Transcript Slide 1

RET
Optics Research Workshop
Workshop #2
Solar Energy
Solar Cells and Solar Ovens
Dr. Mike Nofziger
Professor
College of Optical Sciences
University of Arizona
Dr. Mike Nofziger 2013
Workshop #2 Outline:
● Solar Energy
- Basics of Energy
- Our Sun
- The solar spectrum
- The greenhouse effect
● Solar Cells
● Solar Ovens
Dr. Mike Nofziger 2013
Workshop 2-1
Basics of Energy:
● Two fundamental types of energy:
- Potential Energy : “stored” energy
(work could be done with this available energy)
- Kinetic Energy : “working” energy
(work is being done with this energy)
● Forms of Energy:
-
Light (radiant)
Heat (thermal)
Motion (kinetic)
Electrical
Chemical
Nuclear
Gravitational
Dr. Mike Nofziger 2013
Workshop 2-2
Basics of Energy:
● Renewable Energy Sources:
- Solar energy → electricity or heat
- Wind
- Geothermal energy from heat inside the Earth
- Biomass from plants
- firewood, wood waste
- ethanol from corn
- biodiesel from vegetable oil
- Hydropower from hydro-turbines at a dam
● Non-Renewable Energy Sources:
- Fossil fuels
- oil
- natural gas
- coal
Dr. Mike Nofziger 2013
Workshop 2-3
Basics of Energy:
● Energy is conserved
- Scientifically speaking, “Conservation of energy” does not mean
“saving energy”
“Law of Conservation of Energy”
- The total amount of energy in a closed system remains constant.
- Energy does not disappear, or “get used up.”
- Energy is changed from one form to another when it is used.
www.eia.gov
Dr. Mike Nofziger 2013
Workshop 2-4
Energy vs. Power:
● Energy is defined as the capacity for doing work.
- Fundamental units of energy:
- Joule, Calorie, British Thermal Unit (BTU)
1 J = 0.23889 calories
1 J = 0.947816x10-3 BTU
● Power is defined as the rate of using energy.
- Fundamental units of power:
- Watt, Horsepower
1 Watt ≡ 1 Joule/sec.
1 hp = 746 watts
- Therefore, energy ≡ power x time
- An equivalent unit of energy is:
Watt·hour (Wh), kilo-Watt·hour (kWh)
1Wh  1W  1hour   1J s   1hour   60min/h   60sec /min   3600J
Units! (“love ‘em” or “hate ‘em”…..teach your students to “love ‘em”!)
Dr. Mike Nofziger 2013
Workshop 2-5
Example of Power, Energy, and Photons/sec:
Each photon of light carries a specific amount of energy:
Energy per photon of light:
where h is Planck’s constant
h = 6.626x10-34 J-s
E  h  h
c

Energy and Power in a Laser Beam:
“A typical red laser pointer emits 2-3 mW of power, at a
wavelength of 650 nm. (For simplicity, assume the power is 1 mW)”
• How much energy is delivered by this laser beam, in 1 sec?
E  1mW 
W 1J s
 1s   103 J  1mJ
10 mW W
3
• How many photons per second are in this laser beam?
Photons/sec =  energy sec  photon energy  = photons sec
Photons/sec =  power  energy photon 
-1
= photons/sec
1
J
 1W   J / s   hc 
= 1mW   3

    .001
s
 10 mW   W    
  6.626 1034 Js  3 108 m / s   109 nm  
15


   3.26 10 photons sec

650nm
1
m



1
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Workshop 2-6
Our Sun:
● The “ROY G BIV” solar spectrum:
Wavelength (in nanometers)
700
600
500
400
Visible spectrum
Wavelength (m)
108
107 106 105 104 103 10 2 10
10- 1 10- 2 10- 3 10 - 4 10- 5 10- 6 10- 7 10- 8 10- 9 10- 10 10 - 11 10- 12 10- 13 10- 14 10- 15 10- 16
1
X rays
Radio waves
2
10 10
3
10
4
10
5
10
6
10
7
10
8
10
9
10
10
10
11
10
12
10
13
10
14
10
15
10
16
10
17
10
18
10
Gamma rays
19
10
20
10
21
10
1022 1023 1024
Frequency (Hz)
● The Ideal Blackbody (solar) spectrum:
Dr. Mike Nofziger 2013
Workshop 2-7
Our Sun:
● The “Real” solar spectrum:
● The sun delivers ≈ 1000W/m2 to the surface of the Earth!
● The Earth receives more energy from the Sun in just one hour than the
world uses in a whole year. http://org.ntnu.no/solarcells/pages/Chap.2.php
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Workshop 2-8
Our Sun:
● The Ability to harness solar energy by concentrating it:
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Workshop 2-9
Our Sun:
● The Ability to harness solar energy by using solar cells:
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Workshop 2-10
Basis for the Heating in a Solar Oven (a.k.a. a “Greenhouse Effect”):
• The wavelength of peak output from a blackbody (an ideal emitter, much
like our sun) is given by:
3000   m K   max  T
where T(K) = T(°C) + 273°
max
3000   m K 

T
• The surface temperature of our sun is ≈ 6000K
• The wavelength where our sun emits most energy is, therefore:
max
3000   m  K 
 1000nm 

 0.5 m 
  500nm
6000K
 1 m 
• 500 nm is in the green portion of the visible spectrum.
• The peak sensitivity of human (daylight) vision is at 550nm………?!
Dr. Mike Nofziger 2013
Workshop 2-11
Basis for the Heating in a Solar Oven (a.k.a. a “Greenhouse Effect”):
• The wavelength of peak output from a solar oven cavity, at T≈ 400°F:
5

T [ K ]   400  32   C  273C  477 K
9

3000   m  K 
max 
 6 m
477 K
Dr. Mike Nofziger 2013
Workshop 2-12
Basis for the Heating in a Solar Oven (a.k.a. a “Greenhouse Effect”):
• Typical “window” material for a student solar oven is a single sheet of
Mylar (Xerox Overhead Transparency):
Xerox Overhead Transparency Transmission Spectra (8 cm -1)
0.9
0.8
Transmission (%)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
-0.1
2
4
6
8
10
Wavelength (m)
12
14
16
18
Fig. A.3. Infrared transmission spectrum of one layer of Mylar for wavelengths
from 2 to 18 micrometers
• High transmission in the visible/near IR spectrum
• Low transmission in the thermal IR spectrum (i.e. at 6 μm)
- The cavity absorbs visible light but has trouble emitting (radiating)
thermal energy at 6 μm, therefore the cavity heats up.
• Basic “Greenhouse Effect” (car interiors, the Earth, etc.):
Dr. Mike Nofziger 2013
Workshop 2-13
Basics of Solar Cells:
● A solar cell is a Photovoltaic (“PV”) detector:
- is made of Silicon (not silicone!!)
- absorbs light from ≈ 350nm – 1100nm
- the absorption of light “frees up” electrons
- This creates a voltage at the terminals of the cell
(the “Open-Circuit” voltage)
- If a load resistor is connected to the cell, a current will flow
(the “Photocurrent”)
- If the cell’s terminals are shorted, the maximum current will flow
(the “Short-Circuit” current)
Dr. Mike Nofziger 2013
Workshop 2-14
Basics of Solar Cells:
● The IV Curve of a PV detector is given by:
 qVkT

I  I o  e  1  I ph




● The Photocurrent of a PV detector is given by:
I ph    q   ph
 
  q 
 hc 
Dr. Mike Nofziger 2013
Workshop 2-15
Basics of Solar Cells:
● The Power (Watts) that the cell can produce is
given by:
P V I
● Because of internal resistance in the cell, the
maximum power you can generate is across a load
resistance equal to the internal resistance.
www.keithley.com
Dr. Mike Nofziger 2013
Workshop 2-16
Basics of a Solar Oven:
Dr. Mike Nofziger 2013
Workshop 2-17