# Analysis of a Supercapacitor/Battery Hybrid Power System for a Bulk Carrier
## 4 Results and Discussion
### Economic Study
However, the payback period increased to around 10 years or deceased to around 4 years according to the electricity and fuel costs, as shown in Figure 11(a),b; these were more critical variables compared to the LIB or SC cost during the lifetime of a ship, as shown in Figure 11(c),d. For a 25-year lifespan of a ship, the total savings would be about 0.78 million USD, and the difference was greatly dependent on the fuel and electricity cost, as shown in Figure 13. Even though electricity cost was additionally included for the proposed system, it could be economically beneficial because of the fuel savings (up to 60.2%) compared with the conventional one. In addition, the payback period varied depending on the ship's schedule. As shown in Figure 14, when the cargo handling time was 60 h, payback occurred at around 10.2 years, but this decreased to 4.2 years for 180 h of long cargo handling operations. And, as shown in Figure 15, when the sailing time was 20 d, payback occurred at around 8.2 years, but this decreased to 4.5 years for 5 d in a short sailing time. Therefore, the proposed system is more economical if a ship has a long cargo handling time or visits many ports with a short-term sailing time. Figure 12: Cumulative saving costs for the lifespan of a ship depending on each variation. Figure 13: Total life cycle costs for conventional and proposed systems based on present values (25 years; i = 5%; a, b, c = 2%; electricity cost = 9.2 cents/kWh). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type** & \multicolumn{2}{c}{**Conventional Power System**} & \multicolumn{2}{c}{**Proposed Power System**} \\ \hline  & Equipment & Unit cost (\(\lambda\)W/yr) & No. & Equipment & Unit cost (\(\lambda\)W/yr) & No. \\ \cline{2-6}  & Generator (700 kW) & 15 USD & 3 & Generator (500 kW) & 15 USD & 1 \\ Fixed O6M & Crane converter (300 kW) & 2 USD & 4 & Generator (700 kW) & 15 USD & 1 \\ cost/year & - & - & - & LIB (40 kW) & 3 USD & 2 \\  & - & - & - & SC (1.54 kWh) & 5.55 USD & 4 \\  & - & - & - & Converter (LIB) (40 kW) & 2 USD & 2 \\  & - & - & - & Converter (500 kW) & 2 USD & 4 \\  & - & - & - & AMP converter (750 kW) & 2 USD & 2 \\ \hline
**Total cost** & \multicolumn{2}{c}{**33,900 year USD**} & \multicolumn{2}{c}{**23,674 year USD**} \\ \hline
**Type** & Fuel & No. of visited & Unit & Fuel & No. of visited & Unit \\  & Consumption & ports/yr & price & Consumption & port/yr & price \\ \hline HFO & 17.784 (l/port) & 20 & 400\(\mu\)t USD 1  & 17.426 (l/port) & 20 & 400\(\mu\)t USD 1  \\ MGO & 16.487 (l/port) & 20 & 630\(\mu\)t USD 1  & 0.192 (l/port) & 20 & 630\(\mu\)t USD 1  \\ Electricity & - & - & - & 80,502.41 (\(\lambda\)W/port) (Table 15) & 20 & 9.2 cents/kWh 2 [ENDFOOTNOTE] \\ (AMP) & - & - & - & 80,502.41 (\(\lambda\)W/port) (Table 15) & 20 & 9.2 cents/kWh 2 [ENDFOOTNOTE] \\ \hline
**Total cost** & \multicolumn{2}{c}{**350,008 year USD**} & \multicolumn{2}{c}{**289,952 year USD**} \\ \hline  & & & & & & \\ \end{tabular}
\end{table} 
Table 19: Comparison of the operational expenditure (OPEX) for each system per year.