Energy from the Sun

1
.
.
.
.
.
.
.
.
.
.
.
.
.
Topics in Physics
.
.
.
.
.
.
.
.
Energy from the Sun Part I
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Mr. K., NASA/GRC/LTP
.
.
.
.
.
.
Edited Ruth Petersen
.
.
2
1. The Suns Energy Output
3
The Solar Constant
When we measure the midday intensity of sunlight
at the Earths surface, we find that about 136.7
mW fall on every square centimeter.
We call this number The Solar Constant and
designate it by the Greek letter sigma (?).
At 1 A.U. ? 136.7 mW/cm2.
Check yourself Does everyone know what a watt
(W) is? A milliwatt (mW)?
4
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?
5
1 A.U.
An A.U. is the average Earth-Sun separation,
150,000,000 km.
6
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 suns visible surface?
7
Answer to Question 1
? 136.7 mW/cm2 _at_ 1 A.U.
1 A.U. 1.5 X 108 km
rSun 1.4 X 106 km
?p ?
1 A.U.
The same amount of energy per unit time passes
through the photosphere as through a sphere with
radius 1 A.U. ?
8
Answer to Question 1 Continued a.
Dimensionally Energy per Unit Time ? ? (Area)
b. Conservation of Energy ?P ? (Photosphere
Area) ?? ? (1A.U. Sphere Area)
Area of a Sphere 4?r2
4?(rp2)(?P ) 4?(r1A.U.2)(?)
? ?P ?? ? (r1A.U./rP)2
c. Solving ?P ?136.7 mW/cm2 ? (1.5 ? 108
km/1.4 ? 106 km)2
1.6 ? 106 mW/cm2
.
9
2. What is the total energy output per unit time
of the sun in W?
10
Answer to Question 2 a. Dimensionally Total
Energy per Unit Time ?p ? (Total Surface Area
of Sun)
b. Reminder Area of a Sphere 4?r2
c. Solving With rSun 1.4 ? 106 km 1.4 ? 1011
cm, (1.6 ? 106 mW/cm2) ? 4?(1.4 ? 1011 cm)2
3.9 ? 1029 mW 3.9 ? 1026 W
.
390 Trillion-Trillion Watts
11
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
12
Question 2 Continued
3.9 ? 1026 W
7.8 ? 1017 Avg.Cities
5 ? 108 W/Avg.City
About 780,000 trillion average American cities!
13
Question 2 Continued Again Estimate how much of
this total energy output is actually intercepted
by the Earth. Hint rE 6,400 km
14
Question 2 Continued Yet Again a.
Dimensionally Energy Intercepted at Earth ? ?
Cross Section of Earth ? ? ?r2
b. Solving 136.7 mW/cm2 ? ? (6.4 ? 108 cm)2
1.8 ? 1020 mW 1.8 ? 1017 W
.
180,000 trillion watts, enough to run almost 360
million average American cities!
15
Question 2 is finished at last!!!
Question 2 is finished at last!!!
Question 2 is finished at last!!!
2 Continued 1.) Dimensionally Energy
Intercepted at Earth ? ? Cross Section of
Earth ? ? ?r2
2.) Solving 136.7 mW/cm2 ? ? (6.4 ? 108 cm)2
1.8 ? 1020 mW 1.8 ? 1017 W
.
180,000 trillion watts, enough to run almost 360
million average American cities!
16
3. In what form is this energy transmitted into
space?
17
Answer to Question 3 The energy is transmitted
as light (or, more properly, electromagnetic
radiation).
18
2. Harnessing the Suns Energy
19
Question How can we harness the energy from the
sun?
20
Some possibilities are
Solar thermal collectors
Solar dynamic systems
Solar cells
21
What are Solar Cells?

A solar cell is a solid-state device that
directly converts sunlight into electricity.
-
22
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.
23
Refining Sand Can you fill in the blanks?
O
O
Si
Si
O
O
Words

Chemical Symbols

24
SOLAR CELL
Energy absorbed from incident sunlight
electrically excites the solar cell to produce a
voltage. For silicon, V oc 0.5 V
25
V -
_
Load
When a load is placed across a solar cell,
electrical power is delivered to the load.
Power Current ? Voltage I ? V
26
Questions
1. Is all of the sunlight falling on a solar cell
absorbed?
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?
27
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.
28
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?
29
Answer The cell area (collecting area) is ?r2
? cm2
If the cell were 100 efficient, it would
produce (97 mW/cm2 ) ? (? cm2)
305 mW
But because it is only 18 efficient, it produces
305 mW ? 0.18 55 mW
.
30
3. Using Solar Power
31
Question Now that you know something about
harnessing the suns energy with solar cells,
where do you suppose we can put that energy to
work?
32
Solar System
Earths Surface
Mars
Earth Orbit
33
Do you have any questions or topics you would
like to discuss?
34
For those interested in talking more, contact me
at joseph.c.kolecki_at_grc.nasa.gov