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AN UPDATE ON AIR INDEPENDENT PROPULSION SYSTEMS

A lthough air independent propulsion (AlP) is not new, ~much has been written about it in recent years and some good development work has been accomplished. As is frequently the case during the development phase, much of what has been written has been associated with intentions and hopes instead of actual achievements. Additionally, through the course of development, things have been learned and goals have been adjusted accordingly.

The purpose of this article is to provide an accurate, consolidated update of achievements in AlP to date and to provide the reader factors to consider in assessing which is the best technology for different applications. Unless the requirements for a specific application are clearly specified, the danger exists of comparing apples to oranges.

Historically the USSR, UK and USA early recognized the military importance of AlP and conducted developmental work in this area. The U.S. conducted experimental efforts from 1955 to 1970 with the X-1, a 25-ton submersible powered by a hydrogen peroxide engine. The X-1 suffered at least one major explosion during its period of operation and the U.S. probably saw little reason to continue developmental efforts because of the impressive achievements with nuclear power.

The UK conducted somewhat more ambitious experiments between 1956 and the mid 60s with the EXCALmUR and the EXPLORER. They were 750T (1000T submerged) submarines normally powered by diesel electric. They were fitted with CO~steam turbines for AlP operations using high test hydrogen peroxide and diesel fuel and were capable of 25kts. The success of nuclear power was undoubtedly an influence for UK to also discontinue efforts in this field.

The Soviets have been more dedicated in their development of AlP systems starting in the 1930s with closed cycle diesel (CCD) systems and continuing to the present time (1987BELUGA). Although little information is available, their efforts have probably included units in the 1930s, 10 or more M-class units in the 1940s, QUEBECS (nicknamed ZIPPOS by their crews) with Kreislauf (LOX) CCD’s powering the center of 3 shafts, the 1500-ton WHALE in 1956 powered by a water hydrogen peroxide turbine, and possibly some ZULU, FOXTROTS, JULIETIS and KILO (1982) with CCD’s.

Civilian interest in AlP appears to have begun in about 1965 with General Dynamics’ installation of an Allis Chalmers hydrogen fuel cell (potassium hydroxide electrolyte) in the submersible STAR I. In 1969, Perry Co. installed a Pratt and Whitney hydrogen-oxygen fuel cell in the HABITAT HYDROLAB which operated in 50 feet of water off Palm Beach, FL. This unit could provide 5 kw at 28vdc for 48 hours but cost was a problem. In 1970, the submersible SP350 was operated off Marseille, France, with a hydrazine-hydrogen peroxide fuel cell. It made several dives of up to 15 minutes to depths of 265 feet. In 1980, Lockheed installed a Pratt and Whitney fuel cell in DEEP QUEST, which transited from San Clemente to San Diego submerged.

At present there are four candidate technologies for AlP systems. These are closed cycle diesel, sterling cycle engines, fuel cells and nuclear. Nuclear has been well developed in full scale military submarines (operated by 6 countries and accounting for 40% of the world’s 950 submarines) but some work has been done recently for smaller scale civilian applications and hybrid (diesel and nuclear) military submarines, sometimes referred to as the SSN.

Each technology will be addressed, describing briefly how the system works, current status of development and points for consideration. CLOSED CYCLE DIESEL (CCD) .

A CCD system utilizes stored oxygen (either gas or liquid) and a working gas such as C02 or Argon. The exhaust gases are scrubbed or processed to remove combustion products from the working gas and then chilled, dried and filtered. Excess exhaust products are compressed and stored onboard or discharged overboard. Since combustion is never complete, even in an oxidant rich mixture, this recycling conserves both oxygen and fuel. The processed exhaust is combined with fuel and oxygen at the engine intake where vaporization and mixing {swirl) of components is very important. An oxygen rich mixture is established at the highest temperature practical for complete combustion.

There are currently two ceo programs of interest One is a system called “SPECfRE” based on the earlier “ARGO” system which is being developed jointly by RDM (Dutch) and Cosworth Engineering (British). It uses LOX and Argon as the working gas. Argon was chosen to give the proper specific heat of combustion. The system has a patented exhaust scrubber, water management, overboard discharge system. It is reportedly efficient to 300 meters whereas previous ceo systems have been inefficient at depth.

The other CCD program is that of MARIT ALIA, an Italian group associated with the offshore oil industry. Over the past 10 years they have developed 12 to 250 HP series CCDs for submersible power and are currently developing a 150 ton submersible with a one week endurance. One of their most interesting units is the 3GST9 (3 inch pipe gas storage toroids, 9 meter long hull) which is presently being operated by the U.S. Navy to explore potential uses. The torodial pressure hull is unique and unusually strong. It allows considerable more usable volume for a fiXed diameter vessel with a fiXed volume for gas storage than any other storage arrangement, internal or external to the pressure hull The 3GST9 weighs 24.3 tons dry, has a 620 meter operating depth (430 m DNV certification), an endurance of 34 hours at 6 knots, a 3 man crew and up to 6 passengers.

Technical difficulties and comparisons of the CCD:

  • Short duration of the high temperature spike allows a much higher peak temperature without eroding the combustion chamber than in other designs such as the Sterling Engine.
  • The CCD is more fuel efficient than the external combustion engine (Sterling).
  • The CCD offers reliability at a reasonable cost (especially in civil applications where noise is not a serious consideration.)
  • CCD equipment is compatible with existing infrastructure (especially fuel). It is theoretically possible to improve the efficiency of the CCD to rival other technologies, except nuclear, by turbo-compounding.
  • The ceo direct drive is not subject to efficiency loss in energy conversion required by some other systems.

STERLING CYCLE ENGINE
The Sterling is a reciprocating external combustion engine with thermodynamically connected pistons that transmit mechanical work to a drive shaft and also move a working gas (helium) through a regenerator/cooler (heat sink) between hot and cold sides of the engine. Continuous burning in an external combustion chamber is kept in overpressure to facilitate overboard discharge of exhaust gases down to 300 meters.

Current Sterling Engine developments include the Swedish modified NAEKEN-class submarine (1000 ton) with 2 weeks or more submerged endurance and COMEX’s (French) submersible SAGA which has Swedish built engines (2-150 hp each). Both vessels are presently operational.

Technical difficulties and comparisons of the Sterling Engine.

  • The Sterling Engine is quiet due to external combustion, fewer moving parts, low system VIbration and low RPM.
  • Other external combustion engines include Brayton and Rankine cycle variants that rely on closed cycle turbines. They are smaller but less efficient than Sterling Engines and because of gear reduction they are noisier at low frequencies.
  • External combustion engines can never reach efficiencies attainable in internal combustion engines since the high temperature end of the Carnot cycle is limited by combustion chamber materials.
  • In the Sterling, energy is lost getting the heat from the external combustion chamber to the reaction mechanism.
  • The Sterling is quieter at the high frequency end of the spectrum but roughly equivalent at low frequencies to the internal combustion engine.

FUEL CELLS
Fuel cells convert chemical reactions directly into electrical energy like ordinary storage batteries. Like in a battery, a positive and a negative electrode are activated by an ion conducting electrolyte. The fuel cell, however, produces electricity by a catalyst aided electrochemical reaction between a fuel and an oxidant. Power production continues as long as fuel and oxidant are supplied. The theoretical thermodynamic efficiency of a fuel cell is not Carnot cycle limited and therefore can be very high.

The first fuel cell was built by Sir William Grove in 1839. During the 1930s Francis T. Bacon made signifacant engineering advances and in 1959 he was able to produce a 5kw system to power a fork lift truck. NASA funded fuel cell development for space applications and DOE funded development for vehicle and stationary applications.

A variety of fuel cell types, in various stages of development, are available. In the literature, fuel cells are classified by type according to their electrolyte and catalyst systems. The fuel cell types in current use and development are:

  • Alkaline Fuel Cell (AFC)
  • Solid Oxide Fuel Cell (SOFC)
  • Phosphoric Acid Fuel Cell (P AFC)
  • Molten Carbonate Fuel Cell (MCFC)
  • Proton Exchange Membrane Fuel Cell (PEMFC)
  • Thin Film Fuel Cell (TFFC)

Each fuel cell type has different operating characteristics such as exhaust products, tolerance to impurities, response to cold starts, method of load following, required support systems, preferred fuel and oxidant, preferred power level and range, operating temperature and pressure and response to large and rapid load changes.

Current fuel cell projects include a TYPE 205 German submarine (450 ton) powered by a fuel cell which utilizes LOX and a hydrogen-metal hydride (heat activated). It utilizes a potassium hydroxide electrolyte and has a predicted 1 month submerged endurance.

Additionally Vickers Shipbuilding of Britain is working on a solid polymer fuel cell plastic membrane which holds the catalyst and it uses liquid methanol which produces hydrogen when passed through a reformer.

One of the most successful recent fuel cell projects is one completed by Perry Energy Systems. Theirs was a three year R&D program which married a proton exchange membrane fuel cell (PEMFC) manufactured by Ballard Power systems { 1.5 kw (2kw max)} with a 2 man submersible, the PC 14. The system met all submersible application criteria and the range of the PC 14 was significantly increased (five hours at max speed). The PC 14’s endurance was increased 3.4 times and the payload by 1102 pounds over that available from a lead acid battery pod. The system uses pressurized gaseous oxygen and hydrogen, so¬†onboard processing is required. Resulting pure water is stored onboard so no weight change occurs during operation.

The Ballard Power System’s fuel cell stack uses individual pressure regulators for a simple load following system. It is low cost and has high power density, and long service life. Gases are delivered to stack in excess of need for the reaction and excess gas carries water produced out of the stack where the water is removed and gases are vented. This is an open loop system.

Technical Difficulties and Comparisons of Fuel Cells:

  • When operated below peak power, charge migration through the electrolyte causes a rapid drop in efficiency and a temperature increase requiring cooling pumps.
  • To overcome efficiency drop below peak power, design for nominal power with batteries to take the peaks or alternatively banks of smaller cells to accommodate varying loads.
  • Some question exists about the ability to start and stop cell banks smoothly and quickly on demand
  • Mature designs make use of expensive materials like gold, silver and platinum.
  • H2 stored as liquid requires refrigeration and has poor energy density.
  • Metal hydride or similar compound has better energy density but greater bulk and weight.
  • Newer aluminum/oxygen and iron/acid fuel cells are promising but not mature.
  • Logistics of refueling is a problem.
  • Gas hazards
  • Electrical to mechanical conversion efficiency loss.
  • Estimated 1 month submerged operations.
  • Second highest efficiency (50-70%). Up to 5 times the net energy density of a lead acid battery.
  • Very quiet.

NUCLEAR When talking about a nuclear variant of AlP, we are talking about what has come to be called the SSn or the nuclear-hybrid submarine. Obviously nuclear submarines as built by the U.S., USSR, UK, France and China (India now has a Soviet CHARLIE) are air independent. The SSn, still only conceptual, is an attempt to design a submarine with capabilities and costs somewhere between the diesel and the nuclear. It would be a modern diesel submarine with a relatively small reactor added to allow battery charging without snorkeling.

The French RUBIS – AMETIIYST might actually already fulfill these requirements of costs and capabilities and in fact has been offered for export. The Canadians considered it before dropping their nuclear submarine acquisition program. Pakistan reportedly has shown some interest in the AMETHYST.

Although several other third world countries, including Brazil and Argentina, have shown interest in the SSn, the only known development work on the concept has been performed by a Canadian company, ECS Inc.

ECS designed a 100 kwe nuclear power plant called AMPS, for the French submersible SAGA A test bed of this development has been completed and tested satisfactorily in a laboratory setting at Westinghouse, Hamilton, Ontario. This nuclear source, coupled to an organic Rankine Cycle Engine, is low temperature, unpressurizcd, intrinsically safe and would require minimal manning for operation. The nuclear core is the proven General Atomic TRIGAR reactor core. ECS’s major accomplishments in this project were the development of an innovative passive emergency cooling system and the development of a compact heat source of minimum weight. The passive cooling system has no moving parts and is capable of providing cooling in any altitude. Although production unit construction would present serious challenges, the compactness of the heat source is clever and accomplished, at least in part, by designing component parts (shielding, structure, reflectors, coolant, etc.) to serve multiple purposes.

Because of financial short falls, integration of the AMPS into the SAGA submersible has been delayed and probably will not occur at all. As an extension to the above developed technology, ECS has investigated scaling the AMPS power output upward to that required for an SSn, about 700 kwe to 1700 kwe. In so doing some subtle changes occurred in the design. Higher primary temperatures, and therefore some pressurization, became necessary and the energy conversion unit was changed to a low temperature steam Rankine Cycle Engine. The passive cooling system and the intrinsically safe TRIGAR fuel were retained in the design. However it should be noted that the TRIGAR fuel is manufactured in the U.S. and therefore not readily exportable for foreign military applications.

ECS has conceptualized the AMPS to provide a 2000 ton class submarine speeds up to 14 kts while supplying all ship;s loads. Sprints at higher speeds could be made with the energy taken from the batteries restored later when the ship slowed. The AMPS could be enclosed in a 12m extension in a 7.5 m dia. hull with only slight effects on hull performance. The total weight increase with AMPS integrated into the hull is said to be about 500 Tonnes and refueling interval is 8 to 10 years. A net plant efficiency of 15.7% is calculated.

It is noted that no prototype or test bed for this sized design has been built and tested. Technical Dimcultles and Comparisons – Nuclear

  • Most expensive – large support infrastructure.
  • Licensing (nuclear) and liability insurance
  • Fuel – sources limited – nonproliferation agreements.
  • Highest efficiency.
  • Greatest refueling independence and range.

In summary. the world’s interest in submarines continues to increase rapidly. Although batteries can sustain modern dieselelectric submarines with 4 to 10 days of air independent propulsion, high power density batteries have limited recharge cycles and thus significant cost considerations. Additionally longer periods of AlP are desired since snorkeling is a very noisy evolution with the snorkeler in a vulnerable condition with masts exposed and unfavorable acoustic factors.

Third world countries seek an affordable solution to the AlP submarine and as world economic factors shift, even the major powers may become interested in a high-low mix of AlP capable submarines.

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