The document discusses high concentration photovoltaics (HCPV), including: 1. HCPV aims to decrease expensive III-V solar cell costs by reducing their area through optical concentration of sunlight. 2. HCPV systems use optics to concentrate sunlight onto smaller, higher efficiency multi-junction solar cells. 3. Advanced optics designs for HCPV, like free-form surfaces, aim to increase efficiency, tolerance, and concentration levels compared to traditional HCPV designs like Fresnel lenses.

1. High concentration photovoltaics:
potentials and challenges
J.C. Miñano, P. Benítez
LPI-LLC, USA
Universidad Politécnica de Madrid, Spain
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POLITÉCNICA

2. Outline
1. Why high concentration photovoltaics (HCPV)?
2. Concentrator optics fundamentals
3. Advanced HCPV optics
4. Comparing HCPV systems
5. HCPV versus 2-axis tracked flat-plates
6. Summary
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3. Why high concentration photovoltaics (HCPV)?
Record cell efficiencies
FhG-ISE
41.1%
monolithic
multijunction
tandem III-V
solar cells in
concentration
• From ~30% to 40% during the last decade
• III-V cells are very expensive (~$50,000/m2-$200,000/m2)
• HCPV purpose is to decrease cell cost by reducing its area
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4. What is HCPV?
(High) concentration factor
s unl i g
ht
s unl i g
ht
F PP V
Area A
HCP V
electricity
Area A
FPPV=Flat panel PV C
Solar cell area A /Cg
HCPV=High Concentration Photovoltaics electricity
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5. Why high concentration photovoltaics (HCPV)?
cell cost + other costs
cost
=
energy solar radiation × efficiency
1. Concentration to decrease cell cost
2. Efficiency=(optical efficiency) x (cell efficiency)
3. optics, tracker Tolerance
4. only direct radiation is useful for concentration (90-65%)
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6. Outline
1. Why high concentration photovoltaics (HCPV)?
2. Concentrator optics fundamentals
3. Advanced HCPV optics
4. Comparing HCPV systems
5. HCPV versus 2-axis tracked flat-plates
6. Summary
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7. Classic imaging PV concentrators
Example: Flat Fresnel lens
±α
Rays tilted at the
acceptance angle α:
rays focus
approximately on the
edge of the cell
Cell
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8. Classic imaging PV concentrators
Formal definition of acceptance angle α:
Angle at which transmission drops to 90% of maximum
α
Ideal lens
T(θ) (%)
100
Real lens
90%
75
α
Geometrical
50
and chromatic
25
aberrations
θ (degs)
0.5 1 1.5
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9. Classic imaging PV concentrators
Modifying the geometrical concentration
α
α’
For a given optical design concept:
sin α ≈ constant × cell side
Such “constant” strongly depends
on the optical design concept
L
L’
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10. Some examples of CPV
systems based on flat
Fresnel lens
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12. Illumination non-homogeneity in imaging concentrators
Sun angular diameter= 0.53º (r=±0.27º)
Therefore, imaging concentrators
have to compromise uniformity and
Fresnel
pointing tolerance
lens
Sun image
on the cell
Cell
Perfect aiming Misspointing
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13. Classic non-imaging secondary
α
optical elements (SOE)
Prism
homogenizer
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14. Classic non-imaging secondary
optical elements (SOE)
CPC-type non-
imaging
concentrator
(reduces cell area)
Compare cost and efficiency!
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15. Other imaging concentrator designs
Parabolic mirror Cassegrian two-mirrors
Cell
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16. Other imaging concentrator designs
Parabolic mirror Cassegrian two-mirrors
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17. Outline
1. Why high concentration photovoltaics (HCPV)?
2. Concentrator optics fundamentals
3. Advanced HCPV optics
4. Comparing HCPV systems
5. HCPV versus 2-axis tracked flat-plates
6. Summary
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18. Why advanced HCPV optics?
1. Higher Efficiency
2. Higher Tolerance
3. Higher Concentration?
• To be achieved without increasing the number of optical
elements.
• Each optical surface must perform as many functions
(concentration, homogenization, etc.) as possible.
• The highest Tolerance for a given Concentration will
maximize Efficiency at system level.
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19. Do you need more tolerance?
Symptomatology:
1. Optics surfaces require high accuracy
2. Assembling is expensive because fine adjustments become
compulsory.
3. Efficiency decreases significantly from single unit to array.
Optical mismatch
4. Efficiency increases significantly when the cells are bigger.
5. The electricity production waves in moderate windy
conditions
6. The efficiency decrease due to dirt accumulation is more
severe than in flat modules
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20. Tolerance
Tolerance budget has to be shared among:
1. Sun’s angular extension ±0.27°
2. Optical component manufacturing 0.1°-0.5°
(shape and roughness) present automotive
industry standards
3. Module assembling
4. Array assembling
5. Tracker structure stiffness
6. Tracking accuracy
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21. Advanced HCPV optics: Free-form designs
• Free-form: surfaces with no prescribed symmetry
• New degrees of freedom to the design: A single
optical element can perform multiple functions
• The SMS 3D design method of Nonimaging Optics is
the most advanced method to design free-forms
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22. Free-form XR for HCPV (Boeing-LPI)
Free-form Free-form mirror
lens
Solar cell
Homogenizing prism
Free-form lens
A. Plesniak et al. “Demostration of high performance concentrating photovoltaic module designs for utility scale power generation”, ICSC – 5, (Palm Desert, CA, USA,
2008)
A. Cvetkovic, M. Hernández, P. Benítez, J. C. Miñano, J. Schwartz, A. Plesniak, R. Jones, D. Whelan, “The Free Form XR Photovoltaic Concentrator: a High Performance
SMS3D Design”, Proc. SPIE Vol. 7043-12, 2008
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23. RR free-form Kohler design for HCPV
Primary lens (R)
Secondary lens (R)
Solar cell
A. Cvetkovic et al. “High Performance Köhler Concentrators with Uniform Irradiance on Solar Cell”, ICSC – 5, (Palm Desert, CA, USA, 2008)
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24. RR free-form Kohler design for HCPV
A. Cvetkovic et al. “High Performance Köhler Concentrators with Uniform Irradiance on Solar Cell”, ICSC – 5, (Palm Desert, CA, USA, 2008)
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25. Other free-form designs (for SSL)
Free-form RXI with Kohler
Free-form RXI
integration
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26. Outline
1. Why high concentration photovoltaics (HCPV)?
2. Concentrator optics fundamentals
3. Advanced HCPV optics
4. Comparing HCPV systems
5. HCPV versus 2-axis tracked flat-plates
6. Summary
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27. What should be the criterion to compare
CPV systems?
• Final merit function = cost of electricity
• It is difficult to evaluate before product is very
mature
• Several parameters are usually selected as merit
functions to compare
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28. Some parameters for CPV systems comparison
1. Module electrical efficiency at nominal conditions
2. Concentration
3. Tolerance angle (in degs)
4. Nominal power per unit area of the module, Pmodule (in Wp/m2)
5. Nominal power per unit area of the cell, Pcell (in Wp/cm2)
6. Estimated yearly energy production in certain reference locations
(in kWh/(m2 year))
7. Others: Mounting complexity, numbers of parts per unit area of
the module, materials cost, weight, depth, thermal design, etc
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29. The efficiency-concentration-tolerance
(ECT) space
Electrical efficiency η (%)
Example:
Fresnel lens
concentrator with
η = 27%
27%
Cg=400x
400 α = ±0.5 degs
Concentration Cg
0.5 degs
Tolerance α (degs)
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30. Boundaries of the ECT space
Thermodynamic limits:
• Electrical efficiency (for infinite junctions) limited to: η < 86%
• Concentration × Tolerance2 < n2 ≈ 2.25 (n=refractive index of encapsulant)
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31. Boundaries of the ECT space
Electrical efficiency η (%)
η < 86%
Example:
Fresnel lens
concentrator with
Tolerance > sun radius = 0.26º
η = 27%
Cg=400x
α = ±0.5 degs
Concentration
Concentration × Tolerance2 < n2 ≈ 2.25
Tolerance (degs)
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32. Comparing CPV systems in the ECT space
Fresnel lens concentrator XR free-form concentrator
η = 27% η = 27%
Cg=400x Cg=1,000x
α = ±0.5º α = ±1.8º
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33. Comparing CPV systems in the ECT space
Electrical efficiency (%)
Fresnel lens concentrator
XR free-form concentrator
400 ,000
1
±0
.5º
±1
.8º Concentration
±2
.8º
Tolerance (degs)
Concentration × Tolerance2 ≈ constant
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34. Comparing CPV systems in the ECT space
Electrical efficiency (%)
Fresnel lens concentrator
XR free-form concentrator
40 0
0
,00
2
Concentration
±0
.5º
±2 ±1
.3º
.8
º
Tolerance (degs)
Concentration × Tolerance2 ≈ constant
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35. Comparing CPV systems in the ECT space
A. Plesniak et al. “Demostration of high performance
concentrating photovoltaic module designs for utility scale
power generation”, ICSC – 5, (Palm Desert, CA, USA, 2008)
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36. Comparing CPV systems in the ECT space
Advanced XR HCPV
Target
Target
Advanced XR HCPV
±2.8º 33% 600x
Target ≈ ±2.0º 31% 1,200x
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37. Outline
1. Why high concentration photovoltaics (HCPV)?
2. Concentrator optics fundamentals
3. Advanced HCPV optics
4. Comparing HCPV systems
5. HCPV versus 2-axis tracked flat-plates
6. Summary
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38. HCPV versus 2-axis tracked flat-plates
Concentration-tolerance-efficiency comparison is not possible because
technologies are quite different.
cost cell cost + other costs
=
energy solar radiation × efficiency
• Solar radiation: Diffuse radiation can add 15-30% more for flat-plates.
• Efficiency for flat-plates use to be rated at 25ºC cell temperature while
the efficiency is rated at 20ºC ambient temperature for concentrators.
• Efficiency vs temperature coefficients are different for Si and MJ cells
• Flat plate trackers don’t need accuracy
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39. HCPV versus 2-axis tracked flat-plates
Example: Seville (Spain)
High HCPV High
Conventional
efficiency for equal performance Goal
silicon
silicon output HCPV
Module efficiency at STC
12.0 19.3 - - -
(%)
Average efficiency in
10.6 17.5 22.4 27.0 30.0
operation (%)
Annual solar irradiation 2580 2580 2012 2012 2012
(kWh/(m2·year)) (100%) (100%) (78%) (78%) (78%)
Nominal annual DC
274 451 451 543 604
electrical energy density
(100%) (164%) (164%) (198%) (220%)
(kWh/(m2·year))
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40. HCPV versus 2-axis tracked flat-plates
The most important advantages of HCPV vs flat-plates come from the
comparison of recent time evolution of efficiencies
Record cell efficiencies
• The derivatives of efficiencies
for MJ and Si cells vs time are
FhG-ISE
41.1%
significantly different.
• Si cells are more mature (less
risk and less expected
improvements)
• The same considerations
affects to cell cost of both
technologies
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41. Outline
1. Why high concentration photovoltaics (HCPV)?
2. Concentrator optics fundamentals
3. Advanced HCPV optics
4. Comparing HCPV systems
5. HCPV versus 2-axis tracked flat-plates
6. Summary
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42. Summary
1. The potential of HCPV relies on the fast increase of MJ cells
efficiency
2. The near-term challenge is beating 2-axis tracking flat-panels
3. To succeed, HCPV needs high efficiency, sufficient high
concentration and as much tolerance as possible
4. The best Efficiency-Concentration-Tolerance is being achieved by
Advanced Optics.
5. Scaling-up HCPV will need the synergy with present high-
throughput low-cost industries (such as automotive or solid state
lighting)
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43. LEGAL NOTICE
Devices shown in this presentation are protected by the following US and International Patents and Patents
Pending:
Patents Issued
HIGH EFFICIENY NON-IMAGING US 6,639,733 October 28, 2003
COMPACT FOLDED-OPTICS ILLUMINATION LENS US 6,896,381 May 24, 2005
COMPACT FOLDED-OPTICS ILLUMINATION LENS US 7,152,985 December 26, 2006
COMPACT FOLDED-OPTICS ILLUMINATION LENS US 7,181,378 February 20, 2007
DEVICE FOR CONCENTRATING OR COLLIMATING RADIANT ENERGY US 7,160,522 January 9, 2007
DISPOSITIVO CON LENTE DISCONTINUA DE REFLEXIÓN TOTAL INTERNA Y DIÓPTRICO ESFÉRICO PARA
CONCENTRACIÓN O COLIMACIÓN DE ENERGÍA RADIANTE Spain ES P9902661 December 2, 1999
OPTICAL MANIFOLD FOR LIGHT-EMITTING DIODES US 7,380,962
OPTICAL MANIFOLD FOR LIGHT-EMITTING DIODES US 7,286,296
THREE-DIMENSIONAL SIMULTANEOUS MULTIPLE-SURFACE METHOD AND FREE-FORM ILLUMINATION-
OPTICS DESIGNED THEREFROM US 7,460,985 December 2, 2008
Patents Pending
DEVICE FOR CONCENTRATING OR COLLIMATING RADIANT ENERGY - a continuation of US 7,160,522
FREE-FORM LENTICULAR OPTICAL ELEMENTS AND THEIR APPLICATION TO CONDENSERS AND
HEADLAMPS PCT/US2006/029464 July 28, 2006
MULTI-JUNCTION SOLAR CELLS WITH A HOMOGENIZER SYSTEM AND COUPLED NON-IMAGING LIGHT
CONCENTRATOR PCT/US07/63522 March 7, 2007
OPTICAL CONCENTRATOR, ESPECIALLY FOR SOLAR PHOTOVOLTAICS PCT/US08/03439 Mar 14, 2008
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44. Further reading
R. Winston, J.C. Miñano, P. Benítez, NonImaging Optics, J. Chaves, Introduction to Nonimaging Optics,
Elsevier Academic Press, 2005, ISBN 0127597514 CRC Press, 2008, ISBN: 9781420054293
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45. Contacts
LPI EUROPE SL
LPI LLC
Ramón F. de Caleya, Managing Director
Roberto Alvarez, CEO
rfcaleya@lpi-europe.com
ralvarez@lpi-llc.us
Oliver Dross, Technology Director
Waqidi Falicoff, Exec. VP
odross@lpi-europe.com
wfalicoff@lpi-llc.us
Edificio Cedint
2400 Lincoln Ave.
Campus de Montegancedo UPM
Altadena, CA 91001, USA
28223, Madrid, SPAIN
Fax: (949) 265-0547
Fax: (+34) 91 452 4892
www.lpi-llc.com www.lpi-europe.com
LPI PO
Bill Tse, General Manager
btse@lpi-llc.us
Unit 02, G/F, Photonics Centre, Science Park East Ave., Hong-Kong, CHINA
Fax: +852 2144 2566
www.lpi-po.com
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46. LPI Overview
LPI-Europe
Cologne, Germany
Cologne, Germany
Madrid, Spain
Madrid, Spain
LPI-LLC LPI-PO
Headquarters
Headquarters Hong Kong, China
Hong Kong, China
Altadena, California,
Altadena, California,
USA
USA
Thank you!
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47. Acknowledgements
The authors thank the support under the project
PIE521/2008,“Investigación en nuevos concentradores
FV 1000x con células solares de alta eficiencia” given
by the Instituto Madrileño de Desarrollo and the Fondo
Europeo de Desarrollo regional
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