Optically Transparent Thermally Insulating Silica Aerogels for Solar

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Optically Transparent – Thermally Insulating Silica Aerogels for Solar Thermal Insulation Ahmet Alperen Gunay, Hannah Kim, Naveen Nagarajan, Mateusz Lopez, Rajath Kantharaj, Albraa Alsaati, Amy Marconnet, Andrej Lenert, and Nenad Miljkovic ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18856 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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ACS Applied Materials & Interfaces

Optically Transparent – Thermally Insulating Silica Aerogels for Solar Thermal Insulation

A. Alperen Günay1, Hannah Kim2, Naveen Nagarajan1, Mateusz Lopez1, Rajath Kantharaj3, Albraa Alsaati3, Amy Marconnet3, Andrej Lenert2, Nenad Miljkovic1,4,5,*

1

Department of Mechanical Science and Engineering, University of Illinois at Urbana – Champaign, 1206 W Green St, Urbana, Illinois 61801 USA

2Department of Chemical Engineering, University of Michigan, 2800 Plymouth Road, Ann Arbor, Michigan 48109 USA 3

4

Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana - Champaign, Urbana, Illinois 61801 USA

5

*

School of Mechanical Engineering, Purdue University, 585 Purdue Mall, West Lafayette, Indiana 47907 USA

International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

Corresponding Author. E-mail address: [email protected]

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ABSTRACT Rooftop solar thermal collectors have the potential to meet residential heating demands if deployed efficiently at low solar irradiance (i.e., 1 sun). The efficiency of solar thermal collectors depends on their ability to absorb incoming solar energy and minimize thermal losses. Most techniques utilize a vacuum gap between the solar absorber and the surroundings to eliminate conduction and convection losses, in combination with surface coatings to minimize re-radiation losses. Here, we present an alternative approach that operates at atmospheric pressure with simple, black, absorbing surfaces. Due to their high transmission to solar radiation and low transmission to thermal radiation, silica based aerogels coated on black surfaces have the potential to act as simple and inexpensive solar thermal collectors. To demonstrate their heat trapping properties, we fabricated tetramethyl - orthosilicate (TMOS) based silica aerogels. A hydrophilic aerogel with a thickness of 1 cm exhibited a solaraveraged transmission of 76% and thermally-averaged transmission of ≈1% (at 100 °C). To minimize unwanted solar absorption by O-H groups, we functionalized the aerogel to be hydrophobic, resulting in a solar-averaged transmission of 88%. To provide a deeper understanding of the link between aerogel properties and overall efficiency, we developed a coupled radiative-conductive heat transfer model and used it to predict solar thermal performance. Instantaneous solar thermal efficiencies approaching 55% at 1 sun and 80℃ were predicted. This study sheds light on the applicability of silica aerogels on black coatings for solar thermal collectors and offers design priorities for next generation solar thermal aerogels. Keywords: Silica Aerogels, Optically Transparent – Thermally Insulating, Solar Thermal Collectors, Solar Thermal Insulation, Aerogel Synthesis, Hydrophobic, Hydrophilic.


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1. Introduction Strategies for improving the efficiency of solar thermal collectors can be broadly categorized as optical concentration and suppression of thermal losses. Here, we report on the synthesis and characterization of surface-modified aerogels for suppression of thermal losses. To limit thermal losses, researchers have developed numerous insulation techniques, among which the most prevalent is the vacuum tube - selective surface (VTSS) technology1-2. The VTSS method places a selective surface inside a vacuum environment to limit convective and conductive losses to the surrounding ambient. To fabricate the selective surface, cermet or multilayer based absorbers are typically used to enable high absorption (≈ 90%) in the solar spectrum and limited emission in the IR spectrum (≈ 5%)3-4. Although many advantages to this arrangement exist, the difficulty of maintaining vacuum (< 1000 Pa) limits their lifetime. The cost of maintaining a reliable vacuum system is usually high due to leaks or outgassing 5. Further, vacuum-based deposition methods are typically used to create selective surfaces. Hence, a need exists to develop cheap and scalable methods to create robust and highly efficient solar thermal insulation. One potential candidate to replace VTSS technology are optically transparent - thermally insulating (OTTI) coatings deposited on broadband black absorbers6-7. The ideal OTTI coating is transparent to solar radiation, opaque to IR, and has ultra-low thermal conductivity (~0.01 W/mK). The top OTTI coating serves to transmit sunlight to the absorber while reducing the re-radiation and convection heat losses from the hot absorber to the ambient. The absorber is thermally linked with the heat exchanger, which efficiently transfers the absorbed solar energy to a circulating fluid beneath the absorber. Near-black surfaces can be readily fabricated using carbon black paints8. By using an OTTI coating, the complexity and cost of the VTSS design would be eliminated since neither the selective surface nor the vacuum environment are required9-10. 3 ACS Paragon Plus Environment


Ideal candidates for OTTI coating materials are silica aerogels, consisting of mesoporous materials comprised of air and silica, which have ultra-low thermal conductivities (~0.001 – 0.3 W/mK) and densities (~100 kg/m3)11-15. Moreover, silica aerogels have tunable optical properties which can be tailored for exceptional transparency to solar radiation through careful synthesis16-17. In addition to having a high transparency to solar radiation, silica aerogels are mostly absorptive in the thermal IR spectrum due to the absorption spectra of silica and other gaseous constituents such as water and CO218-20. Thus, IR radiation emitted from the absorber will be re-absorbed by the silica aerogel and in part conducted back to the absorber, creating a heat trap. Therefore, silica aerogels, when synthesized on broadband and black absorbing surfaces, are ideal candidates for the synthesis of realistic OTTI layers for solar thermal applications21-25. One important metric of solar collector performance is the thermal efficiency, i.e. the amount of heat collected relative to the solar irradiance 26-27. Optimizing the thermal efficiency of the collector is a vital characteristic. In this work, we study the solar thermal performance of OTTI aerogels through chemical synthesis of aerogels, radiative characterization, and surface modification (hydrophobicity or hydrophilicity, which impacts the microstructure and surface characteristics of the synthesized aerogel) to achieve maximum optical transmission to solar radiation. The optimal 1 cm thick OTTI materials exhibit high transmission in the solar spectrum (≈95%, < 2.7 µm) and high absorption in the emitted IR spectrum (≈100%, > 2.7 µm). Hydrophobic and hydrophilic aerogels have solar-averaged transmissions of 88% and 76%, respectively. However, synthesized hydrophilic aerogels had higher spectrally averaged absorptivity values (>99%) for IR emission than the hydrophobic ones (≈94%). To help optimize the aerogel properties further and to gain an understanding of the ideal functionalization for maximizing thermal efficiency, a radiative numerical model was developed, showing that thermal efficiencies of 4 ACS Paragon Plus Environment

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52% are attainable with the use of hydrophobic aerogels under 1 sun (no optical concentration) for an absorber/surrounding temperature difference of 50℃. The nominal temperature difference was chosen to have a reasonable comparison with the existing solar thermal absorbers, and the obtained efficiency value is comparable to the collectors that are used in the industry which use selective surfaces and/or higher optical concentrations. In addition to functionalization, optimization of the aerogel thickness was performed for a thickness range of 4 to 120 mm, showing that the efficiency of the system showed an optimal OTTI layer thickness for the same temperature gradient. The outcomes of this work suggest that the use of optimized silica aerogels as OTTI coatings on black surfaces for solar insulation shows great promise for the future of solar thermal collectors. Improving the silica aerogel synthesis process for higher solar averaged transmission, lower infrared transmission and lower thermal conductivity has the potential to achieve enhancements in terms of both performance and cost efficiency.

2. Materials and Methods 2.1. Fabrication of Silica Aerogels. Silica aerogels were chosen due to their ultra-low thermal conductivities and tunable optical properties. The hydrophobic silica aerogel was synthesized by first creating a solution having a molar ratio of 0.017:0.091:1:6.71:12.23 (NH4OH:DMF:TMOS:H2O:MeOH). To create the solution, TMOS was first diluted by methanol, followed by the addition of dimethylformamide (DMF) as the drying chemical control additive. Although DMF was not a required solvent for aerogel synthesis, we found that it helped with the drying stages of the aerogel synthesis process. After creating a solution of TMOS, methanol, and DMF; the catalyst solution which comprises of NH4OH and deionized water was added into the mixture. After completing the synthesis of the alcogel solution, controlled evaporation was used to create a gel structure (gelation). Since gelation is evaporation rate dependent; in other words, 5 ACS Paragon Plus Environment


gelation time was dependent on the environmental conditions such as temperature and humidity, close monitoring of the alcogel solution was required to determine when gelation was complete. This step was of crucial importance; as at the instant the gelation was complete, the gel started its drying process. Since the solid structure of the aerogels was very weak due to the small volume fractions of solid particles, this drying step often resulted in cracks and/or fracture formation. Hence, the gel was kept in a pure alcohol solution as soon as it formed to keep it from further drying. For our experiments, gelation times of 30 minutes to approximately 90 minutes were used to achieve optimum aerogel optical properties. Upon immersion into the pure alcohol solution, the alcogel underwent aging for 21 days in ethanol, while refreshing the ethanol every 24 hours. In addition to preventing dryout, the alcohol helps dehydrate the gels, resulting in a higher alcohol concentration in the aerogels and more uniform solid structures. The longer the gels were aged, the stronger they became and the less preferable their optical properties were. Hence, the process described here (21 days of aging) was optimized with respect to optical transmission. The final stage was supercritical drying of the alcogels. Since the liquid that was present in the alcogel was ethanol (critical pressure, = 6.14 MPa, = 514 K)28, which has a high miscibility with liquid carbon dioxide, exposure of the alcogel to liquid carbon dioxide and repetitive purging resulted in ethanol replacement with liquid CO2, known as solvent exchange. Since solvent exchange is mass diffusion limited, the thickness of the sample played a critical role. Thinner samples resulted in less cracking. In addition, the use of DMF helped the drying process by forming smaller pores and in turn reducing the number of cracks that were visible29 (Figure 1a, c). To create hydrophobic aerogels (Figure 1b), the gels were immersed in trimethylchlorosilane (TMCS) and hexamethdisilazane (HMDS) solutions with hexanes (1:9 volume ratio of the


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hydrophobic agent to hexanes) prior to the aging step. After the silane treatment, the obtained gel was again solvent exchanged with ethanol, aged, and dried with supercritical CO2. The porous structures of the aerogels were also noted (Figure 1c). 2.2. Experimental Characterization of Silica Aerogels. After synthesis, UV-Vis and FTIR spectroscopy was used to characterize the sample optical transmission for a wide range of wavelengths (200 nm < < 12.5 µm). Furthermore, the densities and the average particle sizes of the aerogels were characterized using weight and volume measurements and Dynamic Light Scattering (DLS), respectively. The average particle sizes were also verified using the X-Ray Diffraction method (XRD) and Transmission Electron Microscopy (TEM) imaging. To have a better insight on the thermal properties, the thermal conductivity values were also studied using an IR microscopy technique30. The effects of thermal transport with the environment had been accounted for using ANSYS simulations with the exact conditions at which the IR imaging was performed to get the exact thermal conductivity values (see Supporting Information). Finally, N2 adsorption method was used to determine the average pore size and the average surface area of the aerogels.

3. Theory To better understand the performance of an OTTI absorber and to assess which surface functionalization (i.e. hydrophilic or hydrophobic) would yield better operation, we developed a numerical model to study the thermal behavior of the absorber system. The physical system includes an OTTI coating of a certain thickness () directly mounted on top of a broadband absorber. It is envisioned that by synthesizing the aerogel directly on the broadband absorber and following the necessary processes (aging and drying), the aerogel would adhere directly to the broadband absorber, eliminating any air gaps.



Figure 2 shows a schematic of the proposed physical system under consideration. The combined solar thermal system is exposed to 1 sun radiative flux ( =1 kW/m2), and has a natural convective boundary condition at the top surface of the aerogel with a convective heat transfer coefficient (ℎ ) of 10 W/m2K to the surroundings, assumed to be at =300 K. Radiative transfer to the surroundings is considered since it is the dominant loss mechanism in a well-designed solar collector. Re-radiative heat loss is due to emission from the surface ( is the temperature profile inside the aerogel, =5.67× 10 W/m2K4 is the Stefan-Boltzmann constant, and represents the emissivity of the top of the aerogel, while represents the emissivity of the top of the broadband absorber). For our solution, we used =0.99 and 0.94 for the hydrophilic and hydrophobic aerogels, respectively, while assuming the broadband absorber was a blackbody ( =1). The aerogel emissivity values were obtained by FTIR measurements and averaging of the data with respect to Planck’s function. Experimental optical data was used to determine the optical properties of the silica aerogel OTTI coatings. We neglected thermal boundary and contact resistance between the absorber and the aerogel coating because they are negligible relative to the overall thermal resistance of the system. The thermal resistance of a possible air gap between the aerogel and the absorber would be negligible unless there is a large gap (≫ 1 mm). Furthermore, typical thermal boundary resistance values for dielectric/metal interfaces31-32 are small relative to the aerogel thermal resistance. To obtain the solution, the absorber temperature was varied and a different solution was obtained for changing broadband absorber boundary conditions to evaluate efficiency across the range of expected conditions. 3.1. Governing Equations. To model the system depicted in Figure 2, the radiative transfer equation (RTE) and the heat equation are solved simultaneously. Without any simplifying assumptions, the RTE is given in Equation (1)33.


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∇ Ω" = −%& Ω" + % +

% * Ω+ ", Ω+ → Ω".Ω+ , 4) /0

(1)

where is a unit vector in the Ω direction, %& , % and % are the spectral extinction, absorption and scattering coefficients, respectively. Moreover, , Ω′ → Ω" is the phase function for the change of direction due to scattering from the Ω′ direction into the Ω direction. The presence of the scattering term in Equation (1) makes the analytical solution impossible to obtain, except for a few special cases. For our system, we examine two main bands with numerous sub-bands to obtain the solution. The main bands comprise of a solar band (λ < 2.7 µm) and an IR band (2.7 µm < λ). For the solar band, the heat flux profile due to the absorption of the solar rays inside the aerogel is solved and used as a parametric input to the IR band. Similarly, for the IR band, the solar band results as well as the thermal boundary conditions are incorporated into the heat equation to obtain the steady-state temperature profiles. For the solution of the IR Band, it is assumed that the OTTI layer is optically thick (4 = %& ≫ 1), enabling the use of the Rosseland approximation; we evaluate the validity of this approximation later based on measured extinction coefficients. Hence, a radiative thermal conductivity depending on the temperature can be developed and placed into the heat equation. Thus, for the IR band, Equation (1) simplifies to:

5 d7 9: < " * 5+ ".5+ , = −7 + 1 − 9: + %& d8 2 <

(2)

where 5 is the direction cosine (5 = cos @") and 9: is the single scattering albedo. Using the diffusion approximation, a single radiative thermal conductivity value can be defined, which is substituted into the heat equation as an added thermal conductivity. The Rosseland thermal conductivity is given in Equation (3)34.



AB " =

16DE F , 3%&

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(3)

where is the Stefan-Boltzmann constant ( = 5.67 x 10-8 W/m2K4), D is the real part of the refractive index (approximated as 1 for aerogels), and %& is the mean Rosseland extinction coefficient given by Equation (4).

%&

I H: I . = , 1 I H: % I . &

(4)

where is the spectral hemispherical blackbody flux and is given by Equation (5).

=

J< , K exp JE ⁄ " − 1"

(5)

with J< = 37,413 W·µm4/cm2 and JE = 14,388 K·µm. Hence, the effect of the IR band was calculated by solving Eqs. (3)-(5) for the obtained spectral data. The solution only depends on the imposed thermal boundary conditions on the heat equation. After accounting for the IR band, thermal radiation in the solar band was calculated as described below. For the solar band, negligible emitted intensity exists for the considered temperatures; therefore, the solution was obtained by considering a non-emitting cold medium with isotropic scattering. The boundary conditions for the solution of Equation (2) for the solar band with P7 = 0 are given in Equation (6)35.

5 = 1" = 1000 Q/SE

(6a)

5 ≠ 1" = 0.

(6b)

After calculating the intensity field, the flux and its divergence were obtained by integrating the intensity field with respect to the spectrum in all directions, as described by Equation (7).

Optically Transparent Thermally Insulating Silica Aerogels for Solar

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