In most cases, the ship's electric propulsion system uses an internal
combustion engine to drive a generator set to provide power to the system. Due
to the complex and changeable marine environment, the load changes. When the
load deviates from the optimal load point, the fuel will not be fully burned,
the fuel utilization rate will drop significantly, and a large amount of
nitrogen oxides and sulfur will be produced. oxides, causing pollution to the
environment. Energy storage technology is one solution to this problem.
The energy storage unit is used to store excess energy when the system is
lightly loaded to prevent the impact of this energy on the power grid. When the
system is overloaded, the energy storage unit releases energy to meet the load's
needs. Energy storage technology is already well used in the electric vehicle
industry. The development of large-capacity energy storage technology has made
it possible for energy storage units to be used in ship electric propulsion
systems. Using energy storage units to overcome the impact of power fluctuations
on ship electric propulsion systems will be a new direction for the development
of ship propulsion technology in the future.
1Supercapacitor technology
1.1 Structure of supercapacitor
Supercapacitors, sometimes called electric double-layer capacitors, or
double-layer capacitors, are electrochemical capacitors with high energy
density. A standard battery-sized electrolytic capacitor has a capacitance of
tens of microfarads, but a supercapacitor of the same size can Up to several
farads, the difference can be five orders of magnitude. A capacitor stores
energy through the capacitance effect. Its capacity is directly proportional to
the surface area of the electrodes and the dielectric constant of the
electrolyte, and inversely proportional to the distance between the electrodes.
Supercapacitors use high dielectric constant electrolytes, and their electrodes
use porous activated carbon materials that can greatly increase their surface
area. This is the main reason why supercapacitors can store huge amounts of
energy. Typically, the two electrodes are separated by a porous activated carbon
membrane in the middle, with water or organic electrolyte on both sides. Figure
1 shows the structural principles of double-layer capacitors and ordinary
capacitors.
1.2 Advantages and Disadvantages of Supercapacitors
Advantages of supercapacitors:
1) High current capacity. Supercapacitors are designed with a low
equivalent series resistance, so the capacitor can send and sink very high
currents. The low equivalent series resistance of supercapacitors allows the
capacitors to charge quickly. The characteristics of the capacitors themselves
allow the capacitors to charge and discharge at the same speed, which is not
possible with batteries.
2) Long service life. The energy storage mechanism of supercapacitors is a
highly reversible process. This process only moves charges and ions without
creating or destroying chemical bonds, so the number of charge and discharge
cycles can reach hundreds of thousands of times. Neither small cycle charge and
discharge nor deep cycle charge and discharge will damage the performance of
supercapacitors, making the use of supercapacitors more flexible. In addition,
supercapacitors are easy to store and their performance will not be affected by
long-term placement.
3)Wide temperature range. Because supercapacitors work without resorting to
chemical reactions, they can operate over a wide range of temperatures.
Supercapacitors can operate normally within the temperature range of -40°C to
+65°C.
4) Environmentally friendly. The materials used in supercapacitors are
environmentally friendly and will not produce toxic or harmful substances during
operation.
5) Easy maintenance. Supercapacitors basically require no maintenance, have
no storage performance, no excessive discharge, and can work at any rated
voltage or lower than the rated voltage.
6) Status monitoring is easy. Since the energy stored in a capacitor is
only a function of capacitance and voltage, and the capacitance is relatively
constant, a single open circuit voltage measurement can determine the state of
charge.
7) Extend the service life of other energy sources. Energy sources like
batteries, specialized engines, and fuel cells do not perform well under
transient conditions. For some components, transient processes can significantly
shorten the service life of the component. Combined with the use of
supercapacitors and these energy sources, many of these transients can be
unloaded from the primary energy source.
Disadvantages of supercapacitors:
1) Low energy density. The energy density of supercapacitors is 1~10Wh/kg,
which is 1/10 of lithium batteries.
2) The terminal voltage changes greatly. The terminal voltage of the
supercapacitor constantly changes during the charging and discharging process,
so a voltage regulating device needs to be installed between the energy storage
component and the load to maintain load-side voltage stability, which increases
the cost of the energy storage system.
3) Expensive. The price of a supercapacitor cell is dozens of times that of
a lithium battery.
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