Energy efficient materials for solar water distillation - A review

Renewable and Sustainable Energy Reviews 115 (2019) 109409

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Energy efficient materials for solar water distillation - A review T. Arunkumar a, b, Yali Ao b, c, Zhifang Luo a, b, Lin Zhang a, b, Jing Li b, c, Denkenberger D.d, *, Jiaqiang Wang a, b, c, * a

School of Chemical Sciences & Technology, Yunnan University, Kunming, 650091, China National Center for International Research on Photoelectric and Energy Materials, Yunnan Province Engineering Research Center of Photocatalytic Treatment of Industrial Wastewater, Yunnan Provincial Collaborative Innovation Center of Green Chemistry for Lignite Energy, Yunnan University, Kunming, 650091, China c School of Energy, Yunnan University, Kunming, 650091, China d University of Alaska Fairbanks, Mechanical Engineering and the Alaska Center for Energy and Power, Alaska, USA b

A R T I C L E I N F O

A B S T R A C T

Keywords: Solar energy Solar still Nanomaterials Phase change materials Desalination

Solar energy is one of the most powerful sources for many sustainable applications. Recently, efficient water distillation has attracted significant attention. The fresh water productivity depends on how efficiently the system harvests the incoming solar energy and converts it into useful heat. An ideal blackbody is capable of perfectly absorbing all wavelengths. The absorbed incident photons are converted into thermal energy. To approach the maximum solar absorption of a blackbody, efficient nanomaterials were developed with enhanced absorption in ultraviolet (UV)-visible to near infrared (NIR). Nanomaterials with broadband absorption, efficient heat transfer, minimum surface energy loss, and energy storage have recently emerged exhibiting accelerated the evaporation rate. These nano-enabled materials direct attention back towards traditional solar stills for future sustainable water evaporation for clean water production. Herein, novelty of the review includes (1) direct solar steam generation of highly efficient broadband materials, (2) energy exchange materials including nanoparticles & nano-fluids, (3) energy storage materials including phase change materials & nano-enabled-phase change materials and (4) other sensible energy storage materials for desalination. One result was that the local surface plasmon resonance (LSPR) effect in plasmonic metals and efficient heat trapping capabilities of carbon materials show high evaporation rates.

1. Introduction Water is a basic need for life. Earth is the only planet that has readily accessible water and oxygen for survival. The increase of population and industrialization has slowly polluted our natural water resources. Therefore, researchers have focused on wastewater and seawater treatment through sustainable technologies. Solar stills (SSs) have been identified as one of the natural water purification systems. However, the productivity is generally low. Recently, Arunkumar and his coresearchers [1] reviewed SSs with more than 5 L/m2.day production. They reported that the productivity of the SS was improved by con­ centrators [2–9], phase change materials [10–14], nano-fluids [15,16] and sensible heat storage materials [17–22]. Progress has also been made in efficient light to heat conversion with improved materials. Recently, some materials have achieved absorbance of >95%. Several researchers have reviewed direct steam generation by broadband absorbing materials for clean water production. Zhang et al. [23]

reviewed broadband absorbing materials for wastewater evaporation. The development of direct solar steam generation materials, basic principles and application of photo-thermal conversion processes were reported. Zhang et al. [24] discussed carbon nanocomposites, carbon nanotube based absorbers, and graphene based materials for photo-thermal conversion. Gao et al. [25] reviewed materials for direct solar steam conversion and energy production for desalination. They discussed metals, semiconductors, polymers and other carbon-based materials involved in photothermal conversion. Kim et al. [26] reviewed nanostructured broadband absorbing materials for photo-thermal conversion for clean water production. They analyzed the fundamental heat conversion for semiconductors, organic materials, and plasmonic metal-based materials. Deng and his co-workers [27] classi­ fied suspended and floating systems for photo-thermal conversion and analyzed the factors that influenced the efficiency. Based on the recent reviews, the researchers were mainly focused on materials for efficient heat conversion for clean water production. The boom of nanotechnology creates a need for a revaluation in green

* Corresponding authors. E-mail address: [email protected] (D. Denkenberger). https://doi.org/10.1016/j.rser.2019.109409 Received 11 May 2019; Received in revised form 9 September 2019; Accepted 20 September 2019 Available online 24 September 2019 1364-0321/© 2019 Elsevier Ltd. All rights reserved.

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Nomenclature

NEPCM NGCA NPs NW OLED PBONF PCM PF PIL PLD PM PP PS PSS PTFE PU PVA PW SEM SSPCM SWF TCF TEM TSS US-EPA VACNT WHO

Acronyms AAM Anodic aluminium oxide membrane ACSD Anti-clogging solar desalinator AMS Air melamine sponges CCC Compound conical concentrator CCTSS Concentric circular tubular solar still CF Carbon foam C–H wood Wood cut in horizontal direction CIF Carbon impregnated foam C-L wood Wood cut in longitudinal direction CMF Carbonized melamine foam CNT Carbon nano tube CPC Compound parabolic concentrator CS Carbon sponges CW Carbonized wood EPF Expanded polystyrene foam FPC Flat plate collector F-wood Flame treated wood GA Graphene aerogel HSS Hybrid solar still HX Heat exchanger LED Light emitting diode MF Melamine foam MWCNT Multi-walled carbon nanotube NCAP Nano-coated absorber plate

desalination. One motivation of the present work is to connect the re­ searchers in the field of SSs with those in thermal energy absorbing materials. To date, there have been no reviews that cover (1) direct solar steam generation with highly efficient broadband materials, (2) energy exchange materials including nanoparticles & nano-fluids, (3) energy storage materials including & nano-enabled phase change materials (PCMs-NEPCMs) and (4) other sensible energy storage materials for

Nano enabled phase change material Nitrogen-doped graphene carbon aerogel Nano particles Nano wire Organic light emitting diode P-phenylene benzobisoxazole-nanofiber Phase change material Pin fin Porous insulating layer Pulsed laser deposition Plasmonic membrane Plasmonic active filter paper Polystyrene Pyramid solar still Polytetrafluoroethylene Polyurethane Polyvinyl alcohol Paraffin wax Scanning electron microscope Shape stabilized phase change material Steel wool fiber Titanium dioxide doped carbon fabric Transmission electron microscope Tubular solar still United States-Environmental Protection Agency Vertically aligned carbon nanotube World Health Organization

desalination. The concept of the present review is illustrated in Fig. 1. Herein, the development of efficient materials tested in SSs are discussed from the last three years (2017–2019). 2. Mechanism of SS with thermal energy absorbing materials Absorption takes place in the blackened basin after the short

Fig. 1. Concept of review of materials for desalination. 2

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wavelength of visible sunlight passes through the transparent cover and water. Part of the thermal energy is transferred by convective heat transfer to the basin water and much of the rest is transported by con­ duction to the absorbing materials in the SS (if present). Long wave infrared (IR) radiation is emitted by the water, but the cover is opaque to IR radiation. The water temperature in the SS mainly depends on how efficiently the absorber converts the illumination to heat. Broadband absorbing materials (250–2500 nm wavelength) and other energy stor­ age materials are capable of minimizing the surface energy loss [24]. High achievable conversion efficiency in plasmonic metals is due to localized surface plasmon resonance (LSPR) [25]. This is a photon-induced coherent oscillation. In semiconductors, the electron-hole pairs generated during the illumination, when they return to the ground state, release energy through non-radiative relaxation in the form of phonons. The surface roughness and vertical morphological structures in the carbon materials enhance absorption on the surface [25]. When materials change from liquid from solid, it requires a certain amount of energy from the surroundings. Likewise, the material rejects heat to the environment during the reverse process, i.e. liquid to solid. Therefore, phase change materials in SSs store thermal energy during melting and release it later upon freezing. Sensible materials are capable of absorbing heat without changing their phase and also enhanced the surface area [28]. These materials inside the solar distiller transfer heat to the bulk water. Due to the temperature difference (ΔT) between the water and cover, the evaporated water condenses on the underside of the cover and slides downward across the cover due to gravity.

broadband absorption of the VACNTs were 99% in the solar spectrum. The calculated solar thermal conversion efficiency of the VACNTs was 90% under simulated solar radiation. The initial and final values of the salinity level of the real seawater was 2.6 wt percent (wt%) and <1 wt%, respectively. Fig. 2 (b) reveals the concentration of ions before and after treatment. The water quality parameters greatly improved after desali­ nation and met the World Health Organization (WHO) drinking water quality standards. Higgins et al. [30] coated titanium dioxide (TiO2) nanorods on carbon fabric (CF) for direct solar steam generation. The CF possessed good broadband absorption characteristics of >97%. In addition, the TiO2 nanorods were coated onto the TiO2 doped CF (TCF) for dye-contaminated water treatment as shown in Fig. 2 (c). The TCF was able to effectively remove contaminants under simulated solar ra­ diation from saline water as well as industrial wastewater. Furthermore, the TCF evaporated synthetic saline water (3.5% wt NaCl) under natural solar radiation. The TCF was placed on the bottom of the SS. The top cover of the SS was integrated with a Fresnel lens which concentrates the solar energy onto the TCF. The ionic conductivities of NaCl solution before and after evaporation were 40 milliSiemen (mS)/cm and 0.31 mS/cm, respectively. Thus, the TCF in the SS efficiently removed 99% of the salts from the synthetic salt water. Lin et al. [31] produced carbonized melamine foam (CMF) in a one-step calcination process for solar steam generation. The strong broadband absorption of 95% and an appropriate pore diameter (160 μm) contributed to an efficient water evaporation of 1.27 kg/m2.h under simulated solar radiation of one sun (1000 W/m2). The CMF was tested in the SS under natural sunlight with South China Sea water as shown in Fig. 2 (d). Fig. 2 (e) shows the concentration of ions before and after evaporation. The concentrations of ions (Ca2þ, Mg2þ, Cr, SO2þ 4 ) were reduced 3–5 orders of magnitude after evaporation. Wang et al. [32] developed a paper-based reduced graphene oxide (PrGO) coated silicone layer for seawater evaporation. The broadband absorption of PrGO was ~90% (250–2500 nm). The thermal conductivity of the porous insulating layer (PIL) was 0.038 W/m.K. The evaporation rate

3. Materials for solar desalination 3.1. Energy efficient materials for vapor generation for desalination 3.1.1. Carbon based materials in SSs Yin et al. [29] prepared vertically aligned carbon nanotubes (VACNTs) for solar steam generation, shown in Fig. 2 (a). The

Fig. 2. Solar energy harvesting energy efficient materials in SSs: (a) A schematic view of the VACNT array in a single slope SS and (b) salinity after desalination. (c) Schematic view of CF tested in the SS [30]. (d) Schematic view of integrated solar absorbers (ISA) in the SS (e) seawater quality results [31]. (f) Schematic view of SS with PrGO. (g) Mass change during the solar influx. (h) Salinity level after desalination [32]. (i) Picture of SS on an electronic balance. (j) Infra-red image of SS under 30 min of solar radiation. (k) House-type SS with absorber. (l) Pyramid type SS. (m) Floatable pyramid type SS, and (n) Condensation of freshwater on the sides of the SS [33]. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 3

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and efficiency of PrGO-PIL under one sun illumination with artificial seawater were 1.778 kg/m2.h and 80.6%, respectively. A schematic view of the experimental arrangement is shown in Fig. 2 (f). The NaCl content was reduced from 12710 mg/L to 2.039 mg/L after evaporation (see Fig. 2 (g)). The efficiency of the outdoor experiment was 76%. Fig. 2 (h) shows the salinity levels before and after evaporation. Zhu et al. [33] developed nitrogen doped carbon sponges (CSs) for distillation. The material was tested in simulated solar radiation of one sun. The evap­ oration rate and efficiency of the CSs were 1.31 kg/m2.h and 85%, respectively. Further CSs were tested in house-type SSs (Figs. (i–k)) and floatable pyramid type SSs (PSSs) as shown in Figs. (l–n). The CSs in the PSS produced 1.15 kg/m2.h under natural solar radiation. Feng et al. [34] developed a reduced graphene oxide MoS2 (rGO/­ MoS2) absorber for efficient water evaporation. Initially the experiment was conducted under simulated solar radiation of 0.5 suns. The evapo­ ration rate and efficiency of the rGO/MoS2 absorber were 0.54 kg/m2.h and 74%, respectively. Furthermore, the material was tested in the SS under natural solar radiation. Schematic and pictorial views of the SS designs are shown in (a–b). The productivity of the SS with rGO/MoS2 was 2.0 L/m2.day. The concentrations of ions before and after desali­ nation are shown in Fig. 3 (c). The water quality test revealed that the ions in the seawater were reduced 1–2 orders of magnitude. Zhang et al. [35] prepared a small-size reduced graphene aerogel (srGA) for clean water production. The broadband absorption of the srGA was 99%. An East Bay seawater sample was tested initially with srGA under simulated solar radiation of the equivalent of natural sunlight (1 sun). The net seawater evaporation of srGA was 1.73 kg/m2.h. Furthermore, the srGA was tested in the SS for seawater evaporation under natural solar radi­ ation. The area of the fabricated SS was 750 cm2 as shown in Fig. 3 (d). The insulation and water transport section along with a photograph of the floatable SS are shown in Fig. 3(e–f). The evaporation of seawater in the SS under natural solar radiation was 9.2 kg/m2 in 6 h of solar operation in outdoor conditions. The water quality test also confirmed that the primary ions in the seawater were significantly reduced after evaporation. Kashyap et al. [36] developed a porous polymer embedded with graphite flakes and carbon fibers henceforth called an anti-clogging solar desalinator (ACSD). The broadband absorption of exfoliated graphite was 97% in the wavelength range between 250 and 2500 nm.

Then poly (3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT-PSS) was coated on the dried film surface and formed a flexible ACSD. The initial laboratory scale test was performed under simulated solar radiation. The evaporation rate of the ACSD under 1 sun was 1.01 kg/m2.h with an evaporation efficiency of 62.7%. Furthermore, the ACSD was tested in the SS with synthetic salt as shown in Fig. 4 (a). The top cover of the SS was fabricated with ethylene tetra-fluoroethylene (ETFE) to increase the transmission (93%) of solar energy. The levels of salinity before and after desalination was 3.5% and 0%, respectively (See Fig. 4(b–c)). The calculated solar evaporation efficiency was 60.2%. Weng et al. [37] developed a mixture of beeswax (Apis Cerana), multi-walled carbon nanotube (MWCNT) and polydimethylsiloxane (PDMS) for solar vapor generation. The broadband absorption of PDMS was in the range of 400–1600 nm. The evaporation test used simulated solar radiation of 1 sun. The PDMS evaporated 1.30 kg/m2.h seawater with the corresponding steam generation efficiency of 82.2%. Further­ more, the PDMS was tested in the pyramid SS to evaporate real seawater. The PDMS in the pyramid type SS evaporated 0.90 kg/m2.h of seawater under natural solar radiation. The primary ions in the seawater were considerably reduced after evaporation from the SS. 3.1.2. Metallic based materials in the SS Zhou et al. [38] experimentally demonstrated an aluminium (Al) based plasmonic absorber for desalination (see Fig. 4 (d)). The material underwent testing with simulated solar radiation and seawater. The primary ions (Naþ, Mg2þ, Ca2þ, Kþ, B3þ) in the sea water (Baltic Sea, World Ocean, Red Sea, and Dead Sea) were reduced significantly after evaporation and found to be in the acceptable range according to the WHO and United States Environmental Protection Agency (US-EPA) drinking water quality standards. Further, the material was tested in a double slope SS with real seawater under natural solar radiation. The concentrations of B3þ, Mg2þ and Al3þ were reduced below detection level after evaporation. Chen et al. [39] fabricated a gold nanoparticle (Au-NP) impregnated poly p-phenylene benzobisoxazole nanofiber (PBONF) as a supporting layer for solar steam generation. The system is self-floating and has good water transport and high light to heat conversion. Three different Au amounts namely 1.2, 2.4, and 4.8 mg were named Au-NP/PBONF-1, Au-NP/PBONF-2, and Au-NP/PBONF-3, respectively. Au-NPs/PBONF-3

Fig. 3. (a) Schematic view of rGO/MoS2 in the SS. (b) Photograph of rGO/MoS2 SS. (c) Concentration of ions before and after desalination [34]. (d) Schematic view of SS with srGA. (e) Insulation layer, and (f) photograph of SS with srGA floating on the water surface [35]. 4

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Fig. 4. (a) SS with graphite flakes and carbon fiber, and (b–c) evidence of concentration of salt after desalination [36], and 4 (d) shows the Experimental arrangement of Al-NPs in the house type SS [38].

exhibited the maximum evaporation rate due to the highest concentra­ tion of Au-NPs. Furthermore, Au-NP/PBONF-3 was tested in a SS to evaporate the seawater under natural solar radiation as shown in Fig. 5 (a). Au-NP/PBONF-3 evaporated 3.36 g of fresh water in 2.5 h of oper­ ation (Fig. 5 (b)). The seawater samples were collected from the Baltic Sea, North Sea, Dead Sea, Red Sea and World Ocean and evaporated by Au-NPs/PBONF-3 as shown in Fig. 5 (c). The water quality greatly improved after evaporation (Fig. 5 (d)) and was within the acceptable limit according to the WHO safe drinking water standards. Song et al. [40] demonstrated a black Al2O3 coated Cu-NP/Cu–Si nanowire (NW) and dark brown Al2O3 coated crystal silicon-c-Si/Cu–Si NW for vapor generation. The strong resonance between incident light and the NWs is an advantageous feature of black Al2O3 coated Cu-NPs/Cu–Si NWs. The broadband absorption of black Al2O3 coated Cu-NPs/Cu–Si NWs and dark brown Al2O3 coated c-Si/Cu–Si NWs were 96% and 93%, respectively. The material was used to evaporate pure water and saline water under natural solar radiation. The evaporation rates of pure and saline water were 0.81 and 0.93 kg/m2.h, respectively. The concentration of ions in the seawater were significantly reduced after desalination. The effect of plasmonic membranes (PMs) was

investigated by Wang et al. [41]. The prepared Au-NPs were coated on the porous filter paper with the vacuum filtration process. The optical absorptivity of blank paper and Au-NPs was <10% and ~90%, respec­ tively. Interestingly, the surface roughness of Au-NPs plays a key role in strong broadband absorption because surface roughness creates multi­ ple scattering and absorption. The scanning electron microscope (SEM) images of Au-NPs with different magnifications are shown in Fig. 6(a–c). Two identical SSs of area 0.0625 m2 were fabricated with acrylic covers. Foam with a reflective aluminium film was used to insulate the SS to reduce thermal loss. The synthesized Au-NPs were pasted on the bottom of the SS in a 6 � 6 array. The Bohai Sea (P.R. China) water samples of 1 L were poured in the basin of the SS for evaporation. The Au-NP in­ tegrated SS enhanced the productivity by 80%. The seawater quality result reported that the primary ions were significantly decreased and found to be acceptable according to the WHO standards. Liu et al. [42] coated black gold nanoparticles (BG-NPs) on a mel­ amine sponge for seawater evaporation. The broadband absorption of the BG-NP was 96%. The morphology of BG-NPs with different magni­ fications is shown in Fig. 7(a–c). Three artificial seawater samples and a real seawater sample from Shenzhen Bay were examined under

Fig. 5. (a) Experimental arrangement of SS with Au-NPs, (b) productivity from the distiller, (c) salinity levels of various seawater samples before and after desa­ lination, and (d) concentration of ions before and after evaporation [39]. 5

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Fig. 6. (a–c)SEM images with different magnifications of plasmonic metals [41].

heavy metal ions of Cd2þ, Cu2þ, Cr3þ, and Pb2þ that were present at 1 g/L seawater was reduced to 0.0027, 0.8336, 0.3331, and 0.1793 mg/L, respectively after evaporation. Zhang et al. [46] fabri­ cated a molybdenum sulphide (MoS2) loaded CM for solar steam gen­ eration. The experiment was conducted in two ways: (1) MoS2 loaded CM (MoS2-CM) and (2) CM. The test was conducted in a SS as shown in Fig. 9 (b) under natural solar radiation. The mass loss of MoS2-CM and CM alone in the SS was 0.9 g and 0.079 g, respectively. Yin et al. [47] developed a Ti foam with aerogel insulation and PU sponge for seawater evaporation. The broadband absorption of the Ti foam was >97%. The evaporation rate and efficiency of the indoor experiment under simulated radiation of 1 sun was 1.79 kg/m2.h and ~90%, respectively. Additionally, the Ti foam was tested in a SS under natural solar radiation. Four seawater samples (Baltic Sea, World Ocean, Red Sea and Dead Sea) were used in the SS for evaporation. The con­ centration of ions in the seawater samples (Naþ, Ca2þ, Kþ, and B3þ) were reduced to 0.75, 0.06, 0.03, and 0.01 mg/L from 3600, 190, 90, and 0.54 mg/L, respectively. Yang et al. [48] developed Cu2SnSe3 (CTSe) and Cu2SnSe4 (CZTSe) for efficient seawater distillation. The broadband absorption of CTSe and CZTSe were 97% and 95.5%, respectively. Initially the evaporation rate was tested at the laboratory scale with Bohai seawater under simulated solar radiation of 1 sun. The evapora­ tion rates of CTSe and CZTSe were 1.657 and 1.643 kg/m2.h, respec­ tively. Furthermore, the prepared samples were used in a SS under natural solar radiation. The SS efficiently desalted the seawater samples and delivered high quality freshwater. Gong et al. [49] developed air-calcinated melamine sponges (AMSs) for clean water production. Fig. 10 (a & d) shows the photographs of melamine sponges (MS) and AMS, Fig. 10(b–c) SEM images of MS with different magnifications, and Fig. 10(e–f) SEM images of AMS with different magnifications. Poly­ styrene (PS) foam and air-laid papers were used as the insulation layer and water transport layer, respectively. Initially, four different water samples (seawater, river water, strong acid contaminant, and alkaline waste) were tested under simulated solar radiation of 1 sun. The evap­ oration rate of seawater, river water, strong acid and alkaline waste­ water were 1.91, 1.88, 1.81 and 1.75 kg/m2.h, respectively. Increasing contamination decreased the evaporation rate. The prepared material was also applied in the SS under natural solar radiation. It was seen that the SS with AMS absorber could produce the fresh water rate 5–8 kg under 10 h of direct solar exposure.

simulated solar radiation of 10 suns. The indoor evaporation rate of BG-NPs was 12.74 kg/m2.h and the calculated evaporation efficiency was 90.3%. In addition, the BG-NP was tested in the SS to evaporate synthetic seawater under natural solar radiation. The daily total pro­ ductivity was 7.3–8.0 kg/m2.day with a corresponding efficiency of 80.4–82%. The salt concentrations were decreased 4–5 orders of magnitude from the initial level and found to be acceptable according to the WHO drinking quality standards. Finally, a real seawater sample was tested in the SS and the ion concentrations were reduced 4 to 6 orders of magnitude after evaporation. 3.1.3. Semiconductor based materials in SSs Yi et al. [43] prepared a black Al–Ti–O nanostructure for effective photo-thermal conversion for seawater desalination. The transmission electron microscope (TEM) images with different ball milling time of Al–Ti (AT) are shown in Fig. 8(a–f). The evaporation rate and the solar thermal conversion efficiency of the black Al–Ti–O were 1.03 kg/m2.h and 82.3%, respectively. Furthermore, the Al–Ti–O was tested in a sun-tracking system enabled solar distiller for synthetic seawater evap­ oration under natural solar radiation. The productivity of Al–Ti–O was 4 L/m2.day in 8 h of solar operation. The concentration of primary ions in the seawater (Naþ, Kþ, Mg2þ, Ca2þ, Ti4þ, Al3þ) were reduced six orders of magnitude after desalination. Wu et al. [44] developed a cotton-CuS-agarose aerogel for solar steam generation. The evaporation rate corresponding to 10, 15, 20, 25, and 30 mg CuS was 1.45, 1.51, 1.57, 1.62, and 1.63 kg/m2.h respec­ tively under simulated solar radiation of 1 sun. The maximum evapo­ ration rate of 7.16 kg/m2.h under simulated solar radiation of 10 suns was achieved with 30 mg CuS mass concentration. The CuS-agarose aerogel was also tested under natural solar radiation in a SS as shown in Fig. 9 (a). The outdoor result revealed that the CuS-agarose aerogel produced 1.03 kg/m2.h fresh water during 5 h of solar operation. Mu et al. [45] fabricated a Cu2ZnSnS4 (CZTS) absorber for solar steam generation. The light to heat conversion efficiency was high and its heat loss to the bulk water was low due to the polyurethane (PU) foam and these yielded an evaporation rate of 1.46 kg/m2.h under simulated solar radiation. CZTS was also tested in a SS under natural solar radiation floating on seawater. The top surface temperature of CZTS was increased to 44 � C within 3 min of initiation of solar radiation. The highest evap­ oration efficiency of CZTS in the SS was 58%. The concentration of

Fig. 7. (a–c)SEM images of BG-NPs with different magnification [42]. 6

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Fig. 8. (a,d) TEM images Al–Ti (AT) ball milling time-1 min, (b,e) AT-10 min, and (c,f) AT-20 min [43].

Fig. 9. (a) CuS coated on cotton in a SS [44], and (b) Schematic view of SS with CM [46].

Fig. 10. (a) Photograph of MS, (b–c) SEM images of MS with different magnifications. (d) Photo of AMS and (e–f) SEM images of AMS with different magnifi­ cations [49].

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3.1.4. Salt-rejection based porous fabric material in SSs Ni et al. [50] developed a salt-rejecting floatable SS for seawater evaporation. A hydrophilic black cellulose fabric wick was used as an absorber for the SS. Beneath the black absorber, expanded polystyrene foam (EPF) was used to insulate the SS. They fabricated SS designs for each of the indoor and outdoor experiments. The dimensions of the in­ door absorber were 21 cm � 21 cm. The indoor experiment evaporated fresh water and salt water under simulated solar radiation. The effi­ ciencies of the indoor SS with fresh water and salt water were 57�2.5% and 56�2.5%, respectively. Additionally, a large SS of area 0.3025 m2 was tested outdoor under natural solar influx. The productivity and ef­ ficiency of the outdoor model were 2.8 L/m2.day and 24%, respectively. Finally, the rooftop SS was allowed to float on Pleasure Bay (USA) for evaporation as shown in Fig. 11(a–b). The floatable and salt rejecting SS produced 2.5 L/m2.day and the calculated efficiency was 22%. Liu et al. [51] demonstrated a plasmonic active-filter paper (PP) for seawater evaporation. The broadband absorption of the PP was 88.5% (300–2500 nm). Here, the tripolycyanamide sponge was used as a hy­ drophilic and insulating layer as well. A filter paper made of cotton was used to promote water transport to the absorber. The material was tested under simulated solar radiation of 10 suns. The PP was capable of evaporating 12.46 kg/m2.h and the corresponding evaporation effi­ ciency was 89%. The fabricated material (as shown in Fig. 12 (a)) was used in a SS (Fig. 12 (b)) to evaporate the seawater under natural solar radiation. The maximum recorded PP’s top surface temperature was 65 � C. The salinity level before and after of the water is shown in Fig. 12 (c). The total productivity was 5.93 kg/m2.h (8:00–18:00). The SS reduced the salinity level 3–4 orders of magnitude after evaporation.

the black painted CSS (94%). Thakur et al. [54] developed an Al2O3 NP absorber and tested it in a SS for desalination. The experiment was conducted in three different ways, (1) Al2O3 directly doped in the black paint and (2) Al2O3 mixed with water as nano-fluid and (3) Al2O3 doped in black paint and Al2O3 mixed with water as a nano-fluid. The com­ bined effect of Al2O3 in the black paint and as a nano-fluid gave a pro­ ductivity of 5.56 L/m2.day and the CSS was 4.47 L/m2.day. Mahian et al. [55] developed Cu and SiO2 water nano-fluids and tested them in the SS. Two flat plate collectors (FPCs) were fabricated and linked with the SS through a heat exchanger (HX) as shown in Fig. 13 (a). The nano-fluids were pumped through the FPC’s pipes and heated up. The heated nano-fluids transferred the heat into the SS water though a HX. The Cu/water nano-fluid showed better performance than the SiO2/water nano-fluid. This is because the thermal conductivity of the Cu NPs are 400 W/m.K, while the SiO2 NPs are 1.4 W/m.K. Sukla and Modi [56] developed a hybrid SS (HSS) (single slope at lower basin & double slope for upper basin) for clean water production as shown in Fig. 13 (b). The lower basin had a ZnO nano-fluid and the upper portion used MgCl2 solution as a liquid desiccant. The water evaporated from the lower basin and condensed on the bottom surface of the upper basin. Therefore, the latent heat rejected from the lower basin was utilized by the upper portion for evaporation. The liquid desiccant absorbed the latent heat to evaporate additional water. Here, Arabian Sea water was used. The fresh water yield in the lower and upper basin was 0.62 L/m2.day and 1.3 L/m2.day, respectively under the average solar influx of 642 W/m2. Arunkumar et al. [57] experimentally investigated a SS with CuO-nano-coated absorber plates (NCAPs) to increase the water tem­ perature. The CuO-NCAP was integrated with different thickness poly­ vinyl alcohol (PVA) sponges floating on the water surface. The SS with CuO-NCAP exhibited higher productivity than the CSS. The larger thickness of PVA sponges reduced the productivity. Kabeel et al. [58] experimentally studied Cuprous oxide nanoparticles (Cu2O-NPs) doped in black paint and coated on the basin of the SS. The weight concen­ trations of the prepared Cu2O-NPs were 10%, 20%, 30% and 40%, respectively. An efficiency of 25% was achieved at 40% weight fraction of Cu2O. Kabeel et al. [59] studied 0.1 wt% TiO2-NPs mixed with black paint and coated on the basin of the SS. The TiO2-NPs in the black paint gave a higher productivity of 6.6 L/m2.day than the uncoated SS (5.9 L/m2.day). The nanoparticles in the black paint enhanced the ab­ sorptivity and transferred heat better to the bulk water. Abdelal and Taamneh [60] mixed 2.5 wt% graphene, 2.5 wt% carbon nanotube (CNT), and 5 wt% of CNT with black paint and coated them the bottom surface of three SSs. The productivities of 2.5 wt% graphene, 2.5 wt% CNT, and 5 wt% of CNT were 2.3, 2.7 and 3.3 kg/m2.day, respectively. The calculated maximum efficiency of 83.7% corresponds to 5 wt% CNT. The maximum evaporation efficiency was due to broadband ab­ sorption of CNTs.

3.2. Energy exchange materials for desalination 3.2.1. Nanoparticles in a SSs Nazari et al. [52] experimentally studied a thermo-electric cooling enabled Cu2O nano-fluid in a SS. The thermo-electric cooling reduced the condensing cover temperature, thereby increasing evaporation rate with the larger temperature difference between water in the basin and condensing glass cover (ΔT). Two identical SSs of area 0.50 m2 were designed and tested under natural solar radiation. Two different Cu2O concentrations (0.04 wt% and 0.08 wt%) were examined in the SS. The thermo-electric cooling powered, 0.08 wt% Cu2O nano-fluid in the SS exhibited the maximum productivity of 1.6 L/m2.day with 39.2% pro­ ductivity enhancement over the conventional SS (CSS). Sharshir et al. [53] analyzed a SS with graphite and CuO nano-fluids for heat transfer enhancement. Three identical SSs were designed each with an area of 0.25 m2. The CuO nano-fluids (0.5% CuO þ water) and graphite nano-fluids (0.5% graphite þ water) were used in the basins of two of the SSs. The efficiencies of the graphite nano-fluid, CuO nano-fluid and CSSs were 41.8%, 38.61% and 32.35%, respectively. This was due to the absorptivity of the graphite nano-fluid being higher (99.5%) than the CuO nano-fluid (96%), which in turn was higher than

Fig. 11. (a–b). Schematic and photographic views of the self-floatable salt-rejecting SS [50]. 8

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Fig. 12. (a) Plasmonic active filter paper during illumination (visible (left) and infrared (right), (b) PP tested under natural solar radiation, and (c) salinity level before and after desalination [51].

Fig. 13. (a) Nano-fluids in the solar collector integrated HX SS [55], and (b) Photograph of hybrid solar still [56].

Fig. 14. (a) SS with PCM in the specially designed reservoir [62], (b) SS with shape stabilized PCM in the basin [63], (c) TSS with CPC and PCM in the basin [64], and (d) CCC-SSSS with PCM in the copper balls [68]. 9

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3.3. Energy storage materials for desalination

3.3.2. Nano-enabled phase change material in the SS The mixing of NPs into the PCM dramatically improved the heat transfer rate by increasing the specific surface area [69,70]. Kabeel et al. [71] developed a hybrid storage material (paraffin wax þ graphite NPs) for enhancing the thermal conductivity. The thermal conductivities of the paraffin wax and graphite were 0.24 W/m.K and 195 W/m.K, respectively. Four different concentrations of graphite nanoparticle of 5%, 10%, 15% and 20% were mixed with 17.5 kg of paraffin wax. The productivities of the SS with NPs were 7.4 L/m2.day (5%), 7.9 L/m2.day (10%), 8.2 L/m2.day (15%) and 8.52 L/m2.day (20%). The calculated daily efficiency of the hybrid SS was 65.13% in 20% graphite concen­ tration with 17.5 kg of paraffin wax. Rufuss et al. [72] experimentally studied TiO2, CuO, and GO NPs mixed with PCM. The productivities of the pure paraffin wax, paraffin wax þ TiO2, paraffin wax þ CuO, and paraffin wax þ GO were 3.92, 4.94, 5.28 and 3.66 L/m2.day. The SS with CuO doped paraffin wax exhibited the highest productivity of 5.28 L/m2.day. Rufuss et al. [73] developed a CuO NP (0.3 wt %) doped paraffin wax as a NEPCM in a SS. The NEPCM SS had a higher pro­ ductivity (5.28 L/m2.day) than the SS with pure PCM (3.92 L/m2.day). It was inferred that the NEPCM rejected more latent heat during the phase transition.

PCMs are capable of storing and rejecting heat during the melting and solidification processes [61]. The basin temperature increases as solar radiation intensity increases during the day hours. The PCM in the SS starts to melt when the basin temperature exceeds the PCM’s melting temperature. After this, the PCM stores further sensible heat. Afterward, the solar intensity decreases, and the PCM releases first the sensible heat and then the latent heat to the water and regains its solid form. This improves the evaporation after sunset. PCMs are promising candidates for storing the thermal energy for evaporation and other applications. Normally, paraffin wax (PW) used as a PCM in the SS. Typically the wax does not contact the water directly and is instead stored in an additional tray in the basin. 3.3.1. Phase change material in the SS Yousef and Hassan [62] experimentally studied a SS integrated with PCM, pin fins (PF) and steel wool fiber (SWF) for evaporation enhancement. Paraffin wax was used as the PCM beneath the SS as shown in Fig. 14 (a). The experiment was conducted in the following configurations: SS with PCM, SS with PCM-PF and SS with PCM-SWF. Mediterranean Sea water samples were fed into the SS for evaporation under natural solar radiation. The melting point of the selected PCM was 56 � C. The PFs were used in the SS to increase the surface area as well as solar energy absorption. The SWF acted as a thermal conductivity enhancer of the SS. The fresh water production rates of the CSS, SS with PCM, SS with PF-PCM and SS with PCM-SWF were 3.2, 3.5, 3.8, and 3.6 L/m2.day, respectively. Cheng et al. [63] experimentally tested a SS with shape-stabilized PCM (SSPCM) to enhance the thermal conductiv­ ity. The SSPCM consists of 5% expanded graphite to increase the broadband absorption (94%) for efficient photo-thermal conversion. A schematic view of the pyramid SS integrated with SSPCM is shown in Fig. 14 (b). The productivities of the CSS and modified pyramid SS were 2.3 L/m2.day and 3.4 L/m2.day, respectively, under natural solar radi­ ation. Furthermore, the productivity increases with increasing SSPCM thermal conductivity from 0.2 to 2 W/m.K. Arunkumar and Kabeel [64] experimentally studied a concentrator assisted tubular SS (TSS) with paraffin wax as shown in Fig. 14 (c). The paraffin wax was put around the basin. The melting point of the PCM was 58–60 � C. The PCM loaded TSS was placed at the focus line of the compound parabolic concentrator (CPC) for desalination. The paraffin wax in the TSS gave a higher pro­ ductivity (5.7 L/m2.day) than without PCM (5.3 L/m2.day). Kumar et al. [65] analyzed fins and PCM in a SS. A layer of paraffin wax was placed beneath the still basin. An identical SS except for no PCM was built for comparison. The efficiency of the CSS, SS with fins, and SS with fins and PCM were 23%, 36% and 54%, respectively. Faegh and Shafii [12] developed a SS with PCM and evacuated tubes, the latter of which reduced heat losses, increasing the water tempera­ ture. The combined performance of PCM and evacuated tubes resulted in a productivity of 6.555 kg/m2.day with an evaporation efficiency of 50%. Kabeel and Abdelgaid [66] tested a SS with an oil HX and PCM. A solar collector was used to heat the oil which flowed through the HX. The HX was immersed in the SS water. The productivity of the SS enhanced with the PCM and oil HX was 10.77 L/m2.day and the CSS was 4.48 L/m2.day. Al-harahsheh et al. [67] experimentally studied a SS with PCM tubes in the basin. The SS was integrated with a solar collector to increase the water temperature. The solar collector integrated PCM had a productivity of 4.3 L/m2.day. Arunkumar et al. [68] experimen­ tally studied the performance of a PCM in a CPC-concentric circular tubular SS-pyramid SS (CPC-CCTSS-PSS) and a compound conical concentrator coupled with a single slope SS (CCC-SSSS). The PCM was loaded in six copper balls and the balls were placed in the basin water as shown in Fig. 14 (d) for the CCC-SSSS. 25 g of paraffin was used in each of the copper balls. The productivities of the PCM enabled CPC-CCTSS-PSS and CCC-SSSS were 3.2 L/m2.day and 7.3 L/m2.day, respectively.

3.4. SSs with sensible energy storage materials 3.4.1. Fin type absorber in the SS Jani et al. [74] experimentally studied a double slope SS with cir­ cular and square fins in the basin. Three different water depths were maintained in the basin of 10 mm, 20 mm and 30 mm. The pro­ ductivities of the 10 mm, 20 mm, and 30 mm water depths for the cir­ cular fin integrated SS were 1.4 L/m2.day, 1.2 L/m2.day and 1.3 L/m2. day, respectively. Similarly, the productivities of the square finned SS were 0.967, 0.936, and 0.950 L/m2.day, respectively for the different depths. The circular fins with 10 mm water depth yielded the maximum productivity. The fins improved the energy absorption and enhanced the heat transfer to the water. Manokar and Winston [75] experimentally studied a SS with fins in the basin. The productivities of the SS with and without fins were 2.64 L/m2.day and 2.34 L/m2.day, respectively. El-Sebaii and El-Naggar [76] experimentally and theoretically studied a finned basin made of aluminium, iron, copper, glass, stainless steel, mica and brass. The finned basin yielded a productivity of 5.065 L/m2.day and the basin without fins was 4.235 L/m2.day. 3.4.2. Sand energy storage in a solar still Kabeel et al. [77] experimentally studied a sand-based energy stor­ age in a SS ((Fig. 15 (a)). Two identical SSs were designed and tested under the same climatic conditions. Jute cloth was added to utilize capillary action to ease evaporation. The sand kept the water warm for a prolonged time. The jute cloth with sand energy storage evaporated 5.9 L/m2.day of water under natural solar radiation. 3.4.3. Sponges in a solar still Arunkumar et al. [78] experimentally tested a SS with carbon impregnated foam (CIF) with bubble-wrap (BW) insulation as shown in Fig. 15 (b). The research was extended to analyze the SS with wood insulation and without any insulation. All four SSs were tested under natural solar illumination. The productivity of the SS, SS with BW insulation, SS with BW-CIF and SS with wooden insulation were 1.9, 2.31, 3.1, and 2.2 L/m2.day, respectively. The thermal conductivity of the wooden material and BW were 0.13 and 0.02 W/m.K, respectively. Sellami et al. [79] experimentally studied different thicknesses of blackened sponge layers in the SS. The different sponge thicknesses were 0.5, 1.0 and 1.5 cm. The SS with 0.5 cm thickness of the sponges exhibited the highest productivity of 4.8 L/m2.day. 3.4.4. Cotton based energy storage in a solar still Sakthivel and Arjunan [80] analyzed cotton material as an energy 10

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Renewable and Sustainable Energy Reviews 115 (2019) 109409

Fig. 15. (a) Solar still with sand storage [77], (b) carbon foam [78] and (c) cotton cloth [80].

storage medium in a SS shown in Fig. 15 (c). The cotton cloth thicknesses were 2, 4, 6, and 8 mm. The best result was obtained at 6 mm thickness and the efficiency was 23.8%.

friendly features, making it a good candidate for wastewater evaporation. Liu et al. [85] prepared carbonized wood (Cunninghamia lanceolata) for solar steam generation. The solar absorptivity of untreated wood and carbonized wood (CW) were 53.3% and 97.6%, respectively. The wood slices were carbonized at 500 � C and 900 � C (CW 500 � C and CW 900 � C). The CW-500 � C exhibits better solar steam generation than the CW-900 � C because the CW-500 � C has better broadband absorption (97.6% versus 97.3%). Zhu et al. [86] developed a wood-based absorber to evaporate ground water and seawater. The evaporation efficiencies of ground water under 1 sun and 10 sun illuminations were 57.3% and 80.4%, respectively. Xue et al. [87] prepared carbonized wood for solar steam generation. As the intensity of illumination was increased from 1 sun to 3 suns, the evaporation rate of the flame-treated carbonized wood (F-wood) increased from 1.05 to 3.46 kg/m2.h. It was also concluded that the solar thermal efficiency of the F-wood was 72% at 1 sun illu­ mination. Liu et al. [88] developed a wood-based absorber material for high-performance solar steam generation. They cut the wood in the longitudinal (across the grain) and horizontal direction (along the grain) then carbonized it (C-L wood; C–H wood). The evaporation efficiency of C-L wood under 10 sun illumination was 89%. Chen et al. [89] prepared a CNT-flexible wood membrane (F-wood/CNTs) for desalination by solar steam generation. The evaporation rates of F-wood/CNTs under 1, 3, 5, 7 and 10 suns were 0.95, 2.88, 5.14, 7.65 and 11.22 kg/m2.h. Liu et al. [90] prepared a GO for solar steam generation. The aqueous GO solution was coated on the Basswood surface (GO-wood composite) for photo-thermal conversion. The evaporation efficiency of wood-GO and wood were 82.8% and 59.5% under 12 sun illumination. Dongfang and Xiuchun [91] prepared TiN-NP coated carbonized Beech wood for solar water evaporation. The solar vapor conversion efficiency was 92.5% under 1 sun illumination. Zhu et al. [92] used a carbonized daikon (type of vegetable) for efficient solar absorption and vapor generation. The 5.5 mm thickness of carbonized daikon had an evaporation rate of 1.38 kg/m2.h and a corresponding steam generation efficiency of 74%. They reported that the carbonized daikon can be used for removal of dyes in a wastewater treatment process. The summary of the materials is shown in Table 1.

3.4.5. Black stones in a solar still Mohamed et al. [81] experimentally analyzed different sizes of fine black basalt stones with diameters of 1, 1.5 and 2 cm in a 1 m2 SS. The porous stones acted as sensible heat storage to increase the productivity. The SS with 2 cm stones yielded the maximum productivity (1.05 L/m2. day), as compared to 1.5 cm (1.0 L/m2.day) and 1 cm (0.9 L/m2.day). 4. Discussion In this work, the recent development of efficient materials in SSs are reviewed over the last three years from 2017 to April 2019. Herein, the materials were classified as (1) direct solar steam generation highly efficient materials, (2) energy exchange materials including nano­ particles & nano-fluids, (3) energy storage materials including PCMs and NEPCMs and (4) other sensible energy storage materials for desalination. 4.1. Efficient steam generation materials This criterion was mainly focused on a material’s broadband ab­ sorption being >90% [25]. Photo-thermal is a type of conversion process that produces thermal energy by absorption of light. Superior light to heat conversion process is found in metallic materials, semiconductors, carbon-based materials and polymers. Efficient photo-thermal conver­ sion achieved by the material is due to incident photon frequency matching the natural frequency of the electron in the nanoparticle sur­ face [25]. The materials should possess the following characteristics to work well for solar applications: (1) broadband absorption of incoming solar radiation, (2) good water transport, and (3) stability. The evapo­ ration system comprises of absorber medium on the top and a water transport layer below (porous). Heat is generated by the incident light at the top side of the absorber for efficient evaporation. In a traditional evaporation system, the absorptivity of the black paint was reported as 94%. However, efficient materials including VACNTs [29], Ti nanorods [30,47], CMF [31], graphite flakes/CF [36], MWCNTs [37], srGA [35], Al-NPs [38], Au-NPs [39,41], BG-NPs [42], and Cu2SnSe4–Cu2SnSe3 [48] had absorptivity >95%. Only the carbon based materials exhibited absorptivity of incoming solar radiation >97% [29,35,36]. Many ex­ periments were conducted with broadband absorbing materials under simulated radiation for wastewater evaporation. Finnerty et al. [82] developed a synthetic GO leaf for solar desalination through steam generation. Liu et al. [83] developed paper-based carbon particles (C-paper) for solar steam generation. The maximum evaporation rate and efficiency of the C-paper was 0.964 kg/m2.h and 70%, respectively, under 1 sun illumination (1000 W/m2). A novel carbonized lotus leaf solar steam generation was demonstrated by Liao and his co-authors [84]. They also reported that carbonized lotus leaf had good light ab­ sorption characteristics, effective water transport and environmentally

4.2. Energy exchange materials In this section, the prepared nanomaterials were doped into water and black paint for heat transfer enhancement. The nanomaterials transferred heat to the bulk water when direct contact occurs between them. The nanomaterials minimize the surface energy loss of the system and enhance the thermal conductivity. The nanomaterials possess high specific surface area which enhances the heat conduction of nano-fluids. Stability of nano-fluids is a major issue, which can be ameliorated by adding a suitable surfactant in the base-fluid [93]. The thermal con­ ductivities of various nanoparticles were measured for use in a stills, shown in Fig. 16 [94]. The energy exchange materials, medium, mate­ rial concentration, productivity and efficiency are in Table 2.

11

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Table 1 Advanced absorbers for steam generation in SSs. S. No.

New absorbing Materials

Broadband absorptivity (%)

Type of desalination device

Feed water

Source

Evaporation rate

Efficiency (%)

Ref.

1

VACNTs

99

SS

Seawater

–

–

[29]

2

Ti-nanorods on CF

97

SS with Fresnel lens

3.5 wt% NaCl

–

–

[30]

3

CMF

95

SS

Seawater

–

–

[31]

4

Paper based rGO

90

SS

Synthetic seawater Seawater

Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation Solar radiation

76

[32]

1.15 kg/m .h

–

[33]

2.0 L/m2.day

–

[34]

–

60.2

[36]

–

[37]

1.73 kg/m .h

–

[35]

–

–

[38]

3.36 g/(2.5 h)

–

[39]

–

80

[41]

7.3 kg/m2.day

80

[42]

4 L/m2.day

–

[43]

1.03 kg/m .h

–

[44]

–

58

[45]

–

–

[47]

–

–

[48]

–

–

[48]

5.8 kg/m2.day

–

[49]

5

–

6

Nitrogen doped carbon sponges RGO/MoS2

–

House type & floatable type SS SS

7

Graphite flakes/CF

97

SS

8

Beeswax and MWCNT

–

Seawater Seawater

Pyramid SS

Seawater

9

srGA

99

SS

Seawater

10

Al-NPs

96

House type SS

Seawater

11

Au-NPs

96

SS

Seawater

12

Au-NPs coated porous filter

96

SS

Seawater

13

BG-NPs coated on melamine sponges Black Al–Ti–O

96

SS

–

Sun tracker enabled SS

15

CuS coated cotton

–

House type SS

Synthetic seawater Synthetic seawater –

16

Cu2ZnSnS4

–

Floatable SS

Seawater

17

Ti foam

97

SS

Seawater

18

Cu2SnSe4

95.5

SS

Seawater

19

Cu2SnSe3

97

SS

Seawater

20

Melamine sponges

–

SS

Seawater River water

14

– 2

2

0.90 kg/m .h 2

2

Strong acid water Alkaline waste water

4.3. Energy storage materials

efficiency are in Table 3.

PCMs generally do not contact the water directly and are instead stored in an additional tray [64] in the basin or filled in metal balls [68]. The melting point of the paraffin wax used in SSs is 46–56 � C. Paraffin wax was mostly used in the SSs due to its low cost, low toxicity, high latent heat of fusion, uniform melting and reliability [67]. Moreover, in some cases, NPs were doped into the PCM (NEPCM) to increase the heat transfer [71]. Also adding the NPs to the PCM reduced the melting and solidification temperatures of the PCM [72]. Rufuss et al. [72] studied a mixture of paraffin wax and TiO2 in the SS. The paraffin wax’s melting and solidification temperatures before adding TiO2 were 63.5 and 59 � C. The melting and solidification values were reduced to 58.5 and 55 � C after adding the TiO2 nanomaterial [71]. Kabeel et al. [71] tested paraffin wax mixed with graphite NPs in a SS. The thermal conductivity of the paraffin wax and graphite NPs were 0.24 W/m.K and 195 W/m.K, respectively. The mixture of NPs and paraffin wax had higher thermal conductivity than the wax alone. However, the uniform distribution of the NPs in the paraffin wax is not stable over the time period of days. During the experimentation, the metal NPs settled down at the bottom of the storage container. This reduced heat transfer. The energy storage materials, dopant, melting point, concentration, productivity and

4.4. Other materials Further, other types of sensible energy storage materials like fins [74], sand [77], CIF [78], blackened sponges [79], cotton cloth [80], and black stones [81] have been investigated in recent years for solar distillation. These sensible materials are capable of increasing surface area and enhancing the thermal properties of water in the basin. These materials act as a heat sink and source for evaporation. Sponges of greater thickness decreased the productivity [57], because they reduced the absorptivity of the blackened basin. Rust formation is possible when using stones in the basin. 5. Effect of nanomaterials on the environment and health Nanomaterials play a significant role in improving the environ­ mental and human health nowadays. The developments of nano­ materials to improve day-to-day life are depicted in Table 4. At the same time the negative impact to the environment and health aspects are unavoidable. The long exposure of Ag-NPs induces liver damage in Japanese rice fish [95]. Several studies discuss SiO2 environment 12

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6. Nanomaterials used in various energy sectors Nanomaterials are currently used in various energy sectors due to their unique properties [100–104]. The advantages of nanoparticles in different energy-related sectors including (1) energy production, (2) energy conversion, (3) energy storage and transfer, (4) thermal man­ agement and (5) environment and health are illustrated in Table 4. Each section is sub-divided into applications and materials. In the energy production sector, a nano BaSO4 coating was used as a radiation shielding in a nuclear power plant [105]. Nano Al–Zn–In composites were used as a corrosion resistant coating in tidal power plants [106]. CNTs and polymer composites are used to construct lighter weight wind turbine blades [107]. Nano black silicon is commonly used to fabricate high efficient PV cells [108]. In addition, Ti–Ni nano-composites are used as a water resistant material in geothermal energy production [109]. In energy conversion, Chromium Carbide (Cr3C2), Titanium Carbide (TiC), Molybdenum Carbide (MoS2) nanomaterials are used a corrosion resistors in gas turbines [110]. Nanomaterials such as Li, TiO2, Fe and Ni participate in hydrogen production and storage applications [111]. Plasmonic nanoparticles including Au, Ti are used in a photo-thermal conversion process in wastewater evaporation, steriliza­ tion and desalination [41]. Bismuth (III) Telluride (Bi2Te3), Cobalt Tri­ antimonide (CoSb3) and Silicon–Germanium (SiGe) nanocomposites play a significant role in thermo-electric power generation [112]. Li–Ni nanomaterials are used in Li-ion batteries for power grids [113]. Re­ searchers are working towards the development of efficient super­ capacitors with activated carbon, CNT, and graphene nanomaterials [114]. Carbon-based graphene nanomaterials are used in the fabrication of micro-fuel cells for electronic devices [115]. Moreover, CuO/ethylene glycol, Al2O3/ethylene glycol, Ag, Cerium Oxide (CeO2), and Tungsten disulfide (WS2) nanomaterials were used to construct efficient fuel tanks to reduce hydrogen emission to the environment [104,116]. A mixture of PCM with TiO2, CuO, and GO nanomaterials used in thermal storage applications [73]. Silica nanomaterials were used to fabricate a light weight and low thermal conductivity aerogel for efficient thermal insulation applications [117]. Nanomaterials including Cu, Ag, TiO2, and SiO2 have played a significant role in the development of energy efficient green buildings [118]. Further, Ag, Au, and SiO2 nanomaterials are used in light-emitting diodes (LEDs) and organic light-emitting di­ odes (OLEDs) [104,119,120]. TiO2/SnO2, TiO2/CuO, Au and Ag nano-heterostructures were used to develop gas sensors [105,121]. Moreover, CuO, Al2O3, and CNTs were used for industrial heating ap­ plications [122]. Ag, Au, Si, Pt and Pd nanomaterials have been used to fabricate a smart packaging of food [123]. Gold nanomaterials are used for soil research in agricultural sectors [124]. In addition, Au, Ag2O [124], Ag, Cu, Zr, and Si nanomaterials have been used for cancer treatment [125,126]. Further, nanomaterials including Iron, Zn–Fe2O4,

Fig. 16. Graphical view of different nanomateirials with thermal conductiv­ ity [94].

impacts. Microorganisms were slightly affected when the concentration of SiO2 exceeds few mg/L. Inhalation of SiO2 causes lung disease in human [96]. Some studies indicate that there is no adverse effect of TiO2 on environment and humans [97]. However, TiO2 was identified as harmful if the concentration was 10–100 mg/L [97]. TiO2 is commonly used in sunscreen lotions because TiO2 absorb UV rays in sunlight [98]. However, this study indicates that there is still the possibility of DNA damage in human cells during sun exposure [98]. There is a negligible impact on organisms with low concentrations of ZnO nanoparticles. However, a high dose of ZnO (>70 mg/day) causes lung cell death in rats. This effect is still being actively investigated [97,99]. In addition, the metal oxide nanoparticles such as ZnO and TiO2 destroyed useful bacterial species (organic decomposers) in the environment [95]. Inhalation of 20 nm sized Al2O3 creates infection in rats’ lungs [97]. The inflammatory, fibrotic and genotoxic effects caused by the inhalation of CNTs have been studied in mice and rats. Inflammation and pulmonary fibrosis have been closely associated with the risk of lung cancer. One study on female mice found that exposure to SWCNTs increased the chance of miscarriage. Further studies reported that skin irritation of humans was caused by contact with CNTs [98]. Severe histopathological changes (liver and gill) on juvenile carp species (Cyprinus Carpio) when the ZnO intake was >50 μg/L [98].

Table 2 Summary of energy exchange materials involved in SSs. S. No.

Material

Medium

Material Concentration

Productivity (L/m2.day) [CSS/ Rf.]

Productivity (L/m2.day) [with NPs]

Efficiency with NPs (%)

Design

Ref.

1 2 3 4 5

Cu2O Graphite CuO Al2O3 Al2O3

0.04–0.08% 0.5% 0.5% – –

– 2.6 2.6 4.4 4.4

1.6 – – 5.2 5.5

39.2 41.8 38.6 – –

SSSS SSSS SSSS SSSS SSSS

[51] [53] [53] [54] [54]

6 7 8 9 10 11 12 13 14

Cu SiO2 ZnO CuO Cu2O TiO2 Graphene CNT CNT

Water Water Water Black paint Water & black paint Water Water Water SS plate Black paint Black paint Black paint Black paint Black paint

– – – – 10% 0.1 wt% 2.5 wt% graphene 2.5 wt% CNT 5 wt% CNT

– – – 2.1 3.6 6.2 – – –

– – – 2.9 4.2 6.6 3.3 – –

– – – – 25.0 – 83.7 – –

SSSS SSSS HSS SSSS SSSS SSSS SSSS SSSS SSSS

[55] [55] [56] [57] [57] [59] [60] [60] [60]

13

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Renewable and Sustainable Energy Reviews 115 (2019) 109409

Table 3 Summary of energy storage materials tested in SSs. S. No.

Material

Enhancer

Melting point of PCM (� C)

Material concentration

Productivity (L/m2.day) [CSS/Ref.]

Productivity (L/m2.day) [With PCM]

Efficiency with PCM (%)

Design

Ref.

1

Paraffin wax Paraffin wax SSPCM Paraffin wax Paraffin wax Paraffin wax Paraffin wax Paraffin wax Paraffin wax Paraffin wax Paraffin wax Paraffin wax Paraffin wax Paraffin wax Paraffin wax Paraffin wax Paraffin wax Paraffin wax

–

56

–

3.2

3.6

32

SSSS

[62]

Pin-fins

56

–

3.2

3.8

47

SSSS

[62]

Graphite –

46 58–60

5% graphite –

2.3 –

3.4 5.7

43 –

SSSS TSS

[63] [64]

–

–

–

–

6.5

50

SSSS

[12]

–

56

–

4.4

10.7

–

SSSS

[66]

–

–

4.3

25

SSSS

[67] [68]

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

– –

58–60

–

1.9

7.3

–

Fins

56

–

–

–

44

CPCCCTSS-PSS SSSS

Graphite NPs Graphite NPs Graphite NPs Graphite NPs TiO2 NPs

56

5% graphite

4.3

7.4

54

SSSS

[71]

56

10% graphite

4.3

7.9

59

SSSS

[71]

56

15% graphite

4.3

8.2

62

SSSS

[71]

56

20% graphite

4.3

8.5

65

SSSS

[71]

59–63.5

0.3 wt% TiO2

~1.3

3.9

–

SSSS

[72]

CuO NPs

59–63.5

0.3 wt% CuO

~1.3

4.9

–

SSSS

[72]

GO NPs

59–63.5

0.3 wt% GO

~1.3

5.2

–

SSSS

[72]

–

59

–

–

1.9

–

SSSS

[73]

CuO NPs

63.5

0.3 wt% CuO

–

2.6

35

SSSS

[73]

[65]

Table 4 Nanomaterials in different fields (S. No. is study number). S. No.

Energy production

Energy conversion

Energy storage and transfer

Thermal management

Environment and health

Applications

Materials

Applications

Materials

Applications

Materials

Applications

Materials

Applications

Materials

1

Radiation shielding in nuclear power station Corrosion resistance in tidal power plants

BaSO4 [105]

Corrosion resistance in gas turbines

Cr3C2, TiC, and MO2C [110]

Li-ion batteries and power grids

Co3O4 [113], Ni, and Li [130]

Aerogel insulation

Silica [117]

Food processing

Al-Zn-In Ref. [106], Ag, TiO2, SiO2 [104]

Electrodes in fuel cells

Graphene [114]

Super capacitors

Activated carbon, CNT, and graphene [131]

Buildings

Agricultural

3

Lighter and stronger wind turbine blades

Hydrogen production and storage

Li, TiO2, Fe, Ni [111]

Micro-fuel cells

Graphene [115]

Sensors

4

Nano-PV solar cells

CNT/ Polymer nano composites [107] Black silica [108]

Cu-NPs and Cu nanowires [118], Ag, TiO2 and SiO2 [104] TiO2/SnO2 TiO2/CuO [121], Au and Ag [104]

Ag-NPs [123], Au, Si, Pt, and Pd [104] Ag-NPs [124]

Photothermal conversion

Au-NPs, Ti foam [41],

Fuel tanks for reduced hydrocarbon emission

LED, OLEDs and electronic devices

Ag-NPs [119], ZnS [120], Au, SiO2, and ZnO [104]

Household water treatment

5

Geothermal energy-water resistant equipment

Ti-Ni [109]

Thermoelectric power generation

Bi2Te3, COSb3 and SiGe [112]

Energy storage

CuO/ethylene glycol and Al2O3/ethylene glycol [116], Ag, CeO2, WS2 [104] PW þ TiO2, CuO, and GO [73]

Industrial water heating process

CuO, Al2O3 and CNT nano-fluids [122]

Disaster management

2

14

Medical (cancer treatments)

Au-NPs, Ag2O-NPs [125,126], Ag, Cu, Zr, and Si [104] Iron, Zn–Fe2O4, Ti-Ag [127], activated carbon [104] Ce–NiO [128]

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Renewable and Sustainable Energy Reviews 115 (2019) 109409

Ti–Ag, and activated carbon facilitate household wastewater treatment [127]. Cerium doped NiO (Ce–NiO) nanomaterials are used for making sensors for poisonous gas based disasters [128].

Acknowledgements The authors thank the National Natural Science Foundation of China (21573193 and 21603188). The authors also thank the Key Projects for Research and Development of Yunnan Province (2018BA065), Yunnan Applied Basic Research Projects (2016FD009), and the Industrialization Cultivation Project (2016CYH04). The authors also thank Yunnan Water Conservancy Science and Technology Plan of the Water Resources Department of Yunnan province. Funding was also provided by the national post-doctoral management and Yunnan University: Post­ doctoral approval number 219281. Permission granted by ‘Elsevier’, ‘RSC Publications’, ‘ACS Publica­ tions’, ‘Springer’ and ‘Wiley Publications’ for reproduction of their figures is greatly appreciated.

7. Advantages and disadvantages of energy storage materials One of the ways to store thermal energy is using PCM. Generally, PCMs are classified as organic or inorganic substances. Herein, re­ searchers mostly used paraffin wax as the PCM in SSs, which is organic. The advantages of PCMs include stability, ability to be recycled, noncorrosive, high specific heat, low cost and large latent heat of fusion [28]. However, the disadvantages are low thermal conductivity and requiring proper encapsulation. To improve the thermal conductivity of the PCM, researchers have used nanomaterials within the PCM (NEPCM). Regarding environmental and health aspects, paraffin wax includes formaldehyde and vinyl chloride. Vapors include benzene and toluene, which are identified as an environmental pollutant. In addition, paraffin wax contains benzene, toluene, naphthalene, and methyl ethyl ketone, which can result in severe health impacts [129].

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8. Conclusion and future projections This review covers the last three years of (1) direct solar steam generation highly efficient materials, (2) energy exchange materials including NPs and NFs, (3) energy storage materials including PCMs and NEPCMs and (4) other sensible energy storage materials. The highly solar absorbing materials, including metallic, semiconductor and carbon-based, improved the photo-thermal conversion process. These materials efficiently evaporated seawater, synthetic seawater, river water, strong acid water, and alkaline waste water. The high efficiency in broadband energy efficient materials is due to local surface plasmon resonance effect (LSPR) in plasmonic metals and enhanced heat trapping on the surface of carbon based materials. Energy exchange materials including Cu2O, Al2O3, SiO2, ZnO, TiO2, and CNTs were tested in SSs to enhance the heat transfer with bulk water. PCMs increased energy storage in SSs. However, low thermal conductivity and the requirement of proper encapsulation are the main drawbacks of PCMs. NPs including graphite, TiO2, CuO and GO were doped into paraffin wax to improve the heat transfer. The NPs reduced the melting and solidification tem­ peratures of the PCM. The other sensible materials, including fins, pebbles, cotton cloth, and jute wick, were incorporated into SSs to in­ crease the evaporation surface area. Based on the overall analysis, en­ ergy efficient materials exhibited strong solar absorption (>96%) and reduced heat loss more than energy exchange and storage materials. In energy exchange materials, nanoparticles were dispersed in the liquid or mixed with the black paint to enhance the absorption. This technique improved the evaporation rate somewhat. Sensible materials slightly reduced the absorptivity of the blackened basin. Rust formation is possible when using stones in the basin. Nanotechnology enabled efficient photo-thermal processes have the promise to enable cost effective water treatment in the future. The low greenhouse gas emission freshwater production through solar energy is a much-needed technology to reduce the global water shortage. More­ over, the broadband absorbing materials greatly improve the efficiency over traditional distillation systems. The photo-thermal conversion process is identified as a sustainable and environmentally friendly method of producing fresh water from seawater and polluted water. Commercialization is one of the ways to scale the impact of this research and innovation. The absorber is a priority in this research. High stability and ability to recycle the absorber are important to minimize the eco­ nomic and environmental costs. These systems can be installed in the natural disaster sites, islands, and remote areas to deliver clean water. Declaration of interest There are no conflicts of interest to declare. 15

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