Low power control systems for microbial fuel cell batteries

The URI Ocean Engineering Department has been working with the US Naval Research Laboratory to develop and test microbial fuel cell battery systems for low power seafloor applications of extended duration. These low power fuel cells utilize graphite electrodes in anoxic sediment coupled to bottle brush graphite electrodes in the water column to make up a battery system. These battery systems typically produce power on the order of 10-50 milliwatts per square meter of electrode area, or as much as ten times more if sediment electrode pore water can be exchanged by means of a pump. As part of this program, testing of these microbial fuel cell batteries has been conducted in the Potomac River Washington D.C., Narragansett Bay Rhode Island, Tuckerton New Jersey, and Monterey Bay California. For these tests it has been necessary to develop long term, low power control and monitoring systems to manage the batteries operation and to record their performance without consuming a significant portion of the meager power produced. This paper details the development and testing of these low power monitoring systems and their results. For this project we have developed several related systems. All of the systems utilize Oopic microcontrollers as the core low power computer. The Oopic consumes about 20 mA at 5 volts when on and none when off. Its programs are stored on EEPROM, and they start from the beginning each time the Oopic is powered up. Notes from the previous time awake can be stored in the EEPROM below the program. The microcontrollers are turned on by an electronic alarm from an 12C real time alarm clock chip with its own multi year lithium battery. The microcontrollers command the shut off of power as part of their program. The real time clock maintains crystal controlled date and time without using any system power. The clock can be programmed to wake up the system at a specific date and time, or at any of numerous intervals. Once awake, the Oopic can check the date, monitor/control the system status and even operate a low power pump. Long term measurements over many months can be logged on a micro SD card by means of a serial file management storage chip which uses only 3 ma at 3.3 volts when not writing, and a momentary 40 mA to write. When the system is asleep none of the subsystems consumes any power. Programmed to awake once an hour for 20 seconds to monitor the system and record the results on the SD card, the average power consumption can be kept to less than 2 milliwatts. The control system can be woken up, report data, accept commands, or be reprogrammed; all through an RS232 serial cable. The system utilizes an RS232 chip that manufactures +/-10 V RS232 signals from +5 V with only a few milliamps of additional power consumption. This chip is capable of communicating over a 100 m long cable at 9600 baud. In one instance in Tuckerton New Jersey near a dock, it was possible to continuously cable the control system ashore where a Netburner network based microcontroller was connected. The Netburner was programmed to provide an FTP and Telnet site, allowing remote monitoring and control from across the country over the internet.