Photovoltaic and photocatalytic performance study of SOLWAT system for the degradation of Methylene Blue, Acid Red 26 and 4-Chlorophenol

Applied Energy 120 (2014) 1–10

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Photovoltaic and photocatalytic performance study of SOLWAT system for the degradation of Methylene Blue, Acid Red 26 and 4-Chlorophenol Zhen Wang a, Yiping Wang a,d, Marta Vivar b,d, Manuel Fuentes c,d, Li Zhu d,⇑, Lianwei Qin a a

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China IMDEA Water, Alcalá de Henares 28805, Spain c IDEA Research Group, University of Jaén, Jaén 23071, Spain d School of Architecture, Tianjin University, Tianjin 300072, China b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Electrical and thermal performance of

The SOLWAT system is a combined system, which can use the full solar spectrum for power generation, water purification and thermal. Its photovoltaic and photocatalytic performance was studied in the present paper, analyzing the photovoltaic (PV) electricity production when simultaneously degrading three different pollutants: Methylene Blue, Acid Red 26 and 4-Chlorophenol. The photovoltaic performance of the SOLWAT system was studied comparatively by measuring the Pm and Isc under actual climatic conditions with a reference PV system. The degradation of sample pollutants was detected by analyzing the UV absorption and the TOC to investigate the photocatalytic performance of the SOLWAT system.

SOLWAT system was studied experimentally.  Methylene Blue, Acid Red 26 and 4-Chlorophenol were completely mineralized.  The tested SOLWAT system is selfsufficient even though has lower power output.  Solar cells of SOLWAT have lower working temperature due to the water cooling.  Solar spectrum was fully used for power generation, water purification and thermal.

a r t i c l e

i n f o

Article history: Received 21 September 2013 Received in revised form 10 January 2014 Accepted 14 January 2014 Available online 5 February 2014 Keywords: Hybrid PV system Water purification Methylene Blue Acid Red 26 4-Chlorophenol

a b s t r a c t The SOLWAT system is a combined system for solar water purification and renewable electricity generation. Its photovoltaic and photocatalytic performance, along with the photovoltaic (PV) electricity production was studied under the degradation of three different pollutants: Methylene Blue, Acid Red 26 and 4-Chlorophenol in present paper. The spectrum loss of the system was analyzed theoretically. Spectral transmittance experiments with different medium were conducted and compared with the results of outdoor experiments. The photovoltaic performance of the SOLWAT system was studied comparatively by measuring the Pm and Isc under actual climatic conditions with a reference PV system. To investigate the photocatalytic performance of the SOLWAT system, the degradation of sample pollutants were detected by analyzing the UV absorption and the TOC. In the presence of the additional wastewater layer, PV cells in the SOLWAT system could work under lower temperature. Depending on the spectral absorption of pollutants, if it is within the spectral response of the PV cell, the electricity output is affected by the pollutant degradation (Methylene Blue, Acid Red 26). When the spectral absorption of pollutant is out of the spectral response of the cell, the PV output is not affected by the contaminant

⇑ Corresponding author. Tel./fax: +86 22 27404771. E-mail address: [email protected] (L. Zhu). 0306-2619/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2014.01.039

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Z. Wang et al. / Applied Energy 120 (2014) 1–10

degradation (4-Chlorophenol). The output power of SOLWAT system, though decreased due to the light absorption, was sufficient to drive the whole system. The degradation of sample pollutants was detected by analyzing the UV absorption and the TOC to investigate the photocatalytic performance of the SOLWAT system. For the wastewater with different initial concentrations, the decolorization rate ran up above 99% and the mineralization rate was above 80%. Ó 2014 Elsevier Ltd. All rights reserved.

Nomenclature A T R ni Pm Isc Voc Isun (%) h c Jnp cA WUV QUV

absorption spectrum transmission spectrum reflectivity of interface the refractive index of medium the maximum output power, W short-circuit current, A open-circuit voltage, V percentage of the solar spectrum Planck constant, Js the light speed, m/s the normalized photocurrent density, % the concentration of simulated pollutant, mg/L UV irradiation, W/m2 cumulative UV irradiation, J/m2

1. Introduction Nowadays, the most significant problems were wastewater treatment [1], lack of fresh water [2] and energy crisis [3,4]. Over the past few years, the abusive use of pharmaceutical and personal care products, pesticides, surfactants, industrial chemicals, and combustion byproducts have resulted in undesirable accumulation of organic pollutants in the environment [5]. Moreover, their non-biodegradable character makes conventional wastewater treatment methods unable to remove them completely, which are deleterious to people’s life and health [6]. Photocatalytic processes using semiconductor as photocatalyst under UV irradiation have shown great potential when applied to the oxidation of various refractory organic compounds for the treatment of contaminated groundwater, industrial wastewater [7–10]. Among various photocatalysts, TiO2 remains one of the most promising one due to its high oxidation efficiency, nontoxicity, high photostability, chemical inertness, and environmentally friendly nature [11–15]. Moreover, the abundance of Ti (0.44% of Earth’s crust) provides TiO2 economic foundation for mass application [16]. However, only a small fraction of sunlight can be utilized due to that TiO2 photocatalysts absorbs solely UV irradiation (less than 5% of total solar energy) to initiate catalyzed reaction, which causes low utilization efficiency of solar energy on unit area. This major drawback limits the large scale application of TiO2 [17– 21]. Aiming at solving this problem, many researchers have made tremendous efforts to design and develop photocatalysts to improve the catalytic efficiency or make it workable under visiblelight irradiation. Strategies usually employed in such studies are modification (doping) of titania to give visible-light absorption or use of colored mixed metal oxide and nitride [22–27]. The optimized design of photocatalysis reactors is one of the promising approaches to improve the catalytic efficiency. Goswami [28] had pointed out that nonconcentrating photocatalysis reactors have the potential to be simple in design and low in cost. These nonconcentrating reactors were tested at the University of Florida [29]. Vivar et al. [30,31] proposed a new concept of a hybrid solar water purification and photovoltaic system to meet the rural people’s needs for clean water and electricity in one integrated

kapp Greek k

g

reaction rate constant, m2/J

wavelength of light, nm efficiency of the solar cells, %

Abbreviations SOLWAT solar water purification and renewable electricity generation system IR infrared light UV ultraviolet light EQE external quantum efficiency

autonomous system. Based on this original concept, the SOLWAT (Solar Water Purification and Renewable Electricity Generation) system with photovoltaic cells and photocatalytic reactor fully integrated into a single unit was constructed as shown in Fig. 1. The technology of spectral division was taken advantage of to realize the full use of solar energy. Meantime, nanofluids [32,33] in the channel could reduce the working temperature of the solar cells, and then improve the power generation efficiency of the PV cell [34]. For the SOLWAT system, the utilize of the technology of spectral division were qualitatively analyzed and the MB was chosen as simulated pollutant to preliminarily investigate the photovoltaic and photocatalytic performance by Fuentes et al. [35]. However, the previous study of photovoltaic performance was just limited to output short-circuit current, where the influence of temperature on the photovoltaic performance was ignored. The photocatalytic performance was just evaluated by the detection of UV–Visible spectrophotometer, which cannot prove complete mineralization

Fig. 1. SOLWAT system schematic diagram showing the hybrid module with the photocatalytic reactor integrated on top of the PV module, the pump for circulating the liquid, and the storage tank. The division of solar spectral is also given in the figure.

Z. Wang et al. / Applied Energy 120 (2014) 1–10

of simulated pollutant. Besides, the presence of simulated pollutant have an effect on both photovoltaic and photocatalytic performance, therefore, the comprehensive performance with different simulated pollutants need more investigations to testify its applicability under practical occasions. In the present study, the SOLWAT system was constructed in Tianjin, China and three different simulated pollutants were chosen to investigate the photovoltaic and photocatalytic performance comprehensively. The spectrum loss of the system was analyzed theoretically and spectral transmittance experiments with different medium were conducted. The influences of the temperature and spectrum absorption on the photovoltaic performance of the system were taken into consideration by detecting the shortcircuit current and maximum output power. In order to have a full-scale investigation for the photocatalytic performance of the system, both the characteristic absorption peaks and the total organic carbon content of simulated wastewater were detected and the degrading reaction kinetics were analyzed. 2. System and materials 2.1. Experiment system The solar cells used are standard mono-crystalline cells, encapsulated into a PV module whose dimensions are 536  477 mm. The performance parameters at reference conditions (AM1.5 [36], 25 °C, and 1000 W/m2) are listed in Table 1. The optical losses were minimized by selective adaptation of the refractive indices through carefully selecting the materials adjoining optical interfaces within the receiver as shown in Fig. 2. For the mono-crystalline silicon solar cells, the encapsulation glass is low iron tempered suede glass, with a thickness of 3.2 mm. Its light transmittance of the solar spectral response wavelength (400–1400 nm) can reach 91%, for infrared light (more than 1200 nm) with high reflectivity. The cover glass should possess the following qualities: high transmittance of UV spectrum for photocatalytic degradation of organic matter in the flow channel, high transmittance of visible and near infrared spectrum for photovoltaic power generation and low cost for engineering. Considering these requirements, the high purity boron silicate Borosilicate glass was chosen. It is transmittance of UV spectrum(200–400 nm) can reach 83%, the transmittance of visible and near-infrared spectrum can reach about 90%, and it is considerately inexpensive. A C-profile aluminum frame was built to let the working liquid (water + photocatalyst + pollutant) run between the two glasses. Its height is 25 mm and the photo reactor capacity is approximately 6.5 L. Glazing silicone was used to seal the joints of the frame and then clamps were used to fix the frame to the glass cover of the photovoltaic sub-module to facilitate dismounting the hybrid system during the different experiments. A layer of rubber was put in between the borosilicate glass and the aluminum frame to prevent the glass damage when the clamps were stepped up.

3

Fig. 2. Schematic diagram of the SOLWAT system showed the structure of receiver was composed of borosilicate glass (cover glass), flow channel, aluminum C-profile and mono-crystalline silicon solar cells.

system, are the global radiometer (300–3000 nm) and ultraviolet radiometer (280–400 nm) to measure the total irradiance and UV irradiance respectively. Three temperature sensors (Pt100) were used to record the temperature of the solar modules and the tank water. Two shunt resistors were used to convert the short circuit current to voltage signal. A liquid flow sensor was used to record the cycle flow rate. All these parameters were automatically recorded every 10 s to a datalogger (DI-710, Quatronix company). The meteorological station was used to record the ambient temperature and wind speed at the interval of 5 min. The I–V curves of the two solar modules were measured with the I–V curves (DS-100) instrument at the interval of 10 min, changing the monitoring configuration from Isc with a shunt resistor to a complete I–V curve scan. 2.3. Photocatalyst and simulated wastewater Suspended TiO2 particles (commercial Degussa P25, average particle size 21 nm, surface area 50 m2/g, anatase:rutile = 4:1) were used as the photocatalyst at a concentration of 0.2 g/L. In order to simulate organic pollutants, we selected two types of colorful organic dyes (Methylene Blue-665 nm and Acid Red 26-505 nm) and a type of colorless organic matter(4-Chlorophenol-225 nm), with DI water as the blank sample. The initial pollutant concentration varied from 0.005 mg/L to 0.02 mg/L. Methylene blue (MB) is a kind of aromatic heterocyclic compound, used as a chemical indicator, dye, biological stain and drug use. Acid Red 26 is a kind of single azo aromatic compound which may cause cancer, used as dye and biological staining. Dyes are often detected in waste products from textile industries. Besides, in the present

2.2. Data acquisition and recording system A data acquisition and recording system as showed in Fig. 3 was set up to study the photovoltaic and photocatalytic performance of SOLWAT system. The two radiometers in the same inclination with

Table 1 Performance parameters of solar cells used in the two systems.

g (%)

Pm (W)

Voc (V)

Isc (A)

Vmp (V)

Imp (A)

FF (%)

16.5

30

21.5

1.94

17.5

1.71

71.74

Fig. 3. Data acquisition and recording system including sensors datalogger I–V curve instrument and weather station. Here we changed the current signal of Isc via two shunt resistor. Also control circuit was made to measure the I–V curve at the same time.

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study, they can make the results be reflected directly. In order to inspect comparatively the influence of organic pollutant on the optical performance of SOLWAT, 4-chlorophenol which is used to plant growth promoters, was chosen a as colorless organic matter. The basic information of the simulated pollutants is shown in Table 2, where we could observe the absorption peaks of the two organic pollutants is within the spectral response of mono-crystalline silicon PV-cells, from 300 nm to 1100 nm, and the other one is out of this range. 3. Optical loss 3.1. Theoretical analysis In order to get the spectrum of sunlight which can reach the PV cell surface of cell after passing through several layers, theoretical analysis of optical losses was carried out by revising the measured data from spectrophotometer. With assumption that sunlight reaches the surface of two solar modules vertically and the refractive index can be regarded as constant which do not changes with wavelength (200–2500 nm) Absorption spectrum and transmission spectrum, the function of wavelength, are recorded as A(k) and T(k) respectively. The loss of light reflection on the interface was calculated with Fresnel formula as the following equation:

R¼

ðn1  n2 Þ2

ð1Þ

ðn1 þ n2 Þ2

where R is the reflectivity of interface, n1,n2 are the refractive index of medium. As Fig. 4 shows, for the reference system, the optical loss only includes the reflection loss on the interface between the encapsulated glass (white glass) of cells and air. Its transmission spectrum was calculated as the following equation:

TðkÞ ¼ u03 =u1 ¼ ð1  R1 Þ

ð2Þ

where T(k) is the transmission spectra rate, u is the transmission spectra of reference system, u1 is the incident spectrum. For the SOLWAT system, the optical losses include not only interface reflection loss but also the dielectric absorption loss. The layer of air, cover glass, simulated wastewater and encapsulation glass of cells constitute three interfaces. At the same time, the cover glass and simulated wastewater in the channel have different absorption at different wavelengths of sunlight. Its transmission spectrum was calculated as the following equation: 0 3

TðkÞ ¼ u03 =u1 ¼ ð1  R1 Þð1  R2 Þð1  R3 ÞA1 ðkÞA2 ðkÞ

ð3Þ

where A1(k) is absorption spectrum rate of cover glass, A2(k) is absorption spectrum rate of medium in the channel. 3.2. Spectral transmittance test In order to simulate the SOLWAT system, a tank with thickness 25 mm was made with borosilicate glass. Incident spectra of the samples (cover glass, tank + air, tank + DI water, tank + P25 solution) were measured by an Ultraviolet/Visible/Near-Infrared spectrophotometer (Cary5000, Agilent, US, with resolution 60.05 nm

Fig. 4. Transmission spectrum theoretical analysis showed the optical loss of (a) SOLWAT system and (b) Reference system including the interface reflection loss (R) and the dielectric absorption loss (A).

during UV/Visible area and resolution 60.05 nm during Near-infrared area). The effect of flow channel (photocatalytic reactor) on the spectrum reaching the surface of cells could be obtained by comparing the incident spectrum of the two system. 3.3. Analysis of spectral transmittance For the reference system, sunshine can reach the surface of panel directly, while for the SOLWAT system, sunlight passes through a layer of cover glass and flow channel. The existence of cover glass and flow channel will change intensity and spectral distribution of the incident light on the surface of the panel, so some theoretical analysis on it is needed. The AM1.5G standard spectrum (ASTM G173-03) was showed in Fig. 5. According to Eq. (4), ultraviolet spectrum (280–400 nm), visible spectrum(400–780 nm) and near-infrared spectrum (780–1400 nm) accounted for 4.7%, 52.1% and 32.4% of total energy spectrum respectively.

R k2

I ðkÞdk k1 sun Isun ð%Þ ¼ R 4000 Isun ðkÞdk 200

ð4Þ

Rk where Isun (%) is the percentage of the solar spectrum, k12 Isun ðkÞdk is R 4000 the energy of specific band, and 200 Isun ðkÞdk is the whole energy of incident spectrum. It is known that photons below the band-gap energy can pass through the active area of the cell without being absorbed, and are ultimately dissipated as heat in other parts of the cell. Photons of energy larger than the band-gap can only be partly utilized, and the remainder of their energy is also dissipated as heat. As can be seen obviously from the external quantum efficiency (EQE) curves of mono-crystalline cell, the sunlight spectrum which used to inspire the electron transition absorbed by the silicon cells was concentrated in 300–1100 nm band. The revised transmission spectrums of borosilicate glass, glass tank + air, glass tank + DI water and glass tank + P25 solution were also showed in Fig. 5. By contrasting the transmittance in 400–900 nm band of tank + air and tank + DI water, it could be seen that the presence of DI water reduced reflection loss by adjusting the refractive index, which could benefit the electricity generation of solar cell. Comparing

Table 2 The basic information of the simulated pollutants. Name

Molecular formula

Mol. wt.

kmax (nm)

CAS

Company

Methylene blue Acid Red 26 4-Chlorphenol

C16H18ClN3S C18H14N2Na2O7S2 C6H5ClO

319.85 480.4 128.56

665 505 225

61-73-4 3761-53-3 106-48-9

Aladdin TCI TCI

Z. Wang et al. / Applied Energy 120 (2014) 1–10

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The working temperature of solar cells for the SOLWAT system and the reference system in different seasons were showed in Fig. 6. From the comparison of the two temperatures, it could be concluded that whatever the weather was, the temperature for SOLWAT system was always lower than the one for reference system. With the increase of circulating time, the temperature difference between the SOLWAT system and the reference system reduced slightly. This might be due to the lasting circulation of water in the system. Generally, the presence of flow channel could reduce the temperature of the solar cell, which was consistent with the results of theoretical analysis above.

4. Photovoltaic and photocatalytic performance test 4.1. Test procedure

Fig. 5. Transmission spectra and the external quantum efficiency of silicon assuming that the theoretical losses includes interface reflection loss and the dielectric absorption loss without considering light scattering caused by P25.

the band of 900–2500 nm of them, it could be seen that DI water absorbed infrared spectrum at a high degree, which is desired for the SOLWAT system for the reason that DI water could take away the heat adverse to the electricity generation of solar cell and then keep the system working at lower temperatures. What is more, since the infrared spectrum cannot be used for generating electricity, the loss of infrared spectrum will not influence the electricity generation. In order to quantitatively evaluate the effect of spectral transmittance on photovoltaic and photocatalytic performance of SOLWAT system, the normalized photocurrent density Jnp was introduced, and its definition was as follows:

R J np ðTheoreticalÞ ¼

J np ðActualÞ ¼

TðkÞQ cell ðkÞIsun ðkÞ kq dk hc R Q cell ðkÞIsun ðkÞ kq dk hc

ð5Þ

Isc ðSOLWATÞ Isc ðReferenceÞ

ð6Þ

where T(k) is transmission spectra, Qcell(k) is external quantum efficiency, Isun (k) is AM1.5G, k is the wavelength, h is Planck constant, c is the light speed. Then the Jnp(Actual) and the Jnp(Theoretical) with different medium in the channel, were calculated according to the Eqs. (5) and (6) and the results were showed in Table 3. From the results in Table 4, for pure substance (DI water and Air) the Jnp(Actual) were consistent with the Jnp(Theoretical). However, when the tank was filled with P25 solution, the Jnp(Actual) deviated from the Jnp(Theoretical) severely. This difference might be attributed to optical scattering caused by the suspended particles, which was not considered in this initial theoretical model. This scattering produced losses but also reflected part of the light back into the PV cells, which could increase the real electricity, while in the theoretical model, all the sunlight was assumed to be lost.

Table 3 The results of Jnp(Theoretical) and the Jnp(Actual). Content (%)

Air

DI water

P25 solution

Jnp(Theoretical) Jnp(Actual)

89.4 91.2

83.4 81.9

17.8 51.8

The experiments were conducted from 10:00 am to 2:00 pm in sunny days when the irradiance was relatively stable. All the experiments were carried in Tianjin (latitude 37.1°N and longitude 117.2°E), China. Taking the Acid Red 26 as an example, the color of wastewater changed from red to pink and finally became colorless as Fig. 7 shows. Certain amount of simulated pollutants and P25 powder were put in 8 L DI water and then the mixture was fully stirred in the shadow for 40 min to reach adsorption equilibrium. The simulated wastewater was firstly put into the tank of the SOLWAT system and the pump was turned on to make the system stable running for 10 min. After that, the experiment began. At predetermined time, 50 mL solution was taken out and centrifuged at 12,000 r/ min for 15 min. Then the supernate was stored for further use following the filtration through cellulose membrance of 0.45 lm. A total of 11 experiments with different concentrations and conditions were conducted. The concrete parameters were shown in Table 4. The supernate were measured with UV/Visible spectrophotometer and TOC analyzer (TOC-VCPH bought from SHIMADZU, Japan) after the outdoor experiment.

4.2. Results and discussion 4.2.1. Photovoltaic performance of SOLWAT system It was clear that the presence of working medium reduced the Isc to varying content in Fig. 8. The concrete ratios of loss at the beginning of each experiment were listed in Table 5. The absence of P25 reduced the Isc by 30% in comparison with blank DI water. This was caused by the light scattering effect of TiO2 particles that reduced the incident spectrum on the surface of solar cell. The group of 4-CP simulated wastewater had almost the same Isc as the group of blank P25 solution, while the group of AR26 and MB with the same concentration simulated wastewater had lower Isc at the beginning, especially the group of MB simulated wastewater. The main reason was that the colorless 4-CP has the maximum absorption peak at 225 nm, which is not in the responding band of Si-cell, while for colorful AR26 and MB, the maximum absorption peak are 505 nm and 665 nm. As mentioned above, the sunlight spectrum used to inspire the electron transition absorbed by silicon cells was concentrated in 400–1100 nm band, and therefore, the absorption at 505 nm and 665 nm had significant effect on the electricity generation. As shown in the curves, along with the catalytic reaction, the color of the simulated wastewater faded. Meanwhile the Isc kept rising until it reached the same value as blank P25 solution.

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Table 4 The outdoor experiment content. Simulated wastewater

Concentration (P25 g/L)

Simulated pollutant concentration (mg/L)

DI water

0 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

/ / 5 10 20 5 10 20 5 10 20

MB

Acid Red 26

4-Chlorophenol

4.2.2. The output power of system The normalized max output power of SOLWAT system with AR26 of different concentrations and reference system was illustrated in Fig. 9. From the three curves below representing the max output power of SOLWAT system with AR26, it could be seen that the normalized Pm kept increasing until a stable value, which had the same trend with the Isc. The change trend of Pm also demonstrated that with the photocatalytic reaction, the absorption of simulated wastewater for sunlight decreased, which increased the output power of SOLWAT system. The measured data of Isc and normalized Pm were listed in Table 6. Comparing the Isc and Pm, the Pm of SOLWAT occupied a higher percentage of the one of reference system than the Isc.

Pm ¼ V m  Im ¼ V oc  Isc  FF

Fig. 7. AR26 experiment with the concentration 10 mg/L: (a) the beginning and (b) the end.

ð7Þ

As explained above, the presence of liquid channel could reduce the working temperature of solar cell, and then could increase the value of Voc. Therefore, though the absorption of simulated wastewater for sunlight resulted in a lower Isc, the decrease of temperature improved the power generation efficiency of the PV cell according to Eq. (7). From the practical point of view, the output power of SOLWAT system, though lower than reference system, could achieve above 10 W under cloudy weather, while the maximum consumption for pump operation was 6 W. Therefore, the power produced by SOLWAT system was sufficient to support the system operation. The redundant generated electricity could also be stored [37]. 4.2.3. Photocatalytic performance of the SOLWAT system The results of photocatalytic degradation are summarized in Fig. 10. These curves represented the change trend of concentration for Acid Red 26, Methylene Blue and 4-Chlorophenol with

Fig. 8. The normalization short circuit current (Isc@1sun) of the Reference system and the SOLWAT system with DI water, P25 solution(0.2 g/L) and three simulated wastewater(10 mg/L) with P25(0.2 g/L).

Fig. 6. Working temperature of the SOLWAT system and the Reference system in different seasons: (a) Spring, (b) Summer and (c) Winter.

Z. Wang et al. / Applied Energy 120 (2014) 1–10 Table 5 The loss ratio of the incident spectrum (simulated pollutant 10 mg/L, P25 0.2 g/L).

Isc(@1sun) (A) Percentage (%) Loss ratio (%)

Reference

DI water

P25

MB

AR 26

4-CP

1.857 / /

1.559 84.0 16.0

0.947 51.0 49.0

0.551 29.7 70.3

0.733 39.5 60.5

0.952 51.3 48.7

Fig. 9. The normalized maximum output power of Acid Red 26. As the working temperature of the systems has much less effect on the Pm than the optical loss, only the irradiance was considered in the normalized calculation.

Table 6 The Isc (@1sun) and Pm (@1sun) results of Acid Red 26 with different initial concentration. Acid Red 26 (5 mg/L)

Acid Red 26 (10 mg/L)

Acid Red 26 (20 mg/L)

Isc (@1sun) SOLWAT (A) Reference (A) Percentage (%)

0.933 1.882 49.6

0.958 1.856 51.6

0.941 1.886 49.9

Pm (@1sun) SOLWAT (W) Reference (W) Percentage(%)

15.4 28.4 54.2

16.5 28.9 57.1

16.2 29.1 55.7

7

different initial concentrations. For all the three simulated wastewater, with the catalyzed reaction, the absorbance (Abs) at max absorption peak decreased until the absorbance almost got zero,which indicated the full degradation of contaminants. For the colored simulated wastewater, the process of reaction could also be observed from the change of color. Taking the Acid Red 26 as an example, the color of wastewater changed from red to pink and finally became colorless. TOC of the wastewater was detected for further investigation of the degradation. The TOC of the three simulated wastewater as a function of reaction time are presented in Fig. 11. More than 80% of organic carbon had been degraded at the end, which are consistent with the results of UV-detection. It was proved that not only were organic characteristic functional group destroyed by the photocatalytic reaction, but also the organic matters were decomposed into CO2 and H2O. Based on the two experimental results above, it could be concluded that most of the simulated pollution was degraded during the photocatalytic reaction, confirming the photocatalytic effectiveness of SOLWAT system. From the contrast between the upper part and lower part of Fig. 11, it can be seen that the TOC degradation curves of two complex organic dyes were flat, and for the 4-CP with simple structure, the curve was sharp. 4.2.4. Reaction kinetics analysis To further study the process of photocatalytic degradation, reaction kinetics was analyzed here. Many factors including type and amount of catalyst, pH of solution, initial concentration of organic matter, light source (wavelength and light intensity), illumination time and temperature etc. may affect the photocatalytic reaction. For the SOLWAT system, since the apparent activation energy Ea of photocatalytic reaction is generally low, the temperature has little effect on the speed of photocatalytic reaction .Therefore, with fixed dosage of catalyst, the type and initial concentration of reactants were taken into consideration here. The conventional reaction kinetics method that fits reaction time and concentration of organic matter under fixed light intensity is not suitable here, since the irradiance was different for each outdoor experiment because of the uncontrollable solar irradiance. In this experiment, residual concentration of contaminants was fitted with accumulating UV irradiation for simulated wastewater to determine the reaction rate constant according the Eq. (8), thus experimental results in different days could be compared.



dcA n ¼ kcA dt

Fig. 10. Degradation curve of simulated pollutants with different initial concentrations.

ð8Þ

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Z. Wang et al. / Applied Energy 120 (2014) 1–10

Fig. 11. Mineralization curves of simulated wastewater with different initial concentrations.

Fig. 12. Photocatalytic reaction kinetics fitting. Here the residual concentration of contaminants was fitted with accumulating UV irradiation of simulated wastewater to get the reaction rate constant.



W UV  dcA n ¼ kcA W UV  dt

Q UV ¼ W UV  t The Eq. (9) could be reasoned from Eq. (8). n



dcA kcA ¼ ¼ kapp C nA dQ UV W UV

ð9Þ

where C is sample concentration (mg/L), C0 is initial concentration (mg/L), kapp is reaction rate constant (m2/J), QUV is Cumulative UV irradiation (J/m2). As Fig. 12 shows, by fitting the experimental results to different reaction equations, the reacting process fitted the first-order reaction kinetics. The reaction rate constants for different reactant with different initial concentration were presented in Fig. 13 by fitting the results

Fig. 13. The photocatalytic reaction rate constant kapp of three kinds of simulate pollutant with different initial concentrations.

Z. Wang et al. / Applied Energy 120 (2014) 1–10

to the first-order reaction kinetics. Theoretically, the reaction rate constant for first-order reaction are not related to the initial concentration. However, the result showed that for the same simulated pollutant, the higher initial concentration was, the smaller the reaction rate constant kapp was. It could be explained as following. Both saturated adsorption of TiO2 for organic pollutant and UV transmittance of reactor have effect on the catalytic reaction rate. When the adsorption was unsaturated at a low initial concentration of pollutant, the reaction rate increased with the increase of initial concentration of pollutant. On the other hand, when the concentration was over the saturated content of absorption, more ultraviolet was absorbed by organic pollutant, leading to a decrease of reaction rate. Synthesizing the two factors, the reaction rate constant altered with the initial concentration of reactant. By comparing the photocatalytic reaction rate of three simulated pollutant with same initial concentration (10 mg/L), it could be concluded that degradation rate of colorful contaminants was much faster than the one of colorless contaminant because the presence of color could function as sensitizer, boosting the degradation. 5. Conclusion The photovoltaic and photocatalytic performance of SOLWAT system was analyzed through outdoor experiments in Tianjin, China, where three kinds of common organic matters were chosen as simulated pollutants. The results showed that there were still surplus of electricity for use besides those for system operation and the simulated pollutants were almost degraded completely. For pure medium, the theoretical spectrum loss of the system had good coherence to outdoor experimental data. However, for simulated wastewater with P25, there existed a big difference between theoretical and actual results due to optical scattering in the suspended particles, not considered in the theoretical model. Additionally, the nanofluid in the channel could absorb the solar infrared spectrum, which kept SOLWAT system working under a lower temperature than Reference system, thus increasing the output power. The photovoltaic performance of SOLWAT system and Reference system were compared under the actual working conditions. The presence of P25 mostly affected the photovoltaic performance of system. For colorless 4-CP solution and blank P25 solution, the normalized Isc was stable at 0.9 A, while for colorful MB and AR26 solution, the normalized Isc kept increasing with the catalytic reaction and finally stabilized at 0.9 A.The output power of SOLWAT system could achieve above 10 W under cloudy weather, while the maximum consumption for pump operation was 6 W. Therefore, the power produced by SOLWAT system was sufficient to support the SOLWAT system operation in rural areas. For the outdoor circular degradation experiments with fixed P25 concentration of 0.2 g/L, the two colorful simulated wastewater fully faded and UV spectrophotometer detection showed the degradation rate of initial pollutant achieved 99%. The TOC experiments indicated the minerization of three simulated pollutants could be above 80%, demonstrating that the organic matters were completely mineralized into CO2 and H2O rather than the simple destruction of the functional groups. The study of reaction kinetics indicated that the degradation rates of three organic matters conformed to first order kinetics with different degradation rate constant, which can help optimize the operating parameters of SOLWAT system in practical applications. Acknowledgements This work was funded by the Chinese National Natural Science Foundation for international young scientists, Grant number

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51250110076 and Programme of Introducing Talents of Discipline, Grant number B13011.

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