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Nature-Inspired, 3D Origami Solar Steam Generator toward Near Full Utilization of Solar Energy Seunghyun Hong, Yusuf Shi, Renyuan Li, Chenlin Zhang, Yong Jin, and Peng Wang* Water Desalination and Reuse Center, Division of Biological and Environmental Science and Engineering, King Abdullah University of Science and Technology, Thuwal, Jeddah 23955-6900, Saudi Arabia
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S Supporting Information *
ABSTRACT: Solar steam generation, due to its capability of producing clean water directly by solar energy, is emerging as a promising eco-friendly and energy-efficient technology to address global challenges of water crisis and energy shortage. Although diverse materials and architectures have been explored to improve solar energy utilization, high efficiency in solar steam generation could be accomplished only with external optical and thermal management. For the first time, we report a deployable, three-dimensional (3D) origami-based solar steam generator capable of near full utilization of solar energy. This auxetic platform is designed based on Miura-ori tessellation and is able to efficiently recover radiative and convective heat loss as well as to trap solar energy via its periodic concavity pattern. The 3D solar steam generator device with a nanocarbon composite of graphene oxide and carbon nanotubes being photothermal component in this work shows a very strong dependence between its solar energy efficiency and surface areal density. The device yields an extraordinary solar energy efficiency close to 100% under 1 sun illumination at a highly folded configuration. The 3D origami device can withstand a great number of folding and unfolding cycles and shows unimpaired solar steam generation performances. The unique structural feature of the 3D origami structure offers a new insight into the future development of highly efficient and easily deployable solar steam generator. KEYWORDS: photothermal conversion, solar steam generation, solar energy utilization, Miura origami, carbon nanocomposites
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INTRODUCTION Solar-driven water distillation has been attracting considerable attention as one of the promising technologies with minimum carbon footprint to address the global water crisis that urgently needs to be solved.1−3 It produces clean water directly from seawater without extra energy input and any electronic or mechanically moving parts.4−14 Solar steam generation is the key integral part of any solar-driven water distillation process. From energy efficiency point of view, two primary considerations are rationally integrated in an effective solar steam generation design: (1) selection of photothermal materials having high photothermal conversion efficiency coupled with broadband optical absorptance4−9 and (2) architectural design with proper heat management.10−14 Impressive progresses have been made on both the aspects. Especially, various advanced photothermal materials have been combined with smart designs to almost completely cut off parasitic thermal loss to the bulk water. As a result, the state-of-the-art twodimensional (2D) planar solar steam generator has approached its theoretical energy efficiency limit,15−18 with radiative and convective heat dissipation from the planar photothermal material into the ambient air now accounting for the majority of the energy loss therein. Most recently, it has been reported that the energy efficiency limit of 2D planar devices can be © XXXX American Chemical Society
exceeded by making three-dimensional (3D) structured solar evaporators.18−22 The 3D devices with purposefully enlarged evaporation area lower steady-state temperatures of water evaporation under the same level of light illumination and thus reduce radiative and convective heat loss to the surroundings.19,20,23 In this study, for the first time, we demonstrate a typical origami 3D structure as a new platform of solar steam generator. Origami structure is created by folding and unfolding periodic pleats, which leads to the presence of periodic creases.24−29 The origami has been utilized for designing various deployable structures, such as space-bound solar panel arrays, tunable mechanical metamaterials, and foldable and stretchable electronics.24−29 This auxetic feature of the origami is proven in this work to provide an effective strategy for minimizing radiative and convective heat loss while maximizing the light absorptance in solar steam generator. More specifically, 3D Miura-ori tessellation structure, one of rigid origami and naturally occurring in the context of the packing, unpacking, and deployment of leaves, petals, and Received: May 2, 2018 Accepted: August 1, 2018
A
DOI: 10.1021/acsami.8b07150 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Three-dimensional origami-based solar steam generator. (a) Schematic illustration of Miura-ori tessellation structure for photothermal steam generation. (b) Unit cell of the Miura-ori structure and its geometric characteristics. Aactive and Aproject indicate overall surface area and projected area of the 3D structure, respectively. (c) Surface areal density versus projection angle. The inset shows the theoretical plots of the projection angle versus the dimensions of unit cell. (d) Optical images of the pristine cellulose membrane sheet and the composite-impregnated cellulose evaporator. The scanning electron microscopy images show the surface morphology of the pristine cellulose membrane and the composite-embedded cellulose sheets.
insect wings,30,31 is utilized in this study and is schematically presented in Figure 1a. As can be seen, the Miura-ori tessellation structure is composed of periodically identical rigid parallelogram faces, which are jointed with hinges such as mountain and valley folds.
behavior of the 3D device, the projection angle between two valley ridges in the unit cell is primarily considered. For instance, the planar state, wherein light interactive area (Aactive) is identical to the projected area (Aproject), is described with ϕ = π/2, and the completely compressed state corresponds to ϕ = 0. Furthermore, Figure 1c presents the surface areal density (Aactive/Aproject) at the projection angle (ϕ) experimentally measured at certain compressed state. By compressing the origami device along the x-axis direction, the projection angle gets reduced, and consequentially, decreasing Aproject over constant Aactive results in the increase of the areal density. For the purpose of comparison and a focused discussion, the dimension of individual parallelogram was kept constant in this work, i.e., a = 8 mm, b = 11 mm, and β = π/4. To construct the deployable solar steam generator, a commercially available porous cellulose membrane filter (7.2 × 6.0 cm2) was utilized as the backbone structure while a nanocarbon composite of graphene oxide (GO) and multiwalled carbon nanotubes (CNTs) was chosen as a light absorber. High porosity and flexibility of a solar steam device are structurally favorable to a deployable evaporator.11,32,33 The cellulose membrane filter possesses excellent water absorbency and foldability and thus serves as the desirably flexible structural support and simultaneously as the water conduit path. Moreover, the CNTs, as a filler inside the GO stacked layers, could improve the porosity of the lightabsorbing carbon composite.32,34 The nanocarbon material was impregnated uniformly inside the cellulose matrix by spray deposition, as shown in the scanning electron microscopy
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RESULTS AND DISCUSSION The attractive features of the 3D Miura-ori tessellation structure in deployable solar steam generation are: (1) the two flat and rigid parallelogram surfaces where water evaporation takes place are structurally intact when the origami is compressed to varying degrees; (2) the geometric compressibility and rigidity in turn enable it to have a highly controllable surface areal density, which is critical to a 3D solar steam generation; and (3) more importantly, four parallelogram faces formed from periodic array of mountain-valley folds create a concave structure and efficiently capture and recycle diffuse reflected light and dissipated convective and radiative heat loss from irradiated surfaces, leading to elevation of steam generation efficiency. Figure 1b presents detailed geometric parameters of a unit cell in the Miura tessellation. Each identical parallelogram within any cell is characterized by its lengths a and b, and oblique angle β while a unit cell has its cell length (l), width (w), height (h), dihedral angle (α), and projection angle (ϕ). As a result, the unit cell is described as l = 2b sin(ϕ/2), cos β
w = 2a cos(ϕ / 2) , h =
cot β tan (ϕ/2)]. 2
2
a sin 2 β − sin 2(ϕ / 2)
27,28
cos(ϕ / 2)
, and α = cos−1[1 − 2
To characterize the folding/unfolding B
DOI: 10.1021/acsami.8b07150 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. Solar steam generation measurement. (a) Schematic of the experimental setup for solar steam generation. (b) Temporal change in cumulative water weight and devices’ average surface temperature over time under irradiation, obtained from the 3D origami device with the surface areal density of 2.5.
Figure 3. Solar steam generation performance. (a) Evaporation rate as a function of areal density, obtained from the composite-embedded devices with specific weight in the range of 2.298−4.291 g/m2 under solar illumination (1 sun) and in the dark (0 sun). The deviation in the specific weight had little effect on the evaporation performances. For comparative study, the evaporation rates of the support (cellulose membrane) with no photothermal component is displayed. The curves represent a second-order polynomial (parabolic) fit to the data. (b) Steam generation efficiency under illumination as a function of areal density at a specific weight of 2.298−4.713 g/m2. The solid line is a guide to the eye. The inset demonstrates the averaged interfacial temperatures of the 3D evaporators with increasing areal density and under illumination. The averaged surface temperatures was used over the entire surface of the 3D absorbers in the steady state due to uneven temperature distribution.
effective to minimize the conductive heat dissipation into the bulk water without compromising water supply.11,20,35 The valley crease of the origami device was in direct contact with the T-shaped QGF conduit. The measurement setup is described in more detail in Experimental Section. In addition, the air-filled spaces beneath the 3D devices, regarded as a closed system, were presumed to exchange little heat with the surroundings under illumination. Figure 2b presents a typical cumulative weight loss of pure water under light and temporal evolution of evaporative surface temperature, experimentally obtained from the compressed 3D evaporator. As can be seen, once the light was turned on, the surface temperature of the 3D evaporator increased rapidly, and it was, after a few hundred seconds, followed by a quasisteady state. The fast temperature rise of the 3D evaporator under light demonstrates its fast photothermal conversion from
images (Figure 1d), and its broadband optical absorption capability of >97% was revealed from diffuse reflection spectra of a fully water-wetted material at highly folded state, which will be demonstrated later. Figure 2a schematically depicts an experimental setup for characterization of the steam generation performances. The water evaporation rates were measured by recording the weight change over time under 1 sun illumination within a homemade, thermally insulated system. All in-lab measurements under light were conducted in a well-controlled laboratory environment at an ambient temperature of 21−22 °C with humidity of 50−60%. Water supply from bulk water to the 3D origami structures was driven by capillary force and transpiration action via a water-absorbing quartz glass fiber (QGF) ribbon. The confined water supply through the capillary conduit, different from direct bulk water contact, is C
DOI: 10.1021/acsami.8b07150 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. Mechanism for near full utilization of solar energy. (a) Optical properties of 3D origami evaporator. Standard AM 1.5G solar spectrum and the diffuse reflectance spectra of the 3D device are displayed in the wavelength range of 250−2500 nm. For the folded device, the reflectance spectra from different samples were investigated and plotted with a mean (green solid line) and a standard deviation (shaded band). (b, c) Surface temperature versus projection angle of the (b) dry and (c) wet 3D devices under 1 sun illumination. The arrows indicate the increment of the areal density. The curves represent a second-order polynomial (parabolic) fit to the data. (d) Spatial temperature distribution on the creased surface of the compressed 3D origami device, observed at the areal density of 2.5. The scale bar is 1 cm.
solar energy to evaporation and thermal dissipation via radiation and convection. The stable water mass change and stable temperature at the quasi-steady state indicate the thermodynamic equilibria between solar input and thermal output via evaporation and dissipation to the surroundings. The evaporation rates of the 3D devices were examined at elevated areal density, as illustrated in Figure 3a. The modulation for the projection angle was achieved by uniaxial compression in the x-axis direction. The rate was calculated based on Aproject, which is perpendicular to the sunlight path and in the specific weight range of 2.298−4.291 g/m2. The specific weight of the composite on the support paper was evaluated by weighing the mass of the deposited carbon nanocomposite after fully drying it in a vacuum chamber overnight, and it could be tuned by adjusting the sprayed volume of the carbon composite solution. In addition, it should be noted that the steam generation performance was evaluated from the evaporation rates under light without deduction of natural water evaporation rates in the dark. The spontaneous evaporation in the dark cools the surface to a wet bulb temperature,20,36 where heat gain from the surroundings is energetically equilibrated with heat loss by evaporation. However, under solar illumination, inverse thermal energy flow appears from thermally excited objects to the surroundings, and accordingly, in this work, the water evaporation rate under light was directly used.20
As can be seen in Figure 3a, in the dark, the highly folded origami structures (0.23 and 0.21 kg/(m2 h)) from both the origami cellulose support and the composite with the impregnated GO and CNTs exhibited, respectively, near doubly enhanced evaporation rates compared to those (0.09 and 0.10 kg/(m2 h)) from flat devices, resulting from the enhanced water evaporation areas of the 3D structures. Under simulated sunlight illumination, the water evaporation rates of the original 3D origami cellulose support show a clear incremental dependency on its areal density, while the 3D composite origami structure shows much enhanced steam generation rates as a consequence of the strong photothermal conversion effect of the composite carbon. The evaporation rate of the composite 3D origami structure was 1.59 kg/(m2 h) at an areal density of 4.65, which is higher by 50% than that from the 2D planar device (1.06 kg/(m2 h)) (Figure 3a). There is a plateau region when the areal density is beyond 4.65. We further investigated the solar thermal steam generation efficiency (η) of the origami steam generators based on the measured evaporation rates. The efficiency was calculated by employing the formula η = ṁ hLV/Pin, where ṁ is the water mass flux, hLV is the total enthalpy for evaporation, consisting of latent heat and sensible heat, and Pin is the power density of irradiated sunlight beam on the absorber surface.11,18−20,37 The latent heat varies from 2453 kJ/kg at 20 °C to 2265 kJ/kg at 100 °C, and the sensible heat was calculated based on the D
DOI: 10.1021/acsami.8b07150 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces specific heat of water.20,38 We note that the solar input is energetically balanced with thermal output via evaporation and thermal loss to surroundings in the thermal equilibrium evidenced by stable surface temperature. Suppression of the heat loss can enable enhancement of the steam generation efficiency. Figure 3b presents η as a function of the areal density of the origami structure. The efficiency for the planar devices was merely 71%, while showing convection and radiation losses of approximately 10.9 and 13.5%, respectively. The thermal conduction loss to bulk water was negligibly small. Detailed calculation of the heat loss is provided in the Supporting Information. In contrast, the efficiency of the 3D structure increased along with its areal density and eventually reached close to 100% at Aactive/Aproject of 4.65. The efficiency shows a slight oversaturation (i.e., >100%) beyond the areal density of 4.65, which will be discussed later. Meanwhile, the folding of the origami devices increases the mass density of the GO/CNT composites per unit area illuminated by sunlight. To distinguish the effect of increased specific weight from its 3D feature on efficiency enhancement, the planar devices with higher specific weights were examined (Figure S1). As a result, the evaporation functionality was not improved in the devices even with incremental specific weight from 0.325 to 6.145 g/ m2. To investigate the mechanism associated with near full utilization of solar energy by the 3D origami device, the diffuse reflectance measurement was first conducted as shown in Figure 4a. As can be seen, when wet, the highly folded origami device (i.e., areal density ≥ 2.5) exhibited consistently less reflectance (
