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Cover letter An autonomous flyback converter was designed for energy harvesting from Microbial Fuel Cells (MFCs). The circuit was optimized to minimize the losses and maximize the efficiency. A Maximum Power Point Tracking (MPPT) algorithm was implanted in the converter to extract the maximum power available from MFC. Discontinuous conduction mode operation of the flyback allows controlling the MPP operation by impedance matching. The flyback can start-up at low voltage, around 300 mV. The output open circuit voltage is about 20 V and the voltage at MPP is 6.4 V with a maximum efficiency of 71.2%. Abstract: Microbial fuel cells (MFCs) use bacteria as the catalysts to oxidize organic matter and generate electricity. This energy can be used to supply low power electronic systems. A power management unit between the MFCs and the load is required to adapt the voltage and control the operation. The low voltage and low power characteristics of MFCs prohibit the use of standard converter topologies since the threshold voltage of standard CMOS transistors in CMOS technology is higher than the output voltage of MFCs. A low-voltage start-up sub-circuit is required to charge a primary capacitor to supply the driver. A specific sub-circuit is also required to control the operation of MFCs for Maximum Power Point Tracking (MPPT) issues. An optimized Discontinuous Conduction Mode (DCM) autonomous flyback converter for energy harvesting is presented for ambient sources, like MFCs. The converter is designed, fabricated, and tested. An MPPT algorithm is integrated in the system to control the operation and to extract the maximum available power from the MFC. The converter is able of start and step-up MFC output voltage to a value higher than 3 V under load. The peak efficiency of the converter is 71.2%.

International audience; This energy harvesting solution permits the autonomous supply of low power sensor nodes, without a chemical battery, allowing their extensive use in various environments. The electrical interface between the harvester and the sensor is crucial in order to maximize the harvested energy and boost the voltage to a minimum value required by the sensor. To achieve input impedance and voltage gain independently, this paper presents a flyback converter in discontinuous mode. Using the proposed flyback model validated experimentally, we have studied the impact of each loss source in order to give some trade-off for designing an efficient sub-mW harvesting interface. We underline the effect of the transformer losses due to the magnetic hysteresis as well as the driving loss impact. Following this method, the flyback prototype achieves 71% power efficiency when harvesting from a microbial fuel cell delivering 90µW. Keywords— flyback design method; energy harvesting; DC-DC converter; maximum power point tracking; microbial fuel cell

International audience; This paper aims to show some applications of microbial fuel cells (MFCs), a suitable energy harvesting technique, as clean power source to supply a wireless sensor network for environmental monitoring or wastewater treatment or biosensor. An MFC is a bioreactor that converts energy stored in chemical bonds of organic matter into electrical energy. The performance of MFC is limited by a low output voltage and low output power. However this energy may be enough to supply low power applications to replace their batteries. Moreover considering that the performances of MFCs are related to many operating conditions, MFCs can be employed as a biosensor to measure some parameters (Organic matter concentration, temperature, and pH). The applications targets inaccessible environments where soil and waste-water are nonetheless present. In this case, a terrestrial microbial fuel cell (TMFC), based on soil that acts as membrane and as flow of nutrient, it can function as a pH sensor. Instead a wastewater microbial fuel cell (WWMFC), where waste-water acts as flow of nutrient and bacteria, it can operate as a temperature sensor. Other low power devices can be supplied by the harvested energy from MFCs.

International audience; Microbial Fuel Cells (MFCs) have been receiving increased interest in the last three decades. MFC can produce electrical energy from a wide variety of organic wastes in wastewater treatment plant concurrently with the treatment process. MFCs are considered as low power, low voltage generator. Scaled-up systems are needed to allow for efficient treatment of large volume of wastewater, but also to be able to produce sufficient electricity. Non-uniformities and hydraulic couplings between stacked MFCs are two problems that can negatively affect the performances. A power management unit is required to step-up the output voltage of the association of few MFCs for suitable supply of low power applications. In this paper, the operation of a serial stack, hydraulically coupled, non-uniform MFCs is studied and optimized to supply low power applications in a wastewater treatment scheme.

International audience; Microbial fuel cells (MFCs) show great promise as a concomitant process for water treatment and as renewable energy sources for environmental sensors. The small energy produced by MFCs and the low output voltage limit the applications of MFCs. Specific converter topologies are required to step-up the output voltage of a MFC. A Power Management Unit (PMU) is proposed for operation at low input voltage and at very low power in a completely autonomous way to capture energy from MFCs with the highest possible efficiency. The application of sensors for monitoring systems in remote locations is an important approach. MFCs could be an alternative energy source in this case. Powering a sensor with MFCs may prove the fact that wastewater may be partly turned into renewable energy for realistic applications. The Power Management Unit is demonstrated for 3.6 V output voltage at 1 mW continuous power, based on a low-cost 0.7-L MFC. A temperature sensor may operate continuously on 2-MFCs in continuous flow mode. A flyback converter under discontinuous conduction mode is also tested to power the sensor. One continuously fed MFC was able to efficiently and continuously power the sensor.

International audience; Microbial fuel cell (MFC) converts electrochemical energy from organic matter into electricity by means of chemotrophic bacteria metabolisms. The MFC power output is limited in voltage and in current in the range of microwatts or milliwatts per liter. The output power may be improved by association of MFCs either in series or in parallel. The serial association of a large number of MFCs presents a benefit in effective output voltage but may suffer from several limitations. The most important is that the possible dispersion between the microgenerators leads to a nonoptimal stack efficiency. A power management unit is also necessary to harvest energy from a single MFC or a group of connected MFCs. The converter functions are to step up the voltage and to control the operating point of the MFC(s). The aim of this paper is to build an appropriate architecture for energy harvesting from series-connected MFCs, which feature disparate behaviors. A system to meet low-power sensor consumption specifications is composed of a stack of nonuniform serially connected MFCs, a voltage balancing circuit, and a maximum power point converter. The system is designed, fabricated, and tested. A percent of maximum power achieved of 86% is demonstrated for 1.5 mW of harvested power from four MFCs with large dispersion in characteristics.

International audience; Microbial Fuel Cells (MFCs) harness the metabolism of micro-organisms to generate electrical energy from organic matter. MFCs offer great promise for simultaneous wastewater treatment and green energy production. The association of a large number of individual MFCs offers very interesting perspectives for electrical energy generation. It can scale-up the low output voltage of an individual cell to enable output voltages to levels acceptable by commercially-available DC/DC converters and it permits to mutualize the electrical powers of each cell. The serial association of a large number of MFCs is a challenge itself for many reasons. Firstly the hydraulic couplings (when MFCs share the same substrate) witch cause leakage of electrical-charge careers between the connected reactors. Secondly the non-uniformities between generators which lead to a non-optimal energy recovery because the associated cells do not able to operate at Maximum Power Point (MPP). Non-uniformities can be compensated with electronic circuits to prevent voltage reversal or enable voltage equalizing. In this paper a balancing method is studied and adapted for energy harvesting from a stack of serially connected MFCs. The balancing circuit was simulated, realized and tested for energy harvesting. With balancing method the cell voltage of MFCs in a stack can be equalized and the performance of MFCs can be improved and it leads to an optimal energy recovery of the stack.

International audience; Biofuel cells use catalysts (chemical, enzymatic or microbial) to convert chemical energy from organic substrates into electrical energy. This paper investigates energy harvesting from several low-power biofuel cells to power a load under several volts. As biofuel cells can share the same fuel source, an isolation is required to aggregate energy towards high voltages (compared to a cell voltage). The proposed converter is based on the well known flyback topology. The efficiency of the flyback converter is about 60% for an input power of 10mW. The PCB area is 29 cm2 compared to around 74 cm2 for a previously built boost converter. The topology of the flyback converter includes a control circuit which allows a regulation of the input voltage to keep it around the maximum power point (MPP at around 0.3 V). This control circuit is powered internally through an auxiliary sub-circuit.