## 4 Case Study: A LNG Refueling Station for Maritime Applications
### LNG Production Plant Configuration and Assumptions  
Based on the grid pressure levels available on the installation site, the value of 3 bar was chosen as supply pressure, thus the corresponding optimized layout resulting from the application of the thermodynamic design and optimization model is the one shown in Figure 2d with the Joule-Thompson valve for the LNG production. As already explained, indeed, the choice of considering the JT valve instead of the turbo-expander has been made considering the current technology development.  
For the sake of completeness, the optimal values of the thermodynamic parameters assumed to design the LNG plant and kept constant for the whole off-design analysis are reported in Table 3.  
As it regards the LNG production plant, it has been sized in order to be able to produce the required weekly amount of LNG with a security factor around 2 (corresponding to about 80 equivalent hours/week of operation). Based on these considerations, the LNG design mass flow rate was set at about 47 kg/h (0.013 kg/s).  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline NG composition & 100\% CH\({}_{4}\) \\ \(T_{1}\) & 20 \({}^{\circ}\)C \\ \(p_{1}\) & 3 bar \\ \(T_{2}\) & \(-5\)\({}^{\circ}\)C \\ \(p_{2}\) & 3 bar \\ \(p_{3}\) & 25 bar \\ \(p_{5}\) & 175 bar \\ EER chiller & 1.1 \\ \(T_{8}\) & \(-50\)\({}^{\circ}\)C \\ \(p_{11}\) & 15 bar \\ \(T_{4}\) & 30 \({}^{\circ}\)C \\ \(T_{6}\) & 30 \({}^{\circ}\)C \\ \(T_{amb}\) & 15 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table}
Table 3: Constant thermodynamic parameters for the design and off-design analysis (plant sections with reference to Figure 2d).  
The assumed compressor velocity, instead, was chosen equal to 1500 RPM according with the actual state of the art for what regards the compression of natural gas in the selected range of mass flow rate.  
By the application of the engineering design calculation model, in the basis of the results of the thermodynamic analysis and of the values of LNG mass flow rate and compressor rotational speed, it follows a power plant in design condition with an electrical power consumption slightly higher than 43 kW. Some of the results of the developed calculation are presented in Table 4, while an example of engineering design procedure can be found in the Appendix A of this paper.  
On the basis of the previous results, the application of the off-design model allows the drawn the curves in Figures 6 and 7.  
In more detail, Figure 6 shows the electric energy consumption of the compressors and of the chiller separately, as a function of the total electrical load of the system (i.e., the electric power required by the whole liquefaction process). As it can be noted, starting from the design point, the calculation model shows upper and lower operating limits for off-design conditions. In particular, the operating range can vary from 40% to 200% of the design electric load (this range derives from the operating range of the compressors). As it can be noted, the upper limit seems to be a quite high value and probably in real operation it cannot be reached, but it is presented here in order to show the validity range of the model. Furthermore, in Figure 7 the LNG produced mass flow rate are shown as a function of the plant load: an increase in this quantity with the electric load increase can be observed (with a variation relating to the design point from a minimum of \(-53\%\) to a maximum of about \(+62\%\)), since the electric energy available to the compressors is higher.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline \(P_{d,C1}\) & 17.81 kW \\ \(P_{d,C2}\) & 19.51 kW \\ \(P_{chiller}\) & 6.02 kW \\ \(P_{el,tot}\) & 43.34 kW \\ \(\hat{m}_{ING}\) & 0.013 kg/s \\ \hline \hline \end{tabular}
\end{table}
Table 4: Design parameters of the LNG production plant resulting from the optimization analysis.  
Figure 6: Non dimensional electric energy consumption of compressors and chiller as a function of the total electrical input.