A ship consist of a power plant system which generates enough power for the propulsion plant machines, engine room and deck machines, and navigation equipment of the ship, along with daily life demands of the crew onboard which includes galley, lift and cabin power supply etc. For this, large quantities of fuel are burned within the prime mover which in turns rotates the rotator in the alternator and generates power. The burning of fuel increases the operational cost for owner, burdens the engine crew with maintenance of the generator and the most importantly, it causes air pollution.
To eliminate the usage of independently driven generators when the ship is sailing in the mid sea, shaft generator concept is used. Shaft generator is a clean source of power, which means it does not burn any fuel to generate power, and for the same reason it is also called the green source of power.
Shat Generator: Principle of Working
In an A.C generator, in order to produce power, the stationary armature conductors are cut by the rotating magnetic field, produced by the rotation taken from the propeller shaft of the main propulsion plant or main engine.
The running machinery’s power is supplied through main switch board with constant voltage and frequency by diesel generator. In case of shaft generator, which is driven by main engine, the speed of the former may vary at different situations like ship sailing in traffic water and crossing canals, resulting in variation in voltage and frequency of shaft generator.
To overcome this deficiency, two systems are used Onboard ship
a) Power take off (PTO) system is incorporated with different kinds of frequency control system which makes sure of producing power with constant frequency.
b) Hybrid system consisting of an advanced power electric system for conditioning the power generated from shaft generator so that the supply to the switchboard always remains constant at any engine speed.
The modern application of the shaft generator includes its functioning as a motor by taking power from the electrical plant of the ship to drive the propeller at reduced speed.
This application is expensive to install and is used for vessels which moves very slowly or vessel which stays still most of the time.
Advantages of Shaft generator system:
1) The biggest advantage- it dose not cause air pollution unlike other traditional methods of power production in ship. Moreover, noise level is also low.
2) It is more cost effective as it dose not requires expensive fuel for power generation as main engine itself is a prime mover.
3) The wear and tear and hence the maintenance schedule and costs for the same reduces for independent driven generator.
4) Installation space is less as it is installed close or in line with the shaft of the main engine.
5) The investment cost depends on the type and system of the shaft generator but for a basic designed shaft generator it is low.
6) The installation cost for shaft generator is also low as it doesn’t require separate foundation, prime mover or exhaust system. Even time for installation is also less.
7) Low spare parts cost and man – hour cost as the schedule maintenance period for shaft generator has larger time gap as compared to diesel generator.
Disadvantages of Shaft generator:
1) For a basic shaft generator system, the efficiency of propeller and engine is reduced at low propulsion power. Since the frequency requirement is constant, for a main engine with a CPP, it has to run at constant speed even at low load.
2) No power generation in port as the prim mover is in stop condition.
3) Due to an additional attachment to shaft of the engine, the load in the engine also increases, resulting in increase in specific fuel and cylinder oil consumption when shaft generator is used.
4) Cannot cope up alone when the load demand is high as it may affect the main engine performance and maintenance.
5) It requires gears, couplings and other complicated arrangement for installation in some system.