Critical Review pubs.acs.org/est
Practical Energy Harvesting for Microbial Fuel Cells: A Review Heming Wang,† Jae-Do Park,‡ and Zhiyong Jason Ren*,† †
Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder, Boulder, Colorado 80309, United States ‡ Department of Electrical Engineering, University of Colorado Denver, Denver, Colorado 80204, United States ABSTRACT: The microbial fuel cell (MFC) technology offers sustainable solutions for distributed power systems and energy positive wastewater treatment, but the generation of practically usable power from MFCs remains a major challenge for system scale up and application. Commonly used external resistors will not harvest any usable energy, so energy-harvesting circuits are needed for real world applications. This review summarizes, explains, and discusses the different energy harvesting methods, components, and systems that can extract and condition the MFC energy for direct utilization. This study aims to assist environmental scientists and engineers to gain fundamental understandings of these electronic systems and algorithms, and it also offers research directions and insights on how to overcome the barriers, so the technology can be further advanced and applied in larger scale.
1. INTRODUCTION The microbial fuel cell (MFC) technology has been intensively researched in the recent decade, because it offers a solution for environmental sustainability by simultaneously performing pollutant removal and energy production. MFCs use exoelectrogenic microorganisms to convert the chemical energy stored in biodegradable substances to direct electricity. Furthermore, the electrical current can be utilized for many other functions, including producing value-added chemicals such as H2 in microbial electrolysis cells (MECs) or driving water desalination in microbial desalination cells (MDCs).1,2 The advancements in reactor architecture, material, and operation optimization of these bioelectrochemical systems (BES) have remarkably relieved the physical and chemical constraints of reactor systems,3,4 leading to orders of magnitude increase in power output. However, one main challenge for MFCs or BESs to be used in real-world applications is the low energy output, and to overcome this, one key element that has been largely neglected is how to harvest and practically utilize the MFC energy based on the true potential of the system rather than simply reporting the measured power density using external resistors. Compared to other alternative energy systems such as solar and wind, MFC is a low power system due to its thermodynamic limitation. The theoretical anode and cathode potentials calculated by Nernst equation are −0.3 V (vs NHE) and 0.8 V (vs NHE), respectively, when acetate servers as the electron donor and oxygen serves as the electron acceptor. Therefore, the theoretical voltage across the two electrodes is 0.8 V to −0.3 V = 1.1 V.5−7 However, the experimentally observed open circuit voltage is only around 0.7−0.8 V (Figure 1) due to the losses on the electrode potential, such as activation polarization, concentration polarization and ohmic losses.6 The potential also varies when different electron © 2015 American Chemical Society
Figure 1. Ideal operation conditions for different BESs, including microbial fuel cells (MFCs) and microbial desalination cells (MDCs). The typical polarization (red) and power density curves (blue) were generated using a lab scale recirculation-flow MFC. Microbial electrolysis cells (MECs) are not shown in the figure because their operation points are beyond this range.
donors, electron acceptors, or microbial inocula are used in the system. Traditionally, MFC power output is reported by changing the external resistance (Rext) at a 5−30 min interval or conducting a voltammetry sweep.7−10 Figure 1 shows typical polarization and power density curves obtained from a lab scale MFC. The curves demonstrate that MFC voltage is inversely proportional to the output current, and there exists a pair of voltage and current that delivers the maximum power, when Received: Revised: Accepted: Published: 3267
September 29, 2014 February 6, 2015 February 11, 2015 February 11, 2015 DOI: 10.1021/es5047765 Environ. Sci. Technol. 2015, 49, 3267−3277
Critical Review
Environmental Science & Technology Rext is equal to the system internal resistance (Rint). This peak point is called the maximum power point (MPP), which is the ideal operating point for MFCs and reported by most studies as the top power output.7,11,12 However, top power may not be the goal of all systems. For MFCs used in wastewater treatment, the primary goal may not be high power output but rather more efficient organic removal, so a balance in operation during different phases needs to be considered whether to operate the system at the MPP for maximum power output or at the high current condition for the fastest substrate oxidation rate.8 Similarly, for H2 production in an MEC, the ideal operating point is not MPP but rather the high current region, because H2 production directly correlates with electron flow (current) in the circuit and proton reduction at the cathode. Because an additional voltage is required for MECs, the operation point of MEC is beyond the limiting current of the polarization curve at negative voltages, and the external energy input, as well as energy content of the produced H2 should be considered in addition to the amount of H2 produced, so the actual energy efficiency and energy recovery can be quantified.13 In contrast, the operation of MDCs depends on different needs, because if high energy is desired, the MPP will be the ideal point, but if high salt removal is the primary goal, then high current will be needed (Figure 1).14 Furthermore, the different operational points identified on the power density curve only represent the potential of power output rather than usable energy, as the electricity generated is dissipated into heat through resistors instead of being utilized by electronics. In addition, the fixed Rext cannot always match the system Rint and extract energy at the MPP, because the Rint of an MFC varies constantly with changes in microbial activities and operational parameters. Studies showed that MFCs may lose more than 50% of produced power across the Rint if the operating voltage is not at the MPP.15 To harvest usable MFC or BES energy, resistors have to be replaced with devices that can capture and store energy and boost voltage for practical usage. The direct outputs of a single MFC are primarily in the level of 700−800 mV and 100−2000 mW/m2, which generally cannot directly power common electronics.16 For example, a single light emitting diode (LED) requires a minimum voltage of 2 V and consumes 30 mW,17,18 and many wireless sensors need a voltage of 3.3 V and wattlevel power for temperature, pressure, and humidity monitoring.19−22 While higher power using single or multiple MFCs has been researched, it was reported that larger power production cannot be easily achieved by just building larger MFCs or simply connecting MFCs in series or in parallel due to the nonlinear nature of MFCs.23,24 Therefore, developing tailored energy harvesting systems including MPP tracking and power management systems (PMS) are crucial for MFC and BES scale-up and real-world application. Such systems generally composed of multiple electronics, such as off-the-shelf capacitors, rechargeable batteries, charge pumps, and boost converters, but these devices are not designed for MFC conditions so the efficiency was low and initial voltage boosts were needed. Customized harvesting systems have been reported by several groups, including our group, but there is very limited knowledge base for this important area, because it requires understanding of power electronics, circuitry, and programing, which are not provided in traditional environmental science and engineering education. In this paper, we therefore offer a first comprehensive review of energy harvesting strategies and systems for MFCs to assist researchers
gain fundamental understandings of such methods. We also provide discussions and our insights on the challenges and research needs of this field, so researchers and engineers can help advance the technology development and finally overcome these barriers of MFC application.
2. ENERGY HARVESTING TECHNOLOGIES Since the direct energy production from MFCs is generally not sufficient for practical applications, various circuit topologies have been developed to interface MFCs with electronic loads. Figure 2A shows a concise flowchart of energy harvesting
Figure 2. Schematics of energy harvesting processes: (A) a concise process from MFCs (energy generator) to electronic devices (energy consumer); (B) a classic and widely adopted PMS circuit composed of a charge pump and a boost converter with accessary components; (C) a two-layer energy-harvesting scheme, which is operated in alternative CHARGE and DISCHARGE phases.
process from MFCs (energy generator) to electronic devices (energy consumer), where PMS (e.g., capacitor-based systems, charge pump-based systems, boost converter-based systems, and unreported systems) as the central command aims to control the MFC at its optimal condition and extracts and stores the energy for the uses by external loads. A PMS is an electronic circuit that is composed of electronic components such as capacitors, charge pumps, boost converters, diodes, inductors, power switches, and potentiometers, with the function of harvesting MFC energy and shaping it to a usable form.25 This is different from external resistances, which have been used in most MFC/BES studies to represent the energy output potential but not capture any usable energy, because the current passed through the resistor is dissipated into heat. Table 1 lists all the commercially available parts that have been used in PMS designs for MFCs including the information on manufacturer/model number and the function of each component. Additionally, Table 2 summarizes the energy harvesting performances that have been reported so far and as well the main electronic components utilized in each study. 3268
DOI: 10.1021/es5047765 Environ. Sci. Technol. 2015, 49, 3267−3277
Critical Review
Environmental Science & Technology Table 1. Key Electronic Components Used in Energy Harvesting Systems electronic components capacitor rechargeable battery charge pump boost converter
inductor
transformer
diode
metaleoxideesemiconductor feld-effect transistor (MOSFET)
junction gate field-effect transistor (JFET) Comparator oscillators energy harvesting board
manufacturer and model number
functions
Duracell (DC2400 NiMH rechargeable AAA battery) Seiko Instruments (S-882Z) STMicroelectronics (L6920DB) Linear Technologies (LTC3108) Linear Technologies (LTC3429) Texas Instruments (TPS61200) Texas Instruments (TPS61201) Maxim Semiconductor (max1797evkit) AMI Electronics (T3005P) Triad Magnetics (RC-7) Triad Magnetics (CST206−1A) Triad Magnetics (CST206−3A) Coilcraft (LPR6235−253PML) Coilcraft (LPR6235−752SML) Würth Elektronik (WE749197301) Micro Commercial Components (1N5711) Fairchild Semiconductor (1N755A) Fairchild Semiconductor (BAT54) Avago Technologies (HSMS-286x) Vishay (Si3460BDV) Vishay (Si3499DV) Advanced Linear Device (ALD110800) ON Semiconductor (4906NG) Diodes Incorporated (DMG6968) Vishay (2N4338)
Energy storage in a magnetic field
Energy transfer through electromagnetic induction
A switch that blocks reverse current flow
A transistor that switches electronic signals
A transistor that amplifies electronic signals Compare a voltage/current against a reference and output a digital signal indicating whether the voltage/current reaches the set level Produces a periodic and oscillating signal, such as square waves
Linear Technologies (LTC6906) Advanced Linear Devices (ALD1502) Advanced Linear Devices (EH4295)
An integrated circuit ready for energy harvesting
1 C(Ve2 − Vb2) 2
46 21,22,40,43,45,47 21,43,45 20,21,50 48 42,46 47 19 74 66
61 20 65,69 40,66 59 65 65 21,45,66 21,45 20 59 65 61
57 68
outputs of current, voltage, and power from MFCs. To date five charging/discharging techniques have been reported: direct charging, intermittent energy harvesting (IEH, a.k.a. intermittent charging (IC)), alternate charging and discharging (ACD), charging capacitors in parallel while discharging in series and charging capacitive electrodes (Figure 2A and Table 2).17,27−30 A capacitor circuit can be a simplest PMS, which charges one or more capacitors until enough energy is accumulated for discharging to power electronic devices. The amount of power extracted and system efficiency vary along with the power curve. Capacitor operation is simple and straightforward, but the output voltage is limited at the open circuit potential (OCP) of the MFC because the capacitor stops charging when its voltage reaches the OCP.21 Therefore, MFC stacks with multiple units can be used to charge capacitors and obtain higher voltage outputs. A successful demonstration of such circuitry is the energetically autonomous robot called EcoBot.31 From 2003 to 2013, four generations of EcoBots were developed using MFC stack as the power source and capacitors as the harvesting system.31,32 By using a similar energy harvesting strategy, other studies have been conducted to pulse an artificial heartbeat,33 power a mobile phone,34 and create a self-sustainable MFC stack system.35 The IEH or IC approach cumulates energy extracted from MFCs in a capacitor and discharges it to a load. This mode
Each study is also labeled whether or not its PMS needs an external power supply and whether or not the external power is included in the reported efficiency. The tables may serve the readers as an index for necessary information needed for PMS components and functions, and in the following sections we elaborate on each specific energy-harvesting regime for MFCs. 2.1. Capacitor-Based Systems. A capacitor is composed of two conductive terminals separated by a dielectric material, and energy is stored in the electrostatic field. When a capacitor is directly connected to an MFC, it is charged by the reactor and acts like a variable resistor, because the charging current changes as the capacitor voltage varies.26,27 The required time for a full charge is determined by the charging potential and capacitance.27 The amount of energy W (J) stored in a capacitor when the capacitor is charged from Vb (V) to Ve (V) can be calculated by W=
ref
energy storage in an electric field energy storage through electrochemical reactions A DC/DC converter to step the voltage up or down A DC/DC converter to step up the voltage
(1)
where Vb and Ve are the voltage across the capacitor at the beginning and end of charging, respectively, and C (F) is the capacitance. In energy harvesting systems, capacitors are widely used as either final energy storage before utilization or transitional energy storage during energy extraction. Different arrangements of multiple capacitors in the circuit can manipulate 3269
DOI: 10.1021/es5047765 Environ. Sci. Technol. 2015, 49, 3267−3277
3270
25
0.5 3
sediment MFC 12 two-chamber MFCstack 12 two-chamber MFCstack
3−5
3−10.4
72
1800
3.3 3.3 3.3 2.85 3.6 9 1800
3.3
72
3.25
2.5 0.48
output voltage (V)
1.0
0.73−0.78
input power (mW)
0.633
0.7 0.3
4.2−4.55
input voltage (V)
23 24
main electronic components
2.1 3.25 0.7 0.7
BES
Capacitor-Based Systems Direct Charging 1 40 single-chamber MFC- capacitor stack 2 8 single-chamber MFCstack 3 8 single-chamber MFCstack 4 8 single-chamber MFCstack 5 24 single-chamber MFCstack 6 24 single-chamber MFCstack 7 24 single-chamber MFC- rechargeable battery stack Intermittent Energy Harvesting (IEH, a.k.a. Intermittent Charging (IC)) 8 two-chamber MFC capacitor 9 single-chamber MFC 10 single-chamber MFC Alternate Charging and Discharging (ACD) 11 two-chamber MFC/MEC capacitor Charging Capacitors in Parallel and Discharging in Series 12 single-chamber MFC capacitor 13 single-chamber MFC 14 single-chamber MFC Charging Capacitive Electrodes 15 two-chamber MFC quasi-capacitor (capacitive electrode) Charge Pump-Based Systems 16 two-chamber MFC charge pump, capacitor 17 three-chamber MCDC Boost Converter-Based Systems Capacitor -Boost Converter Systems 18 upflow MDC (UMDC) rechargeable battery, DC/DC boost converter 19 benthic MFC capacitor, DC/DC boost converter 20 upflow MDC (UMDC) 21 benthic MFC 22 sediment MFC
no.
Table 2. Summary of Studies Reported Energy Harvesting Systems for BESs
3.52 36.97
0.73−0.78
0.152
output power (mW)
N N N N N
N N N N N
41.8a 79b 75.3b
Y
N Y
N Y N N
Y
86.6a
N
N N
N N N N
N
N N
N
N N N
N N N
N Y Y Y
Y
N N N
N
N
Y Y Y
N
maximum power point (Y/N)
N
need external power (Y/N)
4.3b 0.94b
100a 90a
>90b
95.2b
efficiency (%)
79
48 74
19 46 47 49
46
40 41
30
17 38 39
29
27 36 37
34
33
32
78
76,77
75
35
ref
Environmental Science & Technology Critical Review
DOI: 10.1021/es5047765 Environ. Sci. Technol. 2015, 49, 3267−3277
BES
main electronic components
3271
two-chamber miniaturized MFC
0.06−0.17 0.36 0.4 0.6−0.7 0.328 0.512
0.9−1.2
>3 2.5
6
0.6−2
0.3 0.3 0.3 0.35
>3
2.5 2.2
5
0.174
3.3 3.3 3.3 3.3 7
3.3
0.316−0.372 0.3 0.28−0.33
0.2−0.4
0.052−0.32 0.6 0.5 1.12−1.44 0.4
0.3
2−7.5 3.3 3.3 1.7−3.3
0.475 0.79 0.18 0.6
output voltage (V) 4
0.37
input power (mW)
0.4
input voltage (V)
85
18
95
95 2500
95 95
output power (mW)
17b 30b
