The diesel powerplant forms the basis of comparison against which all of the subsequent alternative concepts are evaluated.
Ship powering demands two characteristics: Reliability and Economy. Due to excellent economy the diesel Engine remains predominant. The diesel driveline chosen for most large cargo ships consists of a low-speed two-stroke diesel turning a direct-connected single propeller. Such a propulsion plant consists of a single large engine turning the propeller at shaft RPM with no intervening reduction gear.
A leading manufacturer of such engines are MAN B+W, SULZER (Wartisila).
A leading manufacturer of such engines are MAN B+W, SULZER (Wartisila).
Diesel Engines
MAN/B+W have provided an excellent summary of the development of container
ship diesel propulsion:
“A substantial number of recent large cargo ship contracts have called for
main engine outputs up to a the highest ratings available, and for a period, most
large cargo ships were thus specified with main engine MCR outputs of some
65,000 bhp
main engine outputs up to a the highest ratings available, and for a period, most
large cargo ships were thus specified with main engine MCR outputs of some
65,000 bhp
However the launching of ratings up to about 75,000 bhp per unit changed the
picture. Now units with such outputs exist and in anticipation of a market for
above 8000 TEU container ships, engines with even higher outputs have been
introduced.
picture. Now units with such outputs exist and in anticipation of a market for
above 8000 TEU container ships, engines with even higher outputs have been
introduced.
The change in ship size does not in itself explain the substantial increase in the
average engine power seen in recent years. Hence it can be assumed that the
design speed has increased. Increase in the average engine size is an indication of
a changed demand pattern toward higher powered ship types.
average engine power seen in recent years. Hence it can be assumed that the
design speed has increased. Increase in the average engine size is an indication of
a changed demand pattern toward higher powered ship types.
The propulsion power requirement is considerably higher for a container ship
sailing with high-value commodities than for bulk carriers and large tankers
transporting raw materials, for which the sailing time is of less economical
consequence. Hence, the propulsion power requirement for a Post Panamax
container ship is 2-3 times the power requirement for a VLCC.
sailing with high-value commodities than for bulk carriers and large tankers
transporting raw materials, for which the sailing time is of less economical
consequence. Hence, the propulsion power requirement for a Post Panamax
container ship is 2-3 times the power requirement for a VLCC.
The increasing containerization and competition in this market, together with
demands for the lowest possible freight cost per TEU, will imply a continued race
for transporting as many TEUs as possible on the long-haul routes. This means
that an increase in the average power requirement for container ships is to be
expected.”
demands for the lowest possible freight cost per TEU, will imply a continued race
for transporting as many TEUs as possible on the long-haul routes. This means
that an increase in the average power requirement for container ships is to be
expected.”
Other manufacturers have reported their intent to introduce engines larger than the
K98. Examples include IHI’s representation of their intent to introduce a 140,000
hp engine.
RINA reported in June of 2001 that “the two leading designers of low-speed
diesel machinery, Wartsila (Sulzer) and MAN B+W have both launched
extended-cylinder inline versions of their most powerful models. This is being
done to provide suitable plants for future generations of container liners without
branching into twin-engine/twin-screw variants.”
Specifically, “Sulzer can now offer a 14-cylinder RTA96C engine capable of
developing 80,080kW, while MAN B+W has just announced 13- and 14-cylinder
versions of its K98MC and K98MC-C models. These will provide 74,360kW and
80,080kW (K98MC) and 74,230kW and 79,940kW (K98MC-C). (The MC-C
designation indicates a shorter stroke and slightly faster running speed.) Even
more remarkable, this latter designer says both types could be built with up to 18
cylinders and outputs of nearly 103,000kW, if necessary!”
These future engines are not yet in existence, and it appears that when they do
emerge, they will be very similar to the existing K98.
K98. Examples include IHI’s representation of their intent to introduce a 140,000
hp engine.
RINA reported in June of 2001 that “the two leading designers of low-speed
diesel machinery, Wartsila (Sulzer) and MAN B+W have both launched
extended-cylinder inline versions of their most powerful models. This is being
done to provide suitable plants for future generations of container liners without
branching into twin-engine/twin-screw variants.”
Specifically, “Sulzer can now offer a 14-cylinder RTA96C engine capable of
developing 80,080kW, while MAN B+W has just announced 13- and 14-cylinder
versions of its K98MC and K98MC-C models. These will provide 74,360kW and
80,080kW (K98MC) and 74,230kW and 79,940kW (K98MC-C). (The MC-C
designation indicates a shorter stroke and slightly faster running speed.) Even
more remarkable, this latter designer says both types could be built with up to 18
cylinders and outputs of nearly 103,000kW, if necessary!”
These future engines are not yet in existence, and it appears that when they do
emerge, they will be very similar to the existing K98.
ALTERNATIVE POWERING OPTIONS
Gas Turbine Engines
During the period of the 1960s the world enjoyed an affair of preference for new
and “space age” devices. During this period aeroderivative gas turbine engines
saw service in some limited merchant shipping activities. It was during this time,
for example, that the Golden Gate Ferry district first procured gas turbine-driven
high speed ferries to serve San Francisco. However, the Oil Embargo of 1973/74
and the skyrocketing fuel prices associated therewith almost eliminated gas
turbines as prime movers for merchant ships because of their inferior fuel
economy compared to medium and low-speed diesel engines. Today however
there has been a resurgence of interest in gas turbine propulsion.
Gas turbines are small and compact for their power level – especially when
compared to low-speed diesels. They have recently enjoyed a revival as a prime
mover for the growing number of fast ferries that are subject to severe space and
weight restrictions and which transport a “cargo” that appreciates reduced
traveling time. Gas turbines have also seen success in cruise ships, because their
very high operating RPMs result in a nearly vibration-free machinery plant and
thus a potentially quieter, smoother ride.
iy is not evident, however, that these virtues of the gas turbine are sufficient to
qualify it for the propulsion of the greater part of the merchant fleet. Its
disadvantage in terms of its preference for high quality fuel and its relatively low
fuel efficiency, in particular at part load, surely detract from its acceptability.
This is recognized clearly by the turbine manufacturers, and thus a significant part
of their efforts is devoted to increasing the fuel efficiency of their gas turbines.
The U.S. Department of Energy has also recognized this and has sponsored
several cost-shared Advanced Turbine Development programs to boost the
efficiencies of U.S gas turbine engines. Additionally while the gas turbines have
a much lower power-to-weight ratio, they do require a greater amount of interior
space for intakes and exhaust which also becomes a design tradeoff issue.
The latest generation of marine gas turbine – including engines which are still
slightly “over the horizon” – includes intercooled, recuperated or regenerative gas
turbines. These machines capture heat from the turbine exhaust and recover the
energy in order to increase the overall thermal efficiency of the machine. As a
result the fuel consumption per unit power generated is reduced and part load
efficiencies are increased as well.
The recuperators increase the size and weight of the machine and thus somewhat
erode the machine’s advantage in these areas. Turbine manufacturers also claim
their engines to be of greater reliability than a diesel. The absence of
reciprocating parts brings to mind the Mazda car commercials of the 1970s,
wherein we were enjoined to consider that “whirr” was better than “bounce
bounce bounce.” In similar fashion turbine manufacturers state that a modern gas
turbine will run for many thousands of hours with only periodic inspections.
Indeed, in fast ferry applications and land-based stationary power applications the
machines are run completely unattended for hours at a time.
Recent RINA reports suggest that the experience “from the first gas turbinepowered
cruise ships now in service seem to confirm a number of benefits offered
by this form of main propulsion over conventional diesel-mechanical and dieselelectric
systems. The vessels in question are Celebrity Cruises' Millennium and
Infinity, both built by Chantiers de l'Atlantique, and Royal Caribbean
International's Radiance of the Seas, very recently completed by Meyer Werft.
Each of these three ships is powered by two GE LM2500+ gas turbines.
“In the early stages of winning these orders, GE believed (as reported in a paper
presented at the Seatrade Miami conference) that both the owner and the shipyard
would be concentrating most heavily on the following power plant issues when
considering new ship designs:
• space utilisation aboard ship
• environmental friendliness
• passenger comfort
• maintenance costs
• reliability.
“General Electric's original estimates claimed that as many as 50 additional
passenger cabins could be realized as a result of installing a COGES plant
(combined gas turbine and steam turbine with integrated electric drive) in the
original engineroom space designed for a diesel plant. In both Millennium and
Radiance of the Seas, the designers did, in fact, find this much space and the
cabins were added. Additionally, Meyer Werft is refining the design of the
follow-on ships, Brilliance of the Seas and sisters, to move the engineroom aft,
which will result in considerably more public space along with an increase in
passenger cabins.
“Another approach that has come about as a result of the compact and lightweight
design of the GE gas turbine package is placing the gas turbine generator in the
funnel. This is being done on two classes of P&O Princess Cruises vessels, also
onboard Cunard's new Queen Mary 2. On the latter ship, the extremely high
power requirement dictated the use of gas turbines in addition to four Wartsila
diesel engines. Once again, because of the light weight and compactness of the
gas turbine package, the designers were able to place two gas turbo-alternators in
the funnel.”
In addition, significant environmental attractions exist for gas turbines.
According to RINA “Royal Caribbean's decision to utilize gas turbines in its next
generation of cruise liners was heavily driven by its desire to lead the industry in
the construction of environmentally friendly ships. In 1998, GE claimed that its
LM2500+ gas turbine would reduce emissions by 98% from that of current diesel
technology. During the hand-over of Millennium to Celebrity (today a Royal
Caribbean associate), actual exhaust stack emission measurements were taken.
Not only was there no visible smoke, but the NOx emissions were found to be
only 5g/kWh. This is less than half the minimum level targeted by IMO.
“Of course, diesel engine manufacturers have not been standing idly by during the
past three years. MAN B&W has its 'invisible smoke' technology which
incorporates fuel/water emulsification, auxiliary blower, and special turbocharger.
Meanwhile, Wartsila NSD is developing its 'smokeless diesel' which incorporates
a new ultra high-pressure common rail fuel system. In addition, the Finns use
direct water injection to reduce NOx emissions. ‘Both these technologies are not
new and add a considerable amount of complexity to the installation and
operation of these engines,' claims David Whisenhunt, general manager of
commercial marine systems at S&S Energy Products (part of the GE Group). 'On
Millenium, we have proven that our gas turbines operate without visible smoke
and actually meet the 5g/kWh target that we quoted in 1998. No new
development was necessary to accomplish this,'”
Regarding reliability and maintenance, it is reported “Although there have, as yet,
been no major gas turbine-related repair, events on Millenium, the jury is really
still out. It will probably take another year for crews to wholeheartedly believe
what GE has been saying all along about how simple gas turbines are to maintain
onboard.
“As opposed to changing or repairing major components on a set schedule, which
is normally the case with diesel engines, repairs to the LM2500+ sets are carried
out based on condition observed during regular borescope inspections. These are
normally done approximately once every 2500 hours.
“The last borescope inspection on Millenium was carried out in January this year
[2001]. The service engineer stated that internal components still 'looked like
new' after 5000 hours of operation. At that rate, it is the opinion of GE experts
that the predicted 15,000 hour hot-section repair interval will be easily passed.
“Royal Caribbean elected to enter into a long- term maintenance agreement with
GE for its gas turbines. This contract covers all scheduled maintenance activities,
including hot section repairs. GE has been told by the owner that the cost of the
contract was comparable to its diesel engine maintenance at the time on a
cost/MW basis. Today, as diesel engines become somewhat more complex
because of emission requirements, their maintenance cost seems likely to
increase. This could make gas turbines even more attractive in the future.”
Regarding reliability, the RINA article goes on to say: “Another operational
aspect that has been rarely debated by opponents of gas turbine technology is
reliability. With more than 25 years of operating history in the US Navy, the
LM2500 has a proven track record. Because of this, the LM2500 and the
LM2500+ are recognized as the standard for modern gas turbine design
technology when it comes to reliability.
“The turbines aboard Millenium should prove no exception. It is now more than a
year ago since they were first started up in the shipyard, and both units are
reported to have operated flawlessly, requiring no repairs to date. The entire
COGES system is claimed to have operated continuously without any event
causing a delay in the ship's schedule.”
The activity of the cruise ship industry in the adoption of gas turbines may have
important applicability for the container industry as well. Both industries are
conservative and highly competitive. Also, both industries face pressure to
reduce the environmental impact of their service. The RINA article notes
“Because of an emphasis on environment friendly ships, both owners and yards
have changed their attitude towards gas turbine power since the concept was first
considered in 1995 and Royal Caribbean took the lead with the first orders in
1998. Many major cruise shipping companies have now placed orders for ship
with GE LM2500 and LM2500+ gas turbine onboard, Owners appear to have
recognized that gas turbines fulfill the need for cleaner propulsion plant emissions
without adding significantly more complexity. Yards such as Chantiers de
I'Atlantique, Meyer Werft, and Fincantieri are reported to be convinced that gas
turbines are actually easier and less costly to install.”
“S&S Energy Products' David Whisenhunt believes that after a few more years'
experience in operating gas turbines, crews will plead for a total change-over to
this machinery. 'They will find their lives much simpler in the face of
increasingly stringent environmental regulations for waste and sludge, not to
mention the chore of keeping the newer, more complex diesel engines tuned to
limit visible smoke and emissions,' he says.
Marine gas turbines generally are developed either from land-based power units
or from aircraft engines. Since land-based units, such as the Westinghouse 501,
are designed from the beginning to operate on land, weight usually isn't an
important design criterion so most (but not all) units tend to be very large and
very heavy. Aero-derivative turbines, as the name implies, are developed from
engines designed for aircraft use. These units are smaller and lighter than the
land-based units, but their durability is not as good. Since weight and volume
traditionally are important considerations when selecting a ship powerplant, and
since marine engines operate for much fewer hours and at lower power levels than
do land-based units, most large marine gas turbine engines are of the aeroderivative
type. However it is important to note the similarity of evolution of the
land-based turbine and the marine low-speed diesel. In both of these machines
the evidence points to an emphasis upon reliability and efficiency, with little
attention given to weight or size.
Marine gas turbines have power turbines that are either mechanically coupled or
aerodynamically coupled to the gas generator section. Each configuration has its
advantages and disadvantages. Mechanically-coupled engines, such as the
General Electric LM6000, typically are more efficient than the aerodynamicallycoupled
engines. One disadvantage, however, is that minimum power turbine
rotational speed is fixed at a relatively high level because the same shaft also
drives the low-pressure compressor stages, which cannot turn too slowly or the
engine will stall. The aerodynamically-coupled engines are the opposite: the
efficiencies are slightly lower but the power turbine can operate at very low
speeds since the power turbine is not directly coupled to the compressor. Another
advantage of mechanical coupling is that some engines that have it allow power
takeoff from the compressor end as well as from the exhaust end. Most marine
gas turbines also are simple cycle, having only compression, combustion, and
expansion processes typical of a Brayton open cycle. The Northrop Grumman
WR-21 engine now in development, however, is not simple cycle. It has an
intercooler and recuperator (also called a regenerator) so it often is referred to as
the ICR engine. The ICR cycle provides good fuel efficiency even at low power
levels, but it does so at the expense of added complexity, size, and weight.
Reliability is unknown at this time, however, since its development has been
primarily for military naval applications it is assumed to be high.
Current Market Scenario
Turbine options available currently or in the near future are listed below.
Note that there are other turbine manufacturers than those listed, but these
are arguably the leading ones in marine propulsion:
• GE LM2500+
• GE LM6000
• GE 90
• GE Frame 6B
• GE Frame 7
• Rolls-Royce V2500
• Rolls-Royce Trent
• Westinghouse 501
The characteristics of these engines are given in Table 1. They are
described below:
General Electric LM2500+ - An upgrade of the LM2500 aero-derivative
engine, the LM2500+ is a simple cycle gas turbine engine with an
introductory ISO continuous rating of 27,050 kW and a U.S. Navy rating
of 26,100 kW. Initially derived from the TF-39 engine used on DC-10
wide-bodied jets, the two-shaft design has an output speed of 3600 rpm to
permit direct coupling to a 60 Hz generator. This engine has been used
several times for cruise liner electric propulsion. The two-shaft design
consists of a gas generator and power turbine. The gas generator consists
of a variable geometry compressor, an annular combustor, high pressure
turbine, an accessory drive gear box, controls and accessories. The 16-
stage compressor is of the high-pressure-ratio, axial flow design. The
LM2500+ also utilizes a “zero stage” on the compressor with a resulting
increase in airflow, which allows for the upgraded power rating from the
base LM2500. The 6-stage low pressure power turbine is aerodynamically
coupled to the gas generator and driven by the gas generator exhaust.
General Electric LM6000 - This engine is derived from the GE CF6-80C2
aircraft engine used in the Boeing 747 and 767, the McDonnell Douglas
MD-11, and the Airbus A300. Being designed for simple-cycle,
combined-cycle and cogeneration installations the LM6000 has an output
speed of 3600 rpm and can be directly coupled to an electric generator for
60 Hz applications. The LM6000 has an ISO rating of 43,860 kW. The
concentric two-shaft arrangement has the low pressure compressor and
low pressure turbine on one shaft, forming the low pressure rotor, and the
high pressure compressor and high pressure turbine on the other shaft,
forming the high pressure rotor. Utilizing a 5-stage low pressure section
and a 14 stage high pressure section results in a compression ratio for each
section of 2.4:1 and 12:1, respectively. The combustion system is of the
annular type and can be operated with natural gas, liquid fuel, or dual fuel.
The combustion gases expand through a 2-stage, air-cooled, high pressure
turbine and a 5-stage low pressure turbine. Over 160 LM6000 units are
currently in shore-side operation for simple-cycle, combined-cycle or
cogeneration projects worldwide.
General Electric LM9000 - The LM9000 is a nomenclature assigned to a
nominal 125 MW aero-derivative engine which could be developed from
either the CF6-880C2 or from a GE90 core (the engine is currently in
service in the Boeing 777 aircraft.) Although some preliminary studies
have been completed by the manufacturer concerning the possibilities of
such development, no decision has yet been made to proceed with further
development. According to the manufacturer, any decision to proceed
with the development would depend upon assessment of the market for an
aero-derivative gas turbine in this power class.
General Electric Frame 6B – The GE Frame 6B is currently used in 60 Hz
industrial power cogeneration applications worldwide. With a
manufacturer’s nominal rating of 38 MW, the Frame 6B has an estimated
navy continuous rating of 34,525 kW for specified marine applications.
This simple-cycle engine has a 17-stage axial-flow compressor with
modulated inlet guide vanes resulting in a compression ratio of 11.8:1. It
is equipped with a reverse flow, multi-chamber (can annular), single
nozzle combustion chamber with its exhaust expanding into a 3-stage
power turbine.
General Electric Frame 7 – This engine, like the Frame 6B, is also
designed specifically for 60 Hz power generation. Designed to be directly
coupled to a generator, the Frame 7 has a manufacturers rating of 85.4
MW. For proposed naval applications, however, the Frame 7 has been derated
at approximately 77.9 MW. This simple-cycle engine has a 17-stage
axial-flow compressor with modulated inlet guide vanes resulting in a
compression ratio of 12.2:1. It is equipped with a reverse flow, multichamber
(can annular), single nozzle combustion chamber with its exhaust
expanding into a 3-stage power turbine. This gas turbine is available
primarily for electric utility applications, this fuel-flexible power generator
is used in cogeneration and combined-cycle power plants.
Rolls-Royce V2500 - This family of aircraft engines is used exclusively in
the Airbus A319, A320 and A321. Currently, the V2500 is only available
in the aero form and there are no immediate plans by the manufacturer to
convert this engine for use in marine or industrial applications.
Rolls Royce Marine Trent - The Marine Trent is based on the on the Rolls-
Royce Industrial Trent power generation gas turbine which, in turn, is a
derivative of the Trent 700 and 800 aero engine. The result is a mature
powerplant having a marine rating of approximately 47.5 MW. The threeshaft
design Marine Trent engine replaces the industrial dual gas/liquid
fueled combustion system with a simplified liquid-only system. The
engine is equipped with a 2-stage, axial configuration low pressure
compressor with variable inlet guide vanes, an 8-stage intermediate
pressure compressor and a 6-stage high pressure compressor. It has an
annular combustion system. The low pressure turbine consists of 5-stages
of high aspect ratio rotor and stator blades. The low pressure turbine is
followed by a single stage intermediate pressure axial turbine and a high
pressure turbine. In addition, due to the power turbine being able to run
down a typical cube law power/speed curve to idle, the large low pressure
compressor handling bleed and ducting needed in synchronous power
generation applications is not required and has been removed.
Siemens/Westinghouse 501 – The 501 engine has been the key element of
a self-contained electrical power generating system termed ECONOPAC,
which is nominally rated at 160 MW. For naval applications the 501
engine has a reduced rating of 145.4 MW using a conventional combustor
with distillate fuel. Commercial marine rating would likely be similar.
The engine is designed for simple-cycle and heat-recovery applications.
The single-shaft engine has a 16-stage axial flow compressor yielding a
compression ratio of 14:1. The combustion system is composed of 16
single-nozzle combustors in can-annular arrangement. The power turbine
is a 4-stage reaction turbine. This engine primarily has been installed in
industrial power generation applications, and is not currently used in any
marine applications due to its large physical size and weight. Note,
however, that it is not far from the power being discussed in future
generation 10,000+ TEU ships.
presents physical and fuel consumption data on the listed engines.
As will be seen, the fuel consumption for the turbines ranges from 205 to
277 g/kW-hr. This compares to the diesel’s 171 g/kW-hr as a 20% to
60% penalty in fuel consumption. Further, since these engines prefer a
lighter grade of fuel, there is an additional cost increase per pound of fuel
that may be approximately 50%. The net result of this is that the turbines
may cost as much as twice as much in fuel costs, as compared to the
diesels. This of course adds to the total life cycle cost of the gas turbine
propulsion plant alternative as well as a modification of the world-wide
bunkers infrastructure.
Most of the listed turbines turn at about 3600 rpm. Thus a double-stage reduction gear
is required to reduce the rpm to the 100-200 at the ship’s propeller.
Gears of this power and ratio will be large and heavy, often as heavy as
the turbine engine itself. Indeed, a reduction gear weight of about 1 tonne
per MW is likely. Thus the weight of the turbine engine must be increased
from, say, 20% to 100% to account for the weight of required reduction
gears. (A greater weight penalty with the lighter aeroderivative engines.)
The result of this is a range of engine-plus-gear weights as follows. As
may be seen, despite large gear weights these engines are still substantially
lighter than the thousand-tonne-plus diesels. This weight reduction might
in some services be converted into extra revenue capacity. However, due
to the noted fuel consumption penalty, this weight reduction will be
completely eliminated by an increase in the required fuel capacity. The
result is that there is no net reduction in machinery weight, no net increase
in ship revenue, and a substantial increase in recurring fuel costs.
Electric Drive
Electric drive is an alternative prime mover/power generator. This methodology consists of using a steam, diesel, or gas turbine prime mover, or an alternative power generator (fuel
cells or nuclear reactor) to drive a large electric power producer (alternator). The
electricity is then sent via wiring to a propulsion motor that turns the propeller.
This system would more properly be called an electric transmission, as the prime
drive power is still diesel or turbine produced. As may be imagined, the system
introduces some losses, as mechanical energy is converted into electricity and
then back into mechanical energy. Further, the large alternators and motors
required may significantly drive up the weight of the system as compared with a
mechanical transmission, especially when compared to the directly coupled lowspeed
diesel engine configurations.
The attraction of electric drive lies primarily in the ability to distribute power
demand over multiple prime movers. Thus several engines may be working
together to drive one propeller. This in turn offers the possibility of adjusting
load factors so that the engines operate at their most fuel-efficient points
throughout a relatively wide range of ship speeds. Cruise ships are increasingly
turning to electric drive, with the Queen Elizabeth II being a notable example.
Electric drive is also of interest for ships with large hotel electric loads, such as
cruise ships and warships, because it offers the possibility of having one large
machinery “bank”, and tapping power off for propulsion or hotel loads equally.
As has been mentioned, electric drive begs the question of how the electricity is
produced – whether by diesel, turbine, or other means. In this section of this
report we will address only the propulsion motor & generator portion of electric
drive. Sections below will address a variety of propulsion power generation
options. In this way, the present discussion of electric transmission forms a
building block for subsequent discussion of Fuel Cells and Nuclear Power.
Current market scenario
Among the largest electric drive motors currently deployed are those on
the passenger liner Queen Elizabeth II. These motors have the
characteristics given below:
• Length 4.4m
• Width 8.74m
• Height 8.4m
• Weight 285t
• Power 44 MW
• RPM 144
• Power 10kVolt 3 Phase 60 Hz
Additionally, published US Navy reports indicate that the next generation
of naval surface combatant – designated DD 21 – will be electrically
driven. Based on current destroyer-sized warships we may thus expect the
DD-21 to be fitted with two shafts each having 50-70,000 hp electric drive
motors.
Limited data is available on a 35MW GEC Alstom motor, having the
following characteristics:
• Length 11.2m
• Width 4.25m
• Height 3.75m
• Weight 230t
• Power 35 MW
A developmental motor is the superconducting homopolar motor currently
being developed by General Atomics (GA). The following description is
taken from a General Atomics data sheet on this project: “General
Atomics is performing an assessment of superconducting homopolar
motors for ship propulsion as part of the U.S. Navy's quiet electric drive
effort. Homopolar motors are simple in design and offer the potential for
a large weight reduction when compared to conventional motors.
Because there are no multipole components in the motor it is expected to
be acoustically quiet enough to permit hard mounting directly to the ship's
hull, thus greatly simplifying integration.
In order for the homopolar motor to fully exploit the advantages of
reduced size and weight, the field coils must be superconducting. The
coils will be conduction cooled using compact reliable devices called
cryocoolers, which do not require the use of bulk liquid cryogens. GA has
developed and demonstrated the reliability of conduction-cooled
superconducting systems for the Navy under high shock and vibration
environments that are suitable for full-scale homopolar motors. Ongoing
research and development efforts by the Navy are now focusing on
improving the performance and reliability of the motor's current collectors
or "brushes." Dry current collectors presently under development show
promise for reduced wear rates that may result in no maintenance between
ship overhaul cycles.
The conceptual design of the 40,000 HP, 150 RPM motor was developed
by the Naval Surface Warfare Center, Annapolis, MD, and is significantly
smaller in diameter than any other kind of electric drive propulsion motor
of equivalent speed and power, and is expected to have between 1.5% and
2% higher overall efficiency.”
The physical characteristics of these motors may be approximately as
follows:
• Length 4.1m
• Width 4.3m
• Height 4.3m
• Weight 113t
• Power 31 MW
• RPM 100
The efficiency of an electric drive system depends upon a number of
factors. Not least of these is the type of rectifier / inverter used, and how
hard one has “pushed” the rating. For example, adding forced air cooling
to some of the components will increase their rating as much as a third,
but at lower efficiency.
For a general-purpose efficiency estimate it is not unreasonable to expect
electric drive to have a net system efficiency of 90%. This is the ratio
between installed engine power and net delivered propeller power.
Motor------ Technology----- Weight----- Power----- kg/kW
QE-2 -----Conventional AC----- 285 t----- 44 MW -----6.47
GEC /Alstom -----Conventional AC----- 230 t -----35 MW -----6.57
General Atomics----- Superconducting -----113 t------ 31 MW -----3.65
it appears reasonable to state
that current technology motors are available at about 6.5 kW/tonne, and
that future technology motors may become available at about 4kW/t. (The
author has rounded the figure of 3.65 to 4 in order to reflect the
developmental status of the motors. It would be unrealistic to use a
prototype figure to three significant digits to represent a production unit
that may be ten years away.)
Nuclear – Electric
Propulsion
Nuclear power has not been considered since the NS Savannah in the 1950s. The
Savannah experience is complex and cannot be adequately summarized here. In
brief it was that the manning requirements, due to the high degree of training
required, and fearful port regulations impaired further development of nuclear
merchant ships.
A new type of nuclear power plant has been recently proposed, designated the
gas turbine modular helium reactor (GT-MHR). In this type of reactor the heat of
reaction causes helium gas to expand. The helium is “blown” across a turbine
coupled to an electric alternator. Because of the balance of the reaction this type
of reactor is fail-safe: If left uncontrolled it will “wind down” to an idle mode.
The GT-MHR has suggested to several observers an application for shipboard
use. Indeed, a parallel CCDOTT project is studying the application of the GT
MHR to the FastShip Atlantic vessel.
Conceptually the GT-MHR is similar to a gas turbine, except for the existence of
a nuclear reactor instead of fuel burners, and the choice of a closed helium cycle,
resulting in a decrease in the compression ratio. Helium is heated by the nuclear
reaction and expands across the blades of the turbine. The helium is recondensed
and redelivered to the hot side of the reactor. The turning turbine produces
torque, and in some cases is directly coupled to a generator (within the
containment shell) for direct delivery of electrical power.
To further improve the thermal efficiency from that of a simple cycle, a heat
recuperator recovers residual energy from the turbines, reducing the reactor size,
while a precooler and an intercooler reduce the compression power demand. With
23
such characteristics, a nuclear power plant could achieve a 47.6% thermal
efficiency.
Helium is the preferred working fluid for several reasons. This monatomic and
low-molecular-weight gas peaks in efficiency at a relatively low compression
ratio, imposing small mechanical loading on the turbine blades. It has a high
specific heat capacity, high gas constant and a relatively high thermal
conductivity, properties which make compact components possible. On the other
hand, its low density and high gas constant allow high flow rates without Mach
restrictions as in conventional turbines. Its inertness reduces radioactivity within
the turbomachinery. The main limitation of helium is cost.
The conceptual container ship GT-MHR powerplant could be as depicted in
Figure 2. The plant depicted shows a configured envisaged for two-shaft
operation, with one reactor vessel (RV), and two power conversion vessels
(PCV). Both the RV and the PCV are located within a radiological containment
perimeter. The PCV would produce electric power, which would then be fed to
the ship’s propulsion motors. Container ship versions of this system would
probably utilize a single PCV, for a single shaft ship.The main attribute of a GTMHR,
provided that it has a low power or a low power density, is its capacity to
tolerate a full loss of coolant without core meltdown
and the stability of the coolant. Safety resides in a microencapsulated
fuel that can retain fission products during such an accident, its capacity to
passively shut down the reactor if temperature increases (Doppler effect), and a
safety-related favorable core geometry. In the General Atomics GT-MHR, each
fuel element is a hexagonal-prismatic graphite matrix 0.8 m high and 0.3 m
between faces, with 3000 fuel compacts in 94 channels, plus 108 cooling
channels. The elements are arranged in an annular core with internal and external
reflectors. Each fuel compact is 5 cm high and 1.2 cm in diameter, and contains
hundreds of thousands of tiny refractory particles (615 μm), with uranium
encapsulated in several layers of porous carbon, silicon carbide and pyrolytic
carbon (TRISO). This fuel design has been proven at high temperatures for about
three decades, and tested to almost its theoretical burnup.
To remove fission heat, helium is injected to the RV at 7.1 MPa, from the PCV.
It ascends through the RV periphery, descends cooling the reactor core, and
returns to the PCV, expanding through the compression and power turbines. The
ICR cycle is used for a better thermal efficiency, and two compressors make up
for expansion and friction pressure drop. Power level control is provided by gas
pressure adjustment at nominal efficiency, and by a power turbine by-pass.
Figure 6 schematizes the gas flow for one of the GT-MHR modules as applied to
FastShip.
Current market scenario
GT-MHRs, and nuclear reactors in general, experience significant
economies of scale. Thus most development attention is focused on the
deployment of large land-based power generation capabilities. This is in
contrast to most alternative propulsion concepts where the problem of
scaling up to ship size exists. In nuclear power we are challenged to scale
down to ship size.
In the USA the greatest advocate of the GT-MHR has been General
Atomics Corp, in San Diego CA. General Atomics is, on a program
parallel to the present one, developing a conceptual description of a GTMHR
power plant for the FastShip Atlantic (FSA) cargo ship. This ship
application requires about 250 MW total. The GA concept for the FSA
application is a two-reactor plant, with two RV/PCV units operating in
parallel. This is a fortuitous development decision as it allows the present
project to use just one-half of this system for a conventional type container
ship.
The weights for a complete GT-MHR powerplant, including propulsion
motors, is given in Table 4. Added to that table is a column of comparable
line item weights for a direct drive low speed diesel powerplant. Note that
the diesel plant includes fuel for an estimate 6000 nm range. The nuclear
fuel is included as well, but this is not so closely tied to a particular range.
What is surprising is that the nuclear plant is competitive in weight with
the diesel plant. And, in addition, it produces 83% more power. In other
words it has a substantially improved weight per MegaWatt as compared
to the diesel. Of course, it shares this attribute with a gas turbine, which
is also lighter than a diesel, but as will be explored later the nuclear plant
has no additional fuel weight, whereas the gas turbine plant loses nearly
all of its weight advantage due to an increase in the associated fuel weight.
Table 4 - GT-MHR Propulsion plant weight, compared to diesel plant weight
Nuclear Plant Diesel Plant
Description 1/2 FSA MAN B+W K98
Reactors 450t
Shielding 1250t
Generator 800t Engines 2157t
Foundations 250t
Motors 400t
Motor Control 100t
Helium System 7.5t
Heat Exchange 50t Margin 216
Cabling 50t
Margin 336t Fuel 2817t
TOTAL 3693.5t 5190t
Power 125 MW Power 68 MW
Note Nuclear plant is one half of plant being conceived for
FastShip Atlantic
Diesel plant estimates are intentionally optimistic
Fuel Cell – Electric Propulsion
Fuel cells are an emerging technology. A fuel cell converts hydrogen fuel into
electricity directly. There are no moving parts – the electricity is released when
the hydrogen molecule is broken up.
As such, a fuel cell may be thought of as an alternative to a diesel generator. It is
indeed such an alternative, with the advantage of having no moving parts and a
very high fuel conversion efficiency.
The fuel cell reaction works only on the hydrogen in the fuel. When running a
fuel cell with a hydrocarbon liquid fuel it is necessary to first reform the fuel into
hydrogen and CO2. As part of or prior to the reformation, it is also vital to
remove the sulfur from the fuel before it is used. This process represents an
ancillary load on the cell, and requires additional space and weight.
Also, the fuel cell reaction is chemically the same as combustion: Hydrogen is
combined with oxygen and released as H2O vapor. Fuel cells thus have the same
air intake and exhaust uptake requirements as combustion engines. They also
produce waste heat, which is dissipated to cooling water. In all these senses the
fuel cell is a direct replacement of a diesel generator.
The advantages of fuel cells are that they lack moving parts, which implies
reliability. This is only true, however, for the fuel cell itself. The fuel reformer
will certainly be mechanically complex. As will be shown below fuel cells also
demonstrate high power density and high thermal efficiency. Use of fuel cells
may potentially result in a reduction in plant weight, a reduction in plant
complexity, and a negligible reduction in fuel consumption. These advantages
may be enough to draw electric propulsion into the ranks of container ships.
Current market scenario :
Norway is investing in environmental friendly technologies. The Westcon shipyard in Rogaland will start testing fuel cells in a couple of months.Use of fuel cells can reduce the climate gas emissions of maritime transport by 50 %.
The gas-driven supply ship "Viking Lady", owned by the ship owner Eidesvik, will be the first test center in the world for use of fuel cells on board a merchant ship. "Viking Lady" is the third supply ship to be run on LNG (Liquid Natural Gas). The gas will also be fuel for the fuel cell that will produce 320 kw. That is sufficient to function as an auxiliary engine for electricity supply on board, but not to run the ship itself.
The investment in fuel cells for ships represents an environmental revolution for the shipping industry. The Norwegian-German project "Fellowship" is a major step in a positive direction. The fuel cell will be connected to the power supply system and it will contribute to the running of the ship.
Most importantly, the research and experience made on "Viking Lady" will enable future use of fuel cells in the shipping industry. Project leader in Det Norske Veritas, Thomas Hebe Tronstad, believes fuel cells will be competitive in the future. - There is a still a lot of work to be done, but I am convinced that fuel cells is the power of the future, he says. In 100 years I doubt we will find many combustion engines on board Norwegian ships.
It is the German company, MTU Onsite Energy, which has developed the fuel cell that will be tested. Fuel cells are already being used for redundant power supply on land. Using it on-shore in a static environment is very different from using it on board a ship in constant movement with sometimes rough weather conditions. Thomas Hebe Tronstad regards the adjustment of the current fuel cell technology to a marine environment, as one of the biggest challenges.
The next step is to test parts of the equipment on-shore at Wärtsilas facilities at Stord, outside Bergen. This testing will start in the beginning of April.
There are no fuel cells on the market specifically configured for ship
propulsion. However, fuel cells by their very nature are assembled out of
“stacks” of cell elements, in a fashion similar to the way batteries consist
of assembled cells. Because of this inherently modular design fuel cells
can relatively easily be assembled to almost any size. Nevertheless, there
are at present no known fuel cells over 1 MW.
Net fuel cell plant efficiency (from the VINDICATOR project discussed
below) ranges from 42% at 10% load to 51% at most-efficient load. This
translates to an equivalent Specific Fuel Consumption of 165 to 200 g/
kW-hr. This compares quite favorably with a low speed diesel at a catalog
(presumably “best case” fuel consumption of 171 g/kW-hr. Thus total
ship fuel consumptions will be similar between fuel cells and low speed
diesels.
VINDICATOR FUEL CELL ------------ LOW SPEED DIESEL
625 kW --------------- 68000 kW
11 t Power generation total -----------------2157 t Diesel Engine
Net: 56.8 kW / tonne ----------------------Net: 31.5 kW / tonne
Net: 17.6 kg/kW -----------------------------------Net: 31.7 kg/kW
generator sets. The hydrogen fuel cell stack is smaller and more compact
than the portable generators they replace. However, the fuel cell reaction
works only on the hydrogen in the fuel. When running a fuel cell with a
liquid hydrocarbon fuel it is necessary to first reform the fuel into
hydrogen and CO2. This process represents an ancillary load on the cell,
and requires additional space and weight.
Perhaps the most mature fuel cell demonstration project was a project to
install fuel cell propulsion generators on the USCGC VINDICATOR. The
VINDICATOR is a former T-AGOS monohull ship, driven by two 800 hp
motors energized by four 600 kW diesel generators. The project,
performed by JJMA under contract to the US Coast Guard, was to replace
the diesel generators with Molten Carbonate fuel cells.
The project concluded that the replacement was feasible, but that the fuel
cell power plant would be slightly larger and heavier than the medium
speed diesels they were replacing. The figures given in the JJMA final
report are as follows:
Length 26 ft (7.9m)
Width 7 ft (2.1m)
Height 11.5 ft (3.35m)
Weight 12,000 lbs (Stack only)
est. 24,000 lbs complete module (11 t)
Power 625 kW
Referances:
ALTERNATIVE POWERING FOR MERCHANT SHIPS : CCDoTT Chris B. McKesson, PE
FUEL CELL TECHNOLOGY - DNV frode sudmanan berntsn
navy-matters.beedall.com
http://www.fas.org/
http://www.zemships.eu/
Nandkishore Gitte
