- Email: [email protected]
From sky back to sky: Embedded transparent cellulose membrane to improve the thermal performance of solar module by radiative cooling
Journal Pre-proof From sky back to sky: Embedded transparent cellulose membrane to improve the thermal performance of solar module by radiative cooling Tiezheng Lv, Jiangpin Huang, Wei Liu, Rong Zhang PII:
S2214-157X(19)30491-5
DOI:
https://doi.org/10.1016/j.csite.2020.100596
Reference:
CSITE 100596
To appear in:
Case Studies in Thermal Engineering
Received Date: 11 December 2019 Revised Date:
27 January 2020
Accepted Date: 29 January 2020
Please cite this article as: T. Lv, J. Huang, W. Liu, R. Zhang, From sky back to sky: Embedded transparent cellulose membrane to improve the thermal performance of solar module by radiative cooling, Case Studies in Thermal Engineering (2020), doi: https://doi.org/10.1016/j.csite.2020.100596. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
From Sky Back to Sky: Embedded Transparent Cellulose Membrane to Improve the Thermal Performance of Solar Module by Radiative Cooling Tiezheng Lv*, Jiangpin Huang*, Wei Liu*, Rong Zhang Research Institute of Automobile Parts Technology, Hunan Institute of Technology, Hengyang, 421002, Hunan, P.R.China, National Joint Engineering Laboratory of Automobile Pump Parts Design and Manufacturing, Hunan Institute of Technology, Hengyang, 421002, Hunan, P.R.China *Correspondence Tiezheng Lv, Research Institute of Automobile Parts Technology, Hunan Institute of Technology, Hengyang, 421002, Hunan, P.R.China Jianpin Huang, Research Institute of Automobile Parts Technology, Hunan Institute of Technology, Hengyang, 421002, Hunan, P.R.China Wei Liu, Research Institute of Automobile Parts Technology, Hunan Institute of Technology, Hengyang, 421002, Hunan, P.R.China
Email: [email protected], [email protected], [email protected]
Abstract The actual high working temperature severely affects the performance of crystalline Si solar module, and cooling function is favorited. Here we made a wood-based cellulose membrane having good transparent, high emissivity in mid IR atmospheric window, it shows a radiative sky cooling ability. We investigate the membrane’s porous and hierarchical morphology, transmission is averagely higher than 90% in solar spectra for composite of cellulose membrane, EVA and glass, the emissivity of cellulose membrane in 8-10µm is larger than 0.9. Transparent cellulose membranes were prepared and embedded between solar cell and EVA for module
encapsulation process. In the outdoor exposure experiment, we found that the cellulose membrane inside front of solar module can reduce the temperature of it, and around 2℃ cooling effect was obtained. Embedded cellulose membrane is simple, effective way to improve the thermal performance of solar module without weakening of solar spectra transmission and the performance. Keywords: radiative cooling; transparent cellulose membrane; solar module; thermal emissivity
1. Introduction Crystalline Si solar module is a device which converts photon into electricity with certain efficiency, and works under direct solar radiation. Nowadays photovoltaic (PV) is one of the best renewable energy worldwide to replace fossil fuels as the progresses of module in conversion efficiency and production cost. In the practical application, because of limited conversion efficiency, only part of solar radiation is converted into electricity, and the rest is absorbed by solar module as heat, thus the operating PV units accumulate the heats from sunshine and their own temperature inevitably is higher than their surrounding temperature. Especially at noon, the operating temperature of solar cell/module in outdoor condition is about 50-60ºC, or even higher [1]. Because of the intrinsic semiconductor feature of the solar cell, such a high operating temperature severely limits the output and reliability of cell device, it is well known that one degree rising of a Si solar cell can decline 0.4–0.5% to its efficiency. Therefore, it is necessary to investigate proper methods to dissipate heat from device core effectively and keep the operating temperature of device below the environmental temperature. Crystalline Si Solar cell is a core and its thermal conductivity is acceptable for heat spreading. Among all encapsulated materials of solar modules, transparent plastic Ethylene Vinyl Acetate (EVA) has the lowest thermal conductivity, which is the bottleneck of heat flow dissipation from solar cell. Investigated methods include controlled air flow through cell surface [ 2 ], enhancing the thermal conductance of whole solar cell structure, especially adding
highly thermally conductive fillers inside plastic EVA sealing material [3], and so on. Although the intensive work and results have been achieved so far, there are still issues in applying them to mass PV outdoor applications, since of the scalability and costs. EVA is an adhesive material to stick both sides of solar cells, keeping its transparency is also quite important for internal solar cell work. This could limit many opaque traditional thermally conductive fillers, such as SiC, BN powder inside EVA for solar heat dissipation. Radiative cooling is a new concept of passive cooling method [4]. It means that surfaces towards sky lose heat through thermal radiation, and these thermal energies are absorbed by extremely cold universe. In fact, considering the scattering, reflection and absorption phenomena under atmospheric condition, only thermal radiation wavelengths between 8-13µm and 16-25µm have maximal transmittance in sky, these wavelengths ranges are defined as “atmospheric window”, thermal radiation in atmospheric window can really release heat to outer space effectively. Radiative sky cooling is zero energy consumption to cool surface temperature, thus it will play a significant role to resolve the global energy crisis. Moreover, this cooling ability is strongly weather dependent, cooling power at the clear weather is higher than it at cloudy or humid weather [5], thus we believe PV with radiative cooling is a perfect homologous and complementary pair in the energy conversion process , since PV gets all energy from the sky, but radiative cooling release extra unused energy back to the sky. After decades of research, a bundle of materials with radiative cooling ability have been known, such as solid SiO monolayer [6], SiO2/Ag multilayer [7], ammonia or ethylene gas [8], however, to get effective radiative sky cooling for PV device, radiative material located on the front side of device needs to keep high transparency in solar irradiation range of 0.3-2.5µm. Additionally, this radiative cooling material embedded inside the solar module should be compatible with solar module packaging process, and keep insulation and viscosit similar as EVA, therefore proper radiative sky cooling material for PV application still needs further development. Cellulose is a rich natural resource and is widely used in the paper industry [9],
flexible device and environmental protection [10].It is currently used as a thermal insulator, but exhibits good heat-conducting properties when transformed to nano cellulose, because it retains crystalline chain and removes amorphous lignin [11]. Wood with hierarchical and mesoporous structures is composed by lignin, cellulose and hemicelluloses, while cellulose and hemicellulose are colorless, and lignin is more complex with dark color, thus wood always looks nontransparent. Recently Hu and his group have conducted intensive researches on celluloses structure and applications based on delignified woods, such as the development of transparent composits by mixing cellulose-retaining frame with index matching polymers [12], structural material with excellent mechanical properties[13], and ultralow anisotropic thermal conductivity[14]. Wood becomes an interest and abundant source of cellulose and its nanofibers (CNF) in optical, energy, and other applications due to their unique hierarchical nanostructures and wonderful various properties. Cellulose structures based on wood have the following features: high porosity and adjustable long-range pore could make it almost suitable for filling functional polymer by vacuum suction; crystalline chain of cellulose has relatively higher thermal conductivity than amorphous polymer; the thickness of the cellulose structure ranges from µm to cm level, depending on original wood slice thickness. So far, cellulose membrane could be a wonderful matrix while still retains the high transparency by infiltration of various transparent polymers, like PMMA, epoxy resin[15], but filling the EVA of encapsulation glue for solar module is not studied yet. Recently, Science magazine reported that cellulose based structural wood has strong emissivity in atmospheric window [16], these cellulose based fibers are non-absorbing in solar irradiation range, but the molecular vibration and stretching of cellulose generate strong emission in the infrared range[17], so cellulose embedded inside the front side of solar module should radiate heat out. The radiative cooling powers of cellulose based structural wood are also investigated and measured about 63 and 16W/m2 respectively, during night and daytime at 300K [18], these values are exponentially dependent on environmental temperature, which means the higher
surface temperature, the higher radiative cooling power of cellulose, thus cooling ability at 320K (approximated working temperature of solar device) is non-negligible comparing with the energy from AM 1.5 solar irradiation. Considering the all features above, it is expected to study the cooling effects of cellulose inside solar module structure for their thermal performance. Here we fabricated cellulose membranes by delignified process from wood slice. Their morphology structure, transmittance in solar radiation range and emission ability in atmospheric window are investigated. The transparent cellulose membrane/EVA nanocomposites were prepared by over-impregnation of thermal cross-linked EVA into finished membrane as encapsulation layer for solar module, finally the thermal performance of encapsulated solar module was evaluated by outdoor exposure test.
2. Experimental Section 2.1 Wood based cellulose membrane preparation Here a set of basswood slices with a dimension of 100 mm x 60 mm were obtained by multi-wire sawing process using a Si wafering machine in photovoltaic industry as a starting material. Thicknesses are 0.2, 0.5, 1, 2 mm, respectively. We took described chemicals processes to treat them to remove lignin [10]. All reagents were purchased from Aladdin Company (Shanghai, China), analytical grade, can be used directly. Slices were soaked in boiling NaOH (2.5 mol L−1 ) and Na2SO3 (2.5 mol L−1 ) solution to dissolve part of the lignin content as step 1, then the wood slices were moved into boiling H2O2 (2.5 mol L−1 ) solution to remove the residual lignin as step 2. After certain hours of treatments, the wood slice changed from yellow to white as lignin was removed. We stopped delignification and rinsed these slices with DI water several times to remove all chemicals. Finished cellulose membrane was ready for further characterization and impregnation of EVA. We found that among all delignificated slices, 0.5 mm samples are better choice for encapsulation process of solar module. The thick ones are not easy for further impregnation of molten EVA,
thus the stickbility of EVA loosenes. The thin ones often break to pieces at hot water in delignification process. 2.2 Cellulose membrane characterizations Figure.1(a) and (b) display the top and side views of microstructure of cellulose membrane after most of the lignin has been removed from wood slice, respectively. A scanning electron microscopy (Hitachi SU8010) at an acceleration voltage of 10 kV was used to show its morphology, and UV-Vis-NIR transmission spectra were measured by a UV-1601PC spectrophotometer. Transmission test samples are sandwiched structures in which EVA and cellulose membrane embedded between two glasses as shown in the Figure.2(a), and transmission spectra in solar radiation range of the sample were measured in the Figure.2(b), and IR emission spectra range of 6-24µm were measured by a DTGS detector and KBr beam splitter at room temperature as shown in the Figure.3. 2.3 Fabrication of Mini solar module with cellulose membrane To evaluate thermal and PV performance, we inserted above delignified wood slices between an EVA encapsulation sheet and one Metal Wrap Through (MWT) solar cell (Nanjing Sunport Power Corp site). Standard laminating process (vacuum hot pressing at 160 ºC for 20 mins) was done for making mini solar module, but both vacuuming and heating periods were extended by 3 minutes, which is better than impregnating the molten EVA through complete pores of cellulose membrane. Meanwhile, another sister MWT solar cell with almost the same electronic parameters was laminated by standard process as a reference sample. After lamination, peel strength test was done, and peel strengths between EVA and glass of both mini solar modules are larger than 20N/cm, which is standard for production, thus we believe that after excessive impregnation, EVA could penetrate whole cellulose membrane through pores and get tight contact with both sites of glass and solar cell, embedded cellulose membrane will not impair the sticking function of EVA for encapsulation. 2.3 Daytime and nighttime thermal experiment of solar module
Thermal experiment was carried out as following: mini solar module sample inside a closed polyethylene (PE) petri dish, excluding the other thermal influences, like thermal conductance to environment of solar module, and thermal convection above solar module by wind, the side walls and bottom of petri dish were surrounded by thermal insulated aluminium silicate felt, top was covered by a 50 µm thick polyethylene (PE) film window as shown in Figure.4(a). Four k-type thermocouples (TCs) are used and each two measure the surface temperature of the solar module and the reference sample, respectively. The experiment was conducted on the rooftop on a typical sunny day in August 2019 in Changsha, China. Prior to measurement, the test system exposed to the outside, reached the thermal equilibrium state, then every 20 minutes from 8:00am to 12:00pm, we collected the average temperature data, and the results were plotted in Figure. 4(b).
3. Results and Discussions In Figure.1(a), membrane’s pores are founded and wide enough, but not uniform in the range of 10-100µm, and the appearance of finished cellulose membrane is the inset in Figure.1(a), the straight channels along the growth direction are retained as shown in Figure.1(b). These features lead to easily overfilling the molten EVA for embedded cellulose membrane and further sticking top and bottom sides among EVA/cellulose composite layer in the solar module encapsulation process. There are several factors that can influence the transmittance of wood based cellulose/EVA composite, typically like pores distribution of cellulose, dimensions and refractive index matching [19]. The refractive index of cellulose is about 1.54, which matches the traditional transparent polymer PMMA, the refractive index of EVA changes with vinyl acetate content in the range of 1.48~1.51, thus EVA is also a good refractive index matching polymer. In Figure.2(a), we used a pair of glass slides to sandwich cellulose membrane/EVA, then used normal vacuum hot press to laminate for transmission measurement, the transmittance in solar radiation range of cellulose membrane/EVA
composite was shown in the Figure.2(b). Transmittance maintained abov 90% from range 400-1400 nm. It is very interesting that cellulose membrane/polymer film is transparent over the visible range, but the transmittance is inversely dependent on its thickness. In fact, for the application of so called transparent wood instead of glass as an energy-saving building material, centimeter-thick transparent cellulose/polymer composite is preferred, but it is hard to realize. Through interface manipulation, Li can obtain 7mm thick with about 70% transmittance[20]. However in this application, cellulose membrane is thinner and smaller than the thickness of EVA film (0.5mm) for over-filling, which should be a reason of obtaining high transmittance. Li, from Stanford, also investigated the cooling behavior of encapsulated solar cell, experimentally examined its solar absorption curve and thermal emission spectra of IR range[ 21 ]. Although the front glass layer in encapsulated solar module contributes the most thermal emission feature, the current encapsulated structure is not optimal yet, since thermal emissivity shows large drop among 8-10µm in part of atmospheric window, caused by phonon-polariton resonance of bare glass, thus a complex photonic structure was proposed to add on the top of encapsulated solar module. Here we measured the cellulose membrane sample’s emissivity over mid-IR range from 6µm to 24µm at room temperature. The result is shown in Figure.3 and keeps the relative high emissivity over the atmospheric window, especially emissivity of above 0.9 around 8-10µm could cover the emissivity dip of glass, thus the emissivity of whole solar module system gets optimized. Compared with proposed complex photonic structure, such as silica pyramids on the top of solar cell [22], our scenario of cellulose embedded membrane could be simple and straightforward, and can well complement the current encapsulation of solar module. The insertion of cellulose membrane doesn’t impair the adhesion and insulation functions of EVA for fabrication of solar module, because cellulose is non-conductive, and its porosity allows the molten EVA to soak through completely and stick to solar
cell surface. The crystallites of cellulose are the origin of the excellent properties of CNFs, such as the high Young’s modulus of 140~150 GPa, and the low coefficient of thermal expansion (CTE)[23 ]. This low CTE should be helpful to reduce the detachment failure probability when solar module works at repeated harsh thermal condition. In the thermal experiment, Figure.4(a) is the picture of test setup, and temperature profiles are as shown in Figure. 4(b), the surface temperatures of all tested solar modules are rising as morning time lapse until noon, the measured temperature difference between the solar modules with and without cellulose membrane gradually increases as solar radiation enhancement. The maximum difference about 2 ℃ appears at most solar irradiance noon time, then temperature difference reduces slightly in the afternoon until complete sunset. At the nighttime without solar irradiance, this difference still exists and becomes stable around 1℃. Although the temperature difference does not meet our expectation, we still believe that the cooling performance for solar module due to the radiative cellulose membrane is proved and can be improved further, for instance, enhancement of effective radiative area of membrane by optimizing the pore percentage, or combining the membrane with other high emissive, transparent materials, like nano-alumina [24]. Normally, typical passive radiative cooling design under direct sunlight [25] should consider the thermal influence of sunshine, so it always has a metal layer underneath the radiative cooling materials to reflect sunshine, in order to obtain the cooling effect below ambient air temperature. In the PV system, this reflective layer becomes inessential for radiative cooling structure, since some parts of sunshine will be absorbed for electricity. Actually solar cell has certain silver grid pattern on the front surface which collects photo-induced current, for better current collection, the pattern of top metal grids becomes thinner, and further optimized from simple 2 or 3 bus bar to multi bus bar, but there is always 3-5% of the total shadow area of metal grid always exist [26]. From the point of PV conversion, this shadow area is unhelpful, but it is helpful from the point of passive radiative cooling under the sunshine, since it
provides solar reflection sites. Overall we believe the existence of these metal grids on top surface not only collects photocurrent, but also works as a reflection site to strengthen radiative cooling effect. In order to reduce the operating temperature of solar cell core, we need to dissipate outwards generated heat quickly, however, because of low thermal conductivity and stagnation of sealed EVA, heat dissipation by conductance and convection is not feasible for solar module. Cellulose structure embedded EVA is highly transparent in solar irradiation range and radiative in atmospheric window, so it could be a suitable selection for solar cell. Except the thermal performance, this cellulose/EVA composite layer should also benefit for the solar conversion efficiency, since cellulose membrane/EVA composite is an actual heterostructure, causes high haze in visible wavelength range, besides high optical transmittance in the same range of wavelength. With such unique optical properties, light trapping inside cellulose/EVA layer passing through the solar module can be enhanced. Transparent, haze wood composite has better light management and improvement of efficiency of GaAs thin film solar cell as 18% is demonstrated already[27], thus we figure out that this wood-based cellulose/EVA should have similar effect to improve crystalline Si solar module output.
4. Summary and Outlook In conclusion, a wood-based cellulose membrane was successfully made and embedded inside EVA encapsulation layer for solar module. Due to the refractive index matching and high thermal emissivity property, a transparent radiative cooling function can be utilized to reduce the temperature of solar module. Thermal experiment demonstrated that the embedded cellulose membrane can be reduced about 2℃ for solar module under routine working condition. We believe embedded cellulose membrane is helpful to the comprehensive performance of crystalline Si solar module, since it not only contributes the radiative cooling ability for solar module, but also enhances the light trapping by its haze property.
Declaration of Competing Interest The authors declare no competing financial interest.
Acknowledgements The authors gratefully acknowledge the assistance provided by Mr.Baoshan Lv for productive discussions. Authors also acknowledge Nanjing Sunport Power Corp, Ltd, for solar module fabrication. This study was financially supported by the “2016 Hunan Province Technology Planning Program” with contract Nr.2016GK2009 and by the National Natural Science Foundation of China (Grant No. 51602100).
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[23] R. Hori, M. Wada, “The thermal expansion of wood cellulose crystals”, Cellulose, 2005, 12, 479-484. [24] ZB. Zhang, M. Elkabbash, ZH. Li, XY. Li, JH. Zhang, J. Rutledge, S. Singh, CL. Guo, “Enhancing thermoelectric output power via radiative cooling with nanoporous alumina”, Nano Energy, 2019, 65,104060. [25] A. Raman, M. Anoma, LX. Zhu, E.Rephaeli, SH. Fan, “Passive radiative cooling below ambient air temperature under direct sunlight”, Nature, 2014, 515, 540. [26] P. Morvillo, E. Bobeico, F. Formisano, F. Roca, “Influence of metal grid patterns on the performance of silicon solar cells at different illumination levels”, Mater. Sci. Eng. B, 2009, 159, 318. [27] MW. Zhu, T. Li, C. Davis, YG. Yao, JQ. Dai, YB. Wang, F. Alqatari, J. Grilman, LB. Hu, “Transparent and haze wood composite for highly efficient broadband light management in solar cell”, Nano Energy, 2016, 26, 332
Figure.1 top view (a) and side view (b) SEM image of cellulose membrane after chemical delignified process. The inset in upper left corner of (a) is appearance of finished cellulose membrane
Figure.2 (a) transmission test sample image, sample is marked with red square; (b) UV-Vis-NIR transmission spectra of test sample, and has average transmittance over 90% in 400-1400 nm range.
Figure.3 mid IR emissivity spectra, range from 6 to 24 µm, and cellulose membrane has high emissivity (>0.9) at 8-10µm, within atmospheric window.
Figure.4. (a) Thermal experiment setup for mini solar module, (b) measured solar modules’ surface temperature curves with time from 8am to 12pm.
Conflict of interest The authors of this manuscript declared that they have no conflicts of interest to this work with the title “From sky back to sky: embedded transparent cellulose membrane to improve the thermal performance of solar module by radiative cooling” .
Dr.Tiezheng Lv 2019/12/11
S2214-157X(19)30491-5
DOI:
https://doi.org/10.1016/j.csite.2020.100596
Reference:
CSITE 100596
To appear in:
Case Studies in Thermal Engineering
Received Date: 11 December 2019 Revised Date:
27 January 2020
Accepted Date: 29 January 2020
Please cite this article as: T. Lv, J. Huang, W. Liu, R. Zhang, From sky back to sky: Embedded transparent cellulose membrane to improve the thermal performance of solar module by radiative cooling, Case Studies in Thermal Engineering (2020), doi: https://doi.org/10.1016/j.csite.2020.100596. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
From Sky Back to Sky: Embedded Transparent Cellulose Membrane to Improve the Thermal Performance of Solar Module by Radiative Cooling Tiezheng Lv*, Jiangpin Huang*, Wei Liu*, Rong Zhang Research Institute of Automobile Parts Technology, Hunan Institute of Technology, Hengyang, 421002, Hunan, P.R.China, National Joint Engineering Laboratory of Automobile Pump Parts Design and Manufacturing, Hunan Institute of Technology, Hengyang, 421002, Hunan, P.R.China *Correspondence Tiezheng Lv, Research Institute of Automobile Parts Technology, Hunan Institute of Technology, Hengyang, 421002, Hunan, P.R.China Jianpin Huang, Research Institute of Automobile Parts Technology, Hunan Institute of Technology, Hengyang, 421002, Hunan, P.R.China Wei Liu, Research Institute of Automobile Parts Technology, Hunan Institute of Technology, Hengyang, 421002, Hunan, P.R.China
Email: [email protected], [email protected], [email protected]
Abstract The actual high working temperature severely affects the performance of crystalline Si solar module, and cooling function is favorited. Here we made a wood-based cellulose membrane having good transparent, high emissivity in mid IR atmospheric window, it shows a radiative sky cooling ability. We investigate the membrane’s porous and hierarchical morphology, transmission is averagely higher than 90% in solar spectra for composite of cellulose membrane, EVA and glass, the emissivity of cellulose membrane in 8-10µm is larger than 0.9. Transparent cellulose membranes were prepared and embedded between solar cell and EVA for module
encapsulation process. In the outdoor exposure experiment, we found that the cellulose membrane inside front of solar module can reduce the temperature of it, and around 2℃ cooling effect was obtained. Embedded cellulose membrane is simple, effective way to improve the thermal performance of solar module without weakening of solar spectra transmission and the performance. Keywords: radiative cooling; transparent cellulose membrane; solar module; thermal emissivity
1. Introduction Crystalline Si solar module is a device which converts photon into electricity with certain efficiency, and works under direct solar radiation. Nowadays photovoltaic (PV) is one of the best renewable energy worldwide to replace fossil fuels as the progresses of module in conversion efficiency and production cost. In the practical application, because of limited conversion efficiency, only part of solar radiation is converted into electricity, and the rest is absorbed by solar module as heat, thus the operating PV units accumulate the heats from sunshine and their own temperature inevitably is higher than their surrounding temperature. Especially at noon, the operating temperature of solar cell/module in outdoor condition is about 50-60ºC, or even higher [1]. Because of the intrinsic semiconductor feature of the solar cell, such a high operating temperature severely limits the output and reliability of cell device, it is well known that one degree rising of a Si solar cell can decline 0.4–0.5% to its efficiency. Therefore, it is necessary to investigate proper methods to dissipate heat from device core effectively and keep the operating temperature of device below the environmental temperature. Crystalline Si Solar cell is a core and its thermal conductivity is acceptable for heat spreading. Among all encapsulated materials of solar modules, transparent plastic Ethylene Vinyl Acetate (EVA) has the lowest thermal conductivity, which is the bottleneck of heat flow dissipation from solar cell. Investigated methods include controlled air flow through cell surface [ 2 ], enhancing the thermal conductance of whole solar cell structure, especially adding
highly thermally conductive fillers inside plastic EVA sealing material [3], and so on. Although the intensive work and results have been achieved so far, there are still issues in applying them to mass PV outdoor applications, since of the scalability and costs. EVA is an adhesive material to stick both sides of solar cells, keeping its transparency is also quite important for internal solar cell work. This could limit many opaque traditional thermally conductive fillers, such as SiC, BN powder inside EVA for solar heat dissipation. Radiative cooling is a new concept of passive cooling method [4]. It means that surfaces towards sky lose heat through thermal radiation, and these thermal energies are absorbed by extremely cold universe. In fact, considering the scattering, reflection and absorption phenomena under atmospheric condition, only thermal radiation wavelengths between 8-13µm and 16-25µm have maximal transmittance in sky, these wavelengths ranges are defined as “atmospheric window”, thermal radiation in atmospheric window can really release heat to outer space effectively. Radiative sky cooling is zero energy consumption to cool surface temperature, thus it will play a significant role to resolve the global energy crisis. Moreover, this cooling ability is strongly weather dependent, cooling power at the clear weather is higher than it at cloudy or humid weather [5], thus we believe PV with radiative cooling is a perfect homologous and complementary pair in the energy conversion process , since PV gets all energy from the sky, but radiative cooling release extra unused energy back to the sky. After decades of research, a bundle of materials with radiative cooling ability have been known, such as solid SiO monolayer [6], SiO2/Ag multilayer [7], ammonia or ethylene gas [8], however, to get effective radiative sky cooling for PV device, radiative material located on the front side of device needs to keep high transparency in solar irradiation range of 0.3-2.5µm. Additionally, this radiative cooling material embedded inside the solar module should be compatible with solar module packaging process, and keep insulation and viscosit similar as EVA, therefore proper radiative sky cooling material for PV application still needs further development. Cellulose is a rich natural resource and is widely used in the paper industry [9],
flexible device and environmental protection [10].It is currently used as a thermal insulator, but exhibits good heat-conducting properties when transformed to nano cellulose, because it retains crystalline chain and removes amorphous lignin [11]. Wood with hierarchical and mesoporous structures is composed by lignin, cellulose and hemicelluloses, while cellulose and hemicellulose are colorless, and lignin is more complex with dark color, thus wood always looks nontransparent. Recently Hu and his group have conducted intensive researches on celluloses structure and applications based on delignified woods, such as the development of transparent composits by mixing cellulose-retaining frame with index matching polymers [12], structural material with excellent mechanical properties[13], and ultralow anisotropic thermal conductivity[14]. Wood becomes an interest and abundant source of cellulose and its nanofibers (CNF) in optical, energy, and other applications due to their unique hierarchical nanostructures and wonderful various properties. Cellulose structures based on wood have the following features: high porosity and adjustable long-range pore could make it almost suitable for filling functional polymer by vacuum suction; crystalline chain of cellulose has relatively higher thermal conductivity than amorphous polymer; the thickness of the cellulose structure ranges from µm to cm level, depending on original wood slice thickness. So far, cellulose membrane could be a wonderful matrix while still retains the high transparency by infiltration of various transparent polymers, like PMMA, epoxy resin[15], but filling the EVA of encapsulation glue for solar module is not studied yet. Recently, Science magazine reported that cellulose based structural wood has strong emissivity in atmospheric window [16], these cellulose based fibers are non-absorbing in solar irradiation range, but the molecular vibration and stretching of cellulose generate strong emission in the infrared range[17], so cellulose embedded inside the front side of solar module should radiate heat out. The radiative cooling powers of cellulose based structural wood are also investigated and measured about 63 and 16W/m2 respectively, during night and daytime at 300K [18], these values are exponentially dependent on environmental temperature, which means the higher
surface temperature, the higher radiative cooling power of cellulose, thus cooling ability at 320K (approximated working temperature of solar device) is non-negligible comparing with the energy from AM 1.5 solar irradiation. Considering the all features above, it is expected to study the cooling effects of cellulose inside solar module structure for their thermal performance. Here we fabricated cellulose membranes by delignified process from wood slice. Their morphology structure, transmittance in solar radiation range and emission ability in atmospheric window are investigated. The transparent cellulose membrane/EVA nanocomposites were prepared by over-impregnation of thermal cross-linked EVA into finished membrane as encapsulation layer for solar module, finally the thermal performance of encapsulated solar module was evaluated by outdoor exposure test.
2. Experimental Section 2.1 Wood based cellulose membrane preparation Here a set of basswood slices with a dimension of 100 mm x 60 mm were obtained by multi-wire sawing process using a Si wafering machine in photovoltaic industry as a starting material. Thicknesses are 0.2, 0.5, 1, 2 mm, respectively. We took described chemicals processes to treat them to remove lignin [10]. All reagents were purchased from Aladdin Company (Shanghai, China), analytical grade, can be used directly. Slices were soaked in boiling NaOH (2.5 mol L−1 ) and Na2SO3 (2.5 mol L−1 ) solution to dissolve part of the lignin content as step 1, then the wood slices were moved into boiling H2O2 (2.5 mol L−1 ) solution to remove the residual lignin as step 2. After certain hours of treatments, the wood slice changed from yellow to white as lignin was removed. We stopped delignification and rinsed these slices with DI water several times to remove all chemicals. Finished cellulose membrane was ready for further characterization and impregnation of EVA. We found that among all delignificated slices, 0.5 mm samples are better choice for encapsulation process of solar module. The thick ones are not easy for further impregnation of molten EVA,
thus the stickbility of EVA loosenes. The thin ones often break to pieces at hot water in delignification process. 2.2 Cellulose membrane characterizations Figure.1(a) and (b) display the top and side views of microstructure of cellulose membrane after most of the lignin has been removed from wood slice, respectively. A scanning electron microscopy (Hitachi SU8010) at an acceleration voltage of 10 kV was used to show its morphology, and UV-Vis-NIR transmission spectra were measured by a UV-1601PC spectrophotometer. Transmission test samples are sandwiched structures in which EVA and cellulose membrane embedded between two glasses as shown in the Figure.2(a), and transmission spectra in solar radiation range of the sample were measured in the Figure.2(b), and IR emission spectra range of 6-24µm were measured by a DTGS detector and KBr beam splitter at room temperature as shown in the Figure.3. 2.3 Fabrication of Mini solar module with cellulose membrane To evaluate thermal and PV performance, we inserted above delignified wood slices between an EVA encapsulation sheet and one Metal Wrap Through (MWT) solar cell (Nanjing Sunport Power Corp site). Standard laminating process (vacuum hot pressing at 160 ºC for 20 mins) was done for making mini solar module, but both vacuuming and heating periods were extended by 3 minutes, which is better than impregnating the molten EVA through complete pores of cellulose membrane. Meanwhile, another sister MWT solar cell with almost the same electronic parameters was laminated by standard process as a reference sample. After lamination, peel strength test was done, and peel strengths between EVA and glass of both mini solar modules are larger than 20N/cm, which is standard for production, thus we believe that after excessive impregnation, EVA could penetrate whole cellulose membrane through pores and get tight contact with both sites of glass and solar cell, embedded cellulose membrane will not impair the sticking function of EVA for encapsulation. 2.3 Daytime and nighttime thermal experiment of solar module
Thermal experiment was carried out as following: mini solar module sample inside a closed polyethylene (PE) petri dish, excluding the other thermal influences, like thermal conductance to environment of solar module, and thermal convection above solar module by wind, the side walls and bottom of petri dish were surrounded by thermal insulated aluminium silicate felt, top was covered by a 50 µm thick polyethylene (PE) film window as shown in Figure.4(a). Four k-type thermocouples (TCs) are used and each two measure the surface temperature of the solar module and the reference sample, respectively. The experiment was conducted on the rooftop on a typical sunny day in August 2019 in Changsha, China. Prior to measurement, the test system exposed to the outside, reached the thermal equilibrium state, then every 20 minutes from 8:00am to 12:00pm, we collected the average temperature data, and the results were plotted in Figure. 4(b).
3. Results and Discussions In Figure.1(a), membrane’s pores are founded and wide enough, but not uniform in the range of 10-100µm, and the appearance of finished cellulose membrane is the inset in Figure.1(a), the straight channels along the growth direction are retained as shown in Figure.1(b). These features lead to easily overfilling the molten EVA for embedded cellulose membrane and further sticking top and bottom sides among EVA/cellulose composite layer in the solar module encapsulation process. There are several factors that can influence the transmittance of wood based cellulose/EVA composite, typically like pores distribution of cellulose, dimensions and refractive index matching [19]. The refractive index of cellulose is about 1.54, which matches the traditional transparent polymer PMMA, the refractive index of EVA changes with vinyl acetate content in the range of 1.48~1.51, thus EVA is also a good refractive index matching polymer. In Figure.2(a), we used a pair of glass slides to sandwich cellulose membrane/EVA, then used normal vacuum hot press to laminate for transmission measurement, the transmittance in solar radiation range of cellulose membrane/EVA
composite was shown in the Figure.2(b). Transmittance maintained abov 90% from range 400-1400 nm. It is very interesting that cellulose membrane/polymer film is transparent over the visible range, but the transmittance is inversely dependent on its thickness. In fact, for the application of so called transparent wood instead of glass as an energy-saving building material, centimeter-thick transparent cellulose/polymer composite is preferred, but it is hard to realize. Through interface manipulation, Li can obtain 7mm thick with about 70% transmittance[20]. However in this application, cellulose membrane is thinner and smaller than the thickness of EVA film (0.5mm) for over-filling, which should be a reason of obtaining high transmittance. Li, from Stanford, also investigated the cooling behavior of encapsulated solar cell, experimentally examined its solar absorption curve and thermal emission spectra of IR range[ 21 ]. Although the front glass layer in encapsulated solar module contributes the most thermal emission feature, the current encapsulated structure is not optimal yet, since thermal emissivity shows large drop among 8-10µm in part of atmospheric window, caused by phonon-polariton resonance of bare glass, thus a complex photonic structure was proposed to add on the top of encapsulated solar module. Here we measured the cellulose membrane sample’s emissivity over mid-IR range from 6µm to 24µm at room temperature. The result is shown in Figure.3 and keeps the relative high emissivity over the atmospheric window, especially emissivity of above 0.9 around 8-10µm could cover the emissivity dip of glass, thus the emissivity of whole solar module system gets optimized. Compared with proposed complex photonic structure, such as silica pyramids on the top of solar cell [22], our scenario of cellulose embedded membrane could be simple and straightforward, and can well complement the current encapsulation of solar module. The insertion of cellulose membrane doesn’t impair the adhesion and insulation functions of EVA for fabrication of solar module, because cellulose is non-conductive, and its porosity allows the molten EVA to soak through completely and stick to solar
cell surface. The crystallites of cellulose are the origin of the excellent properties of CNFs, such as the high Young’s modulus of 140~150 GPa, and the low coefficient of thermal expansion (CTE)[23 ]. This low CTE should be helpful to reduce the detachment failure probability when solar module works at repeated harsh thermal condition. In the thermal experiment, Figure.4(a) is the picture of test setup, and temperature profiles are as shown in Figure. 4(b), the surface temperatures of all tested solar modules are rising as morning time lapse until noon, the measured temperature difference between the solar modules with and without cellulose membrane gradually increases as solar radiation enhancement. The maximum difference about 2 ℃ appears at most solar irradiance noon time, then temperature difference reduces slightly in the afternoon until complete sunset. At the nighttime without solar irradiance, this difference still exists and becomes stable around 1℃. Although the temperature difference does not meet our expectation, we still believe that the cooling performance for solar module due to the radiative cellulose membrane is proved and can be improved further, for instance, enhancement of effective radiative area of membrane by optimizing the pore percentage, or combining the membrane with other high emissive, transparent materials, like nano-alumina [24]. Normally, typical passive radiative cooling design under direct sunlight [25] should consider the thermal influence of sunshine, so it always has a metal layer underneath the radiative cooling materials to reflect sunshine, in order to obtain the cooling effect below ambient air temperature. In the PV system, this reflective layer becomes inessential for radiative cooling structure, since some parts of sunshine will be absorbed for electricity. Actually solar cell has certain silver grid pattern on the front surface which collects photo-induced current, for better current collection, the pattern of top metal grids becomes thinner, and further optimized from simple 2 or 3 bus bar to multi bus bar, but there is always 3-5% of the total shadow area of metal grid always exist [26]. From the point of PV conversion, this shadow area is unhelpful, but it is helpful from the point of passive radiative cooling under the sunshine, since it
provides solar reflection sites. Overall we believe the existence of these metal grids on top surface not only collects photocurrent, but also works as a reflection site to strengthen radiative cooling effect. In order to reduce the operating temperature of solar cell core, we need to dissipate outwards generated heat quickly, however, because of low thermal conductivity and stagnation of sealed EVA, heat dissipation by conductance and convection is not feasible for solar module. Cellulose structure embedded EVA is highly transparent in solar irradiation range and radiative in atmospheric window, so it could be a suitable selection for solar cell. Except the thermal performance, this cellulose/EVA composite layer should also benefit for the solar conversion efficiency, since cellulose membrane/EVA composite is an actual heterostructure, causes high haze in visible wavelength range, besides high optical transmittance in the same range of wavelength. With such unique optical properties, light trapping inside cellulose/EVA layer passing through the solar module can be enhanced. Transparent, haze wood composite has better light management and improvement of efficiency of GaAs thin film solar cell as 18% is demonstrated already[27], thus we figure out that this wood-based cellulose/EVA should have similar effect to improve crystalline Si solar module output.
4. Summary and Outlook In conclusion, a wood-based cellulose membrane was successfully made and embedded inside EVA encapsulation layer for solar module. Due to the refractive index matching and high thermal emissivity property, a transparent radiative cooling function can be utilized to reduce the temperature of solar module. Thermal experiment demonstrated that the embedded cellulose membrane can be reduced about 2℃ for solar module under routine working condition. We believe embedded cellulose membrane is helpful to the comprehensive performance of crystalline Si solar module, since it not only contributes the radiative cooling ability for solar module, but also enhances the light trapping by its haze property.
Declaration of Competing Interest The authors declare no competing financial interest.
Acknowledgements The authors gratefully acknowledge the assistance provided by Mr.Baoshan Lv for productive discussions. Authors also acknowledge Nanjing Sunport Power Corp, Ltd, for solar module fabrication. This study was financially supported by the “2016 Hunan Province Technology Planning Program” with contract Nr.2016GK2009 and by the National Natural Science Foundation of China (Grant No. 51602100).
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Figure.1 top view (a) and side view (b) SEM image of cellulose membrane after chemical delignified process. The inset in upper left corner of (a) is appearance of finished cellulose membrane
Figure.2 (a) transmission test sample image, sample is marked with red square; (b) UV-Vis-NIR transmission spectra of test sample, and has average transmittance over 90% in 400-1400 nm range.
Figure.3 mid IR emissivity spectra, range from 6 to 24 µm, and cellulose membrane has high emissivity (>0.9) at 8-10µm, within atmospheric window.
Figure.4. (a) Thermal experiment setup for mini solar module, (b) measured solar modules’ surface temperature curves with time from 8am to 12pm.
Conflict of interest The authors of this manuscript declared that they have no conflicts of interest to this work with the title “From sky back to sky: embedded transparent cellulose membrane to improve the thermal performance of solar module by radiative cooling” .
Dr.Tiezheng Lv 2019/12/11