Battery power at 6,000m below
The development of a lightweight, pressure tolerant battery promises to quite literally ‘open up’ very deep-water AUV designs, Guillaume de Forton of Oxis Energy told MJ.
A 500Wh array, the result of an intense R&D programme by cell developer OXIS Energy and battery pack manufacturer Steatite, has recently been integrated by MSubs into Deepbots’ Sperre Subfighter 7500 for final tests in Norway.
It signals a breakthrough on many different levels. Firstly, it is based on the Lithium Sulfur (Li-S) battery technology rather than the usual marine installations which tend to be lithium polymer or similar, and far heavier: “This battery is buoyancy neutral, the same as water,” said de Forton, which means “no extra buoyancy, no chambers, no buoyancy foam... and far less bulk”.
Further, while it’s actually a very safe chemistry, it’s also capable of resisting pressures that would compromise the capability of the electrodes in other lithium cells. In fact, pressure chamber tests at the National Oceanography Centre (NOC) showed the cells can withstand the pressure of 664 bars - that's equivalent to a water depth of 6,640m - at a temperature of 4°C.
Moreover, it should, according to de Forton, yield significantly better performance than the usual lithium marine chemistries “which tend to have a power density of 150 Wh per kg,” he explained. By contrast “these lithium sulphur cells, even at extreme depths, will give around double that, about 300Wh per kg... better than even the best, most advanced car batteries” he added.
These characteristics could fundamentally transform the design he explained, as the batteries no longer have to shelter inside a pressure shell, firstly cutting down complexity and secondly making it possible to explore a range of different configurations.
On a very practical level, it also makes it possible to change out the battery easily, both for maintenance and recharging. This gives operations an extra flexibility as it will be possible to swap the battery without having to take it back to base and open up the shell, setting the AUV back on its path again, with minimal delay.
Finally, this new development means that now more kinds of underwater vessels could slip the leash, get rid of the tether to the mothership and become truly autonomous, powering themselves to even greater depths.
Certainly, the programme has already generated considerable interest within the marine community, and it won’t stop there. As the OXIS chemistry continues to improve in terms of gravimetric energy, de Forton’s partner Paul Edwards from Steatite predicted “we will see greater gains in battery pack performance”.
The next phase, he said, is to deploy these batteries in long term marine autonomous applications.
By Stevie Knight
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