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• W3080187424 abstract "•Radiative cooling for CPV is a cheap, lightweight add-on requiring no power•A 36°C temperature drop is achieved, leading to a 31% increase of VOC•A 4 to 15 times extension of lifetimes for various CPV cells is predicted•Different cooling structures are investigated at a wide range of conditions Concentrating photovoltaics (CPV) aim to focus sunlight on solar cells to improve efficiency and reduce material costs. However, concentration also increases heating of the solar cells, potentially offsetting efficiency improvements and reducing system lifetimes. Active cooling, such as forced air and liquid cooling, is usually required, but increases the cost while reducing net power production. Radiative cooling, on the other hand, uses thermal radiation to dissipate heat, which is cheap, lightweight, and requires no extra power. This is especially beneficial for enclosed CPV systems using solar trackers. Our experiment shows that by coupling radiative coolers on a flat heat sink, the solar cell operating temperature in a passively cooled CPV can be reduced by 36°C under a heat load of 6.1 W. As a result, a 27% relative increase of open-circuit voltage is observed for the GaSb cell. A lifetime extension of 4 to 15 times for typical CPV cells is also projected. Radiative cooling can reject significantly more waste heat than convection and conduction at high temperatures by sending it directly into space. As a passive and compact cooling mechanism, radiative cooling is lightweight and does not consume energy. These qualities are promising for thermal management in outdoor systems generating low grade heat, such as concentrating photovoltaics (CPV) and thermophotovoltaics (TPV). In this work, we first simulate radiative cooling for a wide range of working conditions, including heat loads from 6 to 100 W with different CPV cooling designs. We then demonstrate a CPV system integrated with radiative coolers, achieving a 5°C to 36°C temperature drop and an 8% to 27% relative increase of open-circuit voltage for a GaSb solar cell, under a heat load of above 6 W with different cooling designs. We show that the temperature drops from radiative cooling may significantly improve CPV system lifetimes. Radiative cooling can reject significantly more waste heat than convection and conduction at high temperatures by sending it directly into space. As a passive and compact cooling mechanism, radiative cooling is lightweight and does not consume energy. These qualities are promising for thermal management in outdoor systems generating low grade heat, such as concentrating photovoltaics (CPV) and thermophotovoltaics (TPV). In this work, we first simulate radiative cooling for a wide range of working conditions, including heat loads from 6 to 100 W with different CPV cooling designs. We then demonstrate a CPV system integrated with radiative coolers, achieving a 5°C to 36°C temperature drop and an 8% to 27% relative increase of open-circuit voltage for a GaSb solar cell, under a heat load of above 6 W with different cooling designs. We show that the temperature drops from radiative cooling may significantly improve CPV system lifetimes. Thermal management is extremely important for renewable energy systems, such as photovoltaics (PV), thermophotovoltaics (TPV), and concentrating photovoltaics (CPV). Elevated operating temperatures not only reduce the efficiency of PV modules,1Dubey S. Sarvaiya J.N. Seshadri B. Temperature dependent photovoltaic (PV) efficiency and its effect on PV production in the world – a review.Energy Procedia. 2013; 33: 311-321Crossref Scopus (527) Google Scholar but also substantially reduce their lifetimes.2Sun X. Silverman T.J. Zhou Z. Khan M.R. Bermel P. Alam M.A. Optics-based approach to thermal management of photovoltaics: selective-spectral and radiative cooling.IEEE J. Photovoltaics. 2017; 7: 566-574Crossref Scopus (62) Google Scholar, 3Han Y. Meyer S. Dkhissi Y. Weber K. Pringle J.M. Bach U. Spiccia L. Cheng Y.-B. Degradation observations of encapsulated planar CH3NH3PbI3 perovskite solar cells at high temperatures and humidity.J. Mater. Chem. A. 2015; 3: 8139-8147Crossref Google Scholar, 4Espinet-González P. Algora C. Núñez N. Orlando V. Vázquez M. Bautista J. Araki K. Temperature accelerated life test on commercial concentrator III-V triple-junction solar cells and reliability analysis as a function of the operating temperature.Prog. Photovolt. Res. Appl. 2015; 23: 559-569Crossref Scopus (38) Google Scholar This is an even more critical issue for higher heat load systems, such as TPV and CPV, where low-band-gap solar cells are commonly used, making the system more sensitive to temperature increases. The encapsulated housing of CPV and TPV systems further suppresses convective cooling, leading to dramatic temperature rises. Heat transfer methods potentially relevant to CPV and TPV systems are conduction, convection, and radiation.5Howell J.R. Siegel R. Thermal Radiation Heat Transfer. Taylor and Francis, 2002Google Scholar Conventional PV cooling approaches usually only utilize convective or conductive heat transfer, such as heat sinks, convective or forced air cooling, liquid cooling, etc. Some of these strategies require extra energy input and specially designed cooling systems, which can increase the cost and reduce the overall reliability. Radiative cooling, on the other hand, had been overlooked until recently. Although it is limited for most of the indoor and low-temperature applications, as the temperature difference between the object and ambient is not large enough to fully exploit its potential, radiative cooling becomes powerful for outdoor applications, such as thermal management for buildings and PV systems. This difference is a result of direct access to atmosphere transparency window from 8 to 13 μm. Photons with wavelengths in this range can go through the atmosphere and exchange heat directly with outer space at a temperature around 3 K.6Fixsen D.J. The temperature of the cosmic microwave background.Astrophys. J. 2009; 707: 916-920Crossref Scopus (518) Google Scholar This large temperature difference enables outdoor radiative coolers to reject a great deal of waste heat. Radiative cooling can be classified as either below-ambient cooling or above-ambient cooling, as shown in Figures 1A and 1B . Below-ambient cooling aims for low steady-state temperatures, ideally to use a cooler with unity emissivity in the transparency window and zero elsewhere. However, the cooling power is limited due to the narrow radiation spectrum range. Above-ambient cooling, on the other hand, aims to provide a maximum cooling power, which ideally requires the cooler to absorb no power within the solar spectrum and emit as a blackbody at longer wavelengths. A wide range of materials and structures have been demonstrated to provide radiative cooling.7Sun X. Sun Y. Zhou Z. Alam M.A. Bermel P. Radiative sky cooling: fundamental physics, materials, structures, and applications.Nanophotonics. 2017; 6: 997-1015Crossref Scopus (87) Google Scholar, 8Hossain M.M. Gu M. Radiative cooling: principles, progress, and potentials.Adv. Sci. 2016; 3: 1500360Crossref Scopus (245) Google Scholar, 9Zeyghami M. Goswami D.Y. Stefanakos E. A review of clear sky radiative cooling developments and applications in renewable power systems and passive building cooling.Sol. Energy Mater. Sol. Cells. 2018; 178: 115-128Crossref Scopus (149) Google Scholar In early work, nighttime below-ambient cooling was studied, as it does not require suppression of emission within the solar wavelengths. At this stage, bulk and composite materials with strong emissivity in transparency window were investigated intensively.10Catalanotti S. Cuomo V. Piro G. Ruggi D. Silvestrini V. Troise G. The radiative cooling of selective surfaces.Sol. Energy. 1975; 17: 83-89Crossref Scopus (319) Google Scholar, 11Bartoli B. Catalanotti S. Coluzzi B. Cuomo V. Silvestrini V. Troise G. Nocturnal and diurnal performances of selective radiators.Appl. Energy. 1977; 3: 267-286Crossref Scopus (114) Google Scholar, 12Granqvist C.G. Hjortsberg A. Radiative cooling to low temperatures: general considerations and application to selectively emitting SiO films.J. Appl. 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Energy. 1985; 34: 19-33Crossref Scopus (43) Google Scholar, 18Orel B. Gunde M.K. Krainer A. Radiative cooling efficiency of white pigmented paints.Sol. Energy. 1993; 50: 477-482Crossref Scopus (112) Google Scholar, 19Nilsson T.M.J. Niklasson G.A. Granqvist C.G. A solar reflecting material for radiative cooling applications: ZnS pigmented polyethylene.Sol. Energy Mater. Sol. Cells. 1992; 28: 175-193Crossref Scopus (105) Google Scholar, 20Nilsson T.M.J. Niklasson G.A. Radiative cooling during the day: simulations and experiments on pigmented polyethylene cover foils.Sol. Energy Mater. Sol. Cells. 1995; 37: 93-118Crossref Scopus (148) Google Scholar Daytime below-ambient cooling was not achieved until very recently, due to the challenges of simultaneously producing both strong infrared (IR) emittance and low solar absorption. The emergence of nanophotonic and metamaterial coolers has now made it possible to tailor the emittance spectrum more precisely than has been achieved with traditional bulk materials. Much stronger and flatter emittance plateaus in the atmospheric transmission window have now been achieved while suppressing solar absorption, enabling net cooling even under direct sunlight.21Yeng Y.X. Ghebrebrhan M. Bermel P. Chan W.R. Joannopoulos J.D. Soljacic M. Celanovic I. Enabling high-temperature nanophotonics for energy applications.Proc. Natl. Acad. Sci. USA. 2012; 109: 2280-2285Crossref PubMed Scopus (182) Google Scholar, 22Rephaeli E. Raman A. Fan S. Ultrabroadband photonic structures to achieve high-performance daytime radiative cooling.Nano Lett. 2013; 13: 1457-1461Crossref PubMed Scopus (480) Google Scholar, 23Raman A.P. Anoma M.A. Zhu L. Rephaeli E. Fan S. Passive radiative cooling below ambient air temperature under direct sunlight.Nature. 2014; 515: 540-544Crossref PubMed Scopus (1062) Google Scholar, 24Zhu L. Raman A. Wang K.X. Anoma M.A. Fan S. Radiative cooling of solar cells.Optica. 2014; 1: 32Crossref Scopus (272) Google Scholar, 25Chen Z. Zhu L. Raman A. Fan S. Radiative cooling to deep sub-freezing temperatures through a 24-h day–night cycle.Nat. Commun. 2016; 7: 13729Crossref PubMed Scopus (350) Google Scholar, 26Zhu L. Raman A.P. Fan S. Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody.Proc. Natl. Acad. Sci. USA. 2015; 112: 12282-12287https://doi.org/10.1073/pnas.1509453112Crossref PubMed Scopus (290) Google Scholar, 27Zou C. Ren G. Hossain M.M. Nirantar S. Withayachumnankul W. Ahmed T. Bhaskaran M. Sriram S. Gu M. Fumeaux C. Metal-loaded dielectric resonator metasurfaces for radiative cooling.Adv. Opt. Mater. 2017; 5: 1700460Crossref Scopus (79) Google Scholar, 28Zhai Y. 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Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody.Proc. Natl. Acad. Sci. USA. 2015; 112: 12282-12287https://doi.org/10.1073/pnas.1509453112Crossref PubMed Scopus (290) Google Scholar,29Kou J.-L. Jurado Z. Chen Z. Fan S. Minnich A.J. Daytime radiative cooling using Near-black infrared emitters.ACS Photonics. 2017; 4: 626-630Crossref Scopus (297) Google Scholar,31Li W. Shi Y. Chen K. Zhu L. Fan S. A comprehensive photonic approach for solar cell cooling.ACS Photonics. 2017; 4: 774-782Crossref Scopus (149) Google Scholar, 32Zhou Z. Sun X. Bermel P. Radiative cooling for thermophotovoltaic systems. Proc. SPIE 9973, Infrared Remote Sensing and Instrumentation XXIV.https://doi.org/10.1117/12.2236174Date: 2016Google Scholar, 33Zhou Z. Wang Z. Bermel P. Radiative cooling for low-bandgap photovoltaics under concentrated sunlight.Opt. Express. 2019; 27: A404-A418Crossref PubMed Scopus (19) Google Scholar which can provide a great deal of cooling for objects at high temperatures. Moreover, a recent study showed that given a proper design, broadband coolers can also be used for below-ambient cooling, since the thermal heat exchange with the sky outside of the transparency window can provide additional cooling power at near-ambient temperatures.30Xue X. Qiu M. Li Y. Zhang Q.M. Li S. Yang Z. Feng C. Zhang W. Dai J.G. Lei D. et al.Creating an eco-friendly building coating with smart subambient radiative cooling.Adv. Mater. 2020; : e1906751Crossref PubMed Scopus (32) Google Scholar Different types of radiative cooling can be used, depending on the working temperature of the system. For example, below-ambient cooling is widely used for thermal management of buildings,9Zeyghami M. Goswami D.Y. Stefanakos E. A review of clear sky radiative cooling developments and applications in renewable power systems and passive building cooling.Sol. Energy Mater. Sol. Cells. 2018; 178: 115-128Crossref Scopus (149) Google Scholar,34Goldstein E.A. Raman A.P. Fan S. Sub-ambient non-evaporative fluid cooling with the sky.Nat. Energy. 2017; 2: 9Crossref Scopus (179) Google Scholar, 35Al-Obaidi K.M. Ismail M. Abdul Rahman A.M.A. Passive cooling techniques through reflective and radiative roofs in tropical houses in Southeast Asia: a literature review.Front. Archit. Res. 2014; 3: 283-297Crossref Scopus (110) Google Scholar, 36Gentle A.R. Smith G.B. A subambient open roof surface under the mid-summer sun.Adv. Sci. 2015; 2: 1500119Crossref Scopus (197) Google Scholar, 37Sadineni S.B. Madala S. Boehm R.F. Passive building energy savings: a review of building envelope components.Renew. Sustain. Energy Rev. 2011; 15: 3617-3631Crossref Scopus (664) Google Scholar while above-ambient radiative cooling is more suitable for dissipating low grade heat from PV systems,24Zhu L. Raman A. Wang K.X. Anoma M.A. Fan S. Radiative cooling of solar cells.Optica. 2014; 1: 32Crossref Scopus (272) Google Scholar,26Zhu L. Raman A.P. Fan S. Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody.Proc. Natl. Acad. Sci. USA. 2015; 112: 12282-12287https://doi.org/10.1073/pnas.1509453112Crossref PubMed Scopus (290) Google Scholar,31Li W. Shi Y. Chen K. Zhu L. Fan S. A comprehensive photonic approach for solar cell cooling.ACS Photonics. 2017; 4: 774-782Crossref Scopus (149) Google Scholar, 32Zhou Z. Sun X. Bermel P. Radiative cooling for thermophotovoltaic systems. Proc. SPIE 9973, Infrared Remote Sensing and Instrumentation XXIV.https://doi.org/10.1117/12.2236174Date: 2016Google Scholar, 33Zhou Z. Wang Z. Bermel P. Radiative cooling for low-bandgap photovoltaics under concentrated sunlight.Opt. Express. 2019; 27: A404-A418Crossref PubMed Scopus (19) Google Scholar as the elevated working temperature and high sky transmittance create ideal conditions for maximizing the cooling power. Unlike forced air or liquid cooling for PV systems, which consume 2% to 5% of the total output power,38Wang S. Shi J. Chen H.-H. Schafer S.R. Munir M. Stecker G. Pan W. Lee J.-J. Chen C.-L. Cooling design and evaluation for photovoltaic cells within constrained space in a CPV/CSP hybrid solar system.Appl. Therm. Eng. 2017; 110: 369-381Crossref Scopus (28) Google Scholar,39Xiao M. Tang L. Zhang X. Lun I. Yuan Y. A review on recent development of cooling technologies for concentrated photovoltaics (CPV) systems.Energies. 2018; 11: 3416Crossref Scopus (18) Google Scholar radiative cooling is passive with no extra energy consumption.40Du D. Darkwa J. Kokogiannakis G. Thermal management systems for Photovoltaics (PV) installations: a critical review.Sol. Energy. 2013; 97: 238-254Crossref Scopus (151) Google Scholar It is also compact, lightweight, and reliable, without any bulky heat sinks or moving parts, as in air or liquid cooling. This aspect can benefit PV or CPV modules integrated with tracking systems. More importantly, the radiative power is significantly larger and grows quickly at high temperatures. Its rate of heat dissipation is proportional to the fourth power of the temperature difference of the two objects,5Howell J.R. Siegel R. Thermal Radiation Heat Transfer. Taylor and Francis, 2002Google Scholar which scales much faster than conduction and convection. The efficient, compact, and passive nature of radiative cooling makes it an outstanding cooling mechanism for PV systems. Recent research has shown the effects of radiative cooling in PV, TPV, and CPV systems.24Zhu L. Raman A. Wang K.X. Anoma M.A. Fan S. Radiative cooling of solar cells.Optica. 2014; 1: 32Crossref Scopus (272) Google Scholar,26Zhu L. Raman A.P. Fan S. Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody.Proc. Natl. Acad. Sci. USA. 2015; 112: 12282-12287https://doi.org/10.1073/pnas.1509453112Crossref PubMed Scopus (290) Google Scholar,31Li W. Shi Y. Chen K. Zhu L. Fan S. A comprehensive photonic approach for solar cell cooling.ACS Photonics. 2017; 4: 774-782Crossref Scopus (149) Google Scholar, 32Zhou Z. Sun X. Bermel P. Radiative cooling for thermophotovoltaic systems. Proc. SPIE 9973, Infrared Remote Sensing and Instrumentation XXIV.https://doi.org/10.1117/12.2236174Date: 2016Google Scholar, 33Zhou Z. Wang Z. Bermel P. Radiative cooling for low-bandgap photovoltaics under concentrated sunlight.Opt. Express. 2019; 27: A404-A418Crossref PubMed Scopus (19) Google Scholar Last year, a GaSb-based CPV system with soda-lime radiative cooler was experimentally demonstrated. A 10°C drop of the solar cell was achieved under 13 suns, leading to a relative increase of 5.7% in open-circuit voltage and an estimated 40% increase in lifetime.33Zhou Z. Wang Z. Bermel P. Radiative cooling for low-bandgap photovoltaics under concentrated sunlight.Opt. Express. 2019; 27: A404-A418Crossref PubMed Scopus (19) Google Scholar In this work, we studied the radiative cooling performance of CPV in three different cooling structures, under a range of wind speeds and solar heat loads. We also conducted multiple outdoor experiments covering the worst and best possible working scenarios for radiative cooling to check the overall performance. The experiments showed that radiative cooling, depending on the working conditions, can contribute roughly 25% to 62% of the overall cooling power of a CPV system equipped with flat-plate heat sink, while adding little weight and no extra power consumption. A high-concentration PV system integrated with radiative cooling was designed, refined, and fabricated based on our previous study.33Zhou Z. Wang Z. Bermel P. Radiative cooling for low-bandgap photovoltaics under concentrated sunlight.Opt. Express. 2019; 27: A404-A418Crossref PubMed Scopus (19) Google Scholar The average heat load on the solar cell in our experiment was ∼5 to 6 W. By applying two soda-lime radiative coolers on both sides of the heat sink, the temperature drop of GaSb cell at steady state for worst and best cases were 5°C and 36°C, respectively. To our knowledge, the maximum temperature drop even outperformed some active air cooling methods.39Xiao M. Tang L. Zhang X. Lun I. Yuan Y. A review on recent development of cooling technologies for concentrated photovoltaics (CPV) systems.Energies. 2018; 11: 3416Crossref Scopus (18) Google Scholar,40Du D. Darkwa J. Kokogiannakis G. Thermal management systems for Photovoltaics (PV) installations: a critical review.Sol. Energy. 2013; 97: 238-254Crossref Scopus (151) Google Scholar The temperature decrease also resulted in a 8% to 27% (28 to 75 mV) relative (absolute) increase in the open-circuit voltage of our GaSb PV cell, as well as a projected lifetime extension for various types of solar cells, which potentially can be used in CPV systems.41Ndiaye A. Charki A. Kobi A. Kébé C.M.F. Ndiaye P.A. Sambou V. Degradations of silicon photovoltaic modules: a literature review.Sol. Energy. 2013; 96: 140-151Crossref Scopus (266) Google Scholar, 42Luo W. Khoo Y.S. Hacke P. Naumann V. Lausch D. Harvey S.P. Singh J.P. Chai J. Wang Y. Aberle A.G. Ramakrishna S. Potential-induced degradation in photovoltaic modules: a critical review.Energy Environ. Sci. 2017; 10: 43-68Crossref Google Scholar, 43Park N.C. Oh W.W. Kim D.H. Effect of temperature and humidity on the degradation rate of multicrystalline silicon photovoltaic module.Int. J. Photoenergy. 2013; 2013: 1-9Crossref Scopus (46) Google Scholar, 44Hoffmann S. Köhl M. Effect of humidity and temperature on the potential-induced degradation.Prog. Photovolt. Res. Appl. 2012; 22: 173-179Crossref Scopus (101) Google Scholar, 45Hacke P. Spataru S. Terwilliger K. Perrin G. Glick S. Kurtz S. Wohlgemuth J. Accelerated testing and modeling of potential-induced degradation as a function of temperature and relative humidity.IEEE J. Photovoltaics. 2015; 5: 1549-1553Crossref Scopus (60) Google Scholar Using detailed simulations, a peak radiative power flux of 157 to 310 W/m2 was estimated to be present, thereby increasing the cooling performance per unit weight by 25% to 81%. This improvement is particularly beneficial to PV systems with solar trackers. To better illustrate the concept, we define the specific cooling power Sp to be:Sp=Pr+PcmTcell−Ta,(Equation 1) where Pr and Pc are the radiative and non-radiative cooling power, respectively; m is the total weight of the entire cooling assembly; Tcell is the solar cell temperature; and Ta is the ambient temperature. For cooling systems working at the same temperature, a higher Sp indicates a greater cooling power per unit weight. By integrating radiative cooling into the CPV, the Sp increase can be calculated as a ratio factor f, which is given by:f=Sp,rSp,c≈(Pr+Pc)Pc=σε(Tcell4−Ta4)heff(Tcell−Ta)+1,(Equation 2) where Sp,r and Sp,c are the specific cooling power of our assembly with and without radiative cooling, respectively; heff is the effective coefficient for non-radiative heat transfer; σ is the Stefan–Boltzmann constant; and ε is emissivity of the cooler. The approximation can be made as long as the coolers are much lighter than the remainder of the cooling assembly, and the operating temperatures remain the same. It is straightforward to show that f∼Tcell3 when Tcell is large, which implies that radiative cooling is more resilient to high temperature systems than other cooling methods. Figure 1C gives a better interpretation of the specific cooling power improvement. As temperature goes up, the radiative cooling power grows to quickly dominate the total cooling power, providing a substantially larger Sp. The total cooling power must match the heating power reaching the PV system under thermal steady state. Thus, with radiative cooling, PV can work under higher solar concentrations at the same temperature, to potentially improve efficiencies and power outputs. In the remainder of this paper, we explore how a CPV demonstration setup has been built to achieve the desired metrics of radiative cooling enhancement in a lightweight form factor, and how this design benefits both the operating open circuit voltage as well as long-term reliability. After presenting our experimental methodology, we show our key experimental results, as well as simulations to validate our understanding and interpretation. We then extend this framework to consider coolers that can perform even better for commercial CPV systems at higher heat loads from 6 to 100 W, corresponding to ∼100% to 1600% of the input solar heat demonstrated in our setup. Different cooling designs are also simulated with quantitative data to provide a comprehensive understanding of radiative cooling performance under a wide range of convective heat transfer conditions. Finally, we conclude by summarizing the key results in this work and discuss potential directions for future research. The radiative cooling measurement platform consists of three chambers, as illustrated in Figure 2D . Each chamber is designed for different functions. Chamber 1, as shown in Figure 2A, contains a solar cell and two soda-lime glass radiative coolers, while Chamber 2 has a similar structure without any coolers. Instead, aluminum (Al) reflectors are used in Chamber 2 as a control to minimize the solar heating and suppress the temperature, which is common for cooling outdoor devices under direct sunlight. The top low-density polyethylene (LDPE) films in both chambers can be attached or removed to represent different working conditions. The sealed-chamber structure (with top LDPE) can serve as a reference for two different scenarios. First, it roughly represents the zero wind speed working condition of CPV (natural convection), since the LDPE film can cut off direct convection from the heat sink to ambient air. Second, it can be compared with active air-cooled CPV at zero air-injection rate, since many actively cooled CPV systems require an enclosed fluid channel.39Xiao M. Tang L. Zhang X. Lun I. Yuan Y. A review on recent development of cooling technologies for concentrated photovoltaics (CPV) systems.Energies. 2018; 11: 3416Crossref Scopus (18) Google Scholar In either case, the sealed-chamber structure gives the highest possible temperature drop from radiative cooling. On the other hand, the open-chamber structure (without top LDPE) is best compared to passively air-cooled CPV, which is widely used in commercial CPV. Both structures are tested outdoors and show considerable temperature drops. Electrode probes and type-K thermocouples are mounted to the solar cells in Chambers 1 and 2 to measure their open-circuit voltages (VOC) and temperatures (Texpt), as shown in Figure 2B. Chamber 3 only has a thermal power sensor to monitor the incident solar power. All three chambers are equipped with a Fresnel lens with effective diameter of 6 inches. Considering the zenith angle of sunlight at our field test location, all chambers are tilted at 20°C and fixed on a wood board to maintain the same orientation, which orients the top cooler horizontally during experiments to maximize its view factor to the sky. The wood board is held by a tripod, the tilt and azimuth angles can be adjusted to track the sun. Three first-surface Al mirrors are placed on the board under each chamber separately to reflect sunlight normally to the polymethyl methracylate (PMMA) Fresnel lens, as shown in Figures 2C and 2D. A PMMA rod is fixed in front and tilted 20°C to serve as a solar tracker. Further details are provided in Experimental Producessures section. A daytime outdoor cooling experiment was conducted on September 14, 2019, as shown in Figure 3. LDPE covered both chambers during the experiment. The site had open access to the sky to guarantee the expected cooling performance, as shown in Figure 3A. The role that each of the three chambers played in the experiment is described in the Results section, above. Before the test, all three chambers were aligned to the same level and warmed up by exposing to direct sunlight for 30 min, to reach a steady-state temperature close to ambient. During the test, the tilt and azimuth angles of the setup were manually adjusted approximately every 5 min to focus the beam spot on the center of the solar cells, as shown in Figures 3B and 3C. The temperature Texpt and open circuit voltage VOC of solar cells for both Chamber 1 and Chamber 2 were measured at resolutions of 0.096°C and 0.12 mV, respectively, with a 2 Hz sampling rate. The thermal power meter in Chamber 3 monitored the input solar irradiance at a rate of 1 Hz. A laboratory chair and a tripod were used to stabilize the setup against vibrations caused by the wind. The experiment lasts for at least 1.5 h to ensure that both chambers reach an instantaneous thermal steady state. The measured real-time solar cell temperatures Texpt, as well as a simulation of this experiment, are shown in Figure 4A. The shaded areas of the simulated temperatures account for the errors caused by uncertainties (see Table S3) in the solar power meter measurements in Chamber 3. The experimental data and simulation results exhibited a very good match to the level of uncertainty, sug" @default.
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• W3080187424 date "2020-12-01" @default.
• W3080187424 modified "2023-10-16" @default.
• W3080187424 title "Lightweight, Passive Radiative Cooling to Enhance Concentrating Photovoltaics" @default.
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