Development of a multi-level approach to model and optimise the Kalina Split Cycle

In the marine sector there is a strong motivation for increasing the propulsion system energy efficiency, mainly because of increasing fuel prices and stricter upcoming emission regulations. The Kalina cycle, based on a mixture of ammonia and water as working fluid, exhibits higher conversion efficiencies than conventional power cycles and could be suitable for this purpose. The Split Cycle technique provides a method to further increase the thermal efficiency, by reducing the thermodynamic losses in the heat recovery system. This is achieved by having two separate streams of different ammonia concentrations entering and leaving a first evaporator stage before being mixed at the inlet of a second evaporator stage. It seems that modelling efforts showing the advantages of the Split Cycle have not been presented in the literature yet. Thus, a thermodynamic model of the Split Cycle is introduced in this work. Modelling and optimisation of the rather complex cycle requires approaching the problem at different system levels. This paper investigates tools and methods suitable for demonstrating the feasibility and advantages of the Split Cycle. The integrated model developed and presented in this paper combines three sub-models all using the NIST REFPROP equations of state: a separator and mixing subsystem model to handle the inherent constraints of the Split Cycle, a component-based model to optimise the heat exchanger operating conditions, and a process model to investigate the complete thermodynamic cycle. Results suggest a 9% net power output increase and 7% higher thermal efficiency compared to the baseline case.