# A Comprehensive Review on Efficiency Enhancement of Solar Collectors Using Hybrid Nanofluids

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## Abstract

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## 1. Introduction

## 2. Historical Background

_{2}, TiO

_{2}, Al

_{2}O

_{3}, ZnO, CuO, etc.) or organic particles (carbon nanotubes, graphene oxide, diamond, etc. could be disseminated to create hybrid nanofluids) [50,51] are used to enhance the thermophysical properties and heat transfer efficiency, and hybrid nanofluid synthesis is crucial. The Al

_{2}O

_{3}-Cu nanofluid, for example, was developed using the hydrogen reduction method using Al

_{2}O

_{3}and CuO (90:10 ratio) to improve the viscosity to be steeper than concentration conductivity [52,53]. The MWCNT-Fe

_{3}O

_{4}nanocomposite particle has been synthesized empirically (0–0.3 volume percentage) to test their thermal properties [54]. Improved thermal conductivity was achieved with Ag/MWCNT-HEG hybrid nanofluids at 25 °C by 0.08 percent with 0.04 percent of volume fraction. The rheological properties of nanocomposite MWCNT-Ag can be measured by covalent and non-covalent working methods [55]. A 20.2% increase in the thermal transfer coefficient relative to the base fluid has been discovered in a platform exchanger by the MWCNT-TiO

_{2}/water hybrid nanofluids [56]. The performance of the heat exchanger served by bringing together 0.0111% MWCNT/water nanofluid with 1.89% Al

_{2}O

_{3}/water. The appeal for graphene nanoplatelets (GNPs) has enormously increased despite the excellent use of MWCNTs for hybrid nanofluids [57]. Its diffusion in distilled water showed a 17.77% advancement in thermal conductivity at a 0.1% weight concentration and 40 °C. Another study investigated the impacts of particle concentration (range, 0.0–2.3%) and temperature (range, 25–50 °C) on the thermal conductivity of f-MWCNTs-Fe

_{3}O

_{4}-EG hybrid nanofluid [58]. The effects of various flows and geometrical parameters of solar thermal collector depend on different nanoparticles, base fluids and the thermophysical properties of different nanoparticles. This study indicated that the hybrid nanofluids significantly enhanced the exergy efficiency. The assessment criteria of the examined cases are the thermal, energetic, and overall performance and background of solar collector.

## 3. Preparation of Hybrid Nanofluids

_{2}Cu, and Ag

_{2}Al nanoparticles synthesized by mechanically alloying the prepared nanofluids, the nanoparticles were identified by X-ray diffraction and transmission electron microscopy and the nanofluid thermal conductivity was found by employing a changed thermal comparator. The findings suggest an increase in the thermal conductivity advancement of existing nanofluids by 50–150%. Both experimental findings and empirical analysis suggest that the degree of change strongly depends on the dispersed nanoparticles’ identity/composition, scale, volume fraction, and shape [67]. The two-step method was used to generate a 0.1 percent volume fraction Al

_{2}O

_{3}-Cu/water hybrid nanofluid. As a surfactant, sodium lauryl sulfate (SLS) was used. Before that, over several steps, a thermochemical synthesis process that included spray drying, precursor powder oxidation, hydrogen-atmosphere reduction, and homogenization was used to prepare the nanocrystalline alumina–copper (Al

_{2}O

_{3}-Cu) hybrid powder [68]. The two-step technique was introduced to generate identical hybrid nanofluids as prepared by Suresh et al. Dry f-MWCNT and nanoparticle Fe

_{3}O

_{4}were prepared with a mixture of equivalent volumes. For the development of hybrid nanoparticles (f-MWCNT-Fe

_{3}O

_{4}) dispersed in ethylene glycol, a two-step method was employed [58].

_{3}C

_{2}chemical theorem was synthesized by applying the wet chemistry method and suspended in pure olein palm oil (OPO) to formulate a new type of heat-transfer fluid by applying COMSOL Metaphysics to investigate its thermal and energy efficiency numerically in a hybrid PV/T solar thermal structure. In addition to this research, the hybrid PV/T solar thermal device contrasts Al

_{2}O

_{3}–water-based nanofluid with MXene-OPO nanofluid. With a loading concentration of 0.01, 0.03, 0.05, 0.08, 0.1, and 0.2 percent, the MXene-OPO nanofluid was prepared. At a 0.2 percent loading concentration, the MXene-OPO nanofluid exhibits a 68.5 percent higher thermal conductivity than pure OPO at 25 °C. When the temperature increased from 25 °C to 50 °C for the nanofluid with 0.2 wt. percent of MXene, the maximum viscosity reduction was observed as 61 percent. The MXene-based nanofluid shows about a 16 percent higher thermal efficiency improvement at a 0.07 kg/s flow rate compared to PVT with Al

_{2}O

_{3}–water-based nanofluid. For the PVT with MXene nanofluids, a heat transfer coefficient improvement of approximately 9 percent was observed compared to PVT with Al

_{2}O

_{3}–water heat-transfer fluid. Compared to the stand-alone PV modules, the MXene nanofluid can reduce PV temperature by 40 percent [69].

## 4. Application of Hybrid Nanofluids in the Solar Collector

_{2}/deionized two-fold refined water hybrid nanofluid for ducts within the duct-sort counter stream heat exchanger. They detailed that the surface-functionalized and exceedingly crystalline nature of crossover nanocomposite (Cu–TiO

_{2}) contributed to the creation of successful warm interfacing with the liquid medium; thus, allowing for the accomplishment of an increased heat conductivity and heat-transfer potential for nanofluids [79].

_{2}O

_{3}, MWCNT, Ag, Fe

_{3}O

_{4}, MgO, SiO

_{2}, ZnO, TiO

_{2}, Cu, CNT, graphene, silica, and water is the most used base fluid. Ethylene glycol was also utilized several times as a base fluid in these research studies. Table 3 stated that research has been conducted in various areas such as in a circular tube, warm channel, electronic-warm sink, thermal solar collector, etc. Moreover, the thermo-physical properties such as optical and rheological properties of hybrid nanofluids are still being studied.

## 5. Efficiency Observations of Solar Collectors with Hybrid Nanofluids

_{2}O

_{3}/Fe, Al

_{2}O

_{3}-water, with the mass volume of 0.05–0.2 wt., the volume % increases the efficiency of thermal heat transfer by 6.9%, as found by Harandi, Karimipour [58]. The hybrid nanofluids were developed by dispersing a synthetic ND-CO

_{3}O

_{4}nanocomposite into water, ethylene glycol, and water mixtures to confirm the ND and CO

_{3}O

_{4}phases of synthesized nanocomposites. The thermal properties including thermal conductivity and viscosity were experimentally tested at various weight and temperature concentrations and the ND–CO

_{3}O

_{4}-water maximum efficiency increased to 59% if 0.15 wt. as found by Sundar, Misganaw [105]. The efficiency increased to 89% for the water-based MWCNTs/GNPs/h-BN flat plate solar collector, whereas the mass volume concentration was 0.05 to 0.1 for the weight of water, as reported by Hussein, Habib [37]. For the water-based MWCNTs/MgO, MWCNTs/CuO flat-plate solar collector, the mass volume concentration was 0.25 to 0.2% wt., and the performance increase of CuO-MWCNT was 18.05%, while for MgO-MWCNT it was 20.52% [117]. An efficiency increase of 15.13% was observed for the water/EG-based Al

_{2}O

_{3}, ZnO flat-plate solar collector, whereas the mass volume concentration was 0.25, as reported by Arıkan, Abbasoğlu [116]. Recent studies have investigated this kind of solar collector. The use of hybrid nanofluids is studied in the planned method, and some of the problems in some of the ETSCs with increased heat transfer are evaluated through the general analysis, such as different types of nano-fluids, the nano-fluid scale, volume-fraction, and hybrid nano-fluid application. The efficiency of ETSCs was affected by nanoparticles, using a base fluid [117,118,119,120,121,122,123,124]. In some studies, the enhanced performance was attributed to a higher Nusselt number. The Nusselt number can be improved with the use of hybrid nanofluids to make convective heat transfer more efficient [125,126].

## 6. Mathematical Analysis of Hybrid Nanofluids in Solar Collectors

_{ab}, they are ingested by the heat-exchange medium and transferred into the heat. The valuable vitality selected by the collector, Q

_{u}, is the sum of warmth that the working liquid collects, subtracted by the sum of the heat exchange from the collector to the discussion as the misplaced vitality [151,152,153].

_{R}is calculated as

_{ab}and T

_{a}are the surface temperature of the safeguard and discuss temperature individually, F

^{′}indicates the collector proficiency figure which drops with a rise in the general misfortune coefficient, U

_{L}, from the accepting plate to the environment, which was firstly presented by Hottel and Woertz [154] that was afterward created by Klein [155].

_{c}is the vitality achieved from the collector, m is the mass stream rate, C

_{p}is the heat, Ac is the collector range, T

_{o}and T

_{i}are the outlet and gulf temperatures of the liquid separately. In TSCs, the safeguard range is an imperative parameter characterized as the plate region short the punctured range and demonstrates the sum of the retained vitality (Q

_{ab}= GAcαc). Radiative and convective heat traded from the surface to the encompassing and the back divider are the major components for warm misfortunes [157,158]. For this sort, heat productivity is portrayed as a division of the overall sun-powered energy that comes to the collector’s surface and is accomplished by the discussion as the valuable heat which can be calculated as demonstrated by Leon and Kumar [159].

_{2}Cu hybrid nanofluids was 150 percent. Nanoparticle weight and volume % is the key to the hybrid nanofluid performance of enhancing heat efficiency. can The mathematical correlations related to the design of the solar collector, numerical simulations, efficiency enhancement of solar collectors with different variables such as volume concentrations and viscosity are presented in Table 4.

## 7. Challenges Found Based on the Study

#### 7.1. Physical Characteristics

_{2}O

_{3}nanoparticles at the bottom of the test tube, as portrayed in Figure 5. This discovery reveals the moderate to good stability of both nanofluids, whereas quantitative approaches have been used to study the numerical values of stability as shown in Figure 6 [42].

#### 7.2. Design and Mathematical Relationship

#### 7.3. Cost and Economic Perspective

## 8. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 3.**Portrayal of two-step method of nanofluid preparation [66].

**Figure 4.**Enhancement of thermal conductivity using hybrid nanofluids; (

**a**–

**c**) mirror an accelerated thermal conductivity as a feature of the quantity fraction of the thermal conductivity improvement obtained via researchers (

**d**) as a characteristic of the obtained weight fraction [167].

**Figure 5.**Evaluation of qualitative stability measurement of nanofluids [42].

**Figure 6.**Quantitative analysis of the stability of nanofluids [42].

Reference | Types | Schematic Image | Applications |
---|---|---|---|

Tang, Cheng [86] | Flat-plate solar collector (FPSC) | Solar collector of this sort is abruptly utilized in residential hot water. Additionally, in manufacturing air deicer. 20–80 °C is the working temperature. Thus, it acts as foremost common sort of collector in different kinds of sun-oriented collector frameworks. Provides higher productivity and outlet temperatures when there is less warmth through the cover of glass in collector and the requirement of sunlight. Customary sorts are for the most part planned for warm climates. Efficiencies for 500 and 1000 W/m^{2} are 0.71–0.75 and 0.72–0.75 separately. | |

Arunkumar, Velraj [87] | Compound parabolic collector (CPC) | In terms of flow, these types are rather proficient in collection and concentration of far-off light sources, with a few acceptance points. Basic components in sun-oriented vitality collection, remote contact, sun-focused drying, water purification, biomedical, or any device involving condensation of a disparate source of light. It covers a temperature of 60–240 °C. 500 and 1000 W/m^{2}, with different efficiencies 0.45–0.73 and 0.58–0.72. | |

Papadimitratos, Sobhansarbandi [88] | Evacuated tube solar collector (ETSC) | These types are rather communal in residential hot water. Competent as air deicer. The working temperature of ETSC is 50–200 °C. They are more prudent than routine. In the cold weather, they provide more than FPC. Efficiencies of 500 W/m^{2} and 1000 W/m^{2} are between 0.44–0.82 and 0.62–0.82. | |

Li, Xu [89] | Parabolic trough collector (PTC) | Presently, as the most commercially used and most progressed, these types are rather proficient in control plants. Their capacity leads to the utilization of hybridization and vitality capacity by warm-vitality capacity. The preferences of PTC innovation incorporate the guarantee of the temperate venture, progressed innovation, ample operational involvement, and fossil-filling office and green energy sources. These have a temperature range of 400–500 °C. | |

Beltagy, Semmar [90] | Linear fresnel collector | These types play crucial roles in controlling plants. Currently, in the coordinated steam era, there is a predominance of utilizing this innovation compared to other sun-based frameworks, thereby reducing the cost of heat shift. The frame can be concentrated to provide surcharged steam. Significantly lower than the illustrative trough concentrators of 30–100, the defensive concentration variables are 10–40. Temperatures range between 100–450 °C. | |

Li, Dubowsky [91] | Parabolic dish reflector | This is simply defined as an electrical generator that generates daylight rather than coal or unrefined oil to create power. It was prepared with a dynamic following framework that can indicate the sun reliably. Temperatures can reach as high as 1500 °C. | |

Roca, de la Calle [92] | Heliostat field collector | Overheating during operation; costly choice for a broad range of operating applications including the development of solar energy, solar power, solar assist, carbon capture, water, and home applications. Temperature range of 12–85 °C. |

Author | Nanoparticles | Base Fluids | Research Study |
---|---|---|---|

Ho, Huang [80] | Al_{2}O_{3}, MEPCM | Water | Crossbreed water, primarily based nanoparticle laminar in a round deportation |

Han and Rhi [77] | Ag, Al_{2}O_{3} | Water | Considered warm characteristics for hybrid nanofluids on a notched warm channel. |

Baby and Sundara [54] | Ag, HEG | HEG–Deionized water and Ethylene glycol (EG) | Improvement of heat physical phenomenon and warmth transfer for the arranged hybrid nanoparticle. |

Esfe, Yan [93] | Ag, Al_{2}O_{3} | Water | Arrangement and characterization considered. |

Selvakumar, Suresh [79], Suresh, Venkitaraj [94] | Cu, Al_{2}O_{3} | Water | Exploratory considerations of convective warm exchange and weight drop for crossbreed nanofluids in an electronic warm sink. |

Baghbanzadeh, Rashidi [95] | Silica, MWCNT | Distilled water | Heat transfer and weight drop for hybrid nanofluids in the associated electronic heat sink. |

Chen, Yu [96] | Ag, MWCNT | Water | Considered the upgrade of compelling thermal conductivity. |

Chen, Yu [96] | Graphene, MWCNT | Deionized water and Ethylene glycol (EG) | Upgrade of warm properties for hybrid nanofluids. |

Jyothirmayee Aravind and Ramaprabhu [97] | Al_{2}O_{3}, MWCNT | water | Improvement of warm conductivity for single and half-breed nanofluids. |

Munkhbayar, Tanshen [98] | Ag, MWCNT | Water | Examined the warm characteristics for the prepared cross breed nanofluids. |

Labib, Nine [99] | CNT, Al_{2}O_{3} | Water | Analytical examination along with the impact of associate fluids and cross-breed nanofluid in constrained convective heat exchange. |

Tomar and Chakrabarty [100] | TiO_{2}, ZrO_{2} | - | Considered the auxiliary and optical properties for the arranged nanocomposite. |

Suresh, Venkitaraj [101] | Cu, Al_{2}O_{3} | Distilled water | Turbulent warm exchange and weight sip for hybrid nanofluids in a consistently warmed round tube. |

Madhesh, Parameshwaran [81] | Cu, TiO_{2} | Water | Test considers convective heat transfer and natural philosophy characteristics of hybrid nanofluids in the tube heat exchanger. |

Batmunkh, Tanshen [102] | MWCNT, Fe_{2}O_{3} | Water | Tests consider heat-convective transfer and touch calculates nanofluids in a continuously warmed circular tube for a fully formed, turbulent stream on a crossover. |

Xuan, Duan [103] | TiO_{2}, Ag | Water | Upgrade in sun-based assimilation. |

Takabi and Salehi [73] | Cu, Al_{2}O_{3} | Water | Considered the enlargement of the warm transfer performance of a sinusoidal corrugated enclosure by utilizing crossover nanofluid. |

Baghbanzadeh, Rashidi [104] | Silica, MWCNT | Water | Considered the examination of an upgrade of rheological properties (thickness and density) for crossover nanofluids. |

Sundar, Misganaw [105] | ND, NI | Water and EG | Examined the upgrade of thermal conductivity and thickness for the hybrid nanofluid with distinctive base liquids. |

Syam Sundar, Sousa [106] | CNT, Fe_{3}O_{4} | Water | Examined the warm exchange upgrade in low-quality awareness for the arranged hybrid nanofluids in a tube with bent tape inserts beneath turbulent steam. |

Esfe, Wongwises [107] | Cu, TiO_{2} | Water | Test examination of warm conductivity for the arranged crossover Nanofluids and created Artificial Neural Network (ANN) simulation and correlation for heat conductivity. |

Esfe, Yan [93] | DWCNT, ZnO | Water | The heat conductivity improvement for the organized nanofluids examined for different temperatures (25 °C to 50 °C) and strong volume division of (0.25% to 1%). |

Esfe, Arani [108] | Ag, MgO | Water | Exploratory investigation on warm conductivity and energetic consistency for the arranged crossover Nanofluids with different volume divisions run from (0% to 2%) and created a relationship for warm conductivity and energetic thickness for the arranged cross breed nanofluids. |

Afrand, Toghraie [109] | Fe_{3}O_{4}, Ag | EG | In particular, the effect on the rheological activity of the arranged blended nano-fluid is checked for temperature and nanoparticulate concentration. |

Eshgarf, Afrand [110] | MWCNT, SiO_{2} | EG-water | Experimental change of the temperature range (25 °C to 50 °C) from different suspensions to strong volume distribution and of the rheological behavior of non-Newtonian hybrid nano-coolants in heating and cooling frame applications from (0.0625% to 2%). |

Harandi, Karimipour [58] | f-MWCNT, Fe_{3}O_{4} | EG | The test considers the influence of temperature and concentration on the thermal conductivity of the arranged cross nanofluid from 25 °C to 50 °C, to test different tests of nanofluids with a volume fraction from 0.1% to 2.3% and unused produce. The relationship of the thermal conductivity of the fluid is considered for testing. |

Sundar, Ramana [111] | ND, Fe_{3}O_{4} | Water | Considered the improvement of warm conductivity, thickness for the arranged half-breed nanofluid by shifting the temperature ranges (20 °C to 60 °C) and the volume concentration (0.05 to 0.2%). Additionally, an unused relationship was established for the thermal conductivity and consistency of the semi-aligned nanofluid with exploratory information. |

Soltani, Akbari [112] | MgO, MWCNT | EG | Exploratory consideration of energetic thickness for their arranged half-breed nanofluid with different volume concentrations (0.1% to 1%) by shifting the temperature (30 °C to 60 °C) and created an unused relationship for the energetic consistency from their experimental work. |

Senniangiri, Bensam Raj [113,114] | Graphene/NiO | Coconut oil | The high nanomaterial concentration regenerates the formation of lamellar agglomerated particles and increases the complex viscosity of the basic fluid. To estimate the dynamism of the hybrid nanofluid with a limited deviation margin, it is suggested to use the theoretical correlation artificially (ANN). |

Hussein, Habib [37] | Covalent functionalized graphene nanoplatelets | water | Found that when the mixed hybrid nanofluid was used as the absorption medium and the flow rate was 4 L/min, the solar collector with the highest thermal efficiency increased by as much as 85%. |

Author | Base Fluid | Nanoparticles | Mass Volume % | Solar Collectors | Efficiency Observation |
---|---|---|---|---|---|

Harandi, Karimipour [58] | H_{2}O | Al_{2}O_{3}/Fe, Al_{2}O_{3} | 0.05–0.2 wt. | FPSC | Maximum 6.9% increase |

Sundar, Misganaw [105] | H_{2}O | ND–CO_{3}O_{4} | 0.05–0.15 wt. | FPSC | Maximum 59% increase if 0.15 wt. |

Hussein, Habib [37] | H_{2}O | MWCNTs/GNPs/h-BN | 0.05–0.1 wt. | FPSC | Maximum 89% increase |

[115] | H_{2}O | MWCNTs/MgO, MWCNTs/CuO | 0.25–2 vol. | FPSC | Performance of CuO-MWCNT was 18.05%, while MgO-MWCNT was 20.52%. |

Arıkan, Abbasoğlu [116] | H_{2}O/EG | Al_{2}O_{3}, ZnO | 0.25 vol. | FPSC | Performance was 15.13% positive |

[117] | H_{2}O | SWCNT | 0.2 vol. | ETSC | Optium productivity at 93.43% |

[118] | H_{2}O | Al_{2}O_{3}, TiO_{2} | 0.3 wt. | ETSC | Compared to its based liquid, the system’s performance improved by 16.67% |

Daghigh and Zandi [119] | H_{2}O | MWCNT, CuO and TiO_{2} | Different | ETSC | Performance of the collector using nanoparticles MWCNT, CuO, and TiO_{2}, compared to water, increased by 25%, 12%, and 5%, respectively. |

Peng, Zahedidastjerdi [120] | Water | Al_{2}O_{3}, CuO, TiO_{2} | Different | ETSC | CuO has 1.5% higher collector thermal efficiency than Al_{2}0_{3}, TiO_{2}-water fluid |

Luo, Wang [121] | Oil | C, Ag, SiO_{2}, Al_{2}O_{3}, Cu | 0.01–0.025 wt. | DAC | Efficiency improves by 30–100 K and by 2–25% than the base oil |

Hussain, Jawad [122] | H_{2}O | Ag and ZrO_{2} | 5 vol. | ETSC | Efficiency % not mentioned but improved. |

Kim, Ham [123] | 20% propylene glycol-water | MWCNT, Al_{2}O_{3}, CuO, SiO_{2}, and TiO_{2} | 0.2 vol. | ETSC | Performance 20% increase |

Kaya, Gürel [124,125] | Methanol | CuO | 0.3 vol. | Tube | Performance 63% increase |

Gorji and Ranjbar [126,127] | water | Graphite, Magnetite—15 nm, Silver—20 nm | 5–40 ppm | DAC | According to the results, nanofluids promoted thermal and exergy efficiencies by 33–57% and 13–20%, respectively, compared to base fluid. |

Li, Chang [128] | Di-water | Ti_{3}AlC_{2}, hydrochloric acid, triton X—100 | 100 ppm | DAC | For MXene loading, the maximum photothermal conversion efficiency of 77.49% is achieved. |

Samylingam, Aslfattahi [69] | Di-water | Ti_{3}AlC_{2}, plum oil—MXene-OPO | 0.2 wt. | DAC | A 40% efficiency increase with respect to Al_{2}O_{3}-water-based nanofluid. |

Gupta, Singh [129] | Water | ZnFe_{2}O_{4} | 0.02–0.5 wt. | DAC | Performance enhancement of 42.99% |

Abdelrazik, Tan [130] | Di-water | rGO-Ag, graphene oxide | 0.0005 to 0.05 wt. | DAC | Hybrid system displays improved efficiency at concentrations of less than 0.0235 wt. percent compared to the PV system without integration with optical filtration. The hybrid solar PV/T system with OF using water/rGO-Ag nanofluid can produce thermal energy with efficiencies between 24 percent and 30 percent. |

Kasaeian, Daneshazarian [131] | EG | Nano silica | 0.3 wt. | PTC | Maximum outlet temperature of MWCNT is 338.3 K, and the thermal performance reaches 74.9%. |

Loni, Pavlovic [132] | Water | TiO_{2}, SiO_{2}, Fe_{2}O_{3}, ZnO, Al_{2}O_{3}. | N/A | PTC | Use pure water to enhance the energy performance of low enthalpy parabolic trough collectors. |

Esfe, Alirezaie [133] | EG | SWCNT-MgO | 0.05–2 vol. | PTC | Thermal conductivity enhancement of 18%. |

Bahrami, Akbari [24] | EG-water | Fe-CuO | 0.05–1.5 wt. | PTC | Efficiency increases in the different conditions in different types. |

[134] | Engine oil | MWCNT-ZnO | 0.125–1.0 wt. | PTC | If the viscosity increases then the efficiency increases. |

Afrand [135] | EG | MgO-MWCNT | 0.6 vol. | PTC | Performance increase—21% |

Sundar, Singh [136] | EG-water | graphene oxide/CO_{3}O_{4} | 0.2 vol. | PTC | Performance increase—water based—19.14% Performance increase—EG based 11.75% |

Nine, Batmunkh [137] | Water | Al_{2}O_{3}-MWCNT | 1–6 wt. | PTC | Increasing thermal conductivity is not sharp when compared to simple nanofluids |

Baby and Sundara [54] | Water and EG | CuO-HEG | 0.05 vol. | PTC | Increasing thermal conductivity with volume fraction |

Khan, Abid [138] | Oil-based | Al_{2}O_{3}, CuO and TiO_{2} | 1 wt. | Solar dish collector | Performance increased by 33.73% and 36.27% |

Loni, Pavlovic [132] | Thermal oil | Cu, CuO, TiO_{2}, and Al_{2}O_{3} | 0–5 wt. | Solar dish collector | Thermal efficiency is found to be equivalent to 35% and up to 10% of the exergy efficiency. |

Zadeh, Sokhansefat [139] | Synthesis oil/thermal oil | Al_{2}O_{3} | N/A | Tube | Improve the mean efficiency by 4.25%. |

Huang and Marefati [140] | Thermal oil and water | CuO and Al_{2}O_{3} | N/A | Solar dish collector | Efficiency increase—28.7% |

Loni, Asli-Ardeh [141] | Thermal oil | Al_{2}O_{3}/thermal, SiO_{2}/thermal | N/A | Solar dish collector | Improve efficiency |

Potenza, Milanese [142] | Airflow | CuO, nanopowder | N/A | Transparent receiver tube | Mean efficiency of about 65% |

Aslfattahi, Samylingam [143] | Silicon oil | MXene with a chemical formula of Ti_{3}C_{2} | 0.1 wt. | Photovoltaic thermal collector | Thermal conductivity improvement of 64%. |

Soltani, Kasaeian [144] | Water | SiO_{2}, Fe_{3}O_{4} | N/A | Photovoltaic thermal-thermoelectric system | Maximum energy efficiency at the fixed irradiation of 900 W/m^{2}. |

Sardarabadi, Passandideh-Fard [145] | Water | SiO_{2} | 1–3 wt. | Photovoltaic thermal-thermoelectric system | Total exergy of the PV/T system with nanofluids was increased by up to 24.31%. |

Arora, Singh [146] | Water | SWCNT, MWCNT NP | Different | Photovoltaic thermal-thermoelectric system | Percentage enhancement in total yield obtained using SWCNT and MWCNT was 65.7% and 28.1%, respectively. |

Wahab, Khan [147] | Water | Graphene hybrid | 0.05–0.15 vol. | Hybrid photovoltaic thermal system. | Maximum sustainability index of 1.17 is shown at optimum conditions. |

Soltani, Kasaeian [148] | Water | SiO_{2}, Fe_{3}O_{4} | Mass ratio 0.5 vol. | Photovoltaic thermal collector | Improvement of 54.29% and 1.72% in both power production and efficiency. |

Sardarabadi, Hosseinzadeh [149] | Water | Al_{2}O_{3}, TiO_{2}, ZnO | 0.2 wt. | Photovoltaic thermal collector | Results indicate that the overall exergy efficiencies for the cases of PVT/water, PVT/TiO_{2}, PVT/Al_{2}O_{3}, and PVT/ZnO are enhanced by 12.34%, 15.93%, 18.27%, and 15.45%, respectively |

Sardarabadi, Passandideh-Fard [150] | Water | TiO_{2}, ZnO, Al_{2}O_{3} | 0.2 | Photovoltaic thermal collector | Performance of ZnO is better than for the other types. The numerical model shows that the mass fraction of hybrid nanofluid has a significant impact on the thermal performance of PVT collectors. |

References | Specification | Correlation | Remarks |
---|---|---|---|

Esfe, Behbahani [161] | Functioning fluid: SiO_{2}-MWCNT/EGTemperature field: 30–50 °C Volume area: 0.05–1.95 vol. % | $\frac{{A}_{nf}}{{K}_{bf}}\phantom{\rule{0ex}{0ex}}=0.905+0.002069\phi T\phantom{\rule{0ex}{0ex}}+0.04375{\phi}^{0.09265}{T}^{0.3305}\phantom{\rule{0ex}{0ex}}-0.0063{\phi}^{3}$ | Two design methods and a feed-forward neural network have been provided to model the thermal conductivity of the hybrid nanofluid. R^{2} values of 0.9864 and 0.9981 were obtained for new methods and the artificial neural network (ANN). When these two measurement methods were compared to experimental data, both methods proved to be effective in predicting data. However, ANN’s correlation findings have a much lower error. |

Afrand [135] | Functioning fluid: MgO-MWCNT/EG Temperature field: 25–50 °C Volume area: 0–0.6 vol. % | $\frac{{A}_{nf}}{{K}_{bf}}=0.8341+1.1{\phi}^{0.243}{T}^{-0.289}$ | Maximum increase in nanofluid thermal conductivity is 21.3%. A new connection was proposed to estimate the nanofluid thermal conductivity. |

Sardarabadi, Passandideh-Fard [150] | Functioning fluid: f-MWCNTs-Fe_{3}O_{4}/EGTemperature field: 25–50 °C Volume range: 0–2.3 vol. % | $\frac{{A}_{nf}}{{K}_{bf}}=1+0.0162{\phi}^{0.7038}{T}^{0.6009}$ | Numerical simulation has been validated and used for the effects of mass ZnO-nanoparticles on TiO_{2}, ZnO, Al_{2}O_{3}/water nanofluids (0.2 wt.%). |

Esfahani, Toghraie [162] | Functioning fluid: ZnO-Ag/H_{2}OTemperature range: 25–50 °C Volume range: 0.125–2 vol. % | $\frac{{A}_{nf}}{{K}_{bf}}=1+0.00008794{\phi}^{0.5899}{T}^{1.345}$ | Effect on thermal conductivity of hybrid nanofluid of volume fractions and temperatures is demonstrated. |

Toghraie, Chaharsoghi [163] | Functioning fluid: ZnO-Ag/H_{2}OTemperature field: 25–50 °C Volume percentage: 0–3.5 vol. % | $\frac{{A}_{nf}}{{K}_{bf}}=1+0.004503{\phi}^{0.8717}{T}^{0.7972}$ | Increase in thermal conductivity variance of nano-fluids with a higher solid volume fraction temperature is also greater than that of a lower solid volume fraction. |

Alirezaie, Saedodin [164] | Functioning fluid: f-MWCNT-MgO/engine oil Volume percentage: 0.0625–1 vol. % Heat range: 25–50 °C Shear rate: 50–650 rpm | ${\mu}_{nf}=4\times {10}^{4}+145\phi -240T-\phantom{\rule{0ex}{0ex}}0.061\gamma +1.9\times {10}^{6}\text{}{\phi}^{2}+0.36{T}^{2}$ | Experimental data were calculated with a three-variable correlation, with artificial neural networks modeling the experimental results. The comparison of experimental results with the simulations shows that neural-network modeling is highly accurate. |

Asadi, Asadi [134] | Functioning fluid: f-MWCNT-ZnO/engine oil Volume percentage: 0.125–1 vol. % Heat variable: 5–55 °C | ${\mu}_{nf}=796.8+76.26\phi +12.88T+\phantom{\rule{0ex}{0ex}}0.7695\phi T+\frac{-196.9T-16.53\phi T}{{T}^{0.8441}}$ | At a solid concentration of 2 percent and a temperature of 40 °C, a maximal increase in dynamic viscosity was achieved at 65% while a minimum increase in solid concentration was achieved at 0.25% and a temperature of 25 °C was achieved at 14.4%. |

Esfe, Arani [62] | Functioning fluid: MWCNT-ZnO/10W40 engine oil Volume percentage: 0.05–1 vol. % Temperature difference: 5–55 °C | $\frac{{\mu}_{nf}}{{\mu}_{bf}}=1.035+\frac{\phi {e}^{-1.023}\left(\frac{2.046\phi}{T}+0.4015\phi {}^{2}T\right)}{{T}^{0.8441}}$ | Thermal conductivity at some temperatures was 38% higher than that of ethylene glycol. A new correlation of volume concentration and temperature (R^{2} = 0.9925) is proposed to forecast experimental thermal conductivity. |

Moldoveanu, Ibanescu [165] | Functioning fluid: Al_{2}O_{3}-SiO_{2}/H_{2}O | $\mathrm{For}\text{}0.5\%\text{}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}+0.5\%\text{}{\mathrm{SiO}}_{2}:\text{}{\mu}_{nf}=\phantom{\rule{0ex}{0ex}}0.000005{T}^{2}-0.003T+0.5$ $\mathrm{For}\text{}0.5\%\text{}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}+1.5\%\text{}{\mathrm{SiO}}_{2}:\text{}{\mu}_{nf}=\phantom{\rule{0ex}{0ex}}0.00000{T}^{2}-0.004T+0.571$ | Temperature variation in viscosity for hybrid nanofluid, which underpins viscosity reduction as the temperature increase rises and the action of low hysteresis, has been studied experimentally, proposing two viscosity variation equations as the temperature increases. |

Motahari, Moghaddam [166,167] | Functioning fluid: MWCNT-SiO_{2}/20W50 oilVolume range: 0.05–1 vol. % Heat range: 40–100 °C | $\frac{{\mu}_{nf}}{{\mu}_{bf}}=0.09422-[{\left(\frac{T}{\phi}\right)}^{2}+\phantom{\rule{0ex}{0ex}}0.100556{T}^{0.8827}{\phi}^{0.3148}]\text{}\mathrm{exp}\phantom{\rule{0ex}{0ex}}\text{}(72474.75\mathrm{T}{\mathsf{\phi}}^{3.7951})$ | Increase in solid volume fraction and temperature-improved hybrid nano-lubricant viscosity. Nano viscosity was 171 percent higher than pure 20W50, at its maximum solid volume fraction and temperature. Current models are not capable of predicting the hybrid viscosity of nano-lubricants. A new correlation was thus suggested with an R-squared of 0.9943 with regard to solid volume fraction and temperature. |

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**MDPI and ACS Style**

Mahamude, A.S.F.; Kamarulzaman, M.K.; Harun, W.S.W.; Kadirgama, K.; Ramasamy, D.; Farhana, K.; Bakar, R.A.; Yusaf, T.; Subramanion, S.; Yousif, B.
A Comprehensive Review on Efficiency Enhancement of Solar Collectors Using Hybrid Nanofluids. *Energies* **2022**, *15*, 1391.
https://doi.org/10.3390/en15041391

**AMA Style**

Mahamude ASF, Kamarulzaman MK, Harun WSW, Kadirgama K, Ramasamy D, Farhana K, Bakar RA, Yusaf T, Subramanion S, Yousif B.
A Comprehensive Review on Efficiency Enhancement of Solar Collectors Using Hybrid Nanofluids. *Energies*. 2022; 15(4):1391.
https://doi.org/10.3390/en15041391

**Chicago/Turabian Style**

Mahamude, Abu Shadate Faisal, Muhamad Kamal Kamarulzaman, Wan Sharuzi Wan Harun, Kumaran Kadirgama, Devarajan Ramasamy, Kaniz Farhana, Rosli Abu Bakar, Talal Yusaf, Sivarao Subramanion, and Belal Yousif.
2022. "A Comprehensive Review on Efficiency Enhancement of Solar Collectors Using Hybrid Nanofluids" *Energies* 15, no. 4: 1391.
https://doi.org/10.3390/en15041391