Air-independent Propulsion Research Articles

Enhancing Submarine Propulsion: Hydrogen SI Engine With EGR for NOx Reduction Monitored by AI

AbstractThis research paper underscores the critical importance of advancing submarine technology by proposing a fully hydrogen‐powered submarine integrated with an air‐independent propulsion (AIP) system and artificial intelligence (AI)‐driven safety measures. Such an innovative approach not only addresses the limitations of traditional diesel‐electric submarines but also promises to revolutionize naval operations. The adoption of hydrogen as the primary power source significantly extends the submarine's operational range, reducing its dependence on fossil fuels and minimizing environmental impact. The incorporation of AIP technology enables sustained underwater endurance, reducing vulnerability and enhancing mission effectiveness. Concurrently, AI‐driven safety measures provide robust leak detection and safe hydrogen handling, ensuring the well‐being of the crew while optimizing operational efficiency. This integrated solution represents a transformative step toward achieving a more sustainable, capable, and stealthy submarine fleet for modern warfare scenarios.

Design and Simulation of a Fuel Cell-based Hybrid Underwater Vehicle Propulsion System in Matlab/Simulink

Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs) have many applications in underwater missions. A recurring issue is managing the power source of the vehicle, especially air-independent propulsion sources, such as batteries, accumulators, and fuel cells, to increase diving depth, underwater endurance, and range. The current article proposes the use of both fuel cells and batteries simultaneously. The subject of the study is Pluto Plus ROVs, which are mine sweeping and counter-terrorism underwater vehicles. The considered system is simulated in Matlab/Simulink. The result was a block diagram that simulates a hybrid propulsion system for submersibles in general and serves as the basis for improving the propulsion system for Pluto Plus ROVs.

Autothermal reforming of methanol for on-board hydrogen production in marine vehicles

The current systems for air independent propulsion systems are based on hydrogen stored in a limited number of cylinders for the continuous operation of the fuel cells in marine vehicles. Hydrogen generation by on-board steam reforming of methanol could extend the air-independent period. This study presents an autothermal reactor for the coupled endothermic (methanol steam reforming) and exothermic (oxidation) reactions in a compact reactor. A commercial Cu-based catalyst (ReforMax® M) was used. The kinetics of both reactions were evaluated over a wide range of conditions. This catalyst was found to be very stable under moderate operating conditions (<270 °C and O2/CH3OH <0.03), obtained by injecting oxygen at four different points along the reformer. Based on the catalytic performance, an integrated process was developed by a simulation program. The system was designed to operate for a long time (>500h). The combined process provides operational advantages, high energy efficiency (84.2%) and hydrogen yield (75.2%).

Liquid Cooling Flow Field Design and Thermal Analysis of Proton Exchange Membrane Fuel Cells for Space Applications

Proton exchange membrane (PEM) fuel cells are a promising technology with many features, including having a high energy density, efficiency, lightness, and producing energy directly instead of storing it. PEM fuel cells are currently used in mobile vehicles, military applications, portable systems, stationary systems, and air-independent propulsion systems for sea and space. Elimination of the cooling problems associated with PEM fuel cells in space applications is of great importance to produce more efficient systems. With this motivation in mind, this study examined PEM fuel cell elements and cooling flow channel design for space applications. The effect of the designed flow channel on the PEM fuel cell temperature distribution was investigated. Five different PEM fuel cell cooling channels were designed and modeled computationally, and the most suitable cooling channel design was selected based on the thermal analysis. While modeling the PEM fuel cell, space environment operating conditions, pure reactant supply (oxygen and hydrogen), electrochemical reaction in the membrane, physical properties of liquid coolant, physical properties of fuel cell elements, and heat transfer in the fuel cell were considered. The temperature distribution obtained as a result of the thermal analysis was examined, and it was seen that the PEM fuel cell cooling channel design was successful in maintaining the operating temperature of the PEM fuel cell. According to the data obtained, the cooler inlet temperature is approximately 40°C and the cooler outlet temperature is approximately 75°C in flat serpentine design. These values were obtained as a result of the analysis made with a mixture of 50% ethylene glycol and 50% water selected as the liquid coolant. As a result, a fuel cell can be formed by producing a fit bipolar plate with the analyzed PEM fuel cell cooling flow channel design, and current density and homogeneity can be determined by applying single-cell performance tests to the fuel cell.

Comparing the Effectiveness of Nuclear and Air-independent propulsion Submarine Fleets

This article proposes a methodology that enables one who has actual cost figures to perform the calculations for benefit-to-cost, strictly from the tactical and logistical standpoints, on the performance equivalence not of nuclear vs. air-independent-propulsion (AIP) submarines, but of nuclear v. AIP submarine fleets. With that in hand, it becomes possible for whomever might have sounder figures than what we could find about the complete life-cycle cost of either alternative to figure out, given our results about the benefit assessment, what would be the result of a benefit-to- cost analysis. We didn't perform the latter, due to uncertainty about the methodology by which the public available cost estimates were calculated. By providing a method for benefit calculation of the military worth of alternatives to support an assessment of the SSN/SSP fleet alternatives, we hope to have provided a sounder point of departure for debate, both in general terms, a method, and for the Brazilian case in particular, an application.

Early-Stage Analysis of Air Independent Propulsion Based on Fuel Cells for Small Submarines

This paper presents an analysis of the results of a hybrid air independent propulsion based on fuel cells for small submarines. The weight and volume of the investigated propulsion, and the major tactical and technical characteristics of the 540-tonne submarine equipped with this propulsion have been determined. Five onboard systems for the hydrogen storage or generation, and the oxygen storage are considered. Combining a micro-balloon hydrogen system with the cryogenic oxygen storage is the best solution for the propulsion. The submerged endurance and range improved 32.1-64.8 times, as compared to just battery. The indiscretion ratio equals zero at cruising speeds of less than 7.1 knots, while at higher speeds, this value has dramatically decreased, and submarine secrecy has increased significantly.

Comparative evaluation and selection of submarines with air-independent propulsion system

Conventional submarines produced to date can be more easily detected and exposed to attacks due to their visible, infrared, and radar signatures. Diesel-electric submarines need to come to a close surface at certain time intervals to charge their batteries and power their diesel generators with snorting systems. This situation causes submarines to face threats from land, sea, and air war elements. In order to reduce these threats, it is clear that there is a need for an innovative system in which the need for submarines to the surface is minimized and the timing of emergence is more flexible. To ensure this situation, air-independent propulsion systems have come to the fore from past to present and have gained more and more strategic importance. These air-independent systems significantly improve silent underwater time, and maneuverability and greatly contribute to maintaining the submarine's military stealth strategy. In this context, in this study, six different power system alternatives used as air-independent propulsion (AIP) submarine systems were evaluated, their advantages and disadvantages were compared, and alternatives were sorted by five experts in terms of nine important technical and economic criteria with the fuzzy VIKOR method. It has been seen that the propulsion power with the fuel cell system comes to the fore. The choice of a high power density fuel cell system for a submarine AIP system can allow for superior underwater range and durability and a minimal rate of instability and thermal signature than would be achievable from any heat engine. This will greatly expand the strategic advantage of AIP submarines. In this respect, this article can be a guide in understanding the critical technical features of submarines and in deciding between alternatives in submarine system selection.

Denizaltılarda Stirling Motor Temelli Havadan Bağımsız Tahrik Sistemlerinin (AIP) Değerlendirilmesi

Ülkelerin savunma güçlerinde özellikle donanma kuvvetleri önemli bir yere sahiptir. Donanma kuvvetlerinin mihenk taşı olan denizaltılar II. Dünya Savaşı’nda ne kadar önemli bir güç olduğunu etkili bir biçimde göstermiştir. O günden günümüze denizaltılarda yapılan teknolojik gelişmeler bir hayli hız kazanmıştır. Bahsi geçen gelişmelerin en başında ise kuşkusuz tahrik sistemlerinde yapılan gelişmeler gelmektedir. Özellikle AIP (Havadan Bağımsız Tahrik) sistemlerinin kullanımı ile hızlı bir gelişim gözlenmiştir. Klasik dizel-elektrik denizaltılardan kapalı çevrimlere yakıt hücresi teknolojisine sahip denizaltılardan Stirling temelli denizaltılara kadar birçok varyant uygulanmaktadır. İsveç ve Japonya gibi gelişmiş ülkelerin donanmalarında da Stirling motor kullanımının denizaltı uygulamaları göze çarpmaktadır. Stirling motorlarının Yakıt hücrelerine kıyasla daha düşük yakıt ikmal maliyetleri ve MESMA sistemine göre daha sessiz olması başlıca avantajları olmaktadır. Ayrıca içten yanmalı motorlara kıyasla egzoz gaz atımının daha kontrollü olması tercih edilmesinin başlıca sebepleri arasında yer almaktadır. Stirling motorlarının yakın geçmişte geliştirilmesi ve NASA’ya ait radyoizitop aracında temel güç kaynağı olarak tercih edilmesi, motorun oksijene gerek duymaksızın ne kadar verimli ve bakım gereksiniminin minimum seviyede olduğunun göstergesidir. Bu makalede Stirling motorunun günümüze kadar gelen süreçte nasıl geliştirildiği ve denizaltı uygulamalarında neden tercih edildiği, avantajları ve geleceği hakkında irdelemelerde bulunulacaktır.

Optimal design of a helical coil support for dewars in fuel cell applications.

Fuel cells are gaining popularity because of their efficient energy production without causing environmental pollution. Recently, DRDO has developed a fuel cell-based air-independent propulsion (AIP) system. In this system, the hydrogen is produced onboard while oxygen is carried in liquefied form (LOX) from the land in specially designed insulated storage vessels called dewars. Such vessels are needed because LOX has a low boiling point (NBP ~ 90K) and heat of vaporization (~ 213kJ/kg), due to which it boils off easily even when there is a small amount of heat inleak from the ambient. A typical dewar consists of two vessels separated by insulation. Support members are used to hold the two vessels together. Heat inleak through the supports and the insulation of the dewar causes the boiling of LOX. The vessels are subjected to dynamic loads during the voyage due to the filling and consumption of LOX. Therefore, the support system should be designed to withstand the dynamic loads experienced by the dewar. While the support system should have enough mechanical strength to withstand the loads it is subjected to, it should also restrict the heat inleak from the ambient to minimize the LOX boil-off. To meet this requirement, we need to optimize the support system design. Design optimization of support systems is especially critical in submarines to reduce the snorkeling frequency. Even though the dewars are available commercially for various applications, their design methodologies are not available in the open literature. Cylindrical rods are generally used as support members. In earlier studies, the authors have shown that helical coils give better thermal performance than tension rods as dewar supports. These two support systems involve different design criteria. It is important to evolve an optimal design of the support system to maximize the mechanical strength of the support while minimizing the heat inleak through the support. In this paper, we present a design methodology for optimizing helical support. We have proposed a modified optimization technique derived from the classical genetic algorithm (GA) for this purpose. The modification has been done by ensuring the design feasibility of the coil at each step of the algorithm. The proposed optimization technique has been tested on a LOX dewar, and an optimal design of the helical coil support has been obtained.

Revisiting the dewar design for liquid oxygen storage in fuel cell energy systems

Dewars are used to store liquid oxygen (LOX) in a fuel cell-driven Air Independent Propulsion (AIP) system for subsea vehicles. The dewar should have good storability to reduce snorkeling frequency. It is a double-walled vessel in which the two vessels are held together with a support system. The annular space contains the insulation. Heat flows from the ambient into the LOX through the insulation and support systems. Many types of dewar are available commercially; however, there exist no design guidelines in the open literature. Heat inleak is one of the primary concerns in the dewar design. For the first time, a comprehensive study is presented in this paper to highlight the relative importance of the support system and the insulation in dictating the heat inleak into the dewar. We determined the effects of heat inleak on the dewar design. The study revealed that the heat inleak through the support is considerably larger than that through the insulation. The results clearly emphasize the importance of the support system design in improving the storability of the LOX dewars in a fuel cell system.

Hydrogen energy systems for underwater applications

The most critical development in conventional underwater applications in recent years is to use hydrogen energy systems, including Air Independent Propulsion (AIP) systems. Proton Exchange Membrane (PEM) fuel cell-powered AIP systems increase interest worldwide. They offer many advantages such as longer endurance time without going to the surface for 2–3 weeks or without snorkeling with an average speed, perfectly silent operation, environmentally friendly process, high efficiency, and low thermal dissipation underwater. PEM fuel cells require a continuous source of hydrogen and oxygen as reactants to sustain a chemical reaction to produce electrical energy. Hydrogen storage is the critical challenge regarding the quality of supplied hydrogen, system weight, and volume. This paper reviewed hydrogen/oxygen storage preferences coupled with PEM Fuel Cell applications in the literature for unmanned underwater vehicles. Since underwater vehicles have different volume and weight requirements, no single hydrogen storage technique is the best for all underwater applications.

A Study on the Submarine Air Independent Propulsion System: Focused on Submarines Currently in Operation

Submarines are weapons systems that have been proven to be useful in battle since World War I and have continued to improve the efficiency of propulsion systems in order to be used efficiently on the battlefield. In particular, countries that unable to utilize nuclear propulsion systems make efforts to increase the efficiency of Air Independent Propulsion systems, and typical examples are fuel cells, Stirling engines and MESMA. It is also expected that the development of new propulsion systems such as hydrogen-reformer fuel cells, metal-air fuel cell and direct combustion propulsion systems will continue, so the characteristics of these will be examined and the performance based on the published data be checked in this thesis.

Analysis of phase change heat transfer with annulus 1 kW class methanol steam reforming reaction under high-pressure operation

A fuel cell air independent propulsion (AIP) system of underwater vehicle requires a hydrogen storage system. The methanol steam reforming system is a candidate of hydrogen storage which can produce hydrogen from chemical reaction. Different from reforming system for station fuel cell system, the methanol steam reformer (MSR) for underwater vehicle requires high-pressure operation.Since the longitudinal temperature uniformity is a core parameter of conversion efficiency of steam reforming system, this study is focused on computational analysis of phase change heat transfer through the annulus for methanol steam reforming reaction. The annulus MSR using phase change material was developed to improve the temperature uniformity. The simulation model is verified with safety and performance analysis code (SPACE). The performance parameters of MSR were flow arrangement, steam to carbon ratio (SCR), and gas hourly space velocity (GHSV). The results were analyzed in terms of the hydrogen yield, heat flux, liquid mass flow rate, and methanol conversion rate. The flow arrangement varied the methanol conversion rate to a minor extent of approximately 0.1% because wall temperature was maintained uniformly. In the case of SCR, the hydrogen yield at SCR 2.5 was 0.637 (dry basis), which was the highest yield rate. Also, if GHSV was increased, hydrogen yield decreased from 0.690 (dry basis) to 0.527 (dry basis). The heat transfer pattern was also analyzed and it was found that steam is interactively condensed along with the progress of the reforming reaction.

Fire Safety Risk Analysis of Conventional Submarines

Conventional (diesel-electric) submarines can provide improved stealth compared to nuclear submarines once submerged. This is because nuclear submarines are generally larger and are required to operate their nuclear reactors at all times, unlike diesel-electric submarines which are generally smaller and can run exclusively on batteries when submerged which generally requires fewer moving parts. These characteristics normally result in a smaller acoustic, thermal and magnetic signature which afford diesel-electric submarines greater stealth when submerged. However, the current underwater range and endurance is limited by the energy storage or generation for submerged operation. The application of emerging energy storage technology seeks to address this limitation and provide significant tactical and operational advantages to the conventional submarine operator. From a fire safety perspective, the potential addition of technologies such as rechargeable lithium-ion batteries, Air Independent Propulsion (AIP) systems and increasingly sophisticated electronic equipment dramatically changes the risk space in an already challenging and unforgiving underwater environment. This study reviews the functions, failure modes and maturity of emerging technologies that have serious submarine fire safety implications. A semi-quantitative assessment of the fire risk associated with potential large future conventional submarine design options for batteries and AIP is provided. This assessment concludes that lithium-ion batteries pose the greatest challenge with regard to integration into conventional submarines without compromising reliability or safety.

Conceptual Design Development of a Fuel-Reforming System for Fuel Cells in Underwater Vehicles

An air-independent propulsion system containing fuel cells is applied to improve the operational performance of underwater vehicles in an underwater environment. Fuel-reforming efficiently stores and supplies hydrogen required to operate fuel cells. In this study, the applicability of a fuel-reforming system using various fuels for underwater vehicles was analyzed by calculating the fuel and water consumptions, the amount of CO2 generated as a byproduct, and the amount of water required to dissolve the CO2 using aspen HYSYS (Aspen Technology, Inc., Bedford, MA, USA). In addition, the performance of the fuel-reforming system for methanol, which occupies the smallest volume in the system, was researched by analyzing performance indicators such as methanol conversion rate, hydrogen, yield and selectivity, and reforming efficiency under conditions at which pressure, temperature, steam-to-carbon ratio (SCR), and hydrogen separation efficiency vary. The highest reforming efficiency was 77.7–77.8% at 260 °C and 270 °C. At SCR 1.5, the reforming efficiency was the highest, which is 77.8%, and the CO2 generation amount was the lowest at 1.46 kmol/h. At high separation efficiency, the reforming efficiency increased due to the reduction of reactants, and a rate at which energy is consumed for endothermic reactions also decreased, resulting in a lower CO2 generation amount.

Selection of Optimal Air Independent Propulsion System using Forced Decision Matrix

A project management decision tool viz., forced decision matrix (FDM) is implemented in this paper towards identification of a suitable optimal air independent propulsion (AIP) system for submerged vehicles. FDM is utilised in order to handle the trade-off from amongst multiple propulsion technologies. FDM is based on multi-attribute utility theory used extensively in decision analysis situations involving persuasive multiple alternatives. The efficiency and effectiveness of this methodology to tackle complex solutions is elaborated in this paper with appropriate calculations. A rational decision-making procedure is evolved using the FDM in order to select the best suited AIP technology for a submerged vehicle. It is inferred that FDM is an effective and potential tool towards identification of best suitable solution in a multi-option environment.

The Performance of Low-Pressure Seawater as a CO2 Solvent in Underwater Air-Independent Propulsion Systems

Air-independent propulsion systems have improved the performance and decreased the vulnerability of underwater weapon systems. Reforming systems, however, generates large amounts of water and CO2. The recovery or separation of CO2, a residual gas component generated in vessels, entails considerable cost and energy consumption. It is necessary to understand the characteristics of the interaction between CO2 and seawater under the conditions experienced by underwater weapon systems to design and optimize a CO2 treatment process for dissolving CO2 in seawater. In this study, numerical analysis was conducted using the derived experimental concentration and MATLAB. The diffusion coefficient was derived as a function of temperature according to the CO2 dissolution time. Experiments on CO2 dissolution in seawater were conducted. The concentration of CO2 according to the reaction pressure and experimental temperature was obtained. The diffusion coefficient between CO2 and seawater was found to be 6.3 × 10−5 cm2/s at 25 °C and 7.24 × 10−5 cm2/s at 32 °C. CO2 concentration could be estimated accurately under vessel operating conditions using the derived CO2 diffusion coefficients. Optimal design of the residual gas treatment process will be possible using the derived seawater–CO2 diffusion coefficients under the actual operating conditions experienced by underwater weapon systems.

Waiting for Breakthrough in Conventional Submarine’s Prime Movers

Submarines, as a very expensive and sophisticated type of weaponry, are being intensively exploited by the armed forces of many countries. This means that submarines are sent ever longer patrols, sometimes to distant regions. To meet such requirements, submarine sub-systems and components must have high reliability and operational readiness indicators. Among the many machines and devices found on submarines, the ones that generate, store, and consume energy (mainly electricity) deserve special attention. The largest energy consumers on the submarine are the components of its propulsion system. One of the most complicated and loaded devices on board submarine is a power generator with a diesel engine driving it, on whose continuous and reliable work the safe performance of tasks depends. According to statistical research, despite its importance for the process of performing tasks by submarines, diesel engines are still the least reliable devices on submarines. Despite the constant technological development of piston engines, their work in very difficult conditions under heavy load and at high counter-pressure at the exhaust outlet promotes their malfunction and damage. From this point of view, the development of charge air systems for submarine diesel engines based on the construction experience of MTU Company is described in the paper. The classification of submarines, their propulsion systems, and the working conditions of engines on submarines are presented in the paper. Air-Independent Propulsion (AIP) systems with their applications on chosen submarines are also described. The most significant change in submarine propulsion system observed in 2018, transition from lead-acid to lithium-ion batteries, is also presented.

Pressurized diesel fuel processing using hydrogen peroxide for the fuel cell power unit in low-oxygen environments

A novel concept for diesel fuel processing utilizing H2O2 is suggested to obtain the high-purity H2 required for air-independent propulsion using polymer electrolyte membrane fuel cells for use in submarines and unmanned underwater vehicles. The core components include 1) a diesel-H2O2 autothermal reforming (ATR) reactor to produce H2-rich gas, 2) a water-gas shift (WGS) reactor to convert CO to H2, and 3) a H2 separation membrane to separate only high-purity H2. Diesel and H2O2 can easily be pressurized as they are liquids. The application of the H2 separation membrane without a compressor in the middle of the process is thus advantageous. In this paper, the characteristics of pressurized ATR and WGS reactions are investigated according to the operating conditions. In both reactors, the methanation reaction is enhanced as the pressure increases. Then, permeation experiments with a H2 separation membrane are performed while varying the temperature, pressure difference, and inlet gas composition. In particular, approximately 90% of the H2 is recovered when the steam-separated rear gas of the WGS reactor is used in the H2 separation membrane. Finally, based on the experimental results, design points are suggested for maximizing the efficiency of the diesel-H2O2 fuel processor.

Air Independent Propulsion; Silent Submarines with Stirling Engines

Air Independent Propulsion; Silent Submarines with Stirling Engines