Supporting Information

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1 Supporting Information Commercially Available Activated Carbon Fiber Felt Enables Efficient Solar Steam Generation Haoran Li,, Yurong He *,,, Yanwei Hu,, Xinzhi Wang,, School of Energy Science and Engineering, Harbin Institute of Technology, Harbin , People's Republic of China Heilongjiang Key Laboratory of New Energy Storage Materials and Processes, School of Energy Science and Engineering, Harbin Institute of Technology, Harbin , People's Republic of China Y. Hu and X. Wang contributed equally to this work Corresponding Author * rong@hit.edu.cn. S-1

2 S1 Design of the evaporator The constructed evaporator is shown in Figure S1. Commercially purchased polyacrylonitrile ACF felt (Beihai Carbon Co. Ltd, Qingdao, China) was cut into discs with a diameter of 38 mm and a thickness of 3.0 mm. The discs were washed successively with acetone, ethanol, and water for 15 min, using a 500 W ultrasonic wave cleaner (Jeken PS-100A). The diameter of the floated ACF felt was chosen to be slightly smaller than the inner diameter of the chamber to ensure free movement during the test. Insulation foam with a diameter and a thickness of 34 and 18 mm, respectively, was fixed on the bottom surface to provide floatability for the felt during evaporation (Figure S1a). The insulation foam also ensured low heat conduction loss to bulk water for the evaporator. The water could only contact the ACF felt via a 2-mm annular channel, near the inner wall (schematically shown in Figure S1b). When the water was in direct contact with the sample, it diffused in the sample freely, and infiltrated the whole sample within 5 min. a b Figure S1. (a) Digital image of the ACF felt-based evaporator. (b) Schematic for the water flow process during evaporation. S2 Optical properties measurement Optical reflection and transmission of the ACF felt as well as the bilayer structure were measured using the high-precision instruments assembled by Tan s group (Ref. [41] in the S-2

3 text) for the wavelengths nm. The absorption at any given wavelength, λ, can be calculated using: A( )=1 R( ) T( ) (S1) where A(λ), R(λ), and T(λ) denote absorption, reflection, and transmission at a special wavelength, λ. Subsequently, A(λ), R(λ), and T(λ) were weighted with the AM 1.5 G solar irradiance, as Equations S2 S4. A= A( ) I( )d I( )d (S2) R= R( ) I( )d I( )d (S3) T = T( ) I( )d I( )d (S4) where A, R, and T are the weighted absorption, reflection, and transmission of the sample under AM 1.5 G. S3 Porosity and wettability of the ACF felt The porosity of the ACF felt was determined by measuring the specific weight before and after full absorption of water. A square plate with a length of 3.8 cm and a width of 1.5 cm was used as the test sample. The plate were washed successively with acetone, ethanol, and water for 15 min, and dried at 60 ºC for 24 h. After measured the weight of the dry felt, it was immersed to water for 5 min, and the weight of the wet felt was measured sequentially. The specific weight of the dried and wetted ACF felts are and g cm -3 (Figure S2a), respectively, therefore the porosity of the ACF felt is 93.3%. Contact angles of water on the dried and wetted ACF felts were measured by employing the JCY 2 contact angle meter S-3

4 (Fangrui Instrument Co., Ltd., China) equipped with a CCD camera. After 60 s of standing, the contact angle of the water drop (5 μl in total) on the dried felt surface remains 122 (Figure S2b), showing high hydrophobicity of the dried ACF felt. When the droplet hits the surface of a wet ACF felt, the water permeates into the felt within 0.2 s (Figure S2c), which confirms efficient water supply of the wet felt. a 1.5 b Specific weight (g cm -3 ) cm cm Wet c 0.0 Dry Figure S2. (a) Specific weight of the ACF felts before and after wetting. CCD camera images of 5 μl of water on the (b) dry and (c) wet ACF felts. S4 Thermal conductivity measurement Thermal conductivities of both the dry and wet ACF felts were measured at room temperature using a laboratory-built test-apparatus as described in Ref. [9] of the text, and as shown in Figure S3a. A piece of a sample, with a thickness of 3.0 mm, was sandwiched between two 3- mm glass holders. The sandwich structure was placed between a heating source (silicone rubber heated copper plate) and a cooling source (ice-water bath). An adjustable directcurrent source was employed to supply electricity to the heater. Thermal equilibrium was achieved when the temperature variation within 10 min was less than 0.2 K. Figure S3b shows the schematic of the sandwiching process and the locations for the temperature measurements. The temperature at the three interfaces, e.g., copper plate bottom glass (I1), bottom glass ACF felt (I2), and ACF felt top glass (I3), were monitored using an IR thermal imager (Ti450, Fluke, USA). During heating, the temperature gradient (dt/dx) in the vertical S-4

5 direction appears on the sandwich structure. The heat transfer rate (q) permeating the sandwich structure can be calculated using the Fourier equation: dt T T q k k (S5) dx d1 where k1 = 1.05 W/(m K) is the thermal conductivity of quartz glass, 1 3 T1 and T2 are the average temperatures at the interfaces I1, and I2, respectively, and d1 = 3.0 mm is the thickness of the glass. After the heat transfer rate was obtained, the thermal conductivity of the test sample (k) was calculated using: d k 2 q T T 3 2 (S6) where T3 is the average temperature at the interface I3,and d2 = 3.0 mm is the thickness of the test sample. a b Figure S3. Thermal conductivity measurement. (a) Photography of the test apparatus. (b) Schematic diagram of the sandwiching process showing locations for temperature measurements. S5 Evaporation experiment A laboratory-built evaporator was employed to conduct the evaporation experiment, as shown in Figure S4. Commercially purchased polyacrylonitrile ACF felt (Beihai Carbon Co. Ltd, S-5

6 Qingdao, China) was cut into discs with a diameter of 38 mm and a thickness of 3.0 mm. The discs were successively washed with acetone, ethanol, and water for 15 min, using a 500 W ultrasonic-wave cleaner (Jeken PS-100A). Insulation foam with a diameter and height of 34 and 18 mm, respectively, was then fixed on the bottom surface to provide floatability to the felt during evaporation. The evaporation experiments were conducted at room temperature (ca. 298 K), and each sample was continuously tested three time. A solar simulator (HFM, Beijing Aulight Co. Ltd., China) was calibrated to one sun at the entrance of the thermal receiver using an optical power meter (CEL NP2000, Beijing Aulight Co. Ltd., China). An artificial glass chamber, filled with room temperature water, was placed on an electronic precision balance (Sartorius, SQP, Beijing, China) and under the solar simulator. The distance from the light source to the top surface of all experimental cases was kept 9 cm away from the exit of the light source. The inner diameter and height of the chamber were 40 and 80 mm, respectively. The diameter of the floated ACF felt was chosen to be slightly smaller than the inner diameter of the chamber to ensure easy movement during the test. The evaporating capacity of water was obtained by monitoring the weight loss using the electronic balance. The IR thermal imager was used to measure the temperature of the evaporators during illumination. Figure S4. Schematic for evaporation experiment. S-6

7 To demonstrate the durability of the ACF felt, the evaporation performance was sustainably measured by recording the mass change of the evaporator as a function of time. In this regard, a cyclic experiment was conducted for 20 times under the same conditions. For each cycle, the evaporator was irradiated for 60 min. After 60-min irradiation, the heated water was replaced with the room-temperature water. Figure S5 shows the steady-state evaporation rate for each cycle. The average evaporation rate is kg m -2 h -1, associating with a negligible deviation from the first run even to 20 cycles, thus confirming long-term stability of the ACF felt. Evaporation rate (kg m -2 h -1 ) 1.8 Measurement Average kg m -2 h RECYCLE Cycle number Figure S5. The steady-state evaporation rate for each cycle experiment. S6 Energy balance analysis for the bilayer evaporator The incident intensity, qin, was 1 kw m 2, while, for the bilayer structure evaporator, the heat losses at the steady-state conditions can be subdivided into six categories: (1) phase-change enthalpy for steam generation, (2) convection loss to the surroundings from the top surface, (3) radiation loss to the surroundings from the top surface, (4) reflection loss to the surroundings from the top surface, (5) conduction loss to bulk water through the annular channel and the insulation foam, and (6) convection heat loss to the surroundings from the lateral and bottom surfaces. (1) Phase change enthalpy for the generation of steam The ratio of solar energy transfer into latent heat enthalpy and consumption of steam generation equals the steady-state evaporation efficiency caused by illumination only (65.4%). S-7

8 (2) Convection loss to the surroundings from the top surface Convection loss can be calculated using the Newton s law of cooling: Q ha ( T T ) (S7) conv ts ts f where h = 5 W m -2 K -1 is the convective heat transfer coefficient of air in natural convection, Ats = m 2 is the top surface area, while Tts and Tf are the temperatures of the top surface and the ambient temperature, respectively. Therefore, the ratio of convection loss to the surroundings from the top surface is hats ( Tts Tf ) h( Tts Tf ) 5 ( ) 11.5% q A q 1000 in ts in (3) Radiation loss from the top surface to the surroundings Radiation loss can be calculated using the Stefan-Boltzmann law: Q A ( T T ) (S8) 4 4 rad ts ts f where ε = 0.85 is the emissivity of the ACF felt, and σ = W m -2 K -4, which is the Stefan-Boltzmann constant. Therefore, the ratio of the radiation loss to the surroundings from the top surface is Q rad Ats ( Tts Tf ) ( Tts Tf ) ( ) 0.2% q A q 1000 in ts in (4) Reflection loss from the top surfaces to the surroundings The reflection loss is equal to the reflectance of the ACF felt (5.7%). (5) Conduction loss to bulk water The conduction loss includes the loss to bulk water through both the annular channel and to the insulation foam. It can be calculated based on the temperature gradient of the bulk water and the Fourier s law: Q ka t cond ts (S9) where k is the thermal conductivity of the water, and t / is the temperature gradient for S-8

9 permeating the bulk water. The temperature gradient was calculated using the data captured from the IR image. The water temperatures at the bottom surface of the insulation foam and 0.82 cm away from this surface were 30.9 and 28.8 C, respectively. This is consistent with a temperature gradient of 256 K/m. The thermal conductivity of water strongly depends on its temperature. The average temperature (30 C) was used as reference temperature, and the thermal conductivity was W m -1 K -1. Therefore, the ratio for heat conduction loss to bulk water was katst / kt / q A q % in ts in (6) Convection heat loss from the lateral and bottom surfaces to the surroundings The total amount for the above five categories of energy consumptions is about 98.6%. Therefore, the convection heat loss from the lateral and bottom surfaces to the surroundings should be about 1.4%. REFERENCES (1) Wan, J.; Fan, A.; Yao, H.; Liu, W. Effect of thermal conductivity of solid wall on combustion efficiency of a micro-combustor with cavities. Energy Convers. Manage. 2015, 96, (2) Cuce, E.; Riffat, S. B.; Young, C. H. Thermal insulation, power generation, lighting and energy saving performance of heat insulation solar glass as a curtain wall application in Taiwan: A comparative experimental study. Energy Convers. Manage. 2015, 96, (3) Liu, Y.; Fan, A.; Yao, H.; Liu, W. A numerical investigation on the effect of wall thermal conductivity on flame stability and combustion efficiency in a mesoscale channel filled with fibrous porous medium. Appl. Therm. Eng. 2016, 101, S-9