## 4 Case Study: A LNG Refueling Station for Maritime Applications
### Wind and LNG Plants Integration  
Once the configuration and the size of the LNG plant and the off-design behavior are known, the objective was the determination of the optimal size of a wind plant to be integrated into the system in order to provide the required electricity for the liquefaction process. Since, for the specific application, a certain amount of LNG must be guaranteed every week, the optimal wind size has been determined accounting for the plant operation during a whole year. As a consequence, with this purpose, the production data of existing wind farms and the ones available from TERNA [32], related to the whole Italian territory, have been considered. Thus, starting from these data, the normalized wind production profile (on an hourly basis) has been obtained (Figure 8). Then, a dedicated code, developed in VBA, has been developed in order to consider the coupling between wind and LNG plants: the objective was the determination of the wind size, which allows contemporarily to respect the constraint of LNG production (around 4000 kg/week every week per year) and to minimize the electricity introduction into the national electric grid. The developed code, indeed, allows to evaluate the yearly operating time of the LNG production process, the surplus of  
Figure 8: Normalized wind production profile [32]. Figure 7: Non dimensional total specific energy consumption and the LNG produced mass flow rate, as a function of the total electrical input. electric energy not used by the plant--and then introduced into the electric grid--and the total specific electric energy consumption as a function of the wind plant size. In more detail, the code--for each considered wind plant size--analyzes the coupling of the wind and LNG plants for a whole year, hourly based:  
* if, in the considered hour, the electrical production of the wind plant is lower than the lower operating limit of the LNG production process, then all the electric energy is introduced into the electric grid;
* if, in the considered hour, the electrical production of the wind plant is within the operating range of the LNG plant, then all the electric energy is employed in the process;
* if, in the considered hour, the electrical production of the wind plant is higher than the upper operating limit of the LNG production process, then the difference between the wind electric energy and the maximum electric load of the LNG plant is introduced into the electric grid. The results of the coupling LNG and wind plants analysis are shown in Figure 9, in terms of the yearly LNG production (red curve) and electricity surplus to be sold to the grid (green curve), as a function of the wind size. As it can be seen, the wind size which allows to produce the LNG amount yearly required (about 4000 kg every week for a total of about 208,000 kg/year) is equal to about 78 kW. In this case, the electric energy surplus to be yearly sold to the grid results equal to a value slightly lower than 33 MWh/year. For wind sizes lower than 78 kW, the mass of LNG produced does not fit with the ferry needs constraint and, consequently, should not be considered for this case study. On the other hand, for higher wind sizes, an excess of LNG production is seen with respect to the case study constraint. However, it should be highlighted that, for wind sizes higher than the one selected as the optimal for the presented case study, a further decrease in electricity excess to be sold to the network is registered. This behavior can be explained considering the upper limit of the operational range of the compressors. Finally, in order to evaluate the LNG plant hours of operation, the wind production during a year is shown in Figure 10. As it can be seen, the LNG plant--during a whole year of operation--works for 187 h/year in design conditions and for 4720 h/year in off-design, split between 1455 h/year in which the electrical input is higher than the rated electrical power and 3265 h/year in which the electrical input is lower.