School of Electrical and Computer Engineering

Solar 101for the Duke Energy Academy

June 23, 2014

Peter Bermel

Purdue School of Electrical and Computer Engineering

Outline

› The solar resource

› Approaches to harvesting solar power

› Solar photovoltaics technologies

› Recent and future growth in solar

Purdue School of Electrical and Computer Engineering

In principle, solar energy can scale to supply a large portion of global energy demand.

The solar resource & potential

Over 1000 times extra!

Source: OECD Observer No. 258/59 December 2006

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The solar spectrum

Source: Robert Rohde, Global Warming Art – http://en.wikipedia.org/wiki/Sunlight”

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Insolation

� Incoming solar radiation

� Take into account movement of the sun throughout the day and throughout different seasons

� Also weather patterns

� Energy per unit area per unit time (kWh/m2/day)

Purdue School of Electrical and Computer Engineering

Purdue School of Electrical and Computer Engineering

Solar land area to supply all US power

Source: Nate Lewis (Caltech)

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Electrical power generation

Photo: Brightsource Energy, http://ecotechdaily.com/wp-content/uploads/2008/04/brightsource2_620px.jpg

Source: TGW, http://openlearn.open.ac.uk/file.php/1697/220880-1f1.29.jpg

Photo: Stirling Energy Systems,http://www.wapa.gov/ES/pubs/esb/1998/98Aug/Graphics/Pg5b.jpg

Photo: Ausra, Inc.,http://www.instablogsimages.com/images/2007/09/21/ausra-solar-farm_5810.jpg

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Photovoltaic cell operation

� Light absorption / charge

generation (photovoltaic

effect) � Carrier thermalization

� Charge separation� Photon reemission

� Charge collection� Nonradiative recombination

http://en.wikipedia.org/wiki/Image:Solar_cell.png

Eg

= max. VOC

Conduction band

Valence band

Excess energy above Eg heat

I

VVOC

ISC

dark

light

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I

VVOC

ISC

Maximum power rectangle

VOC, open circuit voltage

ISC, short circuit current

FF, fill factor = max. power rectangle

VOC . ISC

dark

light

IL RP

RS

Circuit model

Solar photovoltaics: electrical engineering

η = VOC x ISC x FF

Pinc

Power conversion efficiency

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Photovoltaic technologies

› Major categories:

– Silicon

• Single crystal

• Polycrystalline

• Amorphous silicon

• Microcrystalline

– CIGS

– Cadmium telluride

– Multijunction Source: Impact Lab,

http://www.impactlab.com/wp-content/uploads/2008/06/solar-energy.jpg

Monocrystalline silicon PV

› One of the first, and still

dominant, cell

technologies

› Advantages:

– Process is mature

– Relatively high efficiencies

› Disadvantages

– High materials usage

– High costs

– Batch processing

Czochralski process for creating monocrystalline silicon ingots

Ingots are then sawed into individual wafers

Source: DOE Solar Energy Technologies Program, http://www1.eere.energy.gov/solar/silicon.html

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Polycrystalline silicon PV

› Manufacturing

improvement; decreases:

– Costs

– Kerf loss

› Disadvantages:

– Lower electronic quality

– Increased fragility

– More difficult to textureEvergreen’s string ribbon process

Source: http://www.evergreensolar.com/images/technology/stringribbon/diagram_string_ribbon_en.jpg

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CIGS (Copper Indium Gallium Diselenide)

› Engineered for direct

bandgap at target

wavelength

› Promising efficiencies:

up to 20.4%

› Sticking point –

manufacturing

processes:

– Vacuum deposition

– Inkjet-style printing

CIGS manufacturingSource: Ibid.

CIGS cell diagramSource: AIST (Japan),

http://www.yet2.com/publish/techofweeks/tow0043972/20080323_cigs01.jpg

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CdTe (Cadmium Telluride)

› Advantages:

– Direct bandgap with

efficiencies up to 19.6%

– Inexpensive fabrication process, proven at GW

scale

› Disadvantages:

– Susceptible to degradation

– Cd toxicity concerns

– Te feedstock issues

CdTe cell diagramSource: http://www.mtl.kyoto-u.ac.jp/groups/awakura-g/index-e.html

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Multijunction PV

› Combines two or more

materials into a stack

› Allows for more efficient

use of each photon in solar

spectrum � record efficiencies

› Challenges with lattice and

current matching can

greatly increase costsSchematic of triple-junction cell

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Best Solar PV Efficiencies

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Solar research is growing

› Solar research has seen

increased investment from

many players:

– US government

– Venture capital

– Manufacturers

› Drivers:

– Rising energy costs

– Environmental concerns

– Energy security Solar R&D Investments (in millions)

Source: Solar Energy Industries Association (2013)

$0

$500

$1,000

$1,500

$2,000

$2,500

2006 2007 2008 2009 2010 2011

PV OEMs

VC&PE

Government R&D

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Purdue Has Unique Expertise in PV + TPV Energy Systems

TPV portable power generator*

Solar PV electricity for homes†

*R. Pilawa-Podgurski et al., APEC 25, 961 (2010); P.

Bermel et al., Opt. Express 18, A314 (2010)

‡ G. Lush & M. Lundstrom, Solar Cells 30, 337 (1991); Q..

Guo et al., J. Am. Chem. Soc. 132, 17384 (2010); M. A.

Alam et al., J. Mat. Res. 28, 541, (2013); L. Varghese

et al., Adv. Opt. Mater. (2013).

† Lafayette Magazine, “Sun Power,” August 17 (2011).

Thin-film PV from new materials ‡

pbermel@purdue

.edu

ECE Grad Open House,

Prof. Bermel

19

Purdue School of Electrical and Computer Engineering

Solar installations growing rapidly

› Solar is transitioning from

niche to mainstream

› Improved efficiencies, lower

costs enable rapid growth

› 32,000 MW installed in 2012

› Over half of new energy

capacity solar in Q1 2014!

› New Duke solar installation at

Indy airportInstalled solar capacity (MW)

Source: Solar Energy Industries Association (2013)

0

20000

40000

60000

80000

100000

120000

2003 2006 2009 2012

Europe

APAC

Americas

China

MEA

ROW

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Solar costs approaching “grid parity”

› Potential for huge drop in installed costs of solar system through variety of sources

› Solar power on track to match grid prices in a variety of locations:

“grid parity”

Source: Emanuel Sachs (MIT)

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Conclusions

› Solar resource is large enough to meet future energy

needs

› Two mechanisms to convert photons into power – solar

thermal and solar PV

› Technological landscape is diverse in terms of

applications, maturity, and costs

› Thin-films expected to be major players in the near-term

› Solar market has been and will continue growing rapidly

› Grid parity already happening in certain places