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Efficient Energy Harvester for Microbial Fuel Cells using DC/DC Converters Jae-Do Park, Ph.D. Zhiyong Ren, Ph.D. Dept. of Electrical Engineering University of Colorado Denver Denver, USA [email protected] Dept. of Civil Engineering University of Colorado Denver Denver, USA [email protected] Abstract— Microbial fuel cell (MFC) is an emerging technology for sustainable energy production. An MFC employs indigenous microorganisms as biocatalysts and can theoretically convert any biodegradable substrate into electricity, making the technology a viable solution for sustainable waste treatment or autonomous power supply. However, the electric energy currently generated from MFCs is not directly usable due to the low voltage and current output. Moreover, the output power can fluctuate significantly according to the operating points, which makes stable harvest of energy difficult. This paper presents an MFC energy harvesting scheme using two layers of DC/DC converters. The proposed energy harvester can capture the energy from multiple MFCs at the most efficient operating point and at the same time form the energy into a usable shape. I. INTRODUCTION The finite resource of fossil fuels and environmental pollution derived from their use are driving the search for renewable and clean energy alternatives. This replacement of fossil fuels will require the utilization of many energy sources suited to meet different end uses. Microbial fuel cell (MFC) technology has been intensively researched in recent years as a novel technology, because it offers a solution for environmentally sustainable energy by treating waste and recovering electricity simultaneously. MFCs use active bacteria to generate electrical energy from the environment electrochemically. MFCs offer a simple, direct method for converting environmentally available biomass into electricity and are very suitable for clean, distributed, and renewable energy source, for example, powering the remote sensors [1], [2]. However, like other micro energy sources such as ambient heat, vibrations, and lights, MFC reactors generate very low power and energy due to thermodynamic limitations and it has been reported that larger power production cannot be easily achieved by just building larger MFCs or simply connecting them in series or in parallel, because of the nonlinear nature of MFCs [3-5]. Figure 1. Schematic of a two chamber microbial fuel cell using ferricyanide as the electron acceptor. The power density from MFCs has increased by orders of magnitude in less than a decade of research. The reported maximum power density from lab scale air-cathode MFCs increased from less than 1 mW/m2 to 6.9 W/m2. [5-7]. This improvement can mainly be attributed to relieving physical and chemical constraints through electrode material and reactor architecture improvement, as well as optimization of operational conditions [2], [8]. However, the reported power output from many MFC studies is based on the power dissipated on a static external resistance instead of the actual attainable power in a usable form, which indicates one crucial missing part before the technology can be commercialized - how to efficiently convert the theoretical potential into a practically meaningful power output. A few energy harvesting systems for sediment MFCs have been reported: they can capture energy from the MFC and convert it into applicable voltage and current levels [9], [10]. However, control scheme that actively harvests energy at an optimal operating point especially from multiple MFCs has not been researched extensively. In this paper, an efficient MFC energy harvesting system using two layers of DC/DC converters is presented. The proposed system can capture the energy from multiple MFCs Figure 2. Lab scale two-chamber MFC using ferricyyanide as electron acceptor (MFC#1, left) and single-chamber MFC usiing air as electron acceptor (MFC#2, right). at their most efficient operating point and at the same time forms the energy into a usable shape. II. MICROBIAL FUEL CELLLS A. Characterization of MFC MFCs use electrochemically active bacteeria (EAB) at the anode to catalyze the conversion of chemiccal energy stored in biodegradable substrate into electricity. In a typical twochamber system in Fig. 1, the anodde and cathode compartments are separated by an ion-exchhange membrane (IEM). Electrochemically active bacteria extract electrons from the donor and transfer them to the anode electrode. These electrons flow from the anode throough an external circuit to the cathode, where they reduce an electron acceptor such as oxygen or ferricyanide. Protons ttransfer from the anode to the cathode and participate in the electron accepter reduction [2]. In a single-chamber, air-caathode MFC, the IEM has been removed and a cathode structuure is constructed for open air diffusion [11]. Lab-scale MFC C reactors in each type are shown in Fig. 2. n the experiment. MFC#1 and Figure 4. Polarization curves of MFCs used in MFC#2 denote two-chamber ferricyanide cath hode MFC and single-chamber air cathode MFC, respectively. output increases. It can be electricaally modeled as a voltage source and a resistance as can be seen in Fig. 3. The MFC internal resistance Rint is the sum of system ohmic resistance, charge transfer resistance, and acttivation resistance. The internal voltage and resistance can vary v nonlinearly as MFC condition changes. Possible causees include instantaneous output power level, accumulated d extracted energy, and bacteria community and activity shifts, and environmental condition changes. An MFC can be treated as a weak voltagge source because the output voltage does not remain constaant as the current The potential difference between anode and cathode when the circuit is open is called open circuit voltage (OCV). The OCV of an air-cathode MFC is generally less than 0.8V, because the MFC anode potentiall is around -0.3 V (vs. Normal Hydrogen Electrode, NHE E), which is set by the respiratory enzymes of bacteria, and d the cathode potential is around +0.5 V when oxygen is used d as the terminal electron acceptor. The anode potential of the two-chamber MFCs me as that of singlewith ferricyanide cathodes is sam chamber MFCs. Typically the po otential of ferricyanide cathode is +0.6 V and it is determin ned by the redox potential of ferricynaide. The thermodynam mic limitation determines the voltage generally less than 0.8 V and the current output in the range of a few mA, which cannot be used directly in most real-world applications. Figure 3. Equivalent circuit of an MF FC. The typical method of charracterizing MFC power production is operating the MFC with w a series of external resistor Rext between the anode and cathode, and monitoring ntinuously to obtain a voltage across the resistor con polarization data. The interval to change the Rext is 5-30 minutes depending on MFC condittion. This could be done either manually or using a potentiiostat controller. Fig. 4 shows the polarization curves for the MFCs used in this o voltage is inversely paper. It can be seen that the MFC output proportional to the output current and a there exists a pair of voltage and current that delivers thee maximum power, which is defined as Maximum Power Poin nt (MPP). It can be shown that this operating point occurs when Rext equals Rint. TABLE I. Polarization test data of MFC#1 and estimated internal resistance Rint and internal voltage Vint at MPP. Measurement Estimation I [mA] vMFC [V] Po [mW] Rext [Ω] Rint [Ω] vint [V] 3.76 0.328 1.233 86.50 86.50 0.653 3.78 0.327 1.236 86.49 86.49 0.654 3.79 0.326 1.234 86.07 86.07 0.652 Although the OCV of an MFC can reach as high as 0.8 V, the actual voltage at MPP is much lower as can be seen in Fig. 4, which makes the direct use of MFC voltage output more difficult. B. Electricity Generation using MFC The power output of an MFC reactor varies according to the load current at a relatively long settling time. The output power Po, which can be measured across Rext, is inversely proportional to the total system resistance squared as follows. 2 Po = v int Rext (Rint + Rext )2 (1) where vint is the internal voltage of the MFC. The output power is also in proportion to the square of the internal voltage. The internal resistance Rint at the maximum power point can be estimated from the polarization curve using the fact that the maximum power is generated when Rint and Rext is same. The internal voltage vint at the maximum power point can also be estimated using Rint and measured current. Estimated values of Rint and vint from the MFC#1 polarization test data are shown in Table I. The power measurement on static Rext on MFC output can simulate the MFC power output to load, but the generated power is dissipated into heat instead of being utilized to support the load. Although the resistors make it straightforward to measure the MFC power generation, this scheme cannot be used for practical purpose. For efficient harvesting and usage of the MFC energy, a power conversion circuitry is indispensable to capture the electrical energy from MFCs and shape it into a usable form. DC/DC switching converters can be used and this can be defined as "active" harvesting compared to the power dissipation on resistance because the power converters actively extract energy from MFC by high frequency switching action. In order to make MFC technology more applicable, following challenges need to be addressed: • • An efficient real-time control scheme without using static resistance needs to be investigated to capture and provide the maximum usable energy from MFCs using power electronics converters. Flexible parallel control of harvesting systems for multiple MFCs need to be developed to address the difficulty of increasing power and energy of stacked MFCs. III. MFC ENERGY HARVESTING TECHNIQUES A. Current Techniques Research on MFC energy harvesting has been focused on three areas: • Increasing output voltage and power capacity. • Optimizing external resistance Rext to find and keep the operating point at the maximum power point. • Development of energy management system to utilize energy harvested from MFCs for devices such as remote sensors and transmitters. Some researchers tried to achieve a larger power from bigger MFC or multiple interconnected MFCs. However, the amount of energy generated by MFCs is not a linear function of their size, thus the power density will not remain constant with the increased electrode surface area. Stacks of MFCs are not operating as same as batteries, and the performance of the stack is limited by the worst performing unit [3], [12]. Popular maximum power point tracking (MPPT) techniques such as perturbation and observation (P&O) or gradient method for photovoltaic systems and hydrogen fuel cells have been introduced to MFC systems to find the optimal value of external resistance [13], [14]. Although it is important to identify the steady-state operating condition for maximum power output, the external resistance should be removed in order to practically utilize the generated energy. Energy management systems have been developed especially for ocean sediment MFCs to power remote sensors and wireless transmitters for naval applications [10, 15, 16]. Given the low energy output from sediment MFCs, intermittent operation rather than continuous harvesting has been suggested [10]. Although the reported systems have supported actual electrical loads, mostly they used passive harvesting techniques to harvest the energy from MFCs using capacitors or charge pumps. Those passive devices do not have proper control over the operating point of MFCs. Hence, the system performance can be improved if the MFC can be controlled to operate at the most efficient operating point. Moreover, an effective way to increase the power/energy capacity of MFC system has not been investigated. Other proposed approaches include multiunit optimization and detailed mathematical model based approach [17], [18]. However, because of the highly nonlinear nature of MFCs and too many parameters to consider, it would be impractical to mathematically model MFCs. Figure 5. Block diagram of proposed system. B. Proposed Harvesting System A harvesting system for maximizing energy recovery from multiple MFCs is proposed in this paper. The system consists of two layers of DC/DC converters. The first converter harvests the energy from an MFC and charges the storage capacitor, and the second layer converter boosts the voltage to an appropriate level for the connected load. Instead of a capacitor-based charge pump, an inductor-based converter has been utilized for more controllability on MFC operation. Compared the single-layer system, which uses a single converter for capturing energy and supporting the load, a double-layer system can achieve better performance by doing energy harvesting and load support in separate subsystems. Fig. 5 shows the block diagram of the overall system. The system operating voltage has been determined from the polarization curve. The proposed controller keeps the harvesting system operating in the vicinity of the maximum power point of the MFC. The hysteresis controller based energy harvesting controller controls the operating voltage by switching the harvesting converter MOSFET Q1. The proposed control scheme changes the power extraction frequency according to the MFC’s condition to maintain the MFC voltage at a pre-defined range and ensures enough recovery time of the MFC reactor. This scheme is effective especially when the MFC internal voltage drops significantly as output current increases. The second layer has a standard DC/DC boost converter that amplifies the output voltage to an appropriate level for powering the external electronic device(s). The operation of the energy harvester in the first layer consists of two modes, CHARGE and DISCHARGE, according to the energy flow on the inductor connected with the MFC. During the CHARGE mode, the harvesting converter's MOSFET Q1 is on, and the energy is extracted from the MFC and stored in the inductor L1. The MFC terminal voltage in this mode decreases due to the increasing current. Assuming negligible inductor resistance and constant internal voltage Vint, the instantaneous MFC output voltage and current in a CHARGE period can be given as follows, where IoC is the initial inductor current when MOSFET Q1 is closed. i (t ) = 1 (Vint − Rint i(t ))dt L1 ∫ ⎛ V = ⎜⎜ I oC − int Rint ⎝ R ⎞ − Lint1 t Vint ⎟⎟e + Rint ⎠ (2) (3) (4) vMFC (t ) = Vint − Rint i(t ) It can be seen that the current would be increasing to a level determined by the internal voltage and the resistance, which is the overall maximum current in the polarization curve in Fig. 4. However, the energy harvesting controller keeps the current at the level of the maximum power operating point. The external inductance and the internal resistance determine the changing rate of voltage and current, which determines the energy extracting frequency as well. During the DISCHARGE mode, the MOSFET switch Q1 is off, and the energy stored in the inductor L1 is discharged to the storage capacitor C1. The MFC voltage increases in this mode as current decreases. The MFC output current and the storage capacitor voltage in the DISCHARGE mode can be given as follows. i (t ) = 1 L1 ∫ (V int − Rint i (t ) − v o1 )dt R int ⎛ V − vo1 (t ) ⎞ − L1 t Vint − v o1 (t ) ⎟⎟e , + = ⎜⎜ I oD − int Rint Rint ⎝ ⎠ v o1 (t ) = 1 C1 ∫ (i(t ) − I )dt L (5) (6) (7) VthL = Vcc × R2 // R3 R1 + (R2 // R3 ) (9) The duty ratio and switching frequency can be controlled by the threshold voltage band, VthH – VthL, and they will also vary depending on the generating capacity and recovery time of the operating MFC. The schematic of proposed hysteresis harvesting controller is shown in Fig. 6. Simulated MFC output voltage and current using the proposed hysteresis controller can be seen in Fig. 7. Figure 6. Schematic of proposed harvesting converter controller In Eqs. (6) and (7), IoD and IL is the inductor current when MOSFET Q1 is open and the load current drawn from the storage capacitor C1, respectively. The MFC output voltage is same as (4). The instantaneous current is a function of internal voltage, internal resistance, external inductance, external capacitance, and load current in DISCHARGE mode. The hysteresis controller turns off the MOSFET Q1 automatically when the MFC voltage reaches lower threshold in the CHARGE mode and turns it back on when the MFC voltage gets to the upper threshold in DISCHARGE mode. Because the MOSFET is on when the output voltage is lower than the CHARGE threshold VthH and off when it is higher than the DISCHARGE threshold VthL, a logic inverter is added to the comparator-based hysteresis controller. The threshold voltage toggles as operating mode changes. The upper and lower voltage thresholds can be determined as follows and easily changed using potentiometers. VthH R2 = Vcc × R2 + (R1 // R3 ) Figure 7. Simulation: MFC#1 operation at measured MPP. (8) C. Parallel Operation Parallel operation will be a viable option to increase the capacity of MFC-based power system because the direct series/parallel connection has difficulties in increasing power and energy capacity. Although sediment MFCs that cannot be connected in series because all the electrodes have to be placed in the same water solution, MFC applications such as wastewater treatment systems can be readily operated in parallel. However, harvesting system to provide flexible control over such designs has not been investigated. It is straightforward to operate multiple MFCs in parallel using the proposed controller. As can be seen in the block diagram in Fig. 8, only the harvesting controllers will be added for multiple MFCs in parallel operation. Each harvesting controller operates independently with separate voltage thresholds based on its MFC’s maximum power operating point. The harvesting controllers share a storage capacitor to put harvested energy into and a single boost converter generates increased voltage to support the load. The output of the second layer boost converter can be given as vo 2 = vo1 × 1 . 1 − DQ 2 (10) Figure 8. Parallel operation with multiple MFCs and a boost converter. Figs. 10 and 11 shows the typical operation cycles of the harvesting system with MFC#1 and MFC#2, respectively. The chambers of MFC#1 have a working volume of 48 mL, and ferricyanide was used as the electron acceptor to provide stable cathode potential. MFC#2 is a single chamber reactor with a working volume 250 mL and an air-cathode. The MPPs for the MFCs have been obtained from the polarization test, which are around 325mV and 300mV for MFC#1 and MFC#2, respectively. The hysteresis band is set as 20mV. It can be seen that each harvesting controller operates connected MFC at its MPP independently and extract continuously the power that is measured by the polarization test. However, it should be noted that the operating condition and generation capacity depends on microbial activity and tends to vary, especially in small reactors. Figure 9. Experimental setup for two MFCs. IV. EXPERIMENTAL RESULT For the experiment, a prototype controller has been implemented for two different MFCs in Fig. 2. It consists of two harvesting controllers and one boost converter. The controller uses ZETEX MOSFET ZXM61N02F and National Semiconductor’s comparator LMC7215 for low conduction resistance and low power consumption, respectively. Also 40V 0.52A schottky diode ZLLS400 has been chosen. For the harvesting converter, various inductance values have been tested. A 14mH inductor has been used for the plots in this paper. A smaller inductance makes the switching frequency faster and voltage/current ripple smaller. Two 2.5V 1F super capacitors have been used in parallel for energy storage. The power for the control circuitry has been supplied by an external power source in this experiment. The developed control system is shown in Fig. 9. Figure 10. The charging/discharging cycles from a two-chamber ferricyanide-cathode MFC#1 controlled by the harvesting system From top, switch state, MFC output voltage, MFC output current. A carrier wave and gating signal generator circuitry for the second layer boost converter is shown in Fig. 12. The duty ratio of this boost converter is set manually using potentiometer R6 in this paper. Fig. 13 shows the carrier wave and output of the boost converter. The output was boosted to 3.3V at 1kΩ load using the energy supplied by two MFCs. The results of the experiments have confirmed the following. • MFC operating point control capability for energy capture using hysteresis controller. • Power mode generation by hysteresis controller based harvesters. • Parallel operation of multiple MFC reactors. • Output voltage boost to a practically usable level using the harvested energy. Figure 11. The charging/discharging cycles from a single-chamber air-cathode MFC#2 controlled by the harvesting system. From top, switch state, MFC output voltage, MFC output current. [3] [4] [5] [6] Figure 12. Carrier wave and reference signal generator for second layer DC/DC converter. [7] [8] [9] [10] [11] [12] Figure 13. Secondary layer DC/DC operation. From top, carrier wave, switching state, input and output voltage of the converter. V. [13] CONCLUSION AND FUTURE WORK In this paper, an efficient MFC energy harvester using DC/DC converters has been presented. 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