# Developments, Trends, and Challenges in Optimization of Ship Energy Systems
## 2 Concepts, Definitions, and Limits of the Present Article
### The Energy System and Its Optimization Levels  
In general terms, an _energy system_ is any system that transforms energy from one form to another or to other forms or transfers energy from one place to another or from one body to another. This article deals with complex systems of interconnected components that convert primary energy to forms of energy useful on ships, i.e., mechanical, electric, and thermal energy. A diesel engine, a gas turbine, a heat exchanger, etc., fall under the general definition of an energy system, but the optimization of those as individual components, which is certainly applicable, is beyond the limits of the present text; instead, they are considered as components of a more general system. In the past, the propulsion, electric, and thermal loads were served by separate units: main engine(s) for propulsion, diesel generator sets for electric loads, and combustion boilers for thermal loads. The need for better exploitation of the chemical energy of fuels led to _integrated energy systems_, i.e., systems that, with proper interconnection of components, aim at covering as many types of loads as possible from the same primary energy source. While improvements and optimization of individual components, a never-ending endeavor, continue increasing their efficiency gradually, system integration and optimization can lead to step-function increase of the total energy efficiency. This is a challenging field, and it is certainly inside the limits of the article. The optimization of an energy system either on land or on a ship can be considered at three levels as initially defined in [4; 5; 6] and further elaborated in [7; 8]:Synthesis optimizationThe term _synthesis_ refers to the components that appear in a system and their functional interconnections. With synthesis optimization, the optimal configuration of the system is determined and the flow diagram of the system can be drawn. Design optimizationThe word _design_ here is used to imply the technical characteristics (specifications) of the components and the properties of the substances entering and exiting each component at the _design point_ (nominal load) of the system. One may argue that design includes synthesis too. However, in order to distinguish the three levels of optimization, the word \"design\" will be used with the particular meaning given here. Operation optimizationFor a given system (i.e., one in which the synthesis and design are known) under specified conditions, the optimal operating state at each instant of time is requested, as it is defined by the operating properties of components and substances in the system (speed of revolution, power output, mass flow rates, pressures, temperatures, composition of fluids, etc.). It is understood that each level cannot be considered in complete isolation from the others. Consequently, the complete optimization problem, taking into consideration that an energy system operates under conditions that change with time, can be stated by the following question:  
What is the synthesis of the system, the design specifications of the components, and the operating state at each instant of time that lead to an overall optimum? In many publications, design optimization is performed at a single set of conditions, usually called _design point_. However, the ship and, consequently, its energy system, operates at conditions that change with time due to, e.g., weather state, load carried, mission requirements, cost of fuel, etc. Thus, if only a single operating point is considered, most probably a suboptimal system will be obtained. Instead, intertemporal optimization is needed, having the following meaning [2; 7]:  
Intertemporal optimization is the optimization that takes into consideration the various operating conditions that a system encounters throughout its life time and determines the operating state at each instant of time that results in the overall minimum or maximum of the general objective function.