Final Year Project ThesisStudent: 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: _______________________________

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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.

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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.

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Table of Contents:

Declaration of originality iAbstract iiAcknowledgements iiiList of Figures viList of Tables vii

Chapter 1 – Introduction 11.1 Project Background 11.2 Project Objectives 21.3 Milestones set out for project 21.4 Tasks Completed 51.5 Applications of a Microbial Fuel Cell 7

1.6 Report Layout 9

Chapter 2 – Fuel Cells 102.1 History of Fuel Cells 102.2 Background of Fuel Cells 132.3 BioFuels 162.4: 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

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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 55Appendices 57

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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

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Fig3.15: Bi-Polar Junction Transistor Layout 47

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

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Chapter 1 – IntroductionThis 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.

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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.

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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.

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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.

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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]

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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]

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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 Cells2.1 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 [33].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 [3]. 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 [3]

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

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Cell worked. Around this time the world was only beginning to understand the basic principles

of Chemistry and Physics [34].

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 [33].

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 [33].

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

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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 [35].

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 [4]. A picture of the van can be viewed in Fig 2.2.

Fig2.2: GM Hydrogen Fuel Cell Van [5]

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

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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 [6]. 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 [7]. 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.

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Fig2.3: Hydrogen Fuel Cell structure [8]

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 [9]. 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 H2 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 [10]. Fig 2.4 conveys the bond between the Hydrogen

atoms in a graphical sense.

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Fig2.4: Ionic bond of Hydrogen Atoms [11]

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

[12]. 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.

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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 [13]. 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

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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 [36]. 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 [36].

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 [14].

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.

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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 [15]. 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 [16]

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 [17]. 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 [18]. A

prokaryote is a unicellular organism having cells lacking membrane-bound nuclei [19]. 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 [37].

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 [20]. 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 [21]. 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 [38].

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 [39].

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:

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

22

the second polarization graph was obtained from. The Polarisation graph can be viewed in

Fig2.7.

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

0

0.1

0.2

0.3

0.4

0.5

0.6

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Current density (mA/cm2)

Pow

er d

ensi

ty (m

W/m

2 )

0

200

400

600

800

1000

1200

Volta

ge (V

)

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[22]

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 [23]

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 [24]

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 [25]. 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

to a relatively higher voltage were impossible to come by. The lowest Converter that was found

31

was the TPS61200 designed and built by Texas instruments [26]. 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 [26]

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 [27]

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 [28]

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 [29]

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.

V 1

0 . 82 V d c

R 1

6 8 0

0

C 110 uF

0

L1

2 . 2 u H

C 410 u

0

C 510 u

0V 2

TD = 0

TF = 10 nP W = 0 .0 0 0 3P E R = 0 . 00 0 8

V 1 = 0

TR = 1 0 n

V 2 = 5

C 61 0 u

0

R 25 0 k

0

V 3

TD = 0 . 00 03TF = 10 nP W = 0 .0 00 5P E R = 0 . 00 0 8V 1 = 0TR = 1 0 nV 2 = 5 0

M 3

M b re ak n

0

0

M 4M b re ak n

C 710

V O L TA G E = 0 . 82

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 [40] 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 [30]. An electric

double layer capacitor uses a physical mechanism which generates an electric double layer

which performs the function of the dielectric [31]. 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[31]

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 [32]

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 [33]

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 ChargersA 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

MFC and the DC-DC boost converter could be fully charged. One would be charged faster

50

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.

Chapter 5 – Conclusion

51

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.

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

52

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.

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

53

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.

References:

54

[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

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

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[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:

64

65

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