Russian diesel-electric submarine “Saint Petersburg”, lead ship of project 677. In the future, submarines of this type may receive VNEU. Photo Wikimedia Commons
The bulk of modern submarines are equipped with diesel-electric power plants. Such devices have characteristic drawbacks, which is why a search for convenient and profitable alternatives is carried out. As practice shows, the modern level of technology makes it possible to create efficient power plants for non-nuclear submarines, and we are talking about systems of different architectures.
Problems and solutions
The main disadvantage of diesel-electric submarines is the need for regular recharging of the batteries by means of a diesel generator. To do this, the submarine must float to the surface or move at periscope depth – which increases the likelihood of detection by the enemy. At the same time, the duration of diving on batteries usually does not exceed several days.
An obvious alternative to diesel is a nuclear power plant, but its use is not always possible and justified due to the complexity and high cost. In this regard, for several decades, the issue of creating air-independent power plants (VNEU) with the desired characteristics and without the disadvantages of diesel-electric systems has been studied. A number of new technologies of this kind have been successfully brought into operation, and the commissioning of others is expected in the near future.
In general, there are several approaches to the creation of VNEU. The first involves rebuilding the diesel generator using a different engine that is less demanding on the incoming air. The second proposes the generation of electricity using the so-called. fuel cells. The third is to improve batteries, incl. up to the rejection of its own generation.
An alternative to Stirling
The first non-nuclear submarine with a full-fledged VNEU, put into service, in 1996 was the Swedish ship Gotland. This submarine had a length of 60 m and a displacement of 1600 tons, and also carried 6 torpedo tubes of two calibers. Its power plant was built on the basis of a standard diesel-electric and supplemented with new components.
Swedish nuclear submarine Gotland in the American dry dock, 2005 Photo by US Navy
Surface running and power generation are provided by two MTU 16V-396 diesels and a pair of Hedemora V12A / 15-Ub generators. The propeller in all modes is driven by an electric motor. In a submerged position, the submarine, instead of diesels, starts a Stirling engine of the Kockums v4-275R type, using liquid fuel and liquefied oxygen. The reserve of the latter allows you to stay under water for up to 30 days without the need for ascent. In addition, the Stirling engine is less noisy and does not unmask the submarine as well.
Three new submarines were built according to the Gotland project; the second and third buildings were commissioned in 1997. At the beginning of the 2000s, a project with the Södermanland code was implemented. It provided for the modernization of two diesel-electric submarines of the Västergötland type with the installation of VNEU from the Gotland project. Japan became interested in Swedish developments. Under license, she assembled VNEU for submarines of the “Soryu” type. Due to their large dimensions and displacement, Japanese submarines carry four v4-275R engines at once.
During the development of the Scorpène project, French shipbuilders proposed their own version of VNEU based on an alternative engine. Such an installation, called the Module d’Energie Sous-Marine Autonome (MESMA), was offered to potential customers for use on newly built submarines.
The MESMA project proposed a special steam turbine engine powered by ethanol and compressed air. The combustion of the alcohol-air mixture was supposed to produce steam for the turbine driving the generator. Combustion products in the form of carbon dioxide and water vapor under high pressure were proposed to be discharged overboard over the entire range of operating depths. According to calculations, the Scorpène submarine with VNEU MESMA could remain under water for up to 21 days.
Nuclear submarine Scorpène in the version with fuel cells for the Indian Navy. Graphics Hisutton.com
The MESMA plant was offered to various customers. For example, it was planned to be used in the Scorpène-Kalvari project for India. However, the pilot plant showed insufficient performance, and interest in the project was sharply reduced. As a result, the new French diesel-electric submarines are still equipped with diesel engines – although the developers have already announced a new modernization with the introduction of other promising solutions.
In 2019, Russian shipbuilders announced the development of a fundamentally new VNEU based on a closed-cycle gas turbine engine. It includes tanks for liquefied oxygen: it evaporates and is supplied to the engine. Exhaust gases are proposed to be frozen and thrown out only when surfacing in a safe area. A similar VNEU is being developed within the framework of the P-750B project.
By the end of the nineties, Germany had created its own version of VNEU. In 1998, construction began on the lead submarine of the new Type 212 project, equipped with a similar system. The German project involved the use of the Siemens SINAVY system, which combines an electric motor and hydrogen fuel cells. For movement on the surface, the diesel generator was retained.
The SINAVY complex includes Siemens PEM proton-exchange fuel cells based on metal hydride from a liquefied oxygen tank. For added safety, metal hydride and oxygen containers are located in the space between the rugged and lightweight housings. During the operation of the VNEU, the hydrogen obtained from the metal hydride, together with oxygen, is fed to special membranes and electrodes, where current is generated.
German submarine U-32, second ship “Type 212”. Photo of the German Ministry of Defense
The autonomy of the submarine “212” reaches 30 days. An important advantage of VNEU SINAVY is the almost complete absence of noise during operation at sufficiently high performance. At the same time, it is difficult to manufacture and operate, and also has other disadvantages.
Six 212 submarines were built for the German Navy. In 2006-2017. four of these ships entered service in the Spanish fleet. On the basis of “212”, the “214” project was created, which provides for the preservation of the existing VNEU. Such submarines are very popular in the international market. Orders received from four countries for more than 20 boats. 15 ships have already been built and delivered to customers.
It should be noted that VNEU based on fuel cells is being developed not only in Germany. In parallel with the MESMA project in France, a variant of the Scorpène submarine with the use of fuel cells was developed. It was these submarines that were sold to India. Now elements of a new generation are being created. Earlier it was reported that its fuel cells are being developed in Russia. VNEU of this type has already passed bench tests, and in the future it will be tested on an experimental ship.
Battery powered submarine
The appearance of fundamentally new engines and generation means does not exclude the need for further development of existing technologies and units. Thus, storage batteries of already known and mastered types retain a high value. In promising projects, they are even considered as the only source of energy for all systems.
The lead submarine of the “Taigei” class before launching. Photo by MSS Japan
Curious processes are observed in Japanese shipbuilding. Japan was one of the first countries to master VNEU with a Stirling engine, but in 2015 and 2017. two submarines of the modified Soryu project were laid without such systems. Space for standard batteries and VNEU units was given for modern lithium-ion batteries. Due to this, the duration of scuba diving has been doubled in comparison with the batteries of the previous generation.
Since 2018, the construction of submarines of the new Taigei project, originally developed using a diesel-electric installation and lithium-ion batteries, has been underway. The lead ship of the new project has already been launched, and two more hulls are under construction since last year. In total, it is planned to build seven submarines with acceptance into service from 2022.
There are many projects of ultra-small submarines, equipped only with batteries. In the future, this architecture may find application in “large” projects. Recently, French shipbuilders presented the SMX31E concept project, which combines a lot of the most daring decisions. In particular, the submarine received only batteries with their placement in all available volumes, incl. between durable and lightweight bodies. The batteries must be charged at the base before going to sea.
It is estimated that when fully charged, the SMX31E will be able to remain submerged for 30-60 days, depending on driving speed and total energy consumption. At the same time, it is planned to ensure the full operability of all standard and additional devices, complexes, etc.
Proposed architecture for the SMX31E submarine. Naval Group Graphics
In the process of evolution
Thus, in recent decades, there has been significant progress in the field of VNEU for non-nuclear submarines. Various variants of such systems with certain features and advantages have been developed, tested, introduced into projects and put into service. However, even the latest air-independent installations have certain disadvantages. They remain complex and expensive, both to manufacture and operate.
Despite the advantages in tactical and technical characteristics, non-submarines with VNEU cannot yet supplant diesel-electric submarines of “traditional” architecture. Moreover, the latter are developing and also use the most modern technologies and components. A striking example of such competition between different classes is the development of the Japanese submarine fleet, which returned to the diesel-electric scheme at a new technical level.
Apparently, the competition between air-independent and diesel-electric installations will continue in the foreseeable future – and there is no clear favorite yet. At the same time, it is obvious that the navies of the world will benefit. They get the opportunity to choose the best option for the power plant that best meets all the requirements.