FYPReport.doc - Compsoc NUIG - NUI Galway by wuzhenguang

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									Final Year Project Thesis
         Student: Stephen Mulryan
           Student ID: 06583725
Discipline: Electronic & Computer Engineering
        Supervisor: Dr. Maeve Duffy
    Co-Supervisor: Professor Ger Hurley


                Project Title:
 Energy Conversion for low voltage sources
                March 2010
                             Declaration of Originality

I hereby declare that this thesis is my original work except where stated.




Date: ___________________________________




Signature: _______________________________




                                                i
                                          Abstract:
With research into renewable energy sources at an all time high and growing year on year
without any sign of stopping every avenue of energy generation is being investigated. It would
seem that no characteristic disqualifies an energy source from being examined. Some of these
energy sources being investigated present a new challenge in the extremely low power which
they output. The voltage levels output by these power sources are quiet low and so there are
problems encountered when converting the energy into a form which can be used. For example
many common batteries are made up of cells which output voltage in an order of anywhere
between a fraction of a volt up to 3.7 volts. To power devices with a higher voltage level such as
12 volts or a device which requires more current than the individual cell can provide two main
solutions have been used through the years consisting of a direct approach and an indirect
approach. The direct approach involves cascading these cells together in series to get the
required voltage and then in parallel to obtain the required current. The indirect approach
involves the use of storage components to store enough charge to meet the current
requirements and the use of conversion circuitry to increase the voltage output of an
individual cell, this approach is usually managed by a power management system.

Researchers in the Energy Research Centre in NUI Galway are inspecting fuel-cells which are
based on bio-fuels, where they are investigating the output power levels achieved using
different bio-waste materials. The aim of this project is to develop circuits to demonstrate the
performance of these cells. Firstly, a relatively simple low power circuit will be designed to
demonstrate the level of power that is produced continuously by a typical cell, while a second
demonstrator will illustrate how the power generator by a cell can be stored for use in
providing higher output power levels.




                                                ii
                                 Acknowledgements:
Over the course of this project the help and guidance received from the many people has
helped to no end in the progress that I have made. I would like to take this opportunity to thank
first and foremost my project supervisor, Dr Maeve Duffy for her guidance throughout the
project and my project Co-Supervisor, Professor Ger Hurley for his guidance throughout the
course of the project. I would like to thank Longlong Zhang of the Power Electronics Research
Center for his good advice and support through the duration of the project.

I would also like to thank the technicians Martin Burke, Myles Meehan & Aodh Dalton for their
continuous support and patience.

Last but not least I would like to thank my family for their support during this important stage
of my education.




                                                iii
                                  Table of Contents:
Declaration of originality                                    i
Abstract                                                      ii
Acknowledgements                                              iii
List of Figures                                               vi
List of Tables                                                vii


Chapter 1 – Introduction                                      1
      1.1 Project Background                                  1
      1.2 Project Objectives                                  2
      1.3 Milestones set out for project                      2
      1.4 Tasks Completed                                     5
      1.5 Applications of a Microbial Fuel Cell               7
      1.6 Report Layout                                       9


Chapter 2 – Fuel Cells                                        10
      2.1 History of Fuel Cells                               10
      2.2 Background of Fuel Cells                            13
      2.3 BioFuels                                            16
      2.4:
              2.4.1 Microbial Fuel Cell Background            18
              2.4.2 Building an MFC                           20
              2.4.3 Characterisation of Microbial Fuel Cell   20
              2.4.4 Microbial Fuel Cell Efficiency            27

Chapter 3 – Conversion Circuitry & Demonstration Circuitry    28
       3.1: DC – DC Boost Converter                           28
       3.2: Commercial Controller IC                          31
       3.3 Storage Capacitors                                 41
       3.4: Demonstration circuit                             43
       3.5 Automation of charging circuit                     46




                                                  iv
Chapter 4 – Battery Chargers           49
      4.1 Charging algorithms          49
      4.2 Trickle Charging             50
      4.3 Possible charge method       50


Chapter 5 – Conclusion                 52

References                             55
Appendices                             57




                                   v
Fig1.1: End goal System Overview                                                          2
Fig1.2: Pilot Scale Microbial Fuel Cell                                                   7
Fig1.3: SMFC and PMS setup                                                                8
Fig2.1: Groves Fuel Cell                                                                  10
Fig2.2: GM Hydrogen Fuel Cell Van                                                         12
Fig2.3: Hydrogen Fuel Cell structure                                                      14
Fig2.4: Ionic bond of Hydrogen Atoms                                                      15
Fig2.5: Microbial Fuel Cell Process                                                       18
Fig2.6: Two chamber Microbial Fuel Cell Polarisation graph                                21
Fig2.7: Single chamber open air MFC                                                       23
Fig2.8: Thévenin equivalent circuit                                                       25
Fig2.9: Cml Innovative Technologies 1 milli-Amp LED                                       26
Fig3.1: Modes of DC-DC Boost Converter Circuit                                            29
Fig3.2:TPS61200 Circuit Layout                                                            32
Fig3.3: Output Current Vs Input Voltage from TPS61200                                     35
Fig3.4: Texas Instruments TPS61200EVM-179 Module Circuit Diagram                          36
Fig3.5: Voltage ripple in output voltage of DC-DC Boost converter with 1k load attached   38
Fig3.6: Voltage Ripple Vs Load Current                                                    39
Fig3.7: Efficiency Vs Load Current                                                        39
Fig 3.8: Efficiency Vs Output Current                                                     40
Fig3.9: Boost Converter Spice model                                                       40
Fig3.10: Circuit layout of electric double layer capacitors                               42
Fig3.11: Circuit layout of demonstration circuit                                          43
Fig3.12: Charge curve of 0.1 Farad capacitor                                              44
Fig3.13: discharge curve of 0.1 Farad capacitor, powering calculator                      45
Fig3.14: discharge curve of 0.1 Farad capacitor, powering LED                             46
Fig3.15: Bi-Polar Junction Transistor Layout                                              47




                                                   vi
Fig3.16: Circuit Layout of the 555 timer                      47



List of Tables:

Table 2.1 Measurements taken from two chamber MFC             22
Table 2.2 Measurements taken from single chamber MFC          24
Table 2.3 Minimum voltage and current required to light LED   25




                                             vii
Chapter 1 – Introduction
This project is concerned with using Microbial Fuel Cells to power everyday applications. This
chapter lays out the background of the project, the objectives hoped to be achieved by the
project, Milestones set out for the project and the tasks that have been completed. It also
discusses the way in which the report will be laid out.


1.1 Project Background:
For years the world’s economy has relied heavily on the combusting of various types of oil to
power everything from automobiles to Jet aircraft. Oil supplies running dangerously low has
resulted in the search for new & efficient ways of creating energy. In the past two decades the
world has already seen the major research and investment in renewable energy sources such as
Wind, Tidal, Hydroelectric energy and Solar power among others.

In conjunction with this energy crisis the world’s population is also increasing rapidly and as a
result waste management is becoming an ever increasing enigma. Bad planning and poor
maintenance in many cases has led to Sewerage schemes operating at full capacity and needing
enormous amounts of tax payer’s money to be upgraded.

Fuel cells may yet hold the answer to these two major problems. A Fuel Cell is in effect a
portable power source similar to a Battery. The difference between the two is that a battery
has a life cycle after which it cannot operate. A fuel cell can operate indefinitely as long as it is
topped up with a fuel at an interval of a certain length of time. Types of Fuel Cells vary and so
do the fuels they operate with.

One type of fuel cell that is of significance to the two problems motioned above and to this
project is the Microbial fuel cell. It is a fuel cell which operates at very low power. Three
important traits of this fuel cell is that it creates power, it can operate using wastewater as a
fuel and in effect treat this wastewater and it can produce Hydrogen gas which can be used to
power hydrogen fuel cells.


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1.2 Project Objectives:
The challenge in this project is to design circuitry which will enable a device which outputs
power as low as the Microbial fuel cell does to power a useful device which operates at a higher
power level. This will involve obtaining a good understanding of Fuel Cells and how they
operate, electrical characteristics of a common Microbial Fuel Cell, acquiring a sound technical
ability with low power Electronic devices and an understanding of battery chargers and charging
algorithms. The end goal would be to design and build circuitry which would enable the
Microbial fuel cell to charge a rechargeable battery.




                                    Fig1.1: End goal System Overview




1.3 Milestones Set Out for Project:
Numerous Milestones were set out from the project specification which was received from the
project supervisor Dr. Maeve Duffy which would be used as a guideline that needed to be
followed in order to complete the project.
Each Milestone was awarded a certain merit. There were five merits laid out which were Pass,
Average, Good, Very Good, Excellent. Each merit indicating the type of grade awarded for the
project.




                                                   2
Pass Milestone:
The first step was to research the numerous types of fuel cells being developed in the modern
day industry. This would involve researching the structure and operation of various types of
fuel cells which would include finding out what type of materials are used in making the fuels
cells, what reactants are used to produce energy, the average efficiency of each fuel cell, the
average power, voltage and current outputs of the fuel cell. The next step was to find out how
bio-fuels are used in the generation of electricity. Once this was completed a Thévenin
equivalent circuit of a fuel cell was to be obtained using readings received from measuring the
power, voltage and current outputs from a microbial fuel cell that was developed by the Energy
Research Centre.
The next step was to customise the electrical performance of the demonstrator bio-fuel cells.
This involved measuring the output voltage versus the load characteristics of a typical bio-fuel
cell, Then determine the energy level for a typical feeding period, This involves finding the
levels of power output from a Bio-fuel cell over a period of time. The last step for this milestone
was to demonstrate the application of the cells in powering a small digital device. For this
demonstrator devices with the lowest possible power consumption needed to be identified,
once this was done the conversion circuitry needed for this had to be designed and built.

Average Milestone:
Investigate the application of Bio-fuel cells in charging a mobile phone battery. This step
involved reviewing the power and energy requirements of a battery charger for a mobile phone
from a DC source and then estimate the number of fuel cells needed to provide sufficient
energy to charge the mobile phone battery under normal conditions. After doing this a DC/DC
Converter solution for connecting between the bio fuel cells and the battery charger circuit
should be designed. Since the bio fuel cells are at such a low power output level this will most
likely be a boost converter. The last thing to achieve in this milestone is to provide spice
simulation results from such a circuit.




                                                 3
Good Milestone:
Design a demonstrator battery charger solution for 2-4 microbial fuel cells, this will depend on
how many the Energy Research Centre can provide. This milestone involves multiple steps, the
first being to review battery charging algorithms for trickle charging. The next thing to be done
is to change the DC/DC converter solution developed above for reduced source power, for this
appropriate switches and passive components should be used in SPICE modeling. A steady state
operation is to be assumed for different load conditions when modeling circuits in SPICE. Then
what combination of connections between the cells provides the maximum output power for
the given load needs to be determined. The final requirement of this milestone involves
building and testing the circuit with a range of loads which simulate the load which will be
applied by the battery charger. A low voltage supply and a series resistor should be used to
model the fuel cell source while variable resistors should be used to model varying load applied
by the charger.

Very Good Milestone:
Develop a controller solution. This involves firstly identifying a suitable commercial controller or
develop a microcontroller solution. Once this has been done a battery charger algorithm for
trickle charging needs to be designed and implemented. After this the combined controller and
power conversion circuitry need to be tested with the fuel cell sources and a variable load. The
last part of this milestone involves testing the conversion circuitry combined with the controller
by using them to charge a rechargeable battery.

Excellent Milestone:
Demonstration of battery charging for mobile phone with bio-fuel cell sources. For this
milestone the efficiency of the converter over all load conditions must be determined and the
main loss contributors have to be identified. Then propose potential improved solutions for
future development. The final step is to test and customise the complete system for different
fuel cell characteristics.




                                                 4
1.4 Tasks completed:
As laid out in the previous section there were several Milestones to be passed in the project.
The first step taken in this project was to fully understand the background information needed
to make a start on the project. At this stage there was a lot of research carried out. The main
point of supply for this research was from articles sourced from books in the library and
websites. Once this early research was completed each objective could then be worked
through.

The first objective was mainly to demonstrate the Microbial Fuel Cell powering a small device
such as a fan or DC motor. This objective was to be achieved by working through a number of
stages.
These stages sat in the following order:

   1. Investigate structure, application and electrical characteristics of Fuel Cells.

   2. Create Circuit model of Microbial Fuel Cell by measuring power, current and voltage
          outputs.

   3. Demonstrate the Microbial Fuel Cell powering an LED.

   4. Demonstrate the Microbial Fuel Cell powering a small device i.e. a Fan / DC Motor

This objective was met to a certain degree. The structure and electrical characteristics of the
Fuel Cell were understood, the circuit model of the Microbial Fuel Cell was created but the
power output of one Microbial Fuel was a lot lower than expected so only a very low power LED
could be lit continuously by the Fuel Cell although by using Capacitors and a Mosfet which can
be switched off and on using a common signal generator the Fuel Cell could light a common
LED. The frequency of this signal generator can then be changed accordingly so as to enable the
Capacitors to store enough energy to light the LED. To enable the Microbial Fuel Cell to light a
small device such as a fan conversion circuitry would need to be either built using equipment in
the Lab or ordered if need be.


                                                 5
The average milestone in the last section was partially met. The theory behind the charging of
rechargeable batteries such as phone batteries was understood. The number of Fuel Cells
needed to charge a rechargeable battery under normal conditions was estimated and a DC-DC
boost converter to step up the voltage was obtained but it was not tested in conjunction with a
battery charging circuit.

Due to the unforeseen extremely low power output of a single Fuel Cell the rest of the
objectives set out by each Milestone left needed more time to work through than was left on
the lifetime of the project so the focus of the project switched to getting a demonstrator
capable of powering a low power device. How to go about completing the design of a battery
charger circuit suitable for the low power output of the MFC’s was researched but no physical
demonstrator was built.




                                                6
1.5 Applications of a Microbial Fuel Cell:
There is a range of applications in today’s world in which Microbial Fuel Cells could be used to
provide long term power. The number of scenarios where the Fuel Cell could be used would be
limited due to the low power output yet there are a range of applications it could be used for.
One very significant use is where a research group named the Advanced Water Management
Centre located in the University of Queensland, Australia has constructed a pilot scale Microbial
Fuel Cell. They have constructed the Microbial Fuel Cell on the site of one of Fosters brewery’s.
The researchers have used brewery wastewater to feed the Fuel Cells. Both the anode and
cathode are made from Carbon Fibre. The Microbial Fuel Cell has a volume of approximately
1m3 and consists of 12 chambers [1]. The following is picture of the Microbial Fuel Cell:




                              Fig1.2: Pilot Scale Microbial Fuel Cell [1]




                                                  7
Another common application of Microbial Fuel Cells is in powering a remote device. A special
type of MFC called sediment Microbial Fuel Cell (SMFCs) are considered to be an alternative
renewable power source for remote monitoring systems. How do SMFCs differ from normal
MFCs? When a Microbial Fuel Cell is in operation in a natural water source such as a river, lake
or sea and takes electrons from Microbial reactions on the anode which is buried under
sediment it is then called a Sediment Microbial Fuel Cell. In a collaboration of academics from
different disciplines within the University of Washington a significant project with MFC’s very
similar to the MFC’s modeled for this project in terms of power output was carried out [2]. In
the SMFC they used, the unit could not provide enough power to operate the remote device in
question. In fact the SMFC could not even guarantee a continuous supply of power to the
electrical device. They needed to design and build a power management system to store the
power output by the SMFC and then power the remote device periodically. They created an
SMFC in a river in Washington to test it by placing an Anode made from graphite under
sediments in the river and a Cathode made from either stainless steel or graphite in the water.
Microorganisms in the sediment act as catalysts, the microorganisms colonise the surface of the
Anode and oxidise natural organic chemicals which allows Electrons to become free. The Anode
and Cathode are connected through wiring to the power management system which first stores
energy and then powers the remote device which will send a signal back to base much like the
graphic in Fig 1.3 portrays.




                                  Fig1.3: SMFC and PMS setup [2]



                                                8
Another possible application is obviously the aim of this project. The use of MFC’s in charging
rechargeable batteries. This would work in a similar way to the way the SMFC worked. Some
form of a Power Management System would store up energy and at regular intervals discharge
current into the battery charger. The battery charger would then trickle charge the battery over
a long period of time.

Other practical applications involve the powering of a calculator, the powering of a low power
fan and the powering of any common LED. These applications are merely for demonstration
purposes and do not possess any real gains.


1.6 Report Layout:
The remainder of this report is structured as follows:

Chapter 2 describes the Microbial Fuel Cells and indeed Fuel Cells in more detail. This chapter
provides an in-depth analysis of Fuel Cells and methods of harvesting energy from such low
power energy sources. It explains topics such as Fuel Cell history, the application of Bio-Fuels in
electricity generation, Microbial Fuel Cell characteristics and Fuel Cell efficiency. It also
describes the procedure to model the Microbial Fuel Cell and how to customise the Fuel Cell
power output for different loads.

Chapter 3 describes the operation of the conversion circuitry and the construction of the
demonstrator circuit. This also gives details on the power consumption and power efficiency of
the different components within the conversion circuit.

Chapter 4 provides information on different algorithms used in the charging of rechargeable
batteries. It also provides possible charging circuits that could be used for this project.

Chapter 5 provides a discussion on the tasks completed and an analysis of Practical and
Theoretical knowledge gained over the course of the project.




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Chapter 2 – Fuel Cells
2.1 History of Fuel Cells:
Fuel Cells gradually evolved from the concept of Electrochemistry. The first steps toward the
creation of Fuel Cells were taken in 1800. In 1800 British Scientists William Nicholson and
Anthony Carlisle took note of the process of using electricity to decompose water into
Hydrogen and Oxygen [3].
The next step came from William Robert Grove in 1838. He discovered that by arranging two
platinum electrodes with one end of each immersed in a container of Sulphuric acid and the
other ends separately sealed in containers of Oxygen and Hydrogen current would constantly
flow between the platinum electrodes [4]. Grove observed that the water level rose in both
tubes as current flowed. He combined several sets of these electrodes in a series circuit; he
termed this circuit as a “gas battery” which is portrayed in Fig 2.1. This discovery was in effect
the birth of the Fuel Cell.




                                     Fig2.1: Groves Fuel Cell [4]

During the nineteenth century after Grove had created the world’s first Fuel Cell two Scientist
among many others lead the charge in the debate of how the Groves Gas Cell operated.
Christian Schönbein and Johann Poggendorff spend long periods debating exactly how Groves




                                                 10
Cell worked. Around this time the world was only beginning to understand the basic principles
of Chemistry and Physics [5].

In 1889, a German Chemist and industrialist by the name of Ludwig Mond and his assistant Carl
Langer recorded experiments with a Hydrogen-Oxygen Fuel Cell which achieved 0.5574 Amps
Metres-2 with a voltage of 0.73 Volts across the output of the Fuel Cell. The reason the current
was recorded as Amps Metres-2 instead of just Amps was because the Current was measured
from the surface area of either the Anode or Cathode Electrode. Mond and Langer used
electrodes made from thin, perforated Platinum. This reduced the energy lost through heat in
the system [3].

Friedrich Wilhelm Ostwald a Baltic German chemist who was born in 1883 devoted much of his life to
Chemistry. He received the Nobel Prize in Chemistry and was accredited with providing much of the
theoretical understanding of how Fuel Cells operate. He determined the roles of many of the
components within the common Fuel Cell such as Anions, Cations, Electrodes, Electrolyte and
Oxidising and Reducing agents. This in-depth review of Fuel Cells in many ways laid the ground
for research in later years [3].

Francis Thomas Bacon was born in 1904. He was an English Engineer. During his life he
contributed many developments in the world of Fuel Cells. He began by researching alkali
Electrolyte Fuel Cells in the late 1930’s. This research culminated in Bacon building a Fuel Cell
which could operate under pressure as high as 20.6843 Mega Pascal’s. During World War II
Bacon worked on a Fuel Cell that could be used by the British Navy’s Royal Navy Submarines. In
1959 Bacon demonstrated a Fuel Cell Stack which output 5 Kilo-Watts of power and had an
operating efficiency of 60%. Although it was said to be very expensive to build it did attract the
attention of Pratt & Whitney, a United States Aircraft Engine Manufacturer. Pratt & Whitney
licensed the patent for the Fuel Cell from Bacon’s Research institute to use for their bid to
provide electrical power for NASA’s Apollo project which was successful. The efficiency of his
Fuel Stack even now seems astonishing. Given that the charging & discharging efficiency of



                                                  11
most batteries made today generally range between 50% and 90% and fossil fuels need to be
burnt in order to provide the energy to charge these batteries [6].

In 1966 General Motors (GM) of America was accredited with being the first automobile
manufacture to use a Fuel Cell to power a vehicle. The vehicle was a 1966 GMC Handivan on the
outside. Its insides were converted into a science lab of new technology that appeared more like
a whisky still of old [7]. A picture of the van can be viewed in Fig 2.2.




                                 Fig2.2: GM Hydrogen Fuel Cell Van [7]

After this demonstration one would expect to see Fuel Cell development increase significantly
but between the mid 1960’s and the early 1990’s Fuel Cell Research seemed to fade into the
background. The last two decades however has seen a massive expansion in Fuel Cell research.
In 1993 the first Bus was powered by a Fuel Cell and since then all kinds of vehicles powered by
Fuel Cells including trains, ships airplanes, Space Shuttles and many more. Many car
manufactures in light of the shortening supply of gasoline are now pursuing Fuel Cell
development as a means of powering their vehicles of the future. This is good news as the



                                                  12
financial clout and research capabilities these companies will provide will inevitably allow the
world of Science to overcome some of the barriers that are preventing Fuel Cells from powering
homes, vehicles and all other devices which we use on a daily basis.


2.2 Background of Fuel Cells:
The first question that must be answered clearly is what exactly is a Fuel Cell? A fuel cell is an
electrochemical device that combines hydrogen and oxygen to produce electricity, with water
and heat as its by-product [8]. An electrochemical device is an instrument which operates by
generating electricity from the process of a chemical reaction. In principle a Fuel Cell operates
in much the same way as a battery does. They both use chemical reactions to produce
electricity; the difference between them being that unlike a battery a Fuel Cell will not run down
or require recharging. It will produce energy in the form of electricity and heat as long as Fuel is
supplied [9]. The two definitions above are the two best explained definitions of what a Fuel
Cell is and how it differs from a battery but yet for people without a background in Biology or
Chemistry this concept is very hard to grasp. The general structure of the device includes two
chambers, although you can get different variations including a single chamber Fuel Cell. In the
most popular variation one chamber is called the Anode chamber and the other is called the
Cathode chamber. There is always an object or some kind of substance separating these two
chambers which is usually referred to as the Fuel Cell membrane. The two chambers are also
connected through a circuit which has some kind of resistive load attached. The structure of the
common Hydrogen Fuel Cell can be inspected in Fig 2.3.




                                                 13
                                Fig2.3: Hydrogen Fuel Cell structure [10]

Of course Fuels vary and are not restricted to Hydrogen. In fig 2.4 the chemical Hydrogen gas or
H2 as it is represented in the picture is being fed into the Fuel Cell. H is the letter used in the
study of Chemistry to denote the chemical Hydrogen while the subscript 2 which follows after
denotes the number of atoms of that particular chemical which are present. The atom is a basic
unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively
charged electrons [11]. It is also well known to any person involved in Science or in Chemistry in
particular that Hydrogen has one Proton and one Electron orbiting it in its natural state and that
because of this Hydrogen is naturally a very unstable Atom. That is why the symbol H 2 is more
commonly come across than H on its own. The Hydrogen Atoms have bonded together. The
type of bond they have formed is known as an ionic bond. An ionic bond is a linking together of
atoms in such a fashion that one atom gives [12]. Fig 2.4 conveys the bond between the Hydrogen
atoms in a graphical sense.




                                                   14
                                Fig2.4: Ionic bond of Hydrogen Atoms [13]

All Atoms have a similar structure to the Hydrogen Atom and so all Atoms can be interfered
with by another Atom or Molecule and it is this concept which allows Fuel Cells to operate. By
using a substance called a Catalyst the rate at which the Fuel Cell operates can be increased. A
Catalyst a substance that causes or accelerates a chemical reaction without itself being affected
[14]. Fig 2.3 shows how Hydrogen Atoms are split into e- and H+. The e- represents the Electrons
and the H+ represents the Hydrogen ions. This process of splitting the Hydrogen Atoms is
accelerated by the fact that on the Anode electrode which conducts Electrons into the circuit
there is usually a Catalyst to speed up the partition of the Atoms. The H+ ions are allowed
through the Electrolyte or Ion Exchange Membrane as it is commonly referred to as which
repels the negative charge attached to the electrons. These electrons are then free to flow
through the circuit attached to the Anode. Provided the circuit offers a resistive load due to
ohms law there will be a voltage across the circuit and current will flow. In cases where the load
is an actual device which will perform a function it should be noted that the device will only
operate if the minimum power requirements of the device are met by the minimum power
output of the Fuel Cell. The internal resistance should also be taken note of as it will create a
voltage division with whatever load is attached. The Electrons then flow out to the Cathode
chamber where they can re-join the Hydrogen ion and react with a chemical held in the
Cathode to create a harmless by product. In the case portrayed in Fig 2.3 the Fuel Cell is a type
of Fuel Cell know as an open air Fuel Cell. In this type of Fuel Cell air is let pass through the
Cathode Chamber. This way the Hydrogen passed from the other side of the Fuel Cell can react
with Oxygen to form water or H2O as it is known in the Science world.



                                                   15
There are various types of Fuel Cells and most of them operate in a very similar fashion to the
operation of the Hydrogen Fuel Cell with the Proton Exchange Membrane (Electrolyte) if not
exactly the same way. Some different Fuel Cells to name a few are Solid oxide Fuel Cells,
Molten Carbonate Fuel Cells and Microbial Fuel Cells. Solid oxide Fuel Cells operate nearly the
same way. The Anode and Cathode are separated by an Electrolyte which is conductive to
Oxygen ions but not conductive to Electrons. This setup is a reverse operation to the ion
Exchange Membrane design. An Oxygen molecule is split in the Cathode chamber, and then the
Oxygen Cation is feed through the Electrolyte to the Anode chamber leaving the Electrons in
the Cathode chamber. A Cation is a positively charged ion. The Electrons then flow from the
Cathode to the Anode where the flow creates a voltage just like the Hydrogen Fuel Cell. Once
the Electrons reach the Cathode the Oxygen atoms react with the Hydrogen molecule to form
water. Molten Carbonate Fuel Cells operate in a similar manner to the Solid Oxide Fuel Cell
except its Electrolyte consists of a liquid carbonate which is an oxidising agent. An oxidising
agent is a chemical compound that transfers Oxygen atoms very quickly. As mentioned
previously the Microbial Fuel Cell will be the type of Fuel Cell investigated in this project. It is a
Fuel Cell which can operate with various types of Biological Waste being used as Fuel, it will be
discussed further on.


2.3 Bio-Fuels:
Bio-Fuel is a fuel made from plant materials or refuse as opposed to petroleum [15]. Under this
definition most Fossil Fuels which are burnt can be termed as being Bio-Fuels as most are
Biological in nature. Often when Bio-Fuels are discussed people are talking about Fuels which
are Biological in nature but are also “Carbon neutral”. Carbon neutral means that when a Fuel
like Bio-Fuel is burnt it does not add any more Carbon to the atmosphere than it has already
taken away. This means that burning the Fuel will not add to the ever increasing problem posed
by the Greenhouse effect. The Greenhouse gas effect is where gases in the atmosphere trap a
percentage of the heat that enters our atmosphere which comes from the Sun. The burning of
Fossil Fuels adds more gas to the atmosphere which will eventually if not controlled lead to our


                                                  16
Planet becoming over heated which will have severe consequences. As will be observed further
on, Bio-Fuels can create energy without being burnt and releasing Carbon Dioxide into the
atmosphere but when Bio-Fuels were first thought to be useful in energy creation it was the
burning of the Fuel which was the main method to create energy. There were many methods.
Biomass was a very straight forward and popular method. It originally involved the burning of
wood shavings and excess straw to provide heat or power. Nowadays in third world countries
such as Brazil in particular farmers are growing Crops especially to provide Fuel for Biomass.
This is leading to a food shortage in parts of the world [16]. Another use of Biomass is Pyrolysis,
this involves allowing the material to be broken down under heat to produce combustible gases
such as Hydrogen and Carbon Monoxide. These gases can then be used normally to heat or
power dwellings or to cook food [16].
Biodiesel is another method used. Biodiesel is a clean burning alternative Fuel. It is generally
produced by reacting vegetable oil or animal fat with an alcohol, this process is termed as
transesterification. The process results in two products being created. Biodiesel is obviously one
of the products and glycerine being the other. Glycerine can be used in the production of soap
so it is a valuable by-product to have. Biodiesel can be used alone or can be mixed with
common diesel. When mixed with Diesel it can be used in current automobiles with little or
alterations to the engine of the vehicle. When used alone the engine will need significant
alterations. There are many positives to using Biodiesel including the fact that Biodiesel outputs
far less emissions than Petroleum Diesel, there is better lubrication in Biodiesel so the life of the
Engine will be increased and the most important point is that Biodiesel has a higher Cetane
number than Petroleum Diesel meaning that BioDiesel is more efficient. Cetane is an alkane
Hydrocarbon with the chemical formula C16H34 [17].
Another method which will be given a mention is the use of Biogas as an alternative fuel to
natural gas. It is a practical alternative as Biogas has practically the same structure as natural
gas, this means that burners for natural gas can be used to burn Biogas. The gas is produced
from either plant or animal waste or a mixture of the two.




                                                 17
The Biofuel used to operate Microbial Fuel Cells can range from Acetic acid obtained from
fermented plants or vegetation to wastewater from polluted water sources.


2.4 Microbial Fuel Cell:

2.4.1Background:
As has been mentioned many times previously this project centers on a type of Fuel Cell called
a Microbial Fuel Cell. What exactly is a Microbial fuel Cell? A microbial fuel cell (MFC) converts
chemical energy, available in a bio-convertible substrate, directly into electricity. To achieve this,
bacteria are used as a catalyst to convert substrate into electrons [18]. Just like any type of fuel
Cell MFC’s have an Anode chamber, a Cathode Chamber, an ion exchange membrane and a
load attached between the two chambers. Open air versions of MFC’s can be created in the
same way that open air versions of the Hydrogen Fuel Cell can be created. Fig 2.5 shows the
layout of an MFC using acetic acid as a Fuel and producing Hydrogen gas as one of its by
products.




                                Fig2.5: Microbial Fuel Cell Process [19]




                                                  18
Projects are taking place right around the globe involving the investigation of energy produced
by MFC’s today. These projects represent the newest approach for generating electricity. It has
signaled a new era in the research of MFC’s after so many decades have passed since it first
became known that MFC’s could generate power. The notion of using Microbial fuel Cells to
generate energy first came about when M.C Potter, a professor of Botany at the University of
Durham succeeded in generating electricity from cultures of enteric bacterium E-Coli. Enteric
bacterium is a large group of gram negative rod-shaped bacteria characterised by a facultative
aerobic metabolism [20]. This work was not continued after this point as at the time there
would have been little interest in pursuing renewable energy sources as Fossil Fuels were
plentiful. The fact that the man’s area of expertise was Botany and not Chemistry or
Microbiology could have also been a contributory factor. In 1931 a very significant development
took place. Barnet Cohen created several half MFC’s that, when connected in series was
capable of producing an output of 35 volts although the current output in this setup was only 2
milli-Amps. DelDuca carried this progress forward by experimenting with the use of Hydrogen
by fermenting Glucose by using Clostridium butyricum as the reactant at the anode. Clostridium
butyricum is an anaerobic prokaryote that requires the absolute absence of oxygen to grow [21]. A
prokaryote is a unicellular organism having cells lacking membrane-bound nuclei [22]. This was

an exciting time yet it ended disappointingly as the Fuel Cell was found to be undependable as
the Hydrogen produced from the micro-organisms was inconsistent. Susuki solved this issue in
1976 by limiting the current flow to a rate proportional the rate at which Hydrogen was being
produced from the Glucose. The next major piece of work on MFC’s came in the 1980’s when
M.J. Allen and H. Peter Benneto both of which were from a London University envisaged the
use of MFC’s in third world countries. During their lifetimes they contributed largely to the
theory behind the MFC’s and the Science worlds understanding of MFC’s. One of the most
significant breakthrough of the modern age came in the 1990’s when B-H. Kim and his
colleagues at the Korea Institute of Science and Technology showed that a bacterium known as
Shewanella oneidensis was electrochemically active. This meant that by using this bacterium in
an MFC electricity could be generated without the need for Electron Mediators [23].


                                                  19
Electron Mediators are usually Phenolic compounds which allow Electrons to be passed directly
from Microbes to the Electrode. A Microbe is an organism too small to be seen with the naked
eye [24]. Phenolic compounds are compounds that occur naturally from the decomposition of
aquatic vegetation or that are manufactured and used in disinfectants, biocides, preservatives,
dyes, pesticides and medical and industrial chemicals [25]. They are often very expensive and
sometimes can be toxic so they are not the most desirable way of transporting Electrons from
the Microbes to the electrode. That is why that discovery was a very important one. The newest
development in the history of the MFC has already been mentioned in the section 5 of chapter
one. It is the recent MFC prototype created by a research group at the University of
Queensland, Australia in 2007 where they developed an MFC with a capacity of 10 litres of
brewery wastewater for Foster’s brewery company. The prototype converts the brewery
wastewater into carbon dioxide, electricity and clean water. As this prototype was very
successful the brewery and the research group are now collaborating to create a 3000 litre
capacity version of the MFC which is estimated to produce 2 kilo-Watts of power [26].


2.4.2 Building an MFC:
A simple two chamber fuel cell can be constructed by collecting common household materials
aswell as some specialised materials. Some common household materials include plastic
bottles, PVC pipe, a drill, salt, flanges, resistors, copper wire and sealing glue among other items
[27].

 2.4.3 Characterisation of Microbial Fuel Cell:
In order to model a Microbial Fuel three characteristics of the Fuel Cell needed to be obtained.
These three characteristics were the current output of a single cell, voltage output of a single
cell and the internal resistance of a cell. Once these three characteristics were known we could
create a Thévenin equivalent circuit. This would benefit the project as the power output could
be acquired and different devices which the MFC could potentially power could be identified.

Two Polarisation graphs were attained through measuring the power density and voltage
outputs at certain current densities and then graphing power density against current density


                                                20
and voltage against current density. One of the Polarisation graphs were attained from a simple
two chamber MFC which used slaughter house waste water as a Fuel source while the other
graph was obtained from a single cell MFC for which a fuel source had not been specified.

The Polarisation graph shown in Fig2.6 is for the simple two chamber MFC which has an Anode
area of 20 cm2. The Anode area is the area which the Current Density and Power Density is
measured from:

     Polarisation graph of MFC fed with Slaughter house wastewater as a fuel source
                     900                                                                    200

                     800                                                                    175

                     700                                                                    150

                     600                                                                    125




                                                                                                  P/mW m-2
          Ecell/mV




                     500                                                                    100

                     400                                                                    75

                     300                                                                    50
                                                          Voltage
                     200                                  Power                             25

                     100                                                       0
                           0   50   100 150 200 250 300 350 400 450 500 550 600

                                                      J/mA m-2

                               Fig2.6: Two chamber Microbial Fuel Cell Polarisation graph




Even though the white legend says Power indicating that it is measured in Watts the
measurement taken is actually in milli-Watts per metre-2. As some points taken are very close
together the current density at the midpoint of these points will be taken and only one value
will be shown in Table 2.1 which defines the measurements that can be calculated at each
point.




                                                          21
                       Table 2.1 Measurements taken from two chamber MFC

As can be seen from the table above the maximum power output from this MFC is 0.364 milli-
Watts which is very low and would not even light the lowest power Light Emitting Diode (LED)
available. The internal resistance of the MFC is also quite high for setup with relatively high
voltage output and as a result the current output is very low. This would mean that powering
devices using a fuel cell stack would also be very difficult. If the minimum voltage of the device
was higher than the output voltage of a single MFC then without the use of conversion or
storage circuitry you would need to connect MFC’s in series to increase the voltage output but
there would then be a trade-off between increase in voltage output and decrease in current
output as doubling the voltage would also double the internal resistance which would decrease
the maximum current output.

The Energy Research Centre here at the National College of Ireland, Galway who created the
two chamber MFC had also created a second MFC which was an open air MFC which is where
the second polarization graph was obtained from. The Polarisation graph can be viewed in
Fig2.7.



                                                22
                    0.6                                                               1200




                                                                                             Power density (mW/m2)
                    0.5                                                               1000
      Voltage (V)




                    0.4                                                               800

                    0.3                                                               600

                    0.2                                                               400

                    0.1                                                               200

                     0                                                                0
                      0.05   0.1   0.15      0.2         0.25     0.3     0.35    0.4
                                      Current density (mA/cm2)

                                    Fig2.7: Single chamber open air MFC

The graph in Fig2.7 is nearly exactly the same as the graph in Fig2.6 in terms of the type of
measurements taken. However the area over which the Power and Current density is measured
is different. The Anode area for this MFC is 5.4cm2. The blue points on the graph represent
Power density Vs. Current density while the white points represent Voltage Vs. Current density.
The Current and Power can be calculated in the exact same way as the way they were
calculated for the two chamber MFC by multiplying the Current density and Power density at
each point by the Anode Area and the internal resistance can be achieved by using ohms law.
Ohms law is stated as V = I*R where V is voltage, I is current and R is resistance.




                                                    23
By following the instructions on the previous page Table 2.2 which shows all the measurements
for each point on the curve.




                      Table 2.2 Measurements taken from single chamber MFC

This MFC showed greater promise as the average internal resistance was far less than the two
chamber MFC and the power output is greater than the other MFC. While in terms of power
Point three has the maximum power output is was decided to use Point two when modelling
the output of a single fuel cell as a higher voltage output can be achieved without a significant
increase in internal resistance or a dramatic drop in current output. From the measurements at
this point on the graph the Thévenin equivalent circuit on the next page can be created.




                                                24
                                  Fig2.8: Thévenin equivalent circuit




Although the power output of the single chamber MFC was an improvement on the power
output of the two chamber MFC it still does not posess enough power to perform a relatively
simple function of powering an LED from the Electronics lab in Nuns Island. After testing of all
Light Emitting Diodes in Nuns Island the minimum voltage and current requirements were
obtained:




                      Table 2.3 Minimum voltage and current required to light LED




                                                  25
The MFC outputs suggest the power supplied would be enough to power a specialised low
power LED such as the Cml Innovative Technologies 1 milli-Amp shown in Fig2.9 which
illuminates with 1 milli-Amps of current running through it although the typical voltage that the
the Light Emitting Diode is rated for is 1.7 volts so the illumination may not be that strong.




                         Fig2.9: Cml Innovative Technologies 1 milli-Amp LED[28]

The optimum power output for the MFC would be where the load resistance matches the
internal resistance of the Fuel Cell. If the load resistance is greater than the internal resistance
then more voltage will be dropped across the load attached than the internal resistance this
will be beneficial in some cases but it is a trade-off with a decrease in current running through
the circuit. If the load resistance is less than the internal resistance then there will be more
current running through the circuit which is again better in certain situations but there will be
less voltage dropped across the load resistance than the internal resistance.




                                                   26
 2.4.4 Microbial Fuel Cell efficiency:
As there is no energy used to generate Fuel for the Microbial Fuel Cell, the efficiency has to be
calculated in a different way to the way power efficiency is calculated for batteries and other
power sources.
While power generation is the principal aim of MFC development, another objective is also to
derive as many of the electrons stored in the Fuel as current as is possible and to reclaim as
much energy from the system as is possible. How well both of these two aspects is carried out
determines the efficiency of the MFC. The retrieval of Electrons is known as the Coulombic
Efficiency which is described as the fraction of Electrons recovered as current against the
amount of Electrons inhabiting the Fuel substance at the start of the process.
Coloumbic efficiency is defined as:



                   CE = Coulombs recovered/ Total Coulombs in substrate [29]

Where CE is the Coulombic efficiency, Coulombs recovered is the amount of Electrons
recovered from the Fuel measured in Amperes and Total Coulombs in substrate is the amount
of Electrons measured in Amperes that inhabit the Fuel.
There are two main loss contributors in MFC’s namely ohmic losses and Activation losses.
Ohmic losses occur when the flow of Electrons is being hindered by the resistance of the
Electrode material. The higher the conductivity of the Electrode and the lower the contact
losses and distance to travel within the Electrode the higher the efficiency. Activation losses are
present as a result of an energy barrier in place which needs to be overcome. It stifles the
transfer of Electrons from electrochemically active microorganisms to the Electrode in the
Anode chamber and stifles the transfer of Electrons from the Electrode to the substance in the
Cathode chamber.




                                                27
Chapter 3 – Conversion Circuitry and
            Demonstration Circuitry
 3.1 DC – DC Boost Converter:
As was shown in the previous chapter the power output from the Microbial Fuel Cell was quiet
low and in particular the voltage output was very low, so much so that a single cell on its own
would not power very much. This leaves two options, cascade the Microbial Fuel Cells in series
or parallel or a mixture of the two to get the required voltage and current output or use some
form of conversion circuitry to step up the voltage output. The latter of the two options is the
more practical for this project as mentioned previously the number of Fuel Cells the Energy
Research centre can provide is limited.

A boost converter is a device which outputs voltage higher than its input voltage. The device
can be inductive, capacitive or a mixture of the two. It is known through an Empirical law of
Physics called the law of conservation of energy that “Energy can neither be created nor
destroyed; it can only be changed from one form to another”. This law means that the power
input to the boost converter must be equal to the power output from the boost converter. This
is true when you neglect very tiny losses in the power through heat dissipation and current
leakage. Therefore if the voltage is stepped up there will be a trade off on the output with
current being decreased. For devices which need a good deal of current, a storage capacitor
could be used in these scenarios as a solution to this problem. The common DC-DC boost
converter implements two modes of operation, a continuous mode and a discontinuous mode.
The discontinuous mode is an unsatisfactory mode to be in from the users’ point of view as in
this mode the amount of power needed by the load can be transferred in a time smaller than
the commutation period. The commutation period is the inverse of the frequency of the
switching device or it can also be defined as DT+ (1-D) T where D is the duty cycle of the
switching device and T is its period. The fact that the power can be transferred in a time smaller
than the commutation period means that by the end of the commutation period the inductor


                                                28
will be completely discharged. This is inefficient and time consuming as the inductor will have
to be charged from the start. The continuous mode is a satisfactory mode to be in as the
current through the inductor never drops to zero. The image shown in Fig3.1 describes exactly
how the circuit changes in continuous mode in order to drive up the voltage on the output in a
typical DC-DC boost converter circuit layout.




                              Fig3.1: Modes of DC-DC Boost Converter Circuit [30]

In the top circuit diagram in Fig3.1 above the operation of the boost converter when the switch
s is closed can be observed. When the switch, s is closed the input DC voltage is applied across
the inductor, L and the switch. Current then builds up in the inductor, increasing its stored
energy. The formula for stored energy is E = (1/2)*L*IL2 where E is the stored energy, L is the
inductor value and I is the current through the inductor. The variation of current through the
inductor when the switch s is closed can be found by using the formula Vi = L(diL/dt) = L(∆I/DT)
where Vi is the input voltage, L is the inductance value, diL is the first derivative of the current
through the inductor, dt is the first derivative of time, ∆i is the change in current through the
inductor, D is the switching devices duty cycle and T is the switching devices period. In the


                                                  29
bottom circuit diagram in Fig3.1 on the previous page the operation of the boost converter
when the switch, s is open can be examined. This causes the inductor to discharge as it now has
a resistive path along to discharge. As it discharges the magnetic field around the inductor
gradually weakens until it is completely discharged although if the duty cycle and feedback loop
built into the boost converter are set right this should never happen. As a result of the inductor
discharging a voltage now appears across the diode D and it is activated. The equation for the
output when the switch is open is as follows:
Vi – Vo = L(diL/dt) = -L(∆I-/ (1-D)T)
where Vi is the input voltage, Vo is the output voltage, L is the inductance value, diL is the first
derivative if the current through the inductor, dt is the first derivative of time, ∆I- is the change
in current coming out of the inductor, D is the duty cycle and T is the period of the switch(time
closed + time open)
Magnetic Flux is the measure of the strength of a magnetic field over a given area [31]. It is
known that the more that current discharges from the inductor the more the magnetic field
around the inductor weakens and therefore the magnetic flux decreases. It is also known that
from one of the basic principles in power electronics that in continuous mode the volt-seconds
of an inductor over a complete cycle (when the switch is on and off) must be zero. The formula
for this basic principle is defined as lambda = Li = integral of V over dt where lambda is the volts
applied to an inductor over time, L is the inductance value, i is the current through the inductor,
V is the voltage applied to the inductor and dt is the first derivative of time. Lambda must be
zero so this means that the inductance value multiplied by the current through the inductor
over the period of the switch being on and off. This makes sense as if the current into the
inductor is greater than the current discharged then the magnetic flux will continue to increase
over a number of periods and eventually drive the inductor into saturation. This should not
happen as the inductor will fail to operate properly when in saturation mode. Therefore the
increase in current through the inductor when the switch is on must be equal to the decrease in
current through the inductor when the switch is off. From the two formula’s obtained from
when the switch is on and off the following formula can be worked with:


                                                  30
                                   (Vi/L)DT = ((Vo – Vi)/L)(1-D)T

From this formula the following formula can be achieved through simplification:

                                            Vo = Vi/1-D

So now it is known that the output voltage can be determined by the duty cycle of the switch.
Due to the increase and decrease of the current over the period of the switch there will be
ripple in the current on the output. This ripple can be worked out through the following
formula:
                                   Change in current = (ViD)/f*L


where Vi is the input voltage, D is the duty cycle of the switch, f is the devices frequency and L is
the inductance value.
It is important to know how much ripple current there is because the variation in the power
output by the device needs to be known in order to know for sure that the output will be
sufficient to consistently power another device.



 3.2 Commercial Controller IC:
Unfortunately due to the low power dissipation of the Microbial Fuel Cell it was not possible to
engineer a boost converter using diodes and BJT’s as Diodes and BJT’s which have diodes
between the base and emitter gates drop at least ≈0.3 volts which is voltage that cannot afford
to be dropped as the output of one Fuel Cell is only 0.42 volts therefore a DC-DC boost
converter that would enable the step up of voltage from the voltage output from the Microbial
Fuel Cell had to be ordered. This was disappointing as it took away the flexibility of the Boost
converter in terms of choosing the speed at which the switching took place within the DC-DC
converter. By choosing a slower frequency you can store more energy in the inductor and any
capacitors placed in parallel with the inductor. DC-DC boost converters which convert 0.42 volts




                                                 31
to a relatively higher voltage were impossible to come by. The lowest Converter that was found
was the TPS61200 designed and built by Texas instruments [32]. The TPS61200 can be started
up with as little as 0.5 volts. So to achieve this, the voltage applied to the input of the DC-DC
boost converter needs to be modeled on the output of two Microbial Fuel Cells connected in
series. Unfortunately using the TPS61200 chip was not an option as there was no translation
board small enough to enable the attachment of the chip to a normal circuit board. This meant
the TPS61200EVM-179 had to be ordered instead which is an evaluation board with the
TPS61200 implanted in it. The TPS61200EVM-179 is a customised version which has set
inductance & capacitance values. This restricts the flexibility of the converter even more as the
size of the inductor and capacitors determine what energy that would be stored in the boost
converter to a certain extent. Fig3.2 shows the circuit layout inside the TPS61200.




                               Fig3.2:TPS61200 Circuit Layout [32]

The following list lays out the function of each pin in the diagram in Fig 3.2:




                                                  32
 VAUX – This pin exists to provide the supply voltage for the control stage of the circuit. A
   capacitor should be connected between the VAUX pin and ground. This has the effect of
   acting as a switch at start up. When power is supplied to the DC-DC boost converter at
   first the output pin VOUT is completely disconnected. Once the capacitor connected
   between VAUX and ground reaches 2.5 volts the converter switches to normal
   operation where 3.3 volts are applied across the output.

 VIN – The supply voltage (voltage output by MFC) is applied between this pin and
   ground.

 PS – This is used to enable/disable power save mode. Power save (PS) mode is enabled
   by connecting the PS pin to ground and disabled by connecting it to VIN. This mode is
   used to improve efficiency when a light load is attached to the output of the converter.
   In the evaluation module which was used for this project the power save mode was
   disabled by placing a shunt between the VBAT pin which was in effect the VIN pin and
   the power save (PS) pin. Although the Power save mode made the DC-DC boost
   converter more efficient for light load it didn’t make a big difference to the project as
   the DC-DC boost converter was attached to a heavy load to keep the power
   consumption to a minimum. Another important point regarding power save mode is
   that in this mode extra current was drawn from the power source at start up to charge
   the capacitors within the circuit faster which wasn’t ideal for the project.

 EN – This pin when connected to VIN enables the device and when connected to ground
   disables the device.

 UVLO – This pins function is to disable the Boost Converter from operating if the device
   drops below a certain voltage. By default the minimum voltage is set to 0.25 volts but
   by connecting two resistors in parallel to the VIN pin and connecting the UVLO pin in
   between these two resistors then through the concept of voltage division a new UVLO
   can be programmed. This UVLO voltage can be set by using the following formula:


                                           33
   R3 = R4 x ((VINMIN/VUVLO) – 1)
   where R3 is the resistor between the VIN pin and the UVLO pin, R4 is the resistor
   between the UVLO pin and ground. VINMIN is the minimum input voltage and VUVLO is the
   voltage applied between the UVLO pin and ground.
   For the evaluation module which was used in the project the Under Voltage Lockout
   value was set to its default value of 0.25 volts as there no resistors connected in parallel
   between VIN and ground.

 GND – acts as a common ground pin for all components and supply source.

 VOUT – This is the output voltage that will be applied to the load resistance. The output
   voltage can be programmed but the maximum recommended output voltage is 5.5
   volts. The output voltage can be changed in much the same way as the UVLO voltage
   can be changed. It can be done by using the following formula:
   R1 = R2 x ((VOUT/VFB) -1)
   where R1 is the resistor connected between the VOUT pin and the FB pin, R2 is the
   resistor connected between the FB pin and ground, VOUT is the output voltage and VFB
   is the feedback voltage
   For the evaluation module used in this project VOUT was configured to output 3.3 volts
   on an open circuit. This was worked out from the formula above. R1 is 1M Ω and R2
   was 178K Ω and VFB was 0.5 volts. The equation is laid out as follows:
                          1M Ω = 178K Ω x ((VOUT/0.5 V) - 1) =>
                                    VOUT = 3.3 Volts

 FB – This pin is the feedback pin and is used to sense the output voltage and changes
   the Duty cycle of the Mosfets inside the circuit accordingly.

 PGND – This pin is for situations where there is high current when the MOSFETs are
   switching to ensure that no ground shift problems arise. This simply means that the
   value of ground does not change.


                                             34
The graph in Fig 3.3 illustrates the maximum output current that can be obtained at various
input voltages. The line generated in red represents the Boost converter setup that is being
used for this project.




                         Fig3.3: Output Current Vs Input Voltage from TPS61200 [33]

It can be seen from the graph in Fig3.3 that the maximum current output when the voltage
input is 0.82 volts is approximately 180 milli-Amps.
It is important to note that if the input or output values change, the feedback loop in the DC-DC
boost converter will cause the Duty cycle of the Mosfets inside the device to change which will
in turn cause current and voltage ripple on the output of the DC-DC boost converter. This will
cause the average power output from the converter to either increase or decrease depending
whether the Duty cycle is increased or decreased.

As mentioned above the TPS61200EVM-179 evaluation module was the DC-DC boost converter
used for this project. The output voltage has a default value of 3.3 volts but can be adjusted
using the concept of voltage division. Fig3.4 shows the outlay of the TPS61200EVM-179 circuit
which incorporates the TPS61200.




                                                    35
                       Fig3.4: Texas Instruments TPS61200EVM-179 Module Circuit Diagram [34]




There are in all three versions of the Evaluation module. The adjustable version which is being
for this project and two other fixed version, this is why some values in the above diagram are
not specified. For the adjustable version R3 is open and R2 is specified as 0 so UVLO is short
circuited to VIN meaning the minimum voltage input is set to its default value of 250 milli-Volts.
R4 is 1 Mega ohm and R5 is 178 Kilo ohms. These two resistors are to stop too much current
from going into the feedback pin, FB. Lastly C4 is an open circuit.

There was a lot of testing of the evaluation module to see how it performed.
Initial tests were done without any internal resistance in series with the input to the DC-DC
boost converter as more current than the Microbial Fuel Cell model was able to provide was
needed. Therefore the input voltage was set to approximately 0.82 volts but the converter was
allowed to draw as much current as it needed. In these initial test the maximum power
efficiency could be obtained through the formula:
Maximum Power Efficiency (%) = (Maximum Power output/Maximum Power input) * 100/1
These measurements were all taken after the converter had entered steady state.



                                                 36
To enter steady state a minimum of 0.5 volts needs to be applied across the input of the
converter and a current of 57 milli-Amps needs to be supplied.

The following measurements taken is a sample test on a 1K Ω resistor attached to the output :
Test 1:
Load Resistance value:                       1000 Ohms
Voltage input:                               0.74 Volts
Current input (estimate):                    0.03 Amps
Voltage output (mean value):                 3.3 Volts
Voltage output Ripple:                       0.16 Volts
Current output (maximum):                    0.0033 Amps
Power input:                                 0.0222 Watts
Power output (maximum):                      0.011154 Watts
Maximum Power efficiency:                    50.24 %

The screenshot taken from the oscilloscope in Fig3.5 portrays the voltage ripple in the output
voltage for test 1.

A full set of tests using different loads attached to the output of the DC-DC boost converter can
be viewed in Appendix A.




                                               37
           Fig3.5: Voltage ripple in output voltage of DC-DC Boost converter with 1k load attached

A few main points can be observed from these tests. One is that as the resistive load attached
to the output of the converter increases the power efficiency decreases. This is proven correct
by the graph in Fig3.8 which was provided by Texas instruments, this is also proven by the
graph in Fig3.7 which was plotted from tests done on the DC-DC boost converter evaluation
module attaching different loads to the output. This graph shows how the output current is
related to the efficiency of the converter. It can be observed that as the output current
decreases as it will do if the load increases, the efficiency also decreases. The second point is
that in steady state if the Microbial Fuel Cell is required to power the load continuously the load
must be greater than 100 kilo-ohms as in the setup where 100 kilo-ohms have been attached
1.53 milli-Amps is being drawn from the power source which is more current than the MFC can
provide. The third point can be seen from Fig 3.8 which shows the relationship between voltage
ripple and load current. As voltage ripple decreases load current increases.




                                                     38
Fig3.6: Voltage Ripple Vs Load Current




  Fig3.7: Efficiency Vs Load Current




                 39
                                                    Fig3.8: Efficiency Vs Output Current [35]

The use of Spice models in this project was limited as the DC –DC boost converter obtained did not
publish the kind of Mosfets and components used in the feedback loop of the TPS61200EVM-179.

                                                                      L1
                                                                                                     Mbreakn M4
                   R1

                   680                                                2.2uH
                                                                    C5
                                                          C4        10u
                                                          10u                                                                            R2
                                             C1                                                                                          50k
               VOLTAGE = 0.82                10uF                                          M3                                  C6
                                                      0         0                                       V3
                                                                                                                               10u
                                    C7                              V1 = 0       V2
                                    10                              V2 = 5             Mbreakn
                                                                    TD = 0
          V1                                                        TR = 10n                            TD = 0.0003        0         0
                                         0                          TF = 10n                            TF = 10n
                                                                    PW = 0.0003                         PW = 0.0005
0.82Vdc                         0                                   PER = 0.0008                        PER = 0.0008
                                                                                                 0
                                                                                                        V1 = 0
                                                                                                        TR = 10n
                                                                                                        V2 = 5         0
          0                                                                        0
                                                      Fig3.9: Boost Converter Spice model




                                                                              40
 3.3 Storage Capacitors:
Even though the Microbial Fuel Cell can drive a device that works on extremely low power in
steady state it still leaves a problem regarding how to supply the DC-DC Boost Converter with
the power it requires to get over the start-up phase.
To get over this issue a Storage capacitor was used to provide the energy.
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 current needed at start up is 57 milli-Amps and voltage needed at
startup is 0.5 Volts.
Using the formula E = 1/2CV2 the size of capacitor needed to supply this power can be worked
out. Where E is the power, C is the capacitance value of the capacitor and V is the input voltage.
0.0285 Watts = 0.5*C*(0.82 Volts)2 -> 0.0285 Watts = 0.3362 Volts2 * C.
C = 0.0285 Watts/0.3363 Volts2 -> C = 0.08477 Farads => 84.77 milli-Farads.

Seen as there is a lot of loss in capacitors it was decided to increase the size of the capacitor by
approximately 18% to a 0.1 Farad capacitor which was easier to come by also. The decision was
also taken to order 3.3 and 10 Farad capacitors [36] in order to demonstrate the MFC powering
a larger power device for a long period of time although it would not be continuous.
The time taken for the 0.1 Farad capacitor to fully charge can be calculated by using the time
constant formula. The formula is stated as follows:
                        Total Time to Charge = 5*R*C
where R is the resistor in series with the capacitor in the RC circuit and C is the capacitance
value of the Capacitor. In this case the resistance in series is the internal resistance of the
Microbial Fuel Cell. This resistance when two MFC’s are connected in series is 697 ohms.

                Total time to charge = 5*697Ω*0.1F = 348.5 seconds
After this time the capacitor is said to be 99% fully charged which can be taken as fully charged.




                                                 41
However the 3.3 & 10 Farad capacitors charging time cannot be calculated in the same way as
they are not standard storage capacitors. They are known as electric double layer capacitors.
Normal capacitors use a dielectric between two opposite electrodes. A dielectric is a material
such as glass or porcelain with negligible electrical or thermal conductivity [37]. An electric
double layer capacitor uses a physical mechanism which generates an electric double layer
which performs the function of the dielectric [38]. This electric double layer acts as an insulator
and does not allow current to flow straight away when an external DC voltage is applied across
the capacitor but as the voltage applied across the capacitor increase a threshold point is
passed after which current begins to flow. A special attribute of these capacitors means that
instead of thinking of the capacitor as one big capacitor it is thought of as a cascade of
capacitors in parallel, each with their own varying internal resistance as Fig3.10 conveys.




                       Fig3.10: Circuit layout of electric double layer capacitors[38]

The effect of this is that the formula which was used on the previous page would need to be
applied to each single path. Each path will have varying internal resistances and capacitance
values. Capacitors which have low internal resistances will take a shorter time to charge relative
to capacitor with high internal resistances as more current will flow through that path so it will
have the effect of making the charging time longer than expected.
The maximum energy each of these capacitors can hold can be calculated in the same way



                                                     42
using the formula E = 1/2CV2. When the capacitance and voltage values of the 3.3 Farad
capacitor is substituted into the formula the maximum energy that could be stored is 1.10946
Joules. When the capacitance and voltage values of the 10 Farad capacitor is substituted into
the formula the maximum energy that could be stored is 3.362 Joules. Both of these capacitors
would be able to provide the DC-DC boost converter with enough current to power a device
which offers a load less than 100 kilo-ohms for quite a long period of time.


3.4 Demonstration circuit:
The demonstration circuit consists of the Microbial Fuel Cell acting as the power source, the
storage capacitor connected in parallel to the power source, a manual switch connected in
series with the power source between the Capacitor and the DC-DC Boost converter and the
Boost Converter itself connected in parallel with the power source and storage capacitor. The
picture in Fig3.13 shows the physical layout of the circuit.




                             Fig3.11: Circuit layout of demonstration circuit




                                                   43
Similar demonstration circuits have been made using the 3.3 & 10 Farad capacitors. The user
can attach the Microbial Fuel Cell at the input terminal, then wait for roughly 300 seconds for
the 0.1 Farad capacitor to fully charge and then close the switch to let the storage capacitor
discharge current into the DC-DC boost converter and whatever load is attached at its output.

By applying the Thévenin's equivalent circuit of the two MFC’s connected in series to the input
of the demonstration circuit and waiting for approximately 300 seconds a charge curve across
the capacitor can be seen similar to the one obtained from the oscilloscope in the Laboratory
which can be viewed in Fig3.12.




                             Fig3.12: Charge curve of 0.1 Farad capacitor

As can be seen 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 so therefore it
takes the capacitor 320 seconds to fully charge.




                                                 44
The demonstration circuit was tested by charging up the storage capacitor, then attaching a DC
battery powered Calculator to the output of the DC-DC boost converter. The DC battery was
obviously removed. Once the capacitor was fully charged the manual switch was turned on so
that the output of the capacitor was connected to the input of the boost converter.
The current discharged from the Capacitor combined with the current being constantly output
from the MFC is enough to power the calculator for about 1 minute. The discharge slope of the
capacitor when a calculator is attached to the output of the DC-DC boost converter can be
examined in Fig3.13.




                   Fig3.13: discharge curve of 0.1 Farad capacitor, powering calculator

The demonstrator was also used to power an LED which was attached to the output of the
converter for 2.13 seconds. It powered the LED for a lot less time than it powered the calculator
due to the fact that Light Emitting Diodes have little or no resistance and will draw as much
current as the power source can provide it with. The discharge slope of the 0.1 Farad capacitor
while powering the LED can be inspected in Fig3.14.


                                                   45
                       Fig3.14: discharge curve of 0.1 Farad capacitor, powering LED

The other capacitors can also be used to power high power devices.


3.5 Automation of charging circuit:
A decision was taken to let the switching in the circuit be controlled by a manual switch as it would give
more flexibility in terms of what device you are powering with the circuit. The switching could be done
by using a 555 timer instead of a manual switch. A 555 timer is a device which when supplied with the
minimum voltage at its input specified by its datasheet it can output a pulse of a certain duty cycle. The
duty cycle of the pulse can be configured by a circuit developer. The pulse output by the timer can be
used to turn a PNP Bi-Polar Junction Transistor (BJT) on and off. This PNP BJT can then be placed
between the storage capacitor and the input of the DC-DC boost converter. A PNP transistor is in effect
a switch controlled by the amount of voltage applied between its two terminals called the base and
emitter terminals. If the voltage applied across these terminals is said to be low which means the
voltage is near zero then the transistor is active and current is allowed to flow between the emitter
terminal and what is called the collector terminal (a third terminal). If the voltage between the base and
emitter terminal is a negative voltage, more negative than a set threshold voltage then the transistor is


                                                    46
not active and current cannot flow from the emitter terminal to the collector terminal. The layout of the
Bi-Polar Junction Transistor can be viewed in Fig3.15.




                              Fig3.15: Bi-Polar Junction Transistor Layout [39]




In Fig3.15 E represents the Emitter terminal, B represent the Base terminal and C represents
the Collector terminal.

There are seven modes in which the 555 timer can be configured to. The mode we are interests
in is Astable mode. In this mode the circuit developer can configure the timer to have a set
period and duty cycle by applying 5 volts to the input pin of the timer and using the voltage
divider rule to set the period and duty cycle of the output. The layout of the 555 timer can be
studied in Fig3.16.




                                Fig3.16: Circuit Layout of the 555 timer [40]

The period of the output pulse can be set by using the formula, Period = 0.693*(RA + 2*RB)*C.


                                                     47
The time the output will be high for can be set by the formula, tH = 0.693*(RA + RB)*C. The time
the output can be low for can be set by the formula, tL = 0.693*(RB)*C.

An example of where you could use this automation is if you wanted to power a DC calculator
for 30 seconds out of every 6 minutes. You could set the period of each output waveform to
350 seconds (not exactly 6 minutes but makes choosing resistors easier) using the period
formula. Then substitute 350 seconds in for the period, choose the value of the capacitor you
want and then you can get the combined value of RA and 2*RB.
Period = 0.693*(RA + 2*RB)*C -> 350 seconds = 0.693*(RA + 2*RB)*0.000010 Farads ->
RA + 2*RB = 50.5 Mega ohms.
The next step taken is to decide on the time for which the output will be low. The capacitor is
been given 30 seconds to discharge so this will be the time that the output will remain low for.
tL = 0.693*(RB)*C -> 30 seconds = 0.693*(RB)*0.000010 Farads -> RB = ~ 4.329 Mega ohms.

The last step is to work out what RA needs to be for the time that the output is high for is 320
seconds.
tH = 0.693*(RA + RB)*C -> 320 seconds = 0.693*(RA + 4.329 Mega ohms)* 0.000010 Farads -> RA +
4.329 Mega ohms = 46.176 Mega ohms -> RA = 41.847 Mega ohms.

Remember the time the output is high is the time for which the capacitor is not connected to
the DC-DC boost converter and so is the time the capacitor is charging for as the Transistor is a
PNP transistor.




                                                48
Chapter 4 – Battery Chargers
A battery charger is a device used to put energy into a secondary cell by forcing current through
that cell.
Although this project did not reach the stage of designing and building a battery charger circuit
based on the output of the existing demonstration circuit this chapter will outline the steps
needed to be taken in order to extend this circuitry to have the ability to charge batteries.


4.1 Charging algorithms:
There are four main types of charging algorithms. These are constant, trickle, timer-based and
intelligent charging.

     Constant charging – This charging algorithm involves supplying the battery with a
         constant DC supply. This charging method is the simplest to implement and as a result is
         inexpensive although it has its drawbacks as when left charging a battery for too long it
         overcharges the battery which can lead to the battery having a shorter life span.

     Trickle charging – A trickle charging algorithm charger charges the battery slowly over a
         long period of time. The advantage of this algorithm is that the battery is never
         overcharged although the obvious drawback is the time needed to charge a battery is
         very long.

     Timer-based – This algorithm works by a battery being charged by a DC power source
         for a set period of time which is set before the charging begins. The advantage of this
         algorithm is that the time taken to charge a battery can be obtained and then the timer
         can be set to this value. The downside of this algorithm is that it operates on the
         assumption that the battery is completely discharged. If they are not completely
         discharged then the battery will be overcharged.




                                                 49
   Intelligent – This charging algorithm operates by first monitoring the batteries voltage,
      temperature or time under charge and then determines what amount if any current
      needs to be applied to the battery. The problem with this algorithm is that the voltage
      applied across the battery when it is fully charged which is known by the intelligent
      battery charger and was worked out from measuring the voltage across a new battery
      and voltage which is across a battery when fully charged in practical terms can be
      different. In these circumstances the battery will be continuously over charging.




4.2 Trickle charging:
  As the current output from the MFC is so small the trickle charging algorithm must be
  implemented. The rate at which a battery is charged is always specified by the charge rate
  denoted by the bold letter C. A battery is always measured in Amp Hours. This
  measurement signifies the amount of current which constantly needs to be applied to the
  battery in order to charge it in one hour. For example if a battery is a 2 Ah (Amp-Hour), then
  when losses in the charge circuit are excluded it should take 2 Amps of current applied to
  the battery for one hour to fully charge the battery. For trickle charging the charge rate C
  can be divided down to provide the battery with a lower constant current but will take a
  much longer time to charge the battery. For instance if you applied a constant current of 0.5
  Amp to the battery for 4 hours (C/4) then again excluding losses in the circuit the battery
  should be fully charged.




4.3 Possible charge method:
  A power management system needs to be developed in order to trickle charge the battery.
  A start up phase could be implemented where two relatively large capacitors between the



                                              50
MFC and the DC-DC boost converter could be fully charged. One would be charged faster
than the other so that when that is fully charged that can act as the power source and
supply a DC current to the charger circuit while the other capacitor is charging up. The
charge and discharge time of both capacitors need to be the same so that there is a
continuous current being supplied to the charging circuit. A complex switching mechanism
needs to be developed to put between the capacitors and the DC-DC boost converter.




                                            51
Chapter 5 – Conclusion
This project was interesting to work on and it showed what can be achieved when people from
a Science background collaborate with people from an Engineering background.

It also gave people involved an insight into a possible solution to the worlds two greatest
problems and mans greatest enigma. Microbial Fuel Cells shows great promise going forward to
someday have the ability to rewrite the history of waste management and energy generation
and help in some way heal the destructive impact humans are having on this planet.

Not alone was a lot learned about Microbial Fuel Cells but knowledge of other types of Fuel Cell
structures as well as a great deal about circuit design and analysis was gained.

This project started out on a steep learning curve as it was a completely new area to work in.
The unbelievable flexibility in terms of Fuel that can be used to power them, maintainability
and ease of construction of this type of Fuel Cell quickly became apparent. At the same time
the challenge of the project also became apparent. The power output by the Microbial Fuel
Cells was extremely low and finding a way of powering devices using it was not an easy task.

Even given that the power was extremely low this was overcome with the use of circuitry and
the potential capabilities of the Microbial Fuel Cell started to become clear.

The main objectives of this project were to first of all build circuitry that enabled the Microbial
Fuel Cells to power a relatively high power device and secondly to build a circuit that would
enable the Microbial fuel Cell to charge a battery.

One of these objectives was met and one wasn’t. The main reason behind the failure to design
a circuit which would charge a battery using an MFC was the surprising low power output of an
MFC.




                                                 52
Nevertheless there was an enormous amount of knowledge gained in the process of this
project. A familiarisation with the structure and electrical characteristics of a Microbial Fuel Cell
was gained. Experience with Electrical components such as the 555 timer, DC-DC boost
converter and storage capacitors were garnered.

It also developed problem solving skills as working with a power source with such a low power
output was difficult in terms of using hardware to harness its energy.

In its current state the demonstrator is made up of the Microbial Fuel Cell, a storage capacitor,
a switching device and a DC-DC boost converter to increase the voltage output from the
Microbial Fuel Cell. There are three different versions of the demonstrator. One version has a
0.1 Farad capacitor acting as the storage element, another version uses a 3.3 Farad capacitor as
the storage element and the last version uses a 10 Farad capacitor as the storage element. The
different version is simply to enable the Microbial Fuel Cell to power devices of different power
ratings. It allows us to observe the trade off between the amounts of time for which we have to
charge the capacitors against the amount of time they can power a device for. The two main
demonstration devices that were used to show the ability of the circuit was a DC powered
calculator and an LED.

If this project is to be continued in the future the first aspect that needs to be looked at is the
design and creation of a power management system to ensure that there is a constant flow of
current coming from a capacitor to the output of the DC-DC boost converter. This will system
will most lightly consist of two or more storage capacitors. There needs to be some way of
limiting the current being discharged by each capacitor to ensure that when one capacitor is
fully discharged the other capacitor that will be allowed to discharge will have had adequate
amount of time to charge up fully. The design of the switching system to implement this will be
very difficult to implement without using an external power source to enable the switching as
power output by the MFC is so low. The problem with this is that it means that the MFC would
not be a self sufficient energy source.



                                                 53
If that was implemented successfully the next step would be to design a charging circuit that
has the ability to trickle charge a battery very slowly using the current output by the DC-DC
boost converter or alternatively it would be a charging circuit that would store up the current in
another storage capacitor and discharge the capacitor as intervals through the battery.




                                               54
References:

[1] http://www.microbialfuelcell.org/www/images/stories/Pilot/5024-
017%20%28Small%29.jpg

[2] http://danu.it.nuigalway.ie/SteveMulryan/Batteryless,%20Wireless%20Sensor.pdf

[3] http://upload.wikimedia.org/wikipedia/commons/c/ce/1839_William_Grove_Fuel_Cell.jpg

[4] http://www.hydrogencarsnow.com/gm-electrovan.htm

[5] http://www.hydrogencarsnow.com/gm-electrovan.htm

[6] http://www.fuelcells.org/

[7] http://www.fuelcells.org/basics/how.html

[8] http://www.grc.nasa.gov/WWW/Electrochemistry/images/fuel_cell.jpg

[9] http://en.wikipedia.org/wiki/Atom

[10] http://www.biologylessons.sdsu.edu/classes/lab3/glossary.html

[11] http://www.hydro.com.au/handson/students/hydrogen/images/h2.gif

[12] http://dictionary.reference.com/browse/catalyst

[13] http://www.gf-5.com/resources/glossary/

[14] http://en.wikipedia.org/wiki/Cetane

[15] http://www.microbialfuelcell.org/www/index.php/Principles/

[16] http://www.making-hydrogen.com/images/hydrogen-microbial-electrolysis-cell.jpg

[17] http://www.biology-online.org/dictionary/Enteric_bacteria

[18] http://en.wikipedia.org/wiki/Clostridium_butyricum

[19] http://wordnetweb.princeton.edu/perl/webwn?s=prokaryote

[20] http://fightaidsathome.scripps.edu/glossary.html

[21] http://www.crd.bc.ca/wastewater/marine/glossary.htm


                                               55
[22] http://ie.farnell.com/productimages/farnell/standard/42767638.jpg

[23] Microbial Fuel Cells, Bruce E. Logan – The Pennsylvania State University, Published by John
Wiley & Sons in 2008

[24] http://www.dos4ever.com/flyback/boost.gif

[25] http://wordnetweb.princeton.edu/perl/webwn?s=magnetic%20flux

[26] http://focus.ti.com/lit/ds/symlink/tps61200.pdf

[27] http://focus.ti.com/lit/ug/slvu207/slvu207.pdf

[28] http://focus.ti.com/lit/ug/slvu207/slvu207.pdf

[29] http://focus.ti.com/lit/ug/slvu207/slvu207.pdf

[30] http://wordnetweb.princeton.edu/perl/webwn?s=dielectric

[31] http://industrial.panasonic.com/www-data/pdf/ABC0000/ABC0000TE2.pdf

[32] http://en.wikipedia.org/wiki/File:BJT_PNP_symbol_(case).svg

[33] http://www.princeton.edu/~chm333/2002/spring/FuelCells/fuel_cells-history.shtml

[34] http://americanhistory.si.edu/fuelcells/origins/origins.htm

[35] http://en.wikipedia.org/wiki/Francis_Thomas_Bacon

[36] http://www.habmigern2003.info/biogas/biofuels.html

[37] http://mfc-muri.usc.edu/public/mfc_history.htm

[38] http://en.wikipedia.org/wiki/Microbial_fuel_cell#History

[39] http://www.microbialfuelcell.org/www/index.php/Tutorials/Building-a-two-chamber-MFC.html

[40] http://docs-europe.origin.electrocomponents.com/webdocs/0969/0900766b80969471.pdf

    http://docs-europe.origin.electrocomponents.com/webdocs/0027/0900766b800274cc.pdf

    http://industrial.panasonic.com/www-data/pdf/ABC0000/ABC0000TE2.pdf




                                                   56
Appendices :
Appendix A – Testing of DC-DC Boost Converter

      Test 1:
      Load Resistance value:                      1000 Ohms
      Voltage input:                              0.74 Volts
      Current input (estimate):                   0.03 Amps
      Voltage output (mean value):                3.3 Volts
      Voltage output Ripple:                      0.16 Volts
      Current output (maximum):                   0.0033 Amps
      Power input:                                0.0222 Watts
      Power output (maximum):                     0.011154 Watts
      Maximum Power efficiency:                   50.24 %

      The screenshot taken from the oscilloscope in Fig3.5 portrays the voltage ripple in the
      output voltage for test 1.




                                             57
Test 2:

Load Resistance value:                      2200 Ohms
Voltage input:                              0.73 Volts
Current input (estimate):                   0.014 Amps
Voltage output (mean value):                3.11 Volts
Voltage output Ripple:                      0.160 – 0.200 Volts
Current output (maximum):                   0.00141 Amps
Power input:                                0.01022 Watts
Power output (maximum):                     0.0045261 Watts
Maximum Power efficiency:                   44.29 %

The screenshot taken from the oscilloscope in Fig3.6 portrays the voltage ripple in the output
voltage for test 2.




                                               58
Test 3:

Load Resistance value:         3200 Ohms
Voltage input:                 0.5 Volts
Current input (estimate):      0.019 Amps
Voltage output (mean value):   3.33 Volts
Voltage output Ripple:         0.200 Volts
Current output (maximum):      0.00104 Amps
Power input:                   0.0095 Watts
Power output (maximum):        0.0035672 Watts
Maximum Power efficiency:      37.55%




                                 59
The screenshot taken from the oscilloscope in Fig3.7 portrays the voltage ripple in the output
voltage for test 3.




Test 4:

Load Resistance value:                      22000 Ohms
Voltage input:                              0.82 Volts
Current input (estimate):                   0.00246 Amps
Voltage output (mean value):                3.33 Volts
Voltage output Ripple:                      0.200 Volts
Current output (maximum):                   0.0001514 Amps
Power input:                                0.0020172 Watts
Power output (maximum):                     0.00049962 Watts
Maximum Power efficiency:                   24.77 %

The screenshot taken from the oscilloscope in Fig3.8 portrays the voltage ripple in the output
voltage for test 4.


                                               60
Test 5:

Load Resistance value:                      82000 Ohms
Voltage input:                              0.82 Volts
Current input (estimate):                   0.0016 Amps
Voltage output (mean value):                3.31 Volts
Voltage output Ripple:                      0.240 Volts
Current output (maximum):                   0.00004048 Amps
Power input:                                0.001312 Watts
Power output (maximum):                     0.0001404656 Watts
Maximum Power efficiency:                   10.71 %

The screenshot taken from the oscilloscope in Fig3.9 portrays the voltage ripple in the output
voltage for test 5.




                                               61
Test 6:

Load Resistance value:                      100000 Ohms
Voltage input:                              0.82 Volts
Current input (estimate):                   0.00153 Amps
Voltage output (mean value):                3.29 Volts
Voltage output Ripple:                      0.320 Volts
Current output (maximum):                   0.0000333 Amps
Power input:                                0.0012546 Watts
Power output (maximum):                     0.000114885 Watts
Maximum Power efficiency:                   9.16 %

The screenshot taken from the oscilloscope in Fig3.10 portrays the voltage ripple in the output
voltage for test 6.



                                               62
63
Appendix B – Information on Gold Storage Capacitors:




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