Although the idea of powering your vessel with the indirect, two-step diesel electric energy transfer system may appear to be unnecessarily complex, its many advantages can make it a compellingly attractive alternative to a conventional direct mechanical prop-shaft drive. When under way all of the electrical power required by the vessel can be supplied from the diesel electric propulsion system, eliminating the need to run a genset. A diesel electric power system can drive multiple propellers from a single engine or use multiple engines to power one or more propeller. In a twin-engine/twin-prop system, one engine can power both props when operating within the speed limits imposed in many areas. Electrical power from the vessel’s genset can be used to propel the boat, providing a built-in backup-especially valuable for vessels with single-engine installations. Conversely, the propulsion system can serve as a backup for the gensets.
The use of electric propulsion in certain vessel types is well-known. In marine applications, nearly all the energy is produced by diesel engines. Using an electric propulsion system, where the energy transmission is electrical and the propulsion and thruster are variable speed electrically driven, fuel consumption can be reduced significantly for many vessel types with environmental benefits.
Diesel Electric Systems have been in use to propel vessels for more than 100 years. Branobel launched the first diesel-electric ship in 1903, and since that day, diesel electric propulsion systems have evolved and today they can be found in all boat sizes and applications.
But how do you know when to utilise diesel electric technology for your vessel?
The investment in Diesel Electric vessels have doubled in the past 4 years, while the construction of purely mechanically propelled vessels have slowed down. But what are the reasons for this growth?
Some of the benefits of the Diesel Electric systems are:
Effective design: The ability to locate your generators in any part of the vessel independent of where the power will be used;
Smaller Engine Rooms: Possibility to replace a big slow speed engine with multiple smaller generators;
Reduced Noise & Vibration: No need for long drivelines;
Flexibility: Capability to share the power of one unit with multiple devices (main propeller, bow thrusters, hotel load, pumps, etc);
Redundancy: Generators can be reassigned to cover any machine downtime;
Efficiency: Depending on the application the system can provide better fuel efficiency (mainly if there are requirements for long periods of low speed/load)
Fuel consumption savings calculation:
The optimum operation point of a diesel engine will typically be around a load of 85 percent of the Max continuous rating (MCR). Moreover, the efficiency level drops quickly as the load becomes lower than 50 percent of MCR. With the help of the electric system, the mechanical propulsion prime mover is replaced by diesel-electric prime movers that will automatically start and stop as load demand varies. In comparison to a conventional vessel with mechanical propulsion, this enhances the efficiency of the energy usage and reduces the fuel consumption by keeping the average loading of each running diesel engine close to its optimum load point. However, in some vessel applications, the load variations can be large and rapid. It is impossible to make the generators switch on and off every five seconds as would be the case with DP vessels.
By using super-capacitors to supply the load variations, and hence let the diesel engines provide the average load, the peak power of the power plant will be reduced, allowing the average loading of the engines to increase to a more optimal point with lower specific fuel oil consumption. The savings in fuel consumption will depend on many parameters such as actual variations in the load, the average load and the number of prime movers.
In many installations the cost of a single-engine twin-prop diesel electric power system will be no more than a conventional twin-engine direct-mechanical drive installation. Properly integrated into the design of the vessel, the diesel electric drive’s many attributes will likely make it the preferred propulsion system for vessel owners and operators now and in the future.