Using abundant solar energy to generate steam offers unique solution overcoming the lack of freshwater resources. Despite extensive explorations, low-cost devices with high efficiency are still critically wanting. In this work, the extremely cheap carbonized kelp with good stability, high solar absorption (∼93%), porous microstructure, and hydrophilic surface is found to be efficient for solar steam generation (SSG) and seawater desalination for the first time. A SSG device meeting the requirements of the “most stable triangle” is thus designed. The carbonized kelp is utilized as the solar absorber, with thermal insulation by an expanded polystyrene foam and water supply by a fiber cotton wick via the capillary force. A high solar to steam conversion efficiency of 84.8% and evaporation rate of 1.351 kg·m-2·h-1 are attained under 1 sun irradiation. This work develops a promising and bioinspired device with long-term stability for SSG that can be produced economically (∼3.8 $/m2).
INTRODUCTION
As a major crisis facing mankind today, global water scarcity affects 2.8 billion people at least one month each year and 1.2 billion people lack access to clean drinking water, especially in the developing countries and remote areas.1,2 Renewable, abundant, and broad-spectrum solar energy as a new energy source under development can be utilized in human daily life by photothermal conversion,3,4 photoelectric conversion,5,6 photochemical conversion7,8 and “solar” fuel products,9 which brings hope and motivation to solve the water crisis. In general, photothermal material absorbs sunlight, then generates hot carries inside the materials, and the energy gained by the carries turns into heat via phonons.10–12 Using obtained heat to generate vapor has become efficient for seawater desalination.13,14 It has been agreed that the “most stable triangle” plays key roles in the SSG process:15–18 (1) light absorption and conversion: photothermal materials with high absorption of the solar spectrum and highly effective light-to-heat conversion. (2) heat management: thermal insulation to limit unnecessary heat loss (thermal conduction, convection and irradiation) to bulk water. (3) mass transportation and steam escape channels: hydrophilic surface and porous structure by using capillary force to transport water from bottom to top.
To date, different kinds of materials, structures,19 and applications20 for interfacial solar evaporation21 have been reported, such as metallic plasmonic nanoparticles,7,22,23 metal oxides,24–26 carbon-based materials,27–31 ceramic or composite materials,32–35 organic polymers,36–39 and semiconductor materials.40,41 In addition, recently, the bioinspired structures42–45 become one of the most exciting developments in this area. For example, Zhou’s group reported that natural wood with ultrahigh solar absorbance, low thermal conductivity, and good hydrophilicity can enable a solar-thermal efficiency of ∼72% under normal 1 sun irradiation after a simple flame treatment.46 It is demonstrated by Hu’s group that tree-inspired design (pristine wood is cut along the transverse direction and top surface is carbonized to create a unique bilayer structure) offers rapid water transport and evaporation, high light absorption, and a low thermal conductivity.47 More interesting, the work by Hu’s group decoupled the fluidic transport direction and the thermal transfer direction for the first time instead of using the intuitive water transport along the wood growth direction, and this device exhibited 80% efficiency with an evaporation rate of 1.15 kg·m-2·h-1.48 Besides wood, other bioinspired structures have also been exploited for SSG. Zhu et al pioneeringly developed carbonized mushrooms achieving ∼78% conversion efficiency.45 Then an artificial mushroom is created by Gao et al, which is made of the common polyvinyl alcohol sponge coated with charcoal exhibiting high efficiency in generating cold water steam of ∼73%.42 Inspired by the kerosene oil lamp, Xu et al demonstrated an advanced solar-evaporation system with efficiency of 88.8%.29 Gan et al designed thermally isolated black paper for efficient solar vapor generation.31 Very recently, Fang et al found hierarchical porous carbonized lotus seedpods possessed highly efficient SSG performance.44
Importantly, the low-cost is one of the most attractive advantages of the bioinspired materials. Since the current efficiency is already high enough, there is only a limited room for improvement. Obviously, further cutting the cost becomes the most crucial factor for practical applications. In this work, kelps grown in seawater, as a kind of ultralow-cost, abundant aquatic plant, are carbonized for SSG and seawater desalination for the first time. The carbonized kelps show high absorption of sunlight, salt tolerance, and micro/nanoporous channels for steam escaping. In order to meet the requirements of the “most stable triangle” for extremely cost-effective and efficient SSG, a device composing three main components is fabricated, in which, carbonized kelp acts as a solar absorber, expanded polystyrene foam insulates thermal conduction, and fiber cotton wick transports water by capillary force. The device achieves a high solar to steam conversion efficiency of ∼84.8% with an evaporation rate of 1.351 kg·m-2·h-1 under 1 sun irradiation, which provides a practical route for seawater desalination. Since the cheap polystyrene foam and fiber cotton can be used repeatedly, and carbonized kelp is ultralow-cost, the estimated price for generating steam of the whole device with a dimension of 1 m×1 m×1.8 cm is below $3.8, which is evidently cheaper than other reported bioinspired materials.31,44,45
EXPERIMENT
Synthesis of carbonized kelp
The fresh kelp was washed in deionized water by ultrasonication for several times, then treated at 400 °C in air for 1h. The obtained carbonized kelp was then grinded into powder.
Characterization
Transmission electron microscopy (TEM) was conducted on the JEOL-2100F electron microscope. Thermogravimetric analysis/differential thermal analysis (TG/DTA) results were obtained by a PerkinElmer Pyris1 differential scanning calorimeter. The reflectance in the range of 400-4000 wavenumbers were recorded by a NEXUS 870 Fourier transform infrared (FTIR) spectrometer. Raman measurements were carried out with a high-resolution Raman microspectrometer (Labram HR-800, Horiba Jobin–Yvon), using a Ne laser source (λ = 585.25 nm). The diffusion reflectance absorption spectra were obtained on a Shimadzu UV-2600 spectrometer with an integrating sphere attachment.
The preparation of experimental setup for SSG and seawater desalination
Fiber cotton (retail price of ∼$0.90 m−2) coated by carbonized kelp power (diameter=3 cm) was placed on a polystyrene foam (thermal conductivity ∼0.04 W m k−1, retail price of ∼$2.83 m−2) with the fiber cotton wick inserted in the center, and the entire device floated on the surface of water with only the bottom side of the wick in direct contact with bulk water.
RESULTS AND DISCUSSION
The carbonized kelp was synthesized by a simple heating treatment. And the micro-structure of the carbonized kelps was characterized by a TEM. As shown in Fig. 1(a) and 1(b), many irregular pores at the nano to submicro-scale are observed, indicating the existence of channels for steam escaping. Size distribution of the pores is shown in Fig. 1(c). Fig. 1(d) shows Raman spectrum of the carbonized kelp. Two obvious characteristic D (1370 cm-1) and G (1578 cm-1) peaks of carbon structure are observed. The two bands represent crystal defects and irregularities, and the carbon atom plane of the hexagonal system, respectively.49 The Raman spectrum indicates the kelp becomes carbon structure after thermal treatment. The functional groups of the carbonized kelp are investigated by FTIR spectroscopy. The distinguishing vibration peak of C=C bonds appears at around 1500 cm-1, further confirming the thermal treatment induced graphitization (Fig. 1(e)). FTIR also contains some other peaks as marked in Fig. 1(e), indicating functional groups such as O-H, C-O and C=O groups are retained after carbonization. These oxygen-containing functional groups make the carbonized kelp hydrophilic and facilitate the escape of steam. Thermogravimetric analysis/differential thermal analysis (TG/DTA) is used to study the thermal stability of the carbonized kelp (Fig. 1(f)). The test was conducted from room temperature to 900 °C at a rate of 10 °C min-1 under air atmosphere. As the temperature reaches 420 °C, a strong exothermic reaction (46 μV) is the observed. When the temperature reaches 579 °C, the residual mass becomes 91% of the original, indicating carbonized kelp is stable under photothermal process and during solar irradiation application (<200 °C).
(a) TEM image of the carbonized kelp. (b) Enlarged TEM image of the pores in carbonized kelp. (c) Pore size distribution counted from multiple TEM images. (d) Raman spectrum of the carbonized kelp. (e) FTIR spectrum showing various functional groups. (f) TG/DTA result of the carbonized kelp tested under air atmosphere.
(a) TEM image of the carbonized kelp. (b) Enlarged TEM image of the pores in carbonized kelp. (c) Pore size distribution counted from multiple TEM images. (d) Raman spectrum of the carbonized kelp. (e) FTIR spectrum showing various functional groups. (f) TG/DTA result of the carbonized kelp tested under air atmosphere.
Fresh kelp was treated at different carbonization temperatures in air for 1 hour to obtain differently carbonized kelps. The photographs of these carbonized kelps obtained at different temperatures are displayed in inset of Fig. 2. All the carbonized kelps exhibit dark surface indicating strong light absorption ability. It is easy to expect that carbonized kelp with a rough surface can effectively scatter, trap and absorb irradiation in the solar wavelength. Here, the light absorption properties of carbonized kelps are investigated by the reflectance spectra recorded via an integrating sphere. As shown in Fig. 2, the carbonized kelp synthesized at 400 °C (red curve) shows the optimal optical absorption ability that the average optical reflectance is below 7% from 200 to 1300 nm wavelength, meaning that ∼93% of the irradiated solar energy in the corresponding spectrum can be absorbed.
Reflectance spectra of carbonized kelps synthesized at different temperatures. Inset: photographs of carbonized kelp under different carbonization temperature.
Reflectance spectra of carbonized kelps synthesized at different temperatures. Inset: photographs of carbonized kelp under different carbonization temperature.
To measure the photothermal effect, the carbonized kelp is placed in a beaker (on the thermal insulation foam) irradiated by a simulated solar irradiation at different power densities (from 0.5 sun to 3 sun) and the surface temperature is recorded by a thermocouple thermometer (Fig. 3(a)). The surface temperatures (T) over exposure to different solar irradiation densities are plotted as a function of time in Fig. 3(b). The temperature rises rapidly that an increased heating temperature of 19.3 °C (after 30 minutes) is observed even if under irradiation as low as 0.5 sun. For 0.5 sun, 1 sun, 2 sun and 3 sun, the temperatures are about 40.2 °C, 53.3 °C, 67.3 °C and 81.7 °C respectively, when irradiation reaches 5 minutes. The temperatures further increase to 45.3 °C, 60.1 °C, 82.9 °C and 93.4 °C, respectively, when the heating curves become stable after 30 minutes irradiation.
(a) Experimental setup for measurement of photothermal effect. (b) The temperature rising curves for carbonized kelps over exposure to different solar irradiations. (c) Thermal images taken for carbonized kelps at different radiation time. From top to bottom, the irradiation density increases from 0.5 sun to 3 sun.
(a) Experimental setup for measurement of photothermal effect. (b) The temperature rising curves for carbonized kelps over exposure to different solar irradiations. (c) Thermal images taken for carbonized kelps at different radiation time. From top to bottom, the irradiation density increases from 0.5 sun to 3 sun.
Infrared thermograms are also used to investigate the photothermal effects. As indicated in Fig. 3(c), at t=0 s, the above picture is a photograph taken with digital camera, and the below is a thermal infrared image taken with a thermal imager. Indiscernible temperature contrast between the carbonized kelp surfaces and the circumjacent zones is observed. After radiation for 30 seconds (second column), compared to the starting thermal image, the infrared thermograms of carbonized kelp display apparent differences on its surface and the temperature is indeed increasing under 0.5 sun, 1 sun, 2 sun and 3 sun irradiation, respectively. As time increases, the results shown in the thermal images are consistent with the temperature measured by the thermocouple thermometer. The thermal imaging characterizations directly confirm carbonized kelp has efficient photothermal conversion efficiency, implying a great prospect for high SSG performance.
Fig. 4(a) illustrates the solar steam device, which contains three main components meeting the requirements of the “most stable triangle”. First, the carbonized kelp is placed at the top surface, which acts as a solar absorber. Second, expanded polystyrene foam is used for thermal insulation. Moreover, the foam can float on the water, thus supporting the carbonized kelp. Third, a hydrophilic fiber cotton wick is inserted in the polystyrene foam, so that the bottom water can be transported to the top surface. In this design, carbonized kelp absorbs solar energy and converts it to thermal energy, then bulk water is transported by fiber cotton wick. The thermal insulation foam ensures that heat loss can be highly suppressed and thermal energy is maximum used for water evaporation rather than heating the bulk water at the bottom (Fig. S1). Fig. 4(b) shows the photograph of our device. The inset of Fig. 4(b) is a vertical view of the device, and diameter of absorber layer is 3 cm. The masses of the multiple layers from bottom to top are 35 g, 0.4921 g and 20 mg, corresponding to original water, insulated pumping layer (foam: diameter=3 cm, thickness=1.8 cm; wick=1 cm×5 cm) and optical absorbent layer (carbonized kelp, thickness=0.1 mm), respectively. The capillary force of the fiber cotton wick is confirmed in movie S1. And, the adiabatic effect of the polystyrene foam is evidenced by infrared thermogram. The temperatures of the bottom water and the surface solar absorber are recorded while measuring the steam generation. As shown in Fig. 4(c), the temperature of the bottom water is remarkably lower than that of the surface solar absorber. After 3 hours, the temperatures of the bottom water only rose by 1.1 °C, 4 °C, 5.4 °C and 7.6 °C corresponding to different solar densities. However, the surface temperature reaches about 56.7 °C under 1 sun irradiation during the process of solar evaporation. These results from infrared thermograms (Fig. 4(c)) display that heat energy stemming from carbonized kelp can be maximum used for water evaporation while minimizing heat loss to bulk water.
(a) Schematic illustration of the SSG device. (b) The photographs of device under 1 sun. Inset: vertical view of device. (c) Thermal images taken during SSG.
(a) Schematic illustration of the SSG device. (b) The photographs of device under 1 sun. Inset: vertical view of device. (c) Thermal images taken during SSG.
The SSG performances of our device are tested under different solar irradiation influences, as plotted in Fig. 5(a). After 3 hours, corresponding to 0 sun, 0.5 sun, 1 sun, 2 sun and 3 sun, the weight losses are 0.3281 g, 1.1399 g, 2.43875 g, 4.92926 g and 7.34713 g, respectively. The movie S2 shows whole SSG process under 1 sun irradiation and obvious steam can be observed. During the SSG process, the temperature of bulk water at the bottom was recorded by a thermocouple thermometer. As shown in Fig. 5(b), the temperature increments are minor due to the thermal insulation by the polystyrene foam, which are in good agreements with the infrared thermograms shown in Fig. 4(c). The evaporation rates and solar-to-steam efficiencies at different solar power densities are shown in Fig. 5(c). More details are listed in Table S2. The overall solar-to-steam conversion efficiency can be evaluated as44
where ηss is the overall solar-to-steam conversion efficiencies, Δm is the mass loss of water during irradiation, ΔvapHm is the phase change enthalpy of water from liquid to vapor which is approximately 40.637 kJ mol−1, M is the molar mass of water, I is the solar power density at the carbonized kelp surface, S is the area of the water surface directly irradiated by the incident light, and T is the duration time of irradiation (3600 s), Qeva is the absorbed energy for water evaporation, Qsolar is the input solar energy, He is the total enthalpy of liquid-steam phase change (2260 kJ·kg-1) and k is the rate of water evaporation. Δm divided by M is k. It is stressed that the SSG can be further improved through optimizing the irradiation power density, and loading weight of carbonized kelp (Fig. S2 and Table S2). When 60 mg carbonized kelp is used for this device, a high solar to steam conversion efficiency of 84.8% and a high evaporation rate of 1.351 kg·m-2·h-1 is attained under 1 sun irradiation. The efficiency of our device is comparable to the competitive values in the previous reports.13,27,28,36,42,44
(a) Plot of the weight loss through SSG as a function of irradiation time with different solar irradiations. (b) The temperature of the bulk water measured by a thermocouple thermometer at location “T” marked in Fig. 4a. (c) The evaporation rates and solar-to-steam efficiencies of carbonized kelp under different solar power densities.
(a) Plot of the weight loss through SSG as a function of irradiation time with different solar irradiations. (b) The temperature of the bulk water measured by a thermocouple thermometer at location “T” marked in Fig. 4a. (c) The evaporation rates and solar-to-steam efficiencies of carbonized kelp under different solar power densities.
To demonstrate the seawater desalination capability of the carbonized kelp, artificial brine (3.5 wt% salinity) was prepared as processed object.13 As shown in Fig. 6 (case 2), the evaporation structure generated is 1.135 kg·m-2·h-1, which is almost as the same as case 1 for evaporation of pure water (1.152 kg m-2·h-1). The insets of Fig. 6(b) are photographs of MS-200 salinometer, which displays the 3.5% salinity water has been desalinized well (0.00% salinity) after SSG by our device. To verify the important roles of the solar irradiation and the optical absorber, case 3-5 (Fig. 6(a)) also are investigated. When the solar irradiation is absent, the evaporation is negligible (cases 3 and 4). While, if there is no optical absorber, only very low efficient evaporation is observed (case 5). Moreover, the thermal insulation effect is studied by cases 2, 6 and 7, when the insulation foam height further increased to 5 cm (case 6), the steam generation efficiency remain unchanged basically. However, when the insulation foam is removed, the SSG performance is very poor (Fig. S1). More details are listed in supplementary material (Table S3).
(a) Table of different conditions for SSG. (b) Plot of the weight loss through SSG corresponding to (a). Inset: photographs of MS-200 salinometer for case 2.
(a) Table of different conditions for SSG. (b) Plot of the weight loss through SSG corresponding to (a). Inset: photographs of MS-200 salinometer for case 2.
The stability and recyclability are also very important for practical applications. Fig. S3 and Table S4, S5 show that the SSG performances of the carbonized kelps slightly changed after continuously irradiated or immersed in brine (3.5 wt%). In addition, as shown in Fig. S4, the carbonized kelp-based SSG device exhibits stable evaporation rates for 50 cycles, suggesting the outstanding recycling stability of the structure. The super stability of the device provides great potential for scalable and recyclable SSG and seawater desalination. Moreover, we would like to emphasize that the most advantage of our device is the low-cost. On one hand, targeting the desalination of seawater, it is almost no cost to locally collect the kelps. Compared with mushrooms45 and lotus seedpods44 that cost a lot in industrialization at large scale, it is commercially valuable to choose salt-tolerant and abundant kelps for SSG. On the other hand, polystyrene foam (∼$2.83 m−2) and fiber cotton (∼$0.90 m−2) are also cheap and can be reused.
CONCLUSION
In summary, carbonized kelp obtained through one-step calcination is developed as a solar absorber (∼93%) for the steam generation. A device composed of carbonized kelp, adiabatic polystyrene foam, and water-transporting fiber cotton wick is designed to meet the requirements of the “most stable triangle” for extremely cost-effective and highly efficient SSG. Our device has achieved a high solar thermal conversion efficiency of ∼84.8% with an evaporation rate of 1.351 kg·m-2·h-1 under 1 sun irradiation. Considering the ultralow-cost, simple preparation, biocompatibility, and good stability of the carbonized kelps, we provides a very promising route for practical steam generation and seawater desalination.
SUPPLEMENTARY MATERIAL
See supplementary material for calculations of light-to-steam conversion Efficiency at different conditions, stability and recyclability of the device.
ACKNOWLEDGMENTS
This work was jointly supported by the National Natural Science Foundation of China (No. 11604155), China Postdoctoral Science Foundation (Nos. 2016M600428 and 2017T100386), and Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1601023A).