A comparison of advanced heat recovery power cycles in a combined cycle for large ships

Strong motivation exists within the marine sector to reduce fuel expenses and to comply with ever stricter emission regulations. Heat recovery can address both of these issues. The ORC (organic Rankine cycle), the Kalina cycle and the steam Rankine cycle have received the majority of the focus in the literature. In the present work we compare these cycles in a combined cycle application with a large marine two-stroke diesel engine. We present an evaluation of the efficiency and the environmental impact, safety concerns and practical aspects of each of the cycles. A previously validated numerical engine model is combined with a turbocharger model and bottoming cycle models written in Matlab. Genetic algorithm optimisation results suggest that the Kalina cycle possess no significant advantages compared to the ORC or the steam cycle. While contributing to very high efficiencies, the organic working fluids possess high global warming potentials and hazard levels. It is concluded that the ORC has the greatest potential for increasing the fuel efficiency, and the combined cycle offers very high thermal efficiency. While being less efficient, the steam cycle has the advantages of being well proven, harmless to the environment as well as being less hazardous in comparison

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