Q - Discuss in detail the advantages and disadvantages of Electrical main Propulsion.
ANSWER :-
Advantages of Electric Propulsion:-
There are many attractive advantages for using electric propulsion for ships. The advantages particular to electric propulsion are divided into the areas of design, operation and cost savings.
The following advantages are given for electric propulsion.
1. Lower fuel consumption and emissions due to the possibility to optimize the loading of diesel engines / gensets.The gensets in operation can run on high loads with high efficiency. This applies especially to vessels which have a large variation in load demand, for example for an offshore supply vessel, which divides its time between transit and station-keeping (DP) operation.
2. High reliability, due to multiple engine redundancy. Even if an engine / genset malfunction, there will be sufficient power to operate the vessel safely. Reduced vulnerability to single point of failure providing the basis to fulfill high redundancy requirements.
3. Electric propulsion provides greater reliability. - Reliability is improved by being able to use any prime mover and generator to power any propeller. With direct drive the failure of the prime movers attached to a specific shaft results in a loss of partial or all power on that shaft. With electric propulsion, power can be supplied from any prime mover to any shaft increasing reliability of the propulsion system.
Even if an engine / genset malfunctions, there will be sufficient power to operate the vessel safely. 4.Reduced vulnerability to single point of failure providing the basis to fulfill high redundancy requirements.
5.There is no direct connection of propeller shaft and prime mover and hence transmission of severe stresses such as torsional and vibration is restricted(low).
6.Flexibility of layout - Ship layout and engineering plant positioning can be more flexible. - Prime movers for shafts no longer have to be located to provide direct coupling to the shaft. An electric propulsion design would allow prime movers be located higher in the hull allowing improved ship design.
7. Increased payload, as diesel-electric propulsion plants take less space.
ANSWER :-
Advantages of Electric Propulsion:-
There are many attractive advantages for using electric propulsion for ships. The advantages particular to electric propulsion are divided into the areas of design, operation and cost savings.
The following advantages are given for electric propulsion.
1. Lower fuel consumption and emissions due to the possibility to optimize the loading of diesel engines / gensets.The gensets in operation can run on high loads with high efficiency. This applies especially to vessels which have a large variation in load demand, for example for an offshore supply vessel, which divides its time between transit and station-keeping (DP) operation.
2. High reliability, due to multiple engine redundancy. Even if an engine / genset malfunction, there will be sufficient power to operate the vessel safely. Reduced vulnerability to single point of failure providing the basis to fulfill high redundancy requirements.
3. Electric propulsion provides greater reliability. - Reliability is improved by being able to use any prime mover and generator to power any propeller. With direct drive the failure of the prime movers attached to a specific shaft results in a loss of partial or all power on that shaft. With electric propulsion, power can be supplied from any prime mover to any shaft increasing reliability of the propulsion system.
Even if an engine / genset malfunctions, there will be sufficient power to operate the vessel safely. 4.Reduced vulnerability to single point of failure providing the basis to fulfill high redundancy requirements.
5.There is no direct connection of propeller shaft and prime mover and hence transmission of severe stresses such as torsional and vibration is restricted(low).
6.Flexibility of layout - Ship layout and engineering plant positioning can be more flexible. - Prime movers for shafts no longer have to be located to provide direct coupling to the shaft. An electric propulsion design would allow prime movers be located higher in the hull allowing improved ship design.
7. Increased payload, as diesel-electric propulsion plants take less space.
8. Increased survivability can be gained from the cross-connect capability and the flexibility in design and layout. - With the flexibility to place prime movers and generators in separate spaces the loss of one space does not cause the loss of several prime movers and generators. The ability to cross connect increases the number of paths that can be used for a prime mover and generator to supply loads.
9. Improved manoeuvrability and station-keeping ability, by deploying special propulsory such as azimuth thrusters or pods. Precise control of the electrical propulsion motors controlled by frequency converters.
10.Ease of control -Propulsion system control is improved, providing a simple and rapid reversal. Speed can be controlled infinitely variable by using electronics to adjust frequency. Reversing can be initiated by changing the sequence in which the electronic devices in the power converter are gated or by the use of a controllable pitch propeller.
11.Noise less-Electric propulsion provides quiet operation. - The prime mover for electric propulsion is not directly coupled to the shaft and, as such, the mechanical vibrations from the prime mover and large reduction gears are eliminated.
12.Improved fuel economy is provided with electric propulsion. - The prime movers can be operated at more efficient speeds and to its maximum rating. Direct drive propulsion is constrained by reduction gears and shaft limitations from the prime mover to the propeller.
13.Reduced cost in maintenance. - Reduced cost for maintenance is gained by running prime movers at a constant speed reducing mechanical stresses which in turn minimizes the failures of moving parts. Also, if prime movers are operated at full load there will be a reduction of operating time for each prime mover. This increases the time between overhauls and the period between required maintenance. It is generally realized that electric equipment costs less to operate and maintain than mechanical equipment. In particular the comparison would be between the reduction gears and the converter and motor operation.
14.Flexibility in the types of prime movers used on the same ship. - Electric propulsion is not concerned with the operating characteristics of the prime movers providing power to the generators. An example would be the use of two diesel engine driven generators and one gas turbine driven generator on the same ship. The output speed and operating characteristic of the two different prime movers could be very different in turn making the speed reduction to the propeller in a direct drives propulsion system very difficult. In the case of an electric propulsion system as long as both prime movers can drive a generator to produce a specified voltage and frequency, the system will work with no increase in equipment or operational difficulty.
15.Efficient performance and high motor torques, as the system can provide maximum torque also at slow speeds, which gives advantages for example in icy conditions
Economical part load running-
system now very popular in LNG ships , where high voltage power generation system is available and LNG as fuel can use in generator .This also prove low emission and economical system .
Disadvantages of Electric Propulsion
As in any technological compromises, there are some disadvantages to electric propulsion. The disadvantages of an electric propulsion system include.
Efficiency between the prime mover and the shaft is reduced. Any time a change in the form of energy occurs there is generally a loss. In the case of a direct drive, the losses introduced by the mechanical transmission are less than those introduced in an electric propulsion system where power is converted from mechanical form to electrical form, manipulated, then converted back to mechanical form. The additional stages of energy conversion usually translate into an efficiency of about 91% as compared to direct drives whose efficiencies are on the order of 97 percent.
2. Power quality problems occur. - Harmonics in the electrical system are created with the use of converters. Harmonics in the propulsion motor are in turn transmitted to the shaft which radiates out as noise and increased signature.
3. The installation cost of electrical propulsion plant is much higher .
4. Different and improved training for ship’s crew as the system is completely different from mechanical system and involves major automation.
5. There is an increase in space and weight required for the electrical equipment. - With electric drives increased space is needed for propulsion generators, wiring, frequency converters and propulsion motors. With the addition of equipment there is an increase in weight for the propulsion system.
9. Improved manoeuvrability and station-keeping ability, by deploying special propulsory such as azimuth thrusters or pods. Precise control of the electrical propulsion motors controlled by frequency converters.
10.Ease of control -Propulsion system control is improved, providing a simple and rapid reversal. Speed can be controlled infinitely variable by using electronics to adjust frequency. Reversing can be initiated by changing the sequence in which the electronic devices in the power converter are gated or by the use of a controllable pitch propeller.
11.Noise less-Electric propulsion provides quiet operation. - The prime mover for electric propulsion is not directly coupled to the shaft and, as such, the mechanical vibrations from the prime mover and large reduction gears are eliminated.
12.Improved fuel economy is provided with electric propulsion. - The prime movers can be operated at more efficient speeds and to its maximum rating. Direct drive propulsion is constrained by reduction gears and shaft limitations from the prime mover to the propeller.
13.Reduced cost in maintenance. - Reduced cost for maintenance is gained by running prime movers at a constant speed reducing mechanical stresses which in turn minimizes the failures of moving parts. Also, if prime movers are operated at full load there will be a reduction of operating time for each prime mover. This increases the time between overhauls and the period between required maintenance. It is generally realized that electric equipment costs less to operate and maintain than mechanical equipment. In particular the comparison would be between the reduction gears and the converter and motor operation.
14.Flexibility in the types of prime movers used on the same ship. - Electric propulsion is not concerned with the operating characteristics of the prime movers providing power to the generators. An example would be the use of two diesel engine driven generators and one gas turbine driven generator on the same ship. The output speed and operating characteristic of the two different prime movers could be very different in turn making the speed reduction to the propeller in a direct drives propulsion system very difficult. In the case of an electric propulsion system as long as both prime movers can drive a generator to produce a specified voltage and frequency, the system will work with no increase in equipment or operational difficulty.
15.Efficient performance and high motor torques, as the system can provide maximum torque also at slow speeds, which gives advantages for example in icy conditions
Economical part load running-
system now very popular in LNG ships , where high voltage power generation system is available and LNG as fuel can use in generator .This also prove low emission and economical system .
Disadvantages of Electric Propulsion
As in any technological compromises, there are some disadvantages to electric propulsion. The disadvantages of an electric propulsion system include.
Efficiency between the prime mover and the shaft is reduced. Any time a change in the form of energy occurs there is generally a loss. In the case of a direct drive, the losses introduced by the mechanical transmission are less than those introduced in an electric propulsion system where power is converted from mechanical form to electrical form, manipulated, then converted back to mechanical form. The additional stages of energy conversion usually translate into an efficiency of about 91% as compared to direct drives whose efficiencies are on the order of 97 percent.
2. Power quality problems occur. - Harmonics in the electrical system are created with the use of converters. Harmonics in the propulsion motor are in turn transmitted to the shaft which radiates out as noise and increased signature.
3. The installation cost of electrical propulsion plant is much higher .
4. Different and improved training for ship’s crew as the system is completely different from mechanical system and involves major automation.
5. There is an increase in space and weight required for the electrical equipment. - With electric drives increased space is needed for propulsion generators, wiring, frequency converters and propulsion motors. With the addition of equipment there is an increase in weight for the propulsion system.
