IEEE Electrification Magazine - December 2019 - 25

Cooling systems,
which are required
to prevent
overheating, also
contribute to
reducing overall
efficiency.

power levels, i.e., cars feature electric motors ranging between 50 and
250 kW, whereas on vessels it can surpass 1 MW of power.
The electrical motor, one of the
main components, transforms electrochemical power from the batteries
to mechanical power in the propellers. The efficiency of this transformation is more than 95% for high-power
electric motors. However, the overall
system integrates other components
that affect its global efficiency and
onboard weight.
Required onboard are ac/dc and dc/ac converters, as
batteries use dc, whereas electric uses ac. Furthermore, to
recharge the batteries, the vessel must connect to the
harbor grid, which is powered by the public grid that runs
worldwide, albeit on variable frequency and voltage.
These devices are composed of magnetic inductors and
copper windings, whose efficiency is high; however, they
add to the weight onboard.
Wires, cables, and safety components are not negligible
in this type of application because high-power dc current
requires thick cables, which affect performance and
weight. Conductors heat up due to the Joule effect, and
they must be accurately designed to optimize efficiency
and preserve safety. The set of safety components includes
circuit breakers, electrical protections, and insulation. Cooling systems, which are required to prevent overheating,
also contribute to reducing overall efficiency.
Finally, there is the battery pack. The main concern
with using batteries as opposed to fossil fuels for transportation purposes, is their lower energy density in electrochemical storage, which is defined as the amount of
energy per unit mass. More specifically, the energy density in diesel fuel is roughly 13,440 Wh/kg, whereas a
lithium (Li)-ion battery has an energy density of approximately 220 Wh/kg. This means that more than 60 times
the weight in batteries would be needed to obtain the
same amount of energy as that which could be derived
from fossil fuels. Fortunately, electric motors have higher
efficiency (over 95%) compared to that of combustion
engines, which are less than 30% efficient in optimal conditions and decrease to below 20% during normal usage.
With this in mind, one can estimate the additional weight
of the batteries to be at between 10 and 20 times. Because
the efficiency of the electric motor is already high, present research efforts are concentrated on increasing the
energy density of batteries and the efficiency of the
recharging process.
Researchers are investigating materials with high electrochemical energy density, transitioning from old, lead acid
batteries to Li-ion batteries and even Li-air batteries, which
are expected to begin operating with an energy density
comparable to that of fossil fuels in the coming decades.

One of the challenges in the field of Liion batteries is the phenomenon of
dendrite formation, i.e., small spikes in
the Li anode that cause short circuits
between the anode and cathode.
Although such a solution is still far
from reaching the market, one possibility is to protect the anode by using a
graphene layer, which reduces the
problem of dendrites and attains a
high energy density of approximately
1,000 Wh/kg. Additionally, recent
research on the same technology
promises to triple the energy density of
graphene-based batteries through the use of an additional
silicon layer.

Onshore Electrification Components
The schematic representation in Figure 3 depicts a ground
line where the vessel connects to recharge. Charging systems can be categorized into two classes: offboard and
onboard. In the first class, the entire battery pack must be
removed and recharged in a specific station, and a new
pack is then placed in the vessel. This operation is fast,
and the vessel is able to continue its operations (maintaining full service). The second method involves the recharging of batteries via a direct connection between the vessel
and the charging inverter in the harbor. This operation
requires the vessel to stop and recharge, making the
length of charging time crucial. Currently, the preferred
solution is to recharge the batteries onboard, with ongoing

Onshore Grid

Magnetic
Plug-In
Coupling
Recharge

Onshore
Onboard

Onboard
Recharge Line
Battery Pack 1

Battery Pack n

Onboard
Services

ac/dc
dc/ac

Onboard
ac Line
ac/ac
M Motor 1 M Motor 2

Motor n

Figure 3. A simplified schematic representation of the required electric components.

IEEE Elec trific ation Magazine / D EC EM BE R 2 0 1 9

IEEE Electrification Magazine - December 2019

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2019