Quietly Efficient

Within the highly competitive marine sector, all-electric vessels are sailing in favourable winds. AC drives used in various applications onboard also help meet the increased demand for efficiency and offer many other benefits

At the Quishan Shipyard in Wuhan, on the Yangtze River in inland China, four all-electric chemical tankers are being built for the UK coastal tanker operator F.T.Everard & Sons. The first tanker, named Speciality, will be delivered at the end of this year with the other tankers following at approximately six monthly intervals. The same customer also has an option on further deliveries of two additional tankers.

Propusion

Dutch main propulsion system integrator Imtech Marine & Offshore will supply full automation, main switchboards, motor control centres, motors including electrical propulsion drives, the bow thruster drive and the cargo, ballast and slop pump drives. All these will be controlled by Vacon NX drives, the total power of which amounts to about 3.2 MW per tanker.

The total power of the Vacon drives on this all-electric chemical tanker will cover the following: 2 x 900 kW liquid cooled frequency converters for the main propulsion, 350 kW bow thruster drive, 6 x 150 kW for cargo pumps and 3 x 52 kW for slops/ballast pumps.

Electrical main propulsion drive is now gaining considerable recognition for the numerous important benefits it offers over a diesel engine direct driven propeller, as Dirk Verschoor, Managing Director of Vacon Benelux explains to The Motor Ship. “Ships with diesel electric propulsion are at a clear break through. The main reason is that the first built electrical propulsion vessels are proving to be more economical in operation then traditional designed vessels. When taking diesel electric into account from first stage in the total design of the ship, the investment costs are about equal.”

Diesel-electric propulsion also offers great flexibility in ship design. Since there is no mechanical link between the diesel engines and the propeller, the various components of the propulsion installation can be ideally positioned on board. Verschoor continues: “There is a clear advantage in freedom of ship lay-out, optimised power plant and advantages using azimuth thrusters ? pods, e-motors for propulsion and space saving liquid cooled frequency converters. The operational costs are proving electric ships are more economical, meaning lower operational costs. Saving fuel with diesel electric propulsion will become even more important in the years ahead. Space saving liquid cooled propulsion frequency converters will have a clear future and Vacon is committed to be a technology leader in the electrical propulsion area from day one.”

Optimising the power generation plant also reduces the emissions of environmentally harmful gasses. Furthermore, in controlling the motor speed according to need, energy demand can be broken down into smaller components.

The new all-electric chemical tankers? main propulsion motor speed will be controlled by two 900 kW liquid-cooled Vacon NX drives. Speed control also decreases electromechanical stress for the electrical system and provides flexibility in power generation and distribution. A Vacon drive will also control the 350 kW bow thruster, essential in dockside manoeuvring.

One of the key characteristics of the all-electric tankers is the requirement for the lowest possible noise levels. Electric motors alone are quiet but this is further enhanced when they are controlled by liquid-cooled drives, which also have a low noise level. Liquid-cooled drives are quiet as no large fan or any additional air conditioning or filtered ventilation systems are required.

The man responsible for energy systems & components at Imtech Marine & Offshore, Cor van Miltenburg, explains more about why AC drives were the preferred option. “The basic requirement for the vessel is 45 dB(A) at 25 meter, this will give the vessel a permit to be in operation 24 hours a day in residential areas. The only way to achieve the 45 dB(A) noise requirement and designing an environmentally friendly cost effective ship is a diesel-electric concept. The diesel engines are running at a constant optimum noise and environment friendly speed driving the main generators. The generators are feeding the AC drives for the propulsion while sealing or feeding the electric power for the low noise frequency converter electrical motor discharge pumps.”

No noise

Consequently, using the six cargo pumps and three slops/ballast pumps for loading and unloading the cargo at ports and harbours does not cause noise problems. A clear advantage for the neighbourhood, especially at night time. In pump applications in particular, speed control also allows considerable energy savings and a short payback time.

At sea where vibration, humidity and salt can be harmful for both mechanical and electronic components, special requirements are placed on the choice of components and materials. The load tolerances of Vacon drives are well within the approved limits and robust designs protect the equipment against vibration and humidity in the harsh environment.

Imtech Marine & Offshore also opted for the Vacon liquid-cooled AC drives because of the considerable space savings allowed by their design. Compared with air-cooled drives, the compact design allows space savings of up to 70%. This is particularly beneficial on ships and vessels with limited space. In the liquid-cooled drives all components requiring cooling are mounted on one or more aluminium cooling elements. The entire power range can be manufactured with only six different chassis sizes.

Even though Vacon drives have an efficiency of 97% or more, the liquid-cooled technology offers major benefits in cooling efficiency as the heat loss can be dissipated outside the installation area. The cooling liquid, which can be pure drinking water, removes about 95% of the heat

generated.

Moreover, when mounted in an IP 54 or higher enclosure, the drives can withstand severe environments and wide variations in temperature, providing an ideal solution for the marine and off-shore use. Vacon NX drives are also built around hardware and software modularity.

The new vessels being built will be sailing primarily in and around UK waters. Consequently, any necessary maintenance and repair can be easily organised from the Imtech location in Billingham (UK). There is also a full 24-hour a day, seven-days a week service worldwide from Vacon.

Efficient

Commenting on the benefits of electrical propulsion in vessels generally, van Miltenburg says: “Diesel electric ships are proving to be more energy efficient, meaning lower operational costs. But also maintenance costs are lower for example on the diesel engines because they are running at optimum speed and load, with partial load one of the engines can be switched off.”

And with such advantages now being acknowledged, such installations are on the increase. “The number of ships equipped with electrical propulsion has shown an enormous growth during the last few years,” van Miltenburg tells The Motor Ship. “During the last two years only, Imtech Marine & Offshore booked orders for electrical propulsion installations for the following ships: Royal British Navy ? four ALSL?s (Alternative Landing Ship ogistics); Royal Netherlands Navy ? one LPD (Landing Platform Dock); A floating sheerleg self propelled; A mega luxury yacht; A light tender; Two Ro-Ro Airbus A380 inland waterway transport vessels; and, of course, the previously described four all-electric tankers for Everard & Sons.”