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. But in some special working conditions, such as dynamic positioning (DP) operation, the load varies substantially, for instance with wave disturbance and weather influence. The sudden load variation is a continuous disturbance of the electric system and the prime movers. Furthermore, to keep to the safety margins of the power generation plants, the average loading of running engines has to be reduced, which increases fuel consumption and environmental emissions.
1. Propulsion and control system layout for a DC grid vessel.
Fast-acting energy storage systems can solve these problems by effectively reducing load power fluctuations in a power system, due to their energy storage capacity. This will smooth sudden changes in power demand, improve the system’s stability and possibly increase the average loading with fewer running engines and thus reduce fuel consumption and maintenance. Super-capacitor technology is one among other solutions, such as batteries, flywheels or possibly in the future superconductors. In this paper, an offshore support vessel (OSV) is chosen as the target vessel. A hybrid converter incorporating supercapacitors will be modeled and simulated in Matlab/ Simulink simulation environment.
2. Structure and outlook of the super-capacitors by Maxell.
An OSV with electric propulsion is equipped with an electric power plant with variable speed drives to control the main propulsion and thrusters. ABB recently released the Onboard DC Grid solution. It adds to the full freedom for integrating and combining different engery sources, including renewables, gas and diesel, and a greater flexibility in placing system components in the vessel design. The main electric propulsion system topology for DC Grid system is shown in Figure 1. The super-capacitor can be used both in AC and DC Grid system to realise the energy storage function and increase the efficiency up to 20 percent.
Super-capacitors technology is a new type of energy storage device used increasingly in industry and automotive applications, such as cars, buses and high-speed trains. Unlike conventional capacitors, super-capacitors have a larger area for storing the charge and closer distance between the electrodes, which is why they achieve much greater capacitance within the same volume.
Compared with the batteries, super-capacitors have several advantages: super-capacitors can be charged extremely quickly, while many battery technologies are damaged by fast charging; supercapacitors can be cycled several hundred thousands of times whereas batteries are capable of only a few hundred cycles. They can deliver frequent pulses of energy without any detrimental effects while batteries experience reduced life-time if exposed to frequent huge power pulses. Super-capacitors can also be charged to any voltage within their voltage rating while batteries operate within a narrow voltage range. On the other hand, batteries can store much more energy than the same size of super-capacitors.
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, as shown in Figure 18. With the help of the electric system, the mechanical propulsion primer 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 DP 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 in the examples above. 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. For example, if one could increase the average loading of the running engines from, for instance, 40 percent to 60 percent, fuel oil consumption would be reduced by more than 10 percent.
Conclusion and further works
In the case studies above it has been shown that the use of super-capacitor for short-term energy storage in a thruster system can effectively limit the power fluctuations seen by the supplying network. The advantages of this are twofold. First, this reduction in power peaks can offset the need for bringing additional diesel engines online, thereby increasing the average loading of each diesel and improving diesel fuel efficiencies. Second, when a diesel engine is loaded and unloaded quickly, the combustion process in the diesel engines is adversely affected.
A reduction in rate at which they are loaded and unloaded will also reduce their fuel consumption significantly. For further work, it is important to quantify possible fuel savings and lifetime costs for larger systems. Investigation into other frequency converter applications found on board ships should also be performed.
Source : https://new.abb.com
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