Shaft generator

As an alternative to auxiliary engines, electrical power can be generated using a shaft generator that converts mechanical energy from the main engine into electric energy. The savings are achieved by the fact that the specific fuel consumption (g/kWh) is normally significantly lower on the main machinery than on the auxiliary machinery and thus should be used to the greatest extent possible.

Shaft generators do not represent any new technology on ships, however, the technology have developed significantly over the recent years. Today, shaft generators can typically be operated in different modes, respectively “power take-off” (PTO):

• PTO: The main engine generates power beyond the propulsion requirements on the shaft which is converted into electrical energy and distributed to the power distribution board. PTO is often used as a full or partial replacement for auxiliary motors in cases where the main machinery can be operated with a steady load over time and produce the necessary power for the grid.

Applicability and assumptions

Shaft generator is applicable for vessels with a diesel mechanic propulsion, for all ages.

Smaller four stroke auxiliary engines are compared to larger two stroke main engines generally less efficient, having a higher specific fuel consumption (g/kWh) resulting in more expensive operations and higher emissions. Onboard different ships there are many different types and configurations of auxiliary and main engines, but the vast majority sail with large two stroke engines in combination with smaller auxiliaries. Installing a shaft generator on this more efficient engine can be done directly to the main propulsion shaft, or with a gear box to the main shaft.

The latest shaft generator configurations can be used independently of shaft speed and maintain a stable voltage and frequency output, this makes it possible to optimize each route with parallel auxiliary operation. Use of shaft generators can reduce the maintenance costs and lubrication costs for the auxiliary engines. The numbers of auxiliary engines or the size of the auxiliary engines can also be reduced. Using shaft generators rather than auxiliary engine for electric power generation typically also reduce noise and vibration levels.

Installing a shaft generator on the typical main engine is as such by itself more efficient than producing the same power via a smaller and less efficient auxiliary engines, but for many cases the shaft generator would in addition increase the total load of the main engine closer to the optimum load point with minimum specific fuel oil consumption.

Shaft generator is an option for many types of vessels, especially those in need of larger amount of power for heating or cooling, and sailing long transits.

Additional measures advised include new torsional vibration calculations.

Cost of implementation

A typical shaft generator will cost around $450 (USD) per kW. Depending on the required power output and vessel type it estimated that the cost of implementation will be in the range of $520,000 – $3,500,000 (USD).

Reduction potential

The reduction potential for fuel consumption and emissions are greatest for shaft generators with PTO functionality. Hence where the shaft generator contributes to propulsion power support and has the capability to replace the auxiliary engine’s power production. Furthermore, smaller, but still significant, reductions can be expected for conventional configurations where the shaft generator exclusively contributes to replacing the auxiliary engine power production.

The emission reduction will also depend on the type of machinery installed. If the main engine is a two-stroke engine and the auxiliary engines are four-stroke, the emission reduction is potentially large since energy for auxiliary purposes can be generated by the more efficient main engine. The energy efficiency of a PTO must be seen in the context of the propeller and operation of the ship. If a ship has a controllable pitch propeller (CPP) and PTO without a frequency converter, the shaft must be operated at a constant RPM to get the correct frequency out of the power output from the PTO (50 or 60hz). When the ship is sailing at lower speeds, the propeller must be turned to reduce the propulsive force as the revolutions are constant. This leads to pressure differences and cavitation which reduces the efficiency of the propeller. To avoid this, a PTO can be installed with a frequency converter so that the shaft does not need to be operated at a constant RPM and increase the energy efficiency of the ship. Such a set-up significantly increases the reduction potential at the same time as the ship can more easily install battery packs and/or fuel cells.

Total emission reduction is estimated to be 1-5%.

Other References

1. MAN Diesel & Turbo (2024) Shaft Generators for Low Speed Main Engines
2. Prousalidis, J., et al. (2012) Exploiting shaft generators to improve ship efficiency
3. ABB (2024) Shaft generator drive for marine
4. Wärtsilä Technical Journal (2016) Shaft Generators: propelling vessels toward leaner, greener power generation
5. Avdeyev, V., et al. (2017) Increase of operating efficiency of ship electrical generating plant with shaft generator