Separating photo-thermal conversion and steam generation process for evaporation enhancement using a solar absorber

Applied Energy 236 (2019) 244–252

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Separating photo-thermal conversion and steam generation process for evaporation enhancement using a solar absorber

T

⁎

Jian Huang, Yurong He , Meijie Chen, Xinzhi Wang School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Heilongjiang Key Laboratory of New Energy Storage Materials and Processes, Harbin, Heilongjiang 150001, China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

C-TiO solar absorber with a good • photo-thermal conversion property 2

was prepared.

photo-thermal conversion • Separating and steam generation process was designed.

The film enhancement effect with dif• ferent evaporation areas was investigated.

changed thermal gradient be• The tween the film and evaporation region was verified.

A R T I C LE I N FO

A B S T R A C T

Keywords: Solar absorber Photo-thermal conversion Steam generation Separating design

Solar steam generation is an effective method combining solar energy utilization with water treatment. Photothermal conversion and steam generation are typically integrated to enhance the evaporation process, which have wide applications in seawater desalination, waste water treatment, sterilization and power plant fields. However, the photo-thermal enhancement for different evaporation areas remains unclear, and there are a number of important issues for membrane process (e.g., blockage of pore structures and contamination of nanoparticles). To overcome these issues, we herein propose a separating design involving a C-TiO2 absorber and a polyvinyl alcohol fiber material as the photo-thermal and steam generation units, respectively. A C-TiO2 absorber with good spectral and photo-thermal conversion characteristics was prepared. And the evaporation enhancement effect was investigated with different evaporation areas by experiments and simulations. The equivalent evaporation rate reached the maxima with the evaporation area and decreased thereafter for this separating design. The optimum behavior was achieved when the evaporation region area to photo-thermal area ratio of ca. 2.06, providing guidance for large-scale use. These results can be explained in terms of the changed thermal gradient generated between the center C-TiO2 film and the evaporation region. The design achieved equivalent evaporation rates and evaporation efficiencies of 1.24 kg·m−2·h−1 and 77.83%, respectively, paving the way for the further improvement of solar steam generation processes.

⁎

Corresponding author at: School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China. E-mail address: [email protected] (Y. He).

https://doi.org/10.1016/j.apenergy.2018.11.090 Received 18 August 2018; Received in revised form 27 October 2018; Accepted 23 November 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.

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Nomenclature u1

ṁ total ṁ 0 ṁ 1

m0

mtotal t u0

the total evaporation mass per unit time of the experiment device under light irradiation) (kg·h−1) the evaporation mass per unit time using the PVA fiber material without light (kg·h−1) the evaporation mass per unit time obtained via photothermal enhancement of the C-TiO2 solar absorber (kg·h−1) the evaporation mass change using PVA fiber material under natural conditions without light for a certain period of time (kg) the total evaporation mass change of the experiment device under light irradiation (kg) the evaporation time (h) the mass change per unit time and area using the PVA fiber material under natural conditions without light

A0 A1 u3 u4 utotal hfs Qs ηtotal

1. Introduction

(evaporation rate, kg·m−2·h−1) the mass change per unit time and area obtained via photo-thermal enhancement of the C-TiO2 absorber (kg·m−2·h−1) the evaporation region area of the PVA fiber material (m2) the photo-thermal conversion area of the C-TiO2 absorber (m2) the evaporation rate of the device relative to the unit photo-thermal area (kg·m−2·h−1) the evaporation rate relative to the unit evaporation region area (kg·m−2·h−1) the total equivalent evaporation rate (kg·m−2·h−1) the latent heat of the water evaporation at 1 atm (2.257 MJ/kg) solar radiation energy density per unit area (1 kW/m2) the total equivalent evaporation efficiency

Zhu et al. [34] found that the interfacial solar steam generation can enable fast-responsive and energy-efficient sterilization. Ni et al. [35] reported that about 100 °C steam under ambient air conditions could be generated and used for power generation. Zhou et al. [36] achieved water productivity of 3.67 kg·m−2 with salt rejection over 99.75% in one cloudy day. Besides, Halas et al. [12] combined solar steam processing and solar distillation for fully off-grid production of cellulosic bioethanol. However, many work with the porous membranes are usually tested in laboratory conditions by pure water or artificial seawater without the actual application process and complex chemical composition environment, making the fouling process inconspicuous. Nevertheless, like other membrane distillation processes for the actual seawater desalination industry, these blockage and fouling problems get high-impact on the fresh water production process for the long term running process [37], and the porous membrane serves both as photo-thermal conversion and evaporation regions, leading to some inevitable issues (e.g., blockage of pore structures and NPs fouling) and hindering current applications. Besides, since the photo-thermal area is equivalent to the evaporation area in these systems, thermal concentration is difficult to improve unless an expensive and complicated light focusing system is used. Therefore, we herein designed a separating photo-thermal and steam generation system to enhance the evaporation process by using a solar absorber. Thus, the photo-thermal conversion was achieved by the solar absorber while the steam generation process took place on a polyvinyl alcohol (PVA) fiber material. In this work, C-TiO2 solar absorbers were first synthesized and their morphology, structure, and composition were characterized. Then, the spectral characteristics and photo-thermal conversion properties of these C-TiO2 solar absorbers were investigated by experimental work and numerical simulations. Subsequently, solar steam generation experiments were carried out. The main contributions of this work are as follows: firstly, separating photo-thermal conversion and steam generation process was designed. The separating and indirect design has the following advantages: (1) this approach avoided pore structures blockage and NPs pollution via indirect contact with the working medium; (2) this separate design allowed us to conduct the photothermal conversion and steam generation in different regions such that the influence of different areas could be investigated; (3) the thermal concentration process could be adjusted to the optimal area ratio of the photo-thermal region and steam generation region. Secondly, a new evaluation system was established to measure the final evaporation capacity of the device. Thirdly, the film enhancement effect with different evaporation areas was investigated. Lastly, the changed thermal gradient between the film and evaporation region was verified.

Water and energy resources, the basis of human life, face serious challenges with the increase of population and environmental degradation [1,2]. The use of solar energy to solve the energy consumption problem of water treatment has great potential for the development [3]. At present, there are two strategies to utilize solar energy: photo-thermal conversion [4] and photo-electric conversion [5]. Therein, solar steam generation, as a high photo-thermal conversion method, is an environmentally friendly approach to providing some potential applications in the seawater desalination [6,7], waste water treatment [8], sterilization and power plant [9] fields. Combining with other processes, this steam generation process could be utilized for the water extraction [10], molecular hydrogen production [11], cellulosic bioethanol production [12], separation of mixed solvents [13] and even food processing [14]. The key to the solar steam generation lies in achieving the efficient solar thermal conversion and water evaporation. In traditional solar steam generation equipment such as solar pond and solar still, solar energy is captured by a solar absorber under an evaporative medium [15]. Although the solar absorber possesses relative great light absorption capacities, the efficiency of the solar steam generation process is still low because of the presence of large thermal losses. Two strategies have been employed to improve the solar steam generation process using nanomaterials and nanotechnology. One strategy lies in the utilization of nanofluids, including noble metal nanoparticles (NPs), metal semiconductor NPs, carbon-based nanomaterials, and other composite nanomaterials (e.g., Ag, Au [16,17], graphene [18], carbon nanotubes [19], graphene-Ag [20], and Ag@TiO2 [21], among others). Although the photo-thermal conversion efficiency of this nanofluid system could reach up to 95% [17], the steam generation efficiency was usually lower than 60% and the solar water evaporation rate was usually smaller than 1.4 kg·m−2·h−1 [19,20]. The second strategy involves the utilization of micro/nano composite structure membranes formed by nanomaterials and porous substrates such as Au NP [22,23], Al NP [24], Au@TiO2 NP, graphene [25,26], waste black polyurethane sponges [27], carbon nanotube [28] films and carbonized wood [29,30], among other materials. And films with diverse structures and materials ranging from an initial simple structure to more complicated three-dimensional (3D) structures have been investigated by many researchers [31,32]. Through these approaches, the light absorption could reach about 99% [10] and the photo-thermal conversion efficiency could be up to 95% [27–29]. Moreover, the solar steam generation rate could reach up to 1.4 kg·m−2·h−1 while the solar water evaporation efficiency could reach up to 85% [32,33]. These improvements promoted the development for the actual application. 245

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2. Experimental

conversion and steam generation process could be separated. A nontransparent cover was made and the central area was dug out to make sure that sunlight is irradiated only on the solar absorber and the PVA fiber material located near the film is not irradiated, which could exclude the effect when the evaporation area was irradiated by sunlight. A constant absorber area as used herein, while the evaporation region area (The evaporation region area was not in accordance with the actual water evaporation area resulting from the porous structure of PVA materials. However, this evaporation region area could be used to evaluate the effect of different evaporation areas when the same congruent relationship between the evaporation region area and the actual water evaporation area for the same PVA material) ranged from 9 to 65 cm2, allowing us to study the effects of the evaporation region area on the solar steam generation process. The entire device floats on water, and the water mass change is measured by a precision balance (PRACTUM313-1CN, Sartorius, Germany). Besides, a thermocouple is provided in the middle of the PVA fiber material to measure the temperature of the water-soaked PVA fiber material, while the temperature of the solar absorber is monitored by an infrared thermal imager (Ti450, Fluke, America).

2.1. Solar absorber synthesis Alcohol (C2H5OH, EtOH, AR99.7%), salicylic acid (C7H6O3, SA, AR99.5%), butyl titanate (C16H36O4Ti, TBT, ACS99.0%), and diacetone (C5H8O2, Acac, AR99.0%) were obtained from Aladdin (Shanghai, China). The C-TiO2 solar absorber was synthesized by a sol–gel method [38]. 1.967 mL of EtOH was thoroughly mixed with 0.276 g of SA and the mixture was ultrasonicated for 10 min. Then, 0.341 mL of TBT were added to 0.204 mL of Acac and the resultant mixture was ultrasonicated for 10 min. Next, the first mixture was added dropwise to the second mixture and the mixture stirred for 2 h. An ageing process was conducted for 24 h (25 °C, 30–50% relative humidity) to prepare the sol. The as-prepared sol was spin-coated on polished Cu substrates (40 mm × 40 mm × 1 mm). The resulting sample was dried for 2 h (80 °C) and subsequently annealed in a tube furnace (nitrogen, 600 °C (5 °C/min ramp) for 1 h).

2.2. Photo-thermal conversion and evaporation experiments 3. Results and discussion The photo-thermal conversion and evaporation experimental system is shown in Fig. 1. The solar light source herein used was produced by a solar light simulation transmitter (S500, China), while the light intensity was measured by a strong light photometer (CEL-NP2000, China). The entire evaporation device consisted of three parts. The top part is the C-TiO2 solar absorber to absorb solar energy and performs the photo-thermal conversion process, which could transfer heat to the working medium through direct contact with the PVA material. The middle part corresponded to the PVA fiber material, which contacted directly with the solar absorber and extended down to the bottom of the working medium. The bottom working medium is continuously transported to the surface of the PVA fiber material by capillarity for the steam generation process while heat exchange was performed with the solar absorber to evaporate the water. The insulating foam in the lower part of the PVA fiber material served as a supporting structure for the entire device. It ensures the floatability on water and reduces heat losses of the PVA fiber material towards the bulk working medium. For the operation of this system, sunlight is irradiated only on the solar absorber while the PVA fiber material located near the film is not irradiated using a non-transparent cover so that the photo-thermal

3.1. Morphologies, structures and composition The surface and cross section morphologies of the prepared C-TiO2 solar absorber is shown in Fig. 2 (a). A relatively stable film with a thickness of ca. 301 nm was formed on the Cu substrate. As shown in Fig. 2 (b), the surface of the solar absorber was relatively rough because of the presence of TiO2 NPs and some raised or sunken structures. These structures might be responsible for capturing light within the C-TiO2 solar absorber. For the smooth surface, a large part of light energy would be taken by the mirror reflection. However, when roughness exists in the surface, multi-scattering and multi-refracting would occur due to the multiscale convex or concave structures, making the incident light easier to be captured by the solar absorber [39]. Therefore, the roughness in the surface could improve the efficiency of solar absorber to a certain extent. As shown by high resolution transmission electron microscopy (HTEM), the spacing between adjacent lattice fringes was 0.348 nm, close to that of the d–spacing of the (1 0 1) crystal plane of anatase–type TiO2 (d = 0.352 nm; JCPDS No.21–1272). This result indicated that the C-TiO2 solar absorber was formed by TiO2 NPs (blue

Fig. 1. Schematic illustration of the solar water evaporation experiment platform. 246

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b

c

d

250

Intensity (a.u.)

a

♥ TiO2 (101)

200 150 100 50 0 0

10

20

30

40

50

60

70

80

90

2θ (degree)

e

2.0x105

f

C1s

1000

Intensity (a.u.)

Counts (s)

1.5x105 O1s 1.0x105

5.0x104

0.0

Ti2p 0

200

1200

400

800

Experiment data Fit D Peak Fit G Peak Cumulative Fit Peak

600 D

400 200

-200 800

800

37.5cm-1 FWHM

231.7cm-1 FWHM

0 600

G

1000

1200

1400

1600

1800

2000

-1

Raman Shift (cm )

Binding Energy (eV)

Fig. 2. (a) SEM (insertion is the image of cross section), (b) AFM and (c) HTEM images of the solar absorber; (d) XRD pattern, (e) XPS pattern and (f) Raman spectra of the solar absorber.

a

100 Polished Cu C-TiO2 (Cu)

60 40 20 0 300

600

900

1200

1500

1800

2100

100 Polished Cu C-TiO2 (Cu)

80

Absorption (%)

Absorption (%)

80

b

c

60 40 20 0 300

600

Wavelength (nm)

900

1200

1500

1800

2100

Wavelength (nm)

Fig. 3. The absorption spectra of the C-TiO2 film obtained by (a) experiments employing an ultraviolet–visible–near-infrared spectrophotometer system and (b) numerical simulations in the UV–Vis-NIR region; (c) the electric field profile in x–z plane at the wavelength of 520 nm. 247

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infrared regions when C-TiO2 thin film was coated on the Cu substrate. As confirmed by the electric field distribution within the x–z plane (Fig. 3 (c)), the electric field intensity near the Cu substrate was almost zero, revealing that the incoming light was absorbed mostly by the CTiO2 composite membrane 300 nm in thickness, indicating that the main effect of Cu substrate was to reduce the emissivity in the infrared region while the good absorption mainly resulted from the light absorption characteristics of C materials. That can be proved through further simulation results. As shown in Fig. S2 (a) and (b), the simulated reflection spectra changed slightly upon increasing the size of the TiO2 NPs from 10 to 40 nm, while the volume fractions increased from 0 to 50%. These results revealed that good light capture capacity of the C-TiO2 absorber resulted from the significant light absorption characteristics of C materials in the full spectrum region while different radius and volume fractions of TiO2 NPs had little effects on light absorption. Therefore, TiO2 NPs immersed in the composite membrane have following effects: (1) providing a lower emissivity compared with carbon materials (as shown in Fig. S3); (2) protecting the infrared emissivity of composite films from deterioration during the photo-thermal conversion process [41]; (3) the amorphous C near the TiO2 NPs could absorb more energy than the C in other regions due to the scattering effect [42]. These spectrum characteristics could be important for the photo-thermal conversion process, which make the film obtain a good photo-thermal conversion capacity (See more in Supporting Information). Furthermore, photo-thermal conversion was experimentally carried out on the experimental apparatus (See more in Supporting Information). And taking the energy balance method mentioned in the work of Carvajal J J [43], the photo-thermal conversion efficiency was calculated and the efficiency of 83.7% was obtained. Compared with other photo-thermal materials reported in the literature, the photo-thermal conversion efficiency of this film was in the middle. However, it would be favorable in the cost and preparation technology for this film, and there is still improved space to further increase the efficiency of the solar steam generation.

circle regions) and amorphous carbon (red square region). The HTEM results were in line with the XRD pattern, which revealed the presence of (1 0 1) crystal planes of TiO2 NPs and the absence of diffraction peaks corresponding to amorphous carbon. Besides, as shown in Fig. 2 (e), the Ti2p3/2 peak was observed at 457.08 eV, which was close to the standard value of Ti2p in TiO2 (458.8 eV). And the peak position of the O1s was at 530.08 eV, which corresponded nearly to the standard value of O1s in TiO2 (529.9 eV). These results revealed the presence of TiO2 in the film. Two strong peaks appeared in the Raman spectrum at 1341 and 1595 cm−1, corresponding to the D and G peaks of the C material. The D peak to the G peak intensity ratio could show the size of the sp2 clusters (i.e., graphitic six-membered ring structure carbon), and the corresponding relationship was as follows [40]:

I (D)/I (G) ≈ 0.55La2

(1)

where I(D) and I(G) are the intensities of the D and G peaks, respectively, and La is the sp2 cluster size (i.e., average size of the graphite nanocrystals). By fitting the Raman spectra, a I(D)/I(G) ratio of ca. 0.62 was obtained, which corresponded to an average size of the graphite nanocrystal of ca. 1.06 nm. Besides, the full width half maximum (FWHM) of the D and G peaks reflect the ordering extent of the C atoms. The fitting results revealed FWHM values of 231.7 and 37.5 cm−1 for the D and G peaks, respectively, indicating that the order of the C atoms in this film was slightly higher than that of amorphous carbon (300 and 100 cm−1, respectively). In addition, since the G peak of the composite film was observed at a slightly higher wavenumber as compared to a completely amorphous carbon (1580 cm−1), the carbon in the film has a certain degree of graphitization. 3.2. Spectrum characteristics To theoretically investigate the spectrum characteristic of the CTiO2 composite membrane, the spatial distributions of E-field intensity and the absorption spectra were numerically calculated using 3D Finitedifference time-domain (FDTD) simulations. More details are available in the Supporting Information. Fig. 3 (a) shows the absorption spectra obtained by absolute hemispherical measurements employing an ultraviolet–visible–near-infrared spectrophotometer system with an integrating sphere, while Fig. 3 (b) shows the FDTD simulated absorption spectra for the C-TiO2 composite membrane in the 300–2100 nm, respectively. Though the peak value was unable to match due to the complex reality, the experimental spectrum was in line with the simulated spectrum over the entire frequency range. For the experimental and simulation spectrum, it is obvious that the C-TiO2 absorber showed strong absorption in the UV–visible region due to the excellent light absorption capacity of carbon materials, and then decreased rapidly in the near infrared region, which was owing to the selective absorption capacity originated from the optical properties of C, TiO2 materials and the Cu substrate. Compared with a polished Cu substrate, the absorption was increased significantly in the UV–visible and near

a

3.5 3.0

3.3. Solar steam generation experiments Before the solar steam generation, a suitable water transport speed was ensured experimentally, as displayed in Fig. 4. After water uptake, the mass change ratio (i.e., mass after water uptake/mass before water uptake) of the PVA material was 4.91, indicating a good water absorption capacity. Combining with the wetting degrees of the PVA material, we concluded that the water transport speed on the material was very high (i.e., it took 30 s for water to spread over the entire material). Thus, the PVA material showed a higher water transport speed compared with others’ work [44], which is suitable for solar steam generation processes. Then, solar steam generation experiments were conducted, and the effects of different evaporation region areas were investigated for this

b

Water uptake of the PVA material Mass change ratio=4.91

Mass (g)

2.5 2.0 1.5 1.0 0.5 0.0

Before

After

Fig. 4. (a) Mass changes of the PVA material before and after water uptake; (b) Wetting degrees of the PVA material with time. 248

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(1 kW/m2); u0 is the evaporation mass per unit time and area using the PVA fiber material without light; u1 is the evaporation mass per unit time and area obtained via photo-thermal enhancement of the C-TiO2 absorber; utotal is the total equivalent evaporation rate (kg·m−2·h−1); ηtotal is the total equivalent evaporation efficiency. u0 and u1 can be calculated according to the following formula [18]:

separate photo-thermal conversion and steam generation system. Since the above-mentioned solar evaporation system separated the photothermal conversion from the water evaporation process, the evaporation region area did not coincide with the photo-thermal conversion area. Thus, the final water evaporation efficiency and the evaporation rate were enhanced by both the photo-thermal effect and the different evaporation region areas. Therefore, a new evaluation system was required to measure the final evaporation capacity of the device (More details in Supporting Information). We assumed that the evaporation and the photo-thermal areas of the device are equal, while the equivalent evaporation rate and evaporation efficiency can be obtained as follows [45]:

utotal = u 0 + u1

(2)

utotal h fs Qs

(3)

ηtotal =

Mass change (g)

Light off Water PVA

-0.05 -0.10 -0.15 -0.20 -0.25

u1 =

ṁ total u A − 0 0 A1 A1

(5)

Evaporation area 9cm2 20cm2 33cm2 48cm2 65cm2

b

0.0 -0.2 -0.4 -0.6 -0.8 -1.0

-0.30 -0.35

(4)

0.2

Mass change (g)

a

0.00

ṁ 0 A0

where ṁ 0 is the evaporation mass change per unit time using the PVA fiber material without light; ṁ total is the evaporation mass change per unit time of the experiment device under light irradiation; A0 is the evaporation region area of the PVA fiber material; A1 is the photothermal conversion area of the C-TiO2 absorber. The experimental results are displayed in Fig. 5. To calculate the

where hfs is the latent heat for water vaporization at 101.325 kPa (2.257 MJ/kg) and Qs is solar radiation energy density per unit area 0.05

u0 =

0

300

600

900

1200

1500

-1.2

1800

0

300

600

Time (s) 55

Temperature (°C)

1200

1500

1800

d

c

50

900

Time (s)

45 40 35

Evaporation area

30

9cm2 20cm2 33cm2 48cm2 65cm2

25 20 15

0

300

600

900

1200

1500

1800

u3 u4

e

1.6 1.2 0.8 0.4 0.0

9cm2

20cm2 33cm2 48cm2 65cm2

Evaporation region area

100

1.4 utotal

f

1.3

ηtotal

90

1.2 20cm2

33cm2

80

48cm2

9cm2

1.1

65cm2

1.0

60

0.9 0.8

70

0

10

20

30

40

50

60

Evaporation region area (cm2)

50 70

Evaporation efficiency (%)

Evaporation rate (kg⋅m-2⋅h-1)

2.0

Evaporation rate (kg⋅m-2⋅h-1)

Time (s)

Fig. 5. (a) Mass changes of the PVA material and pure water VS time without light irradiation; (b) Mass changes and (c) Temperature changes of different evaporation region areas VS time; (d) Temperature distributions with different evaporation region areas; (e) u3 and u4 with different evaporation region areas; (f) utotal and ηtotal with different evaporation region areas. 249

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evaporation rate of the PVA fiber material under natural conditions (25 °C and 20–30% relative humidity (RH)) without light, the mass change in the absence of light was measured (Fig. 5 (a)). The evaporation rate of the PVA fiber material (0.177 kg·m−2·h−1) was slightly larger than that of pure water (0.154 kg·m−2·h−1), which is mainly due to the higher evaporation area with micro structure of the PVA fiber material. The evaporation mass change under the irradiation of 1 sun at constant photo-thermal conversion area (area of the C-TiO2 film, 16 cm2) and varying evaporation area (9–65 cm2) is shown in Fig. 5 (b). The mass change rate increased first and levelled off thereafter. As shown in Fig. 5 (c), the time needed to reach stable temperature was 1200 s for all the evaporation areas, which was consistent with the photo-thermal conversion experiment. These results indicated this solar steam generation process reached the steady evaporation stage at ca. 1200 s, since the mass change was only related to the temperature (i.e., the rest conditions remained constant). Though the mass change increased with the evaporation region area, the effects of the evaporation region area on steam generation enhancement was unclear. As described before, since the evaporation region area did not coincide with the photo-thermal conversion area, there would be two evaporation rates namely, the evaporation rate of the device relative to the unit photo-thermal area (u3) and the evaporation rate relative to the unit evaporation region area (u4, Fig. 5 (e)). Obviously, u3 increased with the evaporation region area since the mass change increased with the evaporation region area and the photothermal area remained invariable with this parameter. In contrast, u4 showed the opposite behavior (i.e., it decreased with the evaporation region area), indicating that the effect of photo-thermal conversion on the steam generation process would be different from that of the evaporation region area. This different trend for u3 and u4 originated from the separate design of the photo-thermal conversion and steam generation processes and the change of the evaporation region area. To clarify it, the equivalent evaporation rate and the evaporation efficiency were both introduced when assuming that the evaporation region area was equal to the photo-thermal area. Thus, an equivalent

conversion was defined as described before (more details in Supporting Information), and the results are displayed in Fig. 5 (f), which could reveal thhe actual evaporation performance of this system. The equivalent evaporation rate and evaporation efficiency both increased with the evaporation region area and then got down, and the maximum equivalent evaporation rate and evaporation efficiency for this design were 1.24 kg·m−2·h−1 and 77.83% at 33 cm2 evaporation region area. This trend could be explained according to Hertz–Knudsen relation [46] (see Supporting Information). When other conditions keep constant, such as the humidity, the pressure and the air flow rate, the working medium temperature is the major factor affecting evaporation. In this experiment, the evaporation region area changed by the separate design, and the photo-thermal enhancement of the thin film for the different evaporation region areas could be seen as an effect on the temperature of the working medium temperature. As shown in Fig. 5 (c) and (d), the center temperature of the working medium decreased upon increasing the evaporation region area from 9 to 33 cm2 since the same absorbed energy is required to be transferred to larger areas to heat more working medium. However, the final center temperature remained nearly invariable for 33, 48, and 65 cm2, indicating that the effect of the center C-TiO2 film on the working medium at the evaporation region could be restricted, as shown in Fig. 5 (d). When the evaporation region area exceded 33 cm2, the temperature of the working medium outside was close to the initial temperature. These results revealed that the heating effect of the center C-TiO2 film on the working medium could be negligible when the evaporation region areas are too large. 3.4. The heat transfer process simulations To verify this, the COMSOL simulation (More details in Supporting Information) was carried out to investigate the thermal gradient generated between the solar absorber and the steam generator. For Fig. 6(a), the temperature distribution of the evaporation region obtained by the COMSOL simulation coincided with the experimental

a

Temperature (°C)

b 50

Evaporation region 9 cm2 45 20 cm2 33 cm2 40 48 cm2 65 cm2

solar absorber

35 30 25 20

-5

-4

-3

-2

-1

0

1

2

3

4

5

Length (cm) Fig. 6. (a) The temperature distribution of the evaporation region and (b) the temperature of center line of different evaporation regions obtained by the COMSOL simulation. 250

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• This upgrade firstly than descending latter tendency could be ex-

result in Fig. 5(d). It is obvious that the wider the steam generator, the lower the thermal gradient in this region, which indicated that the heating effect of the solar absorber would be inefficient and the performance of the system would come down as the area of the steam generator overpasses a certain value. As displayed in Fig. 6(b), the temperature of center line changed with the length of the evaporation region when the existence of the heating effect of the center solar absorber. For different evaporation regions, the highest temperature obtained got a similar trends with the experimental result. However, the thermal gradient generated between the solar absorber and the steam generator would make a difference when the area of evaporation regions changed. With the bigger evaporation region, the thermal loss became lager, causing a larger cooling rate. Therefore, the temperature would rise for small evaporation regions at the same length from the center. When the area of the evaporation region exceeded 33 cm2, the temperature variation curves seemed coincident, indicating the heating effect to improve water temperature for evaporation enhancement could be efficient only in small evaporation region less than 33 cm2. In summary, the separate photo-thermal conversion and steam generation system showed optimum behaviors at an evaporation region area (33 cm2) to photo-thermal area (16 cm2) ratio near 2.06. When taking the cost per unit area into consideration (the cost of materials), the cost of insulating foam, PVA fiber material and C-TiO2 solar absorber are about 1.5 $/m2, 2 $/m2 and 8.5 $/m2. Then the total cost of materials of this system are about 12 $/m2, which would be much cheaper those nanofluid systems [19,21]and noble metals film systems [22,28]. For large-scale application, there would be a repeating arrangement of the photo-thermal conversion and steam generation regions, like displayed in Fig. S6 (Supporting Information). To balance the manufacturing cost and system performance, it is necessary to select a suitable evaporation to photo-thermal area ratio to improve the steam generation process. And this significantly enhanced evaporation process of the separated photo-thermal and steam generation device could help further improve solar steam generation processes.

plained by the limited effect of the center C-TiO2 film over the working medium at the evaporation region. These results demonstrated that is necessary to select a suitable evaporation to photothermal area ratio to improve the steam generation process.

Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 51676060), the Natural Science Founds of Heilongjiang Province for Distinguished Young Scholars (Grant No. JC2016009), and the Science Creative Foundation for Distinguished Young Scholars in Harbin (Grant No. 2014RFYXJ004). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apenergy.2018.11.090. References [1] Elimelech M, Phillip WA. The future of seawater desalination: energy, technology, and the environment. Science 2011;333(6043):712–7. [2] Yan J, Shamim T, Chou SK, Desideri U, Li H. Clean, efficient and affordable energy for a sustainable future. Appl Energy 2017;185:953–62. [3] Duić N, Guzović Z, Kafarov V, KlemeŠ JJ, vad Mathiessen B, Yan J. Sustainable development of energy, water and environment systems. Appl Energy 2013;101:3–5. [4] Chen M, He Y, Zhu J, Wen D. Investigating the collector efficiency of silver nanofluids based direct absorption solar collectors. Appl Energy 2016;181:65–74. [5] Gao X, Liu J, Zhang J, Yan J, Bao S, Xu H, et al. Feasibility evaluation of solar photovoltaic pumping irrigation system based on analysis of dynamic variation of groundwater table. Appl Energy 2013;105:182–93. [6] Liu ZH, Guan HY, Wang GS. Performance optimization study on an integrated solar desalination system with multi-stage evaporation/heat recovery processes. Energy 2014;76:1001–10. [7] Sharshir SW, Peng G, Wu L, Essa FA, Kabeel AE, Yang N. The effects of flake graphite nanoparticles, phase change material, and film cooling on the solar still performance. Appl Energy 2017;191:358–66. [8] Shannon MA, Bohn PW, Elimelech M, Georgiadis JG, Marinas BJ, Mayes AM. Science and technology for water purification in the coming decades[J]. Nature 2008;452(7185):301–10. [9] Guo S, Liu D, Chen X, Chu Y, Xu C, Liu Q, et al. Model and control scheme for recirculation mode direct steam generation parabolic trough solar power plants. Appl Energy 2017;202:700–14. [10] Zhu M, Li Y, Chen G, Jiang F, Yang Z, Luo X, et al. Tree-inspired design for highefficiency water extraction. Adv Mater 2017;29:1–9. [11] Kim S, Piao G, Han DS, Shon HK, Park H. Solar desalination coupled with water remediation and molecular hydrogen production: a novel solar water-energy nexus. Energy Environ Sci 2018;11(2):344–53. [12] Neumann O, Neumann AD, Tian S, Thibodeaux C, Shubhankar S, Müller J, et al. Combining solar steam processing and solar distillation for fully off-grid production of cellulosic bioethanol. ACS Energy Lett 2016;2(1):8–13. [13] Wang Z, Liu Y, Tao P, Shen Q, Yi N, Zhang F, et al. Bio-inspired evaporation through plasmonic film of nanoparticles at the air-water interface. Small 2014;10(16):3234–9. [14] Sajadi SM, Farokhnia N, Irajizad P, Hasnain M, Ghasemi H. Flexible artificiallynetworked structure for ambient/high pressure solar steam generation. J Mater Chem A 2016;4(13):4700–5. [15] Huang H, Shi M, Ge X. The effect of a black insulation sheet on the evaporation rate from a shallow salt pond. Int J Energy Res 2015;23(1):31–9. [16] Neumann O, Urban AS, Day J, Lal S, Nordlander P, Halas NJ. Solar vapor generation enabled by nanoparticles. ACS Nano 2012;7(1):42–9. [17] Amjad M, Raza G, Xin Y, Pervaiz S, Xu J, Du X, et al. Volumetric solar heating and steam generation via gold nanofluids. Appl Energy 2017;206:393–400. [18] Liu X, Wang X, Huang J, Cheng G, He Y. Volumetric solar steam generation enhanced by reduced graphene oxide nanofluid. Appl Energy 2018;220:302–12. [19] Wang X, He Y, Cheng G, Shi L, Liu X, Zhu J. Direct vapor generation through localized solar heating via carbon-nanotube nanofluid. Energy Convers Manage 2016;130:176–83. [20] Sharma B, Rabinal MK. Plasmon based metal-graphene nanocomposites for effective solar vaporization. J Alloy Compd 2017;690:57–62. [21] Li H, He Y, Liu Z, Huang Y, Jiang B. Synchronous steam generation and heat collection in a broadband Ag@TiO2 core-shell nanoparticle-based receiver. Appl Therm Eng 2017;121:617–27. [22] Wang X, He Y, Liu X, Cheng G, Zhu J. Solar steam generation through bio-inspired interface heating of broadband-absorbing plasmonic membranes. Appl Energy 2017;195:414–25. [23] Liu Y, Yu S, Feng R, Bernard A, Liu Y, Zhang Y, et al. A bioinspired, reusable, paper-

4. Conclusions In this work, a separate design involving photo-thermal and steam generation processes using a solar absorber was built to enhance both the evaporation rate and the evaporation efficiency. First, a C-TiO2 solar absorber was prepared and characterized. The spectrum characteristics of the material were experimentally and theoretically (numerical simulations) investigated, and a good light capture capacity was revealed. Subsequently, a photo-thermal conversion process was carried out on the C-TiO2 film by experimental work and numerical simulation. These experiments and simulations revealed highly stable temperature and favorable photo-thermal conversion. Finally, the solar steam generation experiments were carried at constant absorber area and varying evaporation region areas to clarify the enhancement effect of the C-TiO2 film. The main findings are summarized as follows.

• A C-TiO • •

2 solar absorber containing amorphous carbon and TiO2 NPs was synthesized successfully by a sol–gel method and the rough structures in the surface could improve the efficiency of the solar absorber. A good light capture capacity was achieved by the C-TiO2 absorber. That resulted from the significant light absorption characteristics of C materials in the full spectrum region while the main effect of TiO2 NPs is providing lower emissivity and protecting the composite films. Solar steam generation enhancement was achieved by the separate design using C-TiO2 solar absorber. The change trend of equivalent evaporation rate and the evaporation efficiency increased firstly, and then decreased with the evaporation region area. This design showed optimum behaviors when the ratio of evaporation to photothermal area was near 2.06.

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