December 2016 IssueMore Stealth Power
Development of the perfect AIP system cannot be complete without substantial financial backing
Igor V. Vilnit, Rubin Design Bureau's Director General - General Designerhttp://www.forceindia.net/MoreStealthPower.aspx
For as long ago as 15 years the scientific community and defence experts had advocated the introduction of Air Independent Propulsion (AIP) for submarines. Years later, AIP systems have become a regular feature with navies across the world, and so now the first results of AIP usage can be interpreted and conclusions drawn.
The main reason behind adopting AIP systems is to increase a submarine's stealth by eliminating noisy snorkelling and remaining in contact with the atmosphere. The benefits of added stealth outweigh the increased cost of the submarine over its life cycle, stringent requirements for the infrastructure and crew training.From both a theoretical and practical point of view, it is clear that none of today's AIP plant types are ideal in all respect; each has its merits and drawbacks. Besides, none of the navies have similar conditions. Each navy performs its tasks, operates in different geographical zones, and has varied level of crew training and conditions at naval bases.
Most importantly, all navies have separate financial capacity. Hence, from a customer's point of view, availability of different AIP types is a boon as it allows him to select the most suitable solution.
Irrespective of all theoretical diversity of possible AIP types, the experience of recent years has shown that only two types of AIP systems are in demand in the market - Stirling AIP system and fuel cell AIP system.
As for the closed cycle steam turbine MESMA, it has shown its practicality but has remained a niche product. Other exotic types of AIP plants have also remained on paper or in laboratories.Stirling
The Stirling engine-based AIP system has become the first combat-ready system of new age. It is a relatively simple plant where diesel fuel (typical for the submarine) and liquid oxygen are used. Exhaust of the plant is discharged overboard rather easily at small and medium depths. Low power Stirling engines are much quieter than main diesel generators of submarine, which provides for considerable tactical gains.
Accordingly, the introduction of such AIP system gave obvious advantages with acceptable investment in the engineering of the system itself, training of crews and modification of shore infrastructure. It took less than 15 years for the creation of this system from a concept to the implementation in a combat submarine.
Although this system cannot be considered ideal with respect to stealth, everything seems to suggest that the Swedish Navy is quite satisfied with its parameters. In all appearances, it is related to the features of the Baltic Sea - its small area, shallow depths, complex hydrology as well as heavy traffic obviate the reduction of submarine acoustic signature to an absolute minimum. On the other hand, the nature of the Baltic requires the development of relatively small submarines, which agrees well with compactness of Stirling engines and their low aggregate power
. The financial capacity of the Swedish Navy does not allow it to seek expensive "absolute" solutions either.This supposition is confirmed by the fact that Singapore purchased Swedish submarines with Stirling engines. Conditions in the Strait of Malacca and adjacent water areas are quite similar to the Baltic ones.
The acquisition of license for Swedish AIP by Japan stands apart in some way. While the Sea of Japan, just as the Baltic, is an enclosed sea and has a heavy traffic, it is much deeper. And the Japanese submarines are larger than the Swedish ones; they operate not only in inland seas but in the ocean as well. Low power of Stirling engines forced the Japanese designers to go the extensive way to enhance capabilities of AIP system - the Japanese submarines have the AIP system with four engines, not with two as the case with more compact Swedish submarines.
As regards the level of industry development, it is difficult to suppose that Japanese companies would not cope with independent development and manufacture of a necessary AIP system and setup of a required infrastructure. The scale of combat ship/submarine construction in Japan does not allow us to consider the military budget of the country to be extremely limited. However, the Japan Maritime Self-Defence Force preferred licensing the existing system, developed for noticeably different conditions instead of developing the indigenous one. This example once again shows that a choice of AIP system is due to many reasons, some of them not being seen at first glance.Fuel Cells
The second type of AIP system - fuel cells - are firmly associated with German submarines of Class 212A and Class 214 though the works on various types of fuel cells are being conducted in other countries too, including Russia (alkaline fuel cells) and India (phosphoric-acid fuel cells).
Having passed a long way of theoretical and experimental investigations, German designers developed a submarine with a nearly "absolute" AIP system - low noise, low temperature, with ordinary water at the process output. These advantages were achieved at the expense of complexity and high cost of the system as well as considerable increase in submarine dimensions. The Class 212A submarines are three times larger than the previous submarines of the German Navy - Class 206. In addition, the fuel cell-based AIP system requires meticulous training of the crew and the setup of a dedicated infrastructure.
The implementation process of this AIP system turned out to be rather long. Twenty-five years have passed from the first activities on submarine AIP systems till the delivery of a combat submarine. Furthermore, the works of German designers were supported by the long-standing European efforts on bringing hydrogen energy into every field, most notably in automotive industry.
In addition to the development of the fuel cell as such - and complexity is inherent to this product - it was necessary to solve the problem of hydrogen storing. Hydrogen metal hydride storage used in submarines of Classes 212A and 214 makes it possible to achieve a high safety level, but requires considerable weights and volumes. By the way, the safety is confirmed by operational experience of these submarine
s. The efficiency-safety conflict, so common for submarine design, in this particular case is expressed in large weight of metal hydride alloy and low hydrogen content in it. The increase of hydrogen quantity leads to an unacceptable weight of storage system and accordingly to an unacceptable size of the submarine. In addition, there are doubts in the applicability of metal hydride storage system for the submarine operating in the areas with high seawater temperature. A peculiar feature of the metal hydride is to discharge hydrogen exactly when alloy temperature increases, i.e. under tropical conditions, the discharge may take place spontaneously.
The German Navy who conceived their submarines for operations in the Baltic and Northern Seas have not been considering these limitations as essential ones. Submerged endurance of Class 212А submarine is sufficient for these theatres due to their constraints. When the Italian Navy joined the 212A programme, the picture did not change much - realities of the Mediterranean Sea do not require a high submerged endurance. Besides, one should bear in mind that German and Italian submarines both in the Mediterranean Sea and near western and northern coasts of Europe operate in the areas controlled by allied surface and air forces. These submarines may be deployed to the operational areas in any mode. Accordingly, they may use diesels for battery charging the most time switching over to the fuel cell mode only in case of extreme necessity.
Development of the hydrogen infrastructure necessary for hydrogen generation, storage and transfer to the ship is an important part of AIP system engineering. This is related to both stringent requirements for hydrogen purity and its potential hazard. Since the mid-1970s, the activities in the field of hydrogen power including the works on hydrogen storage and transportation have been carried out by many European companies, first of all, by automotive ones. From the late 1990s, these works were actively supported and financed by the European Community. The joint efforts brought positive results. Relying on the achievements of commercial sector, the German industry has successfully coped with the establishment of infrastructure for the Navy.
The Class 214 submarines intended for export also have the AIP system consisting of fuel cells and metal hydride cylinders for hydrogen storage. Their submerged endurance also turned out to be sufficient for the countries that bought these submarines - Portugal, Greece and Turkey. Their navies operate in the same conditions, and purchase of submarines, made with the use of proven technologies, reduces financial and technical risks. It is interesting to note that, in a certain sense, export success of German version of AIP system "feeds itself". The European countries that buy the submarines with this AIP system rely, on the one hand, on the already engineered hydrogen infrastructure, and on the other hand, they expand it because they have joined the users of Class 214 submarine. Each next buyer see a more attractive picture.
In East Asia, the situation with the infrastructure is not so favourable for the time being. The Navy of South Korea, very likely, have selected the Class 214 submarine and a German version of AIP system due to the same peculiar features. Enclosed theatre, relatively short distances of patrol areas from own bases, availability of large own and allied forces - all that has a lot in common with the situation in the Mediterranean Sea.
Thus, the fuel cell-based AIP system and hydrogen storage in metal hydride cylinders has many advantages from the submarine point of view but does not make it possible to create a submarine with submerged endurance over two weeks and requires the availability of expensive hydrogen infrastructure. The deployment of such infrastructure should be supported by existing commercial networks and systems, otherwise it would take dozens of years for its implementation.Types of reforming
These problems made the designers look for new solutions of hydrogen storing. One of the approaches was to store hydrogen in the form of certain chemical compounds with their further disintegration and recovery of hydrogen (reforming). Nowadays the activities are underway in several directions. The most known of them is reforming of methanol, ethanol and diesel fuel. Activities are being carried out for the reforming of other compounds too, for example, sodium borohydride that is stored as water solution. Transfer to the submarine and storage of these liquids is much easier compared to hydrogen and does not affect its safety directly. The volume of hydrogen containing liquids can be very large.
Application of each of the above-mentioned substances has its pros and cons - here again, we cannot get an absolute answer because we have to examine the influence of reforming on the submarine as a whole, not only on the AIP system. During development of submarine reformer, it is necessary to solve among others the issues related to fuel storage and compensation, cooling of the plant, exhaust, etc. All these aspects in their turn affect the submarine's all-round stealth.
Methanol is the easiest to decompose. The least amount of oxygen is required for its decomposition. The produced hydrogen has the highest purity. The decomposition process generates the least amount of carbon dioxide. The volume of carbon dioxide is essential for the submarine AIP plant because the exhaust not only requires an additional system and affect the submarine stealth but also requires the compensation for the weight of discharged carbon dioxide. Unfortunately, drawbacks of methanol are also considerable. It is extremely toxic; tanks, pipelines and fittings intended for this alcohol should be thoroughly sealed and monitored, during both operation and loading the fuel. The methyl alcohol dissolves in water, therefore it cannot be stored and compensated for in the same way as the diesel fuel by taking water in the same tank where the fuel is contained. It is evident that a relevant shore infrastructure will be required. Dedicated procedures and technical means will be necessary for purchase and storage of this toxic alcohol as well as its transfer to the submarine.
Ethanol is close to methanol by its properties, however it demands a more complicated reformer; the process takes place at higher temperatures and produces a larger amount of carbon dioxide. Formally, ethanol is not poisonous, however as one experienced German submariner noticed, "Ethanol is not of a less threat for the crew than methanol". Ethanol storage and compensation are prone to the same problems as for methanol. Its employment also requires a dedicated infrastructure. One can say that the transition from methanol reforming to that of ethanol would enhance the crew safety at the expense of certain additional complexity and a rise in price of AIP plant and the entire submarine.
The obvious advantage of sodium borohydride reforming is that the process does not require oxygen and does not produce a gaseous exhaust.Diesel fuel reforming is attractive from operational point of view. Use and storage of diesel fuel has been mastered long ago, it is not expensive and quite safe; all naval bases of the world have an adequate infrastructure. In addition, only in case of the diesel fuel the submarine gets the possibility of storing only one type of fuel and use it for both diesel-generators, if any, and the AIP system. Hence, submarine operational cost reduces.
Even for diesel submarines the fuel costs during 30-year service life amount to a very high share of operating expenditures. Of course, these advantages do not come easy. The diesel fuel reforming demands the highest consumption of oxygen, it takes place at highest temperatures, produces the largest exhaust volume and the hydrogen generated requires thorough purification. In this respect, it is similar to a nuclear reactor - the nuclear-powered plant is rather complicated too. Many problems should be solved to develop it but it is a "single engine" and provides for the greatest capabilities for the submarine.
Activities on Reforming
German designers commenced the activities on reformers as early as the 1990s and investigated different types of such plants including the building of demonstrators. Based on these results, methanol was selected as fuel in striving to get a compact and relatively cheap system that could be applicable for submarines of Class 212A (2nd batch) planned at that time for construction as well as for export submarines of Class 214. Howaldtswerke-Deutsche Werft (HDW) has developed and tested a methanol reformer demonstrator and then a prototype of methanol reformer. The activities on support systems were performed, primarily for an exhaust gas discharge system. However, the Class 212А submarines have not received the reforming system due to two circumstances: size of the system did not allow its easy integration into the existing project, and the economic crisis forced the German Navy to abandon financing these activities.Nevertheless, HDW (now part of ThyssenKrupp Marine Systems) continues to work on the methanol reformer and actively offers a methanol reformer based AIP for export, first of all, within a power plant of the Class 216 submarines.
It is not the first time when financial crises have a significant effect on the fate of new equipment, and implementation of advanced developments depends on the ability to find a foreign customer.
Rather interesting and ambitious submarine project S-80 (Isaac Peral) of Spain is based on fuel cells and ethanol reformer. Spanish designers successfully created a worldwide cooperation and obtained quite encouraging first results including a low power demonstrator. The transition to a plant of rated power turned out to be more difficult. A number of problems in the submarine design itself and unavoidable difficulties faced during the transition from the demonstrators to actual equipment led to a failure to fulfil the schedule of project, which in its turn, created a problem with project financing. Whether this project will be fruitful, is yet to be seen.
Anyway, one interesting aspect can be mentioned right now. It seems that Spanish and Portuguese navies operate under rather similar conditions and have similar missions. However, the Portuguese Navy has preferred to buy the Class 214 submarines with fuel cell AIP technology and hydrogen storage in metal hydride cylinders, and the Spanish Navy went the way of developing the original submarine project including creation of ethanol reformer from scratch with all subsequent difficulties. This fact once again demonstrates what role in the AIP system choice play the factors, which are not seen to a casual observer but are obvious to a customer.In 2014, it became known about the works of French companies on the second-generation fuel cell system with a diesel fuel reformer. DCNS has developed and now is testing a demonstrator and to all appearances is obtaining positive results.
The AIP system with the diesel fuel reformer is offered for the Scorpene submarines. It is believed that these submarines would achieve the submerged endurance of three weeks and more with such a system.
A large submarine developed by DCNS, first known under the name of SMX Ocean and then as Shortfin Barracuda, deserves a separate mention. This project was selected by the Australian Navy as the basis for their SEA-1000 Programme. Very likely, the availability of export order will help DCNS to bring their AIP system to an operable condition
India develops an indigenous AIP system for the Kalvari-class submarines.
The activities on the AIP systems based on fuel cells and diesel fuel reforming are also underway in Russia. As early as the 1990s, the Rubin Design Bureau and the Russian Navy opted for fuel cells as the most suitable AIP technology. However, the development of the AIP system was slowed by both country's economic problems and lack of pressing need to have an AIP based submarine as the Russian Navy could take advantage of nuclear-powered submarines whose nuclear reactor can be considered an ideal AIP system.
Although, by the early 2000s the necessity of AIP system, including for the export, was recognised and funds were raised for its development.All that time the issue of hydrogen storage did not have a clear answer. After long-term analysis and selection of hydrogen storage method, the Rubin Design Bureau and the Navy came to the conclusion that namely the diesel fuel reforming is an optimal way.
That choice takes into account the political and geographical realities under which the Russian Navy operates since they differ considerably from the European ones. The Russian boats sail in open theatres at long distances from bases with most of the time being under threat of powerful and skilled adversaries and therefore are not able to use unstealthy modes, including during the deployment to operational areas. Accordingly, they require longer submerged endurance that can be provided only if the reforming system is available.
Disconnection of theatres (Northern and Pacific), necessity to rebuild not only one naval base, but a dozen of bases located at long distances from each other and many of them being in little-inhabited areas with severe climate - all that puts the cost reduction for construction and maintenance of the infrastructure as the first priority.All these considerations resulted in the commencement of activities on the demonstrator of low power diesel reformer in 2008. This unit was successfully tested; in 2010, it was additionally developed by adding a hydrogen purification system.
In 2013, the trials were completed. In 2012, the activities began on the demonstrator with a carbon dioxide treatment system. Its trials were successfully completed in 2015. Concurrently from 2012 to 2014, a high power prototype implemented in the size of a submarine compartment was developed and tested. Its trials were also completed successfully. Operation of these plants was demonstrated to high-ranking representatives of the Russian and other navies many times. Results of these efforts allow us to progress in the engineering of AIP systems based on fuel cells and diesel fuel reforming for Russian non-nuclear submarine of Lada type and a number of export submarines.
ConclusionsWhat conclusions can be drawn from the observations described above?
Firstly, the development of AIP system is a long and expensive process demanding a large scope of scientific and design work, engineering and testing a number of demonstrators. Success of the work is ensured not only by a high level of the science and industry but also availability of financing during the entire period of activities. Cooperation of efforts by several countries in the process of such work makes it much easier to get financing in time, and, hence, increases chances for a successful creation of AIP system itself and AIP-based submarine.
Secondly, choice of AIP type depends largely on Customer's requirements. Different requirements and different abilities of a navy lead to different design solutions at different costs. The decisive factors for AIP type selection will be the submarine submerged endurance and noise level as well as the possibility for the development of necessary infrastructure and payment of expenses for submarine operation during its entire life cycle. Difference in these requirements and abilities complicate the cooperative work of navies of various countries.
The Rubin Design Bureau continues the activities on the development of AIP system based on fuel cells and diesel fuel reforming and believes that this work would be interesting for the navies that have similar requirements for the submarine - long submerged endurance, necessity to operate independently from naval bases for a long time, low noise, easy and safe operation, low requirements for basing system.