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The study of an integrated energy system is presented in this article that will cover all types of energy loads (mechanical, electrical, thermal) on a Very Large Crude Carrier (VLCC) with the maximum technically possible and economically feasible exploitation of fuel energy, thus reducing the operating cost and environmental footprint of the ship. There may be a large variety of configurations, design specifications and operating states that can cover the loads, making it necessary to apply synthesis, design and operation optimization of the system. The net present value of the system is selected as the objective function. For this purpose, a superconfiguration of the system is considered, which includes a number of Diesel engines adapted for possible operation in a combined Diesel and Rankine cycle, heat recovery steam generators producing high and low pressure steam, steam turbines that can contribute to propulsion and/or to the electricity production, an exhaust gas boiler and auxiliary boilers, as well as Diesel-generator sets. The synthesis of the system, that is, the components that will finally exist in the system, and their interconnections, the design specification of the components and the operating properties at characteristic operating states of the ship are not predetermined, but they are the result of formal, mathematical optimization. In addition, the speed of the vessel in each state, an important operational variable that has a crucial effect on the propulsion power and thus on the fuel consumption, is also determined by the optimization. The hull characteristics, the loading condition and the weather state are taken into account for the calculation of the propulsion power. For the optimization, proper models of the various components have been developed and the optimization problem is solved by addressing the three levels (synthesis, design, and operation) at a single computational step. The benefits of optimization, as well as the conditions that make the combined cycle economically justified, are demonstrated through an application example. The numerical solution is obtained for various values of fuel price and freight rate, so that the effects of these two crucial parameters on the optimal solution are assessed. [ABSTRACT FROM AUTHOR]

In order to cover the overall energy demands of a vessel (e.g. mechanical, electrical and thermal), energy systems onboard ships are required to produce energy of several forms while at the same time being completely autonomous and cost effective. Optimization of the ship's energy system, as a whole, in terms of synthesis, design and operation, is nowadays an unavoidable aspect of the marine industry. Furthermore, once time dependencies (e.g. time-varying loads and weather conditions) are introduced, the optimization problem is imperatively characterized as dynamic and its complexity increases dramatically. This study is focused on the dynamic optimization of synthesis, design and operation of a marine energy system. The general method, based on differential-algebraic equation formalism combined with mixed-integer nonlinear programming, along with solution procedures is presented first. The three optimization levels (synthesis, design and operation) are tackled simultaneously, with no decomposition. The superconfiguration approach is followed for the synthesis optimization, while time-varying operational requirements with respect to weather conditions and loads are considered. The presentation of the general theory is followed by application examples. A nominal case as described by a particular weather profile and values of parameters is considered, and the optimization problem with minimization of the present worth cost as an objective function is stated and solved. The effect of crucial parameters such as fuel and capital costs on the optimum solution is studied. Also, the problem is solved with a different weather profile, thus revealing the effect of weather on the optimum solution. [ABSTRACT FROM AUTHOR]

The recovery of high temperature thermal energy released by propulsion engines in order to cover thermal loads is commonplace in contemporary ships. However, the medium- and low-temperature thermal energy is only partially exploited or not exploited at all. In the present work, an organic Rankine cycle system driving an electric generator is considered, in addition to the exhaust gas boiler, in order to recover available heat and produce electrical energy. The specifications of the system are determined by an optimization procedure taking economic criteria into consideration, apart from the technical criteria usually used in this kind of studies. More specifically, with the net present value as the objective function and by application of optimization algorithms, the optimal synthesis, design and operation of the organic Rankine cycle system are determined. For the particular vessel considered, the installation of the organic Rankine cycle is technically feasible and economically profitable, with a dynamic payback period of 4 years. The solution of the optimization problem is supplemented with a sensitivity analysis with respect to important parameters. [ABSTRACT FROM AUTHOR]

Copyright of International Journal of Thermodynamics is the property of International Centre for Applied Thermodynamics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of International Journal of Thermodynamics is the property of International Centre for Applied Thermodynamics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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. [ABSTRACT FROM AUTHOR]