Sea Technology

JUL 2017

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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www.sea-technology.com July 2017 / st 19 U nderwater devices that need portable power, such as data loggers and CTD sensors, face sig- nificant challenges. Operating be- neath the sea means facing tidal forces, frigid temperatures and the corrosive nature of the environ- ment. The most daunting challenge is the unrelenting effects of seawa- ter pressure. To keep the effects of constant pressure from imploding power cells, battery packs are placed in- side pressure-tolerant housings. PVC housings can be used to 600 ft. below sea level. At greater depths, other materials such as iron and steel are required; for the deepest applications, titanium and advanced composites are required. The use of these more durable materials significantly increases the cost of portable power underwater. Energy Sales, a custom design and development firm fo- cused on portable power solutions, recently unveiled a new approach to pressure tolerance in the field of battery design. The company worked closely over the last few years with the University of Washington and with Sealand Innovations, an engineering consulting firm, to test its concept and de- sign. The result is the PTBS, pressure-tolerant battery system. Developed for the deep ocean, PTBS addresses a need for cost-efficient battery solutions that will operate safely and successfully at 3,000 m below sea level. This design, ini- tially featuring alkaline cells, removes the need for pressure housings made of titanium or other costly advanced com- posite materials. Seawater Pressure Batteries designed for terrestrial applications are con- fronted with constant pressures in magnitudes beyond the scope of their original design when submerged in the ocean. Unfortunately, constant pressure of this magnitude will cause cells to implode and fail. Normal atmospheric pressure is 14.5 psi at sea level and increases by approxi- mately 14.5 psi per 33 ft. This means that at depths of 33 ft. and 132 ft., that pressure increases to 29 and about 60 psi, respectively. While the average ocean depth is 12,100 ft., consider the 16,099 psi at 36,200 ft. in Challenger Deep, the deepest point in the ocean. To alleviate the effects of seawater pressure, industry best practices call for battery packs to be embedded inside pres- sure-tolerant housings. Beyond that, iron and some of its derivatives can be problematic when used with compasses due to innate magnetic properties, and the use of advanced composites and titanium has the drawback of adding tens of thousands of dollars to the cost of each installation. Battery Chemistries and the Underwater Experience Attempts to address the effects of water pressure have seen varying degrees of success. Lead-acid batteries tend to have a short life cycle. Expense becomes an issue with lithi- um cells due to the difficulty associated with the removal of all the gasses present in the end product. In addition, reports of separator compression-related issues and the number of cells required for the creation of high-endurance batteries also contribute to the significant cost. The Alkaline Challenge Early research and testing conducted by MIT in 1999 showed some favorable results using alkaline batteries. The A New Approach To Pressure Tolerance Deepwater Battery Eliminates Need for Additional Pressure Housing By Vincent Lefebvre • Terry Conrad Energy Sales' PTBS is designed for deep-ocean applications in need of portable power.

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