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# Power Management for Microbial Fuel Cells

Abstract : Microbial fuel cells (MFCs) harness the metabolism of micro-organisms and utilize organic matter to generate electrical energy. They are interesting because they accept a wide range of organic matter as a fuel. Potential applications include autonomous wastewater treatment, bio-batteries, and ambient energy scavenging. MFCs are low-voltage, low-power devices that are influenced by the rate at which electrical energy is harvested at their output. In this thesis, we study methods to harvest electrical energy efficiently. The voltage at which energy is harvested from MFCs influences their operation and electrical performance. The output power is maximum for a certain voltage value (approx. 1/3$^{rd}$ the open-circuit voltage). This noteworthy operating point is favorable in some applications where MFCs are used as a power supply. MFCs can be tested at this point using an automatic load adjuster which includes a maximum power point tracking algorithm. Such a tool was used to evaluate the maximum power, the fuel consumption rate, the Coulombic efficiency and the energy conversion efficiency of ten similarly built 1.3 L single-chamber MFCs. Although structural and operating condition choices will lead to improved performance, these results investigate for the first time the performance of MFCs in continuous maximum power point condition and characterize MFCs in realistic energy harvesting conditions. Harvesting energy at maximum power point is the main thread of the manuscript. This is made possible with dedicated energy processing circuits embedding control feedback to regulate the MFC voltage to a fraction of its open-circuit voltage. Two typical scenarios are developed as outlined below. One critical application concerns autonomous low-power energy scavenging, to supply remote low-power electronic devices (e.g. wireless sensors). In this case, the low-power and low-voltage constraints imposed by MFCs require dedicated self start-up features. The Armstrong oscillator, composed of high turn-ratio coupled inductors and of a normally-on switch, permits to autonomously step-up voltages from a low DC source like MFCs. Although the circuit requires few components, its operation is not trivial because it partly relies on the parasitic elements of the inductors and the switch. Proper sizing of the inductors enables an optimized operation. This circuit can be associated with power electronic AC/DC and DC/DC converters to realize a voltage-lifter and a flyback-based self-starting Power Management Unit (PMU) respectively. The former is suitable for powering levels below 1 mW, while the latter can be scaled for power levels of a few units of mW and facilitates implementation of maximum power point control. A second application of interest concerns the case where energy is harvested from several MFCs. Serial association can be used to step-up voltage but may lead to detrimental consequences in terms of performances because of hydraulic couplings between MFCs sharing the same electrolyte (e.g. if the MFCs are running in continuous flow) or because of electrical non-uniformities between cells. Whereas the former issue can be addressed with galvanically insulated PMUs, the latter can be solved with voltage balancing circuits. Three of these latter circuits were analyzed and evaluated. The "complete disconnection" circuit isolates a faulty cell from the configuration to ensure it does not impede the overall efficiency. The "switched-capacitor" circuit transfers energy from the strong to the weak MFCs to equilibrate the voltages of the individual cells in the stack. The "switched-MFC" circuit alternatively connects MFCs in parallel and in series. Each of the three methods can be implemented at low-cost and at high efficiency, the most efficient one being the "switched-capacitor", that permits to harvest more that 85 % of the ideal maximum energy of a strongly-non-uniform MFC association.
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https://tel.archives-ouvertes.fr/tel-00757996
Contributor : Nicolas Degrenne <>
Submitted on : Tuesday, November 27, 2012 - 6:42:17 PM
Last modification on : Tuesday, September 1, 2020 - 2:44:17 PM
Long-term archiving on: : Saturday, December 17, 2016 - 3:58:14 PM

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• HAL Id : tel-00757996, version 1

### Citation

Nicolas Degrenne. Power Management for Microbial Fuel Cells. Biotechnology. Ecole Centrale de Lyon, 2012. English. ⟨tel-00757996⟩

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