Question

Case 1: ALL ELECTRIC SHIP

Design the electric power system for a ship of about 50 meters long, 7 meters wide, 4 meters deep, 3 meters draft design, and it has a cargo capacity of 1000 tons. The powertrain should be equipped with electric propellers and a mix of supercapacitors and battery pack. The powertrain reportedly should enable a range of at least 100 km on a single charge.

 Search the Internet to find the best type of high capacity batteries to design suitable battery pack with enough energy capacity in MWh. Draw the block diagram of the pack showing the number and way batteries are connected.

 Find the number of electric propellers needed and estimate their sizes. Search the Internet to find equivalents from manufacturers.

 Investigate the power electronics devices involved in the design, size these devices and search for possible manufacturers.

 Propose an innovative way to recharge the battery pac

Answer 1

large cruise liners have become Electric Ships, in the sense that onboard thermal engines (diesel and/or gas turbines) are used exclusively as prime movers of the synchronous generators. Such a  process is shown in below Fig.

• ELECTRIC PROPULSION (EP) SYSTEM in Electric Ship

EP systems are created installing one or more electrical drives for each propeller. EP motors functionally replace conventional slow-running diesel propulsion motors, and are fed through power electronic converters. Generally, all-electric ship liners are equipped with two propellers (so a total of 30-40 MW). Electrical propulsion brings a series of well-proven advantages both to the marine architect and the ship-owner: a) superior dynamics (start, arrest, speed variation) offered by electric motors with respect to conventional diesel motors (or gas turbines); b) possibility of accommodating electrical motors with more flexibility, installing shorter shaft lines thus eliminating the rudder and improving maneuverability; c) reduced fuel consumption due to the modulation of thermal engines running (the number of generators on duty is adjusted in order to keep them working at their minimum Specific Fuel Oil Consumption (SFOC)); d) higher comfort due to vibration reduction (thermal engines run at constant speed, therefore vibrations filtering is much efficient); e) high level of automation of the engine rooms and related reduced technical crew manning. Largest propulsion drives have employed and still adopt synchronous motors.

• SHIPBOARD ELECTRIC POWER GENERATION AND CONTROL

A. Specific properties of power systems onboard AESs
The power system onboard AESs must satisfy requirements which are different from conventional ones “conventional” refers to both land power systems and mechanically propelled ships systems). A first difference regards the distinction between essential and non-essential users. Essential users are loads whose supply and correct service must be assured also in case of a major system fault (defined by rules and regulations), as their functionalities are essential for the ship’s safe operation. These traditionally include propulsion systems, rudder motors, thruster system, fire suppression systems, communication systems, emergency lights, and navigation systems. Nowadays, also air conditioning, ventilation, toilets, and sanitization systems are starting to being considered significant services in some class of ships (e.g. in cruise ships following Safe Return to Port regulation such systems have to be considered essential in certain areas, called Safe Areas). In fact, although not being essential, they assure the on board living standards. A second aspect regards the absence of an infinite power bus onboard a ship (the IEPS is a weak system). Therefore, the insertion or disconnection of both large loads and generators can result in electro-mechanic perturbations, larger in magnitude and longer in recovery times in comparison with conventional power grids (i.e. the EP can absorb more than 50% of the total installed power; therefore strongly affecting the IEPS management, both in steady state and during transients). According to these premises, the IEPS design requires a strong systemic approach, with a particular attention to the functional integration of the different subsystems. Conventional power plants knowledge is not enough, because the IEPS of a large AES includes almost all the possible electrical engineering subsystems: a large power station with generators working in high voltage (HV, intended as voltage above 1kV in shipboard applications); a main HV distribution system; a secondary distribution system working in low voltage (LV); almost each kind of electrical machinery used in industrial applications (both in HV and LV, either direct-on-line or supplied by a variable-speed drive). A modern IEPS exploits also an extended use of power electronics, real time control systems (lower automation layers), and distributed automation systems (higher automation layers), each built and installed by different suppliers, which have to be fully integrated, representing the core of the so called Power Management System (PMS). In a world that in land would be defined as a “micro”-grid, the large power levels and the degree of ICT applications dedicated to power control make the AESs’ IEPS a natural-born multi-MW smart grid. Therefore, its design require the application of the best practices available, also because no series production is foreseen: each ship is different, so each time a different, fully customized, IEPS has to be designed. Finally, it has to be remarked that in an AES almost all the loads are powered by the IEPS, making it a system with high levels of QoS requirements. In fact, it is clear that a total blackout must be avoided at any case, because it results in both the total loss of ship’s maneuverability and in the loss of the life support systems.

• Typical electric propulsion chain with Diesel Engine

• Typical battery propulsion chain

• Best type of battery for ship

The high quality and long life batteries from Furukawa Battery can be supplied either as part of an integrated marine or supplied as stand-alone battery packs (with cables) or individual units.

VRLA (valve-regulated lead-acid battery) technology based Batteries

These are a high quality and long life VRLA battery technology suitable for standby, emergency or cycle use. They are also suitable for applications in which charging and discharging are repeated frequently. On ships and marine platforms these batteries can be used for emergency or back-up power and charged from a range of charging systems including solar power.

• Electric propellers needed.

2 LCI converters (48 pulse reaction) on two propellers with a capacity of 10 MW each can be the best suitable.

• An innovative way to recharge the battery pack

The below figure shows the connection diagram for battery charging.

• Below methods which can be used to charge VRLA Batteries

1. Constant current charging
2. Constant voltage- Unlimited current charging
3. Modified constant Voltage - Limited Current Charging.