Nuclear-powered submarines’ “infinite” source of energy provides them with underwater endurance, speed, range, and stealth that are clearly superior to those of conventional submarines. Some types of missions can be accomplished only by nuclear boats. For coastal defense and littoral combat, however, a different approach could be more efficient. Developments in conventional submarine propulsion, namely, air-independent propulsion (AIP) systems and lithium-ion batteries, could be a game changer, and navies that operate solely nuclear-powered submarines might reconsider including advanced conventional submarines in their fleets.
AIP SYSTEMS
Air-independent propulsion systems provide extra underwater endurance to diesel-powered submarines by generating electricity from a source of energy that does not require external air from the surface. This avoids the submarine having to snorkel to charge its diesel engines’ batteries, a very indiscreet operation that compromises stealth and leaves the submarine temporarily vulnerable.
Currently, there are three main types of AIP technology: steam turbine, Stirling engine, and fuel cell. In the 1970s, the French developed the MESMA (a steam turbine driven by heat from the reaction of ethanol and oxygen) for the Agosta 90B and Scorpène-class submarines. This approach has proved reliable and currently is in service in submarines operated by the Pakistan Navy.
In the 1990s, the Swedish shipyard Saab-Kockums designed an AIP system based on a Stirling engine, which subsequently was installed in the Gotland-class submarines of the Swedish Navy. This system uses liquid oxygen and diesel fuel as its energy source. Later, Saab-Kockums licensed this technology to Kawasaki Heavy Industries, which implemented it in the first ten submarines of the Japan Maritime Self-Defense Force’s Sōryū class. The Chinese Yuan-class submarine also incorporates a Stirling engine AIP system inspired by the Swedish model. The capability of this AIP variant was made evident during joint naval exercises in 2005, when a Gotland-class submarine performed very well against U.S. Navy submarines and surface ships.
The third AIP option is the fuel cell. Fuel cells take an oxidant and a fuel (typically oxygen and hydrogen) and produce electricity directly. This approach was first tested by the German Navy in 1989, with good results. The German and Italian navies’ Type 212 class is equipped with a fuel cell. An export version of this submarine, the Type 214, was acquired by Portugal, Greece, and South Korea. The fuel cell AIP has the inherent danger of having to store oxygen and hydrogen. Oxygen can be handled with relative safety in liquid form. In Type 212/214 submarines, hydrogen is stored in metal hydride accumulators, but they are difficult to refuel. A variation of this strategy, used by the Spanish S-80 class, obtains hydrogen from a hydrocarbon (ethanol) in a chemical process carried out in a reformer.1
These AIP technologies have enhanced the capabilities of conventional submarines by increasing their submerged time. Exact figures on how long they can remain underwater are not easily available, but it is estimated to be around two weeks under low-speed patrolling.
LITHIUM-ION BATTERIES
In 1991, Sony marketed the first lithium-ion battery. Lithium-ion batteries are the most widely used type for consumer electronics, but they also are found in electric vehicles, drones, planes, and satellites or supporting renewable energy sources. They offer several benefits over other battery chemistries, including:
- Increased stored energy per unit of weight and volume
- Less degradation and longer cycle life (~3 times longer than lead-acid)
- Reduced charging times (~2–4 times faster than lead-acid)
- Low level of self-discharge (3 percent per month versus 10 percent for lead-acid)
- Low maintenance and sealed battery cells
- Higher mean voltage per element (3.5V versus 2V in lead-acid)
- Greater useful capacity compared with lead-acid batteries, as the latter should not be discharged below 40 percent (to avoid sulfation) and require special loads periodically to recover 100 percent capacity
The main drawback of lithium-ion batteries is the risk of fire or explosion. A fire can have various causes, but the main risk is thermal. The optimal operating temperature for lithium-ion batteries is between 59ºF and 95ºF. An unexpected impact, a poor manufacturing process, an external-internal short circuit, or overly hot surroundings can cause the battery temperature to rise. Above a certain temperature, typically around 212ºF (this varies with electrode materials), some components of the battery become unstable and start to react, releasing extra heat. This contributes to further temperature rise, in a chain reaction known as thermal runaway. The resulting uncontrolled temperature rise eventually causes a fire or an explosion, which propagates to the adjacent battery cells or surrounding hardware. This is the main reason lithium-ion batteries are not fully implemented in military submarines.
The choice of the cathode material has possibly the greatest impact on the thermal stability of the battery—lithium iron phosphate (LiFePO4) has better thermal behavior than other lithium cathodes but lower energy storage capacity. In addition, a reliable battery surveillance, management, and fire-extinguishing system can minimize the risk of fire/explosion.
One of the last industries to adopt lithium-ion batteries has been submarine construction. In October 2018, the Ōryū became the world’s first lithium-powered military submarine. The 11th boat of the Sōryū-class, a family of Japanese submarines that previously mounted Stirling AIP, the Ōryū is 276 feet in length and about 4,000 tons in displacement (submerged). She was followed in November 2019 by the Tōryū, also equipped with lithium-ion batteries.
Other countries are following suit. South Korea has plans to use lithium-ion batteries in its conventional submarines. While the Republic of Korea Navy does not yet operate any submarine with this technology, the Korean Defence Acquisition Program Administration has announced the expected entry in service of lithium-powered submarines in the next decade. The project will be based on the Dosan Ahn Changho-class submarine, which is similar in size to the Sōryū class. The French Naval Group also is interested in incorporating lithium-ion batteries in its conventional submarines, and the German ThyssenKrupp Marine Systems group is considering lithium-ion batteries for its next-generation submarines.
Closing the Gap
Recent advances in AIP systems together with the adoption of lithium-ion batteries have contributed to greatly increased capabilities in diesel-powered submarines. Although nuclear-powered boats will always outperform conventional ones in range and submerged endurance, the gap is narrowing. When the mission is coastal defense, littoral combat, or operating nearby a forward base rather than long-range power projection, a large fleet of highly capable conventional submarines could be both more economical and tactically efficient than a smaller nuclear fleet.
Spain studying lithium-ion for Isaac Peral
The Spanish Navy is close to launching the first S-80-class submarine, the Isaac Peral (S-81). The S-80 class is a next-generation conventional submarine with ground-attack capability, reduced magnetic and acoustic signatures, an advanced combat system, a high level of automation, and the ability to insert special operations forces. At 262 feet long with a submerged displacement of 3,000 tons, it is similar in size to the Japanese Soryu class. Its power plant consists of three diesel engines used to charge two groups of lead-acid battery cells. The S80-class incorporates a novel air-independent propulsion (AIP) system, which generates electrical power from a fuel cell. The oxygen required by the fuel cellis stored in a tank in liquid form, while the hydrogen, much more volatile and dangerous, is obtained in a reformer via chemical reactions that use ethanol as reactant.1 A separate tank is used to store ethanol. The AIP system is expected to provide 15 days submerged.
Both the Spanish Navy and Navantia, the shipyard constructing the Isaac Peral, are considering replacing the lead-acid batteries with lithium-ion cells. Because lithium batteries have larger energy to weight and volume ratios, they are expected to be accommodated in the current battery chamber with minor modifications. This change, in combination with the existing AIP, would boost the performance of the submarine. Currently, a research-and-development program has been launched to study the implementation of lithium batteries in the S80-class.
1. M. Raska, “Diesel-Electric Submarine Modernization in Asia: The Role of Air-Independent Propulsion Systems,” in R. A. Bitzinger (ed.), Emerging Critical Technologies and Security in the Asia-Pacific (London: Palgrave Macmillan, 2016), 91–106.
