Final Year Project Final Presentation Title: Energy Conversion for low voltage sources. Supervisor: Dr.Maeve Duffy
Aim of Project The aim of this project was to develop circuits to demonstrate the performance of bio fuel cells which are being developed by the Energy research centre in NUI Galway. The ideal end goal would have been where a Microbial Fuel Cell arrangement has the ability to charge a mobile phone battery.
Outline of Presentation This presentation will deal with the following topics: 1. Overview of Project 2. Work Completed 3. Outlook for future development 4. Conclusions 5. Questions
1) Overview of project:
Demonstration Circuit:
2) Work Completed: ThéveninEquivalent circuit: LED Demonstration Demonstration of fuel cell powering low power devices Storage Capacitor Knowledge of charging algorithms Demonstration of fuel cell powering a DC Fan
ThéveninEquivalent circuit: Power Density curve: 0.6 1200 Voltag ge (V) 0.5 0.4 0.3 0.2 0.1 1000 800 600 400 200 Power density (mw/m 2 ) 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Current density (ma/cm 2 ) 0
Blue line represents the power density Vs current density. White line represents Voltage Vs current density. Area across which power density is measured is 5.4cm^2. 1cm^2 = 0.0001m^2 The point at which we have maximum power output is the second from right so we take this point. When worked out the following outputs result: Power ~ 0.486 milli-watts Voltage ~ 0.42 volts Current ~ 1.215 milli-amps Internal Resistance of Fuel Cell ~ 345 ohms
ThéveninEquivalent circuit:
LED Demonstration: On testing the LED s found in the electronics labs it was found that the lowest power LED needed a minimum of 3.8 milli- Amps and a minimum of 1.83 volts to light. This meant the voltage & current output from the fuel cell needed to be stepped up. There is three solutions to this problem: 1) Cascade a number of fuel cells in parallel, this way increasing the current output and then use a DC-DC boost converter to step up the voltage. 2) Use an RC circuit to boost the current using a mosfetfor switching and then use a DC-DC boost converter to step the voltage up. 3) Use a low power LED (1 milli-amp LED can be obtained)
Low power devices identified: Voltage needed: 1.5 Volts DC Power needed: 0.0001 Watts Current needed: 66.66 micro- Amps Voltage needed: 2.7 volts DC Power needed: 1.4 Watts Current needed: 0.42 Amps Voltage needed: ~3.3 volts DC Power needed: unknown Current needed: unknown
Demonstration of fuel cell powering low power devices: To demonstrate these devices a DC-DC boost converter needed to be designed. This caused problems as most common DC-DC boost converters use either diodes or BJT s which have a diode between the base and emitter. The BJT is used due to its fast switching speeds. The diodes cause a minimum of 0.3 voltage drop. As the output voltage from the fuel cell is so low already we can not afford to use BJT s.
Demonstration of fuel cell powering low power devices: Using a boost converter obtained from Texas instruments called the TPS61200 the output voltage could be boosted. This converter gets around the problem of using BJT s by using MOSFET s instead. The TPS61200 can needs 0.8 volts to startup, after which it can operate at a voltage as low as 0.3 volts. As the TPS61200 was to small to fit on a board I needed to order the evaluation module.
Demonstration of fuel cell powering low power devices:
Demonstration of fuel cell powering low power devices:
Demonstration of fuel cell powering low power devices:
Demonstration of fuel cell powering low power devices: From using the formula to work out the minimum inductance needed (Vin = L * DI/DT),I found that the minimum inductance required was 2.1333 micro-henry s. So the 2.2 micro-henry should be satisfactory to induct the input current from the fuel cell.
Storage capacitor: As the DC-DC boost converter needed more power at start up than the MFC could provide a Capacitor needed to integrated into the system to output enough power. It was found through using the equation: E= 1 CV 2 that the Energy which could be obtained from a 0.1 Farad capacitor would be enough to get over the start up power requirements. 3.3 Farad and 10 Farad Capacitors were also obtained to enable the powering of high load devices for longer and devices which require more Energy than the 0.1 Farad capacitor can store. 2
Storage capacitor:
Research of battery chemistries, charging algorithms: Example of type of voltage and current used to charge a phone: My phone (Sony Ericsson) is a lithium-polymer battery which supplies 3.6 volts to the phone. And has 780 milli- Amp hours. The charger for the phone supplies 5 volts and a current of 1Amp. This is probably implementing a charging algorithm known as constant charge where a constant charge is applied to the battery. The type of charging algorithm that could be implemented for this project would be either pulse charging or trickle charging.
Demonstration of fuel cell powering a DC Fan:
Demonstration of fuel cell powering a DC Fan:
3) Outlook for future development Continuous powering of low power device How many capacitors are needed. Possible switching control devices. How many Microbial Fuel Cells are needed Research on more efficient DC-DC boost converters. Further Research on battery charging profiles.
Continuous powering of a low power device There are various ways in which a continuous powering of devices using this circuit can be implemented. Obviously as the load attached to the DC-DC boost converter changes so too does the rate of current discharge from the capacitors. For this reason a system has to be devised for each specific device. An example chosen for this presentation is the continuous powering of a 1.5 volt DC calculator Through testing it has been found that the calculator draws a constant current of 9 μamps from output of the Boost converter and a constant voltage of 3.3 volts is applied across it.
Continuous powering of a low power device A 0.1 Farad capacitor took approximately 194 seconds to charge fully. When tested a fully charged capacitor could power the calculator for 100 seconds. This meant that if the system was going to be implemented by allowing the capacitors to fully charge then three 0.1 Farad Capacitors would have to be used as the charge rate does not equal the discharge rate. This also meant using two extra Microbial Fuel Cells as each capacitor would need to be charged separately.
Continuous powering of a low power device The alternative to this is to charge the capacitor for about 40 seconds. If you do this the calculator can be powered for nearly 40 seconds meaning you will only need two capacitors to continuously power the calculator. This in turn means there is less MFC s needed to charge the capacitors.
Continuous powering of a low power device Possible switching devices: 555 Timer MSP430C1101 Voltage Comparator
Continuous powering of a low power device Advantages: Inexpensive (47 cent) Easy to implement Disadvantages: Sync issues may arise Inflexible 555 Timer: Higher operational voltage & input current (More MFC s used to power it) Minimum of 3 milli-amps & 4.5 volts
Continuous powering of a low power device Voltage Comparator: Advantages: Inexpensive ( 1.65) Easy to implement Low voltage and current input (Less MFC s used to power it) 1.8 volts & 15 μamps Disadvantages: Value of voltage across capacitor has to be very precise
Continuous powering of a low power device MSP430C1101: Advantages: More Flexible MP could also be used if implementing a smart battery charger Low voltage and current input (Less MFC s used to power it) 2.2 volts and 150 μamps Disadvantages: Expensive($49.49 Evaluation module & chip) More complex to implement
Research on more efficient DC-DC boost converters. The following graph shows the efficiency of the DC-DC boost converter at an input voltage of 0.8 volts:
Research on more efficient DC-DC boost converters. An example of this lack of efficiency was observed when powering the calculator. At the start there was from 0.9milli- Amps being drawn from the capacitor into the Boost converter which had 0.8 volts applied across the input. At the end there was 0.3 volts applied across the DC-DC boost converter and 2.2 milli-amps being drawn from it. This means the input power was between 0.72 milli-watts down to 0.66 milli-watts yet the output power was only 0.0297 milli-watts. This is equal to 4.5 % efficiency.
Further Research on battery charging profiles. As the 0.1 Farad capacitor takes so long to charge and outputs relatively so little power it is hard to know if a system like the system proposed for the calculator can be altered to trickle charge a even a 1 milli-amps hour battery without drastically increasing the number of Microbial Fuel Cells on available. For a trickle charge algorithm usually the rate at which the battery is Charged is 15 % of the rate at which constant charging Algorithms are implemented. If implemented it is hard to know whether the current being drawn into the battery would be enough to compensate for the idle discharge of the battery through air.
Further Research on battery charging profiles. Through testing of the discharge rate of a 10 Farad capacitor a possible way to implement a constant voltage\current charging algorithm was identified. The 10 Farad capacitor powered an Led in series with a 1k resistor for 5 minutes supplying the Led with a constant current of 1.6 milli-amps. This means that if twelve 10 Farad capacitors where charged a 1.5 milli-amp Hour battery could be charged.
4) Conclusion: Better understanding of Electronic circuit design & MFC s LED Demonstration Demonstration of fuel cell powering low power devices Knowledge of charging algorithms
5) Questions!!