Fuel Efficient Ship
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08-01-2011, 09:09 AM
The growth of science and technology at an unprecedented pace in the past century was facilitated by the surplus fuel reserves that sustained the developmental requirements in a post industrial revolution era. The oil shock of 1973 brought into existence a sudden setback to this economic growth rate in industrialised nations. The vulnerability of the present developments to be negated by a fuel crisis has shifted the focus of research from better designs during the past century to effecient designs in the last two decades. The transportation industry has partially responded to the situation, as is evident from the magnitude of research for alternative fuels in the automobile sector. But the sheer economics involved have provided for largely insulating the maritime and aviation industry from the impacts of the fuel deficiency. Also the heavy machinery and high load, characteristics of these sectors, reduce the choice of fuels to very narrow levels. Till such time as alternate fuels are explored in economically viable ways, only a comprehensive strategy for energy management compatible to the optimum working conditions of the fuel can sustain the faesible continuation of existent propulsion techniques.
The paper attempts to optimize the existent methods of fuel usage in a ship and to redefine the concept of ‘Fuel Efficient Ship’ at its present form.Some innovative applications of proven scientific phenomena have been applied to ensure energy reclamation and improve overall efficiency.This is besides the general methods of heat recovery,efficient generation and utilisation of steam.
Conventional definitions of efficiency of a system is based on the ratio of input and output energy.As such the focus remains only on the combustion characteristics of fuel and the volume of fuel used is not considered on a discrete basis. The actual measure of efficiency of a system can only be denoted in terms of the economy of usage.ie. in Rs./KJ. This is because, the loss due to improper storage and handling of fuel is not considered in the conventional energy rating methods.
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DIMENSIONS OF ENERGY MANAGEMENT IN MARINE VESSELS.
In a marine vessel, the consumption of net generated energy is carried out in a multitude of channels.The necessity of a well defined strategy encompassing all the areas of energy consumption is evident from the fact that the best of existent norms of energy recovery can only retrieve less than 60% of the actual input energy. The energy consumption statistics vary in accordance with the type and size of vessels,frequency of preventive maintenance and kind of external load variations. Therefore, a decisive and comprehensive strategy of energy management has to be adopted in compatibility to the vessel under study, after a thorough analysis of the associated external parameters, to attain the best results.Only a sustained effort in this direction and a restructuring of strategies at periodic intervals can yield the desired end value.
As a first step in this direction, the optimum speed of travel has to be calculated from the various external and internal parameters like combustion efficiency, wave-making resistance, roughness of sea bed, etc. and other controls can be established in an automated manner on the basis of input from the speed measuring unit and other sensing units for a variety of parameters.The net loss under each value of speed in the optimal range is noted and the best value is selected. The various dimensions to which an enhancement of efficiency is attributed can be explained as follows:
Ø Proper storage and handling of fuel from source to the engine
Ø Effective Generation and tapping of energy from combustion in engine. ie.increasing the combustion efficiency and improving the heat balance.
Ø Efficient transmission of motive power from engine to propeller. ie.Optimal Propulsion Efficiency.
Ø Optimum Path travel facilitated by GPS Routing [existent in most of the vessels].
Ø Heat recovery from exhaust gases and cooling water.
Ø Energy reclamation from potential sources of stored or latent energy.
The requisite technology to enhance optimal energy reclamation from the above methods and their basic functioning has been dealt with in the paper in fair detail.
FUEL STORAGE AND HANDLING.
In a humid environment,under various conditions of pressure and tempereature, there can be a settlement of water vapour in the storage tank. If not drained off properly, this may cause incomplete combustion in the engine and a loss of latent heat thereafter. Incomplete combustion may also cause the soot deposits to be formed on the inside of the exhaust pipes, thus acting as a insulating film diminishing the prospects of heat recovery to enhance production of steam. Due to the above reasons, proper fuel storage is of utmost relevance in enhancing the efficiency of the system.
Fuel Handling is also of particular relevance in the above context. The impurities that may have mixed with the oil will have to be removed before the combustion process.No amount of care in fuel storage would be sufficient to ensure the absence of impurities in the mixture.Hence the two processes are complementary.
To remove the water droplets that may have entered the fuel tank, the installation would have to be made in an oblique manner so that the water would settle down at a corner and may be disposed off promptly by using a drain valve.Fuel Oil should be pre-heated to a suitable temperature that would render it pumpable and at a later stage heated to such a temperarure as would be conducive for proper atomisation and combustion of fuel.
Ø The storage tank should have an obliquity of 1 in 50 towards the side of a drain valve located at the bottom of the tank with the Draw-off or outlet pipes positioned 15 cm above the base level.As the storage tank in a ship is to based on a mobile platform, the sedimentation of water or impurity particles cannot be facilitated and hence the best choice would be to coat a dessicating agent at the top interior of the tank.
Ø Pre-heat fuel to a temperature close to 110 deg. C.
Ø Provide pre-heaters with thermostatic controls to prevent oil carbonisation when flow is stopped.
Ø During Start-ups, cold oil should be drained off and only preheateds oil should be used.
Ø Pipelines should be sized in accordance with the flow rates of oil.
Ø Gear pump is the most suited type to pump oil as it easily adapts to the conditions of excessive pressure drop and cavitation.Centrifugal pumps should not be used for the same reason.
The planned execution of the above considerations in a fuel handling and storage system would be the first step in the path to proper utilisation of energy contained in the fuel. As a sequel to these suggestions, certain additives may be added to the fuel to enhance combustion efficiency in case of a boiler in port use. Also, the optimal amount of pre-heating of oil may be obtained from tables for best combustion characteristics.
HEAT RECOVERY FROM COOLING WATER
Optimum utilisation of energy is attracting a lot of consideration from various sectors of industry and various concepts have been developed for making efficient use of energy. Heat recovery from various sources is one such consideration although the energy thus obtained will be low grade heat and thus may not be utilised effeciently.
Every vessel requires a good amount of fresh water to last the endurance period of the vessel.the production of fresh water on board reduces the required fresh water storage capacity and increases the fuel or cargo capacity of the ships.
Fresh water Generator - Operating principle
When sea water is boiled in a vacuum chamber , vapour can be produced at as low a temperature as 40 deg.C.and it is this principle which is employed in the generation of fresh water from sea water utilising a source of low grade heat to promote the boiling action.
The cooling sea water is circulated through the tubes of the condenser bundlesand then returned to the ships main or discharged overboard.A controlled quantity of pre-heated sea water from the condenser outlet is continuously diverted into the bottom of the evaporator shell as feed water.
Approximately, one-third of this feed water is evaporated, the remaining being continuously discharged in order to control the brine density. The feed water level and discharge rate is maintained by a fixed brine weir.
The low grade heating medium is the heat in fresh water cooling system of the ship engines. The hot engine jacket water is circulated through the tubes of the submerged heater/evaporator bundle, cooling in the process by 5 to 10deg.C, before returning to the engine system. This heat boils the sea water in the evaporator shell and pure water vapour is produced.
This vapour rises through a demister which removes the entrained droplets of sea water and then passes into the condensing section. The vapour condenses on the outside surface of the tubes and collects in a tray from where it is extracted and pumped to storage tank.
The vacuum in the evaporator shell is produced and maintained by a sea water operated air ejector and excess sea water is extracted and discharged by a sea water operaed brine ejector. The sea water for the ejectors is provided by a centrifugal pump which takes its supply from the condenser outlet or from a separate source.
The higher efficiency double and triple effect evaporators are available for applications where the quantity of waste heat is limited, particularly for higher fresh water capacities.Heat consumption of double effect evaporator is approximately 16 kW/m3 for 24 hr fresh water supply and for triple effect generator it is 11 kW/m3.
The multi-effect fresh water generator utilises the pure water vapour produced in one effect as the heating medium for the next effect.The vapour condenses into fresh water in the heater tubes, trensferring its latent heat to the sea water in the evaporator shell.
HEAT RECOVERY FROM EXHAUST GASES.
The heat in the exhaust gases having the highest temperature is most suitable to be converted to useful energy. The heat recovered through exhaust gas economiser is commonly not more than about 10% of the heat loss through exhaust gases. The rate of recovery can be increased by another 30% or even 40%by generating more steam which can be fed to a turbo generator.The maximum capacity of exhaust gas boiler systems can be arrived at by means of the the relation given below:
H=[inlet temperature-exhaust temperature]*weight flow*specific heat.
In the above relation the inlet temperature to exhaust gas boiler and weight of exhaust gas flowing is dictated by the engine. The exhaust gas temperature should not be less than 170 deg. C to avoid dew point corrosion.Application of special corrosion resistant materials will allow bringing the down the outlet temperature of exhaust gases from boiler to be as low as 130 deg. C.,which increases the economiser efficiency by 20%.However, in these cases effective ways of cleaning the soot deposits should be employed.
At present, by using such high efficiency systems steam between 300 kg/hr and 500 kg/hr per 1000 kW of main propulsion engine output power can be generated.Such steam produced can be used for electric generator drive.The power thus developed must be sufficient to cater to the entire sea load.If this power is not enough, it can be supplemented by running a diesel generator set in parallel or by installing a higher capacity steam turbo-generatorand supply extra steam through the oil fired boiler. With an oil fired boiler fed steam generator, ther is no need for even a harbour generator, as the steam from oil fired boiler can be used in ports too.
The steam generated can be utilised more efficiently by the alternatives mentioned below:
• By using the heat of main engine cooling water for heating purposessuch as air-conditioning, fuel pre-heating, etc.
• Higher steam production by heating the boiler feed water with heat from scavenge air.
• Making use of central cooling water system to improve the system efficiency.
• Effectively designing heating systems of fuel tanks.
TURBO COMPOUND SYSTEM or EFFECIENCY BOOSTER
The diesel engine demand for turbo charged efficiency is lower than that obtainable through higher efficiency turbo charger. This enables bypassing a portion of the exhaust gases to a turbine to generate additional power. This system is known as Turbo Compound System[ TCS] or Efficiency Booster.
The power generated in this system is fed back to the engine by a power take-in gear. By using the TCS, the SFC can be decreased and the exhaust gas temperature after turbo charger is raised by 30 deg.C. At lower loads,because of the less amount of exhaust gas available, the overall efficiency achieved with TCS will be less and as such the bypass to TCS can be closedat about 50% of optimum power. As a result,the scavengeair pressure is raised and thus the SFC is reduced by 2 -3 g/kWh
Steering Mechanism :
The core of discussion in this category is the transition from conventional steering gear to an azimuth thruster, proposed for use in vessels with capacity less than or equal to 30,000 dwt. This provides for better manoeuvering of the vessel. The difference between Azimuth thrusters and steering gear can be explained by an analogy between steering mechanisms of airplanes and helicopters. It may be appreciated that the instant manoeuvering of a marine vessel need not be very accurate or sharp under normal conditions of operation, although it is desirable. But the sheer impact of space shrinkage in major ports with increasing traffic is an indication of the relevance of better manoeuvering techniques. Azimuth thrusters can be likened to the tail-ring rotor of an helicopter that provides for rotation about the axis of the chopper rather than a revolution about an external central point
An added advantage of azimuth thruster is the reduction in requirement of steering equipment by 10% of the dead weight of the vessel. Thus the total cargo load can be increased causing an increase in economy of propulsion. Since the equipment is independent of other parts, the cost of inventory can be reduced in manufacture and installation.
The speed of the flow can also be reduced by increasing as far as possible, the span of the propeller blade , thus maintaining the required mass flow rate of water even at low rpm of the propeller.However, the span of the propeller cannot be increased beyond a specified limit.
NAVIGATION and SATELLITE ROUTING
The vessel can be propelled on an auto-pilot mode with optimal route being obtained by using GPS satellites. The use of servomechanisms has fairly facilitated this feature and it is very common in present day vessels.
It is an area where relatively small changes in hull form and geometry can lead to significant changes in ship resistance. A well-designed bulbous bow can facilitate the achievement of the following ends:
q Reduction in drag by lowering of wavemaking resistance through attenuation of ship’s bow wave system.
q Reduction of viscous resistance by smoothening the flow around the forebody of the ship.
The design revolves around the principle of reduction of waterplane area to ensure smooth travel and also for stability considerations. The hull size should be very small as compared to the main hull.
ENERGY RECLAMATION TECHNIQUES
The conventional and neo-conventional methods of energy management notwithstanding, the loss of energy would still come to around 40% in the most efficiently managed ship. The focus therefore shifts to newer methods of energy reclamation from the energy sources in the immediate vicinity of the vessel. This section is an attempt at establishing the possibility and faesibility of some of the innovative applications of a proven scientific phenomenon called ‘magnetohydrodynamics’. By this method, the flow of an electrically conducting liquid when blocked by the field of a permanent magnet ,produces a potential difference at mutually perpendicular direction to both the magnetic field and the flow direction. This energy can be tapped by the introduction of electrodes at the stipulated position. The phenomenon is based on the same principle as Faraday’s law of electromagnetism.
Following are the main sources of energy reclamation :
q MagnetoHydroDynamic ring at the aft of the propeller installation.
q MHD_Diesel Hybrid Engine System.
q Flow Separators
q MHD cavities for smooth propulsion in rough sea beds.
Considering the electrical conductive properties of seawater, there seems to be immense scope for manipulation of the direction of flow of seawater with respect to the hull of the ship in an optimally planned manner. The high velocity of seawater in the immediate vicinity of the propeller can causedrag effect on the ship surface.This can be reduced by the introduction of an “ MHD ring’ at the most suitable distance from the propeller, that varies in accordance with the speed of travel. As such, the ring may either be installed on the basis of a pre-determined optimum speed of travel or be fixed on a mobile platform whose position is governed by the speed of the propeller in an automated manner.
The speed of the flow can also be reduced by increasing as far as possible, the span of the propeller blade , thus maintaining the required mass flow rate of water even at low rpm of the propeller.However, the span of the propeller cannot be increased beyond a specified limit due to a number of reasons.By reducing the velocity of flow in the proximity of the vessel, the ring enhances the propulsion efficiency of the system.
This method of energy reclamation not only manipulates the flow of water along the sides of the ship but also produces a certain amount of high grade (electrical) energy sufficient to cool the electronic components of the vessel.
The benefits would therefore consist of both reduction in drag force and in production of useful high grade energy.
Hybrid Engine System- An Overview
This system provides for tapping of useful energy from the engine in a multi-pronged manner, wherein the combustion efficiency of the engine is enhanced. The basic concept involved in the functioning of the system is the combination of the theories of expansion and MHD in a suitably designed way to provide maximum output. It consists in ‘seeding’ the air at the intake manifold with radioactive caesium particles that enhance electrical conductivity of air to about 10 mho/cm.As this air is compressed and ignited a sudden release of energy occurs in such a way that a huge part of energy is converted to heat and a small part manifests itself as motive force. Since heat is a low grade form of energy and cannot be used efficiently and the possibility of generation of high grade energy would prove more useful.
If a magnetic field is generated at the top quarter of the cylinder head and an electrode is placed in a perpendicular orientation to both the direction of magnetic field and expansion of gases,electrical energy can be tapped from the system along the same lines as an electic generator employing metal armature coils. A randomn movement of the molecules resulting in mutual collision of molecules would be detrimental to the efficiency of the system.The presence of caesium atoms ensures a direct influence on the microscope structure of the system. The motion along radial direction and the interaction of air molecules component molecules can be positively arrested. The magnetic field imparts an influence on the caesium atom preventing or tending to prevent its forward motion. This creates a distinct impact on the atoms surrounding the caesium atom under study, by arresting their random movements. The second stage of uncontrolled combustion in a 2-stroke engine yields the maximum heat that is wasted in a cycle.By effectively controlling the combustion evenwhile maintaining thermal efficiency, a higher overall efficiency can be produced. There would be minimal heat loss at the exhaust and cooling water. As such ,the motive power can be retained even as power is extracted in the form of electricity. Extreme care has to be taken in designing the components of the engine and there should be no mobile component made of magnetic material in the proximity of the magnetic field. The focus should be on components like piston rings, piston head,etc. This is because an electric spark at these positions may lead to explosions and damage to the engine structure.
MHD Cavities and Flow Separators
MHD Cavities in the line of flow with spring loaded shutters to ensure operations only in rough sea conditions would facilitate effective stagnation of the water body in the immediate vicinity of the body. This is the only method to provide some stability to the ship in rough ambient conditions. The cavities work in a two pronged way by reducing the waterplane area as well as by promoting stagnation of rough waves in the proximity of the vessel. The figure depicting the concept involved is shown in subsequent pages. It is a combination of the twin hull and MHD core ideas. Thus it can be compared to a SWATH [ tri-hull ] ship with comparatively miniscule auxilliary hulls with magnets as additional accessories. Permanent magnets may be used in this case unlike in the engine and propeller ring. The shutters will be released only in the event of a rough sea when stability would directly enhance efficiency of propulsion. The comparison to ‘Pulse Jet Engines’ would provide a fair idea of the proposed concept. The method of functioning of the valves is very similar to that of the V2 bombs used in II world war.
Proposed MHD ring Model
Proposed Hybrid Engine Model
Tri-Hull Balancing and Flow Separator Apparatus
An attempt to deal with the energy management techniques in a marine vessel has been done in a comprehensive manner in the paper. The focus has been efficient utilization of available fuel rather than on the possibility of any modifications in system structure to facilitate any breakthrough in the present levels of fuel consumption. As such, the methods would prove useful in enhancing the sustainability of maritime transport for a period of time till the available fuel resources can be economically used. Invariably the thrust in maritime propulsion will have to be based on a new mode of survival if the present levels of economy is to be sustained. Only the advent of a better fuel can promote this end after about 25-35 years. Till such time, the present modes of propulsion can be continued with renewed thrust on optimal fuel utilization.
The suggestions forwarded in this paper would, under ideal conditions, enhance fuel economy by 10 %. The enhancement in engine efficiency comes to around 15 % [ Theoretically ]. There has been an attempt to justify the usage of MHD cavities as auxilliary hulls. From interactions with experienced professionals in this field, it is learnt that such an attempt may cause higher frictional loss if the cavity is placed below water level. But it is also appreciated that relevance and the dynamics of such a system can only be ascertained from actual model tests. Moreover, the considerations of capacity and engine specifications will be of decisive value in determination of the faesibility of such a modification. As such , the thrust has been on suggestions under section A and other theoretical suggestions have been made only on a hypothetical basis with absolute theoretical justifications.
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2. Energy Management in Japan _ Takahashi Sako.
3. Proceedings at VII UN-ESCAP meeting.
4. Efficient Use of Energy _ S.C.Mukherjee.
5. Marine Diesel Engines.
6. Study Manual _ Singapore Maritime Academy
7. P.C.R.A Booklets. Vol 1 - Vol 7.