Comparative Solar Steam Generation using plasmonic, optical-band transition and localized heating mechanisms

Comparative Solar Steam Generation using plasmonic, optical-band transition and localized heating mechanisms | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 31 January 2024 V1 Latest version Share on Comparative Solar Steam Generation using plasmonic, optical-band transition and localized heating mechanisms Authors : Humberto Palza 0000-0001-5246-6791 [email protected] and Patricio Burdiles Authors Info & Affiliations https://doi.org/10.22541/au.170670261.11861277/v1 249 views 215 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Water with three structures was compared for solar-steam generation: a) copper/silica nanoshells (CuSiN) having a plasmonic resonance in the visible region; b) thermal reduced graphene oxide (TrGO) having π-band optical transition; and c) an oxidized carbon foam (OCF) on the top allowing a localized heating. By using an open direct absorbing solar collector under simulated solar radiation, the evaporation rate (in kg/m 2 s) under 3 Suns was 2.1 x 10 -4 for pure water, while values of 5.1, 5.5 and 6.0 were reached for water with CuSiN, TrGO and OCF systems, respectively. Under natural sunlight (60 suns), for pure water an evaporation rate of 7.5 x 10 -4 was obtained after only 10 minutes of irradiation. Noteworthy, this value increased to 1.0 x 10 -2 for the system having the OCF while for all the nanoparticles systems the values were around 8.4 x 10 -3 . Comparative Solar Steam Generation using plasmonic, optical-band transition and localized heating mechanisms Patricio A. Burdiles; Humberto Palza Departamento de Ingeniería Química, Biotecnología y Materiales, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile Advanced Mining Technological Center, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile Beauchef 851, Santiago, Chile. Abstract Water with three structures was compared for solar-steam generation: a) copper/silica nanoshells (CuSiN) having a plasmonic resonance in the visible region; b) thermal reduced graphene oxide (TrGO) having π-band optical transition; and c) an oxidized carbon foam (OCF) on the top allowing a localized heating. By using an open direct absorbing solar collector under simulated solar radiation, the evaporation rate (in kg/m 2 s) under 3 Suns was 2.1 x 10 -4 for pure water, while values of 5.1, 5.5 and 6.0 were reached for water with CuSiN, TrGO and OCF systems, respectively. Under natural sunlight (60 suns), for pure water an evaporation rate of 7.5 x 10 -4 was obtained after only 10 minutes of irradiation. Noteworthy, this value increased to 1.0 x 10 -2 for the system having the OCF while for all the nanoparticles systems the values were around 8.4 x 10 -3 . Keywords: Solar steam generation; plasmonic; nanofluids; Carbon foam; direct absorbing solar collector 1. Introduction Solar energy is one of the most promising renewable energy sources because it is abundantly available and no pollutants are generated during its use [1]. Highly efficient steam generation is a key solar-energy application allowing the development of systems for small-scale water purification, solar still, hygiene processes, large-scale solar-power concentrating systems, chemical plants, and desalination, among other technologies [1]. Since the optical properties of pure water are not appropriate for direct solar absorption, different approaches are used to improve its thermal efficiency. One of the most relevant approaches is the addition of metal nanoparticles having a resonant photon-induced collective oscillation of their valence electrons named surface plasmon resonance (SPR) [2]. The large optical polarization associated with the SPR results in a large local electric field enhancement at the nanoparticle surface as well as in a strongly increase in light absorption and scattering processes comprising its optical extinction [3,4]. Indeed, during the decay process, nanoparticles form energetic charge carriers able to either be transferred to the media as scattered light or be relaxed locally heating the nanostructure [2,4]. The non-radiative component of the extinction, i.e. plasmon absorption, is therefore efficiently converted via electron-electron and electron-phonon relaxations to heat on the time scale of picoseconds [3,4]. Therefore, when some metal nanoparticles dispersed in water are illuminated with a specific light wavelength their SPR induces a superheating at the particle-medium interface, resulting in an explosive vapor-bubble generation around the particles [5-7]. These SPR can be used to design systems interacting with the solar spectrum with applications in heat [2,8,9,10] and steam generation [11]. For instance, gold nanofluids under different solar intensities present a dramatic increase in the total efficiency and evaporation rate as compared with pure water, that is further increased by the solar power [12-14]. From the different approaches to improve the solar steam generation in SPR metal nanostructures, such as using non-spherical metal nanoparticles or hybrid systems [15], those consisting in a dielectric core coated with a nanometer metallic shell (named nanoshells) are highlighted [16]. In particular, gold/silica nanoshells were used for solar steam generation with 80% of sunlight absorption that was converted into water vapor and only 20% converted into heating the liquid [8]. These results allowed the use of these particles in several technological applications such as ethanol distillation, thermal therapy, solar autoclave devices, and cellulosic bioethanol production, among others [8,17-22]. Besides metal nanoparticles with SPR for solar harvesting, during the last years carbon based nanostructures have emerged as promising materials to absorb sun light. Particles such as carbon black, carbon nanotubes (CNT), graphene, and graphene oxide (GO) are able to show solar steam generation and heating with potential lower cost than metallic nanostructures [23,24,25]. Instead of SPR, the process by which carbon materials work as steam generators is their high absorption of light due to the π-band optical transitions explaining that carbon based nanofluids have a broader absorption band than pure water [26,27,28]. In carbon based nanofluids for instance, the extinction coefficient show remarkable improvement compared to the base fluid even at low particle loads. Indeed, CNT can exhibit broadband absorption in the visible range, increasing the solar energy harvested compared to pure water [25]. These carbon-based nanofluids are therefore directly heated by the incident solar light generating heat on the particle surface where strong coupling occurs [25,29]. Moreover, considering both photothermal conversion efficiency and potential economic cost, carbon black nanofluids can be considered more suitable than gold-based nanofluids [30]. In this context, graphene nanoparticles have recently emerged as an outstanding family of carbon-based materials for nanofluids [24]. Graphene can exhibit better thermal conductivity than CNTs, opening a new window to graphene applications in heat management [31,32]. A novel approach to further increase the solar absorption of carbon systems is using 3D structures rather than isolated disperse particles in a nanofluid, that are able to show outstanding steam generation characteristics when they are located on the top area of the solar water collector, for instance as floating structures. In solar harvesting, the main advantage of these 3D structures is based on the radiation focused on the surface of the floating bodies, leading to an enhanced local heat that results in a localized evaporation [33,34]. Another advantage is that the porous structure of the foam acts as an insulation, thus reducing heat losses to the environment and improving steam generation [35,36]. Some 3D structures have further shown enhanced evaporation properties due to the hydrophilic nature of their surface, which modifies the water meniscus profile in the vapor-liquid interface thus enhancing the mass loss rate per area unit [37,38]. Despite the extensive research about solar thermal conversion in water collector systems using plasmonic metal nanoparticles and π-band optical transition using carbon materials based on either nanoparticles or 3D structures, comparative studies are barely reported [15]. Motivated by this lack of a general overview comparing the three main mechanisms for solar harvesting in DASC, this study will analyze the solar steam generation of a core/shell nanofluid having a plasmonic resonance in the visible region with two different carbon structures: a thermally reduced graphene oxide (TrGO) nanofluid and a 3D capillary structure based on an oxidized carbon foam. Moreover, a hybrid TrGO / core/shell system was studied to evaluate potential synergic contributions. The effect of radiation intensity on the steam generation was further studied by using different direct absorbing solar collector set-ups: a laboratory-scale open system and a prototype-scale closed system, under simulated and natural solar radiation, respectively. 2. Methods 2.1 Synthesis of nanoparticles For the synthesis of the copper coreshell (CuSiN) nanoparticles, a procedure based on the method reported by Graf et al. was performed [39]. First, silica nanoparticles of 100 nm were synthesized from tetraethyl orthosilicate (TEOS) precursors by a sol-gel method as detailed elsewhere [40]. Then a 1 % water solution of HAuCl 4 was prepared and stored in the dark for 3 days and afterward Au nanoparticles were made from this solution by using tetrakis hydroxymethyl phosphonium chloride (THPC) as a reduction agent. The surface of the silica nanoparticles was functionalized with (3-aminopropyl) trimethoxysilane for Au nanoparticle attachment on its surface through gold-amine interactions. These silica particles with the Au nanoparticles are the seed for the nanoshell synthesis. A 25 mL Milli-Q water solution with 0.7 gr of potassium sodium tartrate and 0.2 gr of CuSO 4 /5H 2 O was prepared producing the plating solution. Finally, copper nanoshells were prepared by adding into 25 mL Milli-Q water: 5 mL of 1 M NaOH, 0.2 ml of gold/silica seed solution at 1.2%w, different amount of the plating solution (from 0.2 to 4 ml), and 1 mL of formaldehyde into in sequence under vigorous stir. Thermally reduced graphite oxide (TrGO) was obtained from graphite with a two-step process of oxidation and thermal reduction. The oxidation step consists in obtaining graphite oxide (GO) from graphite with KMnO 4 and NaNO 3 in concentrated sulfuric acid, as described by Hummers [41]. In the second step, GO was rapidly heated in nitrogen atmosphere to 600°C for 40 s, for a thermal reduction process forming the TrGO. The oxidized carbon foam (OCF) was obtained oxidizing an 24 pore per centimeter commercial reticulated vitrous carbon foam purchased from Goodfellow Inc. (USA) using the same Hummers method as above discussed for graphite. This method produces a fully open-pore carbon foam with a more hydrophilic nature allowing the free flow of water inside the foam while keeping the high light absorption of a carbon based foam. For instance, while original carbon foam float on the water surface, the OFC was able to be wetted in the water collector. The hybrid TrGO/CuSiN was prepared by mixing a solution with 0.1%wt of TrGO particles with gold/silica seed solution, in a ratio of 1 ml of TrGO per 2 ml of seed solution. This solution was aged for 30 minutes to allow the precipitation of seed particles on TrGO [42,43]. After this time, the copper nanoshell method was completed with the modified seed solution. 2.2 Characterization The extinction spectra of the different nanoparticles dispersed in water were measured using a JENWAY 6320D spectrophotometer operated at room temperature and between 300 and 1000 nm. For the tests under simulated solar radiation, a Sciencetech SF-300B equipment was used producing 1 sun of radiation corresponding to 1 kW/m 2 . The morphology of the nanoparticles was characterized by a FEI-microscope model G2 F20 S-Twin high-resolution transmission electron microscopy (HRTEM) at 200 kV. For tests under direct solar radiation an 18” diameter parabolic dish reflector with a concentration ratio of 80 was used from Edmund Optics (model 80-254). 2.3 Solar steam set ups In order to evaluate the particles under different radiation conditions, four set ups were used through the study. The first set up used simulated solar radiation to irradiate a simple open glass beaker with 50 mL of water (contact area of 0,002 m 2 ) under 1 sun (1 kW/m 2 ) radiation as displayed in Figure 1a. The second set up used the same solar radiation source although an optical lens (Figure 1b) was added between the lamp and the glass beaker concentrating the light irradiation area with an effective radiation of 3 suns. In this case, the receiver was reduced to an open glass beaker with 3 mL of water and an area of 0,0001 m 2 as displayed in Figure 1b. The third set up used direct solar radiation by means of a parabolic dish mirror where an open glass beaker with 50 mL of water is open to the atmosphere enabling the quantification of the vapor generated. The fourth set up was the same as the third one, but a closed glass tube having 25 mL of fluid was located as displayed in Figure 1c. These two set-ups were tested during summer time in Santiago of Chile with a solar radiation displayed in Figure 1d, at the same hours to avoid relevant variation in the sun intensity. For the nanofluids a solution having 0.01 wt% of particles was used. In the case of OCF, a piece of the material was located on the top part of the beaker covering the whole surface for experiments in open systems. For the tests under direct solar radiation in a closed system, the foam covered almost all the system. 3. Results and discussions 3.1. Material characterization 3.1.1. Copper/silice nanoshell particles Nanofluids based on gold nanoparticles are by far the most studied system in direct solar steam generation meanwhile copper nanofluids are barely reported in this topic despite its both lower cost and ability to produce nanoshell structures with tailored SPR [44,45]. Copper/silice nanoshells (CuSiN) are also able to present tailored light absorption as shown theoretically hereafter. The surface plasmon resonance of spherical metal nanoparticles can be understood from the Maxwell electromagnetic equations as considered by the Mie theory assuming a particle with a size much smaller than the light wavelength. By using these equations, the extinction, scattering and absorption cross-section of a coreshell nanoparticle can be estimated depending on the characteristic of the metal [46,47]: where λ is the relative scattering wavelength which is obtained by dividing the incident wavelength (λ o ) by the refractive index of the surrounding media (m o ). The terms and depend of the refractive index of the material of the nanoshell and the radius of the particle [48]. The results for gold and copper nanoshell structures having different metal thicknesses and a dielectric core of 60 nm are displayed in Figure 2a and 2b, respectively. As reported previously, these models predict that the SPR frequency can be tailored according to the characteristics of the nanoshells [16]. By increasing the metal thickness, the resonance is shifted to lower wavelengths becoming broader meaning that is able to absorb a larger portion of the sun radiation. These changes are associated with the effective mean free path of the electrons in the nanoshells that depend on the shell thickness and core diameter [48]. Noteworthy, despite gold/silica nanoshells are mainly studies for solar steam generation, these equations also predicted that copper/silica nanoshells behave as similar as gold nanostructures being therefore suitable for solar steam generation. To validate the relevance of the nanoshell structure on the SPR behavior as observed in Figure 2, different concentrations of plating were used during the synthesis of copper nanoshells [44]. Transmission electron micrographs (TEM) of the resulting nanostructures are displayed in Figure 3 confirming the large effect of this concentration on the particle morphology. By using low amount of plating (0.2 ml), pure silica nanoparticles were mainly observed with some pots of copper structures (Figure 3a). By increasing the amount of plating, some copper shell structures appeared on the silica surface (Figure 3b and 3c) and with the largest concentration of plating copper/silica nanoshells were obtained (Figure 3d). Noteworthy, the UV-vis spectra of these particles confirmed the relationship between particle structure and SPR as displayed in Figure 4. For silica particles without any copper surface layer, synthesized with the lowest concentration of plating, a narrow SPR peak appeared around 610 nm as similar as pure copper metal nanoparticles [49]. By increasing the presence of copper surface layer on the silica particles, the wavelength peak is shifted to higher values becoming broader with an extinction covering almost the whole spectrum over 600 nm at the largest covering. At low plating concentrations, the total copper added formed nanostructures through a small portion of the silica particles, meaning spherical particles. At higher plating concentrations otherwise, there is more copper available to be distributed homogeneously on the silica particles producing a thin layer. At the highest plating concentration, a thick layer of copper is formed that explains the peak movement toward higher wavelengths (see Figure 3). By comparing the experimental spectra with the theoretical prediction (Figure 2b), we concluded that samples synthesized with 0.5, 1.0, and 4.0 ml of plating presented a plasmon spectra roughly similar to the prediction from nanoshell with a metal layer thickness of 30, 50, and 70 nm, respectively. Some mismatch is expected likely due to a non-homogeneous shell (i.e. roughness and thickness), presence of copper particles of different aspect ratio, and presence of higher order multipoles not considered in our simplified model [16]. However, we were able to synthesize a particle with an extinction coefficient covering a large portion of the spectra. Hereafter, we focus on the solar steam generation by using the CuNiS particle synthesized with 1 [ml] of plating, as we concluded from Figure 4 that this particle presents the highest absorption rate in the 300 [nm] to 1100 [nm] wavelength range. 3.1.2. Carbon based systems Three different carbon based systems were used in this study: 1) pure TrGO; 2) hybrid TrGO/CuSiN system; and oxidized carbon foam (OCF). Details about the characterization of the pure TrGO having a low oxidation degree, in particular the presence of functional groups, can be found elsewhere [50]. Regarding the TrGO/CuSiN hybrid system, Figure 5a shows a representative TEM image of this hybrid particle where the thin layered structure from TrGO is clearly observed. On the surface of TrGO, a set of particles having sizes around 100 nm can be further observed associated with nanoshell structures produced in presence of the TrGO. The sizes from these CuSiN particles agree with those observed for pure particles (see Figure 3). Therefore, our method allowed the production of hybrid TrGO/CuSiN nanostructures. Figure 5b otherwise displays a scanning electron microscopy image of the carbon foam used where the open pore size characteristic can be observed with sizes around 500 μm. Based on the pore characteristic (80 ppi) a volumetric surface area of around 50 cm 2 /cm -3 can be estimated [51]. 3.2. Thermal absorption under simulated radiation Light intensity is a relevant variable in plasmonic and carbon based materials for water heating and stem generation [4,12,25]. For instance, by assuming a steady state condition in the heat transfer equations, a direct dependence between the light intensity and the local temperature around a single NP can be concluded [4]. Indeed, the rate of plasmon-induced heat generation in a nanoparticle is proportional to its optical absorption cross-section multiplied by the incident optical intensity [7]. Despite this strong dependence, the effect of solar radiation intensity on the steam generation has been barely studied motivating us to consider different set-ups for a large range of solar intensities: two set-ups at a laboratory scale under a simulated sun radiation and two set-ups at a prototype scale under direct sun radiation coupled to a parabolic dish mirror. The evaporation efficiency at steady-state conditions was determined by dividing the gained enthalpy in the generated vapor by the total incoming solar radiation input: \(\eta=\frac{m\bullet h_{\text{fg}}}{Q_{s}\bullet A}\) Equation 1 where m is the steady-state vapor mass flux, h fg is the latent heat of vaporization for water at 1 atm (2.257 MJ kg -1 ), A is the area of the aperture or irradiance, and Q S is the total incoming solar flux after concentrating optics. Hence the efficiency reported is an internal efficiency [52]. By using a simulated sun radiation to irradiate an open glass beaker with 50 mL of water (area of 0,002 m 2 as displayed in Figure 1a) under 1 Sun (1 kW/m 2 ) of intensity during 60 minutes, the efficiency was measured for pure water and water with the different particles (CuSiN, TrGO, and CuSiN/TrGO) and the OCF, as summarized in Figure 6. At 1 Sun, all the particles and the foam were able to increase the efficiency and while pure water presented an evaporation rate of 0.5 x 10 -4 kg/(m 2 s), meaning an efficiency of 11.3 %, the nanofluids and the OCF increased this value to around 1.2 x 10 -4 kg/(m 2 s), with efficiencies around 20 %. Therefore, in this open system under low radiation intensity the presence of nanoparticles and OCF increased the solar harvest process by a factor of around 2.0. The highest increment was for the hybrid TrGO/CuSiN system confirming previous results regarding advantage of mixing different nanofluids [50]. Nanofluids in volumetric solar collectors must consider their light absorption in the entire solar wavelength range, so this is a recent strategy to improve the photo-thermal efficiency [50,53]. The resulting hybrid fluids has the spectral absorption advantages of each specific nanofluid as the extinction coefficient of binary nanofluids is found to be approximately equal to the sum of those of the constituent components [50,54,55]. By adding an optical lens between the lamp and the water receiver (Figure 1b), the light irradiation zone is decreased having as consequence that the effective radiation increased to 3 Suns. In this case, the receiver was reduced to an open glass beaker with 3 mL of water (area of 0,0001 m 2 ) avoiding a direct comparison with previous results under 1 Sun. However, our goal is to obtain comparative studies regarding the effect of adding different particles to a water solution under a specific intensity and conditions. In this case, the evaporation intensity of pure water was 2.1 x 10 -4 kg/m 2 s meaning an evaporation efficiency of 18 %, after 60 minutes of irradiation. The presence of pure nanoparticles increased the evaporation rate to values of 5.1 x 10 -4 for CuSiN and 5.5 x 10 -4 for TrGO, meaning efficiencies of 40 and 57%, respectively. The TrGO/CuSiN hybrid system obtained an evaporation rate of 6.0 x 10 -4 [kg/m 2 s] with an efficiency of 62%, as similar as results from the carbon foam. By comparing these results with those from 1 Sun, it is concluded that the effect of the different structures on the radiation absorption of pure water is proportional to the solar intensity. For instance, while under 1 Sun the increment by using nanoparticles and OCF was a factor around 2.0, under 3 Suns the different particles and the OCF increased the solar harvest process by a factor of around 3.0 as compared with pure water. The lowest behavior was for CuSiN nanofluids with an increment of 2.4. By analyzing the temperature of the fluid under 3 Suns after 60m minutes, relevant information regarding the mechanism can be obtained. For instance, the temperature of pure water increased from 298 to 311 K meaning that only 36% of the solar energy received by water was used to evaporate and the rest (64%) was used to heat the water bulk. The low evaporation efficiency of water is therefore partially explained by the energy used to heat the water instead of produce steam. Nanofluids otherwise are more efficient due to most of the energy is used to directly evaporate the fluid rather to heat the bulk water [5-7]. In our case, water with nanoshells presented a final temperature of only 305 K meaning that 70% of total solar energy received was used to evaporate and only 30 % of this energy was used to heat the bulk. The nanofluids with TrGO whose mechanism is based on direct heat transfer to the fluid, presented temperatures as similar as pure water. In a similar experiment but using carbon nanotubes dispersed in water, a drastic increase in the water temperature was also observed together with higher solar efficiencies and evaporation rates than pure water [56]. In the fluid having the OCF, the temperature of the fluid was even higher than pure water as in this case the mechanism is based on direct solar heating and its transfer to the fluid. This simple experiment show that different mechanisms are present in each system. 3.3. Thermal absorption under direct sun radiation To go further in our analysis regarding the effect of sun radiation intensity, the different systems were tested under direct solar radiation by using a parabolic dish mirror with a concentration ratio of 60 during 10 minutes as displayed in Figure 1c. This set-up was tested during summer time (noon/afternoon) in Santiago of Chile with a solar radiation displayed in Figure 1d. The data of radiation was obtained from the “Explorador Solar” of the Universidad de Chile [http://ernc.dgf.uchile.cl:48080/inicio]. The evaporation rate was measured using the third set-up obtaining a value of 7.5 x 10 -4 kg/(m 2 s) for pure water representing an efficiency of 3.5 %. An efficiency lower than that obtained under 1 or 3 Suns (11 and 18%, respectively) can be associated with the large radiation producing higher temperatures and therefore higher heat losses [57]. However, the presence of the different particles dramatically increased the evaporation rate to around 8.5 x 10 -3 kg/m 2 for the different nanofluids and to 1.0 x 10 -2 [kg/m 2 s] for the OCF. These values represented efficiencies around 40 % for the nanofluids and 46 % for the OCF, meaning an improvement by a factor of 11 and 13 in the solar absorption as compared with pure water, respectively (Figure 6). Motivated by the high evaporation efficiencies obtained using the parabolic dish mirror, a closed glass tube having 25 mL of water solution was used (fourth set-up). With this closed set-up under this high radiation intensity, large bubbles appeared after some minutes meaning that steam was formed even using pure water. This fact motivated the measurement of the time evolution of fluid properties as displayed in Figure 7. The pure water bulk temperature is displayed in Figure 7a showing a steady increase reaching temperature differences with the initial temperature above 75 K after just 600 seconds of irradiation. Noteworthy, by adding the different nanoparticles and the OCF, the bulk water heating process was faster by a factor around 4.0 as concluded comparing the time needed to reach a difference of 50 K, decreasing from 260 to 70 seconds. After 600 seconds, water with the different nanoparticles and the OCF reached a difference around 100 K as compared with the initial bulk fluid temperature. Regarding the dynamic of the bulk fluid heating process for each system, while the hybrid TrGO/CuSiN system was the fastest further presenting the highest temperatures, TrGO nanofluid was the slowest and pure CuSiN nanofluid reached the lowest temperatures. Despite that OCF in a open system presented the highest efficiency and evaporation rate (Figure 6), in this closed system the carbon foam produced temperature as similar as nanofluids. More information can be obtained analyzing the tendency from the steam properties as larger effects of the nanoparticles and the OCF can be concluded. For instance, the steam temperature was much higher using the nanofluids and the OCF as displayed in Figure 7b showing that while pure water reached differences of 55 K in the top temperature of the system (as compared with the initial temperature), water having CuSiN, TrGO, TrGO/CuNiS and OCF reached differences around 90 K and at much lower times. Similar results were found in gold nanoshells where the temperature of both the fluid and the vapor also increased well above the standard boiling point of water [18]. The different systems not only affect the steam temperature but also the pressure where the largest differences were observed between the particles (Figure 7c). By using pure water, the steam pressure was around 500 kPa after 600 seconds meanwhile by using the CuSiN and TrGO particles this pressure increased to values as high as 1600 and 2000 kPa after the same time period. Noteworthy, the hybrid TrGO/CuSiN system presented the highest value of 2200 kPa further presenting the fastest change. The concentration of nanoparticle is a relevant variable increasing the thermal efficiency of nanofluids [25]. To analyze this effect, different nanofluids based on CuSiN were prepared having concentrations of: 0.002, 0.01, and 0.1 wt%, and the thermal absorption behavior was quantified using direct solar radiation by through a parabolic dish mirror. Figure 8 shows the large effect of particle concentration increasing the thermal absorption of water although two areas were clearly observed. By increasing the concentration from 0.002 to 0.01 wt % a drastic increment was observed in the evaporation rate and thermal efficiency from 4.0 x 10 -3 kg/m 2 and 18 % to 8.5 x 10 -3 kg/m 2 and 38 % respectively. At low concentrations where the ratio of the particle–particle distance to the particle diameter is very large, the nanoparticles act independently and the converted solar thermal energy is proportional to the increase of particle concentration [58]. However, another tendency is found by comparing the results from nanofluid having 0.01 and 0.1 wt% where the evaporation rate and the efficiency barely increased from 8.5 x 10 -3 kg/m 2 and 38 % to 8.8 x 10 -3 kg/m 2 and 40%, respectively. By increasing the concentration of nanoparticles the heat leak through radiation become strengthened and the transparency of the nanofluids decreases [25,58]. The same tendency was observed regarding the temperature of the bulk fluid, the temperature of the steam, and the pressure of steam generated (Figure 9). At low concentrations a large effect of the particle concentration was observed drastically increasing the temperature of the bulk water and the steam, and its pressure, while this effect was barely observed at high concentrations. 3.4. Mechanisms for solar thermal conversion Our results presented the radiation conversion of three different systems, each one having different mechanism for solar absorption. CuSiN particles presented a metal surface layer producing a SPR in the visible spectrum that explain its sun radiation absorption. Moreover, these hybrid metal nanoparticles under plasmonic effects increased the evaporation rate by a mechanism associated with the nano-bubble formation in the water-particle interface rather than by a direct heating of the fluid bulk [8,13,14]. This process explained that under low radiation the temperatures of the CuSiN nanofluid were lower than pure water despite the higher evaporation of CuSiN nanofluid. The reason is that most of radiation energy was used by the nanoparticle to directly evaporate instead to heat the fluid. At high radiation otherwise, CuSiN nanofluid produced steam with higher temperatures and pressures than pure water fluid because of the SPR effect and nano-bubble mechanism. For instance, the bulk temperature of CuSiN nanofluid was as similar as of TrGO nanofluid however the former presented higher steam properties. TrGO nanofluid presented a radiation absorption by a different mechanism that is based on the presence of sp 2 -bonded carbons in its graphite-like structure producing an electronic absorption processes associated with the π-valence bands [26]. The high absorption of light due to this π-band optical transitions explains the good behavior of carbon materials as steam generators [26,27,28]. He et al. recently proposed a mechanism for vapor generation by solar heating in CNT nanofluid based on the strong interaction between carbon based particles and solar light generating heat in their surface [25]. The heat transfer from the carbon particle to the surrounded media can occur due to the relevance of the low-frequency modes for energy transfer across the interface through phonon-phonon interactions [59]. Therefore, the energy absorbed by the CNT can be exchanged by transferring their high frequency phonon modes to low-frequency modes allowing the interaction with the media. The presence of some oxygen functional groups in TrGO (results not shown) reduced this process. On the other hand, OCF presented another mechanism for solar steam generation based on heat localization [60]. This mechanism appears in structures able to absorb solar illumination confining the thermal energy through formation of hot spots near the structure surface while efficiently wicking the fluid to this hot spot. Localization of heat is achieved by using a broad-spectrum absorbing material while the fluid flow is channeled to the hot spot by further using a hydrophilic and porous material. Moreover, the structure should have low thermal conductivity to suppress thermal conduction away from the hot internal region. Our original commercial open-pore (24 pores per centimeter) vitreous carbon foam material presents a free void volume of 96.5% having a density of 0.05 g/cm 3 and a carbon surface specific area of 3750 ± 90 m 2 /m 3 [61]. After oxidation, the system was able to wet allowing water absorption and therefore its conductions. Table 1 shows published results from some representative structures used in DASC for solar steam generation allowing a general picture about our results. Although due to the different set-ups, scales, particles/structures, and solar radiations, among other variables changing the behavior of different DASC, a direct comparison cannot be carried out, at least the table put in context the general tendencies. 4. Conclusions Our results allowed a direct comparison between three systems for solar steam generation based on different mechanisms: surface plasmon resonance, π-band optical transition, and localized heating. At low intensities using an open DASC under a simulated solar radiation, these systems were able to increase by a factor around 2.5 the evaporation rate as compared with pure water, with OCF and hybrid TrGO/CuSiN particles presenting the best results. By increasing the radiation intensity through a parabolic dish mirror with a concentration ratio of 60, the evaporation rate dramatically raised reaching the boiling process after only 10 minutes. Under these conditions, the evaporation rate was improved by a factor higher than 10.0 as compared with pure water, with OCF presenting the best behaviors. Not only the steam generation was improved but also its characteristics as tested in a closed-system. These results show the viability of solar steam generation based on plasmonic nanoparticles and carbon compounds, further confirming the higher efficiency of systems based on optical band effects and localized heating rather than on plasmonic effects. Acknowledgments: The authors gratefully acknowledge the financial support of project CORFO-Innova 14IDL2-30108 and FONDECYT 1150130. Figures and Table Captions Table 1. Examples of different structures reported toward solar steam generation and the efficiencies and steam rate obtained. Figure 1. Photographs of the different set-ups used: a) an open glass beaker with 50 mL of water (area of 0,002 m 2 ) under 1 Sun radiation coming from a simulated solar radiation equipment; b) open glass beaker with 3 mL of water (area of 0,0001 m 2 ) under simulated solar radiation with an optical lens added between the lamp and the water receiver; c) a closed glass tube having 25 mL of water solution located under direct solar radiation by means of a parabolic dish mirror; d) direct solar radiation (W/m 2 ) against time of a day for the latter set-up associated with summer time (noon/afternoon) in Santiago of Chile. Figure 2. Theoretical results from Mie theory for extinction cross-section of coreshell nanoparticles having a silica core of 60 nm and different metal thickness in nm: a) silica/gold and b) silica/copper nanoshells. Figure 3. Transmission electron micrographs (TEM) of copper nanoshell structures synthesized using different amount of plating: a) 0.2 ml; b) 0.5 ml; c) 1.0 ml, and d) 4.0 ml. Figure 4. Experimental UV-vis spectra of copper nanoshell structures synthesized using different amount of plating: a) 0.2 ml; b) 0.5 ml; c) 1.0 ml, and d) 4.0 ml. Figure 5. Representative images of the hybrid TrGO/CuSiN particles and the oxidized carbon foam (OCF). 5a: Transmission electron microscope picture of TrGO/CuSiN; 5b: scanning electron microscopy picture of OCF. Figure 6. Water evaporation rate in kg/m 2 h for pure water and water having the three systems studied: CuSiN particles for surface plasmon resonance mechanism (Copper nanoshells); TrGO for π-band optical transition mechanism; and OCF for localized heating mechanism (Carbon sponge). The hybrid TrGO/CuSiN systems is also displayed (Copper nanoshell + TrGO). The systems were studied under three solar radiations: 1 sun (1000 W/m 2 ) by using using a simulated solar radiation; 3 Sun by using an optical lens between the lamp and the water receiver; and under natural sunlight by using a parabolic dish mirror with a concentration ratio of 60. See Figure 1 for details. The values above each column represents the thermal efficiency according to equation 1. Figure 7. Time evolution of the main fluid characteristics from the set-up associated with a direct solar radiation (Figure 1c) for pure water and water having the three systems studied: CuSiN particles for surface plasmon resonance mechanism (Copper nanoshells); TrGO for π-band optical transition mechanism; and OCF for localized heating mechanism (Carbon sponge). The hybrid TrGO/CuSiN systems is also displayed (Copper nanoshell + TrGO). a) temperature of bulk water; b) temperature of steam; and c) steam pressure. Figure 8. Effect of CuSiN concentration on the solar absorption behavior of water based nanofluids under natural sunlight by using a parabolic dish mirror with a concentration ratio of 60: a) Water evaporation rate (stem generation) and the thermal efficiency according to equation 1 (values above each column); b) temperature of bulk water; c) temperature of steam; and d) steam pressure. Reference [1] Wang X, He Y, Liu X, Cheng G, Zhu J. Solar steam generation through bio-inspired interface heating of broadband-absorbing plasmonic membranes. Applied Energy 2017, 195,414. [2] Linic S, Christopher P, Ingram DB. Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy. Nat. Mat. 2011; 10:911. [3] Jain PK, Huang X, El-Sayed IH, El-Sayed MA. El-Sayed. Review of some interesting surface plasmon resonance-enhanced properties of noble metal nanoparticles and their applications to Biosystems. Plasmonics 2007;2:107. [4] Govorov AO, Richardson HH. 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Keywords nanotechnology optics particle technology Authors Affiliations Humberto Palza 0000-0001-5246-6791 [email protected] Universidad de Chile View all articles by this author Patricio Burdiles Universidad de Chile Facultad de Ciencias Fisicas y Matematicas View all articles by this author Metrics & Citations Metrics Article Usage 249 views 215 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Humberto Palza, Patricio Burdiles. Comparative Solar Steam Generation using plasmonic, optical-band transition and localized heating mechanisms. Authorea . 31 January 2024. DOI: https://doi.org/10.22541/au.170670261.11861277/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu . 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