Battery and Hybrid Propulsion on Ships

“Battery propulsion” means that electrical energy is stored in batteries and used to propel the vessel. On a battery-electric ship, electric motors drive the propellers. Hybrid propulsion, by contrast, combines a battery with at least one other energy source or an additional means of propulsion, such as an internal combustion engine and generator.

Fully electric and hybrid

With battery-electric propulsion, the propulsion motor is powered by stored electrical energy; electrical systems and equipment on board also require power. An energy management system distributes and monitors the electricity. The battery is an energy storage device and must be charged beforehand, for example from a suitable external power supply or by onboard generators in a hybrid system.

In a diesel-electric system, an engine drives a generator that supplies electrical energy to the propulsion motors. A hybrid system can also integrate batteries into the same energy system. Manufacturer Wärtsilä describes both diesel-mechanical arrangements with a PTO/PTI gearbox—which can use engine power to generate electricity or, when needed, feed electrical power into the propeller drive—and diesel-electric variants with generators, electric drives and energy storage. “Hybrid” therefore does not refer to one single, fixed design.

Operating modes depend on the system

An energy management system can control the flow of power between engines, generators and the battery. In one example system from Wärtsilä, the battery buffers short-term fluctuations in demand, allowing generators to run more steadily. The battery can also provide additional power for short periods or—if the design permits—temporarily supply power at low speeds or in port. In this operating mode, the available time is limited by the stored energy and the power demand.

The modes a ship can actually use depend on its specific architecture, battery capacity, generators, power electronics and control system. The word “hybrid” alone does not indicate the vessel’s range, how long it can operate in fully electric mode, or any particular reduction in fuel consumption or emissions.

Distinguishing the energy source from the propulsion system

Two separate questions help clarify the distinction: Where does the energy come from, and how does it reach the propeller? A battery stores energy; an electric motor converts electricity into motion. A ship can therefore be electrically propelled even if its electricity is generated on board by an internal combustion engine and generators. The engine room connects propulsion with the ship’s onboard power supply, while shore power is one possible charging source, but is not a requirement for every hybrid system. Wärtsilä’s overview of hybrid propulsion systems shows several such energy pathways.