AIR INDEPENDENT PROPULSION SYSTEM FOR SUBMARINES
Air independent propulsion (AIP) system is a term that encompasses technologies, which allow a submarine to operate without the need to surface or use a snorkel to access atmospheric oxygen. The term usually excludes the use of nuclear power, and describes augmentation or replacing the electric propulsion system of nonnuclear vessels. AIP is usually implemented as an auxiliary source. Most such systems generate electricity which in turn drive an electric motor for propulsion or recharging the submarineâ„¢s batteries .The submarineâ„¢s electrical system is also used to provide hotel services-ventilation, lighting, heating etc â€œ although this consumes a small amount of power compared to that required for propulsion. A benefit of this approach is that it can be retrofitted into existing submarine hulls by inserting an additional hull section. AIP does not normally provide the endurance or power to replace the atmospheric dependent propulsion, but allows it to remain submerged longer than a more conventionally propelled submarine. A typical conventional power plant will provide 3MW maximum, and an AIP source around 10% of that. A nuclear submarineâ„¢s propulsion plant is usually much greater than 20MW
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“Run deep, run silent, run for ever” appears to be the credo of the submariners all over the world. The prime requirements for an effective submitter are to have prolonged submerged endurance and to have a low indiscretion rate during transit to patrol areas i.e. the main objectives of submarine designers can be destined as
i) Prolonged submerged endurance
ii) Low indiscretion rate during patrolling
iii) Sufficient submerged sprint capability.
iv) Very low self noise levels.
v) Diminished snorkeling during operation.
So we can see that all the objectives are more or less to tally, related to the propulsion plant adage of the submarine. No matter how sophisticated the combat systems of a submarine are, they will not be much worth if the submarine cannot operate silently under water for an extended period time with out being dictated.
Conventional submariners are fitted with diesel electric propulsion system along with lead –acid battery packs. A diesel engine drives a generator, which in turn powers a motor connected, to the propeller through the propulsion system. The generator also supplies the energy for charging the battery packs. During submerged operation, the battery packs supply the power to run the motor. Since the amount of energy that the battery pack can hold is limited, and get exhausted rapidly at higher speeds, the submarine will need to come to surface or at least come to periscope depth to snarl air and the diesel engine to charge the battery pack and propel the submarine. So to avoid this problem of surfacing frequently for access to the atmospheric oxygen demanded by the engines to charge the batteries and to extend submerged endurance as long as possible, designers have come up with Anaerobic propulsion plants called Air independent propulsion systems.
CLASSIFICATION OF AIR INDEPENDENT PROPULSION SYSTEMS
AIPS can be broadly classified into two –
1) Nuclear propulsion systems.
2) Non-nudear propulsion system.
NUCLEAR PROPULSION SYSTEMS
A submarine with a nuclear propulsion system can virtually have an infinite submerged endurance, as the nuclear reactor does not need any external atmosphere for its operation. So this is the most suitable propulsion system for a submarine.
But a nuclear reactor by itself cannot propel the submarine and hence another plant is required to convert the thermal energy into mechanical energy. The reactor and the plant make the submarine considerably large and heavy rendering it unsuitable for operation in restricted waters. Other disadvantages of nuclear propulsion system are .
1) High initial cost
2) Costly maintenance facilities are required and each facility is expensive to operate through its life cycle.
3) Disposal of spent uranium fuel rode presents an environmental hazard.
4) Lead – based shields separating the reactor from crew compartment make the boat mach large and heavier.
5) The coolant pumps are noisy.
6) Hazards in the case of accidents.
Despite all this some navies have exhibited the attractive underwater enduring quality of a nuclear submarine by installing a hybrid system where in a low powered nuclear reactor is used for charging the batteries and providing low cruising speeds when submerged and a diesel engine is used for surface transits.
NON -NUCLER PROPULSION SYSTEMS
Due to the above-mentioned disadvantage and difficulties in running a nuclear submarine, most countries have turned their attention to develop non- nuclear AIP system. The different types of non-nucler AIP system are.
a) Closed cycle diesel engines.
b) Stirling engines
c) Fuel cells
d) Advanced batteries
e) Mesma plants
CLOSED CYCLE DIESEL ENGINE (CCDE)
The diesel engine operating in the closed cycle mode is a strong contender as an AIP system which is a standard diesel engine that can be operated in its conventional mode on the surface, but under water, it runs on an artificial atmosphere synthesized from stored oxygen, exhaust gas (Generally argon) and recycled exhaust products. Thus allow it to be operated in an air independent environment. A major problem in under water conditions is to pump overboard the exhaust gases against the backpressure, which is equivalent to the depth of operation. This would result in the wastage of power.
To circumvent this problem the exhaust gas could be chemically scrubbed off. The combustion inhibiting components replenish used up oxygen and recreate the synthetic air thus generated back to the engine for combustion. This also would lead to loss of power output due to the rise in back pressure.
A general method to scrub the exhaust gas is using KOH. The exhaust gases are chemically scrubbed by KOH solution to remove CO2 and additional oxygen was added to the remaining gases (nitrogen + excess oxygen) before the inlet manifold. Thus it becomes a closed cycle.
The absorption of CO2 and addition of O2 have to be controlled to prevent a buildup in pressure and oxygen concentration within the closed cycle. Due to leakage past the piston rings and the formation of oxides of nitrogen it is necessary to maintain the levels of nitrogen by occasional replenishment.
The main problems encountered in the CCDE are: -
1) The store of oxygen.
2) The disposal of unwanted exhaust without giving away the position of submarine.
3) The reduction of specific heat ratio due to the presence of residual CO2 in combustion mixture.
The storage of oxygen is being tackled by storing it in liquid oxygen (LOX). This LOX is stored in oxygenic double-jacketed tanks on board. Another method of storing oxygen (developed by the Italians) is torroidal storage system in gaseous form. In this system a number of torroidal pipes surround the pressure hull of the submarine into which gaseous oxygen is stored under pressure. The space vacated by oxygen is used to store the unwanted products of combustion, thus ensuing that no exhaust is discharged to the sea.
Another method for handle the exhausts products in which exhaust gas is scrubbed with seawater under pressure to dissolve the carbon dioxide and absorb the water vapour, which is there after, discharged to the sea. The ingenuity of the system lies in the principle where in the ambient seawater pressure is used to move the scrubbing seawater in to the submarine through the CO2 absorber and back outside the hull against the out board pressure. Only small differential pressure pumps are required for the circulation of water inside the submarine. So the system has provided a solution to compression power drain problem and at the same time has improved the overall specific fuel consumption.
The residual CO2 in combustion mixture lower specific heat ratio. This will give rise to a reduction in efficiency to the order of 15 to 20%. This has been resolved by adding small amount of an inert gas like argon to the mixture to bring back the original value.
The Stirling engines are external combustion engines with a closed operating cycle that has many similarities with a conventional I.C engine. The heat is supplied externally and continuously to a working fluid, (helium or hydrogen) which is constrained in a completely closed system.
1 – 2 Compression – Piston go up and air pressure increases.
2 – 3 Heat addition – Displacer go down, so that air enters at hot room. Temp rises from min to maximum.
3 – 4 Expansion – Due the pressure of expanding gas, piston displacer goes down. Which is the power period.
4 –1 Cooling – displacer goes up
ADVANTAGES OF STIRLING ENGINES
1. Multi fuel Adaptability
2. Low noise
3. Long life with high reliability and low maintenance.
4. Comparable size of weight
5. Good plant load performance
A fuel cell is an electro chemical power source, which converts chemical energy directly to electric energy. It combines hydrogen and oxygen to produce electricity, water and heat. Fuel cells are already seeing a no: of promising application in the space and auto motive industries and many authorities believe that fuel cells offer the best potential for developing more capable AIP systems in future.
The hydroxyl (OH) ions are formed at the anode which than migrate to the cathode where they react with hydrogen to form water and releases free electrons. These free electrons flow through the load from anode to cathode to take part in a reaction to produce the hydroxyl ions. Over all reaction are.
2H2 + 4 (OH)- = 4 H2O + 4e – anode
O2 + H2O + 4e- = 4 (OH)- – Cathode
i.e. resulting equation 2H2 + O2 2H2O
Fuel cell performance depends on the type of fuel (alkaline, proton exchange membrane, phosphoric acid, super acids, molten carbonates, solid oxide) as well as the type of fuel and oxidant used. Studies have revealed that cells that using either proton exchange membrane or alkaline electrolyte will best meet submarine needs primarily due to high power densities, low temperature operation and fast startup time.
ADVANTAGES OF FUEL CELL
1) The only exhaust product is pure water
2) Ultra quiet operation.
3) High efficiency (50% to 60%)
4) Depth independence
5) Lower running and maintenance cost
6) Increased battle survivability
7) Lower stored oxygen consumption.
Advanced batteries have the potential to increase the submerged endurance of submarines. The specific power to weight ratio of present submarine using the conventional lead storage batteries has increased about 50% in the power for the same weight of the battery cells has already achieved. In addition to lead acid storage batteries, R & D works on the following types in progress –
a) Lithium Aluminium / Iron sulphide Battery (LAIS)
b) Lithium Tionyl – chloride storage battery ( Li – Thi)
c) Lithium peroxide storage battery (Li H2O2)
d) Ag – Zn battery etc.
Mesma (Module d’ energy sous marine plant) is based on heat generation loop and has high reliability and has depth independent operation due to the condensation and storage of the combustion gases in liquid form at 60 bars. The heat-generating loop operates at 60 bars and comprises the following.
i) Combustion chamber, in which ethanol and oxygen are burnt, cooled by recalculated gases of combustion.
ii) Boiler, which is the heat source of the thermodynamic cycle.
iii) Recirculation loop bringing the cooled gases back to the combustion chamber to cool Flame tube walls to a temp below 10000C and to dilute combustion gases to avoid high temperature on boiler wall.
iv) Condenser for combustion products
v) Storage tank for fuel, oxygen combustion products
Water is used, as working fluid and the main components of Rankine loop comprises boiler, turbine, seawater-cooled condenser and recuperators to preheat water.
The special feature of Mesma plant lies in the condensation of combustion products (CO2 and H2O), which can be stored in liquid state occupying a small volume. CO2 can be stored at 60 bars in liquid state or rejected into sea without any high-pressure compression. Compared to a reciprocating engine. The acoustic levels are low.
The nuclear hybrid AIP system for a submarine can prove to be the best option if political policies are kept aside and if initial cost and dangers of pollution are ignored. Among AIPS, Stirling engines appears to be very promising while CCDE is offering stiff competition with the only disadvantage of having higher noise level and difficulty in exhaust management. With further development in technology, the fuel cell is likely to be the most preferred AIPS of future.
1) A.V.Jayakumar, Rahamathulla.R – AIP Systems for submarines. Journal of institute of engineers, vol – 76, May 1995.
2) Edward C.Whiteman – ‘AIR INDEPENDENT PROPULSION”
3) Karen Winzoski – ‘Fuel cell Technology for Submarines’. CASR – May 2003