thermal systems: A review

Renewable and Sustainable Energy Reviews 76 (2017) 323–352

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Performance of nanofluid-based photovoltaic/thermal systems: A review Farideh Yazdanifard

a,b

b

MARK

c,⁎

, Mehran Ameri , Ehsan Ebrahimnia-Bajestan

a

Department of Energy Conversion and Renewable Energies, Institute of Science and High Technology and Environmental Science, Graduate University of Advanced Technology, Kerman, Iran b Department of Mechanical Engineering, Shahid Bahonar University, Kerman, Iran c Department of Mechanical Engineering, Quchan University of Advanced Technology, Quchan, Iran

A R T I C L E I N F O

A BS T RAC T

Keywords: Photovoltaic/thermal system Nanofluid Laminar regime Turbulent regime Energy Exergy

Recently, applying nanofluids in PV/T systems for improving the performance of these systems has been fascinating many researchers. In this kind of research, nanofluids are employed in the PV/T systems as coolant or optical filter. To emphasis the capability of the nanofluids in PV/T systems, the present study aims first, to comprehensively review the features, structures, and the outcomes of PV/T system that applied nanofluids and investigated the effectiveness of nanofluids, and second, to comprehensively analyze the effective parameters on the performance of a nanofluid-based flat plate photovoltaic/thermal system in both laminar and turbulent regime. In this study, with respect to literature, a vast attempt has been done to study the effects of nanofluids parameters including volume fraction (0–4%), size (21 nm and 100 nm) and type of nanoparticles (TiO2 and Al2O3), as well as type of base fluid (water and mixture of ethylene glycol-water). The accuracy of proposed mathematical model was demonstrated through the comparison of predicted results and the available data in the literature. It can be concluded from the results that, to improve the performance of the system, adding nanoparticles is more efficient in laminar regime compared to turbulent one. The results also indicated that using nanoparticles of larger diameter leads to greater total energy and exergy efficiency in the turbulent regime, while contrary behavior is observed in laminar flow. Moreover, it was observed that employing aluminum oxide in nanofluids improves the system performance more than titanium oxide, where water based nanofluids show higher energy and exergy efficiency compared to ethylene glycol-water based nanofluids.

1. Introduction Nanofluid is defined as a dispersion of nanometer-sized solid particles (lower than 100 nm at least in one dimension) into fluids like water, ethylene glycol and oil. For the first time, Masuda [1] in 1993, introduced the concept of suspending nanoparticles into common heat transfer fluids and Choi [2] in 1995, applied the word “nanofluid” for this kind of colloidal suspensions. Afterward, performing many experimental and theoretical studies, it was found that generally, the nanofluids possess greater heat transfer characteristics compared to the common fluids [3,4]. For example, addition of 0.3% volume fraction of copper nanoparticles into ethylene glycol increased thermal conductivity by 40% [5], or suspending aluminum oxide nanoparticle in water with the volume fraction of 6.8% enhanced heat transfer coefficient of turbulent flow about 40% [6]. Besides, proposing the relations for thermophysical and heat transfer properties of nanofluids has been the subject of numerous studies that some of them are listed in Table 1. Recently, due to the significant heat transfer characteristics of ⁎

nanofluids, several work has been carried out on the application of this novel medium of heat transfer in the solar energy systems, especially solar collectors [7]. Javadi et al. [8] presented an overview of studies in the performance of the solar collectors and concluded that using nanofluid instead of conventional fluid improves heat transfer as well as thermal properties, efficiency, transmittance and extinction coefficient of solar collectors. There are some reports on other applications of nanofluids in solar energy systems such as thermal storage energy systems [9], solar cells [10] and solar distillers [11]. One of the inventive applications of nanofluids in solar energy equipment is in photovoltaic/thermal (PV/T) systems which incorporate photovoltaic module and heat extraction component in a single unit to produce heat and electricity, simultaneously. A survey of literature data shows that various configurations of PV/ T systems, which use air or water as coolant fluid have been investigated. Among them, Shahsavar and Ameri [35] conducted a theoretical and experimental study on a glazed and unglazed directcoupled PV/T system cooled through the natural and forced air convective heat transfer. Their results indicated that glazing system

Corresponding author. E-mail addresses: [email protected] (F. Yazdanifard), [email protected] (M. Ameri), [email protected], [email protected] (E. Ebrahimnia-Bajestan).

http://dx.doi.org/10.1016/j.rser.2017.03.025 Received 11 May 2016; Received in revised form 8 December 2016; Accepted 8 March 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

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Ẋ

Nomenclature

A Ac cb Cp D Di Do ̇ Epv f g hc hr hl I k K L ṁ n Nu pa P Ppump PV PV / T CPV / T Pr Ra Re T Tmax Ts Tsun vw w

exergy rate (W)

Greek letters

area (m2) collector area (m2) thermal conductance of bond (W m−1 K−1) specific heat capacity (J kg−1 K−1) diameter (m) inner diameter (m) outer diameter (m) PV output power (W) friction coefficient gravitational acceleration (m s−2) convection heat transfer coefficient (W m−2 K−1) radiation heat transfer coefficient (W m−2 K−1) head loss solar radiation intensity (W m−2) Thermal conductivity (W m−1 K−1) Minor loss coefficient length (m) Mass flow rate (kg s−1) number of pipes Nusselt number packing factor pressure (Pa) pump power (W) photovoltaic photovoltaic/thermal concentrating photovoltaic/thermal Prandtl number Rayleigh number Reynolds number temperature (K) Maximum cell temperature (K) sky temperature (K) sun temperature, 6000 K wind speed (m s−1) width (m)

α βr δ ΔP ε ε̇ ρ η ηr κ μ σ τ φ

absorption coefficient reference temperature coefficient thickness (m) pressure head loss (Pa) emission coefficient exergy efficiency reflection coefficient, density (kg m−3) efficiency reference solar cell efficiency Boltzmann constant, 1.381×10–23 (J K−1) viscosity (Pa s) Stefan Boltzmann constant (W m−2 K−4) transmission coefficient collector slope, volume fraction

Subscripts

a abs ad b ele bf g i in nf np out s t th w

air layer thermal absorber adhesive tube bonding electrical basefluid glass cover insulation input nanofluid nanoparticle output sky tube thermal working fluid

applications of nanofluids, Mahian et al. [41] concluded that nanofluids have a reasonable capacity to employ in PV/T collectors. To date, several review papers have separately investigated PV/T systems or applying nanofluids in solar energy technologies. Kasaeian et al. [42] considered using nanofluid in solar systems; Verma and Tiwari [43] and Al-Shamani et al. [44] studied nanofluids in solar collectors; Hasanuzzaman et al. [45] and Shukla et al. [46] investigated progresses in cooling PV technologies; Hussien et al. [47] reported cooling technologies based on using nanofluid in mini/microchannels, and Elbreki et al. [48] reviewed effective parameters in PV/T systems. To the best knowledge of the present authors, there is no review paper which especially investigates employing nanofluids in PV/T systems, and all mentioned papers just presented few reports in this field. On the other hand, reviewing the literature indicates that, relatively little information is available on the influence of effective nanofluid parameters on energy and exergy efficiency of PV/T systems. Hence, the main contribution of the present review paper is of twofold. In the first fold, a vast attempt has been done to provide a comprehensive and up-to-date review of configuration, effectiveness and advancement of nanofluid-based PV/T systems. In the second fold, the important parameters that affect the system operation have been examined in details, though proposing a mathematical model for a nanofluid-based PV/T system. Also, several correlations were proposed for thermophysical properties and heat transfer characteristics of some nanofluids, based on available data.

leads to an increase in thermal efficiency and a decrease in electrical efficiency. Ameri et al. [36] performed tests on an air-based PV/T system with the action of natural and forced convection mechanism. They concluded that, installing glass cover, air flow rate increases in natural convection mode, while it decreases in forced convection. Gholampour et al. [37] investigated the performance of photovoltaic/ thermal unglazed transpired solar collectors (PV/UTCs) both experimentally and theoretically. Their results showed that the amount of photovoltaic cooling in this system depends on the amount of air mass flow rate passed through the transpired plate. Kalogirou and Tripanagnostopoulos [38] experimentally and theoretically studied two thermosyphon PV/T systems, one with polycrystalline-silicon (pc-Si) and the other with amorphous silicon (a-Si) PV module. Their results pointed out that system with pc-Si solar cells gives higher electrical efficiency and lower thermal efficiency compared to a system with a-Si solar cells. Huang et al. [39] investigated the performance of a water-based PV/T system experimentally and introduced the concept of primary energy saving efficiency. They demonstrated that the primary energy saving efficiency of the PV/T system is greater than that of the single solar water heater or PV module. Sobhnamayan et al. [40] employed Genetic algorithm (GA) to find an optimum water inlet velocity and pipe diameter of a water-based PV/T system. Although, there is a considerable amount of work reported in the literature on PV/T systems, investigating the ability of nanofluids in PV/T systems is in primary stages. Performing a review on solar energy 324

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Table 1 Proposed relations for thermophysical and heat transfer properties of nanofluids in the literature. Property

Investigator

Relation

Density

Pak and Cho [12]

ρnf = (1 − φ)ρbf +φρnp

Heat capacity

Pak and Cho [12]

Cp, nf = (1 − φ)Cp, bf +φCp, np

Xuan and Roetzel [13]

Cp, nf = Vajjha and Das [14] Cp, nf Cp, bf

Viscosity

+φ(ρCp)

np

⎞ ⎛C ⎛T ⎞ p, nP ⎟ A × ⎜ ⎟ +B × ⎜⎜ ⎟ ⎝ T0 ⎠ ⎝ Cp, bf ⎠ (C + φ )

μnf = (1 + 2.5φ)μbf

Brinkman [16]

μnf =

Frankel and Acrivos [17]

⎛ (φ / φ )1/3 ⎞ 9 m ⎟μ μnf = ⎜ 8 ⎝ 1 − (φ / φ )1/3 ⎠ bf m

Lundgren [18]

⎛ ⎞ 25 μnf = ⎜1 + 2.5φ+ φ2 +f (φ3)⎟μbf 4 ⎝ ⎠

Batchelor [19]

μnf = (1 + 2.5φ + 6.5φ2 )μbf μnf μbf

1 μbf (1 − φ)2.5

⎡ ⎛ ⎛ sp ⎞⎛ = 1 + 2.5φ + 4.5⎢1/⎜⎜⎜ ⎟⎜2 + ⎢ ⎝ d p ⎠⎝ ⎣ ⎝

Maiga [21]

μnf = (1 + 7.3φ + 123φ2 )μbf

Koo and Kleinstreuer [22]

μnf = μstatic + 5 × 104βφρbf

Maxwell [23]

knf kbf

Hamilton and Crosser [24]

knf kbf

Yu and Choi [25]

knf kbf

=

=

sp ⎞⎛ sp ⎞ ⎟⎜1+ ⎟ d p ⎠⎝ dp ⎠

κT ρnp dnp

2 ⎞⎤

⎟⎥ ⎟⎥ ⎠⎦

f (T , φ )

(knp +2kbf ) −2φ(kbf − knp) (knp +2kbf ) +φ(kbf − knp) knp + (n − 1)kbf −(n − 1)φ(kbf − knp) knp + (n − 1)kbf +φ(kbf − knp)

⎛ knp +2kbf +2(knp − kbf )(1 + β )3φ ⎞ ⎟ = ⎜⎜ 3 ⎟ ⎝ knp +2kbf −(knp − kbf )(1 + β ) φ ⎠

Koo and Kleinstreuer [26]

⎛ (knp +2kbf ) −2φ(kbf − knp) ⎞ k nf = ⎜ ⎟k + 5 × 104γφρbf Cbf ⎝ (knp +2kbf ) +φ(kbf − knp) ⎠ bf

Patel et al. [27]

knf kbf

Chon et al. [28]

knf kbf

Prasher et al. [29]

knf kbf

Nusselt number

bf ρnf

Einstein [15]

Graham [20]

Thermal conductivity

=

(1 − φ)(ρCp)

=1+

knpAnp kbf Abf

+ Ck npPe

κT ρnp dnp

g (T , φ )

Anp kbf Abf

⎛ dbf ⎞0.3690 ⎛ knp ⎞0.7476 0.9955 1.2321 = 1 + 64.7φ0.7460⎜ ⎟ Pr Re ⎜ ⎟ ⎝ dnp ⎠ ⎝ kbf ⎠

⎛ [knp(1 + 2α ) +2km] +2φ[knp(1 − α ) − km] ⎞ = (1 + ARemPr0.333φ)⎜ ⎟ ⎝ [knp(1 + 2α ) +2km] −φ[knp(1 − α ) − km] ⎠

Pak and Cho [12]

0.5 Nunf = 0.021Re0.8 nf Prnf ,for turbulent regime

Li and Xuan [30]

Nunf = c1(1 + c2φ c3Pecd4)Renf5 Pr0.4 nf ,for both regimes

Maiga et al. [31]

0.5 Nunf = 0.086Re0.55 nf Prnf ,for laminar regime

Maiga et al. [32]

0.35 Nunf = 0.085Re0.71 nf Prnf ,for turbulent regime

c

Buongiorno [33]

Nunf =

Vajjha et al. [34]

(f /8)(Renf −1000)Prnf ,for turbulent regime 1 + δ v+ f /8 (Pr 2/3 v −1)

0.15 Nunf = 0.065(Re0.65 )Pr0.542 , for turbulent regime nf − 60.22)(1 + 0.0169φ nf

325

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Fig. 1. Schematic of PV/T system with nanofluid as (a) coolant [49], (b) spectral filter [50], (c) coolant and spectral filter with double-pass channel [51], and (d) coolant and spectral filter with separate channels [52].

Fig. 2. (a) Variation of PV temperature with Reynolds number (concentration ratio=40) studied by Rawadan et al. [53] and (b) Variation of PV temperature and increment of PV output power with mass flow rate reported by Karami and Rahimi [55] for different nanoparticle concentrations.

summarized and discussed in the follow sections. It should be noted that, the overall summery will be reported at end, in the conclusion section.

2. Application of nanofluids in PV/T systems A survey of literature shows that the nanofluids have two potential roles in PV/T systems: a) coolant, b) spectral filter, which can be categorized into four structures as displayed in Fig. 1. Among these configurations, most of the research has focused on employing nanofluid as a coolant (Fig. 1(a)), in which the nanofluid flows below PV cells to harvest their heat and reduce the temperature, which can be

2.1. Application of nanofluids as coolant Reviewing the literature, it can be observed that different types of geometries have been studied to investigate the effect of nanofluids in 326

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Table 2 Maximum percentage increment of thermal, electrical and overall energy efficiency of the PV/T systems, using nanofluid as a coolant compared to related base fluid, obtained from literature. Investigator (s)

Nanofluid & concentration

Electrical efficiency increase (%)

Thermal efficiency increase (%)

Overall efficiency increase (%)

Khanjari et al. [61]

Ag/water (10 vol%) Al2O3/water (10 vol%) Fe3O4/water (3 wt%)

3.9 1.83 4.93

12.43 4.54 46.29

11.54 4.26 41.80

TiO2/water (1 wt%) SiO2/water (1 wt%) SiC/water (1 wt%) Al2O3/water (4 vol%) Al2O3/water (5 vol%) Al2O3/water (0.02 wt%)

15.73 10.37 42.97 1.45 9.72 −3.73

9.36 3.98 13.16 – −0.04 57.55

11.89 6.43 18.97 – 1.49 29.47

Al2O3/water (0.4 wt%) Cu/water (0.4 wt%) Al2O3/EG (0.4 wt%) Cu/EG (0.4 wt%)

0.15 0.77 0.16 0.77

8.88 79.97 12.99 216.23

– – – –

Ghadiri et al. [62] Al-Shamani et al. [66]

Xu and Kleinstreuer [65] Xu and Kleinstreuer [64] Tang and Zhu [68] Rejeb et al. [60]

cooling of PV/T systems, which are microchannel, sheet and tube configuration, single rectangular channel, and serpentine shaped channel. Rawadan et al. [53] applied Al2O3/water and SiC/water nanofluids and Lelea et al. [54] used Al2O3/water nanofluid to numerically study microchannel-cooling of PV cells in almost similar CPV/T system under laminar flow regime condition. Both studies showed that nanofluid reduces PV temperature more compared to water, especially at low Reynolds number, also volume fraction was introduced as one of the most influential parameters on the system performance, as shown in Fig. 2(a). Karami and Rahimi [55] experimentally investigated microchannel-cooling of non-concentrated PV cells with Boehmite nanofluid in laminar regime. In contrast to the findings of Rawadan et al. [53] and Lelea et al. [54], Karami and Rahimi [55] concluded that using a lower concentration of Boehmite nanofluid leads to a higher reduction in PV temperature and greater electrical output due to the high surface activity of nanoparticles, as revealed in Fig. 2(b). Thus, according to Fig. 2, conflicting findings have been reported on the effect of nanoparticle concentration on PV temperature for different nanofluids, which can be attributed to different type of nanoparticles and PV/T systems. In the other work, Karami and Rahimi [56] studied two helical and straight channel configuration with Boehmite nanofluid for cooling of PV cells, where using nanofluid instead of water demonstrated more reduction in PV temperature. Sharaf and Orhan [57] modeled high concentration CPV/T, which comprises multi-junction PV cells, segmented thermoelectric generators and minichannel heat extractors, along with Al2O3/water and Al2O3/synthetic oil as coolant. Based on their results, employing Al2O3/synthetic oil nanofluid is unfavorable both thermally and hydraulically compared to water and Al2O3/water. Sardarabadi et al. [58] proved through an experimental work that using silica/water nanofluid in a sheet and tube PV/T system, as well as increasing nanoparticle mass fraction from 1% to 3% enhance overall energy and exergy efficiency, considerably. Sardarabadi and Passandideh-Fard [59] in another investigation applied Al2O3/water, TiO2/water, ZnO/water nanofluids in a sheet and tube PV/T system. Their results showed that TiO2/water and ZnO/water nanofluids have better electrical performance compared to Al2O3/water, while ZnO/ water nanofluid presents the best thermal efficiency among all. Rejeb et al. [60] evaluated the performance of a sheet and tube PV/T with four types of nanofluids containing Al2O3 or Cu nanoparticles mixed into water or ethylene glycol base fluids in three cities of Lyon (France), Mashhad (Iran) and Monastir (Tunisia). Their results specified that water based nanofluids are more efficient in comparison with ethylene glycol based nanofluids, where Cu/water case achieved the best thermal performance. Also, the PV/T system yielded higher electrical

Fig. 3. Variation of average thermal and electrical efficiency by nanoparticle concentration presented by An et al. [50,71].

output in Monastir city. Khanjari et al. [61] studied Al2O3/water and Ag/water in a sheet and tube PV/T system. They indicated that in the case of Ag/water nanofluid, system energy and exergy efficiency enhance more. They also disclosed that, nanoparticle concentration has a positive effect on overall energy and exergy efficiency, whereas fluid velocity decreases both. Ghadiri et al. [62] studied applying the ferrofluid of Fe3O4/water in a sheet and tube PV/T system and tested the system under constant, alternating magnetic field and no field condition. Their results exposed that using the ferrofluid under alternating magnetic field can improve energy and exergy efficiency more compared to the other conditions. Elmir et al. [10] reported that using Al2O3/water nanofluid flowing inside a single rectangular channel in PV/T system increases heat transfer rate, consequently reduces PV temperature, which in turn leads to proper performance. Michael et al. [63] constructed the glazed and unglazed PV/T systems, in which copper absorber directly laminated to the PV cells and Cu/water nanofluid as coolant flowed inside a single rectangular channel. It was concluded that using Cu/ water instead of pure water, along with glazing the system reduce electrical efficiency, but improve thermal efficiency. Xu and Kleinstreuer [64,65] studied the flowing Al2O3/water nanofluid through a single rectangular flow channel in a CPV/T system. They 327

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0.85

surface of the PV. They also suggested designing nanoparticle suspension filters to optimize overall efficiency. An et al. experimentally studied applying Cu9S5/ oleylamine nanofluid [71] and polypyrrole nanofluid [50] as the spectral filters in a Fresnel lens CPV/T system, similar to Fig. 1(b). Also, in their studied system the water channel placed at the back of PV cells to reduce the temperature. They demonstrated that using nanofluid spectral filter can improve system performance. The results indicated that increasing nanoparticle concentration leads to higher electrical and overall efficiency, while thermal efficiency drops, as depicted in Fig. 3. In addition, glazing outside of optical filter improves thermal output, at the expense of reduction in electrical output. Hjerrild et al. [72] conducted an experimental and numerical research on a PV/T system using multi-particle nanofluid filter, composed of core-shell silver-silica nanodiscs and carbon nanotubes mixed into water. Based on their results, the nanofluid contained disc shape nanoparticles results in higher overall efficiency. Otanicar et al. [73] examined using nanofluid-based and thin film-based optical filters in a CPV/T system with five Si, Ge, CdTe, InGaAs, and InGaP photovoltaic cell materials. Their survey indicated that applying thin film-based filter leads to higher overall efficiency compared to nanofluid-based one, however lower cost and lower thickness of the nanofluid-based filter, make them still favorable. Similarly, Taylor et al. [74] who tried to optimize the design of the nanofluid-based optical filter for a CPV/T system, confirmed that nanofluids filters are efficient, compact and potentially low-cost, spectrally selective optical filters.

Thermal and overall energy efficiency

0.8 0.75 0.7 0.65 0.6 0.55 overall efficiency Ag/water, Saroha Au/water, Saroha Ag/water, Mittal Cu/water, Mittal

0.5 0.45 0.4 0.35

0

thermal efficiency Ag/water, Saroha Au/water, Saroha Ag/water, Mittal Cu/water, Mittal

0.01

0.02

0.03

(Tin-Tamb)/G Fig. 4. Comparison of thermal, electrical and overall efficiency of Ag/water, Au/water and Cu/water for spectral filtering and cooling purpose in CPV/T system based on Mittal et al. [77] and Saroha et al. [78] results.

indicated that the employed nanofluid improves electrical and overall efficiencies, while thermal efficiency doesn’t significantly vary [64]. In addition, they expressed that increasing nanoparticle concentration enhances electrical efficiency and reduces entropy generation, also lowering the inlet temperature results in higher electrical output [65]. Al-Shamani et al. [66] examined TiO2/water, SiO2/water, SiC/water as a coolant in PV/T system that contains serpentine shaped absorber design with rectangular cross section. They concluded that SiC/water nanofluid leads to highest overall efficiency. While, the nanofluid coolant widely used under the PV cells in aforementioned studied, few studies considered the effect of nanofluids flowing above the PV cells. Cui and Zhu [67] research showed that employing nanofluid flow over PV cells decreases electrical efficiency, whereas PV/T system can achieve higher overall efficiency due to simultaneously producing of heat and electricity. Moreover, they found that increasing nanoparticle concentration and film thickness reduce nanofluids transmittance. Tang and Zhu [68] indicated that using Al2O3/water instead of water flowing over PV cell, increases thermal and overall efficiency due to better heat characteristic of nanofluids compare to water, while electrical efficiency decreases owing to greater absorption of visible light by nanofluids. Furthermore, a study exists that use cotton wick structures in combination with Al2O3/water and CuO/water nanofluids for cooling PV cells, however extracted heat cannot be used [69]. Table 2 summarizes the maximum percentage of enhancement of electrical, thermal and overall efficiency of the PV/T systems through using nanofluid as the coolant instead of base fluid, obtained from the aforementioned papers. The greatest enhancement in electrical, thermal and overall efficiency corresponds to the SiC/water, Cu/EG and Fe3O4/water, respectively.

2.3. Application of nanofluids as coolant and optical filter simultaneously Furthermore, various studies investigated employing nanofluid for both purposes of cooling and spectral filtering. Zhao et al. [75] optimized a PV/T system with two flow channel, one above the system acted as an optical filter, which separated from PV with an air layer, while the other placed below PV cells for cooling them. They also optimized the working fluid properties such as maximum transmittance of visible light and absorbance of the infrared spectrum and stated that nanofluid can be served for this purpose. DeJarnette et al. [76], Jing et al. [51], Mittal et al. [77] and Saroha et al. [78] investigated using nanofluids in the PV/T systems almost similar to

2.2. Application of nanofluids as optical filter As mentioned before, the other proposed role of the nanofluids in PV/T systems is employing as a spectral (or optical) filter. Spectral filters split light spectrum in a way that visible spectrum passed through the filter which could reach PV cells and produce electricity, while infrared and ultraviolet spectrum are absorbed [48] and increase the temperature of the nanofluid. Otanicar et al. [70] proposed applying liquid based optical filter in CPV/T system. They showed that overall efficiency can be increased using a liquid filter that directly absorbs energy below the band-gap energy along with cooling the back

Fig. 5. Schematic of PV/T system and related differential element used in energy balance analysis.

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Table 3 Energy balance equation for PV/T system components. Component

Energy balance equation

Coefficient [85,86]

Glass cover

I αgwdx = (hc, g − a + hr , g − s )(Tg − Ta )wdx + (hr , pv − g + hc, pv − g )(Tg − Tpv )wdx

*

PV panel

Glazed PV/T:

I τgα pv[1−pa × ηr (1−βr(Tpv − Tr ))]wdx = (hr , pv − g + hc, pv − g )(Tpv − Tg )wdx + (hdA) pv − abs (Tpv − Tabs ) + (hdA) pv − t (Tpv − Tt )

hr , pv − g =

I α pv[1−pa × ηr (1−βr(Tpv − Tr ))]wdx = (hc, pv − a + hr , pv − s )(Tpv − Ta )wdx + (hdA) pv − abs (Tpv − Tabs ) + (hdA) pv − t (Tpv − Tt ) Absorber

(hdA) pv − abs (Tpv − Tabs ) = (hdA)abs − t (Tabs − Tt ) + (hdA)abs − i (Tabs − Ti )

Tube

(hdA)abs − t (Tabs − Tt ) + (hdA) pv − t (Tpv − Tt ) = (hdA)t − nf (Tt − Tnf ) + (hdA)t − i (Tt − Ti )

Insulation

(hdA)abs − i (Tabs − Ti ) + (hdA)t − i (Tt − Ti ) = (hdA)i − a (Ti − Ta )

Working fluid

(hdA)t − nf (Tt − Tnf ) = mC ̇ pdTnf

σ (Tg2 + Tpv 2)(Tg + Tpv ) 1 1 ( + −1) εg ε pv

hc, g / pv − a = 3 vw+2.8 **

Unglazed PV/T:

εgσ (Tg 4 − Ts 4) (Tg − Ta )

hr , g − s =

(hdA)pv − abs =

**

Equivalent sky temperature: Ts=0.0552Ta1.5. + ⎡ 1708 ⎤ ⎛ Air Nusselt relation: Nua = 1 + 1. 44⎣⎢1 − ⎥ ⎜⎝1 − Racosφ ⎦

1708(sin1 . 8φ)1.6 ⎞ ⎟ Racosφ ⎠

(

wdx

(hdA)abs − t =

8kabs δabs wdx w − Do w

(hdA)abs − i =

2ki ⎛ D ⎞ ⎜1− o ⎟wdx δi ⎝ w⎠

(hdA)t − i =

⎞D 2ki ⎛ π ⎜ +1⎟ o wdx δi ⎝ 2 ⎠w

(hdA)t − nf =

⎡ +⎢ ⎣

kad ⎛ D ⎞ ⎜1− o ⎟wdx δad ⎝ w⎠

δpv δ δpvw w2 + ad 8kpv kad Do

(hdA)pv − t =

(hdA)i − a =

*

Nuaka δa

hc, pv − g =

wdx w w + hnf πDi cb

1 δi 1 + 2ki hc, g − a

wdx

⎤+ Racosφ 1/3 −1⎥ . 5830 ⎦

)

Zhao et al. [75] one, that first the nanofluid flowed below PV cells, then preheated nanofluid flowed inside the above channel. DeJarnette et al. [76] made an attempt to present proper details for choosing appropriate nanoparticles for each particular cell band gap in mentioned PV/ T system. Jing et al. [51] demonstrated that using SiO2/water improves exergy efficiency compared to water with or without light concentration. Mittal et al.’s results [77] indicated that their proposed system increases thermal and overall efficiency compared to conventional PV/ T system. Also, they found that Ag/water nanofluid has a superior effect on system performance compared to Cu/water. The survey of Saroha et al. [78] illustrated that nanofluids are efficient media for both optical filtering and cooling purposes. They also reported the system with Ag/water nanofluid achieves higher thermal, electrical and overall efficiency in comparison with Au/water. Fig. 4 shows the comparison of using Ag/water, Au/water and Cu/water nanofluids as both coolant and optical filter according to results of Mittal et al. [77] and Saroha et al. [78], that their studied systems were similar. Hassani et al. [52] investigated using Ag/water and Ag/ TherminolVP-1 nanofluids as the optical filter and coolant in two different design of PV/T systems. One configuration was similar to Zhao et al.’s system [75] with double pass channel, while second contained two separate channel for optical filtering and cooling purpose, as depicted in Fig. 1(c) and (d), respectively. Also, they investigated two silicon and GaAs PV cells. They demonstrated that using separate channels along with two different nanofluids improve system performance, considerably. In the other effort, Hassani et al. [49] compared life cycle exergy of three systems of PV system, conventional PV/T system with water and CNTs/water nanofluid as a coolant, as well as PV/T systems with separate channel (Fig. 1(d)), which they investigated in their previous work [52]. In the third system, they used CNTs/water nanofluid in cooling channel and water and Ag/water nanofluid in optical filtering channel. Their numerical study showed that the PV/T system with separate channels, which used Ag/water nanofluid as optical filter and CNTs/water nanofluid as coolant performs better in term of total exergy, while this system is most pollutant during

manufacturing phase and less pollutant during operational phase compared to other configurations. 3. Effective parameters in nanofluids as coolant In this section, the effect of important parameters in a flat plate PV/ T system using nanofluid as coolant have been investigated and reviewed, comprehensively. Hence, a sheet and tube PV/T system which applies nanofluids in the cooling section has been studied numerically in both laminar and turbulent flows, considering the effect of glass cover and pump power in the proposed mathematical model. It should be mentioned that, despite the substantial role of pump power on the electrical efficiency of the system, in most studies the pump power required for circulating coolant in PV/T systems has been ignored from efficiency analyzing. In addition, normally the researchers have been employing the classical correlations such as those presented by Shah and London [79] to predict the Nusselt number of nanofluid flows. However based on many reports reviewed by EbrahimniaBajestan et al. [7], these classical correlations cannot accurately predict the convective heat transfer characteristics of nanofluids. Therefore, in this paper, a vast attempt has been made to remodel Nusselt number correlations of the studied nanofluids, based on the available data. Finally, the effects of various nanofluid parameters such as nanoparticle concentration, size and type, as well as a base fluid type have been examined in detail on the characteristics of PV/T system including panel temperature, outlet coolant temperature, electrical, thermal and total energy and exergy efficiencies. 3.1. Mathematical modeling In the present work, a nanofluid-based sheet and tube PV/T system, which is shown in Fig. 5, is modeled through writing the energy equation for different components of the system. Due to the periodic geometry of the system in the y direction, a one-dimensional energy balance analysis has been performed along the z direction, where the temperature of the coolant through the tube changes along the x direction. 329

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Table 4 Proposed f and β functions for studied nanofluids used in viscosity model. Nanofluid

Reference of experimental data

f and β functions for viscosity model

Al2O3/water

Nguyen et al. [87]

f (T , φ) = (−0.342×10−3 + 6.043 × 10−3φ)T + (0.113 − 1.916φ)

β = 1.805(100φ)0.855; R 2 = 0.95; 1% ≤ φ ≤ 4% TiO2/water

Turgut et al. [88]

f (T , φ) = (1.207×10 − 2−10.752φ)T + (0.368 + 3577.249φ)

β = 0.00602(100φ)0.1958; R 2 = 0.98; 1% ≤ φ ≤ 3% CuO/water

Nguyen et al. [87]

f (T , φ) = (−1.929×10−3−0.858×10−3φ)T + (0.636 + 1.342φ) β = 0.4275(100φ)0.2387; R 2 = 0.98; 1% ≤ φ ≤ 4. 5%

Al2O3/EG-water 60:40

Vajjha and Das [89]

f (T , φ) = (3.317×10−3−0.349φ)T + ( − 1.058 + 131.034φ) β = 0.2698(100φ)−0.492; R 2 = 0.95; 1% ≤ φ ≤ 6%

CuO/EG-water 60:40

Namburu et al. [90]

f (T , φ) = (−0.069 − 8.160φ)T + (28.008 + 2637.687φ)

β = 0.0241(100φ)0.2578; R 2 = 0.96; 1% ≤ φ ≤ 4%

of pipe, are considered 0.5 and 1, respectively [83]. Additionally, the friction factor is presented for the laminar and turbulent regime by Eqs. (10)–(11), respectively [84].

It is assumed that heat transfer is steady and one-dimensional in the z direction, where specific heat capacity of all components (except the coolant) is negligible compared to that of coolant; also the thermal conductivity of all components are constant, while all thermophysical properties of working fluid (nanofluid) has been considered temperature dependent. Based on these assumptions, energy equations for each component are listed in Table 3 (view more details in [80,81]) The thermal, electrical and total energy efficiency of nanofluidbased PV/T system can be evaluated by Eqs. (1)–(3) [82].

for laminar regime: f =

for turbulent regime: f =

IAc

ηele =

(1)

̇ − Ppump EPV IAc

(2)

ηPV/T = ηth + ηele

(3)

̇ is output power of PV panel and is determined by Eq. In Eq. (2), EPV (4). ̇ = Iτgαpv × pa × A η (1−β (Tpv − Tr )) EPV c r r

(4)

Furthermore, Eqs. (5)–(7) are used for calculating the thermal, electrical and total exergy efficiency.

⎞ mc ̇ p(Tnf , out ⎟ Tnf , out ⎠ ̇ Xth ε̇th = = ⎤ ⎡ ̇ Xin ⎛ ⎞4 ⎢1 + 1 ⎜ Ta ⎟ − 4Ta ⎥IAc T 3 T 3 sun ⎥ ⎢⎣ ⎝ sun ⎠ ⎦ ⎛ − Tnf , in )⎜1 − ⎝

ε̇ PV/T = ε̇th + ε̇ele

3.2.1. Density Density of nanofluid is determined by Eq. (12) [12].

ρnf = φρnp + (1 − φ)ρbf

(5)

nṁ × ρg(L sinφ + hl ) ρ × ηpump

8ṁ 2 L (f + K1 + K2 ) ρ gπ 2Di 4 Di 2

(12)

3.2.2. Heat capacity Heat capacity of nanofluid is obtained from Eq. (13) [13]. (6)

φ(ρCp ) Cp, nf =

(7)

np

+ (1−φ)(ρCp ) ρnf

bf

(13)

where φ is volume fraction of nanoparticles and subscripts np, bf and nf indicate the nanoparticle, base fluid and nanofluid, respectively. (8)

3.2.3. Viscosity For viscosity of nanofluid, the model proposed by Koo and Kleinstreuer [22] has been applied as follows:

where hl is the total head loss that can be obtained from Eq. (9).

hl =

(11)

3.2. Nanofluid properties

The pump power in Eqs. (2) and (6) is defined by Eq. (8):

Ppump =

1 (0. 79ln(Re)−1. 64)2

Ta

̇ − Ppump ̇ EPV Xele = ̇ ⎤ ⎡ Xin ⎛ ⎞4 ⎢1 + 1 ⎜ Ta ⎟ − 4Ta ⎥IAc 3 3 T T sun ⎥ ⎢⎣ ⎝ sun ⎠ ⎦

ε̇elec =

(10)

Considering aforementioned mathematical model, the thermophysical properties, as well as laminar and turbulent convective heat transfer characteristics in the straight tubes (cooling system geometry in this study) for studied nanofluids are necessary for calculations. Therefore an extensive literature survey of useful studies covered all these requirements was undertaken, and finally the nanofluids of Al2O3/water, TiO2/water and Al2O3/ethylene glycol-water 60:40 were found for analyzing in this work. It should be mentioned that ethylene glycol-water 60:40 (Al2O3/EG-water 60:40) is a well-known coolant in cold regions due to its low freezing point of about −50 °C, consisted of 60% ethylene glycol and 40% water (by mass). Followings are the obtained required properties based on the literature data, for employing in the mathematical model.

mc ̇ p(Tnf , out − Tnf , in )

ηth =

64 Re

(9)

The values of K1 and K2 which are loss coefficient in inlet and outlet 330

μnf = μstatic + μBrownian

(14)

μstatic = (1 + 2.5φ)μbf

(15)

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Fig. 6. Comparison between measured viscosity data [87–90] and predicted values using proposed f and β functions for (a) Al2O3/water, (b) TiO2/water, (c) CuO/water, (d) Al2O3/EGwater 60:40, (e) CuO/EG-water 60:40.

μBrownian = 5 × 104βρbf φ

κT f (φ , T ) ρnp dnp

curve fittings (as shown in Table 4) on available data of different nanofluids in literature have been done. Fig. 6 shows the comparison between experimental data and predicted viscosity values resulted from mentioned curve fittings, where reasonable agreements are observed.

(16)

In this study, to define β and f functions appeared in Eq. (16), some 331

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Table 5 Proposed γ and g functions for studied nanofluids used in thermal conductivity model. Nanofluid

Reference of experimental data

γ and g functions for thermal conductivity model

Al2O3/water

Das et al. [91]

g(T , φ) = (3.003 × 10−4 − 2.706 × 10−3φ)T + ( − 8.974×10−2 + 0.9075φ) γ = 0.5668(100φ)−0.2091; R 2 = 0.99; 1% ≤ φ ≤ 4%

TiO2/water

Wang et al. [92]

g(T , φ) = (1.187 × 10−3 − 1. 608 × 10−3φ)T + ( − 0.3509 + 0.596φ) γ = 6.565 × 10−2(100φ)−0.8608; R 2 = 0.95; 1% ≤ φ ≤ 4%

CuO/water

Das et al. [91]

g(T , φ) = (0.5548 × 10−3 − 0.402 × 10−2φ)T + ( − 0.1583 + 1.113φ) γ = 0.8704(100φ)−0.8639; R 2 = 0.96; 1% ≤ φ ≤ 4%

3.2.4. Thermal conductivity Similarly, Koo and Kleinstreuer [26] proposed a model for thermal conductivity of nanofluids as follows:

knf = kstatic + kBrownian

kBrownian = 5×104γCp, bf ρ φ bf

np

knp+2kbf +(kbf − k )φ np

(19)

In this paper, γ and g functions used in thermal conductivity model of Eq. (19) have been obtained from curve fitting performed on the available data of some nanofluids, as presented in Table 5. Fig. 7, proves the accuracy of proposed, γ and g functions, through comparing of available measured data and predicted values. In the case of Al2O3/EG-water 60:40 nanofluid, the γ and g

(17)

knp+2kbf −2(kbf − k )φ kstatic =

κT g (φ , T ) ρnp dnp

kbf (18)

Fig. 7. Comparison between measured thermal conductivity data [91,92] and predicted values using proposed g and γ functions for (a) Al2O3/water, (b) TiO2/water, (c) CuO/water.

332

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Table 6 Constant values employed in Nusselt number model for studied nanofluids and flow regimes. Nanofluid

Flow regime

Reference of experimental data

c1

c2

c3

c4

c5

Al2O3/water

laminar turbulent

Sudarmadji et al. [94] Pak and Cho [12]

0.052 0.01

15.691 0.084

0.565 0.878

0.100 0.001

0.545 0.890

TiO2/water

laminar turbulent turbulent

Ebrahimnia-Bajestan et al. [95] Pak and Cho [12] Vajjha et al. [34]

0.735 0.025 0.027

1.085 0.530 0.001

0.246 0.8 0.694

0.048 0.001 0.001

0.165 0.798 0.731

Al2O3/EG-water 60:40

tioned mathematical model have been considered temperature dependent, determined by Eqs. (27)–(30) for water, as well as Eqs. (31)–(34) for ethylene glycol-water 60:40 obtained by curve fitting on the data of ASHRAE Handbook [96]. • Water:

functions that proposed by Vajjha et al. [93] has been used. These functions are given by Eqs. (20)–(21).

⎛T ⎞ g(T , φ) = (3.917 × 10−3 + 2.8217 × 10−2φ)⎜ ⎟ + (−3.91123 × 10−3 ⎝ T0 ⎠ − 3.0669 × 10−2φ) −1.07304

γ = 8.4407(100φ)

(20)

ρ = − 0.003 × T 2+1.505 × T + 816.781

(27)

(21)

cp = − 0.0000463 × T 3 + 0.0552 × T 2−20.86 × T + 6719.637

(28)

k = − 0.000007843 × T 2 + 0 .0062 × T −0.54

(29)

3.2.5. Nusselt number Nusselt number of nanofluid is a critical property in convective heat transfer calculations, which should be selected attentively. Considering literature review [7], it can be established that the classical correlations presented for Nusselt number of base fluids are not capable of accurately predicting the convective heat transfer of nanofluids. Thus, remodeling of Nusselt number in the case of nanofluids is a crucial need. Consequently, in this study, to model Nusselt number of nanofluids the model proposed by Li and Xuan [30] given by Eq. (22) has been employed.

Nu nf = c1(1 + c2φ

c3

Pecd4)Recnf5

Pr0.4 nf

247.8

μ = 0.00002414 × 10 T −140 • Ethylene glycol-water 60:40:

Prnf =

Renf =

Ped =

hD knf

μnf

4ṁ πμnf D

k = − 0.00000302 × T 2+0.00238 × T −0. 09

(33) (34)

Due to the absence of sufficient available data for nanofluid in PV/T systems, model validation has been performed by considering water as the coolant. Therefore, for validating the results of the present mathematical model, outlet coolant and PV temperature of a waterbased PV/T system (with the properties presented in Table 7) has been compared to experimental data of Hung et al. [39], and numerical results of Sobhnamayan et al. [40]. Considering Fig. 9, the accuracy of the present model is obviously confirmed through the comparison with measured data. 3.4. Results and discussion

(25)

After ensuring the accuracy of the present model, the glazed and unglazed PV/T systems were investigated using nanofluid as coolant flowing under a laminar and turbulent flow regime, according to the information listed in Tables 8 and 9. In the following section, the effects of various nanofluid parameters on the PV temperature, outlet temperature of the coolant, electrical and thermal energy and exergy efficiencies have been examined in detail. Finally, attention must be paid that, all studied cases (excluding Section 3.4.4) have been conducted in constant mass flow rate to make the study more practical, thus Reynolds numbers will vary with changing the nanofluid properties.

ρnf VdnpCp, nf knf

(32)

3.3. Model validation

(24)

=

cp = − 0.00000489 × T 3 − 0.00475 × T 2 + 5.893 × T +1641.327

(22)

μnf Cp, nf

ρnf VD

(31)

453.4

(23)

knf

ρ = − 0.0024 × T 2 + 0.958 × T + 1014.297

μ = 0.00001202 × 10 T −122

where c1, c2 ,…,c5 are constant coefficients, which must be determined for each nanofluid and flow regime, separately. Also, Ped , Re and Pr denote particle Peclet number, Reynolds number and the Prandtl number of nanofluids, respectively, which have been defined by Eqs. (23)–(26).

Nu nf =

(30)

(26)

In the present work, employing curve fitting on available data in the literature, c1,c2 ,…,c5 were found for studied nanofluids, as presented in Table 6. It should be mentioned that, in the case of laminar flow regime of Al2O3/EG-water 60:40 no sufficient data was available, and only turbulent flow was investigated. Finally, to examine the accuracy of the present modified model, the comparison of available experimental data and predicted values of Nusselt number has been depicted in Fig. 8, where reasonable agreement between results is observed. Table 6 includes novel and very valuable data for researchers in the field of nanofluid heat transfer, since just a few models are available for Nusselt number of nanofluids.

3.4.1. Effect of nanoparticles concentration and diameter Fig. 10 depicts the effect of volume fraction and diameter of nanoparticles on PV and outlet nanofluid temperature. In this figure, TiO2/water nanofluid with two nanoparticle diameters of 21 nm and 100 nm at given constant mass flow rates related to the laminar and

3.2.6. Base fluid properties The thermophysical properties of base fluids used in the aforemen333

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Fig. 8. Comparison between measured Nusselt number data [12,34,94,95] and predicted values using the modified model for (a) Al2O3/water in laminar flow, (b) Al2O3/water in turbulent flow, (c) TiO2/water in laminar flow, (d) TiO2/water in turbulent flow, (e) Al2O3/EG-water in turbulent flow.

and flow characteristics of TiO2/water nanofluid at different nanoparticles concentrations, in the case of the unglazed system, have been presented in Tables 10 and 11, for the laminar and turbulent regime, respectively. It should be mentioned that in these tables all thermo-

turbulent regime has been investigated. According to Fig. 10, adding nanoparticles into the base fluid reduces PV temperature for laminar flow, while an opposite manner is observed in turbulent regime. To comprehend the reason of these manners, thermophysical properties 334

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Table 7 Parameters used for validation [40].

Table 8 Parameters needed for simulation [97–100].

Parameter

Value

PV module

Solarex MSX60, Poly-crystalline silicon 0.467 × 1.105 0.0045

Collector Area Reference temperature coefficient Rib spacing Channel material Hydraulic diameter of flow ducts Wind velocity

6 polycarbonate 0.0048 1

Unit

Component

Parameter

Value

Units

Glass cover

Thickness Transmittance Absorptance

0.004 0.92 0.04

m

PV panel

Absorptance Emittance Thermal conductivity Reference open circuit voltage Reference short circuit current Reference maximum power voltage Reference maximum power current Current temperature coefficient Voltage temperature coefficient Reference cell efficiency Reference temperature coefficient Reference temperature

0.9 0.9 100 21.1 3.8 17.1

W m−1 K−1 V A V

3.5

A

2.06 −0.077 15 0.0045

A °C−1 V °C−1 % K−1

298

K

Thermal absorber (copper)

Thickness Thermal conductivity

0.5 310

mm W m−1 K−1

Pipe (copper)

Outlet diameter Thickness Length Number Space between pipes Thermal conductivity

0.008 0.0012 2 10 0.1 310

m m m

insulation

Thickness Thermal conductivity

0.05 0.03

m W m−1 K−1

Working fluid

Inlet temperature Total mass flow rate in laminar regime Total mass flow rate in turbulent regime

298 0.05

K kg s−1

0.4

kg s−1

Wind velocity Collector slope Ambient temperature Solar radiation intensity Packing factor Pump efficiency

1.5 30° 298 800 0.9 0.8

m s−1

m2 °C−1 mm m m s−1

others

Fig. 9. Comparison of variations of PV temperature (Tpv) and outlet water temperature (Tout) during a day, obtained from the present model, available measured data [39] and available numerical study [40].

physical properties have been calculated based on the average temperature of the nanofluids. Considering the definition of Nusselt number by Eqs. (22) and (23), it is clear that the heat transfer coefficient is proportional to Nusselt number and thermal conductivity, again Nusselt number is related to Reynolds and Prandtl number of nanofluid. According to Tables 10 and 11, with increasing nanoparticles concentration, viscosity and thermal conductivity of the studied nanofluid increase, while heat capacity decreases. It is obvious that for studied TiO2/water nanofluid the variation of the viscosity is most dominant compared to other thermophysical properties. Thus, since, Reynolds number is inversely proportional to viscosity, it reduces with increasing nanoparticles concentration at a given mass flow rate. On the other hand, Prandtl number, which is directly proportional to viscosity and inversely related to thermal conductivity, augments with nanoparticle concentration. Consequently, considering Eq. (22), the effect of adding nanoparticle on Nusselt number (and also convective heat transfer coefficient) is determined by a competition of Reynolds number reduction and Prandtl number enhancement. Considering constant coefficient of c5 in Table 6, in the case of TiO2/water, the effect of Prandtl number on Nusselt number is dominant compared to Reynolds number in laminar flow, while Reynolds number is more important in turbulent regime, as expected. Therefore, for studied nanofluid, increasing nanoparticle concentration in the laminar and turbulent flow, respectively enhances and reduces Nusselt number (and consequently heat transfer coeffi-

m W m−1 K−1

K W m−2

Table 9 Al2O3 and TiO2 nanoparticle properties [34,101]. Nanoparticle

Density (kg/ m3)

Specific heat capacity (J/kg K)

Thermal conductivity (W/m K)

TiO2 Al2O3

4157 3600

692 765

8.4 36

cient). Due to this behavior of heat transfer coefficient, it can be concluded that with increasing volume fraction of nanoparticles, PV temperature reduces in laminar regime, while it increases slightly in turbulent regime, as demonstrated in Fig. 10(a). The maximum reduction of PV temperature in laminar flow is about 4.3% and 3.3% for the glazed and unglazed panel, respectively, at 4 vol%. Whereas, in turbulent regime, the small increment of 0.8% and 0.7% are observed for the glazed and unglazed panel, respectively, at 4 vol%. According to Fig. 10(b), the outlet temperature of nanofluid in laminar regime enhances with increasing nanoparticles concentration, while this enhancement is not considerable in turbulent regime. Because, for the given mass flow rate and inlet coolant temperature, in laminar flow, the increment of heat transfer coefficient and reduc335

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Fig. 10. Variation of (a) photovoltaic temperature and (b) outlet coolant temperature against volume fraction for TiO2/water nanofluid with two nanoparticle diameters of 21 and 100 nm, at constant mass flow rates corresponded to laminar and turbulent flows.

reduces in laminar regime, while it increases in turbulent regime. Also, convective heat transfer coefficients follow similar fashions. However, with increasing nanoparticles diameter, PV temperature rises in laminar flow and decreases in turbulent flow, slightly, while nanoparticles diameter has no significant effect on outlet coolant temperature in both regimes. It is obvious from Fig. 10 that, the effect of studied TiO2/water nanofluid on PV temperature and outlet coolant temperature is not noticeable, compared to the effect of flow regime and using glass cover. Due to the importance of energy and exergy analysis in PV/T systems, Figs. 11 and 12 exhibit the effect of concentration and diameter of nanoparticles on energy and exergy efficiencies, respectively. First, the electrical efficiencies variations in the presence of the nanofluid are discussed. As depicted in Figs. 11(a) and 12(a), electrical energy and exergy efficiency gradually increase in laminar flow with increasing nanoparticles concentration, while in turbulent flow these efficiencies first increase slightly and then decrease, though these variations are not considerable. According to Eqs. (2) and (6), the output power of PV and pump power affect the electrical energy and exergy efficiency. It is clear from Eq. (8) that, the pump power is proportional to head loss, which itself with respect to Eq. (9) is directly proportional to the friction factor and inversely related to the square of density. Considering Table 10, with increasing nanoparticles concentration, density increases, while Reynolds number decreases and consequently friction factor enhances. Therefore, the competition between these opposite effects determines the behavior of

tion of specific heat capacity lead to outlet temperature increases. While in turbulent regime, there is a competition between two opposite effects with adding nanoparticles: (i) specific heat capacity reduction and (ii) heat transfer coefficient reduction. Furthermore, in turbulent flow the amount of mass flow rate (and consequently Reynolds number) is high enough, which makes the effect of other heat transfer enhancement techniques, such as adding nanoparticles, insignificant. The maximum enhancement of outlet coolant temperature for the glazed and unglazed panel at 4% volume fraction, in laminar flow is about 2.65%, while a negligible maximum enhancement of about 0.35% is observed in the case of turbulent flow. In general, compared to conventional fluids the impact of nanofluids on the PV temperature is higher than outlet coolant temperature, which is due to the substantial effect of nanofluids on the convective heat transfer coefficient. On the other hand, the effect of nanoparticles diameter on the heat transfer characteristics of the nanofluid has been illustrated in Fig. 10, Tables 10 and 11. According to Eq. (22), Nusselt number is proportional to the nanoparticles diameter, while based on Eqs. (16) and (19), thermal conductivity and viscosity of nanofluids are inversely related to nanoparticles diameter, since the Brownian motion of particles increases by reducing nanoparticles diameter. Therefore, Reynolds number increases and Prandtl number declines using nanoparticles with a larger diameter. Considering aforementioned explanations, consequently with increasing nanoparticles diameter Nusselt number

Table 10 Variations of TiO2/water nanofluid properties with volume fraction for two nanoparticle diameter of 21 and 100 nm in laminar flow for the unglazed system at constant mass flow rate of 0.05 kg s−1. Nanoparticle diameter

Volume fraction

ρnf ρbf

cp, nf cp, bf

μnf μbf

knf

Prandtl number

Reynolds number

Nusselt number

Heat transfer coefficient

kbf

21 nm

0% 1% 2% 3% 4%

1 1.032 1.063 1.095 1.126

1 0.966 0.935 0.905 0.877

1 1.208 1.664 2.388 3.396

1 1.035 1.063 1.092 1.121

5.750 6.460 8.345 11.240 15.020

1106.246 919.808 669.464 468.635 330.884

5.926 6.158 6.738 7.342 7.934

535.800 576.538 648.389 725.661 805.256

100 nm

0% 1% 2% 3% 4%

1 1.032 1.063 1.095 1.126

1 0.966 0.935 0.905 0.877

1 1.108 1.331 1.677 2.152

1 1.029 1.056 1.082 1.109

5.750 5.964 6.728 7.969 9.630

1106.939 1001.416 836.811 666.803 521.340

5.926 6.154 6.541 6.927 7.320

535.800 572.982 624.839 678.642 735.420

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Table 11 Variations of TiO2/water nanofluid properties with volume fraction for two nanoparticle diameter of 21 and 100 nm in turbulent flow for the unglazed system at constant mass flow rate of 0.4 kg s−1. Nanoparticle diameter

Volume fraction

ρnf ρbf

cp, nf cp, bf

μnf μbf

knf

Prandtl number

Reynolds number

Nusselt number

Heat transfer coefficient

kbf

21 nm

0% 1% 2% 3% 4%

1 1.032 1.063 1.095 1.126

1 0.966 0.935 0.905 0.877

1 1.204 1.656 2.380 3.392

1 1.031 1.059 1.087 1.115

6.054 6.826 8.847 11.987 16.130

8453.780 7025.501 5106.305 3554.599 2494.223

69.912 64.119 55.675 47.493 40.634

6228.744 5948.406 5303.189 4643.287 4076.547

100 nm

0% 1% 2% 3% 4%

1 1.032 1.063 1.095 1.126

1 0.966 0.935 0.905 0.877

1 1.106 1.327 1.672 2.150

1 1.027 1.053 1.080 1.107

6.054 6.299 7.126 8.476 10.297

8453.780 7640.030 6370.147 5057.386 3935.404

69.912 66.391 60.918 54.784 48.864

6228.744 6137.168 5774.456 5323.756 4867.088

no important effect on the output power of PV compared to the base fluid, in both laminar and turbulent regime, as shown in Tables 12 and 13. Consequently, the variations of electrical energy and exergy efficiency versus nanoparticles concentration, for the studied TiO2/water nanofluid, are not substantial.

pump power that based on Tables 12 and 13, adding nanoparticles has no significant influence in pump power. Furthermore, According to Eq. (4), the output power of PV declines with increasing PV temperature and pump power, that based on previous results, the studied TiO2/water nanofluid slightly affect the mentioned parameters and consequently have

Fig. 11. Variation of (a) electrical energy efficiency, (b) thermal energy efficiency and (c) total energy efficiency against volume fraction for TiO2/water nanofluid with two nanoparticle diameters of 21 and 100 nm, at constant mass flow rates corresponded to laminar and turbulent flows.

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Fig. 12. Variation of (a) electrical exergy efficiency, (b) thermal exergy efficiency and (c) total exergy efficiency against volume fraction for TiO2/water nanofluid with two nanoparticle diameters of 21 and 100 nm, at constant mass flow rates corresponded to laminar and turbulent flows.

Table 12 Variations of photovoltaic output power, pump power and electrical efficiency with volume fraction for TiO2/water nanofluid with nanoparticle diameters of 21 and 100 nm in laminar flow for the unglazed system at constant mass flow rate of 0.05 kg s-1.

Table 13 Variations of photovoltaic output power, pump power and electrical efficiency with volume fraction for TiO2/water nanofluid with nanoparticle diameters of 21 and 100 nm in turbulent flow for the unglazed system at constant mass flow rate of 0.4 kg s−1.

Nanoparticle diameter

Volume fraction

̇ (W ) EPV

Ppump(W )

Ppump ×100 ̇ EPV

ηele

Nanoparticle diameter

Volume fraction

̇ (W ) EPV

Ppump(W )

Ppump ×100 ̇ EPV

ηele

21 nm

0% 1% 2% 3% 4% 0% 1% 2% 3% 4%

186.012 186.665 186.828 186.945 187.016 186.012 186.651 186.757 186.833 186.855

0.624 0.625 0.629 0.634 0.640 0.624 0.624 0.625 0.627 0.630

0.335 0.335 0.337 0.339 0.342 0.335 0.334 0.335 0.336 0.337

11.586 11.627 11.637 11.644 11.648 11.586 11.626 11.633 11.637 11.640

21 nm

0% 1% 2% 3% 4% 0% 1% 2% 3% 4%

190.989 190.963 190.919 190.865 190.805 190.989 190.971 190.943 190.909 190.869

8.313 8.261 8.343 8.497 8.684 8.313 8.190 8.145 8.156 8.200

4.352 4.326 4.370 4.452 4.551 4.352 4.288 4.266 4.272 4.296

11.417 11.418 11.411 11.397 11.382 11.417 11.423 11.424 11.422 11.416

100 nm

100 nm

are not considerable as demonstrated in Tables 12 and 13. The other important parameter in the studying of PV/T systems is thermal efficiency, which has been showed in Fig. 11(b). Based on Eq. (1), thermal energy efficiency depends on the specific heat capacity of coolant, inlet and outlet coolant temperature and solar radiation, at

On the other hand, the results indicate that increasing the nanoparticle diameter results in a slight reduction in electrical energy and exergy efficiency in laminar flow due to slight increment of PV temperature, while in the turbulent flow electrical energy and exergy efficiency enhance slightly, as PV temperature decreases, although these changes 338

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Fig. 13. Variation of (a) photovoltaic temperature and (b) outlet coolant temperature against volume fraction for TiO2/water and Al2O3/water nanofluids, at constant mass flow rates corresponded to laminar and turbulent flows.

Fig. 14. Variation of (a) electrical energy efficiency, (b) thermal energy efficiency and (c) total energy efficiency against volume fraction for TiO2/water and Al2O3/water nanofluids, at constant mass flow rates corresponded to laminar and turbulent flows.

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Fig. 15. Variation of (a) electrical exergy efficiency, (b) thermal exergy efficiency and (c) total exergy efficiency against volume fraction for TiO2/water and Al2O3/water nanofluids, at constant mass flow rates corresponded to laminar and turbulent flows.

constant mass flow rate. Considering Fig. 10(b), the outlet temperature of the nanofluid rises with increasing nanoparticle concentration, while the heat capacity decreases, as presented in Table 10; therefore, the interplay of these two opposite effects determines the tendency of thermal energy efficiency. As observed in Fig. 10(b), in laminar flow, the amount of outlet temperature enhancement due to nanoparticle addition is sufficient to overcome the heat capacity reduction of

nanofluids, thus thermal energy efficiency slightly augments. But, in turbulent flow the effect of adding nanoparticles on the outlet temperature is not considerable compared to heat capacity reduction, which leads to the slight decline in thermal energy efficiency, as shown in Fig. 11(b). Additionally, thermal exergy efficiency is more influenced by outlet temperature, according to Eq. (5); therefore, thermal exergy efficiency increases in both flow regimes with increasing nanoparticle

Table 14 Variations of TiO2/water and Al2O3/water nanofluids properties with volume fraction in laminar flow for the unglazed system at constant mass flow rate of 0.05 kg s−1. Nanoparticle type

Volume fraction

ρnf ρbf

cp, nf cp, bf

μnf μbf

knf

Prandtl number

Reynolds number

Nusselt number

Heat transfer coefficient

kbf

TiO2/water dp=21 nm

0% 1% 2% 3% 4%

1 1.032 1.063 1.095 1.126

1 0.966 0.935 0.905 0.877

1 1.208 1.664 2.388 3.396

1 1.035 1.063 1.092 1.121

5.767 6.460 8.345 11.240 15.020

1103.938 919.808 669.464 468.635 330.884

4.773 6.158 6.738 7.342 7.934

431.395 576.538 648.389 725.661 805.256

Al2O3/water dp=21 nm

0% 1% 2% 3% 4%

1 1.026 1.052 1.078 1.104

1 0.971 0.944 0.918 0.893

1 1.101 1.297 1.539 1.792

1 1.050 1.120 1.206 1.307

5.767 5.813 6.209 6.623 6.888

1103.938 1011.374 863.086 730.246 629.926

4.773 8.123 9.208 9.751 10.017

431.395 771.853 933.220 1064.860 1186.553

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Table 15 Variations of TiO2/water and Al2O3/water nanofluids properties with volume fraction in turbulent flow for the unglazed system at constant mass flow rate of 0.4 kg s−1. Nanoparticle type

Volume fraction

ρnf ρbf

cp, nf cp, bf

μnf μbf

knf

Prandtl number

Reynolds number

Nusselt number

Heat transfer coefficient

kbf

TiO2/water dp=21 nm

0% 1% 2% 3% 4%

1 1.032 1.063 1.095 1.126

1 0.966 0.935 0.905 0.877

1 1.204 1.656 2.380 3.392

1 1.031 1.059 1.087 1.115

6.054 6.826 8.847 11.987 16.130

8453.780 7025.501 5106.305 3554.599 2494.223

69.912 64.119 55.675 47.493 40.634

6228.744 5948.406 5303.189 4643.287 4076.547

Al2O3/water dp=21 nm

0% 1% 2% 3% 4%

1 1.026 1.052 1.078 1.104

1 0.971 0.944 0.918 0.893

1 1.102 1.297 1.535 1.774

1 1.041 1.104 1.185 1.284

6.054 6.223 7.714 8.193 10.467

8453.780 7672.435 6516.415 5508.552 4768.187

69.912 59.681 53.265 47.203 42.183

6228.744 5589.310 5290.987 5034.960 4872.791

Fig. 16. Variation of (a) thermal and total energy efficiency and (b) electrical energy and total exergy efficiency against volume fraction in laminar flow based on present study and Khanjari et al. [61] investigation.

concentration, which is more considerable in laminar flow, as depicted in Fig. 12(b). Besides, as shown in Figs. 11(b) and 12(b), the effect of nanoparticles diameter on thermal energy and exergy efficiency is not noticeable, since as mentioned before, particle diameter has no observable influence on the outlet coolant temperature in both regimes. Consequently, total energy and exergy efficiency are the summations of electrical and thermal energy and exergy efficiency. Considering Figs. 11 and 12, generally, due to larger amounts of

thermal energy efficiency, it is more effective in total energy efficiency. While, electrical exergy efficiency is about one order of magnitude higher than thermal exergy efficiency, thus total exergy efficiency is strongly affected by electrical part. Figs. 11(c) and 12(c) demonstrate that total energy and exergy efficiency increase in laminar flow with increasing nanoparticle concentration, since electrical and thermal energy and exergy efficiencies increase. Whereas in the turbulent regime, total energy efficiency drops due to the reduction of thermal energy efficiency and total exergy efficiency shows a similar trend as

Table 16 Variations of Al2O3/water and Al2O3/EG-water 60:40 nanofluids properties with volume fraction in turbulent flow for the unglazed system at constant mass flow rate of 0. 8 kg s−1. Volume fraction

ρnf

cp, nf

μnf

k nf

(kg m−3)

(J kg−1 K−1)

(Pa s)

(W m−1 K−1)

Prandtl number

Reynolds number

Nusselt number

Heat transfer coefficient

Al2O3/water

0% 1% 2% 3% 4%

998.807 1024.818 1050.829 1076.839 1102.850

4180.020 4060.055 3946.029 3837.512 3734.113

8.898×10−4 9.587×10−4 1.085×10−3 1.236×10−3 1.387×10−3

0.611 0.634 0.666 0.705 0.752

6.083 6.141 6.429 6.722 6.885

16834.596 15624.003 13810.840 12121.462 10798.258

121.370 111.793 102.147 92.693 84.527

10912.330 10420.433 10000.236 9616.205 9352.275

Al2O3/EG-water 60:40

0% 1% 2% 3% 4%

1086.551 1111.683 1136.815 1161.947 1187.080

3105.952 3030.170 2957.735 2888.432 2822.063

4.497×10−3 4.757×10−3 5.215×10−3 5.788×10−3 6.453×10−3

0.351 0.388 0.397 0.407 0.418

39.776 37.153 38.819 41.030 43.563

3330.755 3148.600 2872.212 2587.884 2321.207

44.278 41.352 39.352 37.282 35.269

2286.676 2359.626 2299.559 2234.113 2168.258

Nanoparticle type

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Fig. 17. Variation of (a) photovoltaic temperature and (b) outlet coolant temperature against volume fraction for two Al2O3/water and Al2O3/EG-water 60:40 nanofluids, at constant mass flow rate of 0.8 kg s−1.

Fig. 18. Variation of (a) electrical energy efficiency, (b) thermal energy efficiency and (c) total energy efficiency against volume fraction for two Al2O3/water and Al2O3/EG-water 60:40 nanofluids, at constant mass flow rate of 0.8 kg s−1.

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Fig. 19. Variation of (a) electrical exergy efficiency, (b) thermal exergy efficiency and (c) total exergy efficiency against volume fraction for two Al2O3/water and Al2O3/EG-water 60:40 nanofluids, at constant mass flow rate of 0.8 kg s−1.

Fig. 20. Variation of PV temperature against Reynolds number at different volume fraction for Al2O3/water nanofluid.

respectively. On the other hand, in turbulent regime, compared to water, total energy efficiency of the nanofluid insignificantly decreases by 0.5% and 0.2%, for the glazed and unglazed system, respectively. Moreover, for the studied TiO2/water nanofluid the effect of nanopar-

electrical exergy efficiency. In this study, the best total energy and exergy efficiency of laminar flow are predicted at 4% volume fraction, which are about 1.8% and 2.3% higher than pure water for the glazed panel and 3.3% and 1.8% higher than pure water for the unglazed one, 343

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Fig. 21. Variation of outlet coolant temperature against Reynolds number at different volume fraction for Al2O3/water nanofluid.

Fig. 22. Variation of total energy efficiency against Reynolds number at different volume fraction for Al2O3/water nanofluid.

Fig. 23. Variation of total exergy efficiency against Reynolds number at different volume fraction for Al2O3/water nanofluid.

efficiency, while it increases thermal energy and exergy efficiency. Also, the glazed system gives higher total energy efficiency and lower total exergy efficiency compared to the unglazed one. Besides, although in turbulent flow electrical output increases more due to the higher reduction in PV temperature, larger mass flow rate results in higher pump power, which consequently leads to lower electrical energy and exergy efficiency, compared to laminar flow, as shown in Tables 12 and 13. Also, despite larger mass flow rate, due to lower outlet coolant

ticle diameter on total energy and exergy efficiency is not considerable. In addition to the findings related to nanofluid parameters, Figs. 10–12 demonstrate the effects of glazing system and flow regime type on the characteristics of PV/T systems, from which, it can be concluded that these two parameters affect temperature values, as well as energy and exergy efficiencies of the nanofluid-based PV/T systems similar to water one. On other words, the glazing system enhances PV and outlet temperature and thus, decreases electrical energy and exergy

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Fig. 24. Variation of PV temperature against Reynolds number at different volume fraction for Al2O3/water and Al2O3/EG-water 60:40 nanofluids at dnp=45 nm.

Fig. 25. Variation of outlet temperature against Reynolds number at different volume fraction for Al2O3/water and Al2O3/EG-water 60:40 nanofluids at dnp=45 nm.

Fig. 26. Variation of total energy efficiency against Reynolds number at different volume fraction for Al2O3/water and Al2O3/EG-water 60:40 nanofluids at dnp=45 nm.

sponded to laminar and turbulent flows. Tables 14 and 15 help in better understanding of the heat transfer behavior of these nanofluids, which show the variation of TiO2/water and Al2O3/water nanofluid properties with nanoparticles concentration in the laminar and turbulent regime. In laminar regime, Al2O3/water expresses higher heat transfer coefficient compared to TiO2/water due to its higher thermal conductivity and Nusselt number, which accordingly leads to further decrease in PV temperature in the case of Al2O3/water, as shown in Fig. 13(a). On the other hand, the variation of heat transfer coefficient with nanoparticles concentration is more complicated in turbulent regime.

temperature, thermal exergy efficiency (which is more influenced by outlet temperature) of turbulent regime is lower than that of laminar one. However, in thermal energy efficiency the effect of mass flow rate enhancement is more considerable than outlet temperature reduction, which leads to the higher thermal energy efficiency in turbulent flow in comparison with laminar flow. 3.4.2. Effect of nanoparticle type In this section, the influence of nanofluid type on the system performance has been examined, by comparing Al2O3/water and TiO2/ water nanofluids in Figs. 13–15, at constant mass flow rates corre345

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Fig. 27. Variation of total exergy efficiency against Reynolds number at different volume fraction for Al2O3/water and Al2O3/EG-water 60:40 nanofluids at dnp=45 nm.

to the present study (298 K), which in turn leads to the higher outlet temperature. On the other hand, the required energy for pump power was ignored by Khanjari et al. [61], which results in greater electrical energy efficiency compared to present work. These differences, consequently lead to higher total energy and exergy efficiency reported by Khanjari et al. Besides, considering Fig. 16, due to considerably greater thermal conductivity of Ag nanoparticles the related overall efficiency is more than that of TiO2 and Al2O3 cases.

Considering Table 15, lower viscosity of Al2O3/water results in higher Reynolds number and lower Prandtl number compared to TiO2/water, and based on Eq. (22), conflicting between these two dimensionless numbers determines the heat transfer coefficient variations. Besides, in view of Table 6, and Eq. (22), Reynolds number affects Nusselt number in turbulent regime stronger than laminar regime. Finally, it can be observed that, for nanoparticles concentrations lower than 2%, the heat transfer coefficient of TiO2/water is slightly higher, while for higher concentrations, Al2O3/water shows higher heat transfer coefficient, which leads to slightly lower PV temperature compared to TiO2/water. In the best situation, using Al2O3/water decreases PV temperature 2.6% for the glazed panel and 2% for the unglazed panel more than TiO2/water at nanoparticles concentration of 2%, in laminar regime. Whereas, in turbulent regime, the insignificant maximum increase of about 0.3% occurs at 4% volume fraction. To explore the effect of nanoparticle type on outlet coolant temperature, the variations of heat capacity and heat transfer coefficient are influential, where lower heat capacity and/or higher heat transfer coefficient may cause outlet coolant temperature to increase. From Table 14, the heat capacity values of Al2O3/water and TiO2/water are nearly same, while heat capacity of Al2O3/water with concentration increases slightly more than TiO2/water. Consequently, according to Fig. 13(b), for the studied nanoparticles, the type of nanoparticles has a minor effect on the outlet coolant temperature. Figs. 14 and 15 display the effect of nanoparticle type on energy and exergy efficiencies, respectively. It can be observed in Figs. 14(a) and 15(a) that, PV/T system containing Al2O3/water provides higher electrical energy and exergy efficiency compared to TiO2/water owing to lower PV temperature gained in the case of Al2O3/water. As depicted in Fig. 14(b), thermal energy efficiencies of the system are nearly same for two studied nanofluids. Considering Eq. (5), at a given mass flow rate, and due to minor variation of heat capacity with particle concentration, the outlet coolant temperature is the most effective parameter on thermal exergy efficiency; thus, exergy efficiency follows the same trends as outlet temperature in both flow regimes, as displayed in Fig. 15(b). Finally, it can be concluded from Figs. 14(c) and 15(c) that, for the studied nanofluids of TiO2/water and Al2O3/water, type of nanofluid has no considerable effect on total energy and exergy efficiency. Among available studies, the work of Khanjari et al. [61] is most similar to present survey, therefore the predicted efficiencies form two studies have presented and compared in Fig. 16(a) and (b).where nearly similar trends and values are observed. However, according to Fig. 16, thermal, electrical and total energy efficiency and total exergy efficiency of Khanjari et al. [61] study are greater than those of present study. Higher thermal energy efficiency of Khanjari et al. [61] study can be attributed to related higher inlet temperature (303.15 K) compared

3.4.3. Effect of base fluid type For investigating the effect of base fluid material on the characteristics of the PV/T system, water as well as a mixture of ethylene glycol and water (with the mass fraction of 60:40) are studied at constant mass flow rates corresponded to laminar and turbulent flows. Ethylene glycol-water mixture was chosen owing to its low freezing point that makes it an excellent heat transfer fluid in cold regions. In the present section, due to lack of available experimental data concerning laminar flow, only the PV/T system containing Al2O3 nanoparticles with diameter of 45 nm mixed into different base fluids in turbulent regime has been investigated (according to available experimental data [34]). Table 16 shows the nanofluid and flow characteristics of the studied Al2O3/water and Al2O3/EG-water nanofluids at different nanoparticles concentrations, in turbulent flow. In this section, as the viscosity of EGwater is considerably higher than water, the investigations were performed at a constant mass flow rate of 0.8 kg s−1, to achieve turbulent regime condition in all studied nanofluids. In view of Table 16, the large viscosity of EG-water based nanofluids (about 4.5 times as much as water), leads to lower Reynolds number and higher Prandtl number, which in turn result in lower heat transfer coefficient compared to water based nanofluids, at a constant mass flow rate. Thus, PV temperature is lower in the case of Al2O3/water in comparison with Al2O3/EG-water, as depicted in Fig. 17(a). The maximum reduction in PV temperature using Al2O3/water instead of Al2O3/EGwater is about 3%. Additionally, Fig. 17(b) demonstrates that the nanofluid type has no considerable effect on the outlet coolant temperature of the PV/T system in turbulent flow at constant mass flow rate, as discussed in previous sections. However, the outlet temperature using Al2O3/EGwater is a little higher due to its lower heat capacity compared to Al2O3/water nanofluid. Comparison of system efficiencies using Al2O3/water and Al2O3/ EG-water nanofluids in Figs. 18 and 19 indicates that employing Al2O3/water nanofluid instead of Al2O3/EG-water as the coolant in the PV/T system improves electrical energy and exergy efficiency dramatically about 5%. This is due to lower PV temperature and lower required pump power in the case of water based nanofluids. In 346

347

2015

2015

2015

2015

2016

Michael and Iniyan [63]

Saroha et al. [78]

Lelea et al. [54]

Khanjari et al. [61]

2014

Karami and Rahimi [56]

Jing et al. [51]

2014

Karami and Rahimi [55]

2015

2014

Sardarabadi et al. [58]

Ghadiri et al. [62]

2014

DeJarnette et al. [76]

2014

2013

Mittal et al. [77]

Tang and Zhu [68]

2013

Otanicar et al. [73]

2014

2012

Taylor et al. [74]

Xu and Kleinstreuer [64]

2012

Cui and Zhu [67]

2014

2012

Elmir et al. [10]

Xu and Kleinstreuer [65]

Year

Investigator

Sheet and tube PV/T

Microchannel cooling CPV/T

CPV/T

Single rectangular absorber PV/T

CPV/T

Sheet and tube PV/T

PV/T

Single rectangular absorber CPV/T

Single rectangular absorber CPV/T

PV/T

Microchannel cooling PV/T

Sheet and tube PV/T

CPV/T

CPV/T

CPV/T

CPV/T

Single rectangular absorber PV/T PV/T

System type

Table 17 Literature review of nanofluid-based PV/T systems.

Numerical

Numerical

–

–

Experimental Experimental Numerical & experimental Experimental

– – –

Laminar

Laminar

–

Numerical

Numerical

Experimental & numerical

Numerical

–

Laminar

Numerical

Experimental

Turbulent

Laminar & turbulent

Experimental

Experimental

– Laminar

Numerical

Laminar

Numerical

Experimental

–

Laminar

Numerical

Method

Laminar

Flow regime

Ag/water, Al2O3/water: 0–12 vol%, 50 nm

Al2O3/water: 1, 3, 5 vol%, 28, 47 nm

Au/water: 0.00025, 0.0093 vol%, 5, 10 nm; Ag/ water: 0.00019, 0.0193 vol%, 5, 10 nm

CuO/water: 0.05 vol%, 75 nm

SiO2/water: 0.5, 1, 2 vol%, 5, 10, 25, 50 nm

Fe3O4/water: 1, 3 wt%, 45 nm

Al2O3/water: 0.02 wt%

Al2O3/water: 5 vol%, 38 nm

Al2O3/water: 0–4 vol%, 38.4 nm

Boehmite: 0.01, 0.1, 0.5 wt%, 5–10 nm

Boehmite: 0.01, 0.1, 0.3 wt%, 5–10 nm

SiO2/water: 1, 3 wt%, 11–14 nm

Cu/water: 0.0043 vol%, 10 nm Ag/water: 0.01937 vol%,10 nm Plasmonic nanofluid

Au, Ag, Al, SiO2, GC495, GC435, GC570, RG715, RG1000 in water and TherminolVP-1: 0–0.1 vol %, 20–50 nm Al, Ag, Au in water and TherminolVP-1: 30– 52 nm

Coolant

Coolant

Spectral filter & coolant

Spectral filter & coolant Coolant

Coolant

Coolant

Coolant

Coolant

Coolant

Coolant

Spectral filter & coolant Spectral filter & coolant Coolant

Spectral filter

Spectral filter

Coolant

Coolant

Al2O3/water: 0–10 vol% MgO/water: 0.02, 0.06, 0.1 wt%, 10 nm

Nanofluid usage

Nanofluid, concentration and size

pump

2

3

max

max

(continued on next page)

Flowing of Al O over PV/T system leads to a higher • total and thermal efficiency than flowing water. ferrofluid by an alternating magnetic field can • Adding improve the overall energy and exergy efficiency. greater exergy efficiency by using nanofluid • Achieving compared to water. system enhances thermal efficiency and reduces • Glazing electrical efficiency. efficiency enhances and electrical efficiency • Thermal declines by employing CuO/water instead of water. Ag/water represent higher overall efficiency than • The Au/water. Nanofluids are practical choice for both optical filter and • HTF in PV/T system. Constant Re: almost the same T by using water and • nanofluid as coolant. P : nanofluid obtains lower T than water • atConstant Re < 1000. energy and exergy efficiency by increasing • Enhancing nanoparticle volume fraction.

of PV/T system.

reduction of PV temperature by using nanofluid • Greater instead of water. channel works more efficient compared with the • Helical straight one. electrical output by increasing flow velocity. • Enhancing improvement of cell efficiency at higher volume • further fractions and lower nanofluid inlet temperatures generation must be considered for determining • Entropy overall efficiency. Using nanofluids improves the electrical and total • efficiencies, but hardly increase the thermal efficiency

particularly at lower concentration.

of PV temperature and achieving higher • Reduction overall efficiency by using nanofluids. to choose appropriate plasmonic nanoparticles for • Help a particular cell bandgap. of the energy and exergy efficiency of the • Enhancement PV/T by using SiO2/water. reduction of average PV temperature by • Significantly applying Boehmite nanofluid instead of water,

conventional thin film filters.

lower overall efficiency and higher thermal • Slightly efficiency of nanoparticle based filters than

• Adding nanofluids improves cooling of the solar cells. of nanofluids transmittance with the • Reduction increasing mass fraction and film thickness. overall efficiency and lower electrical efficiency • Higher of PV/T compared to PV. filters are efficient, compact and potentially • Nanofluids low-cost optical filters.

Remarks

F. Yazdanifard et al.

Renewable and Sustainable Energy Reviews 76 (2017) 323–352

Sheet and tube PV/T

2016

2016

2016

2016

2016

2016

2016

2016

2016

2016

Sardarabadi and Passandideh Fard [59]

An et al. [71]

Radwan et al. [53]

Sharaf and Orhan [57]

Hassani et al. [49]

Al-Shamani et al. [66]

An et al. [50]

348

Hjerrild et al. [72]

Hassani et al. [52]

Rejeb et al. [60]

Sheet and tube PV/T

CPV/T

Fresnel lens CPV/T

Fresnel lens CPV/T

Serpentine rectangular absorber PV/T

PV/T

Minichannel cooling CPV/T

Microchannel cooling CPV/T

Fresnel lens CPV/T

System type

Year

Investigator

Table 17 (continued)

Experimental & numerical

–

Numerical & experimental

–

Numerical

Experimental

–

Laminar

Experimental

Numerical

Numerical

Turbulent

Laminar

Laminar & turbulent

Numerical

Experimental

–

Laminar

Experimental & numerical

Method

Laminar

Flow regime

Al2O3/water, Cu/water, Al2O3/EG, Cu/EG: 0.1, 0.2, 0.4 wt%

Ag/water: 0.001–1.5 vol%, 10 nm Ag/TherminolVP: 0.0002, 0.003 vol%, 10 nm

Ag-SiO2/water: 0.001, 0.003, 0.006, 0.026 wt%, 17.5 nm CNT/water: 0.067, 0.078, 0.083, 0.088 wt%, 6– 13 nm

Polypyrrole: 3.12, 6.25, 12.5 ppm, 13.68 nm

TiO2/water, SiO2/water, SiC/water: 0–2 wt%

Ag/water: 0.001 vol%, 10 nm CNTs/water: 0.1 vol%, 15 nm

Al2O3/water, Al2O3/ synthetic oil

Al2O3/water, SiC/water : 0–4 vol%, 20 nm

Cu9S5/oleylamine: 22.3,44.6, 89.2 ppm, 60.2 nm

Al2O3/water: 0.2 wt%, 20 nm TiO2/water: 0.2 wt%, 10–30 nm ZnO/water: 0.2 wt%, 10–25 nm

Nanofluid, concentration and size

Coolant

Spectral filter & coolant

Spectral filter

Spectral filter

Coolant

Spectral filter & coolant

Coolant

Coolant

Spectral filter

Coolant

Nanofluid usage

pure water.

2

2

3

3

3

2

3

as basefluid is more efficient than the EG. • Water CuO/water shows greater thermal and electrical • The efficiency compared to other HTFs.

concentrated solar radiation.

electrical and overall efficiency of the GaAs • Increasing and Si PV cells by increasing volume fraction. Separate channels of optical and thermal nanofluids is • more suitable than double-pass channel for

2

Ag-SiO /water yields a higher overall efficiency at • The the highest concentration.

total efficiency of system by using • Increasing polypyrrole nanofluid filter. of electrical efficiency and reduction of • Enhancement thermal efficiency by increasing particle concentration.

2

performance of nanofluids-based PV/T system • Better compared to a PV and PV/T systems Decrement of CO emissions by using nanofluids-based • PV/T. thermal, electrical and overall efficiency of SiC/ • Highest water compared to other HTFs.

2

Using Al O /synthetic oil is unfavorable both thermally • and hydraulically compared to water and Al O /water.

2

Higher electrical efficiency of TiO /water and ZnO/ • water nanofluids compared to Al O /water. thermal performance of ZnO/water than other • Better HTFs. dependency of thermal efficiency to volume • Greater fraction than electrical efficiency. Employing the anti-reflect glass can improve system • efficiency volume fraction can adjust heat to electricity • Changing ratio. higher reduction in cell temperature can be • Relatively achieved by using SiC/water than Al O /water. greater electrical efficiency at lower Re and • Achieving higher concentration ratios by nanofluids compared to

performance of Ag/water compared to alumina/ • Better water.

Remarks

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Additionally, at constant Reynolds number, total energy and exergy efficiency of the system used Al2O3/water are higher than the system with Al2O3/EG-water, as shown in Figs. 26 and 27. This fact is attributed to the higher electrical energy and exergy efficiency of Al2O3/water, since related lower mass flow rates lead to the considerably lower pump power. The other interesting result from Fig. 27 is that, at Re=8000 total exergy efficiency of the system used Al2O3/EGwater is negative, as the output power of the PV system is lower than required pump power. Also, it is clear from Figs. 26 and 27 that total energy and exergy efficiency of Al2O3/EG-water decrease with increasing nanoparticle concentration owing to reduction of electrical efficiency. Consequently, weaker characteristics of the system used EG-water based nanofluids present this type of nanofluids as an improper choice in PV/T systems; however, it is an inevitable choice in cold region of the world.

addition, thermal energy efficiency of Al2O3/EG-water nanofluid is lower, which is attributed to its lower heat capacity, while, thermal exergy efficiency of Al2O3/EG-water is higher due to its greater outlet temperature. Finally, total energy and exergy efficiency of the system using Al2O3/water are greater than those of Al2O3/EG-water nanofluid, that this result is more noticeable in energy efficiency. 3.4.4. Analysis of the system at constant Reynolds number In all aforementioned cases, the investigations were performed considering constant mass flow rate, which seems to be more realistic and practical. However, investigating the performance of the system using nanofluids at the same Reynolds number situation is a standard method of studying in many types of research, which will be considered in this section. Figs. 20–23 are column diagrams that illustrate the variation of PV temperature, outlet coolant temperature, total energy efficiency and total exergy efficiency of the unglazed PV/T, respectively at various Reynolds numbers (500, 2000, 4000, 6000 and 8000), for Al2O3/water nanofluid with 21 nm sized nanoparticles. Considering Table 14, increasing nanoparticle concentration leads to an increase in viscosity of nanofluid considerably compared to other thermophysical properties, which in turn leads to an increase in Prandtl number. This behavior, for a given Reynolds number, tends to enhance a Nusselt number and consequently a heat transfer coefficient of nanofluid, which is more considerable in laminar regime as discussed in Section 3.4.1. Therefore, PV temperature reduces with nanoparticles concentration, as shown in Fig. 20. Also, it can be concluded that, with increasing Reynolds number, PV temperature declines, since heat transfer coefficient increases. On the other hand, as nanoparticle concentration increases, the viscosity of nanofluid increases, thus to keep the Reynolds number constant, the velocity of the flow should increases, which in turn leads to an enhancement in the mass flow rate. Accordingly, at a given Reynolds number, adding nanoparticles tends to decrease outlet coolant temperature, as shown in Fig. 21. Also, increasing the Reynolds number decreases outlet temperature, as mass flow rate rises. Besides, total energy efficiency enhances by increasing nanoparticle concentration at low Reynolds numbers (laminar flow regime) due to increasing electrical and thermal energy efficiencies, as PV temperature reduces and mass flow rate rises. While at larger Reynolds numbers (turbulent flow regime) the effect of applying nanofluids is insignificant as mentioned before. As well, for a given Reynolds number of laminar regime, using low concentration nanofluid can improve total exergy efficiency of the system, since electrical efficiency enhances as PV temperature decreases, and the effect of pump power is not significant. However, the value of pump power become significant at higher nanoparticles concentrations, related to viscosity enhancement of nanofluid. In turbulent flow regime adding nanoparticles and increasing Reynolds number decrease total exergy efficiency due to the substantial increase in pump power, since the mass flow rate increases, meaningfully. Finally, the comparison of characteristics of different base fluids at constant Reynolds numbers has been investigated in Figs. 24–27, for the unglazed PV/T system containing Al2O3/water and Al2O3/EGwater 60:40 nanofluids. Similar to Al2O3/water, using Al2O3/EG-water leads to the reductions in PV and outlet coolant temperature of the system at constant Reynolds number and these reductions continue with adding more nanoparticles. The analysis of numerical results indicated that, aside from higher mass flow rate, Al2O3/EG-water provides higher Nusselt number at constant Reynolds number compared to Al2O3/water owing to considerably greater viscosity of Al2O3/EG-water. However, due to the higher thermal conductivity of Al2O3 particles, heat transfer coefficient in the case of Al2O3/water is greater than Al2O3/EG-water. Consequently, at constant Reynolds number, these features lead to achieving slightly lower PV temperature and outlet coolant temperature in the case of Al2O3/EG-water.

4. Conclusion The objectives of this paper were providing a comprehensive review in the field of using nanofluids in the PV/T systems, and comprehensively investigate the effective parameters on the performance of the nanofluid-based flat plate PV/T systems. Generally, nanofluid-based PV/T systems apply nanofluid in two forms, one as coolant and the other as spectral filter. A summary of the papers, which studied nanofluid-based PV/T systems, has been presented in Table 17, clarifying flow regime, survey method, nanofluid type and duty, and related important results. According to this review, in almost all reported nanofluid-based PV/T systems whether use nanofluid as coolant or filter, overall energy and exergy efficiency enhances in comparison to the conventional fluid. Therefore, it seems that these types of systems, which produce both electrical and thermal energy, can provide a part of world energy demands. Besides, the literature review indicated that researchers are interested in examining different nanofluids and system configurations to achieve the best performance and verify the competency of nanofluid-based PV/T systems. For this purpose, the effect of some important parameters such as nanoparticle concentration, nanofluid type, irradiation, inlet temperature, mass flow rate and geometrical properties of the cooling system have been surveyed to find optimum conditions. Furthermore, this paper proposed a mathematical model to extensively analyze the performance of a nanofluid-based PV/T system in both laminar and turbulent regime, considering the influences of the glass cover and pump power, to achieve an extensive overview on the operation of these systems. Besides, the proposed correlations for thermophysical properties and heat transfer characteristics of studied nanofluids will be useful for future studies. According to the results presented in this work, the following remarks were drawn from the studied cases: • In general, the effect of nanofluid as one of improvement techniques of heat transfer characteristics of the system is more considerable in laminar flow compared to turbulent regime. • In the analysis of the nanofluids, it is important to pay attention that the comparisons are performed base on which constant parameter: Reynolds number, mass flow rate, or etc. • At constant mass flow rate, with increasing nanoparticles concentration, PV temperature reduces in laminar regime, while it increases slightly in turbulent regime. Whereas, at constant Reynolds number, PV temperature declines in both flow regimes. • The outlet temperature of nanofluid enhances with increasing nanoparticles concentration at constant mass flow rate, while it decreases at constant Reynolds number analysis, where this changes is more considerable in laminar regime. • Considering constant mass flow rate condition, total energy and exergy efficiency increase in laminar flow with increasing nanoparticle 349

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concentration, while in the turbulent flow total energy efficiency drops and total exergy efficiency doesn’t change considerably. The analysis at constant Reynolds number indicated that, adding nanoparticles increases total energy in laminar flow, dramatically, while it has no significant effect in turbulent flow; however, nanofluid has a noticeable influence on total exergy, where considerable reduction of this quantity was observed in turbulent flow. For laminar regime, there is an optimum volume fraction of particles, in which total exergy is maximized. • For the studied nanofluids the effect of nanoparticle diameter on the performance of the PV/T system is not considerable. Additionally, in the case of the studied nanofluids (TiO2/water and Al2O3/water), nanoparticle material has a minor effect on total energy and exergy efficiency, compared to other parameters. • Type of base fluid affects the performance of system strongly. In this paper, this analysis was performed only in turbulent regime. For the studied nanofluids, at constant mass flow rate, Al2O3/water nanofluid shows a remarkable enhancement of about 5% more than Al2O3/EG-water in electrical energy and exergy efficiency of the system in turbulent regime. Moreover, for a given turbulent Reynolds number, even the exergy of the system used Al2O3/EG-water maybe becomes negative, which introduces this nanofluid as an inappropriate choice in PV/T systems. • Finally, the effect of studied water based nanofluids on the characteristics of the studied PV/T system is not sizeable compared to the effect of flow regime and using glass cover. The glazing system enhances PV and outlet temperature and thus, gives higher total energy efficiency and lower total exergy efficiency compared to unglazed one. Studying the effect of flow regime indicated that, PV temperature and outlet coolant temperature reduce in turbulent flow dramatically, where total energy and exergy efficiency of the system is more in the case of turbulent flow and laminar flow, respectively.

designs with considering the required pump power in determining the efficiency. • Investigating especially experimentally using other base fluids rather than water, such as ethylene glycol-water mixture, oil and ionic liquids in PV/T system both as coolant and spectral filter can be outstanding to find the best media for achieving the highest performance. • A comprehensive investigation on economical viewpoint and reduction of emissions using nanofluids are suggested. Also, the toxicity of nanoparticles used in PV/T systems should be considered, carefully. • In almost all investigated nanofluids increment of viscosity is greater than enhancement of thermal conductivity compared to those of base fluids, which limit improving heat transfer characteristics gained by nanofluids. Thus, manufacturing the nanofluids which enhance the thermal conductivity of base fluids more than viscosity increase can be valuable to enhance the performance of PV/T systems. • Investigating the effect of hybrid nanofluids in PV/T system can be attractive, especially when it is possible to combine different nanomaterials possessed various desired thermal and optical properties. In addition, the effect of some new nanostructures such as borophene on PV/T systems could be investigated. • Research on building-integrated nanofluid based PV/T should be carried out. • The PV/T systems which apply nanofluids as the optical filter and use air, phase change materials (PCMs) or thermoelectric devices for cooling of PV cells can be another attractive new subject. • Based on literature review most of the investigated system examined under laminar flow condition. However, turbulent flow can increase the cooling performance of PV systems; hence, more experimental data about employing nanofluid in turbulent regime is needed. References

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According to the literature review of nanofluid-based PV/T systems, the followings can be subject of future studies: • Since, the key step in applying the nanofluids is to prepare the uniform and stable colloids it is predicted that the method of nanofluid preparation is the main source of scattered and inconsistent data in nanofluid issue. Therefore, it is suggested that the method of preparation, the applied stability method, the stability duration and the method of evaluating the stability of the prepared nanofluids must be reported in all nanofluids’ studies. • It is strongly suggested that, in theoretical and numerical investigations on modeling of nanofluids, the researchers employ appropriate temperature dependent thermophysical properties and especially in the case of Nusselt number, they never use the classical models which are valid for common fluids. Several correlations were obtained in this study for the nanofluids based on available data, listed in Table 6. • Driving comprehensive models for the energy and exergy efficiency of PV/T system using nanofluids, which include all important parameters such as climate conditions, nanofluid characteristics, and geometrical aspects are crucial to make this method practical. Actually, for this purpose, more standard and identical experiments are necessary to enrich the theoretical understanding of this issue. • Complete CFD simulation of PV/T systems, along with using two phase approach for modeling of nanofluid flow, can help detect significant parameters and enhance the understanding of related heat transfer mechanism to propose comprehensive models. • Combination of nanofluids with other methods of heat transfer enhancement in PV/T systems, such as using flow inserts through the nanofluid flow and designing different configuration of nanofluid flow path, seems to be advantageous and practical. Investigation of different channel design such as bionical, sharp, serpentine and other innovate 350

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