# Developments in Electric and Green Marine Ships
## 4 Calculating onboard PVs and Battery Capacity
There will also be additional power entering the batteries coming from the solar panels placed on the top of the ship. In order to calculate all the above things, such as the capacity of the installed batteries, the area they cover, their contribution to the total energy required for a specific trip of the ship, etc., a specific kind of batteries and ship should be chosen. As far as the ship is concerned, the research will focus on the Nissos Mykonos, Hellenic Seaways. For the batteries, the criteria of selection were: maximum power output, total capacity, feasibility of installment, and energy capacity to area ratio. Reminding the different types of marine batteries already operating in Figure 6, the best energy capacity to area ratio was that of the lithium poly-battery. However, lithium-ion batteries are also being used in marine operations. Therefore, the battery system selected for the present research in none of the six types already presented, but another--again, a lithium-ion battery--used on shore for grid energy storage. The prototype battery system is placed in Escondido, California [70]. The system has a power output of 30 MW and a total capacity of 120 MWh. These numbers show that the system provides enough energy, which makes it a considerable idea. Moreover, as far as the feasibility of the installment is concerned, the system consists of 19 containers. Each container has a total capacity 6.316 MWh, and a total of 120 MWh is provided by 19 containers. The length of each container is about 13 m, so a total of 19 containers will occupy a significant, even forbidding amount of space on the ship. For the above reason, a total of nine containers is chosen. These containers have a total capacity of:  
Ampere-Hour Capacity = (Ampere-Hour Capacity of one Container) \(\times\) Number of Containers  
\[ \begin{array}{l}=6.316\text{ MWh/containers}\times 9\text{ containers}\\ =56.844\text{ MWh}\end{array}  \tag{6}\]  
Reminding the power requirements of Nissos Mykonos, \(\text{P}_{\text{NMHS}}\) = 32 MW, in order to find the percentage that the battery system can cover, the duration that the ship is going to operate is required. For example a four-hour trip will require half the energy of an eight-hour trip. Assuming a seven-hour trip, \(\text{T}_{\text{trip}}\), the energy requirements of the Nissos Mykonos, \(\text{E}_{\text{NMHS}}\), will be:  
\[ \begin{array}{l}\text{E}_{\text{NMHS}}\text{=P}_{\text{NMHS}}\times\text{ T}_{\text{trip}}\\ =32\text{ MW}\times 7\text{ h}\\ =224\text{ MWh}\end{array}  \tag{7}\]  
So, the percentage of the batteries system contribution to the total energy needed by the ship is around 25.4%, and 101.6% of the energy required for the electricity needs. Respectively, for a four-hour trip of the Nissos Mykonos, Hellenic Seaways, nine containers provide 44.4% or 177.6% of the total energy for the trip or the electricity needs respectively. It is important to clarify that the required energy is calculated, assuming that the vessel's engine is operating at maximum power, utilizing entirely its 32-MW engines, throughout the trip. Within shipping, ships usually operate at the nominal continuous rating (NCR), which is 85% of the 90% of the maximum continuous rating MCR. The 90% MCR is usually the contractual output for which the propeller is designed. Thus, the usual output at which ships are operated is around 75% to 77% of the MCR. Ferries especially belong to the category of vessels that do not travel at their maximum power. Instead, they very often follow a slow steam (strategy), reducing the fuel consumption to a minimum. The energy required from the battery storage is even less a cause of real NCR, and is using the maximum power of the engines. However, the power stored in the battery system must be converted into propulsion through an electric motor. These transformation from power to propulsion has losses, but these are significantly smaller than those of a fuel engine. Electric motors can easily achieve 85+% efficiency [71; 72].