High concentration photovoltaics: potentials and challenges

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  • TECHNOLOGY ISSUES



    Q: Is HCPV useful for heating or only for electricity generation?

    A: There are some developers working on this concept (for instance, Menova Energy in Canada has a combined parabolic trough (thermal) + CPV design, see http://www.power-spar.com/Power-Spar/index.php). Since PV cell efficiency tends to be lower at higher temperatures, this co-generation of electricity and heat is not well adapted for most of the industrial processes (which need heat at high temperature). Even though the temperature coefficient of the III-V cells efficiency is small compared to other technology options (as silicon), high temperature operation is not recommended for reliability issues. The additional complexity added by the heating system can be justified in PV roof installations where there may be a close need for hot water at medium-low temperature.



    Q: I worked on CPVs in the 1980s. Are heat dissipation and CTE mismatch still problems?

    A: Heat dissipation is still a big issue. The bottleneck in the heat flow is usually the contact between the cell receiver and the heat sink. The point is to conciliate good thermal contact and electrical insulation. That’s why this interface uses to be the most critical from the thermal point of view. To improve the thermal contact between cell and heat spreader it is necessary a good mechanical contact on this interface and this implies that materials at both sides of the interface must have similar Coefficient of Thermal Expansion (CTE).



    Q : Who is developing a system with dish and CPV in the dense array method? Is it suitable with additional thermal use?

    A: Solar Systems (http://www.solarsystems.com.au/) in Australia has developed such a system. They have used Spectrolab cells (http://www.spectrolab.com/) and active cooling.



    Q: What materials are used to make the freeform secondary optics?

    A: Glass is preferred in general. Plastic is a secondary candidate, because of possible degradation due to high radiation. Hollow secondaries can also be used made of conformed reflective metallic foils.



    Q: What are the major technical barriers in making a quantum leap towards the thermodynamic limit of 86% and the current efficiencies of 27%?

    A: It has to be considered the fact that 86% limit assumes a perfect optic system (100% efficient) and infinite cell junctions. In practice, we can increase the number of junctions, but the spectrum sensitivity increases too following daily and annual fluctuations if the cells are series connected. So it makes no sense going further 4 to 5 junctions. Reaching 50% efficiency cell is expected for the coming years but just with 1 or 2 additional junctions, no more. Spectral sensitivity can be improved in the future by making one of the junctions use the principle of Intermediate Band Solar cells (Luque et al), which in principle can behave as three equivalent junctions, one in parallel with the other two in series connection.



    Q: What are the biggest technical hurdles that you see for CPV right now?

    A: We think that CPV has now a unique opportunity to succeed. Concentration cells have reached a very high efficiency and a sufficient maturity level. Optics have also reached a top level with the new high-concentration, high-tolerance designs that we have done. Trackers have also reached a satisfactory level of development.

    The biggest technical hurdle now is to prove that CPV is suitable for mass-production. For this goal, a wide concentrator tolerance angle will be the key to success.





    Q: From the point of view of the power converter (inverter) does CPV have any particularities to be considered?

    A: I don’t see a particular effect of CPV for inverters.





    Q: What are the temperatures encountered at the focal point at X1000 concentration levels?

    A: The thermal design of the concentrator must be such that the junction to ambient temperature drop is not above 50-80 deg Celsius. This optimum temperature is somehow independent of the concentration level. It mainly depends on the heat sink cost and on the efficiency vs temperature curve.

    These temperature drops at 1000x can be achieved with cost-effective passive cooling if the cells are small enough (50 MW), it is difficult to predict when this technology will be ready and mature. Have a look to http://www.slideshare.net/sustenergy/bulk-solar-power-generation-sp-and-cpv-technologies?src=embed



    Q: Do you think that the complexity in manufacturing of the modules could be a bottleneck for their market success?

    A: This becomes a bottleneck unless you have a good optical design. That is why we have stressed the importance of the tolerance angle. This angle has to be wide enough to make room for many aspects related with module manufacturing (optical surface errors, concentrator unit assembling, array assembling, tracking errors, tracker stiffness etc.) For instance, a design with a tolerance angle of ±0.5 degrees leaves no room for manufacturing tolerances: ±0.27 degrees are used by the sun’s angular extension and consequently there is a mere ±0.23 degrees for optical surface errors, concentrator unit assembling, array assembling, tracking errors, tracker stiffness, radiation scattering by dust particles, etc. This tight tolerance angle makes the manufacturing prohibitively expensive due to the necessary high precision. Such ±0.5 degrees design is only good for a prototype.

    We at LPI (http://www.lpi-llc.com/, for more detailed info please contact to info@lpi-europe.com ) have a unique set of design and manufacturing tools to achieve the greatest tolerances, very close to the thermodynamic limit (in particular our SMS design tool). That is why we achieve the highest tolerances even with the highest concentration levels.





    Q: Would you please contrast HCPV vs Si and thin film in terms of land area required (hectares or acres) per MW?

    A: Have a look to http://www.slideshare.net/sustenergy/bulk-solar-power-generation-sp-and-cpv-technologies?src=embed. This presentation states how a CPV system thanks to its increased efficiency can reduce land area requirement by 30% (comparing a flat PV system 14% efficient and a CPV system 25% efficient). However, comparison is not easy as the reference for installed power is not the same in flat PV and CPV. Expected power density in the tracker area is 250 W/m2 for CPV, 170 W/m2 for the most efficient flat PV and 50W/m2 for amorphous silicon thin film.





    Q: What is the state of art of HCPV in terms of your figures of merit ($/KWH, KWH/m2/yr)?

    A: Solfocus announced the installation of 10 MW at a price of $10/W. However, ISFOC in Spain made a call to bid and price of the various companies (Solfocus, Isofotón, Concentrix…) was around 6 – 6,5 €/W (complete PV system). In South Spain this allows a levelized cost of energy around 30 c€/kWh. The expectation is a cost reduction to 4 €/W in 2010 and 3 €/W in 2013, being in this case under the price expectation of flat PV. Have a look to : http://www.leonardo-energy.org/drupal/taxonomy/term/664





    Q: Have you looked at rooftop sized panels being proposed by Soliant or EnFocus. As these compete w non-tracking flat panel, do you expect these to be competitive in terms of cost of electricity produced?

    A: CPV for rooftop has been proposed by several companies at present and in the past (SEA, Sol3G). Competitiveness related with flat PV is not clear for the time being, even though they compete with the retail electricity price. Integrating tracking equipment in buildings is usually considered difficult at this moment.





    Q: Morgan Solar has developed Light-guide Solar Optic. Have you included that in your ECT space?
    A: We have not, but it can be included and the fundamental limitations still apply. The thermodynamic limit, as stated in the ETC space, only assume that there is no frequency shift between the incoming radiation and the radiation leaving the concentrator towards the cell and, apparently, the LSO is not doing frequency shift. Consequently the limitations also apply.
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  • High concentration photovoltaics: potentials and challenges

    1. 1. High concentration photovoltaics: potentials and challenges J.C. Miñano, P. Benítez LPI-LLC, USA Universidad Politécnica de Madrid, Spain
    2. 2. Outline <ul><li>Why high concentration photovoltaics (HCPV)? </li></ul><ul><li>Concentrator optics fundamentals </li></ul><ul><li>Advanced HCPV optics </li></ul><ul><li>Comparing HCPV systems </li></ul><ul><li>HCPV versus 2-axis tracked flat-plates </li></ul><ul><li>Summary </li></ul>
    3. 3. 40.7% monolithic multijunction tandem III-V solar cells in concentration Why high concentration photovoltaics (HCPV)? Record cell efficiencies <ul><li>From ~30% to 40% during the last decade </li></ul><ul><li>III-V cells are very expensive ( ~$ 100,000/m 2 - $ 200,000/m 2 ) </li></ul><ul><li>HCPV purpose is to decrease cell cost by reducing its area </li></ul>
    4. 4. FPPV=Flat panel PV HCPV=High Concentration Photovoltaics (High) concentration factor Solar cell area A / C g sunlight sunlight HCPV FPPV electricity electricity What is HCPV? C g Area A Area A
    5. 5. solar radiation cell cost other costs + efficiency × cost energy = <ul><li>Concentration to decrease cell cost </li></ul><ul><li>Efficiency =(optical efficiency) x (cell efficiency) </li></ul><ul><li>optics, tracker  Tolerance </li></ul><ul><li>only direct radiation is useful for concentration (90-65%) </li></ul>Why high concentration photovoltaics (HCPV)?
    6. 6. Outline <ul><li>Why high concentration photovoltaics (HCPV)? </li></ul><ul><li>Concentrator optics fundamentals </li></ul><ul><li>Advanced HCPV optics </li></ul><ul><li>Comparing HCPV systems </li></ul><ul><li>HCPV versus 2-axis tracked flat-plates </li></ul><ul><li>Summary </li></ul>
    7. 7. Classic imaging PV concentrators Example: Flat Fresnel lens Cell Rays tilted at the acceptance angle  : rays focus approximately on the edge of the cell 
    8. 8. Geometrical and chromatic aberrations Formal definition of acceptance angle  : Angle at which transmission drops to 90% of maximum Ideal lens Real lens Classic imaging PV concentrators  (degs) 100 75 50 25 0.5 1 1.5 T(  ) (%)   90% 
    9. 9. Classic imaging PV concentrators For a given optical design concept: cell side × sin  constant Such “constant” strongly depends on the optical design concept Modifying the geometrical concentration L’  ’  L
    10. 10. Some examples of CPV systems based on flat Fresnel lens
    11. 12. Illumination non-homogeneity in imaging concentrators Cell Fresnel lens Therefore, imaging concentrators have to compromise uniformity and pointing tolerance Sun angular diameter= 0.53º (r= ±0.27º ) Sun image on the cell Perfect aiming Misspointing
    12. 13. Classic non-imaging secondary optical elements (SOE) Prism homogenizer 
    13. 14. Classic non-imaging secondary optical elements (SOE) CPC-type non-imaging concentrator (reduces cell area) Compare cost and efficiency!
    14. 15. Other imaging concentrator designs Cell Cassegrian two-mirrors Parabolic mirror
    15. 16. Other imaging concentrator designs Cassegrian two-mirrors Parabolic mirror
    16. 17. Outline <ul><li>Why high concentration photovoltaics (HCPV)? </li></ul><ul><li>Concentrator optics fundamentals </li></ul><ul><li>Advanced HCPV optics </li></ul><ul><li>Comparing HCPV systems </li></ul><ul><li>HCPV versus 2-axis tracked flat-plates </li></ul><ul><li>Summary </li></ul>
    17. 18. <ul><li>Higher Efficiency </li></ul><ul><li>Higher Tolerance </li></ul><ul><li>Higher Concentration ? </li></ul>Why advanced HCPV optics? <ul><li>To be achieved without increasing the number of optical elements. </li></ul><ul><li>Each optical surface must perform as many functions (concentration, homogenization, etc.) as possible. </li></ul><ul><li>The highest Tolerance for a given Concentration will maximize Efficiency at system level. </li></ul>
    18. 19. Do you need more tolerance? <ul><li>Symptomatology: </li></ul><ul><li>Optics require high accuracy </li></ul><ul><li>Assembling is expensive because fine adjustments become compulsory. </li></ul><ul><li>Efficiency decreases significantly from single unit to array. Optical mismatch </li></ul><ul><li>Efficiency increases significantly when the cells are bigger. </li></ul>
    19. 20. Tolerance <ul><li>Tolerance budget has to be shared among: </li></ul><ul><li>Sun’s angular extension ±0.27 ° </li></ul><ul><li>Optical component manufacturing (shape and roughness) </li></ul><ul><li>Module assembling </li></ul><ul><li>Array assembling </li></ul><ul><li>Tracker structure stiffness </li></ul><ul><li>Tracking accuracy </li></ul>0.1°-0.5° present automotive industry standards
    20. 21. Advanced HCPV optics: Free-form designs <ul><li>Free-form: surfaces with no prescribed symmetry </li></ul><ul><li>New degrees of freedom to the design: A single optical element can perform multiple functions </li></ul><ul><li>The SMS 3D design method of Nonimaging Optics is the most advanced method to design free-forms </li></ul>
    21. 22. Free-form XR for HCPV (Boeing-LPI) 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 Free-form mirror Solar cell Free-form lens
    22. 23. Secondary lens (R) Solar cell Primary lens (R) 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)
    23. 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)
    24. 25. Other free-form designs (for SSL) Free-form RXI Free-form RXI with Kohler integration
    25. 26. Outline <ul><li>Why high concentration photovoltaics (HCPV)? </li></ul><ul><li>Concentrator optics fundamentals </li></ul><ul><li>Advanced HCPV optics </li></ul><ul><li>Comparing HCPV systems </li></ul><ul><li>HCPV versus 2-axis tracked flat-plates </li></ul><ul><li>Summary </li></ul>
    26. 27. What should be the criterion to compare CPV systems? <ul><li>Final merit function = cost of electricity </li></ul><ul><li>It is difficult to evaluate before product is very mature </li></ul><ul><li>Several parameters are usually selected as merit functions to compare </li></ul>
    27. 28. Some parameters for CPV systems comparison <ul><li>Module electrical efficiency at nominal conditions </li></ul><ul><li>Concentration </li></ul><ul><li>Tolerance angle (in degs) </li></ul><ul><li>Nominal power per unit area of the module, P module (in W p /m 2 ) </li></ul><ul><li>Nominal power per unit area of the cell, P cell (in W p /cm 2 ) </li></ul><ul><li>Estimated yearly energy production in certain reference locations (in kWh/(m 2 year)) </li></ul><ul><li>Others: Mounting complexity, numbers of parts per unit area of the module, materials cost, weight, depth, thermal design, etc </li></ul>
    28. 29. Electrical efficiency  (%) The efficiency-concentration-tolerance (ECT) space Tolerance  (degs) Concentration C g  = 27% C g =400x  = ±0.5 degs Example: Fresnel lens concentrator with 27% 0.5 degs 400
    29. 30. Boundaries of the ECT space <ul><li>Thermodynamic limits: </li></ul><ul><li>Electrical efficiency (for infinite junctions) limited to:  < 86% </li></ul><ul><li>Concentration × Tolerance 2 < n 2  2.25 (n=refractive index of encapsulant) </li></ul>
    30. 31. Electrical efficiency  (%) Boundaries of the ECT space Tolerance (degs) Concentration  < 86% Concentration × Tolerance 2 < n 2  2.25 Tolerance > sun radius = 0.26º  = 27% C g =400x  = ±0.5 degs Example: Fresnel lens concentrator with
    31. 32. Comparing CPV systems in the ECT space  = 27% C g =400x  = ±0.5º  = 27% C g =1,000x  = ±1.8º Fresnel lens concentrator XR free-form concentrator
    32. 33. Comparing CPV systems in the ECT space Electrical efficiency (%) Tolerance (degs) Concentration 1,000 400 ± 0.5º ± 2.8º Concentration × Tolerance 2  constant ± 1.8º Fresnel lens concentrator XR free-form concentrator
    33. 34. Comparing CPV systems in the ECT space Electrical efficiency (%) Tolerance (degs) Concentration 2,000 400 ± 0.5º ± 1.3º Concentration × Tolerance 2  constant ± 2.8º Fresnel lens concentrator XR free-form concentrator
    34. 35. Comparing CPV systems in the ECT space Concentrix SolFocus Sol3g Daido Steel Guascor Foton Isofoton Boeing A. Plesniak et al. “ Demostration of high performance concentrating photovoltaic module designs for utility scale power generation ”, ICSC – 5, (Palm Desert, CA, USA, 2008)
    35. 36. Target 2  ±2.8º 33% 600x Comparing CPV systems in the ECT space Target Target Advanced XR HCPV Advanced XR HCPV Target 1  ±2.0º 31% 1,200x
    36. 37. Outline <ul><li>Why high concentration photovoltaics (HCPV)? </li></ul><ul><li>Concentrator optics fundamentals </li></ul><ul><li>Advanced HCPV optics </li></ul><ul><li>Comparing HCPV systems </li></ul><ul><li>HCPV versus 2-axis tracked flat-plates </li></ul><ul><li>Summary </li></ul>
    37. 38. solar radiation cell cost other costs + efficiency × cost energy = HCPV versus 2-axis tracked flat-plates <ul><li>Solar radiation: Diffuse radiation can add 15-30% more for flat-plates. </li></ul><ul><li>Efficiency for flat-plates is rated at 25ºC cell temperature while the efficiency is rated at 20ºC ambient temperature for concentrators. </li></ul><ul><li>Efficiency vs temperature coefficients are different for Si and MJ cells </li></ul><ul><li>Flat plate trackers don’t need accuracy </li></ul>Concentration-tolerance-efficiency comparison is not possible because technologies are quite different. solar radiation efficiency other costs
    38. 39. HCPV versus 2-axis tracked flat-plates <ul><li>With the preceding considerations +optical efficiency, +Si efficiencies, +MJ efficiencies: </li></ul><ul><li>At present, when comparing the most efficient examples of both options the electric energy production/m 2 is about 10-15% greater for HCPV in a sunny place. </li></ul><ul><li>HCPV has not clear advantages yet regarding the cost of energy </li></ul><ul><li>The time evolution of efficiencies and cost shows that HCPV is at the beginning of the learning curve with a big potential for improvements </li></ul>
    39. 40. Record cell efficiencies HCPV versus 2-axis tracked flat-plates <ul><li>The derivatives of efficiencies for MJ and Si cells vs time are significantly different. </li></ul><ul><li>Si cells are more mature (less risk and less expected improvements) </li></ul><ul><li>The same considerations affects to cell cost of both technologies </li></ul>The most important advantages of HCPV vs flat-plates come from the comparison of recent time evolution of efficiencies and cost
    40. 41. Outline <ul><li>Why high concentration photovoltaics (HCPV)? </li></ul><ul><li>Concentrator optics fundamentals </li></ul><ul><li>Advanced HCPV optics </li></ul><ul><li>Comparing HCPV systems </li></ul><ul><li>HCPV versus 2-axis tracked flat-plates </li></ul><ul><li>Summary </li></ul>
    41. 42. Summary <ul><li>The potential of HCPV relies on the fast increase of MJ cells efficiency </li></ul><ul><li>The near-term challenge is beating 2-axis tracking flat-panels </li></ul><ul><li>To succeed, HCPV needs high efficiency , sufficient high concentration and as much tolerance as possible </li></ul><ul><li>The best Efficiency-Concentration-Tolerance is being achieved by Advanced Optics. </li></ul><ul><li>Scaling-up HCPV will need the synergy with present high-throughput low-cost industries (such as automotive or solid state lighting) </li></ul>
    42. 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
    43. 44. Further reading R. Winston, J.C. Miñano, P. Benítez, NonImaging Optics , Elsevier Academic Press, 2005, ISBN 0127597514 J. Chaves, Introduction to Nonimaging Optics , CRC Press, 2008, ISBN: 9781420054293
    44. 45. LPI Overview LPI-LLC Headquarters Altadena, California, USA LPI-PO Hong Kong, China LPI-Europe Cologne, Germany Madrid, Spain
    45. 46. Contacts LPI EUROPE SL Ramón F. de Caleya, Managing Director [email_address] Oliver Dross, Technology Director [email_address] Edificio Cedint Campus de Montegancedo UPM 28223, Madrid, SPAIN Fax: (+34) 91 452 4892 www.lpi-europe.com LPI LLC Roberto Alvarez , CEO [email_address] Waqidi Falicoff, Exec. VP [email_address] 2400 Lincoln Ave. Altadena, CA 91001, USA      Fax: (949) 265-0547 www.lpi-llc.com LPI PO Bill Tse, General Manager [email_address] Unit 02, G/F, Photonics Centre, Science Park East Ave., Hong-Kong , CHINA Fax: +852 2144 2566 www.lpi-po.com
    46. 47. <ul><li>Thank you! </li></ul>

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