Marine Energy Systems Research Articles

A Dynamic Model for Liquefied Natural Gas Evaporation During Marine Transportation

Marine transport of natural gas, predominately in the form of Liquefied Natural Gas (LNG), is of growing importance in the global energy markets. LNG is transported with vessels having cryogenic tanks without any means of external refrigeration. Hence, a significant fraction of the cargo LNG volume evaporates during voyage, which is usually called Boil-Off Gas (BOG). According to the type of the energy system, BOG can be utilized as fuel, reliquefied or burned in an oxidizer. The handling of the boil-off gas during the LNG vessel operation and the assessment of its thermodynamic properties are key issues in the technical and economic assessment of the complete marine energy systems of LNG vessels. A detailed dynamic boil-off model has been developed, which accounts for the variation of BOG mass flow, composition and thermodynamic properties during voyage. The model employs coupled non-linear vapor – liquid phase equilibrium thermodynamic equations and differential conservation equations describing the evolution of LNG quantity and composition with time. Various solution and time discretization schemes have been tested to assess the solution accuracy and stability. The model has been used to investigate the variation of the LNG and BOG quantity, composition and thermodynamic properties during typical voyage profiles of a case study LNG vessel. Variations of the thermodynamic properties and of the quantity of BOG in the range of 6 to 10% during voyage have been found, having a significant impact in operation modes and fuel consumption of the energy system. Results have been also compared with the traditional approaches used in the marine technical practice.

Synthesis, design and operation optimization of a marine energy system

Recent developments in the global fuel markets imposed the need of increased fuel economy and cost effectiveness of sea-going vessels. Optimization of the ship's total energy system, as a whole, is now a demand of the marine industry sector in order to address the significant increase of installation and operational costs. This study is focused on the synthesis, design and operation optimization of a marine energy system. A realistic example of a cruise liner energy system has been selected. Basic technology options have been identified and a generic energy system model has been constructed. Various configuration options, types of technologies and existence of components have been incorporated in the generic system. In addition, time varying operational requirements for this cruise liner ship have been considered, resulting in a time dependent operation optimization problem. The complete optimization problem has been solved using a novel algorithm, inspired by evolutionary and social behavior metaphors. A parametric analysis with respect to the fuel price demonstrated changes in the optimum synthesis of the system.