were designed with in-house codes which have been validated with experiment data. Based on the designed cycle and components, the bottoming S-$CO_2$ cycle performance optimization and establishment of control logic under part load operation of RDE was analyzed by using a quasi-steady state cycle analysis method. Quasi-steady state analysis code improvement was conducted with dual heat sources and with inventory, compressor speed, bypass and throttle methods and optimization of operation condition. In case of inventory control and compressor speed control provided the best result because compressor can operates near the peak efficiency region by minimizing the loss. However, the inventory tank over 9MPa is impractical due to the thickness over 90 mm as well as cost impact. To overcome limitations on inventory control system, constant mass operation was considered and its performance decrease is around 5%. $S-CO_2$ system could be more compact and simple when constant mass operation strategy is accepted due to minimized auxiliary components. Therefore, it was concluded that constant mass operation is more reliable for real ships.