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

Because of its potential to directly transform solar energy into heat and energy, without harmful environmental effects such as greenhouse gas emissions. Hybrid nanofluid is an efficient way to improve the thermal efficiency of solar systems using a possible heat transfer fluid with superior thermo-physical properties. The object of this paper is the study the latest developments in hybrid applications in the fields of solar energy systems in different solar collectors. Hybrid nanofluids are potential fluids with better thermo-physical properties and heat transfer efficiency than conventional heat transfer fluids (oil, water, ethylene glycol) with single nanoparticle nanofluids. The research found that a single nanofluid can be replaced by a hybrid nanofluid because it enhances heat transfer. This work presented the recent developments in hybrid nanofluid preparation methods, stability factors, thermal improvement methods, current applications, and some mathematical regression analysis which is directly related to the efficiency enhancement of solar collector. This literature revealed that hybrid nanofluids have a great opportunity to enhance the efficiency of solar collector due to their noble thermophysical properties in replace of conventional heat transfer working fluids. Finally, some important problems are addressed, which must be solved for future study.


Introduction
A safe and prosperous world now needs more than ever an environmentally friendly and effective source of energy [1][2][3]. In the face of the challenges of eliminating fossil fuels and reducing exhaust emissions from those fuels, one of the major divisions of renewable energy is solar energy. Although the option of energy sources has always been low-priced, solar energy has never really been used globally. More to the point, while the energy of the sun is free, it is expected that the maintenance charge of such systems, including the construction of devices and systems, would surpass the total cost of the usable source [4- The heat-transfer system plays a crucial role in many industries related to thermal and chemical processes. The heat-transfer process is always treated by using fluids. The main aim of the use of mixtures (hybrid nanofluid) is to improve the potency of the mixture's thermal properties, and the mixture of various nano-sized particles with fluid is called hybrid nanofluid. In practice, numerous methods have been applied to enhance the thermal properties of fluids. Nanoparticles are the most common and the most up-to-date technological trend in terms of improving heat-transfer efficiency [27]. Nanofluids have an excellent ability to increase the thermal efficiency of the solar collector by different nanoparticles used in different types of solar collectors [28]. Hybrid nanofluids were found to be the most efficient approach when used as a working fluid in solar energy systems [29]. The hybrid nanoparticles consist of a synthesis of two or more nanoparticles and are propagated into the base fluid. This is due to the synergistic effect of the hybrid nanofluid heat-transfer change compared to a nano-fluid containing a nano product. A hybrid nanofluid can have good thermal characteristics compared to the simple fluid and nanofluid-containing single nanoparticles [30]. The choice of the base fluid, size of nanoparticles, viscosity, fluid temperature and stability, dispensability of nanoparticles, purity of nanoparticles, method of preparation, size and shape of nanoparticles, and compatibility of nanoparticles have contributed significantly to the improvement of hybrid nanofluid heat transfer, resulting in the harmonious nanofluid mixture [31][32][33][34][35]. The primary goal of using hybrid nanofluids is to improve thermal conductivity at a lower cost and with acceptable stability due to the synergistic effect of its constituent materials.
Stability is a term used to describe the long-term performance and thermal efficiency of nanofluids [36,37]. In comparison to mono nanofluids, hybrid nanofluids can significantly improve heat transmission. The pressure drop caused by the friction factor escalation, on the other hand, remains a major concern in hybrid nanofluids. The increase in pressure drop is a direct result of the increase in hybrid viscosity, which results in the penalty of high pumping power [30].
Nanoparticles are subject to various parameters of their thermal conductivity, including concentration, temperature, particle size, pH, shape, material, and perhaps, the production process. Theoretical models for thermal conductivity and viscosity determination. The stability of nanofluids or hybrid nanofluids in terms of settlement and agglomeration is still troublesome for practical applications, particularly in higher concentrations. Hybrid nanofluids, therefore, exhibit greater thermal conductivity compared to the individual nanofluids, which separately contain nanoparticles. Therefore, this paper focuses on evaluating the research of hybrid nano-fluid to recognize and resolve the latest technology by way of a forefront study into the economic feasibility of this technology in the future. The reported theoretical, digital, and experiential work on single nanofluid has revealed that nanofluid cooling, cameras, microcomputers, displays, heat exchangers, and spacecraft applications can be implemented in various possible ways. Many thermal product reviews are found in the literature. However, single nanofluids are characterized and prepared. Few articles are available on hybrid nanofluid preparedness and thermal characteristics. The purpose of this paper is therefore to provide a further consideration of recent developments in various engineering applications in a hybrid nanofluid. Moreover, the critical challenges of hybrid nanofluids are presented, such as long-term stability, cost of preparation, and production. More experimental research is needed to address several issues related to hybrid nanofluids, such as instability and an increase in the friction factor, to reduce pumping power in solar systems. These issues appear to be critical for hybrid nanofluid commercialization and general applications. More importantly, the cost of preparing hybrid nanofluids is high and must be reduced. Future research should concentrate on finding a balance between the hybrid nanofluid's high thermal efficiency and the cost of preparation. This is a critical step toward the commercialization of hybrid nanofluids-based solar systems.

Historical Background
A key parameter of all the specifications which have contributed enormously to the improvement of heat transfer is thermal conductivity. Several studies have reported that the use of nanofluids has improved thermal conductivity undoubtedly [38][39][40][41][42]. Nanofluid hybrid is a brand-new type of nanofluid, which is massed by dispersing two distinct nanoparticles into an agreed heat-transference fluid. Hybrid nanofluids are possible fluids that have improved thermo-physical properties over traditional thermo-transfer fluids, an increased thermo-efficiency (oil, water, and ethylene glycol) and nanofluids with single nanoparticles. The scientific findings have shown that the hybrid nanofluid can be substituted with one single nanofluid as it enhances heat transfer, especially in the automotive, electro-mechanical, manufacturing, HVAC, and solar industries [43].
In solar collectors, a wide ranges of functioning fluids have been tested. Historically, water, grease, ethylene glycol, and various lubricants have been used to promote the performance of solar collectors, as shown in Figure 2 [44-49]. In anchor fluids (water, ethylene glycol or oil/lubricant), nanosized metals (Al, Cu, Zn, Ag, Au, etc.), metal oxides (SiO2, TiO2, Al2O3, 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 Al2O3-Cu nanofluid, for example, was developed using the hydrogen reduction method using Al2O3 and CuO (90:10 ratio) to improve the viscosity to be steeper than concentration conductivity [52,53]. The MWCNT-Fe3O4 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-TiO2/water hybrid nanofluids [56]. The performance of the heat exchanger served by bringing together 0.0111% MWCNT/water nanofluid with 1.89% Al2O3/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-Fe3O4-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.

Preparation of Hybrid Nanofluids
Hybrid nanofluids are new fluids that are generated in a mixture or composite form to increase the heat transmission by suspending two or more nanoparticles [59]. By using hybrid nanostructures consisting of multiple materials with nano dimensions, the thermophysical properties of nanofluids can be further enhanced [43,60]. Water is the basic fluid exposed to radiation. This hybrid mixture uses a larger wavelength combination and absorbs heat. For various concentrations, diameters, and container heights, graphite and the numeric value for the mixture are added into the water of gold, silver, aluminum, graphite, and silicon dioxide gold nanoparticles [61]. Quite a few studies have studied and modeled the thermophysical characteristics of these hybrid fluid forms [62]. It has been found that relative to the base fluid at a volumetric concentration and a temperature of 0.86%, the thermal conductivity ratio of the hybrid nanofluid increased to 20.1% [63]. The reviewed literature indicates that hybrid nanofluids are an attractive candidate for thermal convective fluids in solar systems. Furthermore, hybrid nanofluids have a variety of benefits, which make them more useful for the increase in heat transfer. A summary of solar power is given in this report. Subsequently, the use of hybrid nanofluids is reviewed and the findings are analyzed in various forms of solar-driven technologies [64]. The twostep process of nanofluid preparation includes the induction of the mechanical or chemical action of nanoparticles in powder form, followed mixing them with base fluid as shown in Figure 3. In the base fluid, powdered nanoparticles are dispersed by an intense shearing action known as ultrasonic. Both strength and ultrasonic length play a critical role in the stability of hybrid nanofluids [65]. By scatting around 0.2-1.5 vol.% of these nanoparticles in water and ethylene glycol, Al2Cu, and Ag2Al 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 Al2O3-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 (Al2O3-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 Fe3O4 were prepared with a mixture of equivalent volumes. For the development of hybrid nanoparticles (f-MWCNT-Fe3O4) dispersed in ethylene glycol, a two-step method was employed [58].
MXene with a Ti3C2 chemical theorem was synthesized by applying the wet chemistry method and suspended in pure olein palm oil (OPO) to formulate a new type of heattransfer 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 Al2O3-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 Al2O3-water-based nanofluid. For the PVT with MXene nanofluids, a heat transfer coefficient improvement of approximately 9 percent was observed compared to PVT with Al2O3-water heat-transfer fluid. Compared to the stand-alone PV modules, the MXene nanofluid can reduce PV temperature by 40 percent [69].

Application of Hybrid Nanofluids in the Solar Collector
The research related to the relevance of nanofluids is the talk of the hour. A researchfacility survey of single nano-fluid work covering a wide range of functions has been carried out, with regard to electronic cooling, heat exchange, heat capacity, sun-based building heating and cooling, sun-powered pickers, cooling, room, and security, etc., [70][71][72][73][74][75][76]. Among all the applications, the usage, and the implementation of hybrid nanofluids in solar collectors are breaking new ground. The crossbreed or nanocomposite fluid may be a unique type of nanofluid, and although its utility at the research-facility level has been largely detailed in literary works [77][78][79][80][81], there is also work detailing factors such as heat exchange, electronic cooling, essential limits, and so on. It is now pertinent to focus on the mechanical utilization of crossbreed nano liquids with regard to single nano liquids [81] utilized such as Cu-TiO2/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-TiO2) 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].
Concentrated solar panel (CSP) technology, as a typical PV application, is gaining popularity due to its benefits such as high conversion efficiency and low cost, among others. However, an important issue for CPV technology is non-uniformity in illumination and temperature, which can ultimately affect the overall electrical efficiency of solar cells [82]. Heat transfer applications have been analyzed by fluidic behavior, thermal properties, the size of nanoparticles and the mathematical co-relationships [83]. At higher temperatures, the increase in thermal conductivity with an increasing solid volume fraction is more pronounced. The effect of increasing the volume fraction on thermal conductivity, however, was greater than the effect of increasing temperature. Thermal conductivity was increased by 27.84% compared to the base fluid at a volume fraction of 0.5% and a temperature of 75 °C. Besides, Aguilar, Navas [84] studied the thermal properties of NiObased nanofluids for CSP applications experimentally, and dynamic structures have also been studied as well. They inferred that thermal conductivity increased by up to 96% and that the heat transfer coefficient was enhanced by 50% . They also found that the surfactant has a significant effect on the improvement of thermal properties in CSP. Finally, a model for predicting the thermal conductivity of nanofluids based on the measured data was proposed. This model has a margin of error of 1.44 percent, indicating that the results obtained from model calculations are compatible with the experimental data [85]. The main criteria of the solar collector are to collect heat or increase the efficiency by using different nano or hybrid nanofluids of different conditions. Table 1 illustrates the application characteristics of different solar collectors using hybrid nanofluids. For instance, FPC is used for warm-water in-home applications. Using hybrid nanofluids in these kinds of solar collectors enhances the productivity and outlet temperature, as well as the efficiency, which is also improved significantly. Additionally, ETSC is used to heat the water for residential purposes. ETSC is much better than FPC in cool weather. Generally, CPC is utilized for sun-oriented drying, water cleaning, and biomedical conditions, during which temperature performance is also increased by applying hybrid nanofluids. Besides, PTC is the most developed and commercially used solar collector. PTC is preferred largely due to some specific characteristics such as its hightemperature range. On the other hand, the linear frenal, parabolic-dish reflector, and heliostat-field collector are used to produce electricity by harvesting solar energy. These solar collectors perform at quite high temperatures. Table 2 describes the research area and possible outcomes using hybrid nanofluids. Besides, Table 2 also presents the used nanoparticles and base fluids for these research studies. The repeatedly used nanoparticles are Al2O3, MWCNT, Ag, Fe3O4, MgO, SiO2, ZnO, TiO2, 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. Table 1. Application of hybrid nanofluids in various solar collectors.

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 faroff 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. 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] Fe3O4, 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, SiO2 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, Fe3O4 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, Fe3O4 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%.

Efficiency Observations of Solar Collectors with Hybrid Nanofluids
Hybrid nanofluid and thermal systems play a vital role in heat transfer and the efficiency enhancement of solar collector. Efficiency enhancement is also directly related to nanoparticle size and the mass flow system of fluid, concentration or solid volume fraction of nanoparticles may have a significant effect on the thermal conductivity of hybrid nanofluids. [116]. When a hybrid nanofluid was prepared using Al2O3/Fe, Al2O3-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-CO3O4 nanocomposite into water, ethylene glycol, and water mixtures to confirm the ND and CO3O4 phases of synthesized nanocomposites. The thermal properties including thermal conductivity and viscosity were experimentally tested at various weight and temperature concentrations and the ND-CO3O4water 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 Al2O3, 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].

Mathematical Analysis of Hybrid Nanofluids in Solar Collectors
When sunbeams, G, hit the darkened absorber's surface zone, Aab, they are ingested by the heat-exchange medium and transferred into the heat. The valuable vitality selected by the collector, Qu, 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].
where FR is calculated as and Tab and Ta 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, UL, from the accepting plate to the environment, which was firstly presented by Hottel and Woertz [154] that was afterward created by Klein [155].
Hence, the valuable vitality extricated from the collector can be decided by the sum of the sun-oriented occurrence where the warm productivity of an FPC can be assessed as provided by Fudholi, Sopian [156].
where η (%) is the collector effectiveness, Qc is the vitality achieved from the collector, m˙ is the mass stream rate, Cp is the heat, Ac is the collector range, To and Ti 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 (Qab = 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].
A further calculation of the warm trade adequacy (HEE) proportion (εHX) is additionally taken under consideration to assess the contrast between the real temperature and the greatest conceivable esteem given by Kutscher [160].
Solar collectors are one of the cleanest and most efficient heating systems available. Density, absorbency, temperature, the heat-transmission system, dynamic viscosity and types of nanoparticles are important for efficiency. Table 4 just illustrates the different parameter, which are directly related to efficiency. Normally, temperature and volume are the key parameters of the solar collector co-relation. Besides, if we see an example analysis of hybrid nanofluids as shown in Figure 4, we can see that depending on nanoparticle concentration, it depicts the thermal conductivity of hybrid nanofluids. Remarkable researchers preserved tiny quantities of nanoparticles to prevent particle sedimentation and agglomeration (usually less than 1%). At 1.5% of the volume, the maximal increase in thermal conductivity for Al2Cu 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. 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 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.
Asadi, Asadi [134] Functioning 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.
Motahari, Moghaddam [166,167] Functioning Increase in solid volume fraction and temperature-improved hybrid nanolubricant 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.

Challenges Found Based on the Study
Hybrid nanofluids are newly emerging dynamic liquids, which remain in the study stage. Researchers continue, however, to conduct a feasibility analysis on hybrid nanofluids and solar collectors for better performance across different applications. Some interesting characteristics of hybrid nanofluids were noticed in the performance of heat transfer but there are some challenges with regard to them being a new type of working fluid. First, there is a lack of consensus between researchers and the theoretical model to predict the exact behavior of hybrid nanofluids. Second, there is a lack of understanding between the researchers. Third, in the preparation process, the findings for a particular hybrid nanofluid and volume fraction vary in different methods. The challenges include the design of the solar plate, mixing and making the concentration of the base fluid with different nanoparticles, whether it flows inside the tube or coil, the stability of the hybrid nanofluid, the behavior of surfactants usage, nanoparticles size and volume concentration, pumping power, pressure drop, and most importantly, the cost of hybrid nanofluids.

Physical Characteristics
Stability is one of the main factors in the success of nanofluids and can have a detrimental impact on the hybrid nanofluid, i.e., lack of good stability. The stability of certain nanofluids deteriorates over time, which was observed by previous researchers. Surfactants were used to minimize fluid surface tension and to promote the dispensability of particles in a fluid [53] or no active surface compound. Excess surfactant, however, affects nanofluid viscosity, thermal conductive properties, and stability [168]. Solar power systems are also based on the mass and size of nanoparticles [169]. More research on the effect of nanoparticles on solar energy has been reported for thermal systems [170]. Reference has been made to the appropriate size and quantity of nanoparticles required to optimize the maximum outlet temperature and achieve the desired thermal efficiency of the solar collector [146].
Nanofluids can be prepared by calculating the number of nanoparticles for the required volume concentrations using the following (Equation (7)) [171]: where is the volume concentration of nanofluids (%); is the mass; and stands for the density of nanoparticles. The subscripts p and bf stand for nanoparticles and base fluid, respectively.
A visual inspection of sedimentation in the GNP nanofluids was performed, and it was indicated that the GNP nanofluids were stable even after the heat-transfer run [172] examined the stability of the nanofluids by observing the sedimentation photographs of the nanofluids captured after 30 days of preparation [173]. A combined experimental and statistical method was used to investigate the effective thermal conductivity and relative viscosity of CNC/W-EG nanofluids. The authors used a sedimentation observation to assess the stability of nanofluids. The observation was carried out every day in this investigation. After one week of preparation of the nanofluids, there was no aggregation of CNC and AL2O3 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].

Design and Mathematical Relationship
The total efficiency of the solar collector is based on the design parameters. The evaluation of the efficiency of each upgrading technique centered on the characteristics concerned. Thus, with characteristics such as a higher concentration rate, the effect of sun trackers may be more positive [174]. Hybrid nanoparticles have volume functions in most of the correlations formed by experiments. The efficiency in the heat transfer was characterized by hybrid nanofluids. However, there exist some disagreements with regard to the development of hybrid nanofluids as a new working fluid replacing water or any common working fluid. Besides, the results of a similar hybrid nanofluid analysis appeared to have little contrast as regards to thermal improvement. In particular, the data gained by different researchers are not standardized. Furthermore, expected mathematical correlations are still limited for other applications because of their limitations. The experimental application of nanofluids is therefore limited. Mathematical models rely solely on experimental integrity analytical research [175].

Cost and Economic Perspective
An economic analysis is used to identify industrial effective nanofluids. It is especially critical, therefore, that the type and price of hybrid nanofluids are considered so that the best comprehensive thermal transfer efficiency can be achieved at a lower cost for further industrial applications [176]. The mixing of nanoparticles and preparation of hybrid nanofluids is a challenge in the cost-effective procedure [177]. More importantly, the cost of creating hybrid nanofluids is prohibitively expensive and must be reduced. Future research should focus on finding a balance between the high thermal efficiency of the hybrid nanofluid and the cost of preparation.

Conclusions
In this current paper, a comprehensive study has been performed on the performance of solar collectors with hybrid nanofluids. The literature reviewed that hybrid nanofluids had been applied in different engineering fields to enhance their performance in terms of the circular tube, heat channel, electronic-heat sink and thermal solar collector. Moreover, hybrid nanofluids were implemented in various kinds of solar collectors such as a flatplate collector, compound parabolic collector, evacuated tube solar collector, parabolic trough collector, linear Fresnel collector, parabolic-dish reflector, and heliostat field collector to evaluate the performance in replace of conventional fluids. The study mainly stated the performance of efficiency of solar collectors that has been increased significantly. A maximum increase of 89% efficiency was achieved for a flat-plate collector by MWCNTs/GNPs/h-BN-water hybrid nanofluids. The stable, higher thermal conductive, and lower viscous hybrids nanofluids are preferred so as to improve the performance of a solar collector. Although the hybrid can increase the efficiency of solar collectors to a greater extent, it is also associated with some obstacles. However, a hybrid nanofluid is an impressive fluid to replace conventional fluids and increase the efficiency of solar collectors.  72, 190-196. https://doi.org/10.1016/j.expthermflusci.2015.11.009.