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10 st / July 2015 www.sea-technology.com Whenever there is a change in the MFC's condition, the internal voltage and resistance vary nonlinearly. The poten- tial difference between the anode and cathode when the circuit is open is called the open circuit voltage (OCV). The OCV of the MFC is generally less than 0.7 volts. The typi- cal method of characterizing the MFC's power production M icrobial fuel cells (MFCs), also called biological fuel cells, can use any organic material as a fuel source. They are attractive for low-power applications, such as un- derwater noise data collection, and are a clean and effcient method of energy production. An MFC converts chemical energy to electrical energy by using active bacteria and simulates the interactions be- tween bacteria found in nature. We developed an MFC that generates an output voltage less than 0.7 volts, with low output current in the range of a few milliamperes. This low output requires a power management system between the MFC and the load. Thus, the harvested energy from the MFC is transferred to the load with a boosted voltage to meet the load requirement. The underwater energy harvesting system we developed is based on an MFC that uses marine sediment bacteria and has two major interface circuits: the converter transformer capacitor (CTC) and charge pump capacitor converter (CPC). The CTC requires two supercapacitors and additional circuitry to control circuits, which decreases energy con- version effciency due to large overhead, while the CPC requires one supercapacitor and achieves more effcient energy conversion. The power management system we developed consists of a charge pump, supercapacitor and boost converter. Simu- lation results clearly show the system is capable of powering oceanographic instruments, such as an acoustic modem, underwater sensor nodes using multipath propagation, and instruments measuring environmental variables such as sea state and temperature. It can simultaneously power instru- ments that operate at less than 6.8 volts. MFC Development Modeling of the fuel cell is important to predict accurate output characteristics. When modeling the fuel cell math- ematically and electrically, the voltage drop due to over-po- tential or polarization is considered. The output of the single fuel cell is 0.7 volts. If many cells are stacked in series, then the corresponding output will be of higher voltage. The stacked output can be increased to the required level with the help of a power management system. Microbial Fuel Cell Underwater Power System Harvesting Energy From Seawater for Low-Power Applications By P. Janani • Dr. S. Sakthivel Murugan (Top) The energy harvesting system. (Bottom) The charge pump circuit.