Introduction
Conventional submarines have generally operated underwater through energy stored in lead acid battery banks. They generally must surface every 2 days, at least till snorkel depth, to recharge their batteries, through a diesel engine- generator combination.
This problem was solved with the arrival of the nuclear-powered submarine. A nuclear submarine is a submarine powered by a nuclear reactor. The performance advantages of nuclear submarines over "conventional" (typically diesel-electric) submarines are considerable.
Nuclear propulsion, being completely independent of air, frees the submarine from the need to surface frequently, as is necessary for conventional submarines. The large amount of power generated by a nuclear reactor allows nuclear submarines to operate at high speed for long periods of time.
The long interval between refuelling grants a range virtually unlimited, making voyage times restricted only by the need to restock food or other consumables. Current generations of nuclear submarines never need to be refuelled throughout their 25-year lifespans.
Conversely, the limited power stored in electric batteries means that even the most advanced conventional submarine can only remain submerged for a few days at slow speed, and only a few hours at top speed, though recent advances in air-independent propulsion have somewhat reduced this disadvantage
The high cost of nuclear technology means that relatively few of the world's military powers have fielded nuclear submarines. Huge amount of special shore-based infrastructure is required to maintain nuclear submarines.
The stealth technology weakness of nuclear submarines is the need to cool the reactor even when the submarine is not moving; a substantial part of the reactor output heat is dissipated into the sea water.
This leaves a "thermal wake", a plume of warm water of lower density which ascends to the sea surface and creates a "thermal scar" that is observable by thermal imaging systems, e.g., FLIR (Forward Looking Infra-red).
Another problem is that the reactor is always running, creating steam noise, which can be heard on SONAR, and the reactor pump (used to circulate reactor coolant), also creates noise. This is as opposed to a conventional submarine, which can move about on almost silent electric motors.
Serious nuclear & radiation accidents have occurred within the Soviet nuclear submarines fleet but American naval reactors starting with the S1W, and iterations of designs have operated without incidents since USS Nautilus (SSN-571) launch in 1954.
Development of Air Independent Propulsion Systems like Close Cycle Diesel Engines, Close Cycle Steam Turbines, Stirling Cycle Engines and Fuel Cells have helped to increase underwater endurance to about 2 weeks maximum at a fraction of a cost of nuclear-powered submarines.
New build subs with integrated AIP Modules or AIP Modules fitted into older subs coming in for major refits have become a standard practise with many navies.
However, with the development of new technologies, batteries with high energy storage densities are becoming available. In fact, 2 Soryu Class submarines of the Japanese Maritime Self Defence Force (JMSDF) have replaced their lead acid batteries and Stirling Engine AIP with only Al-Li batteries, and this has improved their underwater performance significantly.
This paper explores both the roots and the future growth in battery technology to increase the underwater endurance of conventional submarines and does some crystal gazing on the future developments in underwater propulsion.
1) Air Independent Propulsion (AIP)
Air-independent propulsion (AIP), or air-independent power, is any marine propulsion technology that allows a non-nuclear submarine to operate without access to atmospheric oxygen (by surfacing or using a snorkel).
Modern non-nuclear submarines are potentially stealthier than nuclear submarines; a nuclear ship's reactor must constantly pump coolant, generating some amount of detectable noise. Non-nuclear submarines running on battery power or AIP, on the other hand, can be virtually silent.
While nuclear-powered designs still dominate in submergence times and deep-ocean performance, small, high-tech non-nuclear attack submarines are highly effective in coastal operations and pose a significant threat to less-stealthy and less-manoeuvrable nuclear submarines.
AIP is usually implemented as an auxiliary source, with the traditional diesel engine handling surface propulsion. Most such systems generate electricity, which in turn drives an electric motor for propulsion or recharges the boat's batteries.
The submarine's electrical system is also used for providing utility power—ventilation, lighting, heating etc.—although this consumes a small amount of power compared to that required for propulsion.
AIP can be retrofitted into existing submarine hulls by inserting an additional hull section. AIP does not normally provide the endurance or power to replace atmospheric dependent propulsion but allows longer submergence than a conventionally propelled submarine.
A typical conventional power plant provides 3 megawatts maximum, and an AIP source around 10% of that (200 to 250 KW -). A nuclear submarine's propulsion plant is usually much greater than 50 megawatts.
There are 4 Major Types of Air Independent Propulsion. These are as Follows: -
a) Closed-Cycle Diesel Engines (CCD)
Closed cycle diesel engines (CCD) employs a technology where the submarine utilizes it conventional diesel engine on the surface and a separate diesel engine for submerged operations. This specialized diesel engine employs combustion of liquid oxygen for its subsurface operations. The closed cycle diesel engine (CCD) is the cheapest among the existing AIP options.
However, the heavy moving parts make the submarine less stealthy. Maintaining the thermodynamic efficiency of the engine and dispensation of exhaust against water pressure, at dived depths is tricky. Overall thermodynamic efficiency of an CCD AIP system is around 30 per cent. U-1/Type 205 (Germany) and Moray Class (Netherlands) submarines use CCD. CCD is not currently in use. It did not prove effective.
b) Closed-Cycle Steam Turbines (MESMA)
Closed cycle steam turbine technology exploits the mechanical energy of the turbine coupled with than alternator to derive electrical energy. The system is based on Rankine cycle.
The system burns liquid oxygen along with ethanol at a temperature more than 600 deg. centigrade. The heat generated is transferred to the steam circuit which in turn drives the turbine. The alternator coupled to the turbine produces the electrical energy required for propulsion machinery and auxiliary circuits.
This technology is only offered by the French with the abbreviation MESMA (Module d'Energie Sous-Marin Autonome). MESMA is available for the Agosta 90B and Scorpène-class submarines. It is essentially a modified version of their nuclear propulsion system with heat generated by ethanol and oxygen.
Specifically, a conventional steam turbine power plant is powered by steam generated from the combustion of ethanol and stored oxygen at a pressure of 60 atmospheres. This pressure-firing allows exhaust carbon dioxide to be expelled overboard at any depth without an exhaust compressor.
An article in Undersea Warfare Magazine notes that: "although MESMA can provide higher output power than the other alternatives, its inherent efficiency [<=25%] is the lowest of the four AIP candidates, and its rate of oxygen consumption is correspondingly higher.”
MESMA has slightly edged out CCD in stealth by employment of a rotating machinery (turbine) instead of a reciprocating system. High power output makes it good for high-speed operations.
c) Stirling Cycle Engines
Stirling engines generate power by combustion of liquid oxygen with diesel fuel oil. The system is based on the Stirling Cycle. The source of energy is extracted from the working fluid which is permanently contained as a part of the system. The engine is run using the heat extracted from the working fluid.
Then the extracted energy is used either to recharge batteries or for direct propulsive load of the submarine. The resultant exhaust gases are thrown overboard the submarine by means of scrubbers.
The major advantage the system could offer is utilization of Diesel as its main fuel source hence reducing the complexity during refuelling operations. These systems are inherently bulkier and poses reduction in stealth when compared with silent fuel cell AIP systems.
The diving depth of the submarine will be restricted due to the interlock with dispensation of exhaust gases overboard due to running of the engine. Due to the flexibility, reduction of retrofit systems and cheaper operational costs feature as the USP of Stirling AIP system.
The biggest advantage of the system is the ability to utilize the onboard fuel. Though the fuel storage space is saved, it is equally compensated by the inclusion of a large internal combustion diesel engine.
Currently, Swedish (Nacken, Gotland & Sodermanland Class), Singapore (Archer Class), Japan (Soryu & Harushio Class) & China (Type -039A, Type – 041 (Yuan), Type – 032(Qing) Class) use Stirling Engines.
The technology is well proven and affordable. It requires the boat to lug around liquid oxygen oxidizer, which can have its own dangers, as well as inert gas to mix with it. It has many moving parts, and which can make noise even when a high degree of sound proofing is designed into the system.
d) Fuel Cells
The system works on the basic principle of combination of hydrogen and oxygen molecules to produce electrical energy with water as its primary waste product. The wastewater produced can be expelled outboard using the submarines water dispensation system.
Fuel cells are heavily researched everyday both in commercial and military sectors due to many distinct advantages including size stealth and exhaust dispensation. Employment of fuel cells on board submarines started back in the eighties and is still progressing ahead at a rapid pace due to innovations in the area.
With an efficiency of 50 to 70% fuel cells provide the much-needed flexibility, ease of operations and enhanced stealth compared to other AIP Systems. Germany (Dolpin, Type 209, Type 212, Type 214 & Type 218 Class), Spain (S-80 Class), Russia (Lada & Amur Class) & India (Kalvari Class- will be fitted in future) utilize fuel cells on board.
Fuel cells are not capable of ramping up power like a MESMA system but are very quiet as they have a few moving parts. Useful to stay silent and stealthy but not very effective at sprints.
e) Comparison of the 4 AIP Systems
i) Investment costs: The operation costs of fuel cells are much higher when compared to that of the CCD/ Stirling Engines. The major cost component of the fuel cell system is the storage required for liquid oxygen & hydrogen.
Use of ethanol makes MESMA slightly less expensive than Fuel Cells. The CCD/ Stirling engines are relatively low priced when compared to Fuel Cells & MESMA, as they utilize the diesel oil of conventional submarines to generate power.
ii) Submerged Endurance: The submerged endurance of the AIP system is directly proportional to the amount of fuel that is present in the storage tanks. The consumption of LOX plays a major role in the determination of endurance.
MESMA is the largest consumer of LOX amongst the existing AIP systems and has the lowest efficiency rate of 25 to 30%. The CCD systems, when in active service, had an efficiency rate of 30 to 35%. The Stirling engines have an efficiency rate of 40%. Fuel Cells have an efficiency rate of 50% to70%.
iii) Ease Of Operations: The Stirling Engine, CCD and MESMA systems will be comparatively easier to operate from the crew’s point of view as the operation of these system will not greatly vary from the operation of conventional diesel engines. Fuel cells though may appear sophisticated in the beginning with adequate exposure in operation of the system will enable the crew to exploit the system in an optimum manner.
Diesel (Stirling Engines) is most easily stored; ethanol requires a more complex stowage, and a reformer is needed to extract the hydrogen in pure form; hydrogen (Fuel Cells), for a given amount of oxygen, generates about a third more energy than diesel but it introduces safety problems if stored in liquid form; metal hydrides are compact and a safer means of storing the hydrogen.
iv) Stealth: The stealth forms the most important parameter during acquisition of any major equipment which is going to be fitted on board a submarine. The CCD, Sterling Engines and MESMA creates a certain amount of vibrational noise.
In addition, CO2, which is expelled over-board, as a bye-product, through a muffler arrangement, creates a disturbance in the ambient environment. Fuel cell is the quietest among all the AIP technologies and paves the way for increasing the overall stealth of the conventional diesel electric submarine.
2) Lithium-Ion battery Submarines (The Japanese Revolution)
Large leaps in battery technology over the last 2 decades may provide serious competition to AIP in the world of submarine warfare. Japan has replaced the Sterling engine-based AIP plus the lead acid batteries, in the last 2 Soryu class boats, with lithium – ion batteries. Like the submarines of World War 1 and 2 these boats will run under water on battery power alone except infused with new technology.
The Advantages of these Lithium – Ion Boats Over the Earlier AIP Boats are as Follows:-
- They are quieter than existing AIP capable submarines
- Lithium-ion batteries can keep up their output even when their charge is low
- Lithium-ion batteries are lighter than lead acid batteries
- They charge exceptionally fast hence the latest Soryu boats have been provided with more powerful diesel engine and generators
- Lithium-ion batteries have a much higher energy capacity for the same volume of a lead acid battery.
- Lithium-ion batteries can provide large output on demand allowing higher submerged dash speeds when compared to AIP.
Disadvantages
The major downside of “runaway” and combust are hard to extinguish using traditional means. Methods to contain these are as follows: -
- Specialized fire extinguishing system can be put in place to lower the risk of a catastrophic fire.
- Building larger lithium-ion cell matrices with reinforced boundaries and enhanced chemistry
- Extensive short-circuit, saltwater intrusion, drop and impact testing to certify batteries for such critical use.
Cost of lithium-ion batteries remains high: Japan's Ouryu submarine had an acquisition cost of $608 million, while its predecessor—which was equipped with lead-acid batteries—had a cost of $488 million, according to Défense News.
3) Future Growth in Battery Technologies
Submarine designers need not actually spend money on researching new battery technologies. They should rather continuously scan the environment for the massive amounts of R&D Investments in batteries for consumer electronics (watt-hour scale), Electric Vehicles (Kilo-watt hour scale) and Balancing Renewables (Mega- watt hour scales) and try to adapt new developments for submarine applications.
Typically, all rechargeable batteries have four major components. An anode, a cathode, an electrolyte, and a separator. During the charging cycle positive ions travel from the cathode to the anode, through the electrolyte and separator.
Electrons travel through an external circuit joining up with the positive ions, at the anode. During the discharge cycle, positive ions travel from the anode to the cathode, through the electrolyte and separator. The electrons travel from the anode to the cathode, through an external circuit doing work.
Diesel has a Volumetric Energy Density (VED)of 9,500 Watt-hour/litre (Wh/L) and a Gravimetric Energy Density (GED) of 12,500 Watt-hours /kg (Wh/kg). Considering engine efficiency of 25% this would lead to effective VED of 2,400 watt-hours/litre and GED of 3,200 watt- hours/kg.
Established chemistries (defined by cathode/ anode materials) like Lead Acid, Nickel cadmium, Nickel -metal hydride and lithium ion (Graphite – Transition Metal Oxide) have increased VEDs from 50 to 600 Wh/L and GEDs from 50 to 300 Wh/kg.
Emerging technologies of Lithium metal (Lithium- Transition Metal Oxide), Lithium-sulphur, Metal – air etc are expected to push VEDs beyond 800 Wh/L and GEDs beyond 800 Wh/kg. Advanced research is on in areas of chemistry, microstructure, cell design and pack design.
Lot of R&D work is also going on in the area of solid-state batteries. Solid-state battery technology is believed to deliver higher energy densities (2.5x), by enabling lithium metal anodes. They may avoid the use of dangerous or toxic materials found in commercial batteries, such as organic electrolytes.
Because most liquid electrolytes are flammable and solid electrolytes are non-flammable, solid-state batteries are believed to have lower risk of catching fire. Fewer safety systems are needed, further increasing energy density.
Recent studies show that heat generation inside is only ~20-30% of conventional batteries with liquid electrolyte under thermal runaway. Solid-state battery technology is believed to allow for faster charging. Higher voltage and longer cycle life is also possible. However, issues relating to prohibitive cost, temperature & pressure sensitivity etc. still need to be resolved.
Super-capacitors have higher power through put but not the high energy densities / specific energy of Li ion batteries. But using super capacitors and battery hybrids can increase the battery performance & life substantially. Research around super capacitors will have a big impact on power systems in future.
The Battery Industry Structure is Generally Organized on the Following Lines: -
1) Mining: Lithium, Nickel, cobalt, manganese, copper aluminium, graphite etc are the key elements that go into battery manufacturing. There is strong competition among countries to control the supply chain, maintain strategic dominance and influence procurement prices.
2) Industrial Chemists: They concentrate on developing Precursors like Lithium Carbonate, Lithium Hydroxide, Cobalt Sulphates, Nickel Sulphates and Manganese Sulphates and other chemicals.
3) Cell Suppliers: They concentrate on 2 areas Electrode + Components and Cells.
- Electrode & Components: Use of active materials, conductive additives and binders through processes of mixing, coating, drying and calendaring to produce separators, current collectors and electrolytes.
- Cells: Making cells in forms like cylindrical, prismatic and pouch of various size and capacity. Electrolyte filling. Cell Conditioning including formation and various quality checks.
4) Assembly/Recyclers: Concentrate on module assembly, battery management system (BMS) integration, Cooling Integration, Pack assembly, Vehicle System Integration ( first use) & Second use ( recycling).
Research on Batteries Need to Improve Performance in the Following Areas: -
- Cost: 130$/ kWh (cell) to 50$/kWh (cell)
- Energy density: 700 Wh/L to 1400Wh/L
- Power Density: 3KW/kg (pack) to 12KW/kg (pack)
- Safety: Eliminate thermal runaway at pack level to reduce pack complexity
- 1st Life: 8 years (Pack) to 15 years (Pack)
- Temperature: -20 deg to +60 deg C (cell)to -40 deg to +80 deg C (cell)
- Predictability: full predictive models for performance and aging of battery.
4) Crystal Ball Gazing
Poor Man’s Nuclear Boat
Main diesel engine will be removed and there will be only an AIP capsule and battery banks, fully charged before departure. The space vacated by the main engines will be filled with more battery banks. The AIP will be used for used for travelling at creep speed of 2.5 to 3 knots for enhanced submerged endurance.
For higher speeds and evasive manoeuvre battery power will be used. This will ensure that the submarine never has to surface during an operational deployment. This pairing could result in highly capable and versatile submarines that feature extreme endurance, very quiet operations, fast acceleration, and high dash speeds. This would be ideal for countries with limited radius of operations.
However, for long range operations a sub will have to use higher transit speeds. Further in an offensive mission off the enemy coast the sub will have to use high speeds to manoeuvre, attack and evade resorting to the power of the main batteries. She will once in safe waters have to recharge the batteries. A high-capacity air breathing generator would be required to top up the batteries in an hour in such situations.
All Electric Submarines
As the energy storage densities improve over time a stage will come when the total energy that can be obtained from an AIP, during a patrol will be less than the equivalent amount of energy that can be obtained if the complete AIP Module was replaced by battery banks.
This would lead to the development of 100% battery operated submarines. These all-electric submarines will be far quieter than Diesel Engine + AIP + Battery Bank submarines of today. For short range operations this may be OK, but for long range operations high-capacity air breathing generators may still be required.
Development of Underwater Charging Stations
If these are developed at various locations along India’s 7000 km coastline these all-electric boats could be vectored to nearest charging station, from its patrol area, for a complete recharge. This would remove the need for submarines to surface which is the time when they are most vulnerable to detection/ destruction.
The submarine would come back to port for repairs, maintenance and other activities which cannot be handled at the charging stations. It is likely the location of these charging stations will be known to the enemy with the passage of time.
Consequently, the enemy could park some of its submarines/ autonomous underwater vehicles (AUVs)/ under water demolition teams (UDTs) close to these charging stations to keep track of friendly boats coming here for re-charging.
The enemy boats could possibly tail the re-charged friendly boats back to their patrol areas and the UDTs could attempt to destroy the charging stations. It would be important to defend each of these charging stations with an all-electric midget submarine (manned or unmanned) to constantly patrol the vicinity of these charging stations, locate and destroy lurking enemy submarines, AUVs or UDTs.
Ocean Thermal Energy Conversion (OTEC) For Charging All Electric Submarines
OTEC uses the Ocean Thermal gradient between cooler deep and warmer shallow seawaters to run a heat engine and produce useful work, usually in the form of electricity. A heat engine gives greater efficiency when run with a large temperature difference. In the oceans the temperature between the surface and deep water is greatest in the tropics, although a modest 20 to 25 deg C.
It is therefore in the tropics that OTEC offers the greatest possibilities. The main technical challenge of OTEC is to generate significant amounts of power efficiently from small temperature differences. It is still considered an emerging technology. OTEC has 3 power cycle types – closed cycle, open cycle, and hybrid. OTEC sites are land based, shelf based and floating.
Floating plants which operate offshore present several difficulties. The difficulties of mooring plants in deep water complicates power delivery. Cables & piping attached to floating platforms are susceptible to be damaged during storms. Floating plants need a stable base for continuous operations which is again a problem in heavy seas.
With sufficient development in OTEC Technology, in future, would it be possible for Indian submarines to have their own OTEC along with their advanced battery banks? Since most of Indian submarine operations would be in tropical waters the boats could sail to areas with the maximum thermal gradients to recharge their batteries.
The submarines would probably have the capability to station themselves optimally between the thermal layers to achieve the best charging conditions. Submarines would also be free from the problems faced by floating plants in storms and high seas. This itself could be a subject of another paper.
About the Author
Amit Kumar Mukherjee studies military history, technology, and tactics as a hobby. He works for L&T, but the views expressed in the paper are his alone and not of the company or anyone he is associated with.
References
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