Energy, Society, and the Environment

Unit 6:Solar Energy

Solar Energy is not a New Concept

Solar cells on a satellite Solar cells on a rooftop

Cookies baking in a solar oven

How do we use sun’s energy?

1. Passive solar: For example, heat your home with south-facing windows

How do we use sun’s energy?

2. Solar-Thermal: Use the heat from the sun to boil water

A solar power plant in Australia

How do we use sun’s energy?

3. Photovoltaic Cell: Directly produce electricity from sunlight using “semi-conductors”

How do we use sun’s energy?

4. The Cheapest Way: The plants know how to

Efficiency: 0.3 %

We can’t meet the world’s energy demands

in this way

Why Solar Energy?

• We need ~ 30 TW of power, the sun gives us 120,000 TW.

• Solar cells are safe and have few non-desirable environmental impacts.

• The sun shines when we need energy the most.

• Using solar cells instead of burning coal to generate electricity is a easy way to reduce carbon emissions.

The Sun

What is Sunlight?

Light is made of waves

Sun Light (Waves)

Frequency: How frequent the peaks areWavelength: How far apart the peaks areSpeed = frequency x wavelength

What is Sunlight?

ALL LIGHT WAVES TRAVEL AT THE SAME SPEED

Speed = frequency x wavelength = c

Composition of Sunlight

Solar Spectrum

• Our atmosphere preferentially absorbs some wavelengths• Most UV and some infrared is blocked• See the handout.

Visible Light

• It is not a coincidence that human/animal eyes see “visible” light, which the sun produces the most of (the peak of its “spectrum”)

• The size of human/animal eyes (lenses) are set by the visible wavelength

• Had we evolved on a planet around a different type of star, we would most likely see a different wavelength!

Photovoltaic Cells

• Concept: How does it work? Makes use of materials called semi-conductors

• Aluminum wire: carries electricity -- a conductor• Plastic: Does not let electricity through -- an insulator• A (doped) Silicon crystal: Can conduct electricity if enough voltage

is provided -- a semi-conductor

Photovoltaic Cells• p-n junction is the basis for all photovoltaic devices• When the two types are brought into contact, electrons (n) and

holes (p) are exchanged over a short region, the interface, known as depletion or junction region

Photovoltaic Cells

• The junction region “band-gap” determines what wavelengths of light the solar cell is sensitive to.

• The bigger part of the solar spectrum it is sensitive to, the higher its “efficiency” e-

usable photo-voltage (qV)

Energy

e-

n-typep-type

ηmax = 32%heat loss

heat loss

hν

h+

Jenny Nelson, The Physics of Solar Cells, 2003.

Photovoltaic Cells

When

Light energy ≥ band gap energy

electrons will gain enough energy to move and electricity is generated

See the handout for the wavelengths that Si crystal is sensitive to.

Photovoltaic Cells

The efficiency of the solar cells drop with increasing temperature: have to cool them well to keep electricity production high

A Si solar cell operating at 0 C has a maximum possible

efficiency of 24%

At 100 C, it drops to 14%

Multi-Junction CellsGrid

p

p

p

p

p

n

nn

++

++

p

n

p

p

nn

++

++

pn

pn

++

++

Ge Substrate (0.67 eV)

GaAs (1.42 eV)

GaInP (1.90 eV)

AlInP

AlGaInP

GaInP

GaInP

n GaAs

GaAs:N:Bi (1.05 eV)

GaAs:N:Bi

GaAs:N:Bi

nn

Slide credit: McGehee, Stanford U.

SpectroLab has achieved 37 % efficiency

Costs are estimated at $50,000/m2, so concentrators must be used.

Thin Film Cells

Slide credit: McGehee, Stanford U.

A Balancing Act

• Research in solar cells tries to optimize efficiency (achieve highest possible) while making it as cheap as possible

• Other concerns are durability, availability of materials, production speed

Module Cost Amortized Capital Expense

Material Yield Substrate PV Efficiency

Crystalline Silicon $2/W $0.16/W <50% Rigid 15%

Vacuum Thin Film $1/W $0.32/W 70% Rigid 10%

Nano $0.3/W $0.04/W 95% Flexible 3%*

Solar-Thermal Energy

• Use the heat of sunlight to boil water and operate a steam engine

• Remember the demonstration Dr. Angel talked about: when sunlight is concentrated by a mirror, can melt 1/4 inch thick steel in 15 seconds!!

Solar-Thermal Energy

A very old new technology!

Solar Energy: Insolation Map

Solar Energy

• Is it feasible? Can provide all of the U.S. electricity needs with solar energy (100 square miles in AZ or Nevada covered with 20% efficient solar cells can do it: e.g., Turner, 1999, Science, 285, 687)

• Currently, the biggest challenge is the cost of photovoltaic cells; focus is on decreasing cost

• Global solar industry has grown 20% every year for the past 10 years (but is still in its infancy)

• 30% annual growth in the U.S.

PV Land Area Requirement (U.S.)

Solar Concentrators

Solar Energy and DemandSolar Energy provides most of the demand, can be supplemented in evening hours

Energy Storage

• Pumped water hydroelectric storage

• Compressed air energy storage

Energy Storage

Energy policy is hard, finally, because there is no technological “silver bullet”: No known

energy option is free of liabilities

• oil and gas: not enough resources

• coal: not enough atmosphere

• biomass: not enough land

• hydropower & wind: not enough sites

• nuclear fission: too unforgiving

• nuclear fusion: too difficult

• photovoltaics: too expensive, intermittent

• end-use efficiency: high consumer discount rate, needs users to pay attention,

inherently diffuse industry

• hydrogen: not a primary source

Holdren, John P. (2006) ENR302 Energy Technology, Markets, and Policy. Lecture 1, February 2, 2006