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Final Year Progress Report

VIEWS: 14 PAGES: 19

									 Final Year Progress Report


              Student: Aidan Walsh
              Student ID: 07540388
        Discipline: Electronic Engineering
           Supervisor: Dr. Maeve Duffy




                  Project Title:
Energy Management System for Microbial Fuel Cells
Progress Report                       20/December/2010   Aidan Walsh


                                     Table of Contents
1 Project Overview

2 Milestones

3 Progress to Date

       3.1 Operation of Fuel Cells

       3.2 Microbial Fuel Cells

       3.3 Characterisation of Microbial Fuel Cells

       3.4 Boost Converter

       3.5 Transmitter, Receiver and Sensor

       3.6 Capacitors

       3.7 Trans Impedance Amplifier

4 Testing

       4.1 Capacitor Testing

       4.2 Boost Converter Testing

5 Problems

6 Where To Next

7. Table Of Figures

8. References




Energy Management System For Microbial Fuel Cell             Page 2
Progress Report                     20/December/2010                            Aidan Walsh




   1. Project Overview

       Researchers in the Energy Research Centre (ERC) in NUI Galway are investigating
       microbial fuel-cells (MFCs) which produce electrical power from microbial and bio-
       fuels; their main aim is to understand the electrochemical processes involved. During
       an initial collaboration with Electrical & Electronic Engineering, a demonstrator
       circuit was built and tested to illustrate the issues involved in converting the low
       power levels produced by MFCs into a useable form. This project will continue the
       collaboration with the ERC to develop some of the ideas identified to date. The main
       aim will be to design, build and test an energy management system that provides
       maximum output power from an MFC to supply a given demonstrator load.




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Progress Report                      20/December/2010                            Aidan Walsh


   2. Milestones

   The completion of the practical milestones identified below does NOT necessarily
   guarantee an award at the associated level. An award at the associated level will only be
   merited if all other aspects of the project (e.g. reports, presentation, attendance in lab
   and at meetings etc.) are completed to an equivalent level.


      Review of collaborative work with ERC to date
          - Fuel cells & MFCs: structure, principle of operation and electrical
             characteristics
          - Energy conversion circuit: storage capacitors and boost converter. Repeat
             tests to confirm circuit operation

      Energy conversion circuit performance characterisation
          - Develop PSPICE circuit model of MFC and apply it to determine the charge
              time required for storage capacitors vs. discharge time for different resistive
              loads
          - Compare circuit models with measurements; incorporate capacitor ESR if
              necessary
          - Determine the average efficiency of energy conversion over a range of charge
              / discharge cycles for different capacitors & load resistors

   Pass

      Comparison of boost converters and switched capacitor solutions for converting
       MFC energy from storage capacitors to a given load; e.g. wireless sensor
          - Determine power / energy requirements of a wireless sensor
          - Review commercial literature and identify suitable controller IC products for
             both power conversion circuit types
          - Analyse and compare the performance of both circuits, when configured to
             supply the power level required by the demonstrator - use PSPICE and
             controller IC datasheets


   Average


      Design and build a demonstrator system
          - Determine the circuit characteristics of the MFC to be used in the
              demonstrator circuit
          - Determine the minimum capacitor required to store sufficient energy for the
              load
          - Build and test the best power conversion solution identified above
          - Build and test the wireless sensor load


   Good

Energy Management System For Microbial Fuel Cell                                      Page 4
Progress Report                      20/December/2010                            Aidan Walsh




      Complete system test
          - Integrate all circuit blocks and confirm the system operation
          - Determine the efficiency of energy conversion over a complete charge /
             discharge cycle: circuit modelling and testing
          - Investigate the scope for varying wireless sensor range / functionality
             through varying capacitor charge / discharge time


   Very good


      System development for improved efficiency / power levels
           - Determine the main limits in efficiency: MFC impedance, storage capacitor,
             power conversion circuit
           - Develop a solution for overcoming the main limits identified, including circuit
             models and analysis to illustrate the level of improvement possible


   Excellent




Energy Management System For Microbial Fuel Cell                                      Page 5
Progress Report                      20/December/2010                             Aidan Walsh




   3. Progress to date.

       The current project assigned to me is based on a similar project undertaken by a
       previous final year student, Stephen Mulryan last year. The initial task was to review
       his work and get up to date on the operation of microbial fuel cells, boost converters
       and storage capacitors.




       3.1 Operation of Fuel Cells
       Fuel cells consist of two chambers filled with chemicals separated by an ion
       exchange membrane. The effect of inserting this membrane between the chambers
       results in the prevention of electrons to flow from one chamber to another.
       Alternatively it allows the positive ions or protons to flow from one chamber to
       another. Each Chamber is connected to external circuitry by means of an anode and
       a cathode. An anode is a positively charged electrode by which electrons leave and
       electric device. An electrode is an electrical conductor similar to the anode except
       that it is negatively charged. The chemical stored in the chamber is normally referred
       to as the fuel and the resulting chemical from the reaction in the chamber connected
       through the cathode is referred to as the reagent. The operation of the fuel cells
       using the ion exchange membrane is by firstly plating the anode and cathode with
       catalysts. These are chemical substances which can either speed up or slow down
       the rate of a chemical reaction without being consumed in that process. In this case
       the catalysts increase the rate of reaction in the anode chamber. In the hydrogen
       fuel cell for example the anode chamber is filled with hydrogen gas (H 2). The catalyst
       on the surface has the effect of speeding up the splitting of the electrons in the H 2
       molecule from the protons. Once split the H2 molecule, which is now a cation, passes
       through the ion exchange membrane into the cathode chamber. This leaves the
       electrons in the anode chamber where they are conducted through the anode and
       the external circuit before coming back into the cathode chamber through the
       cathode. Therefore voltage is created between the anode and the cathode and the
       value of this voltage depends on the total resistance in the external circuitry, the
       total internal resistance in the anode chamber and the amount of current conducted
       through the anode. Once the electrons are conducted back into the chamber they
       rejoin the hydrogen cation and react with the reagent that is held in the cathode. In
       the majority of fuel cells the reagent it oxygen so the reaction of the Hydrogen
       Molecules produces H2O which is water.


Energy Management System For Microbial Fuel Cell                                       Page 6
Progress Report                        20/December/2010                            Aidan Walsh




       Figure 3.1 Hydrogen Fuel Cell

       There are a large amount of other fuel cells that have a very similar if not exactly the
       same operation to the Hydrogen Fuel Cell that uses the proton exchange membrane.
       Other examples of fuel cells include solid oxide fuel cells, molten carbonate fuel cells
       and for this project Microbial Fuel Cells.

       3.2 Microbial Fuel Cells




       A microbial fuel cell (MFC) can be defined as a system that uses microorganisms to
       catalyze metabolic or enzyme catalytic energy into electrical energy (Allen and

Energy Management System For Microbial Fuel Cell                                        Page 7
Progress Report                       20/December/2010                              Aidan Walsh


       Bennetto 1993). This in theory is a good source of renewable energy but due to the
       miniscule amounts of power generated compared to solar/ wind energy systems
       etc., researchers have been hesitant on researching Microbial Fuel Cells as a viable
       source for large scale renewable energy applications.

       Microbial fuel cells are effectively self-renewable power supplies, which can
       continue operating for a very large period of time using local resources. They offer
       numerous advantages over batteries as power sources as they do not require
       recharging. This comes at a cost however as the power output from a typical
       microbial fuel cell is extremely low, much lower than most standard electronic
       components are able to operate with. This can be solved to a certain extent by
       increasing the surface area of the electrodes. If it is not possible to increase the area
       of the electrodes, a typical system could be operated less frequently using a suitable
       power management program, i.e. data transmission occurs only when enough
       energy has been accumulated by the storage solution in place in the system. A
       further advantage of microbial fuel cells is that even when exposed to extreme
       conditions the cells can return to full health when the conditions are removed. This
       is unlike standard batteries which permanently lose a portion of their efficiency
       when exposed to very low temperatures.

       Resources such as common wastewater and acetic acid which is created by plant
       waste fermentation can be utilized as a fuel source for microbial fuel cells and
       natural micro-organisms can act as catalysts which can produce electricity and
       generate hydrogen gas that could be used as a fuel source for a hydrogen fuel cell.

       3.3 Characterisation of Microbial Fuel Cell
       As the new Microbial Fuel Cell has not been fully constructed yet the previous values
       of the old Microbial Fuel Cell are being used in experimentation and replication of
       the previous project’s tests.

       The Thevenin equivalent circuit was calculated by measuring the power density,
       voltage and current density of the Microbial Fuel Cell held at the Energy Research




Energy Management System For Microbial Fuel Cell                                         Page 8
Progress Report                     20/December/2010                          Aidan Walsh


       centre.




       Figure 3.3 Power Density/Voltage Vs. Current Density curve

       The Blue points represent power density versus current density while the white
       points represent Voltage Vs. Current density. The current density is measured from
       the surface of the anode and the power density is measured the same way. The
       second set of points are used to work out the current, resistance and Power to
       achieve the most favorable results.

       The surface area of the anode was 5.4cm2 the second point was chosen, which gives
       voltage = 0.42 volts, power density = 900 milli-Watts/m2 and the current density =
       0.225 milli-Amps / cm2. The current is worked out to be 1.215 milli-amps. The power
       is then worked out by multiplying the voltage by the current which equals 0.5103
       milli-Watts. The internal resistance is calculated by Ohm’s Law and the Thevevin
       equivalent circuit is produced:




Energy Management System For Microbial Fuel Cell                                   Page 9
Progress Report                      20/December/2010                              Aidan Walsh




       Fig 3.4 Thevenin Equivalent of Microbial Fuel Cell

       All of the above calculations are based on the previous Microbial Fuel Cell. Once the
       new Microbial Fuel Cell is manufactured all testing will be redone and all calculations
       will be re-applied to the results of the testing. The proposed Microbial Fuel cell will
       have a much larger anode surface area so should in theory have a much higher
       power output.



       3.4 Boost Converter
       A Boost Converter also known as a Step-Up converter is a power converter with an
       output DC voltage greater than its input DC voltage. The boost converter has two
       main modes of operation which are determined by a switch ‘S’. When the switch is
       closed the Boost Converter is in its ‘On State’ so the current is allowed to increase in
       the inductor. When the switch is open the converter is in its ‘Off state’ so the only
       path offered to inductor current is through the freewheeling Diode ‘D’ the capacitor
       ‘C’ and the load ‘R’. This causes the transfer of energy accumulated during the On
       State into the Capacitor/Load.




Energy Management System For Microbial Fuel Cell                                       Page 10
Progress Report                      20/December/2010                              Aidan Walsh




       Fig 3.5 Operation Modes Of Standard Boost Converter




       Fig 3.6 Voltage and Current Waveforms of Boost Converter

       Due to the low power dissipation of the microbial fuel cell it was not possible to
       engineer the required boost converter conventionally using Bipolar junction
       transistors and diodes. This is due to the diodes between the base and emitter gates
       having a voltage drop of at least 0.3 volts which the system cannot afford to drop as
       it is only receiving 0.41 volts from each microbial fuel cell. This was disappointing as
       it removed a large amount of flexibility of the Boost Converter in terms of the rate
       which the switching took place within the DC-DC converter.

       The converter chosen was the TPS61200 chip which is manufactured by Texas
       Instruments. This was the lowest found and had a start up voltage of 0.5 volts. As
       there was no translation board small enough to enable the attachment of the chip to
       a normal circuit board it was not an option to use the chip on its own. Therefore the
       TPS612000EVM-179 was ordered which has the TPS61200 integrated into it. This
       added more restrictions as the size of the inductor and capacitors have an impact on
       the energy that is stored in the boost converter.


Energy Management System For Microbial Fuel Cell                                       Page 11
Progress Report                      20/December/2010                            Aidan Walsh



       One of the options to automate the charging ad discharging of the capacitors is
       another DC-DC boost converter, the MAX1797 EV kit. This DC-DC converter is
       manufactured by Maxim. The advantage with switching to this converter as opposed
       to staying with the original was the Maxim kit incorporates a voltage comparator
       into the EV kit which would allow the automation of the charging and discharging of
       the capacitor. Fig shows the schematic of the EV kit.




       Fig 3.7 Schematic Of Max1797 EV Kit



       Max1797 EV Kit
       The MAX1797 evaluation kit (EV kit) is a high-efficiency, step-up DC-DC converter for
       portable hand-held devices. Unlike typical boost circuits, the MAX1797 output is
       completely disconnected from the input in shutdown. The EV kit accepts a positive
       input voltage between 0.7V and VOUT and converts it to a 3.3V output for currents up
       to 500mA. The EV kit provides ultra-low quiescent current and high efficiency for
       maximum                                   battery                                life.

       The MAX1797 EV kit is a fully assembled and tested surface-mount printed circuit
       (PC) board. It can also be used to evaluate other output voltages in the 2V to 5.5V
       range. Additional pads on the board accommodate the external feedback resistors
       for setting different output voltages.


       3.5 Transmitter/Receiver and Sensor

       One of the options that could be powered by the systems is a wireless sensor. Similar
       projects have used an off-the-shelf thermocouple/transmitter/receiver kit
       manufactured by MadgeTech. It transmits at a frequency of 418 MHz and has a


Energy Management System For Microbial Fuel Cell                                     Page 12
Progress Report                     20/December/2010                           Aidan Walsh


       maximum transmission distance of 100 ft. The main drawback with the Madgetech
       kits is that they are quite expensive so an alternative will have to be sourced.



       3.6 Storage Capacitors

       To supply the DC-DC boost converter with enough power to get over its start up
       phase storage capacitors were used. The power needed to get past the start up
       phase was worked out using the formula P=I*V where P is power, I is current and V is
       voltage. The power needed worked out to be 28.5 milli-Watts where the current
       needed at start up is 57 milli-Amps and the voltage needed at startup is 0.5 volts.
       Therefore using the E=1/2CV2 the size of the capacitor needed to supply this power
       can be worked out to be 84.77 milli-Farads. (C = 0.0285 Watts/0.3363 Volts2)

       To accommodate for the loss in capacitors the size of the capacitor was increase to
       0.1F which was much easier to come by. 3.3 and 10 Farad Capacitors were ordered
       also.

       To calculate the charging time needed to charge each of the capacitors to 99%, the
       formula 5*R*C is used.

       This works out to be 348.5 seconds for the 0.1Farad capacitor. The 3.3 and 10 farad
       capacitors were worked out to take roughly 187 minutes and 566 minutes
       respectively.

       All calculations are again based on the previous Fuel Cell and will be altered
       accordingly to apply to the new Fuel Cell for choosing the optimum capacitance
       values.

       3.7 Trans-Impedance Amplifier

       This is one of the options explored to deal with the low voltage problem with the
       system. A Trans-Impedance Amplifier is effectively a current-to-voltage converter
       which takes an electric current as an input signal and produces a
       corresponding voltage as an output signal.




Energy Management System For Microbial Fuel Cell                                   Page 13
Progress Report                      20/December/2010                            Aidan Walsh




        Fig 3.8 Trans-Impedance Amplifier

       In the op-amp current to voltage converter the output of the operational amplifier is
       connected in series with the input voltage source and the op-amp’s inverting output
       is connected to point A. As a result the op-amp’s output voltage and the input
       voltage are summed.




Energy Management System For Microbial Fuel Cell                                     Page 14
Progress Report                      20/December/2010                             Aidan Walsh


   4. Testing

       4.1 Capacitor Testing

       Each of the Tests were redone on each of the capacitors to ensure the capacitors
       were still in fully functioning order using the demonstration circuit which consisted
       of the DC power supply with the same characteristics as the fuel cell, the storage
       capacitors (0.1F, 3.3F, 10F) , a manual switch connecter in series with the power
       source between the capacitor and DC-DC boost converter and the Boost Converter
       itself connected in parallel with the power source and storage Capacitor.

       The 0.1F capacitor took roughly 320 seconds to charge fully which was in sync with
       the calculated value of 348.5 seconds. The screenshot below was obtained from one
       of the oscilloscopes in the laboratory and shows the charging of the 0.1Farad
       Capacitor.




       Fig 4.1 Charging Graph of 0.1F Capacitor

       Each division along the x axis represents 40 seconds and it takes the capacitor
       approximately 8 divisions to reach the voltage supplied by the power source which
       corresponds to 320 seconds to fully charge. Similar tests have been completed on
       the 3.3F and 10F capacitors.
        A LED was then attached along with a resistor of 1000ohms to the output of the
       Boost Converter and the switch is closed to allow the capacitor to discharge through
       the boost converter into the resistor to light the LED. Fig below shows the capacitor
       discharging.

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Progress Report                      20/December/2010                            Aidan Walsh




       Fig 4.2 Discharging Of 0.1 F Capacitor

       Similar tests were conducted using different load resistance values and different
       Loads.

       4.2 Boost Converter Testing




Energy Management System For Microbial Fuel Cell                                     Page 16
Progress Report                         20/December/2010                             Aidan Walsh




   5. Problems Encountered

       The main problem encountered was how to get around the very low power output
       of the microbial fuel cell. Although it was able to charge the capacitor which
       powered the boost converter which in turn could power a LED/Calculator etc there
       was no way to automate the discharging and charging of the capacitor. This is
       because the current design of the system uses a manual switch to power the boost
       converter. A separate voltage comparator was not an option as the microbial fuel
       cell was unable to power it and charge the capacitor simultaneously. Therefore
       either more than one Fuel Cell would be needed or some other alternative must be
       used.
       This is why the Maxim Boost Converter was sourced and the Current-to-Voltage
       converter was investigated. Due to the extreme difficulty in keeping the converter
       stable and getting accurate results, the Maxim Converter is seen as a more viable
       option so that will be the first attempt to remedy the problem.




   6. Where To Next?


                 The next steps that I will have to take is to test the new Microbial Fuel cell
                  which shall hopefully be constructed by January and then to re-evaluate all
                  sections of the project to see if there can be any improvements made to the
                  capacitor values, switching mechanism etc.

                 I have to further investigate the wireless sensors and their application and
                  what would be the most suitable type for this project and also investigate the
                  alternative of using the system to charge some sort of battery.

                 The new boost converter will be ordered and I will have to test it in the new
                  circuit set up and set the properties of the built in voltage comparator to
                  cause the capacitor to automatically discharge once it has reached the
                  desired point. This will hopefully increase the efficiency of the system .



Energy Management System For Microbial Fuel Cell                                         Page 17
Progress Report                      20/December/2010             Aidan Walsh



   7. Table of Figures
      Fig 3.1 Hydrogen Fuel Cell
       http://images.yourdictionary.com/fuel-cell

       Fig 3.2 Microbial Fuel

       http://www.making-hydrogen.com/hydrogen-fuel-cell.html

       Fig 3.3 Power Density/Voltage Vs. Current Density curve
       Previous Tests Completed By Stephen Mulryan

       Fig 3.4 Thevenin Equivalent of Microbial Fuel Cell
       Previous Tests Completed By Stephen Mulryan

       Fig 3.5 Operation Modes Of Standard Boost Converter
       http://www.Wikipedia.org



       Fig 3.6 Voltage and Current Waveforms of Boost Converter
       http://en.wikipedia.org/wiki/Boost_converter

       Fig 3.8 Trans-Impedance Amplifier



       Fig 4.1 Charging Graph of 0.1F Capacitor
       Based on Lab Experiments

       Fig 4.2 Discharging Of 0.1 F Capacitor
       Based on Lab Experiments




Energy Management System For Microbial Fuel Cell                     Page 18
Progress Report                     20/December/2010                            Aidan Walsh




   8. References

       Much of the research for this project has been done with the help of the following
       websites:

       www.wikipedia.org

       http://www.madgetech.com/

       http://www.making-hydrogen.com/hydrogen-fuel-cell.html

       http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5497188&tag=1

       http://pubs.acs.org/doi/full/10.1021/es0480668

       Also extensive amounts of this report are based on Stephen Mulryans similar project
       from last year.




Energy Management System For Microbial Fuel Cell                                    Page 19

								
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