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See detailEtude d’un concentrateur solaire basé sur une combinaison optique diffractive/réfractive pour applications spatiales
Michel, Céline ULg

Doctoral thesis (2015)

Continued demand for new applications from spacecrafts leads to larger power needs. For most of them, the primary energy sources are solar panels. However, these solar panels are heavy and expensive ... [more ▼]

Continued demand for new applications from spacecrafts leads to larger power needs. For most of them, the primary energy sources are solar panels. However, these solar panels are heavy and expensive, mainly due to solar cells and their coverglass. The power need increase requires thus new concepts of solar panels. Furthermore, space environment is highly constraining: the vacuum limits heat trans- fer since convection is not allowed, there is contamination modifying the light spectrum reaching cells, ultraviolet light induces a yellowing of silicone glue, radiations degrade pho- tovoltaic cells, etc. Usually, multi-junctions (MJ) cells are used, that are strongly sensitive to spectral modification in their incident spectrum due to their series connected structure (the worst cell defines the whole output performance). The power dimensioning of solar panels is then based on end of life (EOL) PV cells expected performance. Reducing the sensitivity to increase EOL output power is then another challenge for space applications. This thesis aims to answer to both problems, by the proposition and study of a new lightweight solar concentrator with spectral splitting. This conceptual concentrator is composed of a Fresnel lens for sunlight concentration, coupled to a surface relief diffraction grating to spectrally split incident light, forming a single optical element in silicone. The concentration behavior allows a reduction of solar cells area (including its coverglass), replaced by a lightweight silicone lens, reducing the global cost and mass. The lateral spectral separation will permit other types of cells than the usual MJ cells. To demonstrate the concept, two single junctions (SJ) solar cells are placed side by side, the first collecting visible light, the second collecting near IR light. Since cells are electrically independent, sensitivity is lower and EOL output power can surpass standard MJ cells systems. Moreover, cells combination is nearly free: all photovoltaic technology can be used and combined, opening the solar concentration field to other technologies than MJ. This work studies and evaluates the pros and cons of diffraction grating/Fresnel lens combinations as solar concentrator with spectral splitting. The analysis includes the choice of material, optical developments and optimizations (grating selection, freeform lenses, grating period optimization along the lens, etc.), electrical modeling and methods to find the best cells combination as well as the modeling of expected output power, a thermal simulation, weight and deployment considerations, and approaches of sensitivity with spectral modifications. Some experimental results complete the study. Two main configurations were developed: a first with a blazed grating, and a second with a symmetrical lamellar grating. Both configurations, after optimization, show similar performances: a global geometrical concentration ratio around 5-6× (ratio between lens width and the total width of the two cells), a tracking error tolerance up to 0.7◦, no drastic degradation with respect to deformations, fabrication errors, etc., an output power at begin of life (BOL) better than a classical concentrator focusing on a SJ cell or a planar solar panel composed of SJ cells. Both configurations present also a BOL specific mass [kg/W] lower than a classical planar solar panel covered by MJ cells, and a lower sensitivity to space environment, with theoretical larger EOL output power depending on chosen PV cells combinations. Existing deployment systems are proposed and discussed. Also, an experimental prototype of the optical element was realized, with optical results close to simulations. Some adaptations to terrestrial concentrators, other spectra, or using more than 2 different SJ cells are also introduced, highlighting the versatility of our concept. Results demonstrate thus the coherence of the concept, leading to experimentally feasible and quite tolerant concentrator, with interesting cost reduction thanks to concentration and specific weight reduction. However, optical losses due to the non-perfect spectral splitting of the grating is too high to be able to surpass MJ cells systems BOL up to now, and the lower sensitivity of our concentrator cannot completely compensate this lack of performance at EOL. To reach better performances than MJ systems, from cost, mass, radiation resistance, etc. point of view and to exploit the obvious gain in sensitivity, the concentrator needs thus further investigations mainly related to cells combinations. Especially as our concept opens a lot of opportunities thanks to the complete independence of cells: other technologies are allowed, specific coatings can improve performances, ... Applications for Earth or places with different/changing incident spectra is another plausible perspective. Our concentrator with spectral splitting can also be a major advantage for specific scientific space missions like deep space missions. [less ▲]

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See detailOptical study of diffraction grating/Fresnel lens combinations applied to a spectral-splitting solar concentrator for space applications
Michel, Céline ULg; Loicq, Jerôme ULg; Thibert, Tanguy ULg et al

in Applied Optics (2015), 54(22), 6666-6673

This paper presents a new design of a planar solar concentrator with spectral splitting of light for space applications. This concentrator spectrally splits the incident light into mainly two parts. Each ... [more ▼]

This paper presents a new design of a planar solar concentrator with spectral splitting of light for space applications. This concentrator spectrally splits the incident light into mainly two parts. Each part is then focused onto specific spatially separated photovoltaic cells, allowing for independent control of respective cells output power. These advantages of both spectral splitting and light focusing are here combined thanks to a specific diffraction grating superimposed on a Fresnel lens. The theoretical principle of the optical design is presented, with optimization of each element and improvement steps, including optimization of grating period evolution along the lens, and testing of two kinds of gratings (a blazed and a lamellar one). First numerical results are presented, highlighting the possibility to design a concentrator at about 10× or more for each cell, with an output power larger than that of a classical concentrator focusing on a GaAs SJ cell, and less than 10% of losses for tracking errors up to ±0.8°. Some experimental results are also presented. [less ▲]

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See detailOptical Study of a Spectrum Splitting Solar Concentrator based on a Combination of a Diffraction Grating and a Fresnel Lens
Michel, Céline ULg; Loicq, Jerôme ULg; Thibert, Tanguy ULg et al

Poster (2015, April 14)

This poster presents recent improvements of our new solar concentrator design for space application. The concentrator is based on a combination of a diffraction grating (blazed or lamellar) coupled with a ... [more ▼]

This poster presents recent improvements of our new solar concentrator design for space application. The concentrator is based on a combination of a diffraction grating (blazed or lamellar) coupled with a Fresnel lens. Thanks to this diffractive/refractive combination, this optical element splits spatially and spectrally the light and focus approximately respectively visible light and IR light onto electrically independent specific cells. It avoid the use of MJs cells and then also their limitations like current matching and lattice matching conditions, leading theoretically to a more tolerant system. The concept is reminded, with recent optimizations, ideal and more realistic results, and the description of an experimental realization highlighting the feasibility of the concept, and the closeness of theoretical and experimental results. [less ▲]

Detailed reference viewed: 17 (5 ULg)
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See detailOptical Study of a Spectrum Splitting Solar Concentrator based on a Combination of a Diffraction Grating and a Fresnel Lens
Michel, Céline ULg; Loicq, Jerôme ULg; Thibert, Tanguy ULg et al

in AIP Conference Proceedings (2015)

This paper presents recent improvements of our new solar concentrator design for space application. The concentrator is based on a combination of a diffraction grating (blazed or lamellar) coupled with a ... [more ▼]

This paper presents recent improvements of our new solar concentrator design for space application. The concentrator is based on a combination of a diffraction grating (blazed or lamellar) coupled with a Fresnel lens. Thanks to this diffractive/refractive combination, this optical element splits spatially and spectrally the light and focus approximately respectively visible light and IR light onto electrically independent specific cells. It avoid the use of MJs cells and then also their limitations like current matching and lattice matching conditions, leading theoretically to a more tolerant system. The concept is reminded, with recent optimizations, ideal and more realistic results, and the description of an experimental realization highlighting the feasibility of the concept and the closeness of theoretical and experimental results. [less ▲]

Detailed reference viewed: 12 (0 ULg)
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See detailSpectral splitting planar solar concentrator : Design and Experimental testing Preliminary results
Blain, Pascal ULg; Michel, Céline ULg; Clermont, Lionel ULg et al

in Proceedings of SPIE (2014, May 15), 9140

We present a new concept in solar concentrator: spectral splitting. It implies reflective, refractive and diffractive elements that allow two spectrally differentiated beams to reach different and/or ... [more ▼]

We present a new concept in solar concentrator: spectral splitting. It implies reflective, refractive and diffractive elements that allow two spectrally differentiated beams to reach different and/or unmatched lattice solar cells. Those cells efficiencies are then enhanced. The aimed geometrical concentration factor is 5× and the theoretical optical efficiency of that concentrator concept reaches 82%. [less ▲]

Detailed reference viewed: 68 (33 ULg)
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See detailStudy of a solar concentrator for space based on a diffractive/refractive optical combination
Michel, Céline ULg; Loicq, Jerôme ULg; Thibert, Tanguy ULg et al

Conference (2014, April 16)

The present work proposes a theoretical optical study of a new planar solar concentrator for space applications. The concentrator focuses light onto three spatially separated photovoltaic cells (three ... [more ▼]

The present work proposes a theoretical optical study of a new planar solar concentrator for space applications. The concentrator focuses light onto three spatially separated photovoltaic cells (three single-junctions, dedicated to two different light spectra), allowing for independent control of the output power of each cell. The design, based on a blazed diffraction grating superimposed on a Fresnel lens, combines advantages of both spectral splitting and light focusing. Theoretical model and optimization of the optical design are presented, including the variation of the grating period along the lens. Numerical results are detailed, highlighting the possibility to design a concentrator at more than 10× for each cell, with an optical efficiency of ~71% and less than 10% of losses for tracking errors < ±0.85°. A thermal study completes the work, as well as a first theoretical comparison with a planar Fresnel based solar concentrator focusing on a GaAs cell. [less ▲]

Detailed reference viewed: 29 (15 ULg)
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See detailOptical study of a solar concentrator for space applications based on a diffractive/refractive optical combination
Michel, Céline ULg; Loicq, Jerôme ULg; Languy, Fabian ULg et al

in Solar Energy Materials & Solar Cells (2014), 120PA

This paper presents a new design of a planar solar concentrator for space applications. The concentrator focuses light onto two spatially separated photovoltaic cells, allowing for independent control of ... [more ▼]

This paper presents a new design of a planar solar concentrator for space applications. The concentrator focuses light onto two spatially separated photovoltaic cells, allowing for independent control of the output power of each cell. Thanks to a blazed diffraction grating superimposed on a Fresnel lens, advantages of both spectral splitting and light focusing can be combined. The theoretical model of the optical design is presented, with the optimization of either element. Moreover, configuration improvement is proposed: a symmetrical configuration composed of two lenses. First numerical results are presented, highlighting the possibility to design a concentrator at about 10× for each cell, with an optical efficiency of about 75% and less than 10% of losses for tracking errors lower than ±0.9°. [less ▲]

Detailed reference viewed: 74 (37 ULg)
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See detailStudy of a solar concentrator for space based on a diffractive/refractive optical combination
Michel, Céline ULg; Loicq, Jerôme ULg; Mazzoli, Alexandra ULg et al

Poster (2013, April 16)

This paper presents a new design of a planar solar concentrator for space applications focusing on two spatially separated PV cells, allowing independent control of output power of each cell. It has the ... [more ▼]

This paper presents a new design of a planar solar concentrator for space applications focusing on two spatially separated PV cells, allowing independent control of output power of each cell. It has the advantages of both spectral splitting and solar concentration by the combination of a blaze transmission diffraction grating and a flat cylindrical Fresnel lens. An optical optimization has been realized and two variations of configuration have been developed to improve tracking tolerance: first, a design completed by secondary reflective concentrators and second, a symmetrical configuration composed of two lenses. First numerical results are presented, highlighting the possibility to design a concentrator at about 10×, with an electrical output power about 290W/m² lens and less than 10% losses for tracking errors lower than ±0.9°. [less ▲]

Detailed reference viewed: 34 (13 ULg)
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See detailStudy of a solar concentrator for space based on a diffractive/refractive optical combination
Michel, Céline ULg; Loicq, Jerôme ULg; Languy, Fabian ULg et al

in AIP Conference Proceedings (2013), (1556), 97-100

This paper presents a new design of a planar solar concentrator for space applications focusing on two spatially separated PV cells, allowing independent control of output power of each cell. It has the ... [more ▼]

This paper presents a new design of a planar solar concentrator for space applications focusing on two spatially separated PV cells, allowing independent control of output power of each cell. It has the advantages of both spectral splitting and solar concentration by the combination of a blaze transmission diffraction grating and a flat cylindrical Fresnel lens. An optical optimization has been realized and two variations of configuration have been developed to improve tracking tolerance: first, a design completed by secondary reflective concentrators and second, a symmetrical configuration composed of two lenses. First numerical results are presented, highlighting the possibility to design a concentrator at about 10×, with an electrical output power about 290W/m² lens and less than 10% losses for tracking errors lower than ±0.9°. [less ▲]

Detailed reference viewed: 40 (11 ULg)
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See detailStudy of a solar concentrator for space based on a diffractive/refractive optical combination
Michel, Céline ULg; Loicq, Jerôme ULg; Languy, Fabian ULg et al

Conference (2012, November 12)

This paper presents a new design of a planar solar concentrator at 10× with spectral splitting focusing on two separated PV cells, allowing independent control. Optical elements, blazed diffraction ... [more ▼]

This paper presents a new design of a planar solar concentrator at 10× with spectral splitting focusing on two separated PV cells, allowing independent control. Optical elements, blazed diffraction grating and Fresnel lens, are optimized. [less ▲]

Detailed reference viewed: 34 (19 ULg)