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This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE TRANSACTIONS ON ENERGY CONVERSION 1 Hysteresis-Controller-Based Energy Harvesting Scheme for Microbial Fuel Cells With Parallel Operation Capability Jae-Do Park, Member, IEEE, and Zhiyong Ren Abstract—Microbial fuel cell (MFC) is an emerging technology larger MFCs or simply connecting them in series or in parallel, for sustainable energy production. An MFC employs indigenous because of the nonlinear nature of MFCs –. microorganisms as biocatalysts and can theoretically convert any The power density from MFCs has increased by orders of biodegradable substrate into electricity, making the technology a viable solution for sustainable waste treatment or autonomous magnitude in less than a decade of research. The reported maxi- power supply. However, the electric energy currently generated mum power density from lab scale air-cathode MFCs increased from MFCs is not directly usable due to the low voltage and cur- from less than 1 mW/m2 to 6.9 W/m2 –. This improvement rent output. Moreover, the output power can ﬂuctuate signiﬁcantly can mainly be attributed to relieving physical and chemical con- according to the operating conditions, which makes stable harvest straints through electrode material and reactor architecture im- of energy difﬁcult. This paper presents an MFC energy harvesting scheme using a hysteresis controller and two layers of DC/DC con- provement, as well as optimization of operational conditions , verters. The proposed energy harvesting system can capture the . However, the reported power output from many MFC stud- energy from multiple MFCs at individually controlled operating ies is based on the power dissipated on a static external resistance point and at the same time form the energy into a usable shape. instead of the actual attainable power in a usable form –, Index Terms—DC/DC converter, energy harvesting, microbial , –, which indicates one crucial missing part before the fuel cell (MFC). 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 I. INTRODUCTION MFC and convert it into applicable voltage and current levels. HE ﬁnite resource of fossil fuels and environmental pol- T lution derived from their use are driving the search for renewable and clean energy alternatives. This replacement of However, a control scheme that actively harvests energy at an optimal operating point especially from multiple MFCs has not been researched extensively. fossil fuels will require the utilization of many energy sources In this paper, an efﬁcient MFC energy harvesting system using suited to meet different end uses. Microbial fuel cell (MFC) two layers of dc/dc converters is presented . The proposed technology has been intensively researched in recent years as system can capture the energy from multiple MFCs at individu- a novel technology, because it offers a solution for environ- ally controlled operating points and at the same time forms the mentally sustainable energy by treating waste and recovering energy into a usable shape. electricity simultaneously. MFCs use active bacteria to gener- ate electrical energy from the environment electrochemically. MFCs offer a simple, direct method for converting environ- II. MFCS mentally available biomass into electricity and are very suit- A. Characterization of MFC able for clean, distributed, and renewable energy source, for MFCs use electrochemically active bacteria at the anode to example, powering the remote sensors , . However, like catalyze the conversion of chemical energy stored in biodegrad- other microenergy sources such as ambient heat, vibrations, and able substrate into electricity. In a typical two-chamber system lights, MFC reactors generate very low power and energy due to in Fig. 1, the anode and cathode compartments separated by thermodynamic limitations and it has been reported that larger an ion-exchange membrane. Electrochemically active anaero- power production cannot be easily achieved by just building bic or facultative bacteria extracts electrons from the electron donor and transfers them to the anode electrode. These elec- trons ﬂow from the anode through an external circuit to the cathode, where they reduce an electron acceptor such as oxy- Manuscript received November 5, 2011; revised January 26, 2012; accepted gen or ferricyanide. Protons are exchanged from the anode to March 19, 2012. Paper no. TEC-00591-2011. the cathode and participate in the oxidant reduction . In a J.-D. Park is with the Department of Electrical Engineering, University of Colorado Denver, Denver, CO 80204 USA (e-mail: firstname.lastname@example.org). single-chamber air-cathode MFC, the ion exchange membrane Z. Ren is with the Department of Civil Engineering, University of Colorado has been removed and constructs a cathode structure for open Denver, Denver, CO 80204 USA (e-mail: Jason.Ren@ucdenver.edu). air diffusion . Lab-scale MFC reactors are shown in Fig. 2. Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. An MFC can be treated as a weak voltage source because the Digital Object Identiﬁer 10.1109/TEC.2012.2196044 output voltage does not remain constant as the current output 0885-8969/$31.00 © 2012 IEEE This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 2 IEEE TRANSACTIONS ON ENERGY CONVERSION Fig. 1. Schematic of a two chamber MFC using O2 as electron acceptor. Fig. 4. Polarization curves of MFCs used in the experiment. MFC#1 and MFC#2 denote two-chamber ferricyanide cathode MFC and single-chamber air cathode MFC, respectively. Points A and B are the operating points determined by hysteresis controller. Double arrows denote MFC voltage, current, and power in harvesting operation band. is used as the terminal electron acceptor. The anode potential of the two-chamber MFCs with ferricyanide cathodes is same as that of single-chamber MFCs. Typically, the working potential of ferricyanide cathode is +0.6 V and it is determined by the Fig. 2. Lab scale two-chamber MFC using ferricyanide as electron acceptor redox potential of ferricynaide. The thermodynamic limitation (MFC#1, left) and single-chamber MFC using air as electron acceptor (MFC#2, determines the voltage generally less than 0.8 V and the current right). output in the range of a few milliamperes, which cannot be used directly in most real-world applications . The typical method for static characterization of MFC power production is operating the MFC with a series of external resistor Rext between the anode and cathode, and monitoring voltage across the resistor continuously to obtain a polarization data. The interval to change the Rext is 5–30 min depending on MFC condition. This could be done either manually or using a potentiostat controller. The MFC reactors under test have shown a clear activation characteristics but the voltage has not dropped much in concentration region. It can be seen that the MFC output voltage in the ohmic region is practically inversely proportional Fig. 3. Equivalent circuit of an MFC. to the output current. The maximum power point (MPP) can be deﬁned by respective voltage and current, where the maximum increases. It can be electrically modeled as a voltage source and a power is delivered by the MFC system. It can be shown that resistance as can be seen in Fig. 3. The MFC internal resistance this operating point occurs when Rext equals Rint , . Rint is the sum of system ohmic resistance, charge transfer Although the OCV of an MFC can reach as high as 0.8 V, resistance, and activation resistance. It has been reported that the the actual voltage at MPP is much lower as can be seen in internal resistance is reasonably constant in the ohmic region for Fig. 4, which makes the direct use of MFC voltage output more given reactor parameters , . The thermodynamic voltage difﬁcult. Fig. 4 shows the polarization curves for the MFCs used vint can vary nonlinearly as MFC condition changes. Possible for energy harvesting experiment in this paper. causes include instantaneous output power level, accumulated The dynamic response of MFC reactor has also been tested extracted energy, bacteria community and activity shifts, and with MFC#1 reactor. The output terminal of the reactor has been environmental condition changes. short-circuited by a MOSFET switch with a constant 50% duty The potential difference between anode and cathode when the ratio. The MFC generates its maximum current that the reactor circuit is open is called open-circuit voltage (OCV). The OCV condition permits. It turned out that the instantaneous voltage of an air-cathode MFC is generally less than 0.8 V, because the and current response is quite fast, as can be seen in Fig. 5. The MFC anode potential is around −0.3 V (versus normal hydrogen current output has about 2-μs delay, 4-μs rising and falling time electrode), which is set by the respiratory enzymes of bacteria, for step-wise energy extraction and recovery, and 3 × 10−6 time and the working cathode potential is around +0.5 V when oxygen constant τi for steady state. The test has been performed with This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. PARK AND REN: HYSTERESIS-CONTROLLER-BASED ENERGY HARVESTING SCHEME FOR MICROBIAL FUEL CELLS 3 TABLE I POLARIZATION TEST DATA OF MFC#1 AND ESTIMATED INTERNAL RESISTANCE R int AT THERMODYNAMIC VOLTAGE v int AROUND MPP recovered after energy extraction decreases as microbial system continuously generates power, which in turn reduces the output current. The time constant for this voltage decrease is consider- ably slower compared to the current dynamics. For continuous power extraction, the average thermodynamic voltage of MFC can be given as follows, where Vint0 is the initial thermodynamic Fig. 5. MFC#1 output voltage and current to step-wise energy extraction with voltage and τv (t) is a time constant for decaying voltage: 50% duty ratio and 12.5-kHz switching. From top, MOSFET switch state (0: OFF, 1: ON), MFC voltage, and MFC current. vint (t) = Vint0 (e−t/τ v (t) ). (2) The time constant τv is a function of biochemical factors and electrical load as well as the thermodynamic voltage vint . B. Electricity Generation Using MFC The instantaneous power output of an MFC reactor Po , which can be measured across Rext , is inversely proportional to the total system resistance squared as follows: vint (t)2 Rext Po (t) = (3) (Rint + Rext )2 where vint is the thermodynamic voltage of the MFC. The output power is also in proportion to the square of the thermodynamic voltage, which is slowly varying according to load and reactor conditions. The internal resistance Rint at the MPP can be esti- mated from the polarization curve using the fact that for a given voltage, the maximum power is generated when Rint and Rext Fig. 6. MFC#1 output voltage and current to step-wise energy extraction with 50% duty ratio with 10-kHz switching. From top, MFC voltage and MFC is same. The thermodynamic voltage vint at the MPP can also current. be estimated using Rint and measured current Io . The internal resistance in other operating points can be roughly estimated using the slope on the polarization curve or measured accu- different switching frequencies and slightly different reactor rately using electrochemical analysis, such as electrochemical conditions, and results were similar. Because the time constant impedance spectroscopy , . Estimated values of Rint and is small enough, the output dynamics of MFC itself is negligible vint from the MFC#1 polarization test data are shown in Table I. so that it can be modeled as resistive circuit. The instantaneous The power measurement on static Rext on MFC output can MFC output current in generation mode can be given as follows: simulate the MFC power output to a load, but the generated iM FC (t) = IM FC (t)(1 − e−t/τ i ) ≈ IM FC (t) (1) power is dissipated as heat instead of being utilized to support the load. Although the resistors make it straightforward to measure where IM FC is the current magnitude determined by the instanta- the MFC’s power generation, this scheme cannot be used for neous thermodynamic voltage and internal/external impedance. practical purpose. For efﬁcient harvesting and usage of the MFC energy, a power conversion circuitry is indispensable to capture When the MOSFET is OFF, the MFC terminals are open the electrical energy from MFCs and shape it into a usable form. and the voltage across terminals represents the OCV, which is DC/DC switching converters can be used and this can be deﬁned the thermodynamic voltage. It can be seen in Fig. 5 that the as “active” harvesting compared to the power dissipation on voltage recovers instantaneously, but the amplitude is slightly resistance because the power converters actively extract energy smaller than before extraction. Fig. 6 shows the MFC voltage from MFC by high-frequency switching action. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 4 IEEE TRANSACTIONS ON ENERGY CONVERSION Fig. 7. Block diagram of the proposed MFC energy harvesting system. In order to make the MFC technology more applicable, fol- the external resistance should be removed in order to practically lowing challenges need to be addressed: utilize the generated energy. 1) an efﬁcient real-time control scheme without using static Energy management systems have been developed especially resistance to capture and provide the usable energy from for ocean sediment MFCs to power remote sensors and wire- MFCs using power electronics converters; less transmitters for naval applications , , . Given 2) ﬂexible parallel operation of harvesting systems for mul- the low energy output from sediment MFCs, intermittent opera- tiple MFCs to overcome the difﬁculty of increasing power tion rather than continuous harvesting has been suggested . and energy with stacked MFCs; Although the reported systems have supported actual electrical 3) a real-time controller to maintain the operation of MFC at loads, mostly they used passive harvesting techniques to har- an optimal operating point. vest the energy from MFCs using capacitors or charge pumps. In this paper, the ﬁrst two challenges have been addressed. Those passive devices do not have proper control over the op- The proposed scheme can harvest energy from multiple MFCs erating point of MFCs. Hence, the system performance can be at individually controlled operating point (e.g., MPP) and shape improved if MFCs can be controlled to operate at the most ef- the harvesting energy to a usable form. The MPP tracking ca- ﬁcient operating point. Moreover, an effective way to increase pability has not been included in this paper, but can be readily the power/energy capacity of MFC system has not been inves- integrated into the proposed harvesting controller . tigated. Other proposed approaches include multiunit optimization III. MFC ENERGY HARVESTING TECHNIQUES and detailed mathematical model-based approach , . A. Current Techniques Research on MFC energy harvesting has been focused on the B. Proposed Harvesting System following areas: A harvesting system for maximizing energy recovery from 1) increasing output voltage and power capacity; multiple MFCs is proposed in this paper. The system consists 2) optimizing external resistance Rext to ﬁnd and keep the of two layers of DC/DC converters. The ﬁrst converter harvests operating point at the MPP; the energy from an MFC and charges the storage capacitor, and 3) development of energy management system to utilize en- the second layer converter boosts the voltage to an appropriate ergy harvested from MFCs for devices such as remote level for the connected load. Instead of a capacitor-based charge sensors and transmitters. pump that is not designed for linear voltage regulation , Some researchers tried to achieve a larger power from bigger , an inductor-based converter has been utilized for more MFC or multiple interconnected MFCs. However, the amount controllability on MFC operation. Compared to a single-layer of energy generated by MFCs is not a linear function of their system, which uses a single converter for capturing energy and size; thus, the power density will not remain constant with the supporting the load, this double-layer conﬁguration can achieve increased electrode surface area. Stacks of MFCs are not op- better performance by doing energy harvesting and load support erating as same as batteries, and the performance of the stack in separate subsystems. Fig. 7 shows the block diagram of the is limited by the worst performing unit because of the voltage overall system. reversal , . The MFC operation voltage can be determined by the polar- Popular maximum power point tracking (MPPT) techniques ization curve assuming reasonably stable condition. Once the such as perturbation and observation or gradient method for operating point is determined, the proposed real-time operating photovoltaic systems and hydrogen fuel cells have been intro- point controller keeps the MFC output voltage in the determined duced to MFC systems to ﬁnd the optimal value of external operating voltage band. The MFC thermodynamic voltage that resistance , . Although it is important to identify the has been explained in Section II-A determines the dynamics of steady-state operating condition for maximum power output, MFC output voltage and current, i.e., rate of change, with the This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. PARK AND REN: HYSTERESIS-CONTROLLER-BASED ENERGY HARVESTING SCHEME FOR MICROBIAL FUEL CELLS 5 inductance in the power interface circuit. And, these dv/dt and point A. The MOSFET is ON and the MFC voltage decreases di/dt determines the switching period and duty ratio. as current increases. As the MFC voltage decreases to the oper- The hysteresis-controller-based energy harvesting system ating point B, MOSFET turns OFF and the inductor energy is controls the operating voltage by switching the harvesting con- discharged to capacitor. In this DISCHARGE mode, the voltage verter MOSFET Q1 . Unlike the standard boost converter that increases to operating point A as current decrease. It can be controls the output voltage, the proposed control scheme con- seen that the voltage is conﬁned between points A and B and trols the power extraction at input side, i.e., the Q1 switching the MFC can operate around the MPP as shown with double frequency and duty ratio, according to the MFC’s condition to arrows. maintain the MFC voltage at a predeﬁned range and ensures During the CHARGE mode, the harvesting converter’s MOS- enough recovery time of the MFC reactor. This scheme is ef- FET Q1 is ON, and the energy is extracted from the MFC and fective especially when the MFC thermodynamic voltage drops stored in the inductor L1 . The MFC terminal voltage vM FC signiﬁcantly as output current increases. The second layer has a in this mode decreases due to the increasing current. Assum- standard DC/DC boost converter that ampliﬁes the output volt- ing negligible inductor resistance and constant thermodynamic age to an appropriate level for powering the external electronic voltage Vint , the instantaneous MFC output voltage and current device(s). in a CHARGE period can be given as follows, where IoC is the The inductance can be determined with following equation initial inductor current when MOSFET Q1 is closed: , assuming stiff response of MFC generation and edge of vM FC (t) = vint (t) − Rint iM FC (t) (5) continuous conduction: VM FC Ts D 1 L1 = (4) iM FC (t) = vM FC (t)dt (6) 2IM FC L1 where VM FC is the average MFC output voltage in the hysteresis Vint R int Vint = IoC − e− L1 t + . (7) voltage band, IM FC is the average output current, and Ts and Rint Rint D are the average switching period and duty ratio, respectively. It can be seen that the current would be increasing to a level However, it should be noted that the switching period will vary determined by the thermodynamic voltage and the resistance, as the MFC condition changes as well as the duty ratio. For which is the maximum generatable current in the polarization example, the harvesting system does not switch to CHARGE curve in Fig. 4. However, the energy harvesting controller keeps mode from DISCHARGE if the MFC voltage does not recover the current at the level of the speciﬁed operating point. The to the upper threshold voltage due to reduced substrate concen- external inductance and the internal resistance determine the tration or microbial activity. This will decrease the switching changing rate of voltage and current, which in turn determines frequency and duty ratio. The diode is reverse biased and the the energy extracting frequency. converter operates in discontinuous conduction mode in this During the DISCHARGE mode, the MOSFET switch Q1 is case. Fast recovery by strong microbial activity will increase OFF, and the energy stored in the external inductor L1 is dis- switching frequency and duty ratio. The mode changes from charged to the storage capacitor C1 . The MFC voltage increases CHARGE to DISCHARGE similarly. in this mode as current decreases. The MFC output current and The harvesting controller in the ﬁrst layer captures the energy the storage capacitor voltage in the DISCHARGE mode can be from MFC in power mode; in other words, it injects the current to given as follows: the reservoir capacitor C1 ; hence the C1 voltage is not controlled and it is a function of harvested power and load power. Larger 1 iM FC (t) = (vint (t) − Rint i(t) − vo1 (t))dt (8) capacitance can be used for longer power supply for the energy L1 storage capacitor, but it takes longer to charge it up. On the other Vint − vo1 (t) R int Vint − vo1 (t) hand, smaller capacitors are charged faster, but they cannot stand = IoD − e− L1 t + Rint Rint longer. The voltage on C1 affects the efﬁciency of ﬁrst and second layer converters because too high boost ratio reduces (9) the boost conversion efﬁciency . Hence, the selection of 1 capacitor C1 should be application dependent and the tradeoff vo1 (t) = (i1 (t) − i2 (t))dt. (10) C1 between charge time, support time, and converter efﬁciency needs to be considered. In (8) and (9), IoD and i2 is the inductor current when MOS- The design of the secondary boost controller can follow the FET Q1 is open and the load current drawn from the storage standard design procedure . capacitor C1 by the second layer voltage boost converter, respec- tively. The MFC output voltage in the DISCHARGE mode can C. Operation of the Proposed Harvesting System be given as same as (6). The instantaneous current is a function The operation of the energy harvester in the ﬁrst layer con- of thermodynamic voltage, internal resistance, external induc- sists of two modes, CHARGE and DISCHARGE, according to tance, external capacitance, and load current in DISCHARGE the energy ﬂow on the inductor connected to the MFC. The op- mode. eration can be explained with the polarization curve in Fig. 4. The hysteresis controller turns OFF the MOSFET Q1 auto- The CHARGE mode starts when the MFC reaches the operating matically when the MFC voltage reaches lower threshold in the This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 6 IEEE TRANSACTIONS ON ENERGY CONVERSION D. Efﬁciency Calculation The efﬁciency of the energy harvester can be given as Po Pin − Ploss η= = . (13) Pin Pin From Fig. 7, the power loss in CHARGE and DISCHARGE mode are as follows: TCHG PlossC = (PlossL + PlossQ1 ) × (14) TSW TDISCHG PlossD = (PlossL + PlossD1 + PlossCap ) × (15) Fig. 8. Schematic of the proposed harvesting converter controller. TSW where PlossC and PlossD are loss in CHARGE and DISCHARGE modes, respectively. And PlossL , PlossQ1 , PlossD1 , and PlossCap are loss on inductors, Q1 and D1 . TCHG , TDISCHG , and TSW are the time periods for CHARGE, DISCHARGE, and a switching cycle, respectively. Efﬁciency can be calculated using this loss model: VM PP IM PP − PlossC − PlossD − Punm o deled η= (16) VM PP IM PP where VM PP and IM PP are average MFC voltage and current at MPP, respectively, and Punm o deled is a unmodeled miscella- neous losses including switching loss. E. Parallel Operation Parallel operation will be a viable option to increase the capacity of MFC-based power system because the direct se- ries/parallel connection has difﬁculties in increasing power and energy capacity. Although there are number of MFC applica- Fig. 9. Simulation: MFC#1 operation at measured MPP. tions that can be readily operated in parallel for larger power and energy capacity, harvesting systems to provide ﬂexible control over such systems have not been investigated. In this paper, a CHARGE mode and turns it back ON when the MFC voltage multiple-input converter topology is used. The multiple-input reaches the upper threshold in DISCHARGE mode. Because converters have recently been used for applications such as hy- the MOSFET is ON when the output voltage is lower than the brid vehicles, photovoltaics, and wind power systems –, CHARGE threshold VthH and OFF when it is higher than the but it has not been applied to MFC energy harvesting systems. DISCHARGE threshold VthL , a logic inverter is added to the The advantage of the proposed scheme is apparent with comparator-based hysteresis controller. The threshold voltage multiple-reactor operation. The number of converters will be toggles as operating mode changes. The upper and lower volt- N + 1 because N reactors can share one boost converter in the age thresholds can be determined as follows and easily changed second layer. The proposed scheme can achieve better harvest- using potentiometers: ing efﬁciency because it actively extracts the energy contin- uously at individually controlled operating points, while it is R2 difﬁcult to continuously harvest the energy and maintain output VthH = Vcc × (11) voltage at the same time with single converter conﬁguration. R2 + (R1 //R3 ) Typically, a capacitor or a charge pump is used for single con- R2 //R3 verter system, but the operating point changes as capacitor volt- VthL = Vcc × . (12) R1 + (R2 //R3 ) age changes because they just passively takes the power from MFC, which leads to a low harvesting efﬁciency. The duty ratio and switching frequency can be controlled by It is straightforward to operate multiple MFCs in parallel us- the hysteresis voltage band, VthH − VthL , and they will vary ing the proposed controller. As can be seen in the block diagram depending on the generating capacity and recovery time of the in Fig. 10, only the harvesting controllers will be added for operating MFC. The schematic of the proposed hysteresis har- multiple MFCs in parallel operation. Each harvesting controller vesting controller is shown in Fig. 8. Simulated MFC output operates independently with separate voltage thresholds based voltage and current using the proposed hysteresis controller can on its MFC’s maximum power operating point. The harvesting be seen in Fig. 9. controllers share a storage capacitor to put captured energy into This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. PARK AND REN: HYSTERESIS-CONTROLLER-BASED ENERGY HARVESTING SCHEME FOR MICROBIAL FUEL CELLS 7 Fig. 10. Parallel operation with multiple MFCs and a boost converter. and a single boost converter generates increased voltage to sup- Fig. 11. Energy harvesting experiment setup for two MFCs. The experimen- port the load. The output of the second layer boost converter can tal harvesting system consists of two hysteresis controller controlled energy harvesters and a voltage boost converter. be given as 1 vo2 = vo1 × (17) 1 − DQ 2 where DQ 2 is the duty ratio of MOSFET Q2 . IV. EXPERIMENTAL RESULT A. Energy Harvesting From Multiple MFCs For the energy harvesting experiment, a prototype controller has been implemented for two different MFCs in Fig. 2. It con- sists of two harvesting controllers and one boost converter. The controller uses Vishay MOSFET SI3460 and National Semi- conductor’s comparator LMC7215 for low conduction resis- tance and low power consumption, respectively. Also, a Schot- tky diode 1N5711 has been used. For the energy harvesting converter, a 14-mH inductor CST206-1A has been used for the plots in this paper. The inductance is initially determined using (4) with 3 kHz of switching and duty ratio 0.5 to match with the off-the-shelf Fig. 12. Charging/discharging cycles from a two-chamber ferricyanide- cathode MFC#1 controlled by the harvesting system From top, switch state, inductor available, but it should be noted that system opera- MFC output voltage, MFC output current. tion and performance can be changing from calculation because the operating parameters of MFC will vary according to factors such as bacteria community and activity shifts, and environmen- connected MFC at its MPP independently and extract the power tal condition changes. Although a smaller inductance makes the that is measured by the polarization test. However, the operating switching frequency faster and voltage/current ripple smaller, it condition and generation capacity depends on microbial activity could require too small hysteresis band and may not be able to and tends to vary especially in small reactors. The relation be- trigger the switching action because of the small energy extrac- tween MFC bacteria activity and electrical energy extraction has tion. Two 2.5-V 1-F supercapacitors have been used in parallel been researched and MPPT control can be one of the solutions to for energy storage. The developed MFC energy harvesting sys- maintain the operating condition in changing environment . tem is shown in Fig. 11. Fig. 14 shows the carrier wave and output of the second Figs. 12 and 13 show the typical operation cycles of the layer boost converter. A carrier wave and gating signal generator harvesting system with MFC#1 and MFC#2, respectively. The circuitry for the second layer boost converter is shown in Fig. 15. chambers of MFC#1 have a working volume of 48 mL, and The duty ratio of this boost converter is set manually using ferricyanide was used as the electron acceptor to provide sta- potentiometer R6 in this experiment. The output was boosted to ble cathode potential. MFC#2 is a single-chamber reactor with 3.3 V at 1-kΩ load using the energy supplied by two MFCs. A a working volume 250 mL and an air-cathode. For this ex- PI controller can be readily added for constant output voltage periment, the MPP for each MFC has been obtained from the control. If the load demands more than supply power, the storage polarization test, which are around 325 and 300 mV for MFC#1 capacitor voltage decreases and in turn the duty ratio of the and MFC#2, respectively. The hysteresis voltage band is set as second layer boost converter will increase to maintain a constant 20 mV. It can be seen that each harvesting controller operates output voltage. The operation of the second layer converter can This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 8 IEEE TRANSACTIONS ON ENERGY CONVERSION Fig. 15. Carrier wave and reference signal generator for second layer DC/DC converter. TABLE II EFFICIENCY CALCULATION DATA AND LOSS BREAKDOWN Fig. 13. 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. Apparently, the diode drop is too high for such a low-voltage, low-power application. However, the synchronous rectiﬁcation technique can improve the efﬁciency by replacing the diode with a low on-loss MOSFET, although the control circuitry has to be carefully designed for the issues such as reverse power ﬂow pre- vention and ﬂoating gate drive –. Also selecting lower Fig. 14. Secondary layer DC/DC operation. From top, carrier wave, switching loss components, e.g., ones with lower RDS(on) , dc resistance, state, input and output voltage of the converter. and equivalent series resistance, and implementing circuitry in low-power application-speciﬁed integrated circuit (ASIC) , be paused in this case because the efﬁciency drops signiﬁcantly  would be feasible ways to improve efﬁciency. with high boost ratio. V. DISCUSSION B. Efﬁciency The result of the experiments in this paper have conﬁrmed the following: The energy harvesting controller has been tested for efﬁciency 1) MFC operating point control capability for energy capture by charging a 3-F capacitor from 0 to 500 mV. It took 36 min using a simple hysteresis controller; with one MFC#1 type two-chamber reactor. The average switch- 2) parallel operation of multiple MFC reactors in power ing frequency and duty ratio of the switch Q1 depends on the mode; MFC’s generation response and voltage recovery in conjunction 3) output voltage boost to a practically usable level using the with the external power interface. The measured efﬁciency of harvested energy. the harvesting system is 45.21% and breakdown of the loss cal- culated using the data from the polarization test and datasheet A. Controllability of the parts used in the control system are shown in Table II. It can be seen that the power loss due to diode forward drop The control approach proposed in this paper enables the op- is dominant (93%) in overall loss and causes low efﬁciency. erating point control. MFCs lost more than 50% of produced This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. PARK AND REN: HYSTERESIS-CONTROLLER-BASED ENERGY HARVESTING SCHEME FOR MICROBIAL FUEL CELLS 9 power across the internal resistance if the operating voltage is  B. Logan and J. Regan, “Electricity-producing bacterial communities in higher MPP voltage . Other energy extracting techniques microbial fuel cells,” Trends Microbiol., vol. 14, no. 12, pp. 512–518, Dec. 2006. such as resistors or charge pumps cannot sustain the operating  P. Aelterman, R. Korneel, H. Pham, N. Boon, and W. Verstraete, “Con- point at a desirable voltage level, even though the persistant tinuous electricity generation at high voltages and currents using stacked operation at the MPP can deliver stable performance . The microbial fuel cells,” Environ. Sci. Technol., vol. 40, no. 10, pp. 3388– 3394, 2006. proposed control approach enables ﬂexible energy capture based  A. Dewan, H. Beyenal, and Z. Lewandowski, “Scaling up microbial fuel on the MFC condition at a controlled operating point without cells,” Environ. 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Rabaey, “Microbial fuel cells: method- ology and technology,” Environ. Sci. Technol., vol. 40, no. 17, pp. 5181– 5192, 2006. C. Self-Sustainability  L. Woodward, M. Perrier, and B. Srinivasan, “Comparison of real-time methods for maximizing power output in microbial fuel cells,” Amer. Inst. One of the major challenges with use of MFCs is that MFCs Chem. Eng. J., vol. 56, no. 10, pp. 2742–2750, Oct. 2010. produce very low voltage, current, power, and energy to directly  R. Pinto, B. Srinivasan, S. Guiot, and B. Tartakovsky, “The effect of real- time external resistance optimization on microbial fuel cell performance,” support load and its control circuits. It is especially true when Water Res., vol. 45, pp. 1571–1578, 2011. it comes to lab-scale reactors. The implementation of minimal  G. Premier, J. Kim, I. Michie, R. Dinsdale, and A. 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Deukhyoun, and H. Beyenal, “Batteryless, wireless sensor powered by a sediment microbial fuel cell,” Environ. Sci. if the MFC scale is large enough such as the one in waste Technol., vol. 42, no. 22, pp. 8591–8596, 2008. water treatment plants, so that the harvesting system can be  C. Donovan, A. Dewan, H. Peng, D. Heo, and H. Beyenal, “Power manage- more self-efﬁcient and provide more energy. Implementation of ment system for a 2.5 W remote sensor powered by a sediment microbial fuel cell,” J. Power Sources, vol. 196, pp. 1171–1177, 2011. low-power-consuming customized control circuits in an ASIC o  B. Logan, P. Aelterman, B. Hamelers, R. Rozendal, U. Schr¨ eder, J. Keller, chip ,  would be a valid approach for more energy S. Freguiac, W. Verstraete, and K. Rabaey, “Microbial fuel cells: Method- efﬁcient system. Also, a self-start circuitry is required for black ology and technology,” Environ. Sci. Technol., vol. 40, no. 17, pp. 5181– 5192, 2006. start capability.  J. Park and Z. 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Power tem consists of multiple harvesting converters for each MFC and Electron., vol. 26, no. 1, pp. 176–181, Jan. 2011.  J. Starzyk, Y.-W. Jan, and F. Qiu, “A dc-dc charge pump design based a single voltage boost converter. The proposed control scheme on voltage doublers,” IEEE Trans. Circuits Syst. I: Fundamental Theory has been validated experimentally and a successful result has Appl., vol. 48, no. 3, pp. 350–359, Mar. 2001. been shown.  Maxim Integrated Products. (2009) AN725: DC-DC conversion without inductors. [Online]. Available: http://www.maxim-ic.com/app- notes/index.mvp/id/725. REFERENCES  N. Mohan, T. Undeland, and W. Robbins, Power Electronics: Converters, Applications, and Design. New York: Wiley, 2002.  Z. Ren, T. Ward, and J. Regan, “Electricity production from cellulose in a  L. Solero, A. Lidozzi, and J. Pomilio, “Design of multiple-input power microbial fuel cell using a deﬁned binary culture,” Environ. Sci. Technol., converter for hybrid vehicles,” IEEE Trans. Power Electron., vol. 20, vol. 41, no. 13, pp. 4781–4786, 2007. no. 5, pp. 1007–1016, Sep. 2005. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 10 IEEE TRANSACTIONS ON ENERGY CONVERSION  F. Boico and B. Lehman, “Mutliple-input maximum power point tracking Zhiyong Ren received the Ph.D. degree from the algorithm for solar panels with reduced sensing circuitry for portable Pennsylvania State University, University Park, in applications,” Solar Energy, vol. 86, pp. 463–475, 2012. 2008.  Y.-C. Liu and Y.-M. Chen, “A systematic approach to synthesizing multi- He is currently an Assistant Professor of civil en- input DC-DC converters,” IEEE Trans. Power Electron., vol. 24, no. 1, gineering at the University of Colorado Denver, Den- pp. 116–127, Jan. 2009. ver, where he also directs the Environmental Biotech-  E. Carlson, K. Strunz, and B. Otis, “A 20 mV input boost converter with nology Laboratory. His lab uses bioelectrochemical efﬁcient digital control for thermoelectric energy harvesting,” IEEE J. systems and fermentation technology to directly con- Solid-State Circuits, vol. 45, no. 4, pp. 741–750, Apr. 2010. vert cellulosic biomass and waste water into H 2 and  B. Acker, C. Sullivan, and S. Sanders, “Synchronous rectiﬁcation with electricity, and he uses molecular microbiology tools adaptive timing control,” in Proc. 26th IEEE Power Electron. Spec. Conf., and electrochemical analyses to understand the fun- Jun, 1995, vol. 1, pp. 88–95. damental determinant factors of those systems so as to enhance design, op-  J. Park and Z. Ren, “High efﬁciency energy harvesting from microbial fuel eration, and monitoring in concert with traditional approaches. His research cells using a synchronous boost converter,” J. Power Sources, vol. 208, focuses on bioenergy production during waste treatment processes, with the pp. 322–327, 2012. goal of expanding environmental engineering from pollution clean-up to sus-  H. Lhermet, C. Condemine, M. Plissonnier, R. Salot, P. Audebert, and tainable development of energy and environmental systems. M. Rosset, “Efﬁcient power management circuit: From thermal energy harvesting to above-ic microbattery energy storage,” IEEE J. Solid-State Circuits, vol. 43, no. 1, pp. 246–255, Jan. 2008.  A. Richelli, L. Colalongo, S. Tonoli, and Z. Kovacs-Vajna, “A 0.2V DC/DC boost converter for power harvesting applications,” IEEE Trans. Power Electron., vol. 24, no. 6, pp. 1541–1546, Jun 2009.  J. Park and Z. Ren, “Efﬁcient energy harvester for microbial fuel cells using DC/DC converters,” in Proc. IEEE Energy Convers. Congress Expo., Sep. 2011, pp. 3852–3858. Jae-Do Park (M’07) received the Ph.D. degree from the Pennsylvania State University, University Park, in 2007. He is currently an Assistant Professor of electri- cal engineering at the University of Colorado Denver, Denver. Prior to his arrival at the University of Col- orado Denver, he was with Pentadyne Power Corpo- ration, Chatsworth, CA, as a Manager of Software and Controls, where he took charge of control algorithm design and software development for the high-speed ﬂywheel energy storage system. His research inter- ests include various energy and power system research and education including electric machines and drives, energy storage and harvesting systems, renewable energy sources, grid-interactive distributed generation, microturbine control, and microgrid systems.
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