CE SolarSlides

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Copyright 2005 SRI International

Clean Solar Energy

The impact of nanoscalescience on solar energyproduction


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Sources: http://www1.eere.energy.gov/solar/pv_systems.htmlhttp://thomashawk.com/hello/209/1017/1024/Staring%20at%20the%20Sun.jpg

What is Solar Energy?

Energy produced bythe sun

Clean, renewablesource of energy

Harnessed by solarcollection methodssuch as solar cells

Converted intousable energy suchas electricity

Photovoltaic (solar)panel

Set of solar panels

Sun and electrical

power lines


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Energy from the Sun is Abundant

Solar powersystemsinstalled inthe areasdefined bythe darkdisks could

meet the

world'scurrent total

energydemand

Source: http://www.ez2c.de/ml/solar_land_area/


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20 TW

Sources: http://energy.cr.usgs.gov/energy/stats_ctry/Stat1.htmlhttp://www.census.gov/main/www/popclock.html

Current U.S. Energy Demand

The US consumes~25% of the

worlds energy, buthas only 4.5% ofits population

Solar cells wouldneed to cover anarea comparable to

the size of Texas tomeet US energy

demand today


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20 TW

180,000 TWof sunlight

hit the earthevery day

Sources: http://energy.cr.usgs.gov/energy/stats_ctry/Stat1.htmlhttp://www.census.gov/main/www/popclock.html

Projected U.S. Energy Demand in 2050

To meetprojected U.S.

demand in 2050,

solar cells wouldneed to cover

approximatelyone third of theU.S.


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Solar Panel Use Today

Large companies likeGoogle, Walmart, andMicrosoft use solar energyto partially power some of

their facilities

Solar panels on Microsoft buildingSolar panels being tested

on Walmart store

Sources: http://i.n.com.com/i/ne/p/2006/IMG_5396_550x367.jpghttp://www.solarwall.de/assets/images/Walmart_SW.jpg


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Sources: http://www.elp.uji.es/juan_home/images/solar_cell/flex1.JPGhttp://www.wisconsun.org/images/siemen_cell.jpg, http://en.wikipedia.org/wiki/Solar_cell

Silicon-based

solar cell

Dye-sensitizedsolar cell

Photovoltaic Solar Cells

Generate electricitydirectly from sunlight

2 Main types:

Single-crystal silicon

(traditional) Widespread

Expensive to manufacture

Dye-sensitized (nano)

Newer, less proven Inexpensive to manufacture

Flexible


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Sources: http://www.econedlink.org/lessons/EM189/images/cartoon_tv.gifhttps://reader009.{domain}/reader009/html5/0503/5aeab81d56431/5aeab8209db07.jpg, http://www.torpedowire.com/solar.htmhttp://www.uoregon.edu/~stiedeke/a3/assignment03/a3/assignment_images/cartoon-sun.jpg

Solar Cells are Converters of Energy

Solar cells are devicesthat take light energy as

input and convert it into

electrical energyLight energy

Solar cell -converts light

energy toelectricity

Electrical energy(carried through wires)


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Source: https://reader009.{domain}/reader009/html5/0503/5aeab81d56431/5aeab8224722c.jpg

But Not All Energy is Converted

Like chloroplasts in plants, solar cells can onlyabsorb specific wavelengths of light.

In both, light that isnt absorbed is either transmittedthrough or reflected back.

Whether a certain wavelength of lights getsabsorbed depends on its energy.

Chlorophyll moleculesabsorb blue and red light,

but reflect green light


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A Little Background on Light

Different colors of light have different wavelengthsand different energies

Source: http://www.mhhe.com/physsci/astronomy/arny/instructor/graphics/ch03/0305.html


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Absorption of Light byAtoms

Sources: http://members.aol.com/WSRNet/tut/absorbu.htm, http://csep10.phys.utk.edu/astr162/lect/light/absorption.html

Single electrontransition in anisolated atom

Absorption occurs only when the energy ofthe light equals the energy of transition of

an electron

Light


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Molecules havemultiple atomsbonded together

More energy statesin molecules than

atoms More electronjumps possible -light with a range offrequencies areabsorbed

Source: Image adapted from http://www.3dchem.com/molecules.asp?ID=135#

Absorption of Light by Molecules

Smallest DE possible


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Electrons canjump betweenbands

Incident light

with energy than the bandgap energy canbe used to excitethe electrons

Source: Image adapted from http://www.3dchem.com/molecules.asp?ID=135#

Absorption of Light by Ionic Compounds


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So What Does this Mean for Solar Cells?

In dye-sensitized solarcells

Talk about highestoccupied molecular orbital

(HOMO) and lowestunoccupied molecularorbital (LUMO)

Source: Original Images

In single-crystal siliconsolar cells

Talk about conduction

band (excited states)and valence band

(ground states)


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A Closer Look at Solar Cells

How do traditional,silicon-based solarcells and newer,dye-sensitized

solar cells work? What are the

advantages anddisadvantages ofeach type of cell?

Sources: http://www.norfolksolar.co.uk/img/system.gifhttp://www.powerlight.com/newsletters/news_issue/3/newsletter_industry.htm

Silicon-basedsolar cell

Dye-sensitizedsolar cell


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Source: http://nanosense.org/activities/cleanenergy/solarcellanimation.html

How a Silicon-Based Solar Cell Works

A positive holeis left in theelectrons place

This separation ofelectrons and holes

creates a voltageand a current

Light with energy greater than the band gap energyof Si is absorbed

Energy is given to an electron in the crystal lattice

The energy excites the electron; it is free to move

Click image to launch animation

(requires web access)
http://nanosense.org/activities/cleanenergy/solarcellanimation.html


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Silicon-Based Solar Cell Attributes

Expensive

Made in high vacuum at highheat

High manufacturing costs

Need TLC

Fragile, rigid, thick

Long return on investment

Takes 4 years to produceenergy savings equivalent to

cost of production

Source: http://www.powerhousekids.com/stellent2/groups/public/documents/pub/phk_ee_re_001505-2.gif


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How a Dye-Sensitized Cell Works

Click image to launch animation

(requires web access)

Light with high enough energy excites electrons indye molecules

Excited electrons

infused intosemiconducting TiO2,transported out of cell

Positive holes left in

dye molecules

Separation of excitedelectrons and holescreates a voltage

Source: http://nanosense.org/activities/cleanenergy/solarcellanimation.html
http://nanosense.org/activities/cleanenergy/solarcellanimation.html


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Source: https://reader009.{domain}/reader009/html5/0503/5aeab81d56431/5aeab82896b8c.jpg

Dye-Sensitized Solar Cells

Relatively inexpensive

Made in non-vacuum settingmainly at room temperature

Relatively simple manufacturingprocess

Need little TLC

Thin, lightweight, flexible

Short return on investment Takes approx 3 months to

produce energy savingsequivalent to cost of production


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Sources: https://reader009.{domain}/reader009/html5/0503/5aeab81d56431/5aeab82a70050.jpghttp://www.norfolksolar.co.uk/img/system.gif

Dye-Sensitized and Silicon-basedSolar Cells Compared

Dye-Sensitized

Relatively inexpensive

Need little TLC

Short return oninvestment

Traditional

Expensive

Need TLC

Long return oninvestment


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Sources: http://en.wikipedia.org/wiki/Solar_energhttp://www.fplenergy.com/portfolio/solar/facts.shtml#glance Concentrated solar collector (parabolic)

Solar Electric Power Plants

Harness solar power togenerate electricity

Main types:

Solar thermal energy Has mirrored surface thatreflects sunlight to heat upliquid to make steam togenerate electricity

Photovoltaic Uses photovoltaic cells that

absorb direct sunlight (asdiscussed previously)

Array of mirrored solar collectors at

FPL Energy site in California


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Source: http://www.norfolksolar.co.uk/img/system.gif

Example system with water heated by

solar collector and used to supplementhot water radiator

Solar Heating Systems

Direct heating of fluid; no conversion to electricity

Components

solar thermal

collectors fluid system to move

heat (not electricity)

reservoir to stock

heat for later use Common uses

Heat water for home

or pool


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Sources: http://p2library.nfesc.navy.mil/issues/emergejan2007/wind_turbine.jpghttp://science.howstuffworks.com/wind-power1.htm

Wind Energy isSolar Energy

Wind is driven by solarheating

Largest and fastestgrowing solar energy

conversion system

Wind farm in CaliforniaWind turbines


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Important Summary Questions

What are clean and renewable energy sources?

What are current and projected global energy

demands?

How do newer, nanotechnology-influenced solarcells work, and how do they differ from

traditional solar cells?

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