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14 Platform one October/November 2018 | Unmanned Systems Technology Teledyne Energy Systems has developed an underwater charging station that can recharge AUVs without needing a crew nearby or a tether to supply power from above the water, (writes Rory Jackson). The first half of the system, the Teledyne Subsea Power Node, acts as the energy supply. It uses an ejector- driven reactant (EDR) hydrogen fuel cell system to generate 8 kW of power per cell stack, with gaseous hydrogen as the fuel and oxygen as the reactant, which are stored in adjacent canisters. “For about 260 litres of reactants, you’d get about 100 kWh of energy storage; in a standard shipping container, you get 20 MWh of energy storage with three EDR fuel cell modules for 24 kW power delivery,” said Dr Thomas Valdez, chemical engineering manager at Teledyne Energy Systems. “It can be lowered down to the water by boat or helicopter crews, and secured to the seafloor by ROVs.” The containerised version of the Subsea Power Node can also provide up to 40 MWh if solid-state hydrogen is used. To log and communicate data on critical parameters such as charge and system health, Teledyne Benthos acoustic modems with a range of 300 m are integrated into the node. Fuel and oxygen are delivered via Teledyne ODI’s Wet Mate connectors to the second half of the system, which is the charging dock. Typically this consists of four ‘catcher’ ports arranged in an outward-facing square for AUVs to ‘mate’ with and begin charging, “This can happen by inductive charging or using the connectors. The station computer would then download survey or status data from AUVs as they are recharged,” Valdez said. Although the total amount of data captured can take several hours to download, the dock can be programmed to scan an AUV’s data, identify an item of high priority to the operations team, download a small quantity of that data and send up an RF transmitter to alert the team with an appropriate message or indicator. That might be a recommendation to retrieve the AUV to conduct vital maintenance, or an alert that a critical mission target has been found, such as an oil source, airliner wreckage or a pod of whales. As Valdez noted, “Typically, a 2 kW AUV might operate for four hours before being pulled back up, then charged for maybe eight hours. With this system, a group of AUVs can operate independently in deep water for up to a year. “Keeping a ship stationed nearby can cost $40,000 per day, and sailors can become tired and sick out in the open ocean – that doesn’t happen with AUVs.” At the level of energy the containerised Subsea Power Node provides, it is far more expensive to send down lithium-ion or lead-acid batteries – and these are often abandoned because they are too complicated to retrieve. The Subsea Power Node, Valdez said, is slightly buoyancy-negative when full. It therefore sinks gradually to the ocean bed by itself. When spent, it is slightly buoyancy-positive, and can be brought up at a controlled speed, using cabled buoys for example. The system was recently demonstrated in naval training exercises at ANTX 2018, in 14 ft of water from a Rhode Island dock. The company expects to test a 300 m depth-rated version in late 2019, and achieve full ocean depth (6000 m) in late 2021. Standalone recharger Underwater vehicles Teledyne’s subsea fuel cell system undergoing tests