1. Introduction to CPV space heritagePhotovoltaic concentrators proved their feasibility for space missions back in 1998 with the launch of the Deep Space 1 spacecraft, which equipped the SCARLET solar array: a 2.5 kW array comprised of 7.5X refractive linear Fresnel concentrators on 24%-efficient dual-junction GaInP2/GaAs/Ge solar cells able to deliver a specific power of 45 W/kg [1]. Fresnel lenses where molded using space-grade silicone on curved borosilicate glass. During the 3-year mission, it successfully powered instruments and the first-ever ion thruster as primary propulsion, while ending with a lower than expected end-of-life degradation. Since then, several missions have tried to extend this success with improved concentrator architectures, but they have consistently failed. Summarizing quickly, refractive concepts that evolved from the SCARLET array removed the cover glass to enable flexible optics that can be very compact at stow, which eventually led to failure of the silicone lens due to continuous tensile stress and lower radiation protection (Stretched Lens Array [2]). On the other hand, flight experiments using V-trough reflectors only allowed very low concentration ratios and suffered premature power loss due to mirror contamination by out-gassed material (BSS 702 [3]).2. Potential benefits of CPV for deep space missionsConcentration has long been regarded as an interesting solution to reduce the cost of solar arrays because of the reduction of costly semiconductor and cell efficiency increase. However, the long focal distance and bulky concentrators optics required by classic multi-junction (MJ) cell sizes above 1 cm2 posed a challenge to avoid excessive operation temperatures and to achieve high module compactness at stow and low weight. Then again, the renewed interest of space agencies in deep space missions (e.g., Saturn moons) sets up a scenario where concentrators may become mission enablers: a low light intensity (1% AM0 in Saturn), low temperature environment (LILT) can reduce MJ cell performance significantly due to even very small shunts difficult to screen out, but concentrators increase light intensity and temperature in the cell and thus recover efficiency. Furthermore, designs with very small solar cells <1 mm2 (micro-CPV) allow for very compact designs (<1 to a few mm) with broad angular tolerance (>5°) for >10X concentration, which are able to achieve very high specific power exceeding 350 W/kg (vs. typical 200 W/kg of standard cell-interconnect-cover glass) [4].3. State of the art of micro-CPV for spaceDifferent research groups in USA, Europe or Japan have explored refractive and reflective micro-CPV concepts for space in recent years. Notably, the US Naval Research Laboratory-Semprius tandem and Penn State have led some of the most mature concepts: a 30%-eff. 14X refractive concentrator with >5° acceptance angle and 4-mm thickness [5], and a 26%-eff. 18X Ag-coated molded-glass array of parabolic reflectors with >9° acceptance and 1.7-mm thickness [6], respectively. In Europe, CEA-INES has developed a reflective array directly molded in the cavities of a honeycomb structure, thus leading to inherent rigidity and potentially exceeding 150 W/kg specific power [7]. The Japan Aerospace Exploration Agency (JAXA) announced an on-orbit demonstration test for 2023 of two different micro-CPV modules with 2.3X and 3X concentration ratios using 7-mm-wide silicone lenses on 3J cells [8], which, if successful, can lead to further development and interest from other agencies. In this work, we present an overview of this state of the art.4. A new European activity aimed at developing a proof-of-concept CPV module for deep spaceThe European Space Agency (ESA) recently issued a public tender for the development of concentrators for deep space missions, which is part of ESA’s technology development activities applicable to several science program missions, including those in ESA’s Voyage 2050 program. Its objective is to advance space concentrator PV technology to the demonstration of a proof-of-concept CPV module optimized for missions to Jupiter, Saturn and beyond. Thales Alenia Space has gathered a consortium composed of UPM and Fraunhofer ISE to design a ⁓10X refractive concentrator with moderate angular tolerance (>5°) and low thickness, compatible with existing solar array wing architectures, and able to avoid solar cell performance drop under LILT conditions. In this work, we present this new micro-CPV endeavor as well.
1. Introduction to CPV space heritagePhotovoltaic concentrators proved their feasibility for space missions back in 1998 with the launch of the Deep Space 1 spacecraft, which equipped the SCARLET solar array: a 2.5 kW array comprised of 7.5X refractive linear Fresnel concentrators on 24%-efficient dual-junction GaInP2/GaAs/Ge solar cells able to deliver a specific power of 45 W/kg [1]. Fresnel lenses where molded using space-grade silicone on curved borosilicate glass. During the 3-year mission, it successfully powered instruments and the first-ever ion thruster as primary propulsion, while ending with a lower than expected end-of-life degradation. Since then, several missions have tried to extend this success with improved concentrator architectures, but they have consistently failed. Summarizing quickly, refractive concepts that evolved from the SCARLET array removed the cover glass to enable flexible optics that can be very compact at stow, which eventually led to failure of the silicone lens due to continuous tensile stress and lower radiation protection (Stretched Lens Array [2]). On the other hand, flight experiments using V-trough reflectors only allowed very low concentration ratios and suffered premature power loss due to mirror contamination by out-gassed material (BSS 702 [3]).2. Potential benefits of CPV for deep space missionsConcentration has long been regarded as an interesting solution to reduce the cost of solar arrays because of the reduction of costly semiconductor and cell efficiency increase. However, the long focal distance and bulky concentrators optics required by classic multi-junction (MJ) cell sizes above 1 cm2 posed a challenge to avoid excessive operation temperatures and to achieve high module compactness at stow and low weight. Then again, the renewed interest of space agencies in deep space missions (e.g., Saturn moons) sets up a scenario where concentrators may become mission enablers: a low light intensity (1% AM0 in Saturn), low temperature environment (LILT) can reduce MJ cell performance significantly due to even very small shunts difficult to screen out, but concentrators increase light intensity and temperature in the cell and thus recover efficiency. Furthermore, designs with very small solar cells <1 mm2 (micro-CPV) allow for very compact designs (<1 to a few mm) with broad angular tolerance (>5°) for >10X concentration, which are able to achieve very high specific power exceeding 350 W/kg (vs. typical 200 W/kg of standard cell-interconnect-cover glass) [4].3. State of the art of micro-CPV for spaceDifferent research groups in USA, Europe or Japan have explored refractive and reflective micro-CPV concepts for space in recent years. Notably, the US Naval Research Laboratory-Semprius tandem and Penn State have led some of the most mature concepts: a 30%-eff. 14X refractive concentrator with >5° acceptance angle and 4-mm thickness [5], and a 26%-eff. 18X Ag-coated molded-glass array of parabolic reflectors with >9° acceptance and 1.7-mm thickness [6], respectively. In Europe, CEA-INES has developed a reflective array directly molded in the cavities of a honeycomb structure, thus leading to inherent rigidity and potentially exceeding 150 W/kg specific power [7]. The Japan Aerospace Exploration Agency (JAXA) announced an on-orbit demonstration test for 2023 of two different micro-CPV modules with 2.3X and 3X concentration ratios using 7-mm-wide silicone lenses on 3J cells [8], which, if successful, can lead to further development and interest from other agencies. In this work, we present an overview of this state of the art.4. A new European activity aimed at developing a proof-of-concept CPV module for deep spaceThe European Space Agency (ESA) recently issued a public tender for the development of concentrators for deep space missions, which is part of ESA’s technology development activities applicable to several science program missions, including those in ESA’s Voyage 2050 program. Its objective is to advance space concentrator PV technology to the demonstration of a proof-of-concept CPV module optimized for missions to Jupiter, Saturn and beyond. Thales Alenia Space has gathered a consortium composed of UPM and Fraunhofer ISE to design a ⁓10X refractive concentrator with moderate angular tolerance (>5°) and low thickness, compatible with existing solar array wing architectures, and able to avoid solar cell performance drop under LILT conditions. In this work, we present this new micro-CPV endeavor as well. Read More
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