Design studies into the potential of novel submarine power plant configurations
Diesel-electric submarines, also known as conventional submarines, have a non-nuclear power plant that consists of two or more diesel-generators and large lead-acid battery packs. When the submarine is sailing on the surface or on snorting depth, the diesel-generators are used to power the submarine and to charge the submarine’s batteries. When the submarine submerges, the generators are switched off due to the absence of air supply and the batteries are used to power the submarine. Diesel-electric power plants are already used in submarines since the beginning of the 20th century and still used by most navy’s worldwide. With the emerging of new battery and fuel cell technologies, the power plants of non-nuclear submarines are slowly changing and might change radically in the nearby future.
In the recent years, Nevesbu and the TU Delft have developed, through a number of MSc graduation studies, a Mean Value First Principle submarine power plant model. This model is used to perform multiple design studies to investigate the potential of these new technologies for the submarine domain. This article will give an overview of these studies and highlight the potential of new technologies for non-nuclear submarine designs.
Overview of power plant options
With the new emerging technologies, the amount of power plant options for non-nuclear submarines is increasing, especially for the submerged power supply. Fuel cell technology enables an air independent power supply, operating on pure hydrogen and pure oxygen. Lithium-ion batteries can be used as an alternative for the lead-acid batteries. In figure 1, all power plant components considered in this article are shown in an energy flow diagram.
Figure 1: Energy Flow Diagram as representation of the three innovative power plant concepts considered in this article;. ES = Energy Source, M = Mechanical Power, E = Electrical Power.
Both lithium-ion batteries and fuel cells increase the submerged energy storage capacity, enabling submarines to sail submerged for longer periods of time. This is considered a large operational advantage for submarines. Both technologies are also already applied in actual operational submarines. For example, the German Type 212A submarines use a fuel cell system for air independent power supply and Japanse Taigei class submarines have lithium-ion batteries installed.
Mean Value First Principle submarine power plant model
The impact of a selected power plant on the overall submarine design is significant. Thus, selecting the right components of the power plant at an early design stage is key to a successful design. With the increasing amount of powerplant options, this becomes more difficult and time consuming. For this reason Nevesbu developed a Mean Value First Principle power plant model in cooperation with TU Delft MSc graduation students. The model input is a time-based mission profile, stating the propulsion and auxiliary power demand for different phases of the mission. Based on this input, the power plant components are sized and their efficiencies are calculated based on first principle. In figure 2, the overview of the Mean Value First Principle submarine power plant model is shown.
Mean Value First Principle submarine power plant model Power plant sizing & optimization tool
Power plant concept comparison
With the use of the Mean Value First Principle submarine power plant model, multiple powerplant configurations can be compared with each other based on required mass and volume. Both mass and volume are critical design parameters for a submarine. Therefore, the power plant configuration with the lowest mass and volume is preferable. The mission profile shown in Figure 3 is used for the comparison in this article. The submerged surveillance part is varied from 10 hours to 3 weeks. When a fuel cell is applied in the analysed power plant concept, the fuel cell will be used for power supply during the submerged surveillance part, where the battery is used as an submerged power extender for the higher submerged speeds.
Parameterised mission profile of a submarine; the surveillance part is parameterised with different endurances for surveillance operations and two different surveillance speeds (3 and 5 knots)
Figure 4 shows the required volume of the eight different power plant configuration required to perform the mission of Figure 3 with a submerged surveillance speed of 5 knots. In this figure, it is clearly visible that the volume requirements are increasing for an increased submerged surveillance (I-5 is 10 hours vs V-5 is the 3 weeks duration). Furthermore, it is visible that at a longer submerged endurances the use of fuel cells (PEMFC) becomes beneficial since it requires less volume than only using batteries. Also visible in Figure 4, is that the use of lead-acid batteries (LAB) will always require the largest amount of volume (therefore it is not even considered anymore for cases IV-5 and V-5). A similar trend is found for the weight requirements of these power plant concepts. This clearly highlights the benefits of the new technologies.
Figure 4: Required volume for eight different power plant concepts per main component including hydrogen and liquid oxygen storage (if applicable) for 5 knots surveillance speed
Design studies
The power plant concept comparison clearly highlights the benefit of the use of lithium-ion batteries and fuel cells in submarine designs. This raised the question if it might be feasible to eliminate the diesel-generators for the submarine design completely. This would simplify the submarine’s design drastically, since the amount of systems on board will be reduced significantly. Furthermore, it will create more space and weight budget for the installation of lithium-ion batteries and/or fuel cells. Two design and feasibility studies were performed to investigate the feasibility of such a design, from both a technical and an operational perspective.
These studies have taken the same design approach with an existing, detailed and well documented diesel-electric submarine design as starting point. This submarine, with a conventional diesel-electric propulsion plant, a submerged displacement of 1900 tons and a crew size of 34 crew members, has been redesigned into battery-electric concept using lithium-ion batteries (Figure 5) and hybrid-electric submarine concept using PEM fuel cells and lithium-ion batteries (Figure 6). During this re-design process, the submarine’s submerged displacement, pressure hull diameter and design requirements (e.g. top speed, payload, environmental conditions, amount of accommodation) are kept constant to enable a fair comparison between the operational capabilities of the three designs. During the re-design process the technical feasibility of the concepts is determined as well as possible in early-stage design.
Figure 5: Battery-electric (totally battery powered) submarine concept design. Li-ion batteries are shown in red in the bottom compartments.
Figure 6: Hybrid-electric (fuel cell and battery powered) submarine concept design. Hydrogen tanks are shown in green; LOx tank in light-blue and Li-ion batteries in red are shown in bottom compartments
The battery technology used in this study is a lithium-ion battery cell of nickel magnesium cobalt chemistry, which has a specific energy and an energy density of 261 Wh/kg and 505 Wh/l. A packing factor of 1.3 for weight and 1.6 for volume are used for the packing of the cells into modules. Batteries models are placed in separate compartments. As battery safety system, a direct foam injection system is considered to ensure potential battery fires can be suppressed effectively in an early stage limiting the risks of thermal runaway and thermal runaway propagation in the battery packs. For hydrogen storage, high pressure bottles are used with a volumetric storage capacity of 35 gram per litre. This is currently the technology of choice in the automotive industry. For safety reasons, the high pressure bottles are located outside of the submarine pressure hull. Furthermore, the fuel cells themselves are located in an separate airtight compartment. Both aspects ensure the safe integration of lithium batteries and fuel cells onboard of these designs.
To determine the operational feasibility of the two concepts of figure 5 and figure 6, the maximum range and endurance of both concepts are determined and compared with the conventional diesel-electric reference design. The Mean Value First Principle model is used as verification tool for this purpose, based on the power plant design and energy storage capacity of the presented concepts. The submerged range and endurance of both the reference design and concept designs are shown for different speeds in Figure 7. The submerged range and endurance of both concepts is significant compared to the conventional reference design. The battery-electric concept has a maximum submerged range of 1950 nm and submerged endurance of 24 days. The hybrid-electric design has even a higher submerged range and endurance; 2900 nm and 42 days. However, this is at the cost of a more complex design compared to the battery-electric design. In Figure 7, the fuel cell power limit is clearly visible by the drop in range and endurance of the hybrid-electric design.
Figure 7: Submerged range and endurance of the diesel-electric reference design, the battery-electric concept and the hybrid-electric concept
Although the submerged range and endurance of the conventional diesel-electric submarine is significantly less, it still has its re-charging capacity of the diesel-generators and the high energy storage capacity of marine diesel oil. Therefore, the total range of the conventional diesel-electric submarine is still significantly higher than the all-electric and hybrid-electric concepts. This is clearly shown in Figure 8. The total range of the conventional submarine is still more than four times as high as the hybrid-electric and battery-electric concepts. This does not mean that the two concepts are not feasible from an operational perspective. A range of more than 2000 nm and endurance of more than 24 days is expecting to make local to medium range missions feasible, for which long transits to the mission area are not required. To verify this, again the time-domain models are used as designer support tool for verification. Multiple missions are simulated to verify that sufficient battery capacity and/or hydrogen is left after performing a certain mission. The results clearly showed that missions with duration of a maximum of three weeks are feasible for these designs. Based on these results, the conclusion can be made that local to medium range mission are feasible.
Figure 8: Total range and endurance of the conventional diesel-electric reference design, the battery-electric concept and the hybrid-electric concept.
Both the battery-electric concepts and the hybrid-electric concepts are considered to have several advantages compared to the conventional diesel-electric submarine design. Firstly, a significant reduction in systems can be achieved when omitting diesel-generators from the design, since also all diesel-generator support systems can be omitted (e.g. cooling systems, fuel oil and fuel oil compensation systems, air intake system, exhaust gas system, etc.). The reduction in systems will reduce the design complexity and maintenance requirements and will improve the reliability and availability of the submarines. The underlying assumption is that the solid-state technology of fuel cells and Li-ion batteries will require (significantly) less maintenance than the heavily loaded rotating components of an internal combustion engine in conventional submarines. Furthermore, the crew will have less systems to operate, monitor and maintain during operation. This may lead to a crew size reduction as well. A performed manning analysis showed that the crew size of the presented concepts can be reduced from 34 to 23 persons, which will have a positive effect on the range and endurance of the presented concepts which is currently not yet considered.
One of the biggest advantages of both the battery-electric concept and the hybrid-electric concept is their covertness. Both designs have air independent power plant designs, meaning that they have an indiscretion ratio of zero in the operational theatre. This advantage is visualized in Figure 9, where a round trip of the hybrid-electric concept is compared to a round trip with the reference conventional submarine design. Each red block in the voyage is where the reference design needs to sail at snorting depth to charge the batteries. During this period the submarine is vulnerable, since it can be spotted visually and with radars. Furthermore, it experiences a significant increase in noise- and heat signatures. The hybrid-electric concept can sail this complete trip submerged.
Figure 9: Example of a fully submerged feasible mission with a hybrid-electric submarine design. Snorting moments of the reference design are indicated in red.
Future outlook
Both fuel cells and batteries are considered the solution to achieve emission free transport in multiple civil industries, with the automotive industry as clearest example. Therefore, significant amount of research is performed on topics of high capacity batteries, fuel cells and hydrogen storage. It is therefore expected that performance of both the a battery-electric as hybrid-electric submarine design will improve in the nearby future.
It is difficult to assess how soon and how big the technical developments will be. A rough estimation is made, based on multiple public sources and publications, to assess the impact of the expected technical improvements on both the operational capabilities of the battery-electric as hybrid-electric concept. This estimation is shown in 10 and Figure 11.
For battery powered submarines, such as the battery-electric concept, improvements in battery technology will directly lead to improved operational capabilities. If the most positive prospects will become reality, all electric (battery powered) submarines will be able to reach ranges up to 7000 nautical miles. Totally battery powered submarines will be a very realistic design option when these prospects become only partly reality.
Figure 10 (left): Potential of expected improvement of battery capacity on the maximum range of battery electric submarine concept. And Figure 11 (right): Potential of expected improvement of hydrogen storage density on the maximum range of the hybrid-electric submarine concept.
For fuel-cell powered submarines, the prospects are currently slightly lower; up to 5500 nautical miles. There is one important factor to take into account when looking at the prospects of fuel-cell power submarine; the required oxygen storage capacity. An increase in hydrogen storage capacity will require an increase in oxygen storage and oxygen compensation capacity as well. In the hybrid-electric concept, the required oxygen storage and compensation capacity are already limiting design factors. Furthermore, improvement of oxygen storage is not a research topic of civil industries. Therefore, no significant improvements in oxygen storage efficiency are expected. Oxygen storage is therefore expected to be the limiting factor in the development of fuel cell powered submarines.
Submarine power plant selection
With the expected development in technology, the feasibility and capabilities of alternative power plant solutions for submarines are increasing. The importance of a well-considered power plant choice will therefore continue to increase in the nearby future. The choice for a propulsion plant solution should be based on a good trade-off of all technical and operational aspects of the power plant. A power plant model, such as the mean value power plant model presented in this article will enable such a trade-off studies in early design phases.
The most important input for the power plant selection is a clear concept of operations. For example, a diesel-electric submarine (with AIP) is a logical choice when an expeditionary submarine is required by a navy. However, a battery powered or fuel cell/battery powered submarine can provide multiple advantages when a submarine is required for coastal defence and local to medium range missions.