A consortium led by UK-based Steatite completes first phase of pioneering battery development project
As more subsea vehicles become capable of autonomous operation, so too do the demands for longer and more flexible options for powering them. Lithium-ion batteries may be improving by the day in land and aerial-based electric vehicles, but devising systems which can stand up to the temperatures and pressures of subsea work is altogether more challenging. Nevertheless, some have made headway. A consortium led by electronics and communications firm Steatite is currently undertaking a two-year project to produce battery packs which will be pressure tolerant at depths of up to 6,000m, with a view to incorporating them into more Marine Autonomous Systems (MAS). Supported by InnovateUK and the Defence Science and Technology Laboratory (Dstl), the project involves expertise from multiple companies in the supply chain, including Lithium Sulphur (Li-S) cell manufacturer OXIS Energy, underwater vehicle designers and manufacturers MSubs and scientific expertise from the National Oceanography Centre (NOC). The hope is that such systems will not only allow autonomous vehicles to reach these depths, but that they will be able to do so more safely and with increased range, speed and payload capacity. In doing so, the vehicles can remain subsea for longer periods, reducing the costs of deployment and operation.
Under pressure According to the group, pressure testing at the NOC has formed the bulk of the work so far. At 6,000m, subsea pressures exceed 600 bar, yet components must continue to deliver the output required to run the vehicle. Temperature is also problematic (batteries tend not to like the cold), and tests have included cycles run at 0-4°C. However, these tests have produced concrete results. The latest generation of 12Ah Li-S Oxis cells have a raw energy density of 289 Wh/kg, a 70% improvement on comparable Lithium-Polymer performance. By the end of the project, the consortium is aiming to produce a commercial pack with twice the capacity of the current market leader, Steatite battery Paul Edwards told InnovOil. To achieve this, over successive prototype the group has almost managed to overcome the loss of energy density at lower temperatures, allowing Li-S cells to deliver nearly the same amount of charge under working pressures and temperatures as in ambient temperatures. This was done, Edwards said, by close work with cell supplier Oxis, enabling the supplier to redesign the construction of its cells in the wake of each test. Vehicle power supplies must also remain neutrally buoyant over the mission, a requirement which necessarily lowers the available battery storage. The resulting Neutral Buoyancy Energy Density (NBED) value of Li-S is lower, at 126 Wh/kg, but is almost twice as dense as the comparable Li-Po cell, at 66 Wh/kg. According to project documents, the group has successfully performed a number of life cycle tests with several cells, achieving 60 cycles of slow discharge – e.g. an AUV travelling at slow constant speed and recording data – and 80 cycles of “faster” discharge, such as an ROV travelling at speed to a work site and being used to service equipment. That success suggests that these batteries could be used either to substantially extend the operating time of vehicles using current battery architecture, or would allow designers to reduce the battery size by half, whilst maintaining or improving current operating life and freeing up more space for payload. The most recent update marks the project’s halfway point. From now and into 2017, the group will work on into the battery development phase, including work developing on Steatite’s Battery Management System (BMS). Following this, the battery will be demonstrated in a “deep-dive” submarine in the middle of next year.