CONSIDERATIONS REGARDING THE ELECTRIC PROPULSION

Luca Ionel

Constanta Maritime University, Constanta, Romania

Abstract. This paper aims to explain some considerations regarding the electrical propulsion like advantage and disadvantage. By comparing the fuel oil consumption of Azipod with the conventional propulsion we will see a low consumption on Azipod propulsion. The technical development of power semiconductors has therefore been very important for the improvement of the frequency converter. This development has also improved the control of the frequency converter, where PWM (Pulse-width-modulation) is very common.

1. Introduction

The concept of electric propulsion is not new, the idea originated more than 100 years ago. However, with the possibility to control electrical motors with variable speed in a large power range with compact, reliable and cost competitive solutions, the use of electrical propulsion has emerged in new application areas during the 80’s and 90’s.

By introduction of azimuthing thrusters and podded thrust units, propulsion configurations for transit, manoeuvring and station keeping have in several types of vessels merged in order to utilize installed thrust units optimally for transit, manoeuvring and dynamically positioning (dynamic positioning - DP).

The following characteristics summarize the main advantages of electric propulsion in these types of vessels:

Improved life cycle cost by reduced fuel consumption and maintenance; Reduced vulnerability to single failure in the system and possibility to optimize

loading of prime movers (diesel engine or gas turbine). Less space consuming and more flexible utilization of the on-board space increase the

payload of the vessel Improved manoeuvrability by utilizing azimuthing thrusters or podded propulsion. Less propulsion noise and vibrations since rotating shaft lines are shorter, prime

movers are running on fixed speed, and using pulling type propellers gives less cavitation due to more uniform water flow.

These advantages should be weighted up against the present penalties, such as: Increased investment costs. However, this is continuously subject for revisions, as the

cost tends to decrease with increasing number of units manufactured.

Additional components (electrical equipment – generators, transformers, drives and motors/machines) between prime mover and propeller increase the transmission losses at full load.

For newcomers a higher number and new type of equipment requires different operation, manning, and maintenance strategy.

2. Power flow and power efficiency

In any isolated power system, the amount of generated power must be equal to the consumed power including losses. For an electric system consisting of an electric power generation plant, a distribution system, including distribution transformers and a variable speed drive, the power flow can be illustrated in Fig. 1.

Figure 1. Power low in a simplified electric power system.

The prime movers, e.g. diesel engines or gas turbines, supply a power to the electric generator shaft. The electric motor, which could be the propulsion motor, is loaded by a power from its connected load. The power lost in the components between the shaft of the diesel engine and the shaft of the electric motor is mechanical and electrical losses which gives heat and temperature increase in equipment and ambient.

The electrical efficiency of the system in Fig. 1.2 is:

η=Pout

P¿=

Pout

Pout+Plosses

For each of the components, the electrical efficiency can be calculated, and typical values at full (rated) power are for; generator: = 0.95-0.97, switchboard: = 0.999, transformer: = 0.99-0.995, frequency converter: = 0.98-0.99, and electric motor: = 0.95-0.97.

The fuel efficiency characteristics of the diesel engine, with maximum fuel efficiency in the load range of 60 to 100% load, strongly contribute to the difference in power consumption for a traditional mechanical propulsion system, and a diesel electric propulsion system. In a power plant for diesel electric propulsion, the power generation will consist of multiple smaller diesel engines, where the number of running aggregates can be selected to

have an optimum loading of each engine. The rating of the engines can also be adapted to fit the intended operational profile of the vessel, ensuring that it is possible to find an optimal configuration for most of the operational modes and time.

Figure 2. Diesel engine specific fuel consumption (typical), Marintek [3]

Figure 3. Azipod versus conventional, year fuel oil consumptions

3. AC versus DC power

Overall, the ac system today would give the least capital and life-cycle costs since much ac equipment is available off the shelf, and more manufacturers are in the business of making large ac generators, motors, transformers, circuit breakers, and so on. The use of dc has declined over the past few decades. It is hardly used now in new designs, except in special circumstances. As a result, few manufacturers offer dc equipment in their product lines. For this reason, ac is the most likely candidate for new ships and is generally the default choice unless a strong case for dc is made for a special reason. There is one advantage of dc over ac: It makes even less vibration and noise. The U.S. Navy has an ongoing interest in high-voltage dc power for special applications for which a ship has strict acoustic and vibration limits.

4. Variable speed motor drives and control strategies

The most commonly used motor drives are: Voltage source inverter (VSI) type converters for AC motors, i.e. asynchronous,

synchronous and permanent magnet synchronous motors. Current source inverter type (CSI) converters for AC motors, normally synchronous

motors Cycloconverters (Cyclo) for AC motors, normally for synchronous motors

A motor controller contains the speed control, and the control of motor currents by controlling the switching elements of the rectifier and/or inverter. An interface to an overriding control system, vessel management system, maneuvering control, or dynamic position control is normally required. The motor controller acquires measurement signals and feedback signals from sensors in the drive, and motor. Typically motor currents, motor speed, and in some cases temperatures and voltages are measured.

Figure 4. Schematics of a variable speed drive, showing a frequency converter with DC Link, typically for VSI and CSI type converters.

5. Propulsion power requirement

The propulsion power required is basically determined by the hydrodynamic and aerodynamic resistances to the motion of the ship. At the blade front, the propeller load torque varies with α-power of the propulsion speed, that is:

T pop. load=k ¿

Power=Torque × Speed=k ¿

where k = constant and exponent α = 2 at low speeds and 3 at medium speeds and can be 4 or higher at high speeds when the fluid flow is highly turbulent with cavitation’s.

The total propulsion resistance RT of the ship depends on its speed, displacement, and hull form. It comes primarily from three sources: (a) surface friction, (b) air resistance, and (c) wave (residual) resistance. The friction and wave resistances depend on the surface of the hull below the waterline, whereas the air resistance depends on the surface of the ship above the waterline.

A fluid with velocity V and density ρ exerts a force on a steady object that is given by:

F=12

ρ V 2× A

where A = area of the object perpendicular to the fluid velocity. Equation (7.4) — known as Bernoulli’s law—is used as a basis for expressing the propulsion resistances R of the ship by means of dimensionless resistance coefficients C as follows:

R=C × 12

ρ V 2× A

The total resistance of the ship to propulsion coming from three major sources is discussed next.

The total towing resistance RTotal of the ship is the sum of the three components just described, that is:

RTotal=R friction+Rair+Rwave

where the frictional resistance is expressed as follows:

R friction=C friction × 12

ρ V 2× Weeted surface area of hull

The air resistance is expressed as follows, where a typical value of Cair is 0.90:

Rair=Cair ×12

ρV 2×Cross−sectional area of ship above waterline

The wave resistance is expressed as follows:

Rwave=Cwave× 12

ρ V 2 ×Cross−sectional ofshipbelow waterline

The towing power necessary to move the ship through water at speed V is PTow = RTotal × V, which can be expressed as follows:

Towing power in SI units:

kW =Rnewtons × V m/ s

1000

Towing power in British units:

HP=RLbf × V ft / s

550The power delivered to the propeller Pprop, however, is greater than Ptow due to the flow

conditions around the propeller and the propeller efficiency ηprop, that is:

P¿=Ptow

η¿

The propeller efficiency depends on the ship speed, propeller thrust force, revolutions per minute, propeller diameter, and the design of the propeller (i.e., the number of blades, disk area ratio, pitch-to-diameter ratio, etc.). It typically varies from 0.50 to 0.80, the lower number for low-speed ships and the higher number for high-speed ships.

Total shipboard power system rating is the sum of the three components as follows:

Total shipboard power=Propulsion power+Ship service power+Hotel power

Figure 5. Propulsion power versus speed up to wave wall.

Bibliography

1. ABB Oy, Marine (2007-04). Project Guide for Azipod® Propulsion Systems Version 6.1, Helsinki/Finland, ABB.

2. Blokland, A.J. and B. van der Ploeg, Electric ship propulsion

3. Cosulich, G., M. Fracchia, A. Mariscotti and S. Savio, Comparative dependability analysis of electric ship propulsion systems

4. Mukund R. Patel, Shipboard Propulsion, Power Electronics, and Ocean Energy

5. Marintek, Neste generasjon innenriksferjer – optimalt fremdriftssystem