Transcript Final Year Project

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.
2.
3.
4.
5.
Overview of Project
Work Completed
Outlook for future development
Conclusions
Questions
1) Overview of project:
Demonstration Circuit:
2) Work Completed:
• Thévenin Equivalent 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
Thévenin Equivalent circuit:
0.6
1200
0.5
1000
0.4
800
0.3
600
0.2
400
0.1
200
0
0.05
0
0.1
0.15
0.2
0.25
0.3
Current density (mA/cm2)
0.35
0.4
Power density (mW/m2)
Voltage (V)
Power Density curve:
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
Thévenin Equivalent 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 milliAmps 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 mosfet for
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 microAmps
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
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.
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 milliAmp 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
555 Timer:
Advantages:
– Inexpensive (47 cent)
– Easy to implement
Disadvantages:
– Sync issues may arise
– Inflexible
– 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 0.9milliAmps 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!!