Solar Optical Design

…It’s an imaging problem

Mike Sullivan

November 2008

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Outline

• Design Problem

• Solar Spectrum and Detectors

• Concentration Defined

• Direct Imaging

• Pupil imaging

• Concentrator Designs

• Fabrication

DesignProblem

1. Optics provide 100% fill factor with smallest cell footprint

– Concentration improves cell efficiency

– Reduces cost in module with discrete cell backplane

2. Design should allow +/- 1 degree Angle of Acceptance

– Sun is .5 degrees Field of View

– Associated with Tracker: step, alignment, wind, thermal

3. Design should provide 500x concentration or higher

– Concentration trades with Angle of Acceptance

4. Design should provide uniform illumination

– Cell inefficiencies due to hot spots: stay away from 2500x locally

5. Fabrication costs should be lower than $20 per square meter!!!!!!!!

– Goal for a $1 per Watt system installed

– Cost is for lens from vendor

– Very large volume consumer optics fabrication method needed

6. Operate for 20 years in the field

– Hail storms, UV exposure, thermal extremes

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Solar Spectrum

• 50% in Vis, 50% in NIR

• Silicon 20% efficient and limited to 500x concentration

– Auger recombination

• Triple junction GaAs 40% efficient and can go 1500x or more

– Heat dissipation issues

Concentration defined as

ratio of solid angles or

ratio of areas between

lens aperture and

detector

•Independent of FL

Optical Invariant

A11 A22

•FL was determined by detector size and sun FOV

Concentrator Power = A1

A2

A1

A2

Lens area

1

2

Detector area

Geometric Ratio: Lens area/

Detector area

Optical Ratio: Lens area/

Image size

Concentration

Everyone wants to

violate this constant!

0.53 degrees

Angular view of sun from earth

fh

Sin = h/f

What Focal Length lens images the sun onto a 1 mm detector

A 100mm FL lens makes a 920 micron diameter spot

0.53 deg D

Chief Ray

Marginal Ray

Sun

Earth

Direct radiation: 1000 W/m2 = 1 sun

Direct Imaging

Image the sun onto the detector with

defocus to improve off axis performance

•Cell size needs to be 9x larger area to accommodate +/- 0.5 deg.

•Geometric concentration lower than optical concentration

•Local high flux potential on Cell

•Stay away from 2500x hot spots

Direct imaging Issues

Concentrator

Optics

Solar cell

Sun

•For our 100mm FL lens with 1mm image

•Cell needs to be 3mm square to accommodate +/- .5 deg.

•Cost issue back on semiconductor again

Image the sun onto the detector with

defocus to improve off axis performance

•Defocus helps uniformity

•Spherical aberration more uniform spot before focus

Direct imaging w/Defocus

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Immersion Lens

• Secondary Optical Element shortens Focal Length

• Could make Equivalent 38mm FL in one lens though

75 mm

30 mm

• Single lens Power very high though

• Needs to be aspheric

• Curvature issue

• Fresnel lens, but have F/# issue with efficiency

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Folded Optics

• Long Focal length can be folded to save space

• Large cells use long FL designs

• Concept is Galilean Telescope

EFL

• Referred to as Cassegrain in Solar• The "Classic" Cassegrain has a

parabolic primary mirror, and hyperbolic secondary, there’s RC and HK designs

• Obscuration

• Can it be made at low cost

• Use smaller cells

•Pupil imaging gives uniform illumination on

detector

•Secondary Optic forms image of pupil on

cell

•Insensitive to sun position over secondary

lens diameter

•Roland Winston calls this a Kohler design

•Kohler illumination used in microscopes

Pupil Imaging

Concentrator

Primary Optic

Solar cell

Sun

Pupil

Sun Image

Pupil Image

Concentrator

Secondary

Optic

Concentration power trades with AOI sensitivity

Pupil Imagers

•Sandia Labs Patent

•Roland Winston Patent

•Concentration up to 1500x feasible

•Secondary can be lens or light bucket

.5 deg 1 deg

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Fresnels

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CPV - Fresnel Lens

Ammonix Emcore Concentrix CS la Mancha

Sol3g

Abengoa

Arima

Green and Gold

Sunrgi

Opel

Daido Steel

Entech

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CPV– Reflective Optics

Solar Systems Concentrating

Technologies Menova Energy

SolFocus GreenVolts

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CPV – Low Profile

Energy Innovations Soliant

Pyron SolarIsofoton

•Plastic molding

Mold costs, material, process: equal cost in 3rd’s

Can get 100,000 from 1 mold

UV and thermal issues

Low melting point –high cycle times

•Glass molding

Can get 20,000 from 1 mold

Cycle time high due to high melting point

Can get large panels with current methods

Rolled glass line like ornamental shower doors?

•AR coatings

High cost for optics

Can system take the efficiency hit without

Low cost method needed- dip coat, sol gel, moth eye

•Hybrid Methods

Silicone on glass

Polymer on glass

Untested

Fabrication

•Direct Imaging

•Lower cost

•Sun walks over cell so cell needs to be larger

•Potential hot spots

•Pupil imaging:

•Best for uniformity

•Allows beam walk or AOA

•Need 2 optical elements

•Concentrations up to 1500x feasible

•Fabrication

•Aggressive cost targets

•Fresnels nice but lossy

•AR coatings for improved throughput

Summary