POWER SUPPLY

Seawater Battery

The power for the NEREID-191 system is supplied by the SWB system. The battery system consists of four SWB-1200 (Kongsberg Simrad, Norway) cells (Hasvold et al., 1997), a DC/DC converter, the PCS, the DL, and an accumulator (Fig. F20).

The cell is a magnesium/oxygen battery based on a magnesium anode, which uses seawater as the electrolyte and oxygen dissolved in the seawater as the oxidant.

The chemistry of the cell is the dissolution of magnesium at the anode,

2 Mg = 2 Mg2+ + 4 e-,

and consumption of oxygen at the cathode,

O2 + 2 H2O + 4 e- = 4 OH-,

which is written in a simplified form

2 Mg + O2 + 2 H2O = 2 Mg(OH)2.

The formation of an alkaline product at the cathode surface may lead to the formation of a calcareous deposit,

4 Ca2+ + 4 HCO3- + 4 OH- = 4 CaCO3 + 4 H2O.

The alkaline reaction products need to be removed from the cathode surface by the sea current because the calcareous formation disturbs the second reaction at the cathode.

The anode is an AZ61 magnesium alloy rod with a diameter of 0.184 m and length of 2.2 m, including the anode connector device. The anode can be replaced by an ROV. The anode is surrounded by the cathode elements suspended from the titanium frame (Fig. F21). The weight in air of the anode is 120 kg and that of the titanium cathode frame is 40 kg. The cathode element consists of a titanium wire core with carbon fibers oriented radially (Fig. F22). The carbon fibers allow rapid material transport and high current density. The cathode collector lead (titanium wire) is connected to the titanium frame, which is also part of the cathode. The titanium frame is designed to allow seawater to pass easily through the cell so that oxygen-rich seawater is supplied to the cathode and the products of the cell reactions are removed.

The cell voltage obtainable is ~1.6 V, although this largely depends on the conductivity of the seawater, which varies with the temperature and salinity. The catalytic effect of bacteria colonizing on the cathode surface, which has been observed on all seawater cells in previous deployments of the system, is also one of the many factors that affects cell voltage. The maximum cell power is limited by the rate of oxygen supply to the cathode. The oxygen supply rate is proportional to the oxygen concentration in the seawater and the speed of circulation. In order to produce the designed output of 6 W for each cell, a minimum circulation of 20 mm/s, an oxygen concentration of 3 ppm, and a salinity of 20 is required. Because the cells have an open structure, the isolation between cells is low, which leads to large leakage currents in serially connected cells. Thus, the cells are connected in parallel.

The 24-V DC/DC converter changes the low cell voltage (1.6 V) into the output voltage (24.0 V). The output of the 24-V DC/DC converter is fed to the accumulator, which averages the power demand on the 24-V DC/DC converter and the seawater cells. After deployment of the cells, the DC/DC converter is inactive until the cell voltage becomes >1.41 V. After the cells are activated, the 24-V DC/DC converter takes power from the cells and charges the accumulator as long as a sufficient cell voltage (>1.41 V) is available. If the cell voltage becomes <1.20 V, the 24-V DC/DC converter becomes inactive until the cell voltage is restored to 1.41 V.

The accumulator consists of multiple 2-V Cyclon (Hawker Energy) lead acid cells, which form a 7.5-Ah, 24-V cell in total. The accumulator cell is float-charged electrically by the 24-V DC/DC converter output (Fig. F20). The voltage of the accumulator output is 25.7 V when the accumulator cell is fully charged and has no charging voltage applied by the 24-V DC/DC converter. The cell is stored in a 6500-m depth-rated pressure housing, which has a four-pin GISMA series-10 underwater connector for the load output and the 24-V DC/DC converter.

Power Control System

The PCS is a module that monitors and controls the SWB. The SWB system consists of the PCS, four SWB-1200 cells, the 24-V DC/DC converters, and the accumulator for supplying power to an external 24-V DC-driven system (e.g., SAM-191 and MEG-191) (Fig. F20). The 24-V DC/DC converter and the PCS are in the same canister. The purposes of the PCS are to monitor the voltages and currents of the SWB-1200 cells, the accumulator, and the output of the SWB system and to control the power switch of the SWB system to protect the accumulator from overdischarge.

SWB conditions are sampled by a microprocessor, and the sampled data are time-stamped and sent to the external data storage (DL) by an RS232C serial connection. Because the UMC is used for the connection between the PCS and the DL, an ROV can be connected via the same connector to the PCS and then via a serial line, it can check and control the PCS at the time when the DL is disconnected.

The PCS consists of a microcomputer, an interface board, a small DC/DC converter, and a backup battery (Fig. F23). The PCS will monitor the input voltage to the 24-V DC/DC converter and all the output voltage and current distributed from the accumulator. The microcomputer has a four-channel A/D converter to read voltages and currents. The A/D converter in the microcomputer samples the voltage of the SWB cells (Vb), the voltage of the accumulator (Vacc), the current for the load (Iload), and the current of the accumulator input (Iacc) (Fig. F24). The current value of the accumulator includes the direction of current flow (e.g., if the accumulator is charging or discharging). The sampling interval can be changed by sending commands to the microcomputer across the RS232C line. The RS232C line is used for communication between the microcomputer and the interface board.

The power switch on the interface board is controlled by the microprocessor. The microprocessor turns the power switch off to protect the accumulator under certain conditions. When the voltage of the accumulator is <18 V, the PCS immediately turns off the power switch. The PCS also turns off the power switch when the PCS detects three continuous accumulator readings of <20.8 V under small current conditions (<0.5 A). Three continuous readings take ~20 min. When the power switch is turned off and the voltage of the accumulator is >26.5 V, the PCS turns the power switch on. This procedure will be performed after three continuous readings of the PCS (Fig. F25). A small DC/DC converter supplies the power to the PCS from SWB cells. The DC/DC converter is the same as the 24-V DC/DC converter for the accumulator; however, the active voltage of the DC/DC converter is lower than that of the 24-V DC/DC converter. When the voltage from the SWB cells increases, the DC/DC converter becomes active at a voltage of 1.4 V. When the voltage of the SWB cells becomes <0.6 V, the DC/DC converter stops the conversion of voltage. When the DC/DC converter for the PCS does not supply power, the PCS takes power from the backup battery. The lifetime of the backup battery is ~250 days with a sampling interval of 60 s. The PCS consumes 85 mW of power while reading the voltages and the currents and 29 mW in sleep mode. When a lithium backup battery is connected within the canister, the microprocessor starts sampling at an interval that has been set to 60 s. The PCS transmits the data to an RS232C output and no acknowledgement is used in the protocol. After the transmission of the data, the PCS checks the voltage and current limits and takes actions based on these results by turning the external load switch on or off.

The PCS can be configured through an RS232C line using a PC. Usually, we use the program Serikom, which is provided by Kongsberg Simrad. The PCS can be programmed to different interval sampling times by setting the wanted time into the SLEEP_TIME windows on the program. The program has a command (MEASURE_NOW) to read voltages and current immediately. Using this command, a measurement is sent from the PCS, overriding the SLEEP_TIME setting. The power switch can be controlled by sending a command (SWITCH_ON/SWITCH_OFF) to the PCS. After receiving the command, the PCS immediately turns the power switch on or off. The sampling interval and the status of the power switch can be checked using the command READ_REGISTER. RTC time in the PCS can be set from a PC using the DOWNLOAD REAL-TIME CLOCK command. This setting resets the PCS system. The Serikom program also displays ordinary measurements from the PCS.

Data Logger

Data stored in the DL can be retrieved by an ROV. A simple PC program is used to examine the data and empty the DL. The DL consists of a single card computer, provided by Persistor, with a serial interface and a 10-MB flash storage device. A voltage of 7.2 V is supplied to the DL using 24 D-size lithium cells. The cells have a voltage of 3.6 V and a capacity of 16 Ah. After connecting the power line, the DL switches between two modes. In the suspended mode, the DL switches off all devices on the DL, except the serial controller interface, to minimize power consumption. The DL is turned on when the PCS starts sending data over a serial communication line. The data consist of current and voltage readings from the DC/DC converter. The baud rate for the serial communication is set to a fixed rate of 4800 baud. The DL will read the data from the PCS and save them to the flash memory drive in a format as the $M, string. When the DL finishes reading the data from the PCS, it goes back into a suspended mode. When the flash drive has reached its data-storage capacity, logging of data from the PCS is stopped. The recording period of the DL deployed during Leg 191 is ~160 days at a sampling interval of 60 s. The power consumption of the DL is 28 mA in normal operation and 0.3 mA in suspended mode.

When the DL has been retrieved by an ROV, the data stored on the flash card in the DL can be taken out for data retrieval. The data are saved on the flash card in ASCII format to a file called pcsdata.log, and it can be read into a PC. According to limitations of the operating system of the microcomputer (called PicoDOS), the file on the flash cards is never erased or changed using the PC. We show examples of records in the DL from testing of the equipment before Leg 191 in Figure F26.

Battery Frame

The SWB system is mounted on the cylindrical PAT frame, as shown in Figure F27. Four SWB cells are stored in concentric positions. The PAT frame is made of ordinary steel angle but is coated with zinc, and its base part is also coated with tar epoxy paint to protect it from corrosion. The titanium frame of the SWB cell, the stainless steel pressure housings for the DC/DC converter, and the accumulator are mounted on the PAT with polyvinyl chloride insulators. The top of the PAT is a gray, flat panel made of fiber-reinforced plastic drainboard that has access holes for the SWB anodes. The flat panel serves as the ROV platform. The panel is reinforced by a metal frame and is circular when viewed from above. The diameter of the top panel is 3.2 m. The basal "leg" of the PAT is also circular, and the diameter is 3.66 m, which corresponds to the diameter of the reentry cone. The basal leg is 0.24 m in height. The leg lifts the battery cells to improve the seawater circulation through the cells and to keep the PAT stable on the reentry cone. In the center of the PAT frame, coaxial rings are placed to guide the PAT smoothly over the riser assembly during installation. The hole in the center of the top panel provides space for the MEG frame. The total height of the PAT is 2.64 m to accommodate the SWB cells. The vertical position of MEG-191 on the riser is set to allow ROV operations.

The PAT also holds the SAM-191 recorder beneath its top panel. The top part of the SAM-191 sticks out of the panel. The UMC at the bottom of the SAM bulkhead is mated by gravity into a UMC receptacle, mounted on the stab plate placed at the bottom of the SAM hole, when the SAM-191 is dropped in the hole. In the dropping operation, the hole keeps the SAM-191 canister in the upright position. The SAM-191 key is lowered to the guiding wedges prepared inside the hole. At this time, the connectors on the bottom bulkhead of the SAM-191 canister are correctly positioned to the mating orientation of the UMC. The cable from the UMC at the bottom of the SAM hole receptacle has a T-junction; one branch is connected to the SWB system, and the other goes to a UMC receptacle mounted on the PAT top panel. Another cable (the ROV cable) is installed on the panel and is used to connect the UMC receptacle on the top panel to the MEG-191 canister. The ROV cable is fastened to the top panel initially by fastening mechanisms and a parking connector for the ROV UMC plug. An ROV will take off the fasteners and connect the ROV UMC plug to the MEG-191 canister. The SAM-191 can be ejected by an ROV-operated lever mechanism in the recorder frame (Fig. F28). The lever can be locked at two positions, one at the mated position of the SAM-191 and the other at the released position. By the use of the locking positions, an ROV can easily replace the SAM-191 recorder.

The DL was set on the top panel of the PAT (Fig. F28) and was retrieved by an ROV in October 2000.

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