Contribution to Electrical Valorization of Microbial Fuel Cells

Microbial Fuel Cells (MFCs) are bioreactors that convert chemical energy in organic compounds to electrical energy through the metabolism of microorganisms. Organic matters are widely available in the environment that contains a huge amount of energy. This energy could be harvested, converted, by the technology of MFCs, to be used in certain applications. Energy production of a MFC is limited in low voltage value and low-power values what limits the potential applications. However this energy is still sufficient to supply low-power applications (e.g. wireless sensors) to replace batteries and overcome their (environmental and economic) limitations. An electrical model of MFCs appears useful to better understand their dynamics and to validate the design of electrical circuits before fabrication. That also helps to evaluate the internal losses what may lead to another design of MFCs for better efficiency. To step-up the voltage of MFCs to be suitable for these applications, an efficient power management unit (PMU) is required with a specific design to deal with their characteristics. A flyback converter under discontinuous conduction mode (DCM) is the most adapted to such low-power source like MFCs, offers a simple implementation, and low losses conversion system. The DCM characteristics of the flyback converter are very attractive for impedance matching that is investigated as a maximum power point tracking algorithm. An integrated, low-voltage, low-consumption charge pump circuit is a good solution to supply start-up and the oscillator at low voltages. The flyback converter has a good efficiency that can reach 75% with one MFC and about 80% when it is supplied by a serial stack of MFCs. Associations of MFCs are very interesting to increase the output power and expand the domain of application. Parallel association is a method to increase the output current but it imposes limitations in conversion efficiency due to the low output voltage of the stack. Contrarily, the serial association steps-up the voltage what leads to better performance of the converter. However the non-uniformities between cells in a serial stack affect negatively the performance of the stack. Voltage balancing circuits are considered as the solution to compensate this phenomenon. Alternatively connecting the MFCs in series and parallel can achieve the purpose (switched-MFCs) by exploiting the internal capacitor of the MFCs. In the switched-capacitor method, an external capacitor is used to transfer the energy from the strongest MFC(s) to the weakest one(s). The losses in the switched-capacitor circuit are less than the losses of the switched-MFCs. The switched-capacitor offers an efficient, simple, low consumption method to optimize the performance and prevent the voltage reversal of the weak cells. Integration of this circuit can optimize the efficiency. Operating MFCs in a batch mode requires a periodic intervention to add organic matter. Continuous operation mode by hydraulically connection between MFCs overcomes this problem and can continuously refresh the substrate to give an autonomous energy harvesting system. On the other hand, in some applications, e.g. a wastewater treatment plant, MFCs could not be hydraulically isolated. In this configuration, a leakage charge between the associated MFCs will decrease the global efficiency. The flow rate has to be controlled to eliminate this problem. A flow from cathodes to anodes causes additional losses due to the oxygen leakage. A practical investigation of the harvested energy from MFCs may prove the fact that MFCs are good candidates to replace the chemical batteries. A temperature sensor is continuously supplied by alternatively connecting two MFCs. Each MFC supplies the sensor for two days. The flyback converter is able to continuously supply the sensor from the energy harvested from one continuously-fed MFC. This could be a good example, in a wastewater treatment plant (WWTP), to supply the monitoring system or also to supply the low power application of a building from a local WWTP. (author)