Electric Ship. State of the Art and Integration of Components and Systems on Board

In the early days of mechanical propulsion, a combustion engine was associated with the propeller, and the ship's speed was linked to the rotation speed of a diesel engine or a steam or gas turbine. While thermal engines operate satisfactorily at full power, at the lower speeds imposed by mission profiles, they do not operate optimally: poor efficiency, clogging... hence the need for modular solutions for power generation, so as to adjust production capacity to power requirements. This can only be achieved by starting with an on-board network on which the number of generators (two, three, four alternators) can be modulated, and this is how electric motor propulsion came to the fore.

Electric propulsion systems with DC motors and thyristor converters have been used on low-tonnage special-purpose vessels since the early 1960s. Then, at the end of the 1970s, the generalization of the principle of speed variation with synchronous machines and large-calibre thyristor static converters enabled these applications to be extended to powers in excess of 20 MW per shaft line.

As a result, the DC motor was gradually abandoned in favor of AC machines, due to its many constraints.

So, evoking variable-frequency electric motors and fixed-frequency networks, we naturally go through the creation of a rectifier function to obtain a DC voltage and a ripple function to obtain the propulsion machine frequency.

Given this dual AC/DC/AC electrical conversion, the question arises as to whether a global DC power supply might not be more appropriate.

Advances in power electronics have therefore updated this old dilemma - continuous or alternative distribution - for on-board applications.

However, at significant power levels, direct DC power generation can only be achieved by rectifying AC voltages.

Fuel cells and solar systems have yet to reach maturity.

To date, an electric ship is a vessel with electric propulsion and auxiliaries, and AC or DC distribution. Both modes of distribution have their advantages and disadvantages.

So, for military or low-emission vessels, DC distribution is more easily adapted to electricity storage, either in batteries, supercapacitors or electromagnetically, depending on the vessel's mission.