# Analysis of a Supercapacitor/Battery Hybrid Power System for a Bulk Carrier
## 4 Results and Discussion
### Economic Study  
An economic study was conducted to compare the conventional power system and proposed one. Some assumptions were made for the study (below) since it was difficult to obtain exact data from the industry, and the data were changeable depending on the cases. * Only the main equipment was considered;
* The bulk carrier visited 20 ports per year;
* The lifespan of the ship was 25 years. First, the initial capital expenditure (CAPEX) is the sum of the equipment cost for the system. The cost data for the main equipment were obtained from several references [43; 44; 45; 46; 47]. The cost of the LIB was assumed to be 600 /kWh USD, and it was changeable according to the C-rate capacity, cell type, and cooling method, etc. The CAPEX results for the conventional and proposed systems are shown in Table 18. Secondly, the operational expenditure (OPEX) is the sum of each fixed operation & maintenance (O&M) cost, fuel cost for genest(s), and the electricity cost; only the electricity cost for the AMP was considered for the proposed system (Table 19). The fixed O&M cost data of each main equipment were obtained from several references [48; 49; 50]. In this study, the variable O&M costs, which included cooling water or consum \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Sailing Time for** & \multicolumn{3}{c}{**Conventional System (ton/yr)**} & \multicolumn{3}{c}{**Proposed System (ton/yr)**} & \multicolumn{3}{c}{**Emission Reduction (\%)**} \\ \cline{2-10}
**Port-To-Port (d)** & \(\mathbf{CO_{2}}\) & \(\mathbf{SO_{X}}\) & \(\mathbf{NO_{X}}\) & \(\mathbf{CO_{2}}\) & \(\mathbf{SO_{X}}\) & \(\mathbf{NO_{X}}\) & \(\mathbf{CO_{2}}\) & \(\mathbf{SO_{X}}\) & \(\mathbf{NO_{X}}\) \\ \hline
5 (27 ports/yr) & 2174.79 & 13.85 & 39.06 & 1,077.69 & 12.78 & 14.06 & 50.45 & 7.73 & 64.00 \\
10 (20 ports/yr) & 2164.75 & 19.87 & 39.07 & 1,340.72 & 18.88 & 20.34 & 38.07 & 4.98 & 47.94 \\
15 (16 ports/yr) & 2174.84 & 23.58 & 39.36 & 1,506.52 & 22.63 & 24.22 & 30.73 & 4.03 & 38.47 \\
20 (13 ports/yr) & 2127.02 & 25.40 & 38.57 & 1,576.63 & 24.50 & 26.13 & 25.88 & 3.54 & 32.25 \\ \hline \hline \end{tabular}  
* Assuming that the ship stays at a port for 200 h with a cargo handling time of 120 h.  
\end{table} 
Table 17: Comparison of emissions according to different sailing times. Figure 11: Comparison of emissions for each operation mode. maintenance, were assumed negligible because they comprised a relatively small portion in general power systems [51; 52; 53]. It was also assumed that it was necessary to switch the onboard fuel from HFO to MGO in port areas to meet environmental regulations for the conventional power system. The total savings during \(N\) years is the sum of the yearly savings (S\({}_{\text{year}}\)), taking into account the interest rate (i) for the capital, as below [54]:  
\[\text{Total savings}=\sum\
olimits_{n=1}^{\text{N}}\frac{S_{\text{year}}}{\left(1 +\text{i}\right)^{n}}. \tag{11}\]  
In this study, i was set to 5%, and the annual inflation rates for the fixed O&M cost (a), fuel oil cost (b), and electricity cost (c) were set to 2% each. The replacement of the LIB and SC packs was considered with a replacement cost rate of 80% [55] of the initial cost. And the lifespan was assumed to be about 10 years for the LIB [56] and 15 years for the SC [17]. Thus, the payback period was obtained by solving for \(n\) when the initial investment cost was equal to the sum of the yearly savings. Payback occurred at around 5.8 years where the curve passed through the zero of the \(y\)-axis in the case of the below assumptions:  
* The electricity cost for the AMP was 9.2 cents/kWh;
* The HFO cost was 400/t **USD**, and the MGO cost was 630/t **USD**. * The LIB cost was 600/kWh **USD**, and the SC cost was 10,000/kWh **USD**.