## 1 Introduction
[19] carry out a quasi-steady state simulation of a regenerative ORC, integrated in a passenger vessel, over a standard round trip, [PERSON] and [PERSON] [20] investigate the thermodynamic and economic performances optimization for an ORC system for a large marine diesel engine of the merchant ship, considering also different working fluids, while [PERSON] et al. [21] present the design and off-design dynamic and steady-state models of three ORC systems that exploit the waste heat of the cooling circuit of dual fuel diesel Internal Combustion Engines (ICE), on board a LNG carrier, excluding the option of energy recovery from the exhaust gases. The present study analyzes complex solutions considering a lot of energy recovery options at the same time, with the aim of achieving the optimization of the on-board energy system. In the model, the installation on board of both ORC or Stirling groups has been considered, jointly with other technological options like absorption chillers, solar panels or collectors and thermal storages. This optimization must be performed both in terms of energy (minimum fuel consumption) and in terms of the overall dimensions of the system, while respecting the environmental constraint (minimum emissions). Furthermore, it must take into account the effective optimal management of the energy system during a cruise, which is different according to the different design choices. For these reasons, the whole problem is very complex and with a lot of choice variables. It is, therefore, necessary to define a model of the on-board energy system that is able to describe different plant configurations and different operating conditions, on the basis of which the optimal solutions will be subsequently identified, considering also the boundary conditions that have to be respected. In summary, the integrated on-board energy system may be modeled by describing the various components potentially supplying electricity and thermal energy on one hand, and the different possible connections with the electrical and thermal users on board on the other hand, taking also into account the different temperature levels. Models of this type generally assume as input the thermal and electrical demands of the users, including the drive systems (direct or electric) of the propellers. If different options about possible connections among suppliers and users have to be considered, it is not possible to face the problem of feeding each individual user and the management of the connected production units independently by the other users. In fact, the energy flow, for example the thermal flow, transmitted through each connection is a variable that must be determined by the optimization algorithm [23]. It is, therefore, necessary to proceed with the simultaneous optimization of the configurations of all production units, of the energy interconnections and of the optimal management of the whole system. In the present paper this problem is addressed by defining a mixed-integer linear programming model of the system (MILP) [24; 25], looking simultaneously for the optimal solution of its configuration and management. This technique is considered particularly suitable for solving problems of the type in hand, because it allows a sufficiently realistic simulation of the behavior of complex energy systems [26; 27; 28], reducing to an acceptable level the computational effort required. In the simulation, the ORC/Stirling energy recovery system, bottoming the main engines, is considered made up of a variable number of identical units, with defined size and performance [29; 30], possibly integrating the system with the installation of photovoltaic panels (PV), solar thermal collectors (ST) [31; 32], absorption refrigeration machines and thermal storages (TS) [33].