Introduction

The worldwide need for energy goes up constantly. Increasing the energy demand has got up the consumption of fossil fuels. Due to the supply of energy resources, environmental considerations, the use of petrochemicals and the increasing price of fossil fuels, and technological progress, developing and developed countries have a particular view on renewable energy. Converting a natural phenomenon into a proper type of energy is defined as renewable energy technologies. One of the factors that can be stated for using renewable energy instead of fossil energy is the issue of reducing the emission of toxic gases and keeping the environment safe (Georgeson et al. 2017). In general, renewable energies such as solar, wind, and geothermal energy do not pollute the environment and are compatible with nature. Research related to renewable energy sources has gone up widely in recent years.

Sunlight is the source of most of the energy on earth. As a natural nuclear reactor, the sun releases photon energy, which travels a distance of 150 million kilometers from the sun to the earth in approximately 8.5 min. In the past, solar energy was the only energy used by humans (Anderson 1977) (Kreith and Kreider 1978). Exploiting energy from the sun delivers a desirable contamination-free solution for supplying heating systems; thus, solar energy can be expressed as the most prevalent source. It should also be noted that the sun’s energy can be used directly and indirectly to produce different types of energy, such as heat and electricity. The primary challenges of utilizing solar energy arise from its widespread availability and fluctuating nature. Solar energy is categorized into power plant and non-power plant indicators based on its specific application. Solar dryers, solar water heaters, and solar water desalination are among the most critical non-power plant applications (Alawaji 2001). Additionally, production of hot water stands out as a crucial application of solar energy due to its favorable economics. Rewarding the significance of solar energy, increasing the efficiency of this energy has obtained growing consideration. One of the most challenging issues in enhancing the thermal efficiency of solar collectors is trying to absorb as much solar radiation energy as possible. For this purpose, a solar energy tracker and a suitable designer are needed (Motahhir et al. 2019). Generally, solar collectors are one of the particular heat transfer devices that transfer the sun’s radiant energy to the internal energy of the carrier environment. The solar collector is responsible for absorbing the energy of the sun’s radiation and converting it into the required heat of the fluid. The two basic types of solar collectors are decentralized or fixed and centralized. In decentralized collectors, the process of absorbing solar energy is done by a single surface. While in the concentrated type, direct solar radiation is received by reflective concave surfaces and concentrated on a smaller surface. Recently, a lot of endeavors have been made by researchers to go up the collector’s efficiency.

Types of solar collectors

All solar collectors are categorized into three main groups according to the maximum temperature produced (fluid outlet temperature). This classification includes solar collectors with low temperatures (temperatures less than 100 ℃), medium temperatures (temperatures between 100 and 300 ℃), and high temperatures (temperatures more than 300 ℃) (Kalogirou 2004). The high-temperature collector is utilized for power plant applications; the medium-temperature collector is employed for food industries, hospitals, and office applications; and the low-temperature collector is implemented as a solar water heater.

Flat plate solar collectors (FPSCs)

Flat plate solar collectors are one of the most common and widely used solar collector models. They are commonly utilized in low-temperature heating applications owing to their simple design (it consists of a flat, rectangular box-like structure with a transparent cover, typically made of glass or plastic, that allows sunlight to pass through. Inside the collector, there is an absorber plate, usually made of metal, which is painted black to maximize its ability to absorb solar radiation), effortless installation, and lower cost than other models (Mustafa et al. 2021). For the reasons mentioned, according to reports spanning from 2011 to 2021, the utilization of flat plate solar collectors witnessed a 15% increase, constituting 35% of all solar collectors employed during that period. The importance of using this type of collector is determined when this number was reported to be nearly 72% for Europe (Weiss and Mauthner 2010).

Flat plate collectors are usually placed in a fixed position and do not need to follow the sun. Also, their placement direction is usually directly along the equator, towards the north in the southern hemisphere, and towards the south in the northern hemisphere. The collector curvature angle is equal to the latitude position with a deviation angle of more or less than 10 to 15°, which depends on its application (Kalogirou 2004). For instance, a picture of the flat plate collector is demonstrated in Fig. 1.

Fig. 1
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A view of the flat plate collector (Tang et al. 2010)

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Parabolic tube solar collectors (PTSCs)

A parabolic solar collector, also known as a parabolic trough collector, is a type of solar thermal technology used to harness solar energy for various applications. The parabolic collector is one of the most widely used types of collectors; installed collector areas (end of March 2023) are reported to be 670,000 m2 (Weiss and Mauthner 2010). The parabolic shape of the reflector allows the concentration of incoming sunlight into a receiver tube positioned along the focal line of the trough. As the sunlight is reflected off the parabolic surface, it converges towards the receiver tube, maximizing the amount of solar energy captured (Nazir et al. 2021). Synthetic oil was used as a heat transfer fluid in the first parabolic collector solar power plants (Pal and Kumar 2021). The most critical applications of the linear parabolic collector can be mentioned in heating and cooling loops, drying processes, power generation plants, and desalination (Nawsud et al. 2022). For instance, a picture of the parabolic solar collector is displayed in Fig. 2.

Fig.2
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A view of the parabolic solar collector (Jamal-Abad et al. 2017)

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Linear Fresnel solar collector (LFSCs)

A linear Fresnel solar collector is a type of solar thermal technology that utilizes a series of flat mirrors with a small width and a large length and with a fixed receiver to concentrate sunlight onto an absorber tube. It is named after the French physicist Augustin-Jean Fresnel, who invented the Fresnel lens. Linear Fresnel solar collectors use a curved mirror to concentrate sunlight onto a single focal line, Linear Fresnel collectors focus sunlight along multiple lines. This design allows for a wider absorption area and reduces the need for complex tracking systems. Linear Fresnel collectors are often used in large-scale solar thermal power plants. It can be said that the length of these collectors is more than 100 m. One advantage of linear Fresnel collectors is their modular design, allowing for easier installation and maintenance than other concentrated solar power technologies. The linear arrangement of mirrors simplifies their manufacturing and assembly. Also, the core disadvantage of this type of collector can be considered low optical performance (Rungasamy et al. 2021). It should be noted that the installed area of this type of collector reaches 24,000 m2 (Weiss and Mauthner 2010). For instance, a picture of the Fresnel solar collector is displayed in Fig. 3.

Fig. 3
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A view of the Fresnel solar collector (Beltagy et al. 2017)

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Evacuated tube solar collectors (ETSCs)

The supply of these collectors started in the late 1970s, about 70 years after the first use of flat plate collectors. There are various kinds of vacuum tube collectors, in which the absorbent surface is usually surrounded by a double-walled glass tube with a vacuum between the walls. The most significant properties of vacuum tube collectors are the low influence of the sun’s motion during 24 h on the heat flux received by the absorber and the fact that the working fluid inside the collector does not freeze due to cold. Evacuated tube solar collectors are the most appropriate technology solar for generating beneficial heat in both low and medium temperature levels (Kumar et al. 2021a). It can be noted that the installed area of this kind of collector reaches 91,000 m2 (Weiss and Mauthner 2010). For instance, a picture of the evacuated tube solar collectors is displayed in Fig. 4.

Fig. 4
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A view of the evacuated tube solar collectors (Papadimitratos et al. 2016)

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Parabolic dish collector (PDC)

The parabolic dish collector is a type of solar energy system that uses a parabolic-shaped dish to concentrate sunlight onto a receiver located at the focal point of the dish. The collector consists of a large parabolic dish made of reflective material, such as mirrors or shiny metal surfaces. The parabolic shape of the dish allows it to focus incoming sunlight onto a small area at the focal point. Parabolic dish collectors are known for their high concentration ratio, which means they can achieve extremely high temperatures and generate significant power output in a small area. They are particularly suitable for applications requiring high-temperature heat (e.g., solar hydrogen production) or when a concentrated beam of light is needed. (Cherif et al. 2019). For instance, a picture of the parabolic dish collectors is presented in Fig. 5. Also, Fig. 6 is given for easy access to the types of collectors examined in the study.

Fig. 5
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A view of the parabolic dish collectors (Lovegrove et al. 2011)

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Fig. 6
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Types of collectors examined in the study

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Methods of improving heat transfer in FPSCs

Nowadays, new ways are carried out for better heat exchange in variant thermal systems. In this regard, multiple approaches are implemented to increase heat exchange. Based on the literature (Bergles et al. 1983, Bergles et al. 1991), the ways of increasing heat transfer can be divided into three gangs: active, passive, and combined techniques.

Active methods

The existence of at least one external energy source is the difference between this method and the passive method. This can include surface vibration (Zhou et al. 2022), magnetic or electric field (Giwa et al. 2021, Hamida and Hatami 2021, Izadi et al. 2023a), jet impact (Baghel et al. 2021), suction (Mamori et al. 2021), injection (Jalali et al. 2019), and mechanical aids (Léal et al. 2013).

Passive methods

Passive techniques deal with changes created in the thermal systems to enhance the thermal efficiency of the systems while no longer requiring external energy sources (Rashidi et al. 2019, Alshuraiaan et al. 2023). Various techniques have been used, containing the utilization of porous materials (Izadi et al. 2019, Peng et al. 2021), microchannel heat sinks (Izadi et al. 2013a, Mehryan et al. 2020b, Lanjwani et al. 2021), inserts (such as twisted strips, coils, swirling flow devices, and turbulators) (Zaboli et al. 2019, Shehzad et al. 2021a, Ajarostaghi et al. 2022, Noorbakhsh et al. 2022, Izadi et al. 2023b), and rough surfaces. For example, wavy surfaces (He et al. 2022), elongated surfaces (such as fins) (Goel and Singh 2021, Shehzad et al. 2021b), depressions and ridges (Cao et al. 2021a), change material (Shehzad et al. 2021a, Izadi et al. 2022a, Xiong et al. 2022), nanofluids (Izadi et al. 2013b, Valipour et al. 2017, Mohammadpour et al. 2022), spiral tubes (Xu et al. 2022), and helical tubes (Rashidi et al. 2021). Some passive methods focused on improving the rate of heat exchange are further discussed in the following.

Twisted tapes

Twisted tapes are a type of heat transfer enhancement device used in various industrial applications, particularly in heat exchangers. These kinds of inserts are commonly metalliferous strips that are twisted in some specific shape to form an orderly pattern. The twisted tape, such as a tube or pipe, is typically inserted into the flow passage to enhance heat transfer between the fluid flowing inside the passage and the surrounding walls (Zheng et al. 2017, Gnanavel et al. 2020). Enhanced heat transfer, compact design, energy savings, and versatility can be mentioned among the advantages of twisted tapes. Conical tapes are an example of this method of heat transfer enhancement (Liu et al. 2018; Bahiraei and Gharagozloo 2020).

Baffles

Baffles are used as flow-directing panels for liquid or gas flow. By using the baffle, the dead areas are eliminated, and better mixing of the flow in the system is done, and as a result, the heat transfer is improved (Bahiraei et al. 2021, El-Said et al. 2021, Uosofvand and Abbasian Arani 2021). Enhanced heat transfer, flow control, residence time control, and vibration reduction can be mentioned among the advantages of twisted tapes.

Winglets and vortex generator

The “wing” portrays the situation when the wing’s dorsal edge is connected to the plate. If the wing’s arch is connected to the end, its name is “winglet.” A vortex generator (VG) is an aerodynamic machine, including a small vane generally connected to a lifting plate. In the attendance of winglets and vortex generators, the resulting rotational flow leads to the appropriate dispensation of temperature in both the longitudinal and radial directions (Zhai et al. 2019, Modi et al. 2020).

Wire coil

This sector focuses on using helical or spiral coil tubes to improve thermal efficiency in thermal systems. The use of spiral tubes increases the heat exchange area, resulting in better heat exchange (Alimoradi et al. 2017, Zheng et al. 2018, Saydam et al. 2019, Fadaei et al. 2023).

Extended surface (Fin)

A fin is a thin component or appendage attached to a larger body or structure plane that continued from an object to go up the heat exchange rate. A pin fin, a ring fin, and a straight fin with constant and variable areas can be mentioned as types of fins. (Borhani et al. 2019, Gong et al. 2021, Izadi et al. 2023c, Saedodin et al. 2023).

Nanofluids

With worldwide competition in the field of different industries and the importance of energy in the cost of production, these industries are intensely moving towards developing new and advanced fluids with high thermal indices. Nanotechnology is one of the factors of progress in various industries. Nanotechnology involves a series of activities at the nanometer scale. One of the fields of action of this new technology is the production of particles with nanometer dimensions (nanoparticles). Among the applications of nanoparticles, we can mention the increase in thermal and chemical resistance and improvement in the strength of the produced materials. The nanoparticles’ high surface-to-volume ratio is another one of the properties of this material (Izadi 2020). Conforming to this feature, strong catalysts can be made on the nanoscale. Nanofluids are manufactured of stable carbon suspensions with high thermal conductivity, based on metal, and non-metal, which are suspended in fluids called base fluids such as glycol, oil, acetone, water, and ethylene (Buongiorno 2006, Taylor et al. 2013, Izadi et al. 2018). Nanofluids, a cutting-edge category of fluids, have garnered significant attention in research circles. Increasing evidence suggests that nanofluids outperform traditional fluids in diverse heat transfer applications (Mehryan et al. 2020a). In 1995, Choi from the Energy Technology Department of the Argonne National Laboratory of the United States first proposed the issue of nanofluid as a new environment for heat exchange. Recently, many researchers checked the influence of nanofluid applications and the alteration of the thermophysical properties of these fluids on different devices. These properties consist of specific heat capacity, density, adhesion force, thermal conductivity coefficient, viscosity, etc. (Choi and Eastman 1995). Eminent characteristics and some problems with using nanofluids of nanofluids are shown in Figs. 7 and 8.

Fig. 7
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Eminent characteristics of nanofluids

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Fig. 8
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Problems of using nanofluids

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Models of thermo-physical properties for mono nanofluids and hybrid nanofluids

There are generally two single-phase and two-phase models in the modeling process of nanofluid flow in FPSCs. In the first one, the base fluid and nanoparticle are formed into a single-phase fluid, and thermos-physical properties varying with temperature have been presented according to the experimental outcomes of various research (Izadi and Assad 2021, Xiong et al. 2021a, Xiong et al. 2021b). The interaction between the two steps is considered in the two-phase model, where the nanoparticle is in the solid phase, and the base fluid is in the liquid phase. Among the two considered standards, the most common model for nanofluid flow modeling is the single-phase model, whose advantages are the simplicity of the equations and, consequently, the decline in cost and simulation time (Izadi et al. 2015a). Furthermore, the results presented in related works have shown that the single-phase model has acceptable precision. The main challenge in using single-phase models for nanofluid modeling is the use of appropriate temperature-dependent experimental correlations for different thermos-physical properties of nanofluids.

Assuming nanofluids to homogenized (single phase) mixtures, the following equations are often used to approximate the thermophysical properties (Izadi et al. 2015b, Hu et al. 2021, Sajjadi et al. 2021, Yang et al. 2021, Kazaz et al. 2022):

$${\rho }_{nf}=\phi {\rho }_{np}+(1-\phi ){\rho }_{bf}$$
(1)
$${k}_{nf}={k}_{bf}\left[\frac{{k}_{bf}+{k}_{np}+n{k}_{bf}+\phi ({k}_{np}-{k}_{bf})-n\phi ({k}_{bf}-{k}_{np})}{{k}_{bf}+{k}_{np}+n{k}_{bf}+\phi ({k}_{bf}-{k}_{np})}\right]$$
(2)
$$({C}_{p}{)}_{nf}=\frac{\phi (\rho {C}_{p}{)}_{np}+(1-\phi )(\rho {C}_{p}{)}_{bf}}{{\rho }_{nf}}$$
(3)
$${\mu }_{nf}=\frac{{\mu }_{bf}}{(1-\phi {)}^{2.5}}$$
(4)

If two or more nanoparticles are used in the nanofluid instead of one type of nanoparticle, the obtained nanofluid is a hybrid type. These models for hybrid nanoparticles take the following form (Li et al. 2020, Javadi et al. 2021, Asadi et al. 2022, Mousavi Ajarostaghi et al. 2022).

$${\rho }_{hnf}={\phi }_{np1}{\rho }_{np1}+{\phi }_{np2}{\rho }_{np2}+(1-{\phi }_{np1}-{\phi }_{np2}){\rho }_{bf}$$
(5)
$$({C}_{p}{)}_{hnf}=\frac{{\phi }_{np1}(\rho {C}_{p}{)}_{np1}+{\phi }_{np2}(\rho {C}_{p}{)}_{np2}+(1-{\phi }_{np1}-{\phi }_{np2})(\rho {C}_{p}{)}_{bf}}{{\rho }_{hnf}}$$
(6)
$${\mu }_{hnf}=\frac{{\mu }_{bf}}{(1-{\phi }_{np1}-{\phi }_{np2}{)}^{2.5}}$$
(7)
$${k}_{hnf}=\frac{2\left({\phi }_{np1}{k}_{np1}+{\phi }_{np2}{k}_{np2}\right)-2{k}_{bf}\left({\phi }_{np1}+{\phi }_{np2}\right)+2{k}_{bf}+\left[\frac{{\phi }_{np1}{k}_{np1}+{\phi }_{np2}{k}_{np2}}{{\phi }_{np1}+{\phi }_{np2}}\right]}{-\left({\phi }_{np1}{k}_{np1}+{\phi }_{np2}{k}_{np2}\right)-{k}_{BF}\left({\phi }_{np1}+{\phi }_{np2}\right)+2{k}_{bf}+\left[\frac{{\phi }_{np1}{k}_{np1}+{\phi }_{np2}{k}_{np2}}{{\phi }_{np1}+{\phi }_{np2}}\right]}$$
(8)

Combined methods

In a combined method, two or more active and passive methods are used together to improve the system’s thermal efficiency, producing a higher heat exchange rate than individually provided by either technique. By combining passive and active methods, it is possible to achieve synergistic effects and maximize the overall heat transfer performance. For example, passive methods can create an optimized heat transfer environment. In contrast, active methods can provide additional control and adjustability to match specific heat transfer requirements or accommodate varying operating conditions. Factors such as cost-effectiveness, system complexity, energy consumption, and available resources should be considered when selecting and integrating passive and active methods for increasing heat transfer efficiency. (Valipour et al. 2018, Izadi et al. 2020, Saedodin et al. 2020, Saedodin et al. 2021, Alshuraiaan et al. 2022, Izadi et al. 2022b, Mashayekhi et al. 2022, Shang et al. 2022).

Flat plate solar collectors with nanofluids

Some recent research focusing on ways to better the rate of heat transfer with nanofluid in flat plate collectors is mentioned below. In 2012, Yousefi et al. (Yousefi et al. 2012) experimentally checked the impact of aluminum nanofluid on the performance of an FPSC. In this study, the ASHRAE standard was used to compute efficiency. The outcomes displayed that compared to water as the absorption medium, the usage of nanofluid as the base fluid increases the efficiency. For example, in 0.2% by weight, the efficiency increase was 28.3%. Pressure drop and heat transfer of an absorbent medium with suspended nanoparticles (aluminum oxide, copper oxide, titanium dioxide, and silicon dioxide dispersed in water) inside a flat plate solar collector were reviewed by Alim et al. (Alim et al. 2013). Based on the analytic outcomes, the copper oxide nanofluid average Nusselt number increased by 22.15% compared to base fluid as an absorbent fluid and reduced the entropy production by 4.34%. In 2014, Said et al. (Said et al. 2014) investigated entropy generation, heat transfer improvement, and pressure drop capabilities for a flat plate solar collector with nanofluid-based single-walled carbon nanotubes (SWCNTs). According to the report, the single-walled carbon nanotubes nanofluid decreased the entropy production by 4.34%, and the pumping power of the nanofluid solar collector was 1.20% higher than the base fluid. Safarian et al. (Saffarian et al. 2020) assessed the increase of heat transfer in a flat plate collector using nanofluids in different concentrations. For investigating the changes in the average Nusselt number in the pipes, numerical simulations were performed at speeds of 0.5, 1, 2, and 4 m/s. The results showed that adding nanoparticles caused an increment in the heat transfer coefficient. Gupta et al. (Gupta et al. 2021) checked the performance of flat plate solar collectors with and without nanofluid containing aluminum oxide nanoparticles. The water temperature at the flat plate solar collector outlet without nanofluid was 5–10 °C lower than when the nanofluid was used. Sundar et al. (Sundar et al. 2020a) experimentally checked the energy performance, economic influence, and heat exchange aspects of solar flat plate collectors using aluminum oxide nanoparticles. Based on the reported outcomes, using nanofluid increased the collector’s efficiency by 20%. Some recent research focusing on ways to ameliorate the heat transfer rates with nanofluid in flat plate collectors are listed in Table 1.

Table 1 Overview on ways to ameliorate the heat transfer rates with nanofluid in flat plate collectors
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Type of study using nanofluids in FPSCs

In recent years, several kinds of research have been conducted on flat plate collectors with different nanofluids. These researches include experimental and non-experimental studies. As shown in Fig. 9, experimental studies comprise the majority of these studies (69.5%). It can also be seen that in the last eight years, non-experimental studies played a more diminutive role in this review, with nearly 40%.

Fig. 9
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Breakdown of the type of analysis about employing nanofluids in flat plate solar collectors

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Type of the base fluid of the employed nanofluids in FPSCs

The working fluid used considerably affects the efficiency of flat plate collectors and other collectors (Xiong et al 2021c), so the different types of working fluids are explained in this section. Since the base fluid plays a considerable role in FPSCs as a heat carrier, paying attention to factors such as avoiding excessive viscosity in the solution, heat capacity, etc., to choose the working fluid is necessary. The results show that storing and recovering more thermal energy by water is possible compared to other base fluids. Figure 10 illustrates the distribution of the usage of several kinds of carrier fluid applied to flat plate collectors. Water is often utilized as the working fluid in the collector (83.33%).

Fig. 10
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Breakdown of the use of various working fluids in flat plate collector

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Type of the nanoparticle of the employed nanofluids in FPSCs

Figure 11 shows a usage breakdown of several kinds of nanoparticles applied in the flat plate collectors for 8 years. It is observed that Al2O3 is often used as a nanoparticle in flat plate collectors (26%), and after that, CuO is in second place (12%). It should also be noted that the number of investigations on using combined nanofluid in FPSCs is relatively high during the last 8 years (about 10%).

Fig. 11
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Breakdown of the various nanofluids usage in flat plate collector

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Volume concentration of employed nanofluids in FPSCs

Figure 12 illustrates the distribution of the use of various volume concentrations of nanofluids applied to FPSCs. Accordingly, it can be seen that about 62.34% of the works employed nanofluids with the volume concentration in the range of 0–0.5% in which 63.19, 12.5, 10.42, 9.03, and 4.86% of the works belong to the cases with the volume concentration range of 0–0.1%, 0.1–0.2%, 0.2–0.3%, 0.4–0.5%, and 0.3–0.4%, respectively. Furthermore, based on the plotted data in Fig. 12, it can be concluded that about 16.88, 11.26, 4.76, 2.6, 1.3, and 0.87% of the studies evaluated the usage of nanofluids with the volume concentration in the range of 0.5–1, 1–2, 2–3, 3–4, 4–5, and 5–6%, respectively.

Fig. 12
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Distribution of the usage of various volume concentrations of nanofluids in FPSCs

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Thermal efficiency of FPSCs utilizing nanofluids

Different assessments have been conducted to facilitate the thermal performance of flat plate solar collectors. This study aims to assess the current two methods, including nanofluids and inserts (enhancement devices), to improve the thermal performance of FPSC.

In total, attaining better heat exchange rates is one of the original targets in industrial applications. By adding nanoparticles to the base fluid and creating a nanofluid, the conductivity of the working fluid may be increased. According to prior analyses, using nanoparticles and investigating the concentration, size, and types of nanoparticles have led to a noticeable improvement in thermal efficiency (Pandey and Chaurasiya 2017). Tang et al. (2010) investigated FPSCs with nanofluid aluminum oxide and copper oxide. They declared that utilizing nanofluid instead of water increased the collector's efficiency by 3.7%. Figure 13 shows the cent distribution of collector efficiency increase using nanofluid.

Fig. 13
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The percentage distribution of collectors’ thermal efficiency considering various ranges of the volume concentration of nanoparticles

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Categorized outcomes of works concerning the FPSCs utilizing nanofluids

Researchers have turned to nanofluids as a promising way to improve the efficiency or performance of FPSCs. Numerous factors have been investigated in this study, which consists of the analysis of the type of nanoparticles employed, the type of nanofluid base fluid, the type of study using nanofluids, the volume concentration of employed nanofluids, and lastly, the thermal efficiency achieved through these advancements. The choice of base fluid is significant in the nanofluid formulation. Researchers have checked a wide range of base fluids, containing but not limited to water, engine oil, ethylene glycol, and even molten salts, to determine their impact on heat exchange efficiency. By changing the nanoparticles and the base fluid, researchers have sought to find the most effective compounds to increase the thermal performance of solar collectors.

Several types of study using nanofluids have been included, spanning experimental, numerical, and theoretical analyses, which in this study is divided into two parts of experimental and non-experimental research. Each kind of research brings unique theories into the action of nanofluids in solar collectors, allowing for a comprehensive understanding of their heat exchange specifications and performance gains.

The volume concentration of the employed nanofluids significantly impacts the heat transfer efficiency. Researchers have investigated a wide range of volume concentrations to determine the collector’s thermal efficiency. Achieving the apropos equipoise is necessary, as excessively high concentrations may lead to particle aggregation, hindering the desirable enhancement.

Lastly, the thermal efficiency achieved through nanofluids in flat plate solar collectors is a significant parameter for appraising the success of this progress. Researchists have meticulously analyzed and measured thermal efficiency to assess the practical applicability of nanofluids in real-world solar collectors.

According to the studies conducted in the last 10 years regarding the selection of the type of nanoparticle, it can be said that the investigation of nanoparticles such as aluminum oxide and copper oxide alone does not justify innovation and great application. Another noteworthy point is the comprehensive investigation of these nanoparticles in different concentrations, which can be said to cover other further research to a large extent. On the other hand, according to the authors, due to its magnetic properties, hybrid nanoparticles, wall carbon nanotubes, and iron oxide can be a new approach for research and investigation of future researchers in this field.

Considering the increase in thermal efficiency of flat plate solar collectors along with the use of nanoparticles, it can be pointed out that the thermal efficiency increases with increasing concentration. Increasing concentration involves increasing the cost, increasing the pressure drop, severe sedimentation, etc., and that is why most of the research reviewed (as mentioned in “Volume concentration of employed nanofluids in FPSCs”) is in the range of 0–0.5.

Furthermore, as indicated by the research presented in this study, particularly in non-experimental analyses, there are instances where various outcomes have been reported. These discrepancies may stem from several sources, containing simulation errors and calculation, as well as potential inaccuracies in the measurement process.

Employing inserts in FPSCs

As stated in the previous sections, new methods, including confusing agents, have been implemented to elevate heat exchange in various thermal systems. In the following, some research focusing on ways to ameliorate and optimize the heat exchange rate in flat plate solar collectors is given along with the insert.

In 2000, Kumar and Prasad (Kumar and Prasad 2000) tested a flat plate solar water heater implementing twisted tape inserts with various torsion ratios. The outcomes showed that the solar collector efficiency performed better than conventional samples despite the twisted tapes. According to the report, the performance improvement reached 30% (Fig. 14).

Fig. 14
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A flat plate solar collector with twisted tapes (Kumar and Prasad 2000)

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In 2009, Jaisankar et al. (Jaisankar et al. 2009) assessed the presence of a twisted tape in a flat plate solar collector. Experimental data indicated that the presence of twisted tape went up the average Nusselt number and the coefficient of friction by 5.35 and 8.80%, respectively (Fig. 15).

Fig. 15
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A view of the utilized twisted tape inserts with various lengths (Jaisankar et al. 2009)

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Martin et al.) Martín et al. 2011 (checked the improvement of heat exchange in a flat panel solar collector by inserting wire coils with various working fluids. Conforming to the reported results, the collector’s thermal efficiency increased up to 4.5%. Garcia et al. (García et al. 2013) experimentally studied the heat transfer improvement in a flat plate solar water heater with coil insertion at five different mass flow rates. The outcomes illustrated that average thermal efficiency and useful power increased by 17% and 4%, respectively (Fig. 16).

Fig. 16
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A view of a flat plate solar collector with wire coils (García et al. 2013)

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Using a twisted tape, Ananth and Jaisankar (Ananth and Jaisankar 2014) found that heat transfer got down with the rising distance between the strip and the length of the rod. Also, the heat transfer coefficient increased by 2.64 times when using twisted tape. Sandlow et al. (Sandhu et al. 2014) investigated the influence of a wire coil and a twisted tape on a flat plate collector. Based on the result, the maximum heat transfer coefficient was obtained for the concentric wire coils, so this increase even reached 460%. Some recent research focusing on solutions to ameliorate heat exchange rates by using inserts in flat plate collectors are listed in Fig. 17 and Table 2.

Fig. 17
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A view of a flat plate solar collector utilizing different types of inserts (Sandhu et al. 2014)

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Table 2 Overview of the studies of inserts used on flat plate solar collectors
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According to Fig. 18, a low-velocity area immediately behind each winglet can be seen for winglet turbulators, known as fluid recirculation areas. These recirculation areas are more drastic for the higher angles of attack. Moreover, these zones are also seen for their insignificant heat exchange rate regarding low velocity and the slight temperature difference between the walls and fluid. Due to the plane’s situation in the tube, the primary target is to go up the mass flow on the winglet to produce more powerful longitudinal vortices.

Fig. 18
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The velocity contour with winglet (da Silva et al. 2019)

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Figure 19 illustrates how the temperature is distributed in a flat plate solar collector. As you can see, the axial fins cause more circulation of the flows inside the collector. For this reason, the average temperature distribution of the collector is higher in the base model.

Fig. 19
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The temperature contour with fins (Nabi et al. 2022)

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Type of study in the field of FPSCs using inserts

Conforming to Fig. 20, the considered studies are divided into two parts, experimental and non-experimental, in which the share of experimental research is about 42%, which is less compared to non-experimental studies. According to these outcomes, it can be said that conducting experimental studies has been given less attention than non-experimental studies due to the complexity of making all kinds of inserts and also the higher cost.

Fig. 20
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Usage breakdown of inserts on a flat plate collector

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Type of the employed inserts in FPSCs

Based on the investigation, it can be seen from Fig. 21 that twisted tapes and turbulator had a significant contribution in past studies, with 33% and 16%, respectively. Also, fin inserts are attractive to researchers, with a share of 14% in the reviewed study. These observations may be attributed to these items’ simple manufacturing or lower cost than other inserts.

Fig. 21
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Breakdown of the use of various inserts on a flat plate collector

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Thermal efficiency of FPSCs equipped with inserts

Different studies to ameliorate the thermal efficiency of FPSCs have been conducted. This paper aims to assess the current two methods, namely, nanofluids and inserts (enhancement devices), to increase the thermal performance of FPSC. Inserts such as twisted tapes, wire coils, and turbulator improve heat transfer by increasing turbulence and swirling flow and decreasing the thickness of the boundary layer. Garcia et al. (García et al. 2013) studied the thermal efficiency improvement with coil insertion. The outcomes depicted that the average thermal efficiency went up by 17%. Figure 22 shows the percentage distribution of collector efficiency increase implementing inserts.

Fig. 22
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The percentage distribution of collector efficiency using inserts

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Categorized outcomes of works concerning the FPSCs equipped with inserts

This part of the study delves into a comprehensive investigation of the vital influence resulting from the implementation of several inserts in flat plate solar collectors (FPSCs). Inserts, strategically placed within the collectors, have illustrated promising potential to go up their thermal efficiency. In this study, several factors are essential to understanding the effectiveness of using inserts in FPSCs, containing the type of study, the variety of employed inserts, and the assessment of thermal efficiency. Using inserts in FPSCs represents an approach to ameliorating heat exchange and thermal efficiency. Researchists have conducted many studies in this domain, utilizing experimental and non-experimental methods. By exploring the findings of these studies, valuable insights into the benefits and limitations of incorporating inserts into FPSC can be gained.

A varied range of inserts has been used to improve the thermal efficiency of FPSCs. These inserts come in different configurations and materials, each presenting unique heat exchange and flow increase characteristics. Some usual insert examples consist of twisted tapes, fins, and vortex generators, among others. Furthermore, this part examines the synergistic impacts of combining various kinds of inserts within FPSCs. Researchers have investigated diverse combinations of inserts to realize how they interact. In conclusion, integrating inserts into flat plate solar collectors illustrates a great way to enhance thermal efficiency and improve energy conversion.

Over the past decade, a significant trend in solar collectors has been the growing adoption of inserts to enhance and improve heat transfer, even at the expense of increased pressure drop. As mentioned in “Type of the employed inserts in FPSCs,” twisted tape is the most popular insert for researchers due to its simplicity and well-established manufacturing processes.

In fact, the implementation of inserts introduces a transformative influence on the flow regime and structure within the flat plate solar collectors. Each unique insert geometry brings about a distinct alteration, offering various approaches and methodologies for researchers to check. The versatility and adaptability of inserts contribute to a wealth of possibilities, making the future of research in this field significantly more precise and promising. As researchers continue to delve into novel shapes and configurations of inserts, the potential for further advancements and breakthroughs in enhancing heat transfer within flat plate solar collectors becomes increasingly evident. Thus, pursuing new and varied insert designs holds the key to unlocking even more significant potential in this area of research.

Compound

Sometimes two or more active and passive approaches are used to meliorate thermal performance, producing higher heat transfer rates than would be provided by either technique individually. In the following, some research is given on ways to ameliorate and optimize the heat exchange rate in flat plate solar collectors, including inserts and nanofluids.

Sandra et al. (Sundar et al. 2018a) in 2018, with a laboratory study of the flat plate collector using aluminum oxide and inserting a longitudinal strip, found that the simple collector’s efficiency using nanofluid and longitudinal strip went up by 58 and 84%, respectively. Also, in 2020, they scrutinized the effect of employing twisted tape with copper nanoparticles on the thermal efficiency of the flat plate collector, which conforms to the obtained outcomes; the average Nusselt number increased to 46.90% compared to the simple collector. Also, the solar collector’s efficiency with base fluid is 52%, and the nanofluid’s presence went up by 58% (Sundar et al. 2020b).

Khatib et al. (Khetib et al. 2022), by numerical and laboratory investigation by combined nanofluid with turbulator, found that with the installation of turbulator, the average Nusselt number increased up to 63.46%. In addition, with the presence of nanofluid, energy efficiency was obtained about 22.19%. Table 3 shows some of the recent research focusing on ways to ameliorate and optimize the heat transfer rate in flat plate solar collectors, along with the inclusion of insert and nanofluids.

Table 3 Overview of the studies of nanofluids and inserts used on flat plate solar collectors
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In this section, the recent investigations aimed at improving the heat transfer rate in flat plate solar collectors by simultaneous use of nanofluids and inserts are presented. These cutting-edge techniques offer promising avenues to improve the overall performance and efficiency of flat plate solar collectors.

Conclusions

This work reviews the findings of the previously published research on flat plate solar collectors, in which the working fluid is nanofluid with a specific volume concentration, and a turbulator inside the collector has been used to ameliorate heat exchange. The use of nanofluid, owing to its enhanced thermos-physical properties compared to the pure fluid, leads to an increment in heat transfer rate from the solar collector to the working fluid. The results have shown that the selection of the type of nanoparticle and its volume concentration in the nanofluid, has a noteworthy impact on the value of augmentation the heat transfer rate. Employing a turbulator (vortex generator) inside the flat plate solar collector causes the formation of swirling and secondary flows in the fluid flow. Then, the swirling flow increases heat transfer between the working fluid and the collector. The investigations have shown that different types of turbulators have been used to enhance heat transfer in the collector model, and positive results have also been announced, which shows the importance of using the desired method in improving the collector’s thermal performance. Moreover, the investigations carried out in this work have shown that in a number of articles, both methods of using nanofluid (single or hybrid) and turbulator have been used as passive methods to ameliorate heat exchange in the FPCSs, and the obtained results were promising in enhancing the thermal performance of the collector.