Download Efficient Energy Harvester for Microbial Fuel Cells using DC/DC

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