Storage systems

The use of onboard DC grids for large ships brings up a new and interesting issue : storage systems. The main goal of the MVDC Large Ship project has been the study of new applications of storage systems in order to improve performances and the safety and security levels of the onboard electrical system .

Dynamic power supplied to a thruster with storage systems (green line) and without storage systems (red line).

The project investigated the use of storage units to solve DC network stability problems occurring in the presence of constant power loads.

The analysis of the different applications clearly showed that storage systems can play a key role not only in reliability and safety but also in improving the performances and lifetimes of diesel generators.

The first topic considered in the project concerns the docking manoeuvre of a large ship. During this manoeuvre, the use of storage systems reduces the time needed for the thrusters to reach their full power. Another benefit connected with the use of storage systems during docking manoeuvres is connected to the reduced stress on diesel generators, implying an increase in their lifetime and a reduction in maintenance costs. The size of a storage system capable of achieving this goal for the tested ship is about 1 m³ with a load power of 1 MW and a total stored energy of 2.5 kWh.

The second area of the analysis regarded the use of storage systems to supply the onboard grid during reconfiguration periods due to a fault in one generator. The size of the storage system for this application, and for the tested ship, is a 7 MW unit capable of working for several hundred milliseconds.

The third area of the study concerns the application of storage systems to elevate the fault current in AC networks supplied by means of DC/AC converters. If high power loads are present in these networks, the fault current, limited by the DC/AC converter, can be too low for the normal operation of common protection systems. Using storage systems this problem can be solved by increasing the fault currents and making the protection systems act correctly . A storage unit of 0.1 m³ is necessary for a 250 kW AC network.

Network diagram, including storage systems.

The studies focused in particular on the size and weight of the storage systems. By combining the storage systems dedicated to different solutions into concentrated units, significant results could be achieved, of an acceptable size and weight for naval applications.