AIP BEST system and future hybridization on S-80 submarines

Navantia argues that its bioethanol-based solution allows conventional submarines to operate longer, secretly and safer – even in hybrid configurations with lithium-ion batteries. Is there really a breakthrough or just a well-packed attempt to catch up with competition?

AIP BEST system and its relevance to S-80 submarines

This material provides a detailed analysis of the possibilities offered by the third generation bioethanol-based AIP system in technological, tactical and operational terms. We will also look at its potential role in hybrid gym configurations using lithium-ion batteries and whether and to what extent S-80 units can respond to needs in the Polish Navy In the future.

I encourage all interested – to independently assess the value of this offer in the light of published data. Do Spaniards really have a competitive proposition? Do you AIP BEST Is it groundbreaking technology, or is it just an attractively wrapped compromise?

The importance of conventional submarines is widely recognised. To the point that even countries that do not have conventional submarines, such as the major powers of Western Europe, ultimately rely on their allies. Their units supplement escorts for their atomic counterparts in some underwater operations that do not require long-term deployment but require maximum discretion.

These include intelligence operations for shore objects, docks, transition bases or task forces manoeuvres at periscope depth. The reduced patrol speed is therefore necessary, and the mere penetration through the surface of the water, in order to obtain intelligence in the electromagnetic spectrum, becomes an important source of indiscretion for this type of platform.

The required level of cover is achieved through a platform capable of operating for weeks in open confrontations, using heavy torpedoes and supersonic anti-ship missiles. These prolonged confrontations are more effective compared to the first impact of the water task force standing off the enemy's coast, openly exposing both its air defense to a new anti-aircraft combat demonstrated during the war in Ukraine and its own platform, which currently cannot face the attack due to the saturation of new supersonic and hypersonic maneuvering effects that immobilize its launch deck.

In addition to intelligence operations, the vital element is the time spent in immersion after the batteries are charged on a conventional submarine. Its ability to operate without detection depends directly on the battery capacity, as well as the diesel power of alternators (DAR).

Historically, conventional submarines used lead-acid technology in their batteries. Their exploitation is widely known, requires a number of auxiliary systems and safe and effective management of hydrogen emitted in battery pits. A lower charging regime generates a higher battery capacity, about 50 percent more than a higher regime, which is directly related to submarine speed in various scenarios. In addition, it should be noted that the load intensity cannot be maintained at a high level at all times.

When the set voltage is reached, hydrogen gets out of the batteries, which requires a reduction in the charging intensity, wasting DAR energy at the moment when submarine is exposed to detection during snoring. However, one of the advantages of such batteries is that the capacity remains unchanged, so achieving a deep discharge at high speed does not mean that the battery has the same discharge state at low speed, and in fact it is larger.

Anaerobic systems as a solution to improve detection

The aim of designers is to develop solutions that will extend the time during which submarines will remain undetectable. Research on anaerobic power generators based on hydrogen peroxide has been documented since World War II. Since then, different concepts have been developed to achieve more efficient and safe solutions within the projects we call the first generation.

Initially, they were based on the use of thermal machines such as closed cycle diesel engine systems (Stirling engines), an external combustion piston system or systems based on external combustion steam turbines. Stirling's engine has been quite successful in its installation on 20 submarines and in 25 submergable floating units, which China has estimated. However, the turbine based system was not so lucky – probably due to dubious results – and was only installed on four submarines of Pakistan Navy.

The use of fuel cell technology with metal hydride storage in the 1990s was a qualitative leap in the development of anaerobic systems to such an extent that it is still successfully sold and installed on over 50 submarines with a displacement of about 2000 tonnes. This technology, which we call second-generation technology, avoids the use of spinning elements as the basis for energy generation, present in the previous generation, as well as the need to drain exhaust gas outside.

On the other hand, storage capacity allows several days of work, and the integration of bulky and heavy cylinders with hydrogen limits the design capabilities of the submarine, which makes it unsuitable for ocean submarines of the SSK type. For this reason, the advantages of this system on submarines with a buoyancy of more than 3000 tonnes are much lower than the solution based on the integration of additional battery chambers using lithium-ion technology. This is the attitude taken recently by Japanese Polish Navy in the latest Sōryū submarines.

S-80 submarine is a vessel with a displacement of 3000 tonnes, the size of which allows the transport of a large load of weapons and sensors, as well as a high durability and high standard of living conditions. Such options would not be feasible if the AIP was used with hydrogen stored in large quantities of metal bottles. Having such a large number of cylinders that would be needed to achieve the same efficiency as the Navantia BEST AIP system would undermine the balance of mass and buoyancy which would entail a drastic increase in the rigid hull volume.

Best AIP distinguishing factor for submarines S-80

Ambitions of new conventional submarine construction programs require a new qualitative technological leap in order to obtain larger underwater platforms, which exponentially reduces the likelihood of detection. This means that the requirements for range and duration of low-speed immersion will now be measured in weeks rather than in days, which poses a challenge in providing more energy on board without prejudice to the design or other capabilities of the platform.

Addressing this problem can be successfully achieved with third generation AIP systems, state-of-the-art AIP systems such as BEST (Bio Ethanol Stealth Technology) developed by Navantia for S-80 class submarines. The Navantia BEST AIP system produces hydrogen on demand from liquid fuel (bioethanol) which is stored in conventional construction tanks at ambient temperature and pressure. This solution is more efficient, safer and cheaper to maintain than hydrogen stored in metal bottles. The AIP BEST system produces the appropriate battery charging power and allows the submarine to patrol at maximum undetectability.

From a safety point of view, risks arising from the use of flammable substances are in many cases equivalent to those associated with battery-based systems, as well as the storage of metal hydrides, since the management of hydrogen and pure oxygen on board is similar. As in first generation systems, there is a need to drain the exhaust gas outside the hull of the ship, so the footprint of such a submarine operating in the AIP mode does not differ from the trace of a conventional submarine moving exclusively on an electric drive.

^{Fig. Navantia}

BEST system Spanish Navantii address these problems by using conventional systems to detect and eliminate potential risks from their operation and by ensuring the level of undetectability by draining a marine water stream with a crystalline exhaust gas solution which is not detectable in offshore waters. In addition, the use of bioethanol instead of fossil fuel eliminates the potential risk of chemical trace detection using chemical sensors.

Integrity versus detection: analysis of the effectiveness of SSK and SSN platforms in ASW activities

Apart from the mathematical details of the statistical model, the quantitative results for 3000 simulated cases are presented in the Montecarlo chart, in a scenario with only one edge where the search area does not exceed 1,000,000 km2 (a 1000 km long coast strip and 100 km wide), the stay in the immersion lasted 300 hours when SSK conducted reconnaissance activities with two raised masts.

During this exercise we obtained results that require attention. Ultimately, there is a 50% probability that we were detected by the opponent. A hypothetical scenario, an ideal search with the exact overlap of search lanes, but using previous experience, can be assessed realistic distortions equivalent to excessive sweeps and introduced into the search within 300 hours for over 78 percent of the patrol area.

This approach, which reduces the efficiency of ASW, assumes an increase in masking capacity of up to 70%, but is still unsatisfactory due to assumed tactical costs. The remaining 30% probability of detection is associated with the continuous presence of SAR interference in the trail to maintain 300 hours of detection without interruption. It should be stressedthat the interruption of this process leads to a decrease in masking values to levels relevant to operations.

Therefore, it was assessed that the campaign of safe communication combined with the automatic collection of intelligence data could take place in cycles "4/4", i.e. 4 useful minutes (except for manoeuvres) every 4 hours, devoted in part to speeding up manoeuvres and speed with acid-lead batteries ( twice as much in the lithium battery, where the advantage here would increase). The same calculation now indicates that the probability of the total lack of detection by the opponent increases to over 90%. This value indicates operationally the importance of this type of platform.

It should be noted that by repeating this type of calculation for SSN, in the most popular Western standard, the same model generates the probability for the same condition slightly below 50%, essentially because the same time and area have been set. Since in the event of an increase in the patrol area and thus a reduction in the time of immersion provided by the nuclear power plant, the difference between ‘capacity for actual detection’ and the noise emitted in the SSN broadband band becomes 6 to 12 dB times greater at minimum continuous speeds.

New horizons, lithium-ion batteries

Work is currently underway to develop a system of lithium-ion batteries for submarines. It represents a significant improvement in relation to lead-acid batteries, allowing charging with very high intensity and significantly reducing the number of auxiliary systems. From an operational point of view, the lithium battery system allows diving with more power, which translates into improved IR. A longer time to stay under the water surface using the total power of auxiliary devices means more immersion time because the battery is more charged, and a significant improvement in the time during which the submarine can move at high speed.

However, the development of lithium battery systems for submarine applications poses technical challenges, especially in terms of safety.

New generation hybrid propulsion on S-80 ships

Finally, the hybridization of 3rd generation AIP technology with technology lithium-ion accumulators allows fast charging with a diesel generator system and greater durability at maximum speed, where increasing speed is of high tactical importance. This approach will increase the capabilities of the next generation submarines, providing better performance and tactical advantage.

The combination of the AIP system and the Lithium Battery System (LIB) significantly prolongs the submarine immersion time thanks to:

• Third generation AIP system performance.
• Fast charging while swimming with a periscope.
• The ability to maintain the submarine at maximum speed for more hours.

The advantages of replacing AIP with more LIB modules were assessed. First, it should be noted that this configuration involves less complexity of the structure, as it does not require the installation and maintenance of a chemical installation inside a submarine, but ensures a shorter stay time in immersion than a submarine equipped with an AIP system.

On the basis of the above considerations, the following conclusions can be drawn:

1. Conventional submarines can achieve maximum discretion at reduced speed during patrol, distinguishing themselves in intelligence operations at periscope depth compared to atomic submarines.
2. A submarine capable of maintaining maximum discretion for many weeks represents a new advantage for SSK submarines: it prevents the opponent from controlling the waters with surface forces. Lead-acid accumulators are well-known technology, but the very application of the exhaust-electric configuration has an impact on the immersion time and therefore on the discretion rate.
3. Anaerobic systems of the first or second generation were presented as a solution increasing the duration of medium submarines during immersion; however, their storage capacity limits the durability to a few days, and the integration of bulky and heavy metal hydrogen cylinders of second generation systems makes this design unprofitable for the conventional submarine market.
4. Third generation AIP systems, such as Navantia's BEST, are the most optimal design solution for ocean submarines.
5. With a secure communication campaign combined with automatic intelligence collection in 4/4 cycles on a submarine such as the S-80, equipped with the state-of-the-art AIP system, the probability of the opponent's total lack of detection increases to over 90%, indicating an operational advantage of this type of platform (SSK type) over a nuclear submarine (SSN type) which would be less than 50%.
6. Lithium-ion battery technology has significant advantages compared to lead-acid batteries, such as speed and load utilisation; however, there are safety aspects which cannot be overlooked at the design stage and which may pose a technical challenge. However, it seems obvious that the combination of AIP third-generation technology with lithium-ion battery technology will allow to exploit the advantages of fast charging time based on a diesel generator system and to extend the duration of the p-submarine position at maximum energy load, where operation at maximum speed is of high tactical importance.

To conclude, I would like to point out that I am leaving the reader space for his own assessment, both in terms of the technological solutions presented and the overall value of the Spanish offer. Can the S-80 with the AIP BEST system really be competitive with the other proposals in the Orka programme? I leave the answer to this question to those concerned and to the decision-makers responsible for selecting operational capacity in the Polish Navy In the coming future.

Source: spsnavalforces.com/Adam Woznicki