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Final Year Progress Report
Student: Aidan Walsh
Student ID: 07540388
Discipline: Electronic Engineering
Supervisor: Dr. Maeve Duffy
Project Title:
Energy Management System for Microbial Fuel Cells
Progress Report 20/December/2010 Aidan Walsh
Table of Contents
1 Project Overview
2 Milestones
3 Progress to Date
3.1 Operation of Fuel Cells
3.2 Microbial Fuel Cells
3.3 Characterisation of Microbial Fuel Cells
3.4 Boost Converter
3.5 Transmitter, Receiver and Sensor
3.6 Capacitors
3.7 Trans Impedance Amplifier
4 Testing
4.1 Capacitor Testing
4.2 Boost Converter Testing
5 Problems
6 Where To Next
7. Table Of Figures
8. References
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Progress Report 20/December/2010 Aidan Walsh
1. Project Overview
Researchers in the Energy Research Centre (ERC) in NUI Galway are investigating
microbial fuel-cells (MFCs) which produce electrical power from microbial and bio-
fuels; their main aim is to understand the electrochemical processes involved. During
an initial collaboration with Electrical & Electronic Engineering, a demonstrator
circuit was built and tested to illustrate the issues involved in converting the low
power levels produced by MFCs into a useable form. This project will continue the
collaboration with the ERC to develop some of the ideas identified to date. The main
aim will be to design, build and test an energy management system that provides
maximum output power from an MFC to supply a given demonstrator load.
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Progress Report 20/December/2010 Aidan Walsh
2. Milestones
The completion of the practical milestones identified below does NOT necessarily
guarantee an award at the associated level. An award at the associated level will only be
merited if all other aspects of the project (e.g. reports, presentation, attendance in lab
and at meetings etc.) are completed to an equivalent level.
Review of collaborative work with ERC to date
- Fuel cells & MFCs: structure, principle of operation and electrical
characteristics
- Energy conversion circuit: storage capacitors and boost converter. Repeat
tests to confirm circuit operation
Energy conversion circuit performance characterisation
- Develop PSPICE circuit model of MFC and apply it to determine the charge
time required for storage capacitors vs. discharge time for different resistive
loads
- Compare circuit models with measurements; incorporate capacitor ESR if
necessary
- Determine the average efficiency of energy conversion over a range of charge
/ discharge cycles for different capacitors & load resistors
Pass
Comparison of boost converters and switched capacitor solutions for converting
MFC energy from storage capacitors to a given load; e.g. wireless sensor
- Determine power / energy requirements of a wireless sensor
- Review commercial literature and identify suitable controller IC products for
both power conversion circuit types
- Analyse and compare the performance of both circuits, when configured to
supply the power level required by the demonstrator - use PSPICE and
controller IC datasheets
Average
Design and build a demonstrator system
- Determine the circuit characteristics of the MFC to be used in the
demonstrator circuit
- Determine the minimum capacitor required to store sufficient energy for the
load
- Build and test the best power conversion solution identified above
- Build and test the wireless sensor load
Good
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Progress Report 20/December/2010 Aidan Walsh
Complete system test
- Integrate all circuit blocks and confirm the system operation
- Determine the efficiency of energy conversion over a complete charge /
discharge cycle: circuit modelling and testing
- Investigate the scope for varying wireless sensor range / functionality
through varying capacitor charge / discharge time
Very good
System development for improved efficiency / power levels
- Determine the main limits in efficiency: MFC impedance, storage capacitor,
power conversion circuit
- Develop a solution for overcoming the main limits identified, including circuit
models and analysis to illustrate the level of improvement possible
Excellent
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Progress Report 20/December/2010 Aidan Walsh
3. Progress to date.
The current project assigned to me is based on a similar project undertaken by a
previous final year student, Stephen Mulryan last year. The initial task was to review
his work and get up to date on the operation of microbial fuel cells, boost converters
and storage capacitors.
3.1 Operation of Fuel Cells
Fuel cells consist of two chambers filled with chemicals separated by an ion
exchange membrane. The effect of inserting this membrane between the chambers
results in the prevention of electrons to flow from one chamber to another.
Alternatively it allows the positive ions or protons to flow from one chamber to
another. Each Chamber is connected to external circuitry by means of an anode and
a cathode. An anode is a positively charged electrode by which electrons leave and
electric device. An electrode is an electrical conductor similar to the anode except
that it is negatively charged. The chemical stored in the chamber is normally referred
to as the fuel and the resulting chemical from the reaction in the chamber connected
through the cathode is referred to as the reagent. The operation of the fuel cells
using the ion exchange membrane is by firstly plating the anode and cathode with
catalysts. These are chemical substances which can either speed up or slow down
the rate of a chemical reaction without being consumed in that process. In this case
the catalysts increase the rate of reaction in the anode chamber. In the hydrogen
fuel cell for example the anode chamber is filled with hydrogen gas (H 2). The catalyst
on the surface has the effect of speeding up the splitting of the electrons in the H 2
molecule from the protons. Once split the H2 molecule, which is now a cation, passes
through the ion exchange membrane into the cathode chamber. This leaves the
electrons in the anode chamber where they are conducted through the anode and
the external circuit before coming back into the cathode chamber through the
cathode. Therefore voltage is created between the anode and the cathode and the
value of this voltage depends on the total resistance in the external circuitry, the
total internal resistance in the anode chamber and the amount of current conducted
through the anode. Once the electrons are conducted back into the chamber they
rejoin the hydrogen cation and react with the reagent that is held in the cathode. In
the majority of fuel cells the reagent it oxygen so the reaction of the Hydrogen
Molecules produces H2O which is water.
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Progress Report 20/December/2010 Aidan Walsh
Figure 3.1 Hydrogen Fuel Cell
There are a large amount of other fuel cells that have a very similar if not exactly the
same operation to the Hydrogen Fuel Cell that uses the proton exchange membrane.
Other examples of fuel cells include solid oxide fuel cells, molten carbonate fuel cells
and for this project Microbial Fuel Cells.
3.2 Microbial Fuel Cells
A microbial fuel cell (MFC) can be defined as a system that uses microorganisms to
catalyze metabolic or enzyme catalytic energy into electrical energy (Allen and
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Progress Report 20/December/2010 Aidan Walsh
Bennetto 1993). This in theory is a good source of renewable energy but due to the
miniscule amounts of power generated compared to solar/ wind energy systems
etc., researchers have been hesitant on researching Microbial Fuel Cells as a viable
source for large scale renewable energy applications.
Microbial fuel cells are effectively self-renewable power supplies, which can
continue operating for a very large period of time using local resources. They offer
numerous advantages over batteries as power sources as they do not require
recharging. This comes at a cost however as the power output from a typical
microbial fuel cell is extremely low, much lower than most standard electronic
components are able to operate with. This can be solved to a certain extent by
increasing the surface area of the electrodes. If it is not possible to increase the area
of the electrodes, a typical system could be operated less frequently using a suitable
power management program, i.e. data transmission occurs only when enough
energy has been accumulated by the storage solution in place in the system. A
further advantage of microbial fuel cells is that even when exposed to extreme
conditions the cells can return to full health when the conditions are removed. This
is unlike standard batteries which permanently lose a portion of their efficiency
when exposed to very low temperatures.
Resources such as common wastewater and acetic acid which is created by plant
waste fermentation can be utilized as a fuel source for microbial fuel cells and
natural micro-organisms can act as catalysts which can produce electricity and
generate hydrogen gas that could be used as a fuel source for a hydrogen fuel cell.
3.3 Characterisation of Microbial Fuel Cell
As the new Microbial Fuel Cell has not been fully constructed yet the previous values
of the old Microbial Fuel Cell are being used in experimentation and replication of
the previous project’s tests.
The Thevenin equivalent circuit was calculated by measuring the power density,
voltage and current density of the Microbial Fuel Cell held at the Energy Research
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Progress Report 20/December/2010 Aidan Walsh
centre.
Figure 3.3 Power Density/Voltage Vs. Current Density curve
The Blue points represent power density versus current density while the white
points represent Voltage Vs. Current density. The current density is measured from
the surface of the anode and the power density is measured the same way. The
second set of points are used to work out the current, resistance and Power to
achieve the most favorable results.
The surface area of the anode was 5.4cm2 the second point was chosen, which gives
voltage = 0.42 volts, power density = 900 milli-Watts/m2 and the current density =
0.225 milli-Amps / cm2. The current is worked out to be 1.215 milli-amps. The power
is then worked out by multiplying the voltage by the current which equals 0.5103
milli-Watts. The internal resistance is calculated by Ohm’s Law and the Thevevin
equivalent circuit is produced:
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Progress Report 20/December/2010 Aidan Walsh
Fig 3.4 Thevenin Equivalent of Microbial Fuel Cell
All of the above calculations are based on the previous Microbial Fuel Cell. Once the
new Microbial Fuel Cell is manufactured all testing will be redone and all calculations
will be re-applied to the results of the testing. The proposed Microbial Fuel cell will
have a much larger anode surface area so should in theory have a much higher
power output.
3.4 Boost Converter
A Boost Converter also known as a Step-Up converter is a power converter with an
output DC voltage greater than its input DC voltage. The boost converter has two
main modes of operation which are determined by a switch ‘S’. When the switch is
closed the Boost Converter is in its ‘On State’ so the current is allowed to increase in
the inductor. When the switch is open the converter is in its ‘Off state’ so the only
path offered to inductor current is through the freewheeling Diode ‘D’ the capacitor
‘C’ and the load ‘R’. This causes the transfer of energy accumulated during the On
State into the Capacitor/Load.
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Fig 3.5 Operation Modes Of Standard Boost Converter
Fig 3.6 Voltage and Current Waveforms of Boost Converter
Due to the low power dissipation of the microbial fuel cell it was not possible to
engineer the required boost converter conventionally using Bipolar junction
transistors and diodes. This is due to the diodes between the base and emitter gates
having a voltage drop of at least 0.3 volts which the system cannot afford to drop as
it is only receiving 0.41 volts from each microbial fuel cell. This was disappointing as
it removed a large amount of flexibility of the Boost Converter in terms of the rate
which the switching took place within the DC-DC converter.
The converter chosen was the TPS61200 chip which is manufactured by Texas
Instruments. This was the lowest found and had a start up voltage of 0.5 volts. As
there was no translation board small enough to enable the attachment of the chip to
a normal circuit board it was not an option to use the chip on its own. Therefore the
TPS612000EVM-179 was ordered which has the TPS61200 integrated into it. This
added more restrictions as the size of the inductor and capacitors have an impact on
the energy that is stored in the boost converter.
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Progress Report 20/December/2010 Aidan Walsh
One of the options to automate the charging ad discharging of the capacitors is
another DC-DC boost converter, the MAX1797 EV kit. This DC-DC converter is
manufactured by Maxim. The advantage with switching to this converter as opposed
to staying with the original was the Maxim kit incorporates a voltage comparator
into the EV kit which would allow the automation of the charging and discharging of
the capacitor. Fig shows the schematic of the EV kit.
Fig 3.7 Schematic Of Max1797 EV Kit
Max1797 EV Kit
The MAX1797 evaluation kit (EV kit) is a high-efficiency, step-up DC-DC converter for
portable hand-held devices. Unlike typical boost circuits, the MAX1797 output is
completely disconnected from the input in shutdown. The EV kit accepts a positive
input voltage between 0.7V and VOUT and converts it to a 3.3V output for currents up
to 500mA. The EV kit provides ultra-low quiescent current and high efficiency for
maximum battery life.
The MAX1797 EV kit is a fully assembled and tested surface-mount printed circuit
(PC) board. It can also be used to evaluate other output voltages in the 2V to 5.5V
range. Additional pads on the board accommodate the external feedback resistors
for setting different output voltages.
3.5 Transmitter/Receiver and Sensor
One of the options that could be powered by the systems is a wireless sensor. Similar
projects have used an off-the-shelf thermocouple/transmitter/receiver kit
manufactured by MadgeTech. It transmits at a frequency of 418 MHz and has a
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Progress Report 20/December/2010 Aidan Walsh
maximum transmission distance of 100 ft. The main drawback with the Madgetech
kits is that they are quite expensive so an alternative will have to be sourced.
3.6 Storage Capacitors
To supply the DC-DC boost converter with enough power to get over its start up
phase storage capacitors were used. The power needed to get past the start up
phase was worked out using the formula P=I*V where P is power, I is current and V is
voltage. The power needed worked out to be 28.5 milli-Watts where the current
needed at start up is 57 milli-Amps and the voltage needed at startup is 0.5 volts.
Therefore using the E=1/2CV2 the size of the capacitor needed to supply this power
can be worked out to be 84.77 milli-Farads. (C = 0.0285 Watts/0.3363 Volts2)
To accommodate for the loss in capacitors the size of the capacitor was increase to
0.1F which was much easier to come by. 3.3 and 10 Farad Capacitors were ordered
also.
To calculate the charging time needed to charge each of the capacitors to 99%, the
formula 5*R*C is used.
This works out to be 348.5 seconds for the 0.1Farad capacitor. The 3.3 and 10 farad
capacitors were worked out to take roughly 187 minutes and 566 minutes
respectively.
All calculations are again based on the previous Fuel Cell and will be altered
accordingly to apply to the new Fuel Cell for choosing the optimum capacitance
values.
3.7 Trans-Impedance Amplifier
This is one of the options explored to deal with the low voltage problem with the
system. A Trans-Impedance Amplifier is effectively a current-to-voltage converter
which takes an electric current as an input signal and produces a
corresponding voltage as an output signal.
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Progress Report 20/December/2010 Aidan Walsh
Fig 3.8 Trans-Impedance Amplifier
In the op-amp current to voltage converter the output of the operational amplifier is
connected in series with the input voltage source and the op-amp’s inverting output
is connected to point A. As a result the op-amp’s output voltage and the input
voltage are summed.
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Progress Report 20/December/2010 Aidan Walsh
4. Testing
4.1 Capacitor Testing
Each of the Tests were redone on each of the capacitors to ensure the capacitors
were still in fully functioning order using the demonstration circuit which consisted
of the DC power supply with the same characteristics as the fuel cell, the storage
capacitors (0.1F, 3.3F, 10F) , a manual switch connecter in series with the power
source between the capacitor and DC-DC boost converter and the Boost Converter
itself connected in parallel with the power source and storage Capacitor.
The 0.1F capacitor took roughly 320 seconds to charge fully which was in sync with
the calculated value of 348.5 seconds. The screenshot below was obtained from one
of the oscilloscopes in the laboratory and shows the charging of the 0.1Farad
Capacitor.
Fig 4.1 Charging Graph of 0.1F Capacitor
Each division along the x axis represents 40 seconds and it takes the capacitor
approximately 8 divisions to reach the voltage supplied by the power source which
corresponds to 320 seconds to fully charge. Similar tests have been completed on
the 3.3F and 10F capacitors.
A LED was then attached along with a resistor of 1000ohms to the output of the
Boost Converter and the switch is closed to allow the capacitor to discharge through
the boost converter into the resistor to light the LED. Fig below shows the capacitor
discharging.
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Progress Report 20/December/2010 Aidan Walsh
Fig 4.2 Discharging Of 0.1 F Capacitor
Similar tests were conducted using different load resistance values and different
Loads.
4.2 Boost Converter Testing
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Progress Report 20/December/2010 Aidan Walsh
5. Problems Encountered
The main problem encountered was how to get around the very low power output
of the microbial fuel cell. Although it was able to charge the capacitor which
powered the boost converter which in turn could power a LED/Calculator etc there
was no way to automate the discharging and charging of the capacitor. This is
because the current design of the system uses a manual switch to power the boost
converter. A separate voltage comparator was not an option as the microbial fuel
cell was unable to power it and charge the capacitor simultaneously. Therefore
either more than one Fuel Cell would be needed or some other alternative must be
used.
This is why the Maxim Boost Converter was sourced and the Current-to-Voltage
converter was investigated. Due to the extreme difficulty in keeping the converter
stable and getting accurate results, the Maxim Converter is seen as a more viable
option so that will be the first attempt to remedy the problem.
6. Where To Next?
The next steps that I will have to take is to test the new Microbial Fuel cell
which shall hopefully be constructed by January and then to re-evaluate all
sections of the project to see if there can be any improvements made to the
capacitor values, switching mechanism etc.
I have to further investigate the wireless sensors and their application and
what would be the most suitable type for this project and also investigate the
alternative of using the system to charge some sort of battery.
The new boost converter will be ordered and I will have to test it in the new
circuit set up and set the properties of the built in voltage comparator to
cause the capacitor to automatically discharge once it has reached the
desired point. This will hopefully increase the efficiency of the system .
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Progress Report 20/December/2010 Aidan Walsh
7. Table of Figures
Fig 3.1 Hydrogen Fuel Cell
http://images.yourdictionary.com/fuel-cell
Fig 3.2 Microbial Fuel
http://www.making-hydrogen.com/hydrogen-fuel-cell.html
Fig 3.3 Power Density/Voltage Vs. Current Density curve
Previous Tests Completed By Stephen Mulryan
Fig 3.4 Thevenin Equivalent of Microbial Fuel Cell
Previous Tests Completed By Stephen Mulryan
Fig 3.5 Operation Modes Of Standard Boost Converter
http://www.Wikipedia.org
Fig 3.6 Voltage and Current Waveforms of Boost Converter
http://en.wikipedia.org/wiki/Boost_converter
Fig 3.8 Trans-Impedance Amplifier
Fig 4.1 Charging Graph of 0.1F Capacitor
Based on Lab Experiments
Fig 4.2 Discharging Of 0.1 F Capacitor
Based on Lab Experiments
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Progress Report 20/December/2010 Aidan Walsh
8. References
Much of the research for this project has been done with the help of the following
websites:
www.wikipedia.org
http://www.madgetech.com/
http://www.making-hydrogen.com/hydrogen-fuel-cell.html
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5497188&tag=1
http://pubs.acs.org/doi/full/10.1021/es0480668
Also extensive amounts of this report are based on Stephen Mulryans similar project
from last year.
Energy Management System For Microbial Fuel Cell Page 19
