2016_1 Shaft Generators, propelling vessels toward leaner, greener power generation master

Shaft Generators: propelling vessels toward leaner, greener power generation

Energy efficiency is the name of the shipping game these days. In addition to lowering operational costs, increased efficiency often translates to fewer emissions and can mean better compliance with environmental legislation. Over the last few decades, shaft generator systems have become a more common option to help ship owners and operators achieve profitability in a sea of fierce competition and keep those waters cleaner for future generations.

Text: REINHARD SWOBODA Photo:

A vessel generates power for both the ship’s operational equipment as well as crew accommodation, such as the galley and cabin lighting. To achieve this, a standard generator burns a large volume of marine diesel fuel, which increases the operational cost, requires more frequent maintenance of the generator and contributes to air pollution. Merchant vessels typically spend most of their operational life sail long distances and fuel economy is the most important factor after safety and reliability. 

As an alternative, a shaft generator (SG) is driven by the ship’s main engines which, when compared to auxiliary diesel generators, generally have lower fuel consumption and can run on less expensive heavy fuel oil or liquefied natural gas (LNG). However, saving money on fuel is only part of the equation as tightening regulations on ship emissions means that reduced fuel burnt not only increases efficiency and reduces cost but also lowers overall emissions. 

Traditionally, the downside of this arrangement was that the propulsion machinery could only be run at constant speed to maintain the network frequency to within limits when operating with a Shaft Generator arrangement. This is because the ships network frequency is tied tightly to propeller rotational speed and any change in speed has a direct impact on network frequency which was traditionally overcome by controlling propulsion thrust and ship speed by changing the propeller pitch, which can lead to decreased efficiency and increased CO2 emissions. 

The ideal operational scenario is to enable efficient and reliable power generation when the ships propulsion system operates at varying speeds needed to maintain optimal propulsion efficiency. Adding a frequency converter to the shaft generator makes this possibility a reality. 

The frequency converter, designed using the most modern pulse-width modulation (PWM) technology and power switches, enables the option of taking power from the main engine whilst maintaining a stable frequency and voltage while the main engine speed is changing, such as at reduced speed, manoeuvring, or due to heavy sea conditions. Such Power Take Off (PTO) systems are available in a power range of 500 kW to 3000 KW in low voltage (450 V) technology and above 3000 kW up to 7000 kW in medium voltage (6600 V) technology. Direct connection to the ship mains without requiring a step-up transformer ensures an efficiency factor above 91%. 

These systems are available in slow speed technology with the shaft generator directly mounted in the main propulsion shaft line or driven by a gearbox, resulting in a higher speed range. Due to the simple design, installation in the propeller shaft line accounts for 90% of shaft generator applications. PTO shaft generators are offered with a variety of generator technologies but the most common design is made using a synchronous machine with top mounted rectifier and water cooler for an efficient, compact, low cost and light weight package.  These systems are also available in the same power and system voltage range as PTO/PTI (booster motor) drives and as PTO/PTH (Power Take Home) drives.

Once fitted as part of the shaft generator system the frequency converter can also be used to adapt various shore supply voltages and frequencies, with no need for additional panels in the main switchboard.  Connection to a shore based power supply is possible using the existing synchronizing equipment having the advantage of not using the auxiliary diesel engines in harbour, an additional environmental benefit. The arrangement is often used to supply the thruster motors via the shaft generator system with split busbars during manoeuvring, further improving efficiency and system flexibility. 

 
 

As an option for vessels with a Waste Heat Recovery Turbine System, Wärtsilä Electrical & Automation also offers a PTO/PTI system. A higher efficiency of the Waste Heat Recovery Turbine System can be achieved by integrating the turbo-alternator system using the same technology.

Due to the numerous advantages of shaft generator systems, more and more vessels are equipped with them. In summary, some of the greatest benefits include the following:

  • Lowering of fuel and lubrication costs
  • Reduction of maintenance costs and personnel on board 
  • Return on investment in 3 to 4 years
  • Increased safety for ship and crew
  • Low noise power generation
  • Smaller or fewer diesel generator sets
  • Continuous parallel operation together (two SG systems) or combined with diesel generator sets

The more than 450 systems delivered indicate that shaft generator systems are a valuable technical solution for ship owners who are looking for economical and cleaner electrical power generation during their sea voyage. In addition, the number of installations underlines Wärtsilä Electrical & Automation’s position as the world leader in this market segment. 

 

Shaft generators

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