Energy saving technologies and new analysis methods in cargo ship machinery design

During the recent years, considerable efforts have been made to improve the fuel efficiency of ocean-going ships. The main focus of the design improvements has been an efficient hull form and main engine choice and tuning. Furthermore, the new global and local rules for energy efficiency or emission limits in shipping are creating opportunities for introducing new alternatives to the machinery and fuel choice in even the simplest cargo ships.
Today the majority of prime movers (propulsion configuration) and auxiliary plants of ships are diesel engines. In current ships approximately half of the fuel energy is utilized as propulsion power or electricity to ship processes and
the other half is lost and delivered to sea as waste heat. The four main sources of waste heat in a marine internal combustion engine are the flue gases, scavenge air, jacket water and lubrication oil cooling. These waste heat
streams can be divided into different temperature categories. Furthermore, depending on the temperature level a portion of this waste heat could be utilized as electricity or heating energy purposes in ships. Several up-to-date energy saving technologies, suitable for ships were reviewed. The selected energy saving technologies included Organic Rankine cycle (ORC), natural circulated boiler, absorption chilling process, thermal storage, compression heat pump, onboard direct current distribution, and efficient ship cooling circuit. The waste heat recovery technologies require individual power inputs and they absorb and reject heat at individual temperatures. The target of the study was to apply these technologies to simple cargo ships that represent the most common type of ship machinery set up. The technologies should be integrated in such a way that total fuel consumption of the vessel
is minimized.
In practice, evaluating the profitability of the various energy saving methods for a ship can be a challenging task due to many reasons, but most of all due to the often limited time and resources for making a holistic evaluation of the
ship systems in concept design phase. Therefore, the tools for supporting the energy and environmental ship design process have been developed recently, such as system level energy flow simulators. At the side of the system model
entropy generation or exergy analysis can be performed for concretizing the energy saving potential in especially the heat flow systems.
A multi-domain ship energy flow model in Simscape simulation environment was utilized for the study. The ship model included all main components in the ship machinery, including the most relevant auxiliaries. The model and its
components included the necessary functionalities and the system requirements and limits that are relevant for concept level design. Once the system components were defined, the expected yearly operation profile was included
in the model, including estimation of the electricity consumption, propulsion power consumption and heating- and cooling requirements. After this, the true saving potential in the system was evaluated by modelling the ship fuel
consumption and performing entropy generation analysis in the main components. Based on the analysis and optimization process, the characteristics of improved, energy efficient machinery for a cargo ship could be recognized.
As a result of the study, various types of waste heat recovery technologies available onboard ships were studied from the perspective of technical principle and application feasibility. Furthermore, an optimal solution for improved, energy efficient cargo ship machinery was defined, considering the pre-defined system limits. With the studied technologies the excess engine heat could be utilized to meet the cooling, heating and power demand onboard that otherwise
would require additional fuel.
The ship-specific fuel saving potential depends always on the actual operation profile that can be to some extend always evaluated at the early phases of ship design project. The developed solution included a new system analysis
method for supporting the ship concept development. The whole method aims for holistic optimization of ship energy efficiency.