Battery

 

The battery group at FFI has been involved in the development of power sources for unmanned submersibles since 1990. Before that, we had developed seawater batteries for buoys and seabed sensors using magnesium anodes and oxygen dissolved in the water. This system has a very high specific energy as the oxidant (oxygen) and the electrolyte (seawater) is free, but the power capability was very low, making it suitable only for discharge over years. The challenge for AUV use was to get sufficient power out of the battery. Compared to more common battery electrolytes, the specific resistance of seawater is high, limiting the current density between the electrodes in the battery. Contributing to the low power capability is also the low concentration of oxygen in water of only 4 to 8 ml/ liter. (In contrast, air contains 200 ml of oxygen per liter, making the required work for breathing much less).

The concept had to be based on a forced flow of seawater through the battery. The concept was to make an AUV with a water intake in the front and a water exit in the back of the vehicle located at the point of lowest water pressure (close to the maximum diameter of the vehicle). When the vehicle moves through the water, water is forced through the battery, supplying oxygen and removing the reaction products. Calculations showed that a cable to the vehicle would induce too much drag, consuming too much power. So we needed to make a remotely controlled but un-tethered vehicle as a test platform for the battery. For a power production of 300W, a water flow of 20 liter per second (or 72 cubic meter per hour) was needed. Because of the open structure of the cells and the water channels, only parallel concoction of cells was possible, so a 1V in, 28V out DC to DC converter was developed. A 25 Ah lead acid battery served as a load leveler and allowed start-up of the system.

A section of a full scale battery was built and proved the battery concept using pumped seawater indicating a range capability of 1200 nm for an AUV at 4 knots. At the same time a number of model studies with flow-through AUV models were built and tested. The resulting final design was christened AUV-Demo and tested in the spring of 1992 during a 109 nm dive. After the dive, only one tenth of the battery capacity was consumed. FFI had now demonstrated its capability to design, produce and control unmanned un-tethered underwater vehicles and started to look for applications for the technology. The original applications were no longer of interest as the cold war had ended and civilian applications had to be considered, such as seabed mapping. The seawater battery did not have power for more than the propulsion of the AUV and other battery technologies had to be considered.

The long-range capability is still of interest however and a French Norwegian program concept study “CLIPPER” is evaluating the possibility of developing a long-range demonstrator based on the seawater battery concept.

Aluminum hydrogen peroxide semi fuel cell

As we started to look on sensors, it became immediately obvious to us that the power capability of the seawater battery used in AUV-Demo was not sufficient. The next AUV, HUGIN 1 was powered by a conventional NiCd battery giving an endurance at 4 knots and all sensors operating of ca 6 hours. This was the test vehicle for the development of control and navigation systems and it was also the carrier of a multibeam sonar developed by Simrad AS (Later Kongsberg Maritime ASA). In addition to serve as a test and development platform, HUGIN 1 mapped 460 line km of the seabed for the Åsgard Transport pipeline in 1997, demonstrating the superior capability of the AUV for seabed mapping compared to state of the art ROV technology.

At the same time as HUGIN 1 was designed, the battery group started the development of the aluminum hydrogen-peroxide semi fuel cell (AlHP) in order to extend the discharge time to 36 hours compared to 6 hours with NiCd technology. It was in many ways similar to the magnesium seawater cell, but used KOH as electrolyte; thereby reducing the internal resistance of the battery by a factor 20. Oxygen was added to the electrolyte in the form of hydrogen peroxide (HP). HP as a 50% solution was contained in plastic bags and was supplied continuously to the electrolyte with a metering pump. The cathode catalyst had to be developed from scratch, but knowledge of the aluminum alloy composition and the metallurgical treatment of the anode was inherited from the groups earlier work on the aluminum air fuel cell. HUGIN 2 was produced and the first test of the Al/HP semi fuel cell in the ocean started. The vehicle was transferred to NUI in 1999 and christened NUI Explorer. NUI Explorer had a 4 cell battery, in HUGIN 3000 the number of cells was increased to 6 and in HUGIN 4500 8 serially connected cells are in use.

 
The AlHP cell operates at ambient pressure (outside the pressure hull) and dives to more than 3000 m sea depth have been undertaken. Endurance of the AUVs using AlHP fuel cells with all sensors operational is 2 to 3 days, but longer missions at 4 knots have been undertaken. The specifications for HUGIN 3000 is 45 kWh/60 h, the specification for HUGIN 4500 is 60 kWh/80 h. The very long endurance possible with high energy density AlHP cell makes it a favorite choice by a number of survey companies (e.g. C&C Technology, Fugro, DOF), but the logistics involved, with containers for KOH, HP and spent electrolyte makes the system unsuitable for military use as well as operation from small platform for which electrically rechargeable batteries is a better option.

Traditional batteries inside a pressure hull can be used, but the weight of the pressure hull increases with design depth, making a battery based on pressure tolerant lithium ion polymer cells a more attractive solution for deep diving AUVs. After some encouraging experiments in the 90s with single cells, pressure tolerant batteries were made and a number of safety tests performed. The first battery was produced in the autumn of 2003 and field tested in HUGIN 1000 in the spring of 2004. The battery is qualified for a depth of 4500 m and is used in all HUGIN 1000 AUVs. The battery consists of one to three 6 kWh modules, each 48V/120Ah. With 3 modules, the endurance is approximately 24 hours at 4 knots and all sensors operational.

Hydrogen oxygen fuel cells

Hydrogen oxygen fuel cells have been used in space for a long time, but AUV application has been rare. Given that one can use the positive buoyancy from pressurized carbon composite cylinders, fuel cells in combination with lithium ion batteries may well be the ultimate power source for AUVs and the battery group is working to make this a reality.

The battery group is well equipped with laboratories and equipment for characterizing power sources at ambient and high external pressures and as well as under abuse conditions for safety studies. Today the staff is:

Jon Øistein Hasvold, Chief Scientist
Martin Gilljam, Senior Scientist, fuel cells
Torleif Lian, Principal Engineer, battery qualification, engineering and testing
Tom Cato Johannessen, Principal Engineer, battery safety and engineering
Dr Sissel Forseth, Senior Scientist, power sources for soldier systems

 

Power Sources for AUVs

Ø Hasvold, S Forseth, T C Johannessen, T Lian (2007): Safety aspects of large lithium batteries. FFI/RAPPORT- 2007/01666

Ø Hasvold (2010): Sikker anvendelse av litium batterier  FFI/RAPPORT- 2010/00215