Final Year Progress Report

Transcription

1 Final Year Progress Report Student: Stephen Mulryan Student ID: Discipline: Electronic & computer Engineering Supervisor: Dr. Maeve Duffy Co-Supervisor: Professor Ger Hurley Project Title: Energy Conversion for low voltage values

2 Table of Contents 1. Abstract.page 2 2. Milestones Set Out for Project:....page Progress to Date 3.1 BIO Fuel & Fuel Cell Research...page Characterisation of microbial fuel cell..page Demonstration of fuel cell powering small device page Energy requirements of a battery charger for mobile phone page What to do next?..page Figure index page References..page 17

3 1.Project Abstract: With some of the new electrical power sources being considered, the voltage levels are very low and therefore there are issues in converting the energy to a useable form. For example, the voltage developed across an individual photovoltaic cell is of the order of 0.6 V, and therefore the cells need to be connected together in series in order to provide power at a higher voltage level; 12 / 24 / 36 V is typical for domestic solar panels. Similarly, the output voltage from fuel cells is low and individual cells need to be connected in series in order to drive different loads. Researchers in the Energy Research Centre in NUI Galway are investigating 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.

4 2.Milestones Set Out for Project: Numerous Milestones were set out from the project specification which I received from my project supervisor Dr. Maeve Duffy which would guide me through steps which I would need to complete in order to complete my 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. 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 biofuels 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 characterise the electrical performance of the demonstrator biofuel cells. This involved measuring the output voltage versus the load characteristics of a typical biofuel cell. Then determine the energy level for a typical feeding period, this involves finding the levels of power output from a biofuel 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 I needed to identify demonstrator devices with the lowest possible power consumption, then design, build and test any conversion circuitry needed.

5 Average Milestone: Investigate the application of biofuel 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 I should then design a DC/DC Converter solution for connecting between the bio fuel cells and the battery charger circuit. Since the bio fuel cells are at such a low power output level this will most lightly be a boost converter. The last thing to achieve in this milestone is to provide spice simulation results from such a circuit. 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

6 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 biofuel cell sources. For this milestone we must first of all determine the efficiency of the converter over all load conditions and identify the main loss contributors. Then propose potential improved solutions for future development. The final step is to test and characterise the complete system for different fuel cell characteristics.

7 3.Progress To Date: 3.1BIO Fuel & Fuel Cell Research: The first thing to find out was what exactly was a fuel cell. The most self explanatory explanation of a fuel cell can be found on Wikipedia where it defines a fuel cell in the following words: A fuel cell is an electrochemical conversion device. It produces electricity from fuel (on the anode side) and an oxidant (on the cathode side). It is the most self explanatory definition yet for a lay person to understand this definition needs to be broken down further. A fuel cell consists of two chambers filled with chemicals separated by an ion exchange membrane. The effect of putting this membrane between the chambers results by not allowing any electrons flow from one chamber to the other. Instead it only allows positive ions or protons flow from the one chamber to the other. 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 an electric device. An electrode is an electrical conductor with a non-metallic part of a circuit. The cathode is similar to the anode except that it is negatively charged. The chemical stored in the chamber connected to the anode is normally referred to as the fuel and the resulting chemical from the reaction in the chamber connected through the cathode is often referred to as the reagent. The way the fuel cells which use ion exchange membranes work is by firstly plating the anode and cathode with catalysts, catalysts are chemical substances which either speed up or slow down the rate of a chemical reaction without being used up in the process. In the case of fuel cells have the effect of speeding up the reaction in the anode chamber. In the most popular type of fuel cell, the hydrogen fuel cell, the anode chamber is filled with hydrogen gas (H2). The catalyst on the surface has the affect of speeding up the splitting of the electrons in H2 molecule from the protons. Once these have been split the H2 molecule which is now a Cation passes through the ion exchange membrane into the cathode chamber. A Cation is defined as an atom or a molecule in which the total number of protons is greater than the total

8 number of electrons so the atom or molecule in this case has a overall positive charge. 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. This creates a voltage between the anode and cathode. The value of the 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 cathode chamber they rejoin the Hydrogen Cation and react with the reagent that is held in the cathode. In most hydrogen fuel cells the reagent is oxygen. So when the Hydrogen molecules react with the oxygen atoms it produces H2O which is of course water. Figure 3.1Hydrogen Fuel Cell Operation There are various types of fuel cells and most of them operate in a very similar way to the operation of the hydrogen fuel cell with the proton exchange membrane 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 seperated by an electrolyte which is conductive to oxygen ions but not conductive

9 to electrons. This setup is a reverse operation to the ion exchange membrane design. An oxygen molecule is split in the cathode chamber, then the oxygen Cation is feed through the electrolyte to the anode chamber leaving the electrons in the cathode chamber. 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 way to the solid oxide fuel cell except it s electrolyte consists of a liquid carbonate which is an oxidising agent. An oxidising agent is a chemical compound that transfers oxygen atoms very quickly. A microbial fuel cell is the most important of the three of these as it is the type of fuel cell that will be investigated in this project. The first thing is to define this type of fuel cell. According to Wikipedia this type of fuel cell is a bio-electrochemical system that drives a current by mimicking bacterial interactions found in nature This means that things such as common wastewater and acetic acid which can be obtained from plant waste fermentation can be used as a fuel source in fuel cells and natural microorganisms can be used as catalysts which can produce electricity and generate hydrogen gas among other types of scenarios. In the case where hydrogen gas is produced from the reaction, it can then be used as a fuel source for a hydrogen fuel cell. Figure 3.2 Microbial Fuel Cell

10 Biofuels are fuels which have been produced from plants and vegetation such as trees, starch crops and grasses. Once such Biofuel is ethanol which can be used as fuel for vehicles but is usually used as an additive to gasoline to increase the octane and improve the vehicle emission levels. Another example of Bio fuels is BioDiesel which is made from vegetable oils and animal fats or recycled greases, this too can be used for fuel but is usually used as a diesel additive to reduce levels of carbon monoxide and hydrocarbons in the emissions. 3.2Characterisation of microbial fuel cell: The thevenin equivalent circuit needed to be calculated from measurement of power density, voltage and current density obtained from measuring a Microbial fuel cell held at the Energy Research centre. Voltage (V) Power density (mw/m 2 ) Current density (ma/cm 2 ) Figure 3.3 Power Density/Voltage Vs. Current Density curve The blue points represent power density Vs. Current density while the white points represent Voltage Vs. current density. The current density is measured from the surface area of the anode and the power density is obtained in the same way. By taking the second set of points from the curve and working out the current, resistance and power we will achieve the most favorable results. We

11 were able to obtain the surface area of the anode from the Energy Research Centre. The area of the anodes surface was 5.4cm 2. So if taken at the second point along the curve, voltage = 0.42 volts, power density = 900 milli-watts/m 2 and the current density is 0.225milli-Amps/cm 2. Firstly we can work out the current by multiplying the current density by the surface area of the anode. The current works out to be milli-amps. The next element we can calculate is the power output. This can be calculated by multiplying the voltage which can be read from the curve and the current which we have just calculated. The power output works out to be milli-watts. The final calculation to be made is the internal resistance of the fuel cell. This can be calculated through ohms law and works out to be ohms. From this the following thevenin equivalent circuit can be produced: Figure 3.4 Thevenin equivalent ciruit. There is also internal resistance but this is negligible.

12 3.3Demonstration of fuel cell powering small device: The first thing I needed to test was an LED. After testing all of the LED s in the lab I found that the lowest power LED needed 8.2 milli-amps to light and needed a minimum voltage of The maximum current that can be obtained from the microbial fuel cell by reading from the curve in figure 3.3 is 1.89 milli-amps. So to light an LED in the lab you would need to connect 4 or 5 microbial fuel cells in parallel to get enough current. Then we would need to use either a charge pump or some other kind of DC-DC converter to boost the voltage to the required level. We could also use lower powered LED s which operate of 1 milli-amp. The next thing we need to do is to demonstrate how an arrangement of microbial fuel cells can power a more sophisticated device such as a DC servo motor. To do this I needed to design a DC-DC boost converter. This proved very difficult as the fuel cell had such a low voltage and low current. As most common DC-DC boost converters use diodes and bipolar junction transistors due to their fast switching speeds it is impossible to boost a voltage as low as 0.42 volts up to 5 volts seen as there is a drop of ~0.3volts across every diode and bipolar junction transistor. For this task I needed to order a low power DC-DC boost converter from Texas Instruments. Although the circuit diagrams do not show what components are used for switching it is assumed that they have used an arrangement of Mosfets. It has a minimum input voltage of 0.3 volts and a maximum output of 5 volts. The following shows the functional diagram of the device: Figure 3.5 Functional block diagram of TPS61200

13 In the diagram above Vin is you input voltage, Vout is the output voltage, Vaux is connected to a capacitor which is used to ramp up the output voltage at startup. FB is the feedback voltage and can be used to control the output of the circuit. PGND is the power ground, Ps is a pin which when set high will enable power save mode and make the chip consume less energy, EN enables the chip, UVLO is the under voltage lockout comparator input this is set to a certain voltage and if the input voltage goes below this the device shuts down and GND is ground. The more voltage we input this device the more current we will get out so ideally we would use multiple fuel cells connected in series to give us maximum voltage input. The following graph shows just how the input voltage effects the output current: Figure 3.6 Output Current Vs Input Voltage from TPS Energy requirements of a battery charger for mobile phone: A battery charger is a device used to put energy into a secondary cell by forcing current through that cell. The way this current is forced into the cell depends upon the type of rechargeable battery being used and the capacity of the battery. Even for phone batteries there are various types of chargers available. For example the charger that currently charges my phone supplies 5 volts across the battery and 1 amp current through the battery at all times until the battery is

14 fully charged. There are different methods used to charge batteries. These include constant charge; this operates by applying a constant Dc charge of to the battery which may be done by using a step down converter to decrease the mains voltage. Constant current is another type of charging method which operates by varying the voltage in order to keep a constant current flow through the battery. Another popular charging method is pulse charging, this method feeds the charge current to the battery in pulses. This way the rate at which the battery is charged can be precisely controlled by the width of the pulse. The final charging method that I will mention and the one that I am most lightly going to use in this project is trickle charging as it charges the battery very slowly and compensates for the self discharge rate of the battery.

15 4.What to do next?: The next thing to do is firstly test the low power DC-DC boost converter which I have received from Texas Instruments in the lab to see if it can drive a DC motor. After that I should research different battery specifications to see which one has the lowest power consumption. I should also check if each battery is compatible with trickle charging seen as some are not. Once the battery has been chosen I should design a DC-DC boost converter using spice to convert the voltage output from one microbio fuel cell if possible to the voltage needed to power the mobile phone charger. After this I should design a DC-DC boost converter solution using multiple microbial fuel cells to power the phone charger. Before building the DC-DC boost converter I should figure out the arrangement of microbial fuel cells which will give me the maximum power output from the fuel cells whether it be connecting them in parallel to get increase current or in series to get increased voltage.

16 1. Figure Index: Figure 3.1 Hydrogen Fuel Cell Operation - This image was taken from the website Figure 3.2 Microbial Fuel Cell This image was taken from the Figure 3.3 Power Density/Voltage Vs. Current Density curve This graph was obtained from measuring the voltage, current and power outputs of a microbial fuel cell Figure 3.4 Thevenin equivalent ciruit. This circuit diagram was got from using Orcads Spice software to draw the circuit. Figure 3.5 Functional block diagram of TPS61200 This functional diagram was taken from the data sheet for the TPS61200 module which can be found here: Figure 3.6 Output Current Vs Input Voltage from TPS61200 This graph was taken from the data sheet for the TPS61200 evaluation kit which can be found here:

17 1. References: Most of the research for this project has been done with the help of the following websites: