Direct current (DC) is back in the news. You might have noticed it mentioned in relation to charging batteries for electric cars. DC lost its biggest battle in the 1890s, when it fought against alternating current (AC) as the method of choice for transmitting large amounts of electricity. But DC is back with us, flowing from batteries not only to power torches and smartphones but also aboard vessels. It has a new role in marine propulsion.
In an alternating current (AC) the flow of electrons switches back and forth at regular intervals. Direct current (DC) flows consistently in one direction.
For many years, electric propulsion plants employed AC systems. But times are changing. Thanks to new DC components and an innovative engine control philosophy that operates the gensets on variable speed, electric propulsion has evolved, creating a much more efficient solution with a wide range of potential applications.
The war of currents
In 1890 Thomas Edison (for DC) and George Westinghouse (for AC) were fighting for control of the large-scale supply or electricity and the construction of power grids in the USA. Westinghouse won. The advantages of AC predominated at the time thanks to advantages such as the easy transformability of the voltage and simple current interruption. The technology for efficient DC handling was not ready.
Efficiency affects the choice of currents
DC in marine propulsion technologies
MAN EPROX-DC uses almost the same components as a classic AC-based propulsion system, but arranged in an innovative way. DC removes the AC main switchboard.
Fuel-saving benefits of EPROX-DC
Using a DC grid allows the diesel engines to operate independently on variable speeds for more fuel efficiency. If you then integrate an energy storage system, like batteries, you get the following benefits:
- Operation of engines close to their design point with minimal fuel oil consumption
- Optimized loading of engines which are in operation
- Peak shaving and load equalizing on the engines
- Dynamic and transient response support of the propulsion system
- Electrical spinning reserve reducing the number of standby engines
- Reduction of running hours on the engines
- Less number of cylinders to be installed to achieve the same performance
- Zero emission mode, i.e. in harbors
EPROX-DC propulsion solution on anchor handling tug supply vessel
For applications of over 20MW installed power, MAN Energy Solutions applies an EPROX-AC solution. The “Dynamic AC” concept, developed by ABB, provides substantial fuel savings for large vessels, like cruise liners. Fuel consumption in the vessel is again optimized by adjusting the rotational speed of the engines and allowing the system frequency to vary within a specified range. Here all engines run on the same (variable) frequency.