Photothermal Conversion Performance of Fe₃O₄/ATO Hybrid Nanofluid for Direct Absorption Solar Collector

Abstract

In order to enhance the efficiency of direct absorption solar collectors, this study carried out an experimental analysis about the optical and photothermal conversion performance of Fe₃O₄, ATO (Antimony-doped tin oxide), and Fe₃O₄/ATO nanofluids with a total concentration of 0.1 wt%. According to the results of the experiments, Fe₃O₄ nanofluid outperforms ATO nanofluid in terms of optical absorption; nevertheless, at wavelengths shorter than 600 nm, it also shows significant scattering reflection. The solar-weighted absorption coefficient of Fe₃O₄/ATO nanofluid rose from 0.863 ($m_{Fe3O4}/m_{Total}$= 0.2) to 0.932 ($m_{Fe3O4}/m_{Total}$= 0.8) when the optical path length increased from 0.01 m to 0.06 m. Moreover, the Fe₃O₄/ATO hybrid nanofluid achieved a photothermal conversion efficiency of 0.932 when the mass ratio of Fe₃O₄ to total mass was 0.2, surpassing the efficiencies of 0.892 and 0.898 recorded for 0.1 wt% ATO and Fe₃O₄ nanofluids, respectively. When present together, the opposing optical characteristics of Fe₃O₄ and ATO boost photothermal conversion performance, which is anticipated to raise the efficiency of direct absorption solar collectors.

1. Introduction

Due to global industrialization and population growth, the demand for energy has continuously increased. This increase has been accompanied by a rise in greenhouse gas emissions from fossil fuel consumption, contributing to serious issues such as global warming and climate change. These challenges underscore the urgent need for sustainable alternative energy sources, with solar energy offering significant potential as an abundant and clean energy source [1].

Solar energy converts radiant energy emitted by the sun into thermal energy, making it applicable across various fields. This conversion facilitates two critical objectives: reducing energy costs and minimizing environmental impact. Solar collectors, devices that convert solar radiation into thermal energy, play a pivotal role in applications such as heating, hot water supply, and industrial processes [2,3,4]. Solar collectors are generally classified into indirect and direct absorption types. In the indirect absorption method, the heat-transfer fluid absorbs solar energy indirectly from the absorbing surface. In contrast, the direct absorption method involves the heat-transfer fluid directly absorbing solar radiation, potentially achieving higher efficiency [5,6,7].

In direct absorption solar collectors (DASCs), the heat-transfer fluid directly absorbs solar radiation and converts it into thermal energy, thereby reducing thermal resistance in the energy absorption process and minimizing heat losses. However, the performance of these systems is highly dependent on the photothermal conversion efficiency of the fluid. Conventional heat-transfer fluids, such as water, antifreeze, and oil, exhibit low optical absorption, limiting their ability to absorb sufficient solar radiation and achieve optimal performance in the solar collector [8,9]. Nanofluids are an efficient solution for enhancing thermal conductivity, convective heat transfer, and optical absorption [10,11,12].

Recent efforts to enhance the photothermal conversion performance of nanofluids have concentrated on systematically examining the composition, concentration, and structure of nanoparticles to identify optimal operating conditions. Studies employing various materials, including metals [13,14,15], metal oxides [16,17,18,19], CNTs [20,21], graphene [22,23], and environmentally friendly materials, have shown significant progress in improving optical absorption performance. These investigations have confirmed notable enhancements in the optical properties and photothermal conversion efficiency of heat-transfer media [7,24,25,26,27,28]. While nanofluids have typically been synthesized with single-component nanoparticles, they face a key limitation in expanding the optical absorption peak band [28]. To address this issue, ongoing research is exploring the photothermal conversion characteristics of nanofluids, incorporating coexisting nanoparticles with distinct optical absorption spectrum peaks. Chen et al. [29] developed nanofluids by combining spherical and rod-shaped Au nanoparticles to extend the absorption wavelength range. They demonstrated that this approach allows for red and blue shifts, leading to a broader optical absorption spectrum. Wang et al. examined the photothermal conversion efficiency of Hedgehong-Oil nanofluids, which incorporate a combination of Au and ZnO nanoparticles, and observed a 240% enhancement compared to the base oil. Shang et al. [30] sought to improve optical absorption by creating Ag@Al₂O₃ nanocomposites with a core-shell structure using atomic layer deposition, aiming to maximize localized surface plasmon resonance. Their findings revealed that Ag nanoparticles coated with Al₂O₃ exhibited a red shift in the absorption band due to localized surface plasmon resonance, resulting in a wider absorption band and enhanced optical absorption performance.

To address the limitations of single-component nanofluids with narrow absorption bands, extensive research has been conducted on multi-component or hybrid nanofluids, which contain two or more types of nanoparticles with different absorption spectra. Joseph et al. [31] used ANOVA analysis to examine the factors that affect the enhancement of photothermal conversion properties in binary nanofluids and to identify the optimal mixing ratio of SiO₂/Ag-CuO. Their findings revealed that a nanofluid consisting of SiO₂/Ag: 206.3 mg/L, CuO: 864.7 mg/L, and SDS (surfactant): 1996.2 mg/L achieved a solar-weighted absorption fraction as high as 82.82%. This indicates that multi-component nanofluids can significantly boost photothermal conversion efficiency. Hazra et al. [32] explored the photothermal conversion characteristics of a BN/CB hybrid nanofluid, composed of carbon black and hexagonal boron nitride (BN), and observed a 34.55% improvement in photothermal performance compared to the base fluid, ethylene glycol (EG). This demonstrates the potential of hybrid nanofluids in enhancing photothermal efficiency. Similarly, Kim et al. [32] investigated the photothermal conversion performance of SiC/ITO nanofluids, aiming to improve the limited infrared absorption of SiC by incorporating ITO. The study showed that a SiC mixing ratio of 8:2 resulted in an increase in photothermal efficiency by 38.7% over that of the SiC nanofluid alone, with a maximum efficiency of 34.1%. Collectively, these studies provide clear evidence that hybrid nanofluids can effectively enhance photothermal conversion performance.

Despite considerable progress in nanofluid research, challenges remain in developing nanofluids that can achieve an optimal balance of absorption across different spectral regions. Current single-component nanofluids, although effective to some extent, often have limited absorption capabilities, especially when broad-spectrum absorption is needed to enhance the photothermal conversion efficiency of direct absorption solar collectors (DASCs). This limitation suggests the need for approaches that combine different nanoparticles to improve overall performance.

Fe₃O₄ and ATO (Antimony-doped tin oxide) nanofluids present an approach to address this issue. Fe₃O₄ nanofluids [33,34] exhibit strong absorption primarily in the visible light spectrum, while ATO nanofluids [35,36] show enhanced absorption in the near-infrared region. Individually, these nanofluids have specific limitations, but when combined, they can potentially utilize the complementary absorption characteristics of both materials. This combination may expand the overall absorption spectrum and improve the photothermal conversion efficiency of DASCs. However, despite this potential, research specifically focusing on Fe₃O₄/ATO hybrid nanofluids is limited, and comprehensive studies evaluating their effectiveness in DASCs are scarce.

This study aims to address this gap by evaluating Fe₃O₄/ATO hybrid nanofluids as heat-transfer fluids for DASCs. A comparative analysis with single-component nanofluids was conducted to assess the potential advantages of the hybrid approach in enhancing photothermal conversion performance. The influence of varying the Fe₃O₄ and ATO mixing ratio on key performance metrics, such as optical absorption, absorption rate, solar-weighted absorption fraction, and photothermal conversion efficiency, was systematically examined. The results provide insights into the feasibility of using Fe₃O₄/ATO hybrid nanofluids for DASCs and their potential to perform more effectively than single-component nanofluids in this application. The contribution of this research lies in its systematic evaluation of Fe₃O₄/ATO hybrid nanofluids and their ability to address some of the limitations associated with existing single-component nanofluids. The findings aim to support the optimization of solar collector systems for improved energy efficiency and offer insights that may inform the development of sustainable energy technologies.

2. Experimental Method

2.1. Preparation of Fe₃O₄/ATO Hybrid Nanofluid

In this study, Fe₃O₄/ATO hybrid nanofluids were synthesized by combining Fe₃O₄ and ATO single-component nanofluids. Fe₃O₄ was synthesized using the coprecipitation method, and surface modification with PAA (Polyacrylic acid) [18,37] was performed to ensure dispersion stability. The ATO nanofluid was prepared by diluting a 50 wt% suspension provided by K&P Nano Co. (Jecheon-si, Republic of Korea) with distilled water to reach the desired concentration. To ensure proper dispersion, both the single-component and hybrid nanofluids underwent stirring at 500 rpm for 2 h, followed by a 5 h ultrasonication process. The characteristics of the Fe₃O₄ and ATO nanoparticles are detailed in

Table 1. Specification of nanoparticles.

NP	Fe3O4	ATO
Purity		99%
Color	Dark brown	Blue
Outer diameter	5–20 nm
Thermal conductivity	$80 \mathrm{W}/\mathrm{m}·\mathrm{K}$	$4.4 \mathrm{W}/\mathrm{m}·\mathrm{K}$
True density	5.1 g/cm3	6.8 g/cm3
Manufacturing method	Coprecipitation	50% aqua solution

.

Figure 1 illustrates the synthesized Fe₃O₄, ATO, and Fe₃O₄/ATO nanofluids, while Figure 2 shows the TEM images and particle size distributions of the dispersed nanoparticles. The Fe₃O₄ and ATO nanoparticles exhibited distorted spherical shapes, and the Fe₃O₄/ATO nanoparticles in the hybrid nanofluid were found to be agglomerated. The average sizes of the clustered Fe₃O₄ and ATO nanoparticles were 10.9 ± 4.2 nm and 11.72 ± 4.2 nm, respectively, with the Fe₃O₄ nanoparticles showing a distorted morphology.

According to the study by Tong et al. [28], the optimal concentration range for achieving maximum photothermal conversion efficiency for Fe₃O₄ and ATO is between 0.075 and 0.1 wt%. Based on this result, Fe₃O₄/ATO hybrid nanofluids were prepared at a total concentration of 0.1 wt%, with mass ratios set at 0.2, 0.4, 0.6, and 0.8. Previous research identified the critical concentration for achieving maximum photothermal conversion efficiency in Fe₃O₄ nanofluids as 0.1 wt%. Guided by this result, Fe₃O₄/ATO hybrid nanofluids were formulated at a total concentration of 0.1 wt%, with mass ratios of $m_{Fe3O4}/m_{Total}$ set at 0.2, 0.4, 0.6, and 0.8. The zeta potential measurements of the Fe₃O₄, ATO, and Fe₃O₄/ATO nanofluids were −40.4 ± 2 mV, −49.9 ± 0.41 mV, and −40.2 ± 1.8 mV, respectively. The zeta potential is a critical parameter for evaluating the stability of colloidal systems. A zeta potential magnitude greater than 30 mV (in absolute value) typically indicates a stable dispersion, as the strong electrostatic repulsion between particles minimizes aggregation. In this study, the pH of Fe₃O₄, ATO, and Fe₃O₄/ATO nanofluids was measured at 7. Therefore, the prepared Fe₃O₄, ATO, and Fe₃O₄/ATO nanofluids were assessed to have adequate dispersion stability based on this criterion. Furthermore, previous research [28] conducted in this study confirmed that Fe₃O₄ and ATO nanofluids exhibited no change in optical transmittance after five cycles of photothermal conversion experiments, as well as no variation in optical transmittance two weeks after preparation. Therefore, the hybrid nanofluid demonstrates reliable dispersion stability.

2.2. Optical Property Measurement Method

When solar energy passes through a nanofluid, some of the energy is reflected at the interface between the air and the nanofluid, while the rest is absorbed as it moves through the fluid. The portion of solar energy that is not absorbed is transmitted through the nanofluid. The relationship describing the transmission of solar energy through the nanofluid is given by Equation (1).

$$A+T+R=1$$

(1)

where $A$, $R,$ and $T$ are absorbance, reflectance, and transmittance.

In this research, the optical properties of the nanofluids, including absorption, reflection, and transmission, were analyzed using the Novel Double-Thickness Transmittance Method (NDTTM). This method involves measuring the transmittance of the nanofluid through two cuvettes with different optical path lengths to determine its optical characteristics. By applying the NDTTM, the optical extinction coefficient ($\kappa$) and the reflectance at the interface between the cuvette and air (${\rho }_{op}$) are derived. These values are calculated using the transmittance data from cuvettes of different optical lengths and applying Equations (2) and (3).

$$\kappa =-{\displaystyle \frac{\lambda \ln \left(\frac{T_1}{T_2}\right)}{4\pi \left(L_{op,1}-L_{op,2}\right)}}$$

(2)

$${\rho }_{op}={\displaystyle \frac{1-\sqrt{T_1^2+T_1\left[\exp \left(\frac{4\pi \kappa L_{op,1}}{\lambda }\right)-\exp \left(\frac{-4\pi \kappa L_{op,2}}{\lambda }\right)\right]}}{1+T_1\left(\frac{-4\pi \kappa L_{op,1}}{\lambda }\right)}}$$

(3)

where $T_1$ and $T_2$ are transmittance at $L_{op,1}$ and $L_{op,2}$, respectively, and $\lambda$ is the wavelength.

Subsequently, the extinction optical coefficient is calculated by incorporating the reflectance and transmittance values at different optical depths into Equation (4). This calculation is repeated until the condition $\left|{\kappa }_{ex,as}-{\kappa }_{ex}\right|<{10}^{-8}$ is met. During this iterative process, optical properties such as $n$, $\kappa$, $T\left(\lambda \right)$, $R\left(\lambda \right)$, and $A\left(\lambda \right)$ are continuously tracked and refined.

$$\kappa ={\displaystyle \frac{\lambda }{4\pi L_{op,2}}}\ln \left[{\displaystyle \frac{\left(1+\sqrt{1+4{\left(\frac{T_2}{{\left(1-{\rho }_{op}\right)}^2}\right)}^2{\rho }_{op}^2}\right)}{2\left(\frac{T_2}{{\left(1-{\rho }_{op}\right)}^2}\right)}}\right]$$

(4)

To perform the NDTTM analysis, nanofluids were placed in cuvettes with optical path lengths of 5 mm and 10 mm, and their optical transmittance was measured using a visible-infrared spectrometer (AVANTES-2048, Avlight-DHc, Apeldoorn, The Netherlands), because longer path lengths could lead to low transmittance at the concentrations used, increasing the likelihood of errors in the NDTTM analysis. To comprehensively evaluate the solar-radiation absorption by the nanofluid across different wavelengths, the solar-weighted absorption coefficient $S\left(L_{op}\right)$ was calculated. This coefficient was determined using Equation (5), with $I_{AM1.5}$ representing the spectral solar irradiance based on ASTM G173-3 AM 1.5 Global [38].

$$S\left(L_{op}\right)={\displaystyle \frac{{\displaystyle \int I_{AM1.5}\left(\lambda \right)\left(1-\exp \left(\alpha \left(\lambda \right)L_{op}\right)\right)d\lambda }}{{\displaystyle \int I_{AM1.5}\left(\lambda \right)d\lambda }}}$$

(5)

2.3. Experimental Setup for Photothermal Conversion

Figure 3 depicts the experimental setup used to assess the photothermal conversion performance. This setup consists of a solar simulator, an acrylic container, and a supporting stand. The solar simulator (Oriel Xenon Arc lamp, LCS-100, Newport Co., Franklin, MA, USA) is equipped with an AM 1.5 filter to replicate the solar spectrum. The solar irradiance intensity at the top of the acrylic container, where the nanofluid is placed, is set at 1000 W/m². The container has internal dimensions of $\varphi$40 mm × 42 mm, with three T-type thermocouples installed at intervals of approximately 10–11 mm.

The photothermal conversion experiment involves a heating phase, where the nanofluid is exposed to light for 2.5 h, followed by a cooling phase lasting 0.5 h to measure the heat loss coefficient. During the cooling phase, the light source is turned off after the nanofluid reaches its equilibrium temperature under light exposure, allowing the heat dissipation rate from the receiver to be evaluated. The data gathered during the experiment were recorded at 1 s intervals using a data logger (MX-100, Yokogawa Inc., Tokyo, Japan).

The photothermal conversion efficiency of the nanofluid, which represents the proportion of solar energy absorbed and converted into thermal energy, is calculated using Equation (6).

$${\eta }_{PTC}={\displaystyle \frac{B{m}_{nf}{c}_{p,nf}\left(T_{eq}-T\left(0\right)\right)}{IA}}$$

(6)

In this equation, $B$ denotes the thermal diffusivity to the surroundings, $T_{eq}$ is the equilibrium temperature, $T\left(0\right)$ is the initial temperature, $c_{p,nf}$ is the specific heat capacity of the nanofluid, $I$ represents the solar irradiance, and $A$ is the area of the receiver exposed to light. The expression for $B$ is provided in Equation (7) and is derived during the cooling phase. The value of $B$ was measured to be within the range of 4.1$\times$10⁻⁴ to 4.1$\times$10⁻⁵ s⁻¹.

$$In{\displaystyle \frac{T\left(t\right)-T_{sur}}{T_{eq}-T_{sur}}}=-{\displaystyle \frac{h{A}_{dis}}{{\displaystyle \sum _i{m}_i{c}_{p,i}}}}t=-Bt$$

(7)

In Equation (7), $T\left(t\right)$ is the temperature of the nanofluid at time $t$, $T_{sur}$ is the ambient temperature, $h$ denotes the heat loss coefficient, $m_i$ refers to the mass of both the receiver and the nanofluid, and $c_{p,i}$ is the specific heat capacity of the receiver and the nanofluid.

The receiving efficiency, ${\eta }_{rec}$, is defined as the efficiency with which the nanofluid captures thermal energy and is expressed by Equation (8).

$${\eta }_{rec}={\displaystyle \frac{H{\rho }_{nf}{c}_{p,nf}\left(T\left(t\right)-T\left(0\right)\right)}{{\displaystyle {\int }_0^{t}IAdt}}}$$

(8)

In this equation, $H$ indicates the height of the receiver, and ${\rho }_{nf}$ represents the density of the nanofluid.

The uncertainties in photothermal conversion and receiving efficiency were calculated using Equations (9) and (10).

$${\displaystyle \frac{\delta {\eta }_{PTC}}{{\eta }_{PTC}}}=\sqrt{{\left({\displaystyle \frac{\delta m_{nf}}{m_{nf}}}\right)}^2+{\displaystyle \sum _{i=1}^3{\left(\frac{\delta T}{T}\right)}^2+{\left(\frac{\delta A}{A}\right)}^2+{\left(\frac{\delta I}{I}\right)}^2+{\left(\frac{\delta B}{B}\right)}^2}}$$

(9)

$${\displaystyle \frac{\delta {\eta }_{rec}}{{\eta }_{rec}}}=\sqrt{{\left({\displaystyle \frac{\delta m_{nf}}{m_{nf}}}\right)}^2+{\displaystyle \sum _{i=1}^3{\left(\frac{\delta T}{T}\right)}^2+{\left(\frac{\delta A}{A}\right)}^2+{\left(\frac{\delta I}{I}\right)}^2+{\left(\frac{\delta t}{t}\right)}^2}}$$

(10)

The uncertainties in measuring the mass of the nanofluid, the temperature of the nanofluid, the light exposure area, the irradiance intensity, time, and the heat dissipation rate to the surroundings were 2.36%, 2.47%, 0.5%, 0.5%, 0.01%, and 2.35%, respectively. The uncertainties in photothermal conversion and receiving efficiency are 4.21% and 3.5%, respectively.

3. Results and Discussion

3.1. Optical Characteristics of Fe₃O₄, ATO, and Fe₃O₄/ATO Hybrid Nanofluid

The optical properties of nanofluids are critical factors influencing their photothermal conversion efficiency. Figure 4 presents the optical absorbance and transmittance spectra for 0.1 wt% Fe₃O₄, ATO, and Fe₃O₄/ATO nanofluids. As depicted in Figure 4a, Fe₃O₄ and ATO nanofluids exhibit distinct absorption characteristics within the wavelength range of 400 to 1200 nm. The ATO nanofluid shows a peak absorbance near 1100 nm, but overall, it demonstrates a lower optical absorbance compared to the Fe₃O₄ nanofluid. In contrast, the Fe₃O₄ nanofluid displays strong absorbance between 800 and 1100 nm, with a marked reduction in absorbance at wavelengths below 600 nm. This reduction can be attributed to the significant scattering reflection that occurs in the ultraviolet region for Fe₃O₄ nanofluids.

Figure 4b illustrates this scattering phenomenon, where the Fe₃O₄ nanofluid exhibits increased reflectance at wavelengths shorter than 600 nm, with reflectance intensity increasing as the wavelength decreases. On the other hand, the ATO nanofluid maintains a reflectance below 0.1 in the same wavelength range, indicating minimal scattering. These findings suggest that, although Fe₃O₄ nanofluids possess superior optical absorption properties, their high reflectance at shorter wavelengths can hinder their performance. Therefore, the combination of ATO and Fe₃O₄ in a hybrid nanofluid presents an opportunity to enhance overall optical absorption by leveraging the complementary properties of both materials. It is well known that the optical absorbance of hybrid nanofluids, containing multiple types of nanoparticles, tends to follow a linear combination of the optical absorption properties of each individual nanoparticle [39]. Therefore, the combination of ATO and Fe₃O₄ in a hybrid nanofluid presents an opportunity to enhance overall optical absorption by leveraging the complementary properties of both materials.

Figure 5 shows the solar-weighted absorption coefficients of the Fe₃O₄/ATO hybrid nanofluids (NFs). For the Fe₃O₄ nanofluid, the solar-weighted absorption coefficient exhibited a modest increase from 0.854 to 0.883 as the optical path length increased from 0.01 m to 0.06 m. In comparison, the ATO nanofluid displayed a more pronounced increase, with the solar-weighted absorption coefficient rising from 0.529 to 0.876 over the same range. This indicates that, although the Fe₃O₄ nanofluid has superior optical absorbance, the improvement in photothermal conversion performance with increasing optical path length is limited due to scattering reflection.

The Fe₃O₄/ATO hybrid nanofluids demonstrated enhanced solar-weighted absorption coefficients relative to both Fe₃O₄ and ATO nanofluids. At an optical path length of 0.01 m, the solar-weighted absorption coefficient of the Fe₃O₄/ATO hybrid nanofluid (where $m_{Fe3O4}/m_{Total}$= 0.8) was 0.861, indicating a slight improvement over the coefficients of 0.854 for the 0.1 wt% Fe₃O₄ NF and 0.529 for the 0.1 wt% ATO NF. However, at an optical path length of 0.06 m, the solar-weighted absorption coefficient of the Fe₃O₄/ATO hybrid nanofluid (where $m_{Fe3O4}/m_{Total }$= 0.2) increased from 0.748 to 0.938. As the optical path length extended from 0.01 m to 0.06 m, the optimal $m_{Fe3O4}/m_{Total}$ for maximizing the solar-weighted absorption coefficient gradually decreased. These findings suggest that, with an increase in optical path length, enhancing optical absorbance and suppressing scattering reflection are crucial for the performance of Fe₃O₄/ATO hybrid nanofluids.

3.2. Photothermal Conversion Performance of Fe₃O₄/ATO Hybrid Nanofluid

The photothermal conversion performance of nanofluids is a critical parameter that directly influences the solar radiation harvesting efficiency of direct absorption solar collectors. This performance is affected by several factors, including the optical absorbance of the nanofluid, the fluid temperature, and the optical path length of the containment vessel. Figure 6 presents the temperature variation of 0.1 wt% Fe₃O₄/ATO nanofluids over time under solar-radiation exposure. The results indicate that the temperature of the 0.1 wt% Fe₃O₄/ATO nanofluids increases progressively with prolonged exposure to solar radiation, demonstrating the effective conversion and storage of solar energy into thermal energy. Over an exposure period of 9000 s, the nanofluid with $m_{Fe3O4}/m_{Total}$= 0.2 exhibited the highest temperature increase, reaching 12.2 °C. In contrast, the nanofluid with $m_{Fe3O4}/m_{Total}$= 0.6 showed the lowest temperature increase of 11.7 °C, a result that is comparable to the temperature increase observed for the ATO nanofluid.

Figure 7 depicts the energy conversion efficiency of water and nanofluids (0.1 wt% Fe₃O₄ NF, 0.1 wt% ATO NF, and 0.1 wt% Fe₃O₄/ATO NFs) during solar exposure. The 0.1 wt% Fe₃O₄/ATO nanofluids exhibited photothermal conversion efficiencies that were either comparable to or greater than those of the 0.1 wt% Fe₃O₄ and 0.1 wt% ATO nanofluids. Among the mixtures, the Fe₃O₄/ATO hybrid nanofluid with $m_{Fe3O4}/m_{Total}$= 0.2 achieved the highest collection efficiency, reaching 76.8%. As the solar exposure duration increased from 300 to 9000 s, a decrease in collection efficiency and an increase in thermal losses were observed across all fluids, with nanofluids showing higher thermal losses compared to water. Notably, the Fe₃O₄/ATO hybrid nanofluid with $m_{Fe3O4}/m_{Total }$= 0.2 not only enhanced collection efficiency but also reduced thermal losses. Its thermal loss rate increased from 13.2% to 64.7% as exposure time extended from 300 to 9000 s, which is lower than the thermal loss rates observed for the single-component nanofluids: 16.2–65.7% for the 0.1 wt% Fe₃O₄ nanofluid and 15–64.9% for the 0.1 wt% ATO nanofluid. However, increasing the proportion of Fe₃O₄ nanoparticles in the Fe₃O₄/ATO nanofluid led to a rise in thermal loss, indicating a potentially adverse effect. This behavior suggests that three-dimensional absorption of solar energy within the receiver contributed to a reduction in heat loss to the surroundings.

Figure 8 shows the temperature increase and local collection efficiency of the nanofluids after 9000 s of solar exposure. The temperature distribution within the receiver is primarily influenced by the solar energy absorption characteristics of the nanofluids. Both water and nanofluids exhibited higher energy storage in the upper section of the receiver, attributed to the concentration of absorbed thermal energy in this region. The average temperature increases and receiving efficiency for water were observed to be 8.16 °C ($T_{r,tp}$= 10.3 °C, $T_{r,md}$= 8.4 °C, $T_{r,bt}$= 7 °C) and 17.5% (${\eta }_{tp}$= 7.02%, ${\eta }_{md}$= 5.73%, ${\eta }_{bt}$= 4.77%), respectively. The single-component nanofluids, specifically 0.1 wt% Fe₃O₄ and 0.1 wt% ATO, demonstrated enhanced solar energy absorption capabilities compared to water, resulting in elevated temperature increases and receiving efficiencies. The 0.1 wt% Fe₃O₄ nanofluid achieved an average temperature increase of 11.8 °C ($T_{r,tp }$= 15.8 °C, $T_{r,md}$ = 11.7 °C, $T_{r,bt}$= 9.1 °C) and a receiving efficiency of 25% (${\eta }_{tp}$= 10.8%, ${\eta }_{md }$= 8%, ${\eta }_{bt}$= 6.2%), indicating that the primary absorption of solar radiation occurred predominantly in the upper region. In contrast, the 0.1 wt% ATO nanofluid exhibited an average temperature increase of 11.7 °C ($T_{r,tp}$= 15.1 °C, $T_{r,md }$= 11.8 °C, $T_{r,bt}$= 9.5 °C) and a receiving efficiency of 24.82% (${\eta }_{tp}$= 10.2%, ${\eta }_{md}$= 8.05%, ${\eta }_{bt}$= 6.48%), demonstrating its capacity to absorb solar energy more evenly across the receiver, including the lower regions.

The 0.1 wt% Fe₃O₄/ATO hybrid nanofluid showed a further improvement in collection efficiency over the single-component nanofluids. Notably, the 0.1 wt% Fe₃O₄/ATO hybrid nanofluid with $m_{Fe3O4}/m_{Total }$= 0.2 recorded the highest performance, with an average temperature increase of 12.2 °C ($T_{r,tp}$= 16.5 °C, $T_{r,md}$= 16.5 °C, $T_{r,bt}$= 13.4 °C) and a receiving efficiency of 26.6% (${\eta }_{tp}$= 11.3%, ${\eta }_{md}$= 11.3%, ${\eta }_{bt}$= 9.14%). This enhancement is attributed to the combined presence of Fe₃O₄ and ATO nanoparticles, which effectively broadens the solar energy absorption spectrum, thereby facilitating more efficient solar energy capture compared to single-component nanofluids. The 0.1 wt% Fe₃O₄/ATO hybrid nanofluid demonstrated superior solar energy absorption performance, particularly in the upper and middle sections of the receiver.

Interestingly, the Fe₃O₄/ATO hybrid nanofluid with $m_{Fe3O4}/m_{Total}$= 0.2, despite having a relatively lower absorption rate for solar energy at wavelengths above 800 nm, exhibited the most efficient photothermal conversion. This outcome is likely due to its lower reflectance at 400 nm and the optical path length of the receiver, which was adequate for optimal solar absorption. These results indicate that for the efficient performance of direct absorption solar collectors (DASCs), it is crucial to optimize both the optical properties of the nanofluids and the design parameters of the receiver. However, a temperature gradient still develops between the top and bottom of the receiver, as light absorption primarily occurs at the upper section. This temperature gradient leads to localized heating of the nanofluid, which may negatively impact its dispersion stability. Therefore, mitigating stratification through flow disturbances within the receiver may be beneficial to improve the performance of conventional direct absorption solar collectors using hybrid nanofluids [40].

Figure 9 shows the photothermal conversion efficiency and the solar-weighted absorption coefficient of 0.1 wt% Fe₃O₄/ATO hybrid nanofluids as a function of $m_{Fe3O4}/m_{Total}$ at an optical path length of 0.04 m. The photothermal conversion efficiency of the 0.1 wt% Fe₃O₄/ATO hybrid nanofluid reached its peak value of 0.932 at $m_{Fe3O4}/m_{Total}$ = 0.2. Beyond this ratio, a decline in photothermal conversion efficiency was observed. This trend is consistent with the behavior of the solar-weighted absorption coefficient, which also attained its maximum value of 0.918 at $m_{Fe3O4}/m_{Total}$= 0.2 and subsequently decreased.

To enhance photothermal conversion performance using hybrid nanofluids, it is crucial to achieve a complementary interaction between the optical properties of the coexisting nanoparticles. While Fe₃O₄ nanofluids exhibit high scattering reflection in the ultraviolet and near-infrared regions, the coexistence of Fe₃O₄ and ATO nanoparticles in the base fluid reduces scattering reflection, thereby allowing for increased optical absorption and, consequently, higher photothermal conversion efficiency. Therefore, by carefully designing the $m_{Fe3O4}/m_{Total}$ of the hybrid nanofluid in consideration of the optical path length of the direct absorption solar collector, the optimal performance of the system can be achieved.

Table 2. Comparison of photothermal conversion efficiency and solar-weighted absorption coefficient for different nanofluids.

Nanofluid	Concentration	SWAC	PTEC
MWCNT [28]	0.002 wt%	0.935	0.933
Fe3O4 [28]	0.075 wt%	0.891	0.9
	0.1 wt%	0.888	0.898
ATO [28]	0.1 wt%	0.904	0.892
Fe3O4/ATO hybrid nanofluid	0.1 wt% ($m_{Fe3O4}/m_{Total}$= 0.2)	0.938	0.932

compares the solar-weighted absorption coefficient and photothermal conversion efficiency of single-component nanofluids, including MWCNT, Fe₃O₄, and ATO, with those of hybrid nanofluids. The Fe₃O₄/ATO hybrid nanofluid achieved a photothermal conversion efficiency of 93.2%, closely matching the 93.3% observed for MWCNT. Additionally, the solar-weighted absorption coefficient of the Fe₃O₄/ATO hybrid nanofluid was 0.938, which is comparable to the 0.935 recorded for MWCNT. While MWCNT exhibits excellent photothermal conversion efficiency, its practical use is limited due to agglomeration and sedimentation at elevated temperatures [41]. Therefore, the Fe₃O₄/ATO hybrid nanofluid presents itself as a potential alternative to MWCNT.

4. Conclusions

This study experimentally examined the optical properties and photothermal conversion performance of $m_{Fe3O4}/m_{Total}$ nanofluids to improve the efficiency of direct absorption solar collectors. While Fe₃O₄ nanofluids demonstrated superior optical absorption compared to ATO nanofluids, they also exhibited significant scattering reflection at wavelengths below 600 nm, limiting their ability to absorb additional solar radiation. In contrast, ATO nanofluids, despite having slightly lower optical absorption, showed reduced scattering reflection. The solar-weighted absorption coefficient of the Fe₃O4/ATO nanofluid notably increased with the optical path length, reaching a peak of 0.938 at an optical path length of 0.06 m and a mixing ratio of $m_{Fe3O4}/m_{Total}$= 0.2

The Fe₃O₄/ATO hybrid nanofluid achieved more uniform optical absorption within the receiver, owing to the reduced scattering reflection and lower optical transmittance provided by the ATO component. This led to an enhancement in photothermal conversion efficiency. At a mixing ratio of $m_{Fe3O4}/m_{Total}$ = 0.2, the Fe₃O₄/ATO hybrid nanofluid attained a photothermal conversion efficiency of 0.932, surpassing the efficiencies of Fe₃O₄ and ATO nanofluids, which were 0.883 and 0.821, respectively.

To effectively harness the benefits of hybrid nanofluids, it is vital to integrate complementary optical absorption characteristics that mitigate the limitations of single-component nanofluids. The contrasting optical properties of Fe₃O₄ and ATO nanofluids can enhance both the solar-weighted absorption coefficient and the photothermal conversion efficiency. However, sufficient optical path length is necessary for this effect to be significant. At shorter optical path lengths, the impact of scattering reflection is minimal, thereby reducing the improvement offered by the hybrid nanofluid. Thus, in designing direct absorption solar collectors with hybrid nanofluids, it is crucial to carefully consider both the optical path length and the nanofluid mixing ratio to achieve optimal performance.