Download Practical Energy Harvesting for Microbial Fuel Cells: A Review

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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.
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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
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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
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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
<85b
N
N
N
Y
Y
Y
Y
N
N
N
N
36.0a
46.1a
75.9a
73b
66.5−80.6b
74b
85b
Y
Y
3−13a
<67.7a
60b
22.54−37.80b
<70b
5.33b
N
N
N
N
N
N
N
N
N
N
N
N
58b
4.29b
N
need external power
(Y/N)
55b
efficiency (%)
N
N
Y
Y
Y
Y
Y
Y
Y
Y
Y
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
maximum power point
(Y/N)
67
68
16
40
59
60
12
65
69
70
51
57
66
21
22
44
45
42
43
73
71
61
20
21
50
72
ref
a
The efficiency presents the circuit efficiency (η1) only. External power was provided but not included in the calculation. bThe efficiency presents both the circuit efficiency (η1) and the overall system
efficiency (η2). No external power was provided or external power is included in the calculation.
51
33 sediment MFC
34 benthic MFC
35 sediment MFC
36 benthic MFC
37 benthic MFC
38 sediment-MFC
Custom-Designed Systems
39 two-chamber MFC
capacitor, inductor, diode
40 two-chamber MFC
Maximum Power Point-Based Systems
41 two-chamber MFC
capacitor, inductor, diode
42 two-chamber MFC
capacitor, transformer
43 two-chamber MFC
capacitor, inductor
44 two-chamber MFC
45 single-chamber MFC
capacitor, transformer, diode
46 single-chamber MFC
47 single-chamber MFC
48 benthic MFC
unknown
Integrated Circuit-Based Systems
49 single-chamber MFC
commercial IC, capacitors
50 two-chamber MFC
custom-designed IC
Capacitor -Boost Converter Systems
26 sediment-MFC
Capacitor-Transformer-Boost Converter Systems
27 single-chamber MFC
capacitor, transformer
28 single-chamber MFC
capacitor, transformer, DC/DC boost
converter
29 single-chamber MFC
30 sediment MFC
Capacitor-Charge Pump-Boost Converter Systems
31 single-chamber MFC
capacitor, charge pump, DC/DC boost
converter
32 sediment MFC
no.
Table 2. continued
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pump circuit are first charged by the power source and then
connected in different combinations to generate various
voltages for different applications. The S-882Z series charge
pump from Seiko Instruments has been widely used in BES
studies, and it requires a minimum input voltage of 0.3 V in
order to generate a discharge voltage of 1.8−2.4 V (Figure 2A).
The charge pump consumes a minimum 0.1−0.5 mA current
during operation when the input voltage is 0.3−0.6 V, which
may limit its charging speed when the current is low and leads
to long charging/discharging cycles and low energy harvesting
efficiency.20,40 For example, using a 316 mL air−cathode MFC
as the power source, it took 22 h for the charge pump-based
circuit to output a voltage of 3.3 V during the start-up phase,
but a transformer-based circuit only took 2.5 h to output the
same voltage,21 suggesting that the energy extraction rate of the
charge pump was much slower compared to the transformer.
Similar performance was observed by Wang et al., who found
that due to charge pump’s input current limitation, its operating
point was maintained at the low current region of the power
curve which was far away from the MPP (Table 2).40 When
using a charge pump to conduct capacitive discharge from a
microbial capacitive desalination cell for energy production,
Forrestal et al., found the Coulombic efficiency was only 0.94%,
indicating that the charge pump is not sufficient for energy
harvesting during desalination regeneration (Table 2).41
Therefore, charge pumps can accommodate low-voltage
MFC sources and be used for intermittent energy harvesting
when low charging rate is acceptable, such as for remote
sensors. The performance of the charge pump can be greatly
improved when input current increases. Furthermore, charge
pumps can also be used as dynamic switches in the circuit to
automatically control on/off and prevent reverse current
flows.42,43 S-882Z (Seiko Instruments) has been the most
commonly used commercially available charge pump in BES
studies, and because its maximum output voltage 2.4 V,
sometimes it is not sufficient to power common electronic
devices. To further increase the output voltage, another layer of
power converter may be placed after the charge pump for
voltage boost.
2.3. Boost Converter-Based Systems. A DC/DC
converter is an electric circuit to convert direct current (DC)
power from one voltage level to another level, so an
unregulated DC input can be converted to a controlled output.
The input voltage can be stepped down (buck converter),
stepped up (boost converter) or inverted. Boost converters are
widely used in MFC research (Figure 2A), and the circuit of a
boost converter includes both semiconductors, such as diodes
and transistors, and energy storage components, such as
capacitors and inductors, with a more complex structure than
that in the charge pump. While the commonly used charge
pump can step up the voltage from 0.3 V to 1.8−2.4 V, the
output may still be low for many electronics such as marine
sensors (3.3 V)19,44 In such cases, DC/DC boost converters
can be used to boost the output voltage further. For example,
several studies used a boost converter (L6920DB, STMicroelectronics) to obtain an output voltage of 3.3 V with a
minimum start up input voltage of 0.8 V.21,43,45
Most commercially available low input voltage boost
converters require a minimum input voltage of 0.7 V
(max1797evkit, Maxim Semiconductor) or 0.8 V (L6920DB,
STMicroelectronics), which are practically beyond the voltage
capability of a single air-cathode MFC or a parallel-linked MFC
stack. There are two off-the-shelf boost converters that require
delivers power in intermittent pulses when the source is not
capable of supplying continuous power to the load. Compared
with the conventional continuous energy harvesting (CEH) in
which current is continuously passing through the load, one
study showed that the IEH approach harvested 152 μW from a
500 mL two-chamber MFC which is 111% higher than that in
the CEH mode (72 μW).27 The results were explained by using
an analogy that plug flow reactors (PFR) are more efficient
than continuous stirred tank reactors (CSTR). In the IEH
mode, the electrical current decreases slowly as the process
progresses, similar as a PFR operation, so within a given period
of time more electrical charges can be harvested than CEH, in
which the reaction rate keeps stable like a CSTR. The effect of
charging and discharging frequency during the IC mode can
affect system performance, and it was suggested that lower
frequency led to higher current output and chemical oxygen
demand (COD) removal, though an optimum frequency
should also consider charge recovery efficiency.36 Similarly,
progressively switching MFC units from parallel to serial
connection in the stack reduced capacitor charging time in half
and increased current generation by 35% when comparing with
MFC stack with serial connections.37
In the ACD mode, an MFC charges capacitors first for
energy collection, and then the charged capacitors discharge the
energy back to the system for MEC operation. Liang et al.
showed that the ACD mode could increase the current by 22−
32% compared to the IC mode, which was attributed to the
shorter discharging time than the charging time, as well as the
higher anode potential caused by discharging the capacitor.29
However, power densities in the ACD mode were lower than
those in the IC mode.
The voltage output can be increased when charging an array
of capacitors connected in parallel and then discharging them in
series. By using two groups of capacitors with alternative
charging and discharging sequence, Kim et al. found the output
voltage was constantly enhanced from 0.7 V to as high as 2.5
V.17 Moreover, this approach does not require a minimum
input voltage threshold, so voltage can be increased without
using initial boost. It also effectively alleviated voltage reversal
problem with negligible energy losses in the circuit. However,
external energy supplied to control relay switches was not
considered in the energy calculation. Another study used the
same array to harvest energy from multiple MFCs and power
an MEC, and it was found that energy recovery improved from
9% to 13%, and H2 production rate doubled from 0.31 to 0.72
m3/m3/day.38 A similar study used three capacitors separately
charged by three MFCs and then linked them in series to
power an electrochemical deposition system (ECD), which
obtained a sulfur recovery efficiency up to 46.5 ± 1.5%.39
Another new capacitor-based energy harvesting is quasicapacitor-based method. In such systems, a capacitive bioanode
was constructed by coating a capacitive layer consisted of a
mixture of activated carbon and polymer, and then the anode
can be used as an internal capacitor.30 The MFC equipped with
the capacitive bioanode produced a peak current density up to
1.7 A/m2, which almost doubled the output of a control
noncapacitive anode (0.9 A/m2). Future studies are needed to
investigate the longevity of the capacitive electrodes.
2.2. Charge Pump-Based Systems. Charge pumps are
low cost devices with simple circuit topologies. In general, a
charge pump is an inductor-less DC/DC converter that uses
capacitors to store and transfer energy in order to generate
either higher or lower voltages. The capacitors in the charge
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requires a much longer charging time (11.3 h vs 1.06 h) and
a higher minimum input voltage (0.3 V vs 0.18 V).21
To obtain high energy efficiencies, a classic PMS circuit
composed of a charge pump, a boost converter, and the load
with accessary components, such as capacitors and switches, has
been widely adopted by benthic MFCs (BMFCs) (Figure
2B).22,42−45,47 BMFCs utilize naturally occurring potential
difference between the anoxic sediment and oxic water to
generate electricity and therefore provide long-term power
source for remote sensors.51−54 One challenge of BMFCs is the
low power output due to poor ion transfer between the
sediment anode and the air cathode in the natural water
body.55,56 This classic PMS first uses charge pumps to harvest
energy from the low voltage/current BMFCs and then boosts
the voltage via a boost converter to provide intermittent power
for wireless sensors, telemetry systems or hydrophones.44,45
The intermittent energy harvesting is a practical approach for
BMFC operation, as it allows a small power source such as
BMFC to power larger electronic devices with higher energy
demand, and the energy efficiency is higher than continuous
operation. When coupling the PMS with a two-cathode BMFC
(one floating and one settling), Zhang et al. found that
continuous sensor charging was possible, but the charging rate
was faster when only using the floating cathode.42 A multianode
decoupling circuit could be used to separately connect charge
pumps with different anodes, so interactions among anodes
with different performances could be avoided.43
2.4. Maximum Power Point Tracking and Active
Energy Harvesting. In addition to commercially available
devices such as capacitors, charge pumps, and boost converters,
customized energy harvesting systems with maximum power
point tracking (MPPT) capability have been developed to
increase power harvesting. While different systems have been
reported with various performances, Table 1 shows the
common components used in such systems, and each unit
has a specific function. For example, inductor stores energy in
the magnetic field; transformer transfers and amplifies energy
through electromagnetic induction; diode, metal-oxide-semiconductor field-effect transistor (MOSFET), and junction gate
field-effect transistor (JFET) are utilized as switches to prevent
current reverse flow. Figure 2C shows a two-layer energyharvesting scheme, which can be used in conjunction with
various converters such as a boost converter or flyback
converter to further increase the output voltage. The energyharvesting scheme was operated in alternative CHARGE and
DISCHARGE phases.40,57,58 During the CHARGE phase (the
first half of the circuit in Figure 2C), the controller extracts
energy from MFCs and temporarily stores it in the inductor;
during the DISCHARGE phase (the second half of the circuit
in Figure 2C), the controller discharges the energy from the
inductor to the capacitor for storage. To increase the harvesting
efficiency, the inductor was replaced with a transformer, and the
diode was replaced by a MOSFET.59,60 Adami et al. developed
a flyback converter by using a step-up transformer and a
normally on N-channel JFET transistor, and they obtained an
output voltage up to 7.5 V, which was much higher than the
3.3−5.0 V obtained from commercial boost converters.61
An MFC is a dynamic system that its internal resistance and
power density curve vary constantly with changes of microbial
activities and operational parameters, such as substrate
concentration, pH, and temperature. This means that static
energy harvesting without adaptation to MFC real time
condition cannot capture the peak energy all the time, and
very low operating input voltages, for example, LTC3108 (0.02
V, Linear Technologies) and TPS61200/TPS61201 (0.3 V,
Texas Instruments), but their low input voltages can also limit
the output voltages, making it hard to be used for real world
applications. Although the OCP of MFCs could be around
0.7−0.8 V, the output voltage of a single MFC or parallelconnected MFCs decrease rapidly during current extraction by
the boost converter, which is likely the reason for system failure
when connecting a boost converter directly with three parallelconnected upflow MDCs (UMDCs).46 Hence the coordination
between MFC outputs and electronic components must be
carefully controlled to avoid system collapse. Series-connected
MFCs could provide a higher input voltage, but it is at the risk
of voltage reversal and performance is not stable due to changes
in environmental conditions.
To bridge the gap between the MFCs and the boost
converter, electronic components like capacitors/rechargeable
batteries, transformers, charge pumps, etc. are placed before
boost converters to cumulate energy and jumpstart the
converter. Table 2 summarizes different adopted components
in related studies. Capacitors/rechargeable batteries are the
most commonly used for storing energy. For example, Bo et al.
used three serial-connected UMDCs as a predesalination
process and harvested their energy into a rechargeable battery
or a capacitor, then boosted the voltage via a boost converter to
power a electrodialysis (ED) cell for further desalination.46
Compared with ED, this two-step desalination process can
effectively reduce both energy consumption and desalination
time. A similar approach was used by a benthic MFC with a
biocathode and a sacrificial anode, which first charged a
capacitor to 1.2 V and then boosted the voltage by a boost
converter to 3.3 V, the minimum requirement as an
intermittent power source for a wireless sensor.19 A higher
efficiency was reported when two capacitors were charged by
two MFCs individually, and then linked them in series and
further boosted the voltage by the boost converter for higher
voltage/current output.47 This method was used to develop a
bulk energy storage for more efficient power conversion. By
implementing two groups of supercapacitors with one group
(12 supercapacitors) charged in parallel and the other switched
in series, the harvesting approach was able to boost output
voltage to 9 V.48 To provide a continuous power supply to
sensors such as a submersible ultrasonic receiver (SUR) that
listens and records time and signals, Donovan et al. developed a
novel SMFC PMS composed of two boost converters to
continuously power a real-time clock (RTC) in a sensor
system.49
The second option is using transformers coupled with boost
converter to amplify the voltage by transferring energy through
electromagnetic induction. The advantage of transformers is
that they extract energy much faster and can take lower input
voltage than charge pumps. Yang et al. reported that by
connecting an MFC with a capacitor and a transformer, the
PMS worked well under a low input voltage of 0.18 V and
successfully boosted output to 3.3 V.20 Without using
capacitors, Thomas et al. connected an MFC directly with a
transformer and boosted output voltage to 1.7−3.3 V.50 As
discussed in section 2.2, charge pumps can also be used to
connect low-voltage MFCs/BESs and the boost converter. A
comparative study showed that a charge pump-capacitorconverter PMS had a higher energy efficiency (5.33%) than a
capacitor-transformer-converter PMS (4.29%), but it also
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therefore the efficiency is low. Great efforts have been made in
the MPPT techniques which can successfully track the
maximum power production of MFCs in real-time, such as
perturb and observe (P&O) method, partial OCP method, and
multiunit method, etc.11,15,62 An MPPT technique not only
maximizes the power production of MFCs, but also reduces the
start-up time and increases exoelectrogenic activity and
Coulombic efficiency.63 Moreover, the control of MPPT can
be applied on each MFC separately to achieve a high stack
voltage without the issue of voltage reversal.64 However,
traditional MPPT techniques still adjust external resistances to
demonstrate the power production potential with no actual
energy harvested. To actually use the MPPT real time tracking
and produce usable energy, Park and Ren built a hysteresis
controller based MPPT energy harvesting system, which can
track the MPP and maintain the energy harvesting at the peak
level in real-time.12 Degrenne et al. developed an original
converter system which contains a voltage controller for
maintaining the input voltage at the maximum power
production stage.65
Based on the real-time MPPT, a maximum power point
circuit (MPPC) was developed to control a BES at any
operation point along the power density curve, especially at the
MPP for MFC operation.40 This is a new energy harvesting
approach that not only can capture the maximum power from
an MFC, but also harvest energy actively without using any
external resistance. Compared with traditional circuits using
capacitors or charge pumps, which passively receiving electrons
from the reactor, this controller can actively extract energy from
the MFC at any operating point, especially at the peak power
point to maximize energy production. Using this active
approach, the MPPC extracted 76 times more energy than
the commonly used Seiko charge pump, and the Coulombic
efficiency increased by 21 times.40 Despite this dramatic
improvement, the efficiency of this diode-based boost converter
was only about 36% with nearly 60% of energy lost, which
means much more potential can be tapped. Follow-up studies
showed that by replacing the diode with a P-channel MOSFET
or using a newly designed synchronous flyback converter, the
efficiencies were improved by 37.6% and 73.0%, respectively.15,59,60 The effects of inductance, duty ratio, and switching
frequency on these power electronic converters for MFC
energy extraction have also been investigated, and results
indicated that these factors play important roles for the
performance of MFC and energy harvesting, and their effects
can be cross-linked. While current and voltage are generally
proportional and inversely proportional to the inductance,
respectively, the total harvested energy and efficiency vary
significantly by combinations of duty ratio and switching
frequency.66
perspective and stimulate more interests and research activities,
we think the following areas will require more investigations:
1. The main challenge for MFC harvesting circuit or PMS
design is to build an efficient system that can operate at
the low-level voltage/energy supplied by MFCs yet
support high-level voltage/energy electronic devices.
Energy loss inevitably happens during each conversion
process, so it is imperative to develop a circuit with an
acceptable complexity but with high efficiency. This is
especially critical for long distance distributed power
applications such as benthic MFCs, because the transmission loss can be significant. So far, almost all PMSs
developed for MFCs are discrete circuits that are built
with various electric components, which lead to low
efficiency. Integrated circuits (ICs) or chips have been
commonly used in all common electronic devices
nowadays due to their small volume, low cost, low
energy consumption, and quick switch among components, so developing ICs for MFC energy harvesting
should be a primary task. This would inevitably need
interdisciplinary collaboration, and some groups have
already started creating the high efficiency and high
performance ICs for MFCs16,67 or adopting commercially available IC energy harvesters.68
2. Another main challenge for many developed PMS
circuits is that they are not autonomous, which means
that they require an external power source to either
jumpstart or operate the circuit for energy extraction
from MFCs. Although the circuit has a better
controllability when supplied by an external power, this
is not considered sustainable especially for stand-alone
sensor-type systems. SMFC-powered PMSs for monitoring environmental parameters can become autonomous
when intermittent operation is possible, which allows
long energy harvesting time. Reactor type MFCs used in
wastewater treatment and other applications are
generally capable of maintaining the PMS operation
due to their scale, but the direct voltage or current from
the MFC may not always meet the minimum requirements of the circuits to carry out tasks continuously, and
inverters requiring grid frequency or voltage maybe
needed for practical applications. To solve this problem,
the MFC energy output and the PMS operational
requirement should be carefully evaluated, and more
importantly, self-starting and self-powering systems need
to be developed. Several recent studies have reported
such systems, which require either a small jump start57 or
no extra power16,61,65,67,69,70 for self-sustaining operation.
The two key factors to develop self-sustaining PMSs are
effective energy harvesting strategies and low-power
consuming circuits. On one hand, MPPT function might
be integrated in the PMSs to dynamically maximize the
energy harvesting in real time; on the other hand, the
PMSs should be managed effectively with minimum
power consumed. Further studies are still required to
improve the system efficiency, lower the start-up voltage,
shorten the start-up time, investigate long-time performance and robustness, implement pilot-scale and full-scale
studies on field, etc.
3. More on the evaluation of energy harvesting efficiency,
we think quantitative methods need to be developed
similar as general MFC parameters like Coulombic
3. CHALLENGES AND PERSPECTIVES
The generation of practically usable power is a critical
milestone for further MFC development and application, and
how to effectively and efficiently harvest and utilize MFC’s
energy remains a key challenge. This review discusses the
different methods and systems that have been developed for
MFC energy extraction and conditions for practical use, but it is
very clear that more work needs to be done to optimize the
design, improve harvesting efficiency, and reduce the cost. We
consider this is a main bottleneck for MFC application and
should be a new frontier of MFC research. To put it into
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Environmental Science & Technology
efficiency. To optimize capacitor charging, an MFC tester
(MFCT) was developed to determine the optimum
capacitor sizes, charging/discharging potentials, the
frequency of charging, the limiting electrode and even
the optimum size of the electrodes required to power a
particular sensor.26 It is also necessary to optimize each
component in the circuit to accommodate different MFC
capabilities. Wu et al. explained how to determine the
value of each component for a voltage boost circuit
design.57 In literature, there are two ways to calculate
energy harvesting efficiency (η) (eqs 2 and 3).
η1 =
η2 =
Eoutput
EMFC
Eoutput
E input
× 100%
(2)
× 100%
(3)
where Eoutput is the energy applied on the electronic
devices or the energy output at the final step of PMS;
EMFC is the energy produced by MFC only; Einput is the
total energy input into the system including energy
produced by BES and the extra energy added on the
circuit; Many studies used eq 2 to calculate energy
harvesting efficiency, but it actually presents the energy
loss through the PMS or harvesting circuit, so η1 cannot
reflect the overall energy harvesting efficiency when extra
power is supplied. Instead, eq 3 is suggested to replace
EMFC with Einput to present the total energy added on the
whole system. If no extra energy is supplied on the
circuit, the two calculations can be the same, that is η1 =
η2. Therefore, η1 focuses on the efficiency of the energy
harvesting circuit, while η 2 is to emphasize the
importance of net energy harvesting.
4. The scale up of MFC/BES technology has been largely
focused on the reactor itself, while current PMS has
primarily focused on benthic MFCs and sediment MFCs
because such devices could meet the lower demand of
remote sensors in practical operations.22,48,49,71−73
Though the efficiency can be low and a long charging
time is required, it is still acceptable since most sensors
do not need to work continuously. However, for
wastewater treatment and bioremediation, multiple
MFC units have to be connected as stacks in order to
obtain a higher treatment efficiency and applicable power
output, which requires high efficiency power harvesting
systems. The development of MFC stacks has been very
challenging, because the efficiency of MFC stacks was
low and the performance was not stable due to the
nonlinear nature of MFCs. Unlike traditional fuel cell
stacks, which depend on stable chemical reactions in each
unit to provide a higher system voltage output, MFCs
rely on relatively unstable microbial activity to provide
potential and current outputs. The microbial activity and
resulted voltage output are very sensitive to environmental and operation condition changes and can
fluctuate significantly. Moreover, the overall performance
of an MFC-stack is generally limited by the worst
performing unit(s), resulting in a reduced efficiency.23
One solution for obtaining and maintaining power
output from MFC stacks is to connect each MFC unit
with a separate energy harvesting controller and then
make different combinations of these pairs as needed.71
■
Each active harvester stores the captured energy in a
common reservoir. Once enough energy is captured in
the reservoir, a separate boost converter can readily
generate an appropriate voltage for the load. Connecting
MFCs through controllers allows real-time tracking and
harvesting capability, and the power output can avoid the
issue of voltage reversal. If necessary, individual units can
be simply removed from the stack without affecting other
units and the overall system performance.
5. While most research focuses on system development,
little is known that how energy harvesting will change
microbial activity and community. While passive harvesting using charge pumps or capacitors may not affect such
parameters much because these devices just receive
whatever amount of power provided by the MFC
without controllability, power electronics converters use
pulse-shaped power extraction in high frequency may
lead to microbial community shifts and electron transfer
mechanism changes. Our preliminary results support the
hypothesis that microbial activity, biofilm viability, and
mix culture community may shift and evolve during
active power extraction. Such process creates a selective
pressure on the microbial community to regulate
respiratory pathways for more efficient electron transfer
and ATP synthesis. Specifically, cells with multiple
extracellular electron transfer mechanisms may shift
their mechanisms to more efficient pathways, such as
from mediated indirect transfer to direct transfer, while
bacteria with more efficient electron transfer mechanisms
in a mixed culture may outcompete less efficient species
as they are more likely able to meet the requirements of
high rate electron delivery. This will be a very interesting
topic to investigate.
AUTHOR INFORMATION
Corresponding Author
*Phone: (303) 492-4137; fax: (303) 492-7317; e-mail: jason.
[email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank the financial support from Dr. Linda Chrisey
at the Office of Naval Research (ONR) under Award
N000141310901.
■
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DOI: 10.1021/es5047765
Environ. Sci. Technol. 2015, 49, 3267−3277