ENGR110 T2 2017Engineering Modeling and Design

Elf*, Howard Lukefahr, Arthur Roberts

School of Engineering and Computer ScienceVictoria University of Wellington

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Next time on ENGR110...

To increase voltages with capacitors we need diodes

Today: Diodes, Transistors, Semiconductors andwhy almost all solar cells are crap.

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Topic 1

Topic 1: Circuit Elements

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Diodes

Allow current to flow in a single direction only.

Have an associated voltage drop across them.

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Voltage Multiplier

Allows voltages to be increased above what is input (in AC)

This WONT be examined.

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Rectification

Diodes allow RECTIFICATION (conversion of AC to DC). Adding acapacitor helps smooth the DC that’s generated.

This WONT be examined.

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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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Other semiconductors

Why don’t we use something else?!

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We do!

We just don’t use much of it...yet...

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Because Money!

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Question:

Why are all these efficiencies so low?

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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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Topic 2

Topic 2: Other Energy Sources

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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?

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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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Geothermal Energy

Wairakei Geothermal Plant

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Hylink

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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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Blue Smoke Refiller

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Topic 2: Review Questions

Core: Why are alternative to solar cells investigated?Completion: Which of these might be most appropriate to power aschool in Maungaraki?Challenge: Which of these technologies would you choose toinvest in?

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