Energy from the Sun Part I

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Topics in Physics:

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. . . . . . . . . . . . . Topics in Physics:. . . . . . . . . Energy from the Sun Part I. . . . . . . . . . . . . . . . . . . Mr. K., NASA/GRC/LTP . . . . . . . Edited: Ruth Petersen. . . 1. The Sun’s Energy Output. The Solar Constant. - PowerPoint PPT Presentation

Transcript of Energy from the Sun Part I

Page 1: Energy from the Sun Part I

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Topics in Physics:

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1. The Sun’s Energy Output

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The Solar Constant

Check yourself: Does everyone know what a watt (W) is? A milliwatt (mW)?

We call this number “The Solar Constant” and designate it by the

Greek letter sigma ().

When we measure the midday intensity of sunlight at the Earth’s

surface, we find that about 136.7 mW fall on every square centimeter.

At 1 A.U.: = 136.7 mW/cm2.

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A watt (W) is a unit of energy flow - Joules per second.

A milliwatt (mW) is 10-3 W.

Does everyone know what an “A.U.” is?

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An A.U. is the average Earth-Sun separation, ~ 150,000,000 km.

1 A.U.

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Questions:If the mean distance from the Earth to the Sun is 1.5 108 km, and the solar radius is 1.4 106 km, then

1. What is the value of the solar constant P at the photosphere, i.e.,

the sun’s visible surface?

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1 A.U.

The same amount of energy per unit time passes through the photosphere as through a sphere with radius 1 A.U.

Answer to Question #1: = 136.7 mW/cm2 @ 1 A.U.

rSun = 1.4 X 106 km1 A.U. = 1.5 X 108 km

p = ?

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Answer to Question #1 Continued:

a. Dimensionally:Energy per Unit Time = (Area)

~ 1.6 106 mW/cm2

b. Conservation of Energy: P (Photosphere Area) = (1A.U. Sphere

Area)

c. Solving:P = 136.7 mW/cm2 (1.5 108 km/1.4 106 km)2

4(rp2)(P ) = 4(r1A.U.

2)() P = (r1A.U./rP)2

Area of a Sphere = 4r2

.

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Questions (continued):If the mean distance from the Earth to the Sun is 1.5 108 km, and the solar radius is 1.4 106 km, then

2. What is the total energy output per unit time of the sun in W?

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Answer to Question #2:a. Dimensionally: Total Energy per Unit Time = p

(Total Surface Area of Sun)

390 Trillion-Trillion Watts

~ 3.9 1029 mW = 3.9 1026 W

b. Reminder: Area of a Sphere = 4r2

c. Solving:With rSun = 1.4 106 km = 1.4 1011

cm, (1.6 106 mW/cm2) 4(1.4 1011

cm)2 .

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Question #2 Continued:If an average American city has a peak

power consumption of 500 MW, estimate how many average American

cities this total energy output (390 trillion-trillion watts) is equivalent to.

1 MW = 106 W

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About 780,000 trillion average

American cities!

3.9 1026 W

5 108 W/Avg.City~ 7.8 1017 Avg.Cities

Question #2 Continued:

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Question #2 Continued Again:

Estimate how much of this total energy output is actually intercepted

by the Earth.

Hint: rE = 6,400 km

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Question #2 Continued Yet Again:

a. Dimensionally:Energy Intercepted at Earth =

Cross Section of Earth= r2

180,000 trillion watts, enough to run almost 360 million average American

cities!

~ 1.8 1020 mW = 1.8 1017

W

b. Solving:136.7 mW/cm2 (6.4 108 cm)2

.

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Question #2 is finished at last!!!

#2 Continued:

1.) Dimensionally:Energy Intercepted at Earth =

Cross Section of Earth= r2

180,000 trillion watts, enough to run almost 360 million average American

cities!

~ 1.8 1020 mW = 1.8 1017

W

2.) Solving:136.7 mW/cm2 (6.4 108 cm)2 Question #2 is finished at last!!!Question #2 is finished at last!!!

.

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Questions (Continued):If the mean distance from the Earth to the Sun is 1.5 108 km, and the solar radius is 1.4 106 km, then

3. In what form is this energy transmitted into space?

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Answer to Question #3:

The energy is transmitted as light (or, more properly, electromagnetic

radiation).

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2. Harnessing the Sun’s Energy

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Question:How can we harness the energy from

the sun?

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Some possibilities are:Solar thermal collectorsSolar dynamic systems

Solar cells

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What are Solar Cells?

136.7 mW/cm 2

A solar cell is a solid-state device that directly converts sunlight into

electricity.

+

-

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What is the most common raw material from which solar cells are made?

The most common raw material is white sand, specially refined to remove unwanted impurities.

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+“Refining” Sand: Can you

fill in the blanks?

Words

+Chemical Symbols

+

SiliconDioxide

SiO2

SiliconSi

Oxygen

O2

Si O

O

Si OO

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Reflected light

SOLAR CELLV oc

Energy absorbed from incident sunlight electrically excites the solar cell to produce a voltage. For silicon,

V oc ~ 0.5 V

Absorbed light

Incident sunlight

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Load

I

When a load is placed across a solar cell, electrical power is delivered to

the load.

+V-

Power = Current Voltage = I V

+_

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Questions:

2. Is all of the energy absorbed by the solar cell converted into electricity?3. If the answer to Question #2 is, “No,” then what other energies might be involved?

1. Is all of the sunlight falling on a solar cell absorbed?

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Answer to Question #1:No. Some of it is reflected back into

space.Answer to Question #2:

No. Silicon solar cells are nominally 20% efficient.

Answer to Question #3:The rest of the energy goes into

heating the solar cell.

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Problem:A given circular solar cell has a 1 cm radius. It is 18% efficient. Because

today is cloudy, the solar constant is a mere 97 mW/cm2. What is the

maximum power output you can expect from the cell?

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Answer:The cell area (collecting area) is

r2 = cm2

If the cell were 100% efficient, it would produce

(97 mW/cm2 ) ( cm2)

But because it is only 18% efficient, it produces

~ 305 mW

305 mW 0.18 = 55 mW

.

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3. Using Solar Power

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Question:Now that you know something about

harnessing the sun’s energy with solar cells, where do you suppose we can

put that energy to work?

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Earth’s Surface

Earth Orbit

Solar System

Mars

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Do you have any questions or topics you would like to discuss?

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For those interested in talking more, contact me at:

[email protected]