View
218
Download
0
Category
Preview:
Citation preview
RETOptics Research Workshop
Workshop #2Solar 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 2013Workshop 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 2013Workshop 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 2013Workshop 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.
Dr. Mike Nofziger 2013Workshop 2-4
www.eia.gov
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 calories1 J = 0.947816x10-3 BTU
● Power is defined as the rate of using energy. - Fundamental units of power:
- Watt, Horsepower1 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)
Units! (“love ‘em” or “hate ‘em”…..teach your students to “love ‘em”!)
Dr. Mike Nofziger 2013Workshop 2-5
1Wh 1W 1hour 1J s 1hour 60min/h 60sec /min 3600J
Example of Power, Energy, and Photons/sec:
Each photon of light carries a specific amount of energy:
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?
• How many photons per second are in this laser beam?
Dr. Mike Nofziger 2013Workshop 2-6
Energy per photon of light: where h is Planck’s
constant h = 6.626x10-34 J-s
33
W 1J s1mW 1s 10 J 1mJ
10 mW WE
-1
134 81 9
153
Photons/sec = energy sec photon energy = photons sec
Photons/sec = power energy photon = photons/sec
6.626 10 3 10 /1 / 10= 1 .001 3.26 10 se
10 650 1
Js m sW J s hc J nmmW photons
mW W s nm m
c
cE h h
Our Sun:
● The “ROY G BIV” solar spectrum:
● The Ideal Blackbody (solar) spectrum:
Dr. Mike Nofziger 2013Workshop 2-7
81 0 71 0 61 0 51 0 41 0 31 0 21 0 1 0 11 0 - 21 0 - 31 0 - 41 0 - 51 0 - 61 0 - 71 0 - 81 0 - 91 0 - 101 0 - 1 11 0 - 121 0 - 1 31 0 - 141 0 - 151 0 - 1 61 0 -1
700 600 500 400
W a v e le n g th ( in n a n o m e te rs)
V is i b l e s p e c tru m
W a v e le n g th (m )
F re q u e n c y (H z )
1 0 21 0 31 0 41 0 51 0 61 0 71 0 810 91 0 1010 111 0 121 0 131 0141 0 151 0 161 0 1710 1810 191 0 201 0 211 0 221 0 231 0 2410
R a d io w a v e s X ra y s G a m m a ra y s
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 Dr. Mike Nofziger 2013
Workshop 2-8
Our Sun: ● The Ability to harness solar energy by concentrating it:
Dr. Mike Nofziger 2013Workshop 2-9
Our Sun: ● The Ability to harness solar energy by using solar cells:
Dr. Mike Nofziger 2013Workshop 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:
where T(K) = T(°C) + 273°
• The surface temperature of our sun is ≈ 6000K• The wavelength where our sun emits most energy is, therefore:
• 500 nm is in the green portion of the visible spectrum.• The peak sensitivity of human (daylight) vision is at 550nm………?!
Dr. Mike Nofziger 2013Workshop 2-11
max
max
3000
3000
m K T
m K
T
max
3000 10000.5 500
6000 1
m K nmm nm
K m
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:
Dr. Mike Nofziger 2013Workshop 2-12
max
5[ ] 400 32 273 477
9
30006
477
T K C C K
m Km
K
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):
• 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
2 4 6 8 10 12 14 16 18-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Wavelength (m)
Tra
nsm
issi
on (
%)
Xerox Overhead Transparency Transmission Spectra (8 cm-1)
Fig. A.3. Infrared transmission spectrum of one layer of Mylar for wavelengths from 2 to 18 micrometers
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 2013Workshop 2-14
Basics of Solar Cells:
● The IV Curve of a PV detector is given by:
● The Photocurrent of a PV detector is given by:
Dr. Mike Nofziger 2013Workshop 2-15
1qV
kTo phI I e I
ph phI q qhc
Basics of Solar Cells:
● The Power (Watts) that the cell can produce is given by:
● 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 2013Workshop 2-16
P V I
Basics of a Solar Oven:
Dr. Mike Nofziger 2013Workshop 2-17
Recommended