solar energy part i
DESCRIPTION
solar energyTRANSCRIPT
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Topics in Physics:
1. The Sun’s Energy Output
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.
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?
An A.U. is the average Earth-Sun separation, ~ 150,000,000 km.
1 A.U.
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?
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 = ?
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
.
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?
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 .
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
About 780,000 trillion average American cities!
3.9 1026 W
5 108 W/Avg.City~ 7.8 1017 Avg.Cities
Question #2 Continued:
Question #2 Continued Again:
Estimate how much of this total energy output is actually intercepted
by the Earth.
Hint: rE = 6,400 km
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
.
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!!!
.
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?
Answer to Question #3:
The energy is transmitted as light (or, more properly, electromagnetic
radiation).
2. Harnessing the Sun’s Energy
Question:
How can we harness the energy from the sun?
Some possibilities are:
Solar thermal collectors
Solar dynamic systems
Solar cells
What are Solar Cells?
136.7 mW/cm 2
A solar cell is a solid-state device that directly converts sunlight into
electricity.
+
-
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.
+
“Refining” Sand: Can you fill in the blanks?
Words
+Chemical Symbols
+
SiliconDioxide
SiO2
Silicon
Si
Oxygen
O2
SiO
O
SiO
O
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
Load
I
When a load is placed across a solar cell, electrical power is delivered to
the load.
+V-
Power = Current Voltage = I V
+
_
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?
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.
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?
Answer:
The cell area (collecting area) isr2 = 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
.
3. Using Solar Power
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?
Earth’s Surface
Earth Orbit
Solar System
Mars
Do you have any questions or topics you would like to discuss?
