Three-Dimensional Porous Solar-Driven Interfacial Evaporator for High-Efficiency Steam Generation under Low Solar Flux.

Solar steam generation is critical for many important solar-thermal applications, but is challenging to achieve under low solar flux due to the large evaporation enthalpy of water. Here, we demonstrate a three-dimensional porous solar-driven interfacial evaporator that can generate 100 °C steam under 1 sun illumination with a record high solar-to-steam conversion efficiency of 48%. The high steam generation efficiency is achieved by localizing solar-thermal heating at the evaporation surface and controlling the water supply onto the porous evaporator through tuning its surface wettability, which prevents overheating of the evaporator and thus minimizes conductive, convective, and radiative heat losses from the evaporator. The design of steam outlet located at the sidewall of the evaporator rather than from the solar absorber surface not only facilitates the collection of generated steam, but also avoids potential blockage of solar radiation by the condensing steam. The high-efficiency solar-driven evaporator has been used to generate hot steam for outdoor removal of paraffin on the wall of oil pipelines, offering a promising solution to mitigate the wax deposition issue in petroleum extraction processes.

Introduction

The Sun is a promising renewable energy source to propel sustainable development of human society.[1,2] In addition to direct conversion into electricity through photovoltaic process, a facile and efficient way to harness the abundant sunlight is converting it into heat through solar-thermal technology.[3,4] The harvested thermal energy can drive many important processes such as domestic heating,[5] seawater desalination,[6,7] sterilization,[8−10] distillation,[11,12] and electrical power generation.[13] In these applications, high-efficiency steam or vapor generation at high temperatures is often desired and critical to achieve high performance of the solar-thermal systems. The dilute natural 1 sun solar flux (1 kW/m2), however, does not provide sufficient solar-heating density to generate hot steam due to the high vaporization enthalpy of water (2.26 × 106 J/kg at 100 °C).[14] In traditional concentrated solar power steam generation technologies, it requires high optical concentrations (10–1000×) to generate hot steam. The expensive optical concentrators not only add significant cost to the evaporation systems, which would limit the deployment of solar-thermal systems in undeveloped regions, but also sacrifice the overall energy conversion efficiency of the system due to the increased heat losses from the hot solar receiver surfaces.

In recent years, solar-driven interfacial evaporation, which localizes solar heating at the air–water interface rather than heating up the bulk liquid, has emerged as a new form of evaporation design.[15−18] Compared to conventional bulk heating-based evaporation, the interfacial evaporation systems have demonstrated significantly increased solar-to-vapor conversion efficiency and faster response due to reduced heat losses and smaller thermal masses.[15,17] In the past, great efforts have been devoted to increasing the solar-to-vapor efficiency to ∼90% through synthesizing new solar-absorbing materials,[19−21] tailoring microstructure of the solar absorbers,[22−25] improving thermal insulation,[26,27] capillary water-supplying design,[28,29] and optimizing evaporator structure.[30,31] More recently, the favorable interaction between water and the hydrogel-based evaporator has been used to reduce the water evaporation enthalpy, thereby further increasing the evaporation mass flux.[32,33] Such high-efficiency solar-driven interfacial evaporation has enabled high-performance steam generation,[34−36] solar desalination,[29,33,37−39] multifunctional clean water generation,[40−43] and electricity generation[44,45] under reduced solar concentration. Most recent research interest has been directed to vapor generation under natural 1 sun illumination condition,[46−48] thus fully eliminating the requirement for optical concentrators. Steam generation under low solar flux, in particular, 1 sun illumination with a power density of 1 kW/m2, however, is still challenging.[9,10,17,49−51] In spite of its important medical sterilization applications, previous generation of steam at 100 °C or above requires high optical concentration.[9,10,49] Only until recently, Chen et al.[52] have reported a thermal concentration scheme and successfully generated 100 °C steam under 1 sun irradiation. The evaporator generated steam when the thermal concentration ratio was larger than 200 with the solar-to-steam conversion efficiency at ∼20%. Additionally, in the solar-driven interfacial evaporation systems reported so far, the generated vapor or steam is evaporated from the front surface of the solar absorbers. The broad distribution of steam over a large area not only adds difficulty in collecting the generated steam, but also tends to affect solar absorption by the absorber. For instance, in solar stills when the evaporated hot vapor or steam is in contact with the transparent condensing cover, it often forms a layer of blurry mist, which interrupts the normal receiving of sunlight for solar-thermal conversion.[47,48]

In this work, we report a three-dimensional (3D) porous solar-driven interfacial evaporator with minimized heat losses for high-efficiency steam generation under low solar flux. In this new porous evaporation structure, the effective surface area for occurrence of evaporation is amplified, thereby minimizing the conduction, radiation, and convection heat losses from the evaporation system. Controlled water supply, which is realized through tailoring the surface wettability of the evaporator, is used as a means to tune the temperature of the porous evaporator and vapor temperature. Steam is successfully produced at an efficiency of ∼48% by the porous evaporator with hybrid hydrophilic–hydrophobic surfaces under 1 sun illumination. Additionally, the steam outlet is designed to be located at the sidewall of the evaporator to facilitate steam collection without blocking normal solar irradiation onto the absorber. Outdoor experiments show that the collected steam can effectively heat up and melt the paraffin deposits on the wall of pipelines, thus offering another wax mitigation technology for petroleum industry.

Experimental Section

Materials

The selective absorber was purchased from Dezhou Jinheng Solar Company. Copper foam (30 ppi) was supplied by Kunshan Jiayisheng Electronics Company. Potassium hydroxide and potassium peroxodisulfate were purchased from Sinopharm Chemical Reagent. 1H,1H,2H,2H-Perfluorooctyltrichlorosilane was ordered from Shanghai Macklin Biochemical. Paraffin wax was purchased from Aladdin Reagent (Shanghai).

Fabrication of Copper Foam with Hybrid Surface Wettability

The hybrid hydrophobic–hydrophilic copper foams, which are hydrophilic at the bottom section and hydrophobic at the top section, were prepared by a three-step surface treatment of commercial pristine copper foams. In the first step, the surface of the whole copper foam was oxidized by a strong alkali solution to be hydrophilic. In the second step, the obtained hydrophilic copper foam was deposited with a fluorosilane to convert the surface into hydrophobic. In the third step, half of the hydrophobic copper foam was immersed within the strong alkali solution again to convert that portion into hydrophilic. Specifically, the commercial copper foam was first cleaned by acetone, ethanol, and deionized water through sonication before being immersed in the mixed solution of 0.065 M K2S2O8 and 2.5 M KOH at 60 °C. After 1 h, the oxidized hydrophilic copper foam was washed by deionized water and dried at room temperature. The hydrophilic copper foam was further modified with fluorinated silane through a vapor deposition process. The hydrophobic foam was obtained by placing the oxidized hydrophilic foam within a plastic vacuum chamber, in which 1H,1H,2H,2H-perfluorooctyltrichlorosilane is loaded. In the final step, the obtained hydrophobic copper foam was partially immersed in the K2S2O8/KOH solution again to convert that part of foam surface into hydrophilic.

Steam Generation and Paraffin Removal Test

Simulated sunlight generated by a solar simulator (EOS-150, EOS Technologies Inc.) or natural sunlight focused by a plastic Fresnel lens was used to induce steam generation. The evaporation mass change of the system was measured by a digital balance with a resolution of 0.1 mg (BSM-220, Shanghai Zhuo Jing Electronic Technology). K-type thermocouples were placed at the steam outlet to measure steam temperature or were attached to the surface to monitor the real-time temperature distribution of the evaporation system, and the measured temperatures were recorded by a multichannel data acquisition system (Agilent 34972A). The outdoor paraffin removal test was carried out on the roof of MSE Building H at Shanghai Jiao Tong University. The steam generator outlet was connected to a plastic bottle by a silicone tube with a length of 5 cm, and the silicone tube was wrapped by expandable polyethylene (thickness, 1 cm) to suppress heat loss. Within the plastic bottle, a copper tube with an inner diameter of 1 cm was used as a simulated oil pipeline. Paraffin (3 g) was deposited along the inner surface of the copper tube. One thermocouple was placed at the inlet of the paraffin removal chamber (T1), which directly measures the steam temperature when the solar steam enters the removal chamber. Three thermocouples (T2, T3, and T4) were uniformly attached to the inner surface of the copper tube (6 cm long) and their surfaces were covered by a layer of paraffin.

Measurement and Characterization

Optical absorption spectra of the spectrally selective solar absorber and transmittance spectra of quartz were measured by a UV–vis–NIR spectrometer (PerkinElmer Lambda 750 S). The microstructure of air-laid paper and copper foam was observed by field emission high-resolution SEM (Sirion 2000, FEI). The solar illumination power density was measured by a solar power meter (CEL-NP2000, Beijing China Education Au-light). Theoretical analyses of the heat loss, temperature distribution, and energy conversion efficiency of the evaporation system based on energy conservation principle and 3D COMSOL simulation are provided in the Supporting Information.

Results and Discussion

Minimizing heat loss is critical to achieve high-efficiency solar steam generation, in particular under low solar illumination flux. As shown in Figure , besides vaporizing water, part of the converted solar-thermal energy is lost through convection and radiation from the front surface and part of the converted heat is lost through downward conduction. The thermal concentration approach[52] relies on concentrating the harvested solar-thermal energy toward a small evaporation area to boost the vapor temperature (Figure a). Such design limits the available surface area for water evaporation, and the suppressed evaporation in turn causes overheating of the solar absorber. In the 3D porous interfacial evaporator (Figure b), an open-pore foam is used to connect the solar absorber and water-supplying layer to amplify the evaporation area. Through tuning the surface wettability of the foam, the evaporation mass could be optimized to ensure steam generation at 100 °C. With increased evaporation flux that stores the converted solar-thermal energy as latent heat in the hot steam, the surface temperature of the absorber decreases, which in turn reduces the heat losses from the evaporation system. With a temperature of 100 °C at the surface of the solar absorber, the calculated evaporation efficiency of the porous interfacial evaporator reaches 51% (Figure S1). Another unique design of the steam generator is the steam outlet, which is confined and located at the sidewall rather than on the top surface of the solar absorber. Such design would avoid potential blocking of solar incidence onto the absorber by the generated steam and facilitate steam collection.

Figure 1

Structure design of a solar-driven steam generator under low solar flux. (a) Schematic structure of thermal concentrated interfacial evaporator. A small hole is drilled through the surface of solar absorber, which allows the escape of the generated steam. (b) Schematic structure of 3D porous interfacial evaporator. The generated steam at the surface of the porous evaporator escapes from the sidewall.

As schemed in Figure a, the 3D porous interfacial steam generator contains five key components. First, a spectrally selective solar absorber is used to absorb the broad-band sunlight with minimized radiation loss. Second, a transparent cover made of quartz is placed on top of the absorber. Third, an air-laid paper is utilized as the capillary wicking material to spontaneously pump water from the reservoir at the bottom to the solar-heating region. In between the absorber and the air-laid paper is the copper foam that provides ample space for the vapor generation. Finally, a thermal insulator that is constructed from PDMS (poly(dimethylsiloxane)) foam is placed at the bottom to float the evaporator. The entire evaporator is sealed within a plastic beaker forming an integrated portable solar steam generator. To reduce heat leakage from the sidewalls, the steam generator is wrapped by thermal-insulating expandable polyethylene.

Figure 2

Key components of the 3D porous solar-driven interfacial steam generator. (a) Schematic structure of the steam generator. (b) Absorption spectrum of spectrally selective absorber. (c) Water-wicking test of air-laid paper. (d) SEM image of the air-laid paper under low magnification showing its porous structure. (e) SEM image of the air-laid paper under high magnification showing its fiber component. (f) Schematic process for preparing copper foam with different surface wettability. (g) Photograph of untreated copper foam (top) and treated hydrophilic copper foam (bottom). (h) SEM image of treated hydrophilic copper foam under low magnification showing its rough surface. (i) SEM image of treated hydrophilic copper foam under high magnification showing the formation of a needlelike nanostructure on the surface.

Optical spectrum measurement (Figure b) presents that the low-emissivity (0.05) spectrally selective absorber (TiNO) has a broad-band absorption, which fully covers the solar irradiance spectrum with a high absorptance (0.95). The quartz cover is highly transparent to the incident sunlight from 250 to 2500 nm with a transmittance of 93% (Figure S2), thus allowing for the efficient transmission of sunlight to the absorber while suppressing the convective heat loss. To avoid direct contact between the solar absorber and the bulk water body, the thermal insulation PDMS foam is used as both the physical separator and floater. The low thermal conductivity of the foam (0.03 W/(m K)) mitigates the downward heat leakage to the water body.

The hydrophilic air-laid paper is used as the wicking material that delivers water to the evaporation region. To test the capillary wicking capability of the air-laid paper, we placed it in direct contact with a red aqueous solution that contains Rhodamine dye to observe water movement. Figure c shows that the air-laid paper (5 cm long) is fully wetted by the red solution within 30 s. By comparing the mass change before and after sorption of water, the water sorption rate of the air-laid paper was measured to be 35.84 kg/m2 h, which is much higher than the theoretical maximum evaporation mass flux (1.584 kg/(m2 h)) under 1 sun illumination. Meanwhile, it should be noted that after absorbing water, the wetted air-laid paper would become more hydrophilic and the resultant water sorption rate would be even higher.[53] Hence, the air-laid paper can provide sufficient water supply to compensate the evaporation mass loss. SEM observation indicates that the air-laid paper is composed of interweaved fibers (Figure d,e), and the strong capillary pumping effect results from its surface hydrophilicity and porous microstructure.[25]

The porous foam is the key component to achieve large evaporation flux and high solar-to-steam conversion efficiency. Here, copper foam (30 ppi, 5 mm thick) was employed to construct the porous evaporation chamber due to its high thermal conductivity and wide availability. The converted solar heat can be timely conducted along the network of the copper foam to induce evaporation. For the occurrence of evaporation on the heated surface, it also requires delivery of water from the air-laid paper onto the surface of the copper foam. Similar to the air-laid paper, the copper foam also relies on capillary force to pump water from the air-laid paper to its surface. Surface wettability is used as a means to tune the water-wicking capability of the copper foam, thus controlling the amount of water supplied onto the foam surface. As schemed in Figure f, through different surface treatment, we prepared copper foams with hydrophilic, hydrophobic, and hybrid hydrophilic–hydrophobic surfaces and comparatively studied the evaporation performance of the corresponding evaporators. The hydrophilic surface was obtained through oxidizing the copper foam with a strong alkali solution.[54,55] After oxidation, Figure g shows that the foam turned black as a result of formation of copper hydroxide on the surface and the water contact angle decreased from 76° for the pristine copper foam (Figure S3) to fully wetted state for the hydrophilic copper foam. SEM observation indicates that the oxidized copper foam surface is rough (Figure h) and the surface is covered with a layer of densely distributed needlelike nanostructure (Figure i). Hydrophobic surface with a water contact angle of 130° (Figure S3) was prepared by coating the oxidized hydrophilic copper foam with fluorosilane (1H,1H,2H,2H-perfluorooctyltrichlorosilane). To obtain hybrid wettability, the hydrophobic copper foam was partially immersed into the alkali solution to chemically cleave the silane coating and convert that portion of foam into hydrophilic copper again. For the treated copper foam with hybrid wettability, the hydrophilic side is placed in direct contact with the air-laid paper to wick water upward.

Figure a shows the experimental setup for evaluating the steam generation performance of the 3D porous solar-driven interfacial evaporator. A solar simulator was used to generate solar irradiation, which penetrates the transparent quartz cover and shines onto the spectrally selective absorber. The evaporation mass loss was recorded by a digital balance, and a thermocouple was placed near the outlet to measure the vapor temperature. The evaporation efficiency (η), i.e. solar-to-vapor or solar-to-steam conversion efficiency, is calculated by the following formulawhere ṁ is the evaporation mass change rate, hLV is the enthalpy change of water during the evaporation process that includes both the liquid–vapor phase change enthalpy and sensible heat, qsol is the solar flux, and A is the area of the spectrally selective absorber (13.85 cm2).

Figure 3

Tunable steam generation performance. (a) Schematic experimental setup for measuring evaporation performance. (b) Temperature evolution of steam generators with different surface wettability. The dashed line marks the practical steady-state steam temperature at 97 °C. (c) Steady-state evaporation mass flux of steam generators. (d) Comparison of evaporation efficiency. (e) Simulated temperature evolution of generated vapor. (f) Simulated steady-state temperature distribution of the steam generator with hybrid surface wettability.

The evaporation performance of three evaporators consisting of hydrophobic, hydrophilic, and hybrid hydrophobic–hydrophilic copper foams was studied. As shown in Figure b, the hydrophobic evaporator has shown the fastest vapor temperature rise and reached a steady temperature after 1 sun illumination for 1000 s. It should be pointed out that due to rapid cooling of the generated steam, the achieved steam temperature is 97 °C, which is slightly lower than the theoretical steam temperature of 100 °C. Under such condition, the steam is successfully generated when the evaporator surface temperature reaches 100 °C and the evaporation process enters the steady state.[52] After illumination for 1200 s, the evaporator with hybrid surface wettability also started steady steam generation. By contrast, the hydrophilic evaporator has lower vapor temperature, and even after illumination for 30 min, it did not reach 100 °C. The measured surface temperature of the selective absorber indicates the same trend that the hydrophobic evaporator has the highest temperature of 108 °C, the hybrid evaporator has reached 103 °C, and the hydrophilic evaporator has the lowest temperature of 92.8 °C after the same 1 sun illumination for 30 min (Figure S4). During the experiment, it was found that even extending the illumination time to 1 h, the vapor temperature curve of the hydrophilic evaporator became flat, which indicates that the system has reached a thermal balance with the atmosphere, but the vapor temperature and surface temperature of the solar absorber could not reach 100 °C. To characterize the evaporation rate at the steady state, the evaporation system was subject to 1 sun illumination for 30 min before measuring the mass change. Figure c presents the evaporation mass change for the three evaporators as a function of illumination time. It shows the opposite trend to the vapor temperature. The hydrophilic evaporator has the largest mass loss and the hydrophobic evaporator has the smallest mass loss. The corresponding evaporation efficiencies for the evaporator with hydrophobic, hydrophilic, and hybrid copper foam are 37, 52, and 48% (Figure d), respectively. The solar-to-steam conversion efficiency for the evaporator with hybrid hydrophilic–hydrophobic copper foam (48%) is more than double the steam generation efficiency achieved in the thermal concentration scheme (∼20%).[52]

It can be seen that water supply, which is controlled by the surface wettability of the copper foam, is the key factor that determines the vapor temperature and evaporation efficiency. Slow water supply from the hydrophobic copper foam facilitates the increase of the vapor temperature to the steam temperature, but it limits the evaporation efficiency. Fast water supply from the hydrophilic surface increases the evaporation flux but lowers the vapor temperature. We evaluated the water supply toward the evaporator with different hydrophilicity by measuring the amount of water adsorbed onto the copper foam surface. The adsorbed water affects the physical properties of the copper foam evaporator, which in turn adjusts the evaporation performance. The energy conversion efficiency of the evaporator can be analyzed by deducting the heat losses from the incident solar energy input. Based on the energy balance principle, we can analyze the evaporation efficiency for the three evaporation systems. We measured the temperatures at the quartz cover surface, absorber surface, and top and bottom surfaces of the PDMS foam as the input parameters (Figure S5). Based on these measured temperatures, we estimated the combined conduction, convection, and radiation heat losses from the evaporator and calculated the evaporation efficiency (Figures S6 and S7). As shown in Figure d, the calculated evaporation efficiency is in good agreement with experimental measurement.

To gain insight into the temperature evolution of the vapor generated within the porous copper foams, we further carried out 3D COMSOL simulations for the evaporation system.[56] By measuring the weight of treated copper foams before and after wetting with water, the amount of water adsorbed by the copper foams with different wettability was estimated. The water was adsorbed onto the copper foam through the capillary wicking effect, which pumps water from the air-laid paper layer toward the copper foam evaporation surfaces. In our case, the adsorbed water only coats the surfaces of the copper foam, considering that the large pore size (∼1 mm) of the copper foam cannot provide the needed capillary force to fill the entire pore with water. As schemed in Figure b, the porous foam provides large surface areas for the evaporation to proceed in a 3D manner. The adsorbed water affects the thermophysical properties of the original copper foam, including density, specific heat capacity, and thermal conductivity. In particular, the adsorbed water significantly increases the effective heat capacity of the copper foam. Thus, the same amount of received solar heat can only induce slower and smaller temperature rise in the copper foam with adsorbed water. Figure e shows the simulated vapor temperature evolution profiles as a function of illumination time. It can be seen that the surface wettability of the copper foam determines the temperature of the vapor generated, and the simulated results are in good agreement with experimentally measured temperature curves in Figure b. Figure f shows the 3D temperature distribution of the whole evaporation system with hybrid hydrophilic–hydrophobic copper foam. It clearly shows localized heating at the solar absorber and the copper foam, and other parts of the evaporation system are not heated up. After 1 sun illumination for 30 min, the temperature of the copper foam reached 100 °C, indicating that the evaporator can successfully generate steam.

We further investigated the influence of increased solar illumination intensity ranging from 2 sun (2 kW/m2) to 5 sun (5 kW/m2) that is attainable with common low-cost plastic lenses on the steam generation performance for the evaporator with hybrid wettability. Figure a presents that the evaporation mass loss increased with larger solar illumination flux. Figure b shows that the evaporation efficiency gradually increases with solar flux. The solar-to-steam conversion efficiencies reach 65 and 81% when the solar flux increases to 2 and 5 sun, respectively. The evaporation efficiency under different solar flux can also be estimated through deducting the heat losses from the received solar energy, and it matches well with measured values (Figure b). Temperature measurement of the generated vapor in Figure c shows that stronger solar flux shortens the heating-up time for the evaporator to reach the steady state. For example, when the solar flux is 5 sun, it generates steam within 4 min. By utilizing the aforementioned 3D COMSOL model, we simulated the response time for the evaporator to reach the steady steam generation state. As shown in Figure d, the heating-up time quickly decreases with increased solar flux, implying that solar illumination power density could be used as another means to tune the steam generation behavior. In general, the simulated trend and experimentally measured results are in good agreement. The slightly overestimated drop of the heating-up time by simulation is due to the fact that the thermal contact resistance between each component within the evaporator was not considered in the model.

Figure 4

Solar steam generation performance by the porous solar-driven interfacial evaporator with hybrid wettability under different solar flux. (a) Steady-state evaporation mass flux. (b) Steady-state solar-to-steam conversion efficiency. (c) Evolution of steam temperature. (d) Simulated heating-up time for the steam generator to reach steady state and comparison to experimental measurement.

Finally, we demonstrated such porous solar-driven interfacial evaporator for outdoor steam generation and explored its application for the removal of paraffin deposits from the wall of oil pipelines, which would restrict normal crude oil flow and may eventually block the pipeline.[57,58]Figure a presents that the outlet of the steam generator is connected to a plastic bottle in which a copper tube filled with paraffin inside was placed to simulate the deposition of paraffin wax on the wall of oil pipelines. The generated hot steam warms up the copper tube, which in turn melts the paraffin deposit. The melted paraffin liquids slip along the tube and fall into the bottom of the container, thus avoiding blockage of the pipeline in oil wells. Four thermocouples were placed at the inlet (T1) and copper/paraffin interface (T2, T3, and T4) to monitor the temperature change in the paraffin removal chamber (Figure a). The outdoor experiments were carried out on the roof of a building in Shanghai Jiao Tong University (SJTU) in Shanghai, China. The main test day was June 28, 2018, which was a cloudy day. The time-dependent global horizontal solar irradiance was recorded by a solar power meter. Due to roaming cloud, the solar flux from 13:00 to 14:00 varied between 150 and 420 W/m2 (Figure b). A Fresnel lens with a geometric concentration ratio of 10.5 was used to focus natural sunlight. As the testing time only lasted 1 h, we did not use any tracking or aiming system during our experiment. Figure c presents that after solar irradiation for 700 s, the inlet temperature of the plastic bottle (T1) quickly reached steady state at 97 °C, indicating that the hot steam has diffused into the paraffin removal chamber. After illumination for 1200 s, the temperature measured at the interface between the paraffin wax and the inner copper tube (T2, T3, and T4) gradually rises as a result of heat exchange between the hot steam and the copper tube. When it passes the melting temperature of paraffin (60 °C) at 1500 s, the temperature quickly increases as the deposited paraffin fell apart from the copper tube. After removal of the paraffin wax, the copper tube is subject to sensible heating that has a higher temperature increasing rate (Figure c). The inset photographs of the treated copper tube confirm that the initially deposited paraffin was fully melted and removed from the inner surface of the copper tube.

Figure 5

Outdoor steam generation and paraffin removal application. (a) Schematic setup for outdoor removal of paraffin deposit on the wall of oil pipelines. (b) Outdoor natural solar flux. (c) Temperature evolution profiles of the paraffin removal chamber. The dashed line marks the melting temperature of the paraffin wax at 60 °C. The inset photograph shows the simulate pipeline deposited with paraffin wax in the inner wall before (left) and after (right) steam treatment.

Conclusions

In summary, we demonstrated an alternative solar-driven interfacial evaporator structure to efficiently generate steam under low solar flux. The control over the amount of water supplied onto the porous evaporator surface is the key to simultaneously increase the vapor temperature and prevent overheating of the evaporator, which minimizes heat losses from the evaporation system. The capillary pumping-based water supply allows us to change the surface wettability of the porous evaporator, thereby tuning the water supply toward the evaporation region. The confined steam outlet design at the sidewall rather than directly perfuming from the whole surface of the solar absorber facilitates the steam collection and consequent utilization of generated steam. Besides the demonstrated wax removal application, the porous interfacial evaporator can also be used for high-efficiency solar desalination with improved performance. For example, it can avoid the formation of blurry mist on the surface of the condensing cover, thus ensuring continuous and stable operation of the evaporation process. Additionally, the separated solar absorber and evaporation region design can mitigate salt fouling of the absorber. The generated steam can be directly utilized or be further pressurized to achieve even higher temperature for sterilization applications. Given the wide availability of low-cost materials for fabricating the evaporation system, no requirement of costly optical concentrators, and the broad range of enabled applications, we believe that the high-efficiency portable steam generator will help accelerate the widespread utilization of solar-thermal technologies, in particular, under low solar flux.