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ENGR110 T2 2017Engineering Modeling and Design
Elf*, Howard Lukefahr, Arthur Roberts
School of Engineering and Computer ScienceVictoria University of Wellington
Elf Eldridge (ECS) ENGR110 1 / 30
Next time on ENGR110...
To increase voltages with capacitors we need diodes
Today: Diodes, Transistors, Semiconductors andwhy almost all solar cells are crap.
Elf Eldridge (ECS) ENGR110 2 / 30
Diodes
Allow current to flow in a single direction only.
Have an associated voltage drop across them.
Elf Eldridge (ECS) ENGR110 4 / 30
Voltage Multiplier
Allows voltages to be increased above what is input (in AC)
This WONT be examined.
Elf Eldridge (ECS) ENGR110 5 / 30
Rectification
Diodes allow RECTIFICATION (conversion of AC to DC). Adding acapacitor helps smooth the DC that’s generated.
This WONT be examined.
Elf Eldridge (ECS) ENGR110 6 / 30
Triodes and Vacuum Tubes
In the beginning there were Triodes (1906) and Vacuum tubes (1904)
Electronic control of current.
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TransistorsAllow currents to be switched on and off via an applied voltage.
Electronically controlled switches.https://www.youtube.com/watch?time_continue=39&v=6HkCGgIYNps
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Electronic Control
Allow you to use small voltages/current to control much largervoltages/currents!
The h-bridge you used in the AVC was a series of transistors.Elf Eldridge (ECS) ENGR110 9 / 30
Question: what is so special about Silicon
Why is Silicon in all of these?
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Silicon is a cheap semiconductor
And we can precisely control its electrical properties through chemistryand the application of electric fields
Doping or applying a voltage to silicon can allow (or prevent) electronsjumping from the valence to the conduction band.
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Solar cells and LEDs
Silicon has a band gap of 1.11 eV (equivalent to a wavelength of 1100nanometers)
Which mean it also interacts with light!
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Look at how little of the spectrum this covers!
The rest of this is absorbed as heat!
Peak solar intensity is about 550nm (or about 2.2eV)
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Why are all these efficiencies so low?
It’s actually because of the definition of efficiency for solar cells...
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Max efficiency
We can calculate a maximum solar cell efficiency based on the BandGap of a semiconductor..
N.B. This is HIGHLY temperature dependant (best at about 0◦C).Elf Eldridge (ECS) ENGR110 19 / 30
Topic 1: Review Questions
If you want to know more about this consider doing a second yearECEN course (maybe ECEN202 - digital electronics).
Core: Why is Silicon used in most current solar cells?Completion: Why are solar cell efficiencies so much lower than forturbines?Challenge: How might you increase the efficiency of a cell beyondthis limit?
Student Debthttps://www.youtube.com/watch?v=P8pjd1QEA0c
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Alternative Solar
Also known as Solar Thermal generators:
Focus sunlight at a point to boil water. Max efficiency of about 40%.
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Turbine Design matters!
https://www.youtube.com/watch?v=-YJuFvjtM0s
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Wind Turbine Design
Why the difference between these designs?
Allows power generation in lower wind and/or from more directions.
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Thermoelectric Generator (or cooler!)
Pretty much just solar cells for absorbing heat...
Typical efficiencies are between 5 and 8%
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