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