1. Introduction

F1000Research

2046-1402

F1000 Research Limited

London, UK

10.12688/f1000research.175920.1

Research Article

Articles

The Experimental Development of Solar Collector with Different Types of Nanofluid

[version 1; peer review: 1 approved with reservations, 2 not approved]

Awad

Afrah Turki

Formal Analysis Resources Validation Writing – Review & Editing https://orcid.org/0000-0003-3967-0821 a 1 Taifor

Mustafa Naozad

Conceptualization Investigation Visualization Writing – Original Draft Preparation https://orcid.org/0000-0001-7146-1160 1 Hussein

Adnan M.

Data Curation Formal Analysis Funding Acquisition Methodology Project Administration Supervision 1 1Renewable Energy Research Center- Kirkuk, Northern Technical University, Kirkuk, 36001, Iraq

a afrah.turki@ntu.edu.iq

No competing interests were disclosed.

29 1 2026

2026

137

13 1 2026

2026

This is an open access article distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In the present study, the flow rate and nanofluid effects on a parabolic trough solar collector were examined experimentally in Kirkuk city climate conditions during the period from May to July. Three flow rates, 0.1, 0.2, and 0.3 l/min were utilized. The theoretical and experimental results prove that lower flow rates enhance the thermal performance significantly as they increase the fluid residence time. According to the obtained results, two nanofluids, ZnO-water and MgO-water (at 0.2 wt. %), were experimentally evaluated at the optimal flow rate of 0.1 l/min. Both nanofluids showed better results than base fluid (water). Thus, MgO exhibited a better thermal efficiency of 66.9% at 12 pm than ZnO (62.7%) and water (57.19%). Directly, MgO generated the better thermal efficiency with maximum outlet temperature of MgO was 75.08°C. This could be due to the higher thermal efficiency of MgO-water, which is attributed to its much higher thermal conductivity (48.4 W/m·K) than ZnO (29 W/m·K). The exergy efficiency was nearly the same and negligible, that is, 13.8% for MgO, owing to the thermodynamic limitations. The practical results show that MgO nanofluid at a low flow rate could be an optimal solution for the parabolic trough solar collector.

Solar Energy Parabolic Trough Solar Collector Nanofluid ZnO Nanoparticles MgO Nanoparticles.

The author(s) declared that no grants were involved in supporting this work.

1. Introduction

Energy production from renewable sources has attracted worldwide attention. Solar energy, as a renewable resource, has the potential to satisfy the growing global energy demand. ¹ Solar energy can be divided into photovoltaic (PV) and concentrated solar thermal power plants (CSTPP). CSTPP are subdivided into tower power plants, parabolic trough solar collectors (PTSC), flat-plate solar collectors, and dish-type collectors. ² It consists of a solar collector, working fluid, heat exchanger, turbine, and generator to produce electricity. ^{3,
4} Furthermore, energy storage systems can be added to store solar energy and provide it under request or at night. ^{5–
7} The main component of the CSTPP is the solar collector, which captures sunlight and converts it into heat. There are different numerical studies on the heat transmission of PTSC in order to analyze the results. ^{8–
11} Furthermore, both simulation and experimental studies were conducted to provide insight into the heat transfer on PTSC. ^{12,
13} To improve the efficiency of PTSC, new techniques have been developed, such as nanoparticles. Dispersing nanoparticles into the working fluid leads to improved heat transfer, resulting in a higher increment in the solar collector efficiency. ¹⁴ Furthermore, extensive studies on the performance of nanofluids have been conducted. ^{15,
16}

According to Raza et al. (2023), nanoparticles improved the efficiency of PTSC. ¹⁷ They prepared two types of nanoparticles, namely, multi-wall carbon nanotubes and alumina nanoparticles. It was found that nanoparticles improved the efficiency of the solar collector by 18% compared with the base fluid without any additives. In addition, alumina nanoparticles were dispersed in the working fluid. ¹⁸ Their results showed that the efficiency of the PTSC when using alumina-nanofluid outperformed base fluid (water) by 3.9%. Another study showed that alumina nanofluids can improve the performance of PTSC. ¹⁹ They studied the effect of different geometries of absorber tubes along with nanofluids. ¹⁹ A simulation study was conducted using CFD to compute the performance of different types of nanoparticles (alumina and copper oxide) on a PTSC. It was found that copper oxide nanofluid has a higher thermal efficiency of 7.19% (with a porous obstacle insert) than the others. ²⁰ Other researchers used CFD to analyze PTSC using nanofluid. ^{21–
25}

Moreover, Kaloudis et al. (2019) numerically investigated a PTSC using Al ₂O ₃/Syltherm 800 nanofluid as the heat transfer fluid via a two-phase CFD model. ²⁶ The simulations, validated against experimental data with a maximum relative error of 0.3% in the outlet temperature and 7.3% in the collector efficiency, demonstrated that a 4% nanoparticle concentration enhanced the efficiency by up to 10% compared to pure oil.

Two-phase modeling was more accurate than the single-phase approach in representing nanofluid behavior and provided new insights into potential thermal performance enhancements in PTSCs. Ekiciler et al. (2021) performed a numerical study to analyze the heat-transfer performance of a PTSC receiver using hybrid Ag-based nanofluids of Ag–ZnO, Ag–TiO ₂, and Ag–MgO in Syltherm 800 at 1-4 vol. % concentration and a Reynolds number of 10,000-80,000. ²⁷ They conducted 3D turbulent flow simulations based on C++ and non-uniform heat flux numerical methods and found that Ag–MgO/Syltherm 800 at 4% concentration was the best performing nanofluid, as it demonstrated the highest thermal efficiency compared to other fluids and the base fluid. They reported an improvement in the Nusselt number, heat transfer coefficient, and Peclet number with an increase in nanoparticle concentration, while the efficiency deteriorated at higher Reynolds numbers. This work suggests that Ag–MgO hybrid nanofluids are the most recommended choice for implementation in PTSC receivers, and that nanofluid component selection is crucial for enhancing the performance of solar collectors. Farooq et al. (2022) also conducted a CFD analysis to examine a PTSC’s thermal performance using Al ₂O ₃ and CuO nanofluids of 0.01% concentration and flow rates of 0.0112 and 0.0224 kg/s and acquired efficiencies of 13.92% and 14.79%, respectively, indicating that Al ₂O ₃ reached 13.01-13.1%. ²⁸ The authors also explored the influence of the absorber tube material, proving that copper had the highest value of 311 K, while steel and aluminum reached 307 K and 308 K, respectively. Tube length was also studied, and similar to a previous study, the CuO nanofluid always performed better. Their work also indirectly makes a case for implementing nanofluid-enhanced PTSC, while validating their CFD-based thermal data with laboratory experiments. Ram et al. (2023) assessed a PTSC experimentally using a CuO-water nanofluid with a 0.05-0.1% mass fraction and flow rates of 70-140 L/h and reported peak efficiencies of 55.26% and 69.07%, respectively, compared to water; thus, efficiency depended on the nanoparticle volume fraction and flow rate. ²⁹ They confirmed that the cost is 1.08% greater for 0.05% nanofluid at a flow rate of 70 L/h, affirming that using more concentrated nanofluids requires a large investment but pays off at 69% efficiency, making CuO-water nanofluid an attractive choice for PTSC implementations. Awad and Hussien (2024) assessed the effects of Al ₂O ₃ and SiO ₂ nanoparticle-based water-assisted thermal systems at a fixed 0.5 vol. % concentration over a cold Iraqi period from January to March and reported that Al ₂O ₃ consistently outperformed SiO ₂ at up to 4.8% higher efficiency rate due to the better-defined thermal conductivity characteristics. ³⁰ The authors repeated the sentiment that nanoparticle component selection should be the subject of detailed analysis to improve solar system performance. Abu-Zeid et al. (2024) compared a flat-plate solar collector and a PTSC for water heating based on CNTs-based CNT/water and CNT/EG fluids at 0.47-1.75 kg/min mass flow rate and stated that PTSC’s 80.6% efficiency with CNT/EG was substantially higher than flat-plate solar collector’s 64.1%. All energy-based indicators favored the PTSC, and a higher outlet temperature and useful energy were noted. ³¹ The study also presented an ecological impact, claiming that 31.26 and 39.28 kg/day was saved during water heating process since the flat-plate and PTSC implementations respectively reduced CO ₂ emissions. This study links multiple factors that impact the selection of a sustainable water-heating solution for a private household, where a consistent nanofluid-PTSC configuration is perceived as the most optimal.

Furthermore, recent research indicates that metal-oxide and hybrid nanofluids markedly improve the parabolic trough collector by increasing the thermal conductivity, Nusselt number, and heat transfer fluid temperature with moderate to significant increases in efficiency depending on the nanoparticle type and operating conditions. ^{32–
36} Recent advances include the increased utilization of hybrid nanofluids, nano-enhanced coatings, and direct-absorption schemes, together with enhanced photothermal conversion, improved exergetic performance, and broader operating conditions in advanced solar collectors. ^{36,
37}

To date, theoretical studies have generally focused on the dispersion of nanoparticles in a base fluid (water, etc.) to enhance the thermal enhancement of nanofluids. The resulting working fluid also has an advantage in its thermophysical properties for its performance characteristics as an applicant for solar collectors. Various types of nanoparticles, such as CuO, SiO ₂, Ag, TiO ₂, and Al ₂O ₃, have been dispersed in various working fluids. Studies have focused on the concentration of nanoparticles and their distributions in such fluids, which are key to improving the heat transfer.

A significant challenge is to achieve a stable nanoparticle distribution. Generally, nanoparticles with a diameter of 20 nm are used. The main goal of this study was to illustrate the effect of nanoparticles in PTSC. As part of this investigation, an assessment of the energetic and exergetic performance of PTSC will be conducted, experimentally testing various nanofluids (0.2 wt. % ZnO-water and 0.2 wt.% MgO-water). Additionally, different volumetric flow rates, namely 0.1 l/min, 0.2 l/min, and 0.3 l/min, will be examined. The purpose of this research is to demonstrate the optimum type of nanoparticles that provide a higher thermal efficiency of PTSC under Iraqi weather conditions. To the authors’ knowledge in this important and distinctive thermal application, ZnO-water and MgO-water nanofluids have not been compared in the previous literature. We focused on MgO and ZnO nanoparticles because of their high thermal conductivity, stability, low cost, and availability.

2. Experimental setup

The Al ₂O ₃ –water nanofluids were prepared using a two-step method. The NPs were dispersed in deionized water with the assistance of a magnetic stirrer for 30 min, and ultrasonic treatment was performed for 1 h to maintain homogenization. The loading of the particles was 0.2 wt. % were precisely weighed by sensitive weight balance. ³⁸ The mixture was well dispersed using a magnetic stirrer and ultrasonication for different periods. ³⁹ The prepared samples were stable with no sedimentation or agglomeration.

The nanofluid stability was qualitatively tested. After preparation, each nanofluid sample was stored in a clear capped container and visually monitored for sedimentation, agglomeration, or color change at regular intervals. The samples were visually examined at the time of mixing and after specific times (0, 6, 12, 24, etc.).

No apparent sedimentation formation or layer splitting was observed during the first 48 h of all the experimental measurements. Because all the experiments were performed within the stability time lapse and the nanofluids were freshly prepared before each test operation, the quality of dispersion was assumed to be relatively stable for the scope of this research.

Table 1 lists the specifications of the material used in this study. ^{40,
41} The size of the nanoparticles was determined using a TEM device, as shown in Figures 1 and 2, to clarify the nanosize. The thermophysical properties of the nanofluid were calculated according to equations ( 1- 3) ³⁵: ρ nf = ( 1 − φ ) ρ f + φ ρ p (1) cp nf = ( ( 1 − φ ) ρ f cp f + φ ρ p cp p ) / ρ nf (2) k nf = k f [ ( k p + 2 k f − 2 φ ( k f − k p ) ) / ( k p + 2 k f + φ ( k f − k p ) ) ] (3)

Table 1. Properties of the materials used in this research. <sup> <xref ref-type="bibr" rid="ref41">40</xref>, <xref ref-type="bibr" rid="ref42">41</xref> </sup>

Properties	Water	ZnO nanoparticles	(0.2% ZnO-water)	MgO nanoparticles	(0.2% MgO-water)
Density (kg/m ³)	996	5600	1916.8	3580	1512.8
Thermal Conductivity (W/m. K)	0.615	29	1.04159	48.4	1.0550136
Specific heat capacity (J/g.K)	4178	544	2054.628	903	2627.9603

Figure 1. TEM of ZnO nanoparticles. Figure 2. TEM of MgO nanoparticles.

After the materials were prepared, the ZnO-water nanofluid and MgO-water nanofluid were supplied as the working fluid. The experiments were conducted over three different months (May, June, and July). There are three different types of working fluids: water, ZnO-water nanofluid, and MgO-water nanofluid. In the experiments, we used three different values of the volume flowrate (0.1, 0.2, and 0.3 l/min). All data for these experiments were collected at the same location as the northern technical university in Kirkuk City. The experimental setup contained PTSC, receiver tube, thermocouples (± 0.5°C), and flow rate meter (± 0.02 l/min). In addition, devices to measure ambient temperature (± 0.2°C) and wind speed (± 0.02 m/s). All the instruments employed in this study were calibrated before conducting the experiments. The accuracy of the thermocouples was calibrated using a two-point reference procedure (ice bath at 0°C and boiling water at 100°C) to an accuracy of ±0.5°C, while the flowmeter was calibrated against a known volumetric tank with an uncertainty of ±2%.

Data were collected between 8 A.M. and 5 P.M. The specifications of the PTSC are listed in Table 2. Figure 3 shows the experimental apparatus.

Table 2. Specifications of PTSC.

Description	Values
Focal distance	250 mm
Rim angle	90°
Aperture Area	1.85×10 ⁻⁶ mm ²
Length of collector	1880 mm
Thickness of reflector	3 mm
Thickness of receiver pipe	2 mm
Collector reflectance	0.85
Absorptivity of receiver	1
Material of receiver	Copper

Figure 3. Experimental rig: (a) photography, (b) scheme. 3. Energetic and exergetic analysis 3.1 Energetic analysis

Energetic (first law) analysis evaluates the thermal performance of the PTSC by quantifying how effectively the solar energy is converted into useful heat.

As a result, the obtained analysis considered the main parameters of heat gain, outlet temperature, and thermal efficiency as influenced by nanoparticle type, flow rate, and solar irradiance. Comparing the energy output of the two nanofluids, ZnO-water and MgO-water, with the baseline pure water, the present study shows which working fluid can be used to achieve the maximum heat transfer and system efficiency in practice. According to Ref. 42, from the perspective of energy conservation, the energy input to the PTSC is Q in = A c I (4) where Q in : input energy (W), A c : the collector aperture area (m ²), I : Solar irradiance (W/m ²).

The output energy calculates by: Q out = m ̇ C p ( T out − T in ) (5) where Q out : outlet energy (useful energy) (W); m ̇ : fluid flow rate (kg/s); T in , T out : Input and output temperatures, respectively (°C).

Now the thermal efficiency ( η th ) is calculated from equation (6): η th = Q out Q in = m ̇ C p ( T out − T in ) A c I (6)

3.2 Exergetic analysis

However, exergetic or second law analysis is not restricted to energy conservation alone; it also examines the quality of energy conversion. While energetic analysis only includes the inner energy from heat, exergy analysis includes irreversibilities, such as losses linked to entropy creation. This method on the efficiency of the solar collector has highlighted some inefficiencies experienced in the process, which include thermal resistance and temperature differencing. It has also provided a sharp understanding of how a nanoparticle-containing fluid optimizes the heat transfer and overall performance of solar thermal systems.

The exergy rate of the available solar energy can be calculated using the Petela model. ⁴³ E s = A c I [ 1 − 4 3 ( T a T s ) + 1 3 ( T a T s ) 4 ] (7) where E s : exergy rate of the available solar energy (W), T a : the ambient temperature (K), T s : the solar temperature (5770 K). ⁴⁴

The useful exergy rate ( E u ) (W) of the PTSC used water and nanofluids as the working fluids, as given by equation (8) ⁴⁵: E u = m ̇ C p ( ( T out − T in ) − T a ln T out T in ) (8)

The exergy efficiency ( η ex ) represents the exergy rate of available solar energy to the useful exergy rate, which can be expressed by equation (9). ⁴⁶ η ex = E u E s (9)

3.3 Uncertainty and error analysis

The uncertainty (error) in the experimental tests was evaluated using the uncertainty propagation equation ⁴⁷ δ R = [ ( ∂ R ∂ x 1 δ x 1 ) 2 + ( ∂ R ∂ x 2 δ x 2 ) 2 + ( ∂ R ∂ x 3 δ x 3 ) 2 + ⋯ + ( ∂ R ∂ x n δ xn ) 2 ] 1 2 (10) where R : the calculated result, such as exergy efficiency, x n is the independent measured variable, δ xn is the respective uncertainty, ∂ R ∂ x n is the sensitivity coefficient (partial derivative of R with respect to each x ).

The accuracy of this study was deemed acceptable because the uncertainty did not exceed 3.5%. Table 3 shows the uncertainty of the measurements.

Table 3. Uncertainty of the measurements.

Measurements	Uncertainty values
Temperature	±0.5°C
Solar irradiance	±3%
Flow rate	±2%

4. Validation of results

The obtained thermal efficiency results of the current PTSC study were compared with literature data from Hamad (1987). ⁴⁸ The comparison is shown in Figure 4. Good agreement between both datasets, with the peak efficiencies around solar noon, enables validation of the accuracy of the measurement methodology and the performance of the baseline collector. Peak efficiencies were estimated to reach 57% in the current study and 58% in the literature. Minor differences below 1% were probably caused by different local weather conditions, collector parameters, or measurement techniques. The small differences (<1%) in the current work compared to the literature were due to slight test environmental deviations. For this investigation Solar irradiance (850–920 W/m ²), outdoor ambient temperature (30–36°C), and limited to low wind speeds (<3 m/s) exert a minor influence on heat losses and thermal behavior, clarifying the very small but non-identical efficiencies.

Figure 4. The validation of current data with literature. <sup> <xref ref-type="bibr" rid="ref49">48</xref> </sup>

This validation step is essential, as it establishes confidence in the subsequent nanofluid efficiency comparisons, providing evidence that the obtained enhancements were produced by the presence of nanoparticles rather than artifacts of the measurement equipment. The majority of the current results are in close relation to published literature data, thereby ensuring the methodological robustness of the study and providing a baseline for interpreting novel nanofluid results.

5. Results and Discussion

This section discusses the effects of changing the water flow rate on the performance of the solar collector and the selection of the optimal flow for its application to nanofluids. The second subject addressed was nanofluids and their impact on improving thermal efficiency.

5.1 Effect of flow rate on thermal performance

The relationship between time and the outlet temperature of the water from the PTSC is shown in Figure 5. The X-axis represents the time of the day in hours, while the y-axis shows the outlet temperature (in °C). The figure contains three plots of the relationship between the outlet temperature and time for flow-rate volumes of 0.1, 0.2, and 0.3 L/min. Generally, the data for all the flow rates exhibit the same trend, indicating a diminished outlet temperature owing to the reduced residence time of the fluid in the collector. At a flow rate of 0.1 L/min, the difference in the heat absorption within the day fluctuates between the least at 54.22 °C by 8 A.M. to the peak of 70.3 °C by 12 P.M. The curve for the flow of 0.2, 0.3 L/min hardly increases the peak temperature of the water from the inlet temperature graph. However, the relationship between temperature and the rate of increase of intensity for all flows is equally similar to the wavelength. The steepest gradient was observed at a higher rate of 0.1 L/min shows the steepest gradient. This means that the impact of solar intensity on the PTSC is higher than that for high flow rates, which has a dampening effect. These results are consistent with the broad statement from the study that lower flow rates conserve heat better by consuming more heat. However, they are likely to trade off flow volume. The gradient curve for the 0.1 L/min reflects the extent to which the heat absorption process can accelerate or decelerate. In high-irradiance areas, such as Iraq, the flow rate needs to be consistently optimized.

Figure 5. Effect of volume flow rate on outlet temperature in a PTSC over time.

Figure 6 shows the heat gain of the PTSC over time for the three volume flow rates. The heat gain exhibits a diurnal pattern and peaks at noon before declining symmetrically in the morning and afternoon, following the trajectory of solar irradiance. Regardless of the volume flow rate, heat gain achieves its maximum values at midday, and this range from 446.24 W at 0.1 L/min, 429.8 W at 0.2 L/min, and 400.5 W at 0.3 L/min. These data confirm the previous preliminary results that increased volume flow rates reduce the thermal performance. This reduction is due to the lower fluid residence time in the collector, which limits the amount of heat that can be transferred. However, the differences between the volume flow rates decreased slightly during peak hours. This indicates that, while the volume flow rate significantly influences the heat gain, the impact is visible at lower solar irradiance times. The gradual decline in values from noon is linked to the reduction in solar radiation. Consequently, the variation was minimal in the late afternoon. Subsection 5.2 utilized a 0.1 L/min volume flow rate, because its results were optimal in nanofluid analysis. Hence, it demonstrates the best results with peak outlet temperatures of 70.3°C and heat gain of 446.24 W. The fixed volume flow rate addresses the variance and allows a direct review of the effects of nanoparticles on system performance. This approach is critical, as it aligns with the premise of reduced volume flow rates for optimal thermal energy output.

Figure 6. Effect of volume flow rate on heat gain in a PTSC over time. 5.2 Effect of nanofluids rate on thermal performance

Figure 7 shows the outlet temperature profiles of three fluids (pure water, ZnO-water nanofluid (0.2 wt. %), MgO-water nanofluid (0.2 wt.%)—in PTSC with flow ratio of 0.1 L/min. The results showed that the nanofluids performed better than pure water. During the daytime (10:00 A.M. to 4:00 P.M.), MgO achieved the highest temperatures; for example, at noon (12 PM) it reached 75.08°C.

Figure 7. Comparative outlet temperatures of water, ZnO, and MgO nanofluids in a PTSC over time.

The performance disparity is greatest during solar hours of 10 A.M.–2 P.M.; MgO nanoparticles have a higher thermal conductivity, to wit, 48.4 W/m·K (versus ZnO nanoparticles owns 29 W/m·K), allowing for greater heat absorption. The difference in ΔT between the nanofluid and water also decreases around the early morning and late afternoon, which is attributed to the decrease in solar irradiance but has always been better for MgO. The results highlight two main conclusions: (1) the enhancement of PTSC thermal convection through nanoparticles and (2) MgO is more efficient than ZnO because of its higher conductivity and low density. These tendencies justify the decision in this study to consider NP selection as a major leverage for solar collector optimization, especially in high insolation regions such as Iraq. The figure also emphasizes the correlation between the timing of the daylight temperature maxima and solar noon, illustrating that the system behavior is decoupled from irradiance intensity.

The data in Figure 8 affirm the significant improvement in heat gain when nanofluids are used compared to pure water in the PTSC. This can be observed in all three cases, where MgO-water delivers the highest heat gain in all three cases, with a maximum value of 489.78 W occurring at noon, followed closely by ZnO-water at 466.89 W and pure water at 446.24 W. This difference is particularly large at noon, when high solar intensity allows the thermal attributes of nanoparticles to express themselves. These variances indicate the differences in heat transfer between the nanofluids and pure water, which is expected considering the superior thermal conductivity and convection properties of the nanofluids. Furthermore, it can be observed that the variations in heat gain as a function of solar intensity, as measured for all fluids, are consistent with the expected day/night and high/low solar irradiance patterns. However, it is clear that the performance gap between MgO nanofluids and pure water is more substantial (up to 9.8% at noon) than the gap between ZnO and pure water (approximately 4.6% at the same time). The entire performance curve for MgO is consistently above water at all times, including when the solar intensity is relatively low. Therefore, it appears that the MgO nanofluids in this experiment continually outperform water under the same conditions.

Figure 8. Comparative heat gain performance of water and nanofluid working fluids in PTSC.

Figure 9 presents the thermal efficiency of the PTSC using three types of working fluids: pure water, ZnO-water nanofluid 0.2 wt. %, and MgO-water nanofluid 0.2 wt. %. Therefore, the use of both types of nanofluids as working fluids leads to a significant improvement in collector efficiency, and MgO provides the best performance during any measured time of the day. At solar noon, the maximum efficiency values for all the working fluids were recorded. For example, the efficiency of the MgO nanofluid was 66.9%. Simultaneously, the efficiencies of ZnO and water were 62.7% and 57.19%, respectively. The efficiency variation with time is equal to the variation in the solar irradiance, which is propagated during the day starting in the morning, reaching peak values, and then falling in the afternoon. Moreover, the best performance of the nanofluid-enhanced working fluids was observed during the two hours around solar noon starting from 10 A.M. and finishing at 2 P.M. in the afternoon, when the thermal transfer processes were the most intensive. MgO also exhibited the best efficiency among the working fluids used during this part of the day. The increased thermal conductivity and better stability in the suspension of MgO compared to ZnO 2 facilitated the heat transfer processes, which led to the higher efficiencies observed at this time. The results suggest that the addition of nanoparticles to the working fluid, especially in the form of MgO as a nanoparticle enhancer, shows promising results and improves the thermal efficiency of PTSC systems. The performance was consistent across the day, which means that such a system can operate at an improved energy output.

Figure 9. Thermal efficiency comparison of water and nanofluid working fluids in PTSC over time.

Table 4 captures the main findings for water, ZnO nanofluid, and MgO nanofluid, highlighting the outlet temperature, heat gain, and efficiency, respectively.

Table 4. Main results of base fluid and nanofluids.

Parameter	Water	ZnO-nanofluid	MgO-nanofluid
Outlet temperature (°C)	70.3	73.19	75.08
Heat gain (W)	446.24	466.89	489.78
Efficiency (%)	57.19	62.7	66.9

Figure 10 shows the PTSC exergy efficiency for water, the ZnO water nanofluid, and the MgO water nanofluid. The PTSC exergy efficiency for the working fluids shown in Figure 10 was notably smaller than the thermal efficiency values presented in Figure 9. At solar noon, the peak exergy efficiency recorded by the PTSC was only 13.8% when the MgO nanofluid was used. This value is much lower than the corresponding thermal efficiency of 66.9%, owing to the fact that exergy analysis takes the quality of the energy used into account, with the thermodynamic irreversibilities and entropy generation ignored by thermal efficiency. The modest values attained by exergy reflect the inherent inefficiency of converting the high-exergy solar energy produced by the sun at 5770 K into low-exergy thermal energy usable for work by a working fluid. The processes and equipment used can cause such reductions. The MgO nanofluid continues to perform the best in this analysis because of reducing system irreversibilities by streamlining the heat transfer, but the gap is not large compared to the efficiency of water in exergy terms. This suggests that the sole aspect of the system affected by nanoparticle modifications is the relative energy quantity and quality remaining. Consequently, both values are critical for analyzing the performance of a solar collector.

Figure 10. Exergy efficiency comparison of water and nanofluid working fluids in PTSC over time. 6. Conclusions

This experimental work presents a performance enhancement of a PTSC using ZnO-water and MgO-water nanofluids compared to the base fluid (water) in the Kirkuk climate. It was found that the nanofluid species and flow velocity play important roles in thermal performance. The highest outlet temperature (water = 70.3°C) and maximum heat gain (446.24 W) were achieved at the optimum flow rate of 0.1 L/min, i.e., owing to longer residence time and better absorption of heat.

The MgO-water nanofluid showed a maximum thermal efficiency of 66.9%, or 16.98% higher than that of pure water at noon working conditions, and a higher thermal conductivity of 48.4 W/m). This measurable enhancement indicates the potential of MgO nanoparticles as an effective heat-transfer augmentation material in PTSC systems. While the thermal efficiencies were quite high, the corresponding exergetic performances were moderate (13.8% for MgO), which is inherent when applying thermodynamic properties in solar-thermal conversion.

From a technical perspective, these results have significant implications for large-scale solar thermal adoption at Iraq and other hot-dry sites. Iraq is more than 1800–2200 kWh/m ² yr for enhanced collector efficiency, meaning more thermal output and eventually a smaller system size, for example, industrial heating, desalination, or district hot water pump and pump installation. According to the demonstrated ~17% efficiency increment, it is believed that the use of MgO-based NF for PTSC loops may potentially enhance daily thermal energy production, lower fuel consumption in hybrid systems, and reduce overall life cycle costs. Furthermore, the good stability of both MgO-water and ZnO-water nanofluids for the period of experimentation indicates their applicability under field conditions.

In summary, this study demonstrates that MgO nanofluids flowing at optimum rates represent a viable and cost-effective method to improve the solar-thermal performance in high-irradiance environments and national objectives for sustainable and affordable energy.

To enhance future research, the experimental period should be extended to include other seasons. Multi-season measurements would assist in evaluating the effect of changing solar and ambient conditions on the nanofluid performance and PTSC efficiency, which will present a comprehensive overview of year-round operation.

Ethics and consent statement

This research did not involve human subjects, human tissue, animals or individual personal data. Hence, no ethical approval and informed consent were necessary for this study.

Data availability statement

The underlying data of the current study can be access from https://doi.org/10.5281/zenodo.18029577 ⁴⁹ under the Creative Commons Attribution 4.0 International license (CC-BY 4.0).

Copyright: © 2025 Afrah Turki Awad et al. This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0). This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.

References 1

Ellabban

Abu-Rub

Blaabjerg

: Renewable energy resources: Current status, future prospects and their enabling technology. Renew. Sust. Energ. Rev. 2014;39:748–764. 10.1016/j.rser.2014.07.113

Awad

Taifor

Yaseen

: Enhanced energy storage via one-step preparation of Cuo-nanosalt. Eur. Phys. J. Plus. 2025;140(10):995. 10.1140/epjp/s13360-025-06924-7

Awad

Muayad

: Experimental heat transfer study of an enhanced storage medium. J. Energy Storage. 2023;73:108953. 10.1016/j.est.2023.108953

Elghamry

Hamdy

Hawwash

: A parametric study on the impact of integrating solar cell panel at building envelope on its power, energy consumption, comfort conditions, and CO2 emissions. J. Clean. Prod. 2020;249:119374. 10.1016/j.jclepro.2019.119374

Awad

Ahmed

Waleed

: Nanotechnology for energy storage. Emerging nanotechnologies for renewable energy. Elsevier;2021; pp.495–516. 10.1016/B978-0-12-821346-9.00009-2

Rashid

Dhaidan

Mahdi

: Heat transfer enhancement of phase change materials using tree shaped fins: A comprehensive review. Int. Commun. Heat Mass Transf. 2025;162:108573. 10.1016/j.icheatmasstransfer.2024.108573

Song

Zhang

Chi

: Research on the coordinated optimization of energy storage and renewable energy in off-grid microgrids under new electric power systems. Glob. Energy Interconnect. 2025;8:213–224. 10.1016/j.gloei.2024.12.004

Hachicha

Rodríguez

Capdevila

: Heat transfer analysis and numerical simulation of a parabolic trough solar collector. Appl. Energy. 2013;111:581–592. 10.1016/j.apenergy.2013.04.067

Yuan

: Three-dimensional numerical study of heat transfer characteristics of parabolic trough receiver. Appl. Energy. 2014;113:902–911. 10.1016/j.apenergy.2013.07.050

Coccia

Di Nicola

Colla

: Adoption of nanofluids in low-enthalpy parabolic trough solar collectors: Numerical simulation of the yearly yield. Energy Convers. Manag. 2016;118:306–319. 10.1016/j.enconman.2016.04.013

Koprulu

Kareem

Yaseen

: The Effects Of Temperature, Dust, And Cement-Coal Particles On PV Panel Performance: A Case Study In Iraq. Eurasian J. Sci. Eng. 2025;11(2):357–367. 10.23918/eajse.v11i2p25

Natarajan

Srinivas

: Experimental and simulation studies on a novel gravity based passive tracking system for a linear solar concentrating collector. Renew. Energy. 2017;105:312–323. 10.1016/j.renene.2016.12.060

Murtuza

Byregowda

Imran

: Experimental and simulation studies of parabolic trough collector design for obtaining solar energy. Resour. Eff. Technol. 2017;3(4):414–421. 10.1016/j.reffit.2017.03.003

Hamada

Khalil

Abou Al-Sood

: An experimental investigation of nanofluid, nanocoating, and energy storage materials on the performance of parabolic trough collector. Appl. Therm. Eng. 2023;219:119450. 10.1016/j.applthermaleng.2022.119450

Omer

Ikram

: Effect of addition of zirconium oxide nanoparticles on flexural strength and porosity of heat cure acrylic resin. Al-Kitab Journal for Pure Sciences. 2018;2(2):96–119. 10.32441/kjps.02.02.p7

Kadhim

Hammoodi

Askar

: Feasibility review of using copper oxide nanofluid to improve heat transfer in the double-tube heat exchanger. Results Eng. 2024;24:103227. 10.1016/j.rineng.2024.103227

Raza

Qamar

Noor

: Experimental analysis of thermal performance of direct absorption parabolic trough collector integrating water based nanofluids for sustainable environment applications. Case Stud. Therm. Eng. 2023;49:103366. 10.1016/j.csite.2023.103366

Vijayan

Rajasekaran

: Performance evaluation of nanofluid on parabolic trough solar collector. Therm. Sci. 2020;24(2 Part A):853–864. 10.2298/TSCI180509059G

Khan

Yan

Ali

: Comparative performance assessment of different absorber tube geometries for parabolic trough solar collector using nanofluid. J. Therm. Anal. Calorim. 2020;142:2227–2241. 10.1007/s10973-020-09590-2

Panja

Das

Mahesh

: Numerical study of parabolic trough solar collector’s thermo-hydraulic performance using CuO and Al2O3 nanofluids. Appl. Therm. Eng. 2024;248:123179.

Samiezadeh

Khodaverdian

Doranehgard

: CFD simulation of thermal performance of hybrid oil-Cu-Al2O3 nanofluid flowing through the porous receiver tube inside a finned parabolic trough solar collector. Sustain. Energy Technol. Assess. 2022;50:101888. 10.1016/j.seta.2021.101888

Byiringiro

Chaanaoui

Halimi

: Heat transfer enhancement of a parabolic trough solar collector using innovative receiver configurations combined with a hybrid nanofluid: CFD analysis. Renew. Energy. 2024;233:121169. 10.1016/j.renene.2024.121169

Shaker

Gholinia

Pourfallah

: CFD analysis of Al2O3-syltherm oil Nanofluid on parabolic trough solar collector with a new flange-shaped turbulator model. Theor. Appl. Mech. Lett. 2022;12(2):100323. 10.1016/j.taml.2022.100323

Bozorg

Doranehgard

Hong

: CFD study of heat transfer and fluid flow in a parabolic trough solar receiver with internal annular porous structure and synthetic oil–Al ₂O ₃ nanofluid. Renew. Energy. 2020;145:2598–2614. 10.1016/j.renene.2019.08.042

Hong

Yang

Rashidi

: Numerical simulations of a Cu–water nanofluid-based parabolic-trough solar collector. J. Therm. Anal. Calorim. 2021;143:4183–4195. 10.1007/s10973-020-09386-4

Kaloudis

Papanicolaou

Belessiotis

VJRE

: Numerical simulations of a parabolic trough solar collector with nanofluid using a two-phase model. Renew. Energy. 2016;97:218–229. 10.1016/j.renene.2016.05.046

Ekiciler

Arslan

Turgut

: Effect of hybrid nanofluid on heat transfer performance of parabolic trough solar collector receiver. J. Therm. Anal. Calorim. 2021;143:1637–1654. 10.1007/s10973-020-09717-5

Farooq

Farhan

Ahmad

: Thermal performance enhancement of nanofluids based parabolic trough solar collector (NPTSC) for sustainable environment. Alex. Eng. J. 2022;61(11):8943–8953. 10.1016/j.aej.2022.02.029

Ram

Ganesan

Saini

: Performance assessment of a parabolic trough solar collector using nanofluid and water based on direct absorption. Renew. Energy. 2023;214:11–22. 10.1016/j.renene.2023.06.016

Awad

Hussein

: Influence of Nanofluid Types on the Enhancements of Solar Collector Performance. Int. Innov. J. Appl. Sci. 2024;1(2). 10.61856/f7m6ty09

Abu-Zeid

MAR

Elhenawy

Bassyouni

: Performance enhancement of flat-plate and parabolic trough solar collector using nanofluid for water heating application. Results Eng. 2024;21:101673. 10.1016/j.rineng.2023.101673

Shirole

Wagh

Kulkarni

: Thermal Performance Comparison of Parabolic trough collector (PTC) using various Nanofluids. Int. J. Renew. Energy Dev. 2021;10(4):875–889. 10.14710/ijred.2021.33801

Khaledi

Saedodin

Rostamian

: Energy, hydraulic and exergy analysis of a compound parabolic concentrator using hybrid nanofluid: An experimental study. Int. Commun. Heat Mass Transf. 2022;136:106181. 10.1016/j.icheatmasstransfer.2022.106181

Ajbar

Hernández

Parrales

: Thermal efficiency improvement of parabolic trough solar collector using different kinds of hybrid nanofluids. Case Stud. Therm. Eng. 2023;42:102759. 10.1016/j.csite.2023.102759

Talem

Mihoub

Boumia

: Thermal performance of parabolic trough collector using oil-based metal nanofluids. Appl. Therm. Eng. 2024;256:124128. 10.1016/j.applthermaleng.2024.124128

Zhang

Liang

Zheng

: Advances in enhancing the photothermal performance of nanofluid-based direct absorption solar collectors. Nano. 2025;15(18):1428. 10.3390/nano15181428

Al-Rabeeah

Seres

Farkas

: Experimental investigation of parabolic trough solar collector thermal efficiency enhanced with different absorber coatings. Int. J. Thermofluids. 2023;19:100386. 10.1016/j.ijft.2023.100386

Hussein

Awad

: Solar Collectors with Nanofluid for Domestic Applications: A Review. NTU J. Renew. Energy. 2024;7(1):57–64. 10.56286/dskpxb94

Kadhim

Askar

Saleh

AAM

: An enhancement of double pipe heat exchanger performance at a constant wall temperature using a nanofluid of iron oxide and refrigerant vapor. J. Therm. Eng. 2024;10(1):78–87. 10.18186/thermal.1429191

Khalid

Zakaria

Wan Mohamed

WAN

: Comparative analysis of thermophysical properties of Al2O3 and SiO2 nanofluids. J. Mech. Eng. 2019;1:153–163.

Abdulhamid

Aboul-Enein

Ibrahim

: MgO and ZnO nanofluids passive cooling effects on the electricity production of photovoltaic panels: a comparative study. Eur. Phys. J. Plus. 2024;139(10):864. 10.1140/epjp/s13360-024-05627-9

Husseın

Awad

Alı

HHM

: Evaluation of the thermal efficiency of nanofluid flows in flat plate solar collector. J. Therm. Eng. 2024;10(2):299–307. 10.18186/thermal.1448578

Petela

: Exergy of undiluted thermal radiation. Sol. Energy. 2003;74(6):469–488. 10.1016/S0038-092X(03)00226-3

Garg

: Solar energy: fundamentals and applications. Tata McGraw-Hill Education;2000.

Ibrahim

OAAM

Kadhim

Ali

: Enhancement the solar box cooker performance using steel fibers. Heat Transfer. 2024;53(3):1660–1684. 10.1002/htj.23008

Hammoodi

Dhahad

Alawee

: Energy and exergy analysis of pyramid-type solar still coupled with magnetic and electrical effects by using matlab simulation. Front. Heat Mass Transf. 2024;22(1):217–262. 10.32604/fhmt.2024.047329

Moffat

: Describing the uncertainties in experimental results. Exp. Thermal Fluid Sci. 1988;1(1):3–17. 10.1016/0894-1777(88)90043-X

Hamad

FAW

: The performance of a cylindrical parabolic solar concentrator. Energy Convers. Manag. 1988;28(3):251–256. 10.1016/0196-8904(88)90031-3

Awad

Taifor

Hussein

: The Experimental Development of Solar Collector with Different Types of Nanofluid. f1000research. Zenodo. 2025. 10.5281/zenodo.18029577

10.5256/f1000research.193943.r460497

Reviewer response for version 1

walke

pramod V

1 Referee 1Department of Mechanical Engineering, G H Raisoni College of Engineering, Nagpur, Maharashtra, India

Competing interests: No competing interests were disclosed.

19 3 2026

2026

This is an open access peer review report distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

recommendation

reject

Major Concerns (Technical Issues)

Inconsistency in Nanofluid Preparation:-The manuscript states that Al₂O₃–water nanofluids were prepared, while the study actually investigates ZnO and MgO nanofluids. This is a serious inconsistency.

Table 2 lists the aperture area as 1.85 × 10⁻⁶ mm², which is unrealistic for a solar collector. The unit is likely incorrect and must be verified (probably m².

Experiments were conducted only from May to July, which does not represent annual operating conditions. Seasonal variation should be considered or discussed more thoroughly.

Nanofluid stability was evaluated only visually for 48 hours. More reliable techniques should be used.

The manuscript does not provide detailed regarding solar radiation profiles, hourly irradiation data and uncertainty in irradiance measurement

No statistical validation or repeatability analysis is presented. The results appear to be based on single experimental runs.

Although many references are cited, the research gap is not clearly defined. Authors should explicitly explain.

Some figures lack sufficient explanation. Axes labels and units should be clearer

The paper lacks a list of symbols and abbreviations, which is recommended for technical clarity.

Overall Evaluation:

The manuscript addresses an interesting topic related to nanofluids in solar thermal systems; however, it currently contains several major technical inconsistencies, methodological limitations, and insufficient experimental validation. These issues significantly affect the scientific reliability and clarity of the work.

In its current form, the manuscript is not suitable for indexing. The authors are advised to substantially revise the experimental description, correct technical inconsistencies, improve data validation, and clearly establish the research gap before considering resubmission to a suitable journal.

Is the work clearly and accurately presented and does it cite the current literature?

Partly

If applicable, is the statistical analysis and its interpretation appropriate?

Partly

Are all the source data underlying the results available to ensure full reproducibility?

Is the study design appropriate and is the work technically sound?

Are the conclusions drawn adequately supported by the results?

Are sufficient details of methods and analysis provided to allow replication by others?

Reviewer Expertise:

Thermal Engineering

I confirm that I have read this submission and believe that I have an appropriate level of expertise to state that I do not consider it to be of an acceptable scientific standard, for reasons outlined above.

Awad

Afrah

Northern Technical University, Mosul, Nineveh Governorate, Iraq

Competing interests: No competing interests were disclosed.

22 3 2026

Dear Prof. Walke,

We would like to express our deepest appreciation to you for your thorough evaluation, insightful comments, and the significant time and effort invested in refining our manuscript. The constructive feedback has been invaluable in strengthening the clarity, quality, and scientific rigor of this study. In the following, we present a comprehensive point-by-point response to all the reviewer’s comments.

We gratefully acknowledge your valuable feedback once again, which has significantly contributed to improving the quality of our manuscript.

On behalf of all the authors,

The Corresponding Author

Assist. Prof. Dr. Afrah Turki Awad,

PhD in Mechanical Engineering, University of Leeds, UK

Associate professor at the Northern Technical University, Iraq.

Q1: Reviewers' point:

• Inconsistency in Nanofluid Preparation:-The manuscript states that Al₂O₃–water nanofluids were prepared, while the study actually investigates ZnO and MgO nanofluids. This is a serious inconsistency.

Our Response:

We appreciate this comment. We mean “The ZnO –water and MgO-water nanofluids were”

It is corrected.

………………………………………………………………………………………………………………………..

Q2: Reviewers' point:

• Table 2 lists the aperture area as 1.85 × 10⁻⁶ mm², which is unrealistic for a solar collector. The unit is likely incorrect and must be verified (probably m².

Our Response:

Apologies for this typo error it meant to be 1.85 × 106 mm².

………………………………………………………………………………………………………………………..

Q3: Reviewers' point:

• Experiments were conducted only from May to July, which does not represent annual operating conditions. Seasonal variation should be considered or discussed more thoroughly.

Our Response:

We appreciate your recommendation. We acknowledge that testing only between May and July does not consider seasonal variability in solar and ambient conditions. This limitation is mentioned in the conclusion section, where we stated that it is important for this type of experiment to be extrapolated over multiple seasons.

"To enhance future research, the experimental period should be extended to include other seasons. Multi-season measurements would assist in evaluating the effect of changing solar and ambient conditions on the nanofluid performance and PTSC efficiency, which will present a comprehensive overview of year-round operation."

………………………………………………………………………………………………………………………..

Q4: Reviewers' point:

• Nanofluid stability was evaluated only visually for 48 hours. More reliable techniques should be used.

Our Response:

Thank you for your comment. We conducted a zeta potential to observe the stability of the nanofluids.

The following paragraph has been added before the conclusion section

“In addition, the stability of MgO-water and ZnO-water nanofluids performed by use of Dynamic Light Scattering (DLS) is illustrated in Particle Size Distribution Figures (11-12).

The DLS results indicate a narrow single-peak distribution with average particle size of 100 nm for both nanofluids, confirming homogeneous dispersion and stability of the nanoparticles. In particular, the ZnO-water nanofluid displays a small peak at 10 nm in addition to the main peak at ~100 nm (with sharp peaks for MgO-water). Both graphs show narrow and well-defined peaks around 100 nm, which proves that MgO-water and ZnO-water nanofluids can be termed stable in accordance with the definition mentioned above. The relatively close size distributions indicate negligible aggregation and good MgO and ZnO nanoparticle dispersion in water, thus suitable for utilization in heat transfer, cooling or other relevant nanofluid technologies.”

………………………………………………………………………………………………………………………..

Q5: Reviewers' point:

• The manuscript does not provide detailed regarding solar radiation profiles, hourly irradiation data and uncertainty in irradiance measurement

Our Response:

We appreciate your feedback. We have added the required Figure 13 as requested.

………………………………………………………………………………………………………………………..

Q6: Reviewers' point:

• No statistical validation or repeatability analysis is presented. The results appear to be based on single experimental runs.

Our Response:

Thanks for your precious feedback. We would like to highlight that each experiment was conducted three times in order to guarantee the credibility of the data. Second, we have added error bars to the revised figures (particularly figures (5,6, 9)) in response to your suggestion, reflecting variance from repeated experiments. We hope if this revised version allay your concern.

………………………………………………………………………………………………………………………..

Q7: Reviewers' point:

• Although many references are cited, the research gap is not clearly defined. Authors should explicitly explain.

Our Response:

Thank you for your valuable comment. We know how important it is to clearly state the research gap. We changed the text of introduction to critically refer to the literature and clearly state the research gap that our study aims to fill.

……………………………………………………………………………………………………………………..

Q8: Reviewers' point:

• Some figures lack sufficient explanation. Axes labels and units should be clearer

Our Response:

Thank you for your feedback. The required modifications have been conducted.

………………………………………………………………………………………………………………………..

Q9: Reviewers' point:

• The paper lacks a list of symbols and abbreviations, which is recommended for technical clarity.

Our Response:

Thank you for your suggestion. We have added a list of symbols and abbreviations in the revised manuscript (Table 5).

………………………………………………………………………………………………………………………..

10.5256/f1000research.193943.r460496

Reviewer response for version 1

Bhatti

Muhammad Mubashiri

1 Referee 1North-West University (Mafikeng Campus), Mmabatho, South Africa

Competing interests: No competing interests were disclosed.

16 3 2026

2026

recommendation

reject

Manuscript Title: The Experimental Development of Solar Collector with Different Types of Nanofluid

General Evaluation:

The manuscript presents an experimental investigation of a parabolic trough solar collector (PTSC) operating with pure water, ZnO-water nanofluid, and MgO-water nanofluid under climatic conditions in Kirkuk, Iraq. The study evaluates the influence of nanoparticle type and volumetric flow rate on the thermal and exergetic performance of the collector.

The topic is relevant to the field of solar thermal energy systems and nanofluid heat transfer enhancement. inconsistencies in the experimental description, incorrect physical parameters, and insufficient characterize. However, the manuscript contains several methodological, technical, and presentation issues that significantly weaken its scientific rigor. In particular, addition of nanofluid properties raise concerns regarding the reliability of the results.

Therefore, substantial revision is required before the manuscript can be considered scientifically sound.

Major Comments

1. The manuscript states that “Al 2 O 3 â€“water nanofluids were prepared using a two-step method”. However, the study investigates ZnO-water and MgO-water nanofluids. This inconsistency suggests that part of the experimental description may have been reused from previous work or incorrectly edited. The preparation procedure must clearly correspond to the nanofluids actually used in the experiments.

2. Table 2 reports the aperture area of the collector as 1.85 ×10 -6 mm 2 . This value is physically unrealistic and dimensionally incorrect. Typical parabolic trough collectors have aperture areas on the order of square meters. The authors must verify and correct the collector dimensions and associated units.

3. The manuscript claims nanofluid stability based only on visual observation for 48 hours. Visual inspection alone is insufficient to confirm nanoparticle stability in scientific experiments. Standard characterization methods include:

- Zeta potential measurement

- UV–Vis spectroscopy

- Sedimentation analysis

- Dynamic light scattering (DLS)

Without such analyses, the stability of the nanofluid suspension cannot be properly verified.

4. The thermophysical properties of the nanofluids are estimated using empirical correlations. However, the study does not include experimental measurements of thermal conductivity or specific heat. Since these parameters strongly influence heat transfer enhancement, experimental validation or comparison with literature values should be provided.

5. The manuscript does not consider the viscosity of the nanofluids. This omission is important because viscosity directly affects pumping power, Reynolds number, and pressure drop within the collector system. Without viscosity analysis, the practical applicability of the nanofluid cannot be fully assessed.

6. The validation of the experimental setup is performed using a comparison with data from a study published in 1988. This validation approach is limited and outdated. The authors should compare their results with multiple recent experimental studies to better establish the reliability of the measurements.

7. Although exergy efficiency is calculated using the Petela model, the discussion remains superficial. The manuscript does not adequately analyze the sources of irreversibility such as thermal losses, entropy generation, or optical losses in the collector system.

8. The manuscript provides measurement uncertainties for temperature, irradiance, and flow rate but does not propagate these uncertainties into the calculated performance parameters such as thermal efficiency or heat gain. The final results should include uncertainty bounds or error bars.

9. The manuscript claims that ZnO-water and MgO-water nanofluids have not been compared previously in PTSC applications. However, numerous studies have already investigated metal-oxide nanofluids in solar collectors. The novelty of the present work should be clarified and supported with a more detailed literature analysis.

Minor Comments

1. The thermophysical properties table mixes nanoparticle properties and nanofluid properties in a confusing format. The table should be reorganized to clearly distinguish between base fluid, nanoparticle, and nanofluid properties.

2. The logarithmic temperature term in the exergy equation must use absolute temperature in Kelvin. The manuscript should explicitly clarify the units used in this equation.

3. Numerous grammatical and stylistic issues appear throughout the manuscript. For example, expressions such as “ZnO nanoparticles owns 29 W/mÂ·K” should be corrected. A thorough language revision is recommended.

4. Several parts of the results section repeatedly explain that lower flow rates increase fluid residence time and heat absorption. The discussion could be condensed to avoid redundancy.

5. The figures presenting outlet temperature, heat gain, and efficiency do not include error bars or uncertainty ranges. Including these would strengthen the reliability of the experimental conclusions.

6. Units such as L/min and l/min are used inconsistently. Additionally, spacing and formatting of symbols (e.g., W/mÂ·K) should be standardized throughout the manuscript.

7. The introduction lists numerous references but does not critically analyze the existing literature. The authors should more clearly identify the research gap addressed by the present study.

8. The conclusion claims that MgO nanofluid represents a cost-effective solution for large-scale solar systems. However, no economic or cost analysis is provided. Such claims should be moderated unless supported by additional analysis.

Recommendation:

The manuscript addresses an important topic in solar thermal energy systems. However, the presence of several methodological inconsistencies and insufficient experimental characterization currently limits its scientific robustness.

Is the work clearly and accurately presented and does it cite the current literature?

Partly

If applicable, is the statistical analysis and its interpretation appropriate?

Not applicable

Are all the source data underlying the results available to ensure full reproducibility?

Partly

Is the study design appropriate and is the work technically sound?

Partly

Are the conclusions drawn adequately supported by the results?

Partly

Are sufficient details of methods and analysis provided to allow replication by others?

Partly

Reviewer Expertise:

CFD and Material Science

Awad

Afrah

Northern Technical University, Mosul, Nineveh Governorate, Iraq

Competing interests: No competing interests were disclosed.

22 3 2026

Dear Assoc. Prof. Bhatti,

We sincerely thank you for your valuable insights and for the considerable time and effort devoted to improving our manuscript. We are truly grateful for the constructive comments, which have significantly contributed to enhancing the quality of this work. Below, we provide a detailed point-by-point response addressing all the comments raised.

We would like to express our sincere gratitude once again for your valuable and constructive feedback.

On behalf of all the authors,

The Corresponding Author

Assist. Prof. Dr. Afrah Turki Awad,

PhD in Mechanical Engineering, University of Leeds, UK

Associate professor at the Northern Technical University, Iraq.

General Evaluation:

Q1: Reviewers' point:

1. The manuscript states that “Al 2O 3â€“water nanofluids were prepared using a two-step method”. However, the study investigates ZnO-water and MgO-water nanofluids. This inconsistency suggests that part of the experimental description may have been reused from previous work or incorrectly edited. The preparation procedure must clearly correspond to the nanofluids actually used in the experiments.

Our Response:

We appreciate this comment. We mean “The ZnO –water and MgO-water nanofluids were”

It is corrected.

………………………………………………………………………………………………………………………..

Q2: Reviewers' point:

2. Table 2 reports the aperture area of the collector as 1.85×10-6 mm 2. This value is physically unrealistic and dimensionally incorrect. Typical parabolic trough collectors have aperture areas on the order of square meters. The authors must verify and correct the collector dimensions and associated units.

Our Response:

Apologies for this typo error it meant to be 1.85 × 106 mm².

………………………………………………………………………………………………………………………..

Q3: Reviewers' point:

- Zeta potential measurement

- UV–Vis spectroscopy

- Sedimentation analysis

- Dynamic light scattering (DLS)

Without such analyses, the stability of the nanofluid suspension cannot be properly verified.

Our Response:

Thank you for your comment. We conducted a zeta potential to observe the stability of the nanofluids.

The following paragraph has been added before the conclusion section

“In addition, the stability of MgO-water and ZnO-water nanofluids performed by use of Dynamic Light Scattering (DLS) is illustrated in Particle Size Distribution Figures (11-12).

………………………………………………………………………………………………………………………..

Q4: Reviewers' point:

Our Response:

We appreciate your comment. Table 1 has been updated, and the calculated data have been validated against the literature.

We apologize for not being able to perform experimental measurements of thermal conductivity or specific heat in this study, as we did not have access to an accurate device for such measurements. As a result, we relied on equations (2 and 3) to calculate the specific heat capacity and thermal conductivity, which is a widely accepted approach in the literature for nanofluid systems. Additionally, we have included experimental validation of these properties against the literature in Table 1.

………………………………………………………………………………………………………………………..

Q5: Reviewers' point:

Our Response:

Thank you for your valuable suggestion. We have now included the viscosity calculation in Table 1, and equation 4. We appreciate your suggestion and hope that this addition addresses your concern regarding the viscosity estimation.

………………………………………………………………………………………………………………………..

Q6: Reviewers' point:

Our Response:

We really appreciate your suggested, thank you. Another reference has also been added to support the validation, and this section has been lengthened.

The following paragraph is added: “It was found that when 0.1% CuO nanofluid was used as HTF, efficiency increased up to 69.07%, which is somewhat close to the maximum efficiency (66.9%) we observed in the current study, seeming all these results directed toward the observations made during our research study [22]. The differences in values could be explained by differing nanofluid concentrations (0.2% used in our study as opposed to 0.1% in the reference 53) and different nanoparticles (MgO nanofluid in this paper against CuO nanofluid investigated in the referenced research53.

………………………………………………………………………………………………………………………..

Q7: Reviewers' point:

Our Response:

We appreciate your constructive comments. Thank you for your suggestion to provide a more in-depth analysis of the sources of irreversibility present in the collector system. In response, we have elaborated and highlighted a more comprehensive discussion of the following factors in the manuscript: thermal losses, entropy production, and optical losses.

After figure 10, we added this section " The exergy efficiency is calculated using the Petela model in this study, which provides a convenient basis for assessing the performance of the system. However, it is clear that instead of being directly proportional to the efficiency of exergy conversion in our system, a more detailed view about the sources of irreversibility is required. These sources include thermal losses and entropy generation as well as optical losses, which all play a major role in the inefficiencies of the system.

Firstly, the thermal losses in the collector system are mainly caused by heat dissipation to the environment, resulting in lower useful energy that can be extracted and accumulated. These losses happen at the collector surface, storage medium, and conduction in the rest of the system. We found that decreasing the temperature difference between the collector and surrounding air can greatly increase system efficiency. Improving insulation or finding a way to store heat more effectively could help counteract such thermal losses.

Secondly, entropy production, the second irreversible reaction within a system, is an unavoidable process of energy transfer or transformation. In our evaluation, we found that the factors affecting entropy generation in the collector are irreversible heat transfer and fluid friction processes. Having temperature gradients between the collector surface and the working fluid increases entropy production which negatively impacts overall exergy efficiency. This means that to minimize entropy generation the optimal heat transfer characteristics must be exploited while simultaneously minimizing irreversibilities in fluid flow paths throughout the system.

Thirdly, the optical losses in the collector of solar radiation are mainly due to reflection and absorption inefficiencies and imperfections in transmission through the collector. This affects the overall amount of solar energy can be harnessed into thermal energy. The collector surface and the glass cover loses heat by reflection, reduces those losses can be done with the use of anti-reflecting coatings or materials that are better transmitters. The exergy efficiency of the entire system can thus be improved by increasing collector optical efficiency.

Finally, irreversibility causes like thermal losses, entropy production, optical losses strongly reduces the collector system exergy efficiency. By doing so, all these factors will get addressed through design improvements and optimization strategies, increasing the overall system performance while at the same time eliminating inefficiencies."

……………………………………………………………………………………………………………………..

Q8: Reviewers' point:

Our Response:

Thank you for your valuable suggestion. We agree that including error bars or uncertainty ranges would enhance the reliability of the experimental. In response, we have updated the figures (5,6, 9) to include error bars, reflecting the experimental uncertainties.

………………………………………………………………………………………………………………………..

Q9: Reviewers' point:

Our Response:

We appreciate the reviewer’s valuable comment regarding the novelty of our work and agree that metal‐oxide nanofluids have been studied in various solar collector applications. To clarify our contribution, we have now revised the manuscript to position our study more precisely within the existing literature.

We added this paragraph into the introduction section " Despite several studies reported on the efficacy of metal‐oxide nanofluids in solar collectors (e.g., Alsagri and Alrobaian 2026 15), little attention has been directed toward this comparative performance of ZnO–water against MgO–water nanofluids under the same operating conditions in parabolic trough solar collectors (PTSC). The effects of individual nanofluids, including ZnO overcoming thermal performance in solar collectors have been explored, yet no studies exist that directly compare the exergy and energy of the two forms of nanofluid, ZnO-water to that of MgO-water within the same experimental setup.

On the other hand, a direct comparison of ZnO–water and MgO–water nanofluids has not yet been shown in terms of thermal and exergy performance for a PTSC system. Our study addresses this gap in the existing literature by performing a comprehensive investigation of these two particular nanofluids under unified conditions, which we trust will help guide optimal selection of nanofluids to enhance PTSC performance."

………………………………………………………………………………………………………………………..

Minor Comments

Q1: Reviewers' point:

Our Response:

Thank you for this comment.

The table has been reorganized into clearly defined sections (base fluid, and nanofluids) within a unified format to improve clarity.

………………………………………………………………………………………………………………………..

Q2: Reviewers' point:

2. The logarithmic temperature term in the exergy equation must use absolute temperature in Kelvin. The manuscript should explicitly clarify the units used in this equation.

Our Response:

Thank you for this comment the following sentence is added to clarify the unit of temperature in Equation 8:

“T represents the temperature measured in Kelvin”.

………………………………………………………………………………………………………………………..

Q3: Reviewers' point:

Our Response:

Thank you for your feedback. We have carefully revised the manuscript to address the grammatical and stylistic issues, including the example you provided. The expression “ZnO nanoparticles owns 29 W/m·K” has been corrected to “ZnO nanoparticles have a thermal conductivity of 29 W/m·K.”

………………………………………………………………………………………………………………………..

Q4: Reviewers' point:

4. Several parts of the results section repeatedly explain that lower flow rates increase fluid residence time and heat absorption. The discussion could be condensed to avoid redundancy.

Our Response:

Thank you for your valuable comment. We have revised the results section to condense the discussion and eliminate redundancy.

………………………………………………………………………………………………………………………..

Q5: Reviewers' point:

Our Response:

………………………………………………………………………………………………………………………..

Q6: Reviewers' point:

6. Units such as L/min and l/min are used inconsistently. Additionally, spacing and formatting of symbols (e.g., W/mÂ·K) should be standardized throughout the manuscript.

Our Response:

Thank you for this comment the units are checked thought the manuscript to ensure the consistence.

………………………………………………………………………………………………………………………..

Q7: Reviewers' point:

7. The introduction lists numerous references but does not critically analyze the existing literature. The authors should more clearly identify the research gap addressed by the present study.

Our Response:

Thank you for your valuable feedback. In response, we have revised the introduction to not only include a more comprehensive review of the relevant studies but also to explicitly highlight the gaps that have not been sufficiently addressed in the current literature.

While many studies have explored various aspects of parabolic trough solar collector, few have directly focused on energy and exergy analysis of nanofluid in parabolic solar collector. In current study, we provided an experimental study energy and exergy analysis for MgO-nanofluid and ZnO-nanofluid under identical climate conditions. Our revised introduction now clearly explains how our study uniquely addresses this gap. We hope this revision better aligns with your expectations.

………………………………………………………………………………………………………………………..

Q8: Reviewers' point:

Our Response:

Thank you for your valuable comment. We acknowledge that a detailed economic or cost analysis was not included in the study. We have added the following sentences into the conclusion section "Although MgO nanofluids have shown notable thermal performance, their viability as an economical fluid in large-scale solar systems needs to be further economically analyzed for a more accurate understanding of the cost-effectiveness."

………………………………………………………………………………………………………………………..

10.5256/f1000research.193943.r458165

Reviewer response for version 1

Tayebi

Tahar

1 Referee https://orcid.org/0000-0001-5524-385X 1Laboratory of Innovative Materials for Energy, Environment, and Sustainable Development, University Mohamed El Bachir El Ibrahimi of Bordj Bou Arreridj, El-Anasser, Bordj Bou Arreridj, Algeria

Competing interests: No competing interests were disclosed.

25 2 2026

2026

recommendation

approve-with-reservations

The manuscript presents an experimental investigation of a parabolic trough solar collector (PTSC) operating under Kirkuk (Iraq) climate conditions using Pure water, 0.2 wt.% ZnO-water nanofluid and 0.2 wt.% MgO-water nanofluid. The topic is relevant and experimentally oriented. However, several methodological and technical weaknesses must be addressed before the article can be considered scientifically robust:

- "The Al2O3-water nanofluids were prepared…" However, the study investigates ZnO and MgO nanofluids, not Al2O3.

- The manuscript claims nanofluid stability based only on visual observation for 48 hours. This is not sufficient for a scientific study.

- No validation of calculated properties is provided.

- No experimental measurement of thermal conductivity.

- No viscosity estimation is provided (important for pumping power).

- Density and Cp values in Table 1 are confusing and inconsistently formatted.

- Table 2 lists: Aperture Area = 1.85 × 10-6 mm². This is clearly incorrect (dimensionally and physically unrealistic).

- Clarify temperature units in Equation 8

- Validation against Hamad (1988) is insufficient.

- Language editing required throughout (grammar inconsistencies).

Is the work clearly and accurately presented and does it cite the current literature?

Yes

If applicable, is the statistical analysis and its interpretation appropriate?

Yes

Are all the source data underlying the results available to ensure full reproducibility?

Yes

Is the study design appropriate and is the work technically sound?

Partly

Are the conclusions drawn adequately supported by the results?

Yes

Are sufficient details of methods and analysis provided to allow replication by others?

Yes

Reviewer Expertise:

Heat and mass transfer. Nanofluids, Entropy generation, Porous medium, Natural convection, Heat storage, CFD.

I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.

Awad

Afrah

Northern Technical University, Mosul, Nineveh Governorate, Iraq

Competing interests: No competing interests were disclosed.

22 3 2026

Dear Prof. Tayebi,

We greatly appreciate the reviewer's insights and the time and effort invested in shaping the revised version of our manuscript. Below is a detailed point-by-point response, addressing all the points raised by the reviewer.

Thank you once again for your valuable feedback.

On behalf of all the authors,

The Corresponding Author

Assist. Prof. Dr. Afrah Turki Awad,

PhD in Mechanical Engineering, University of Leeds, UK

Associate professor at the Northern Technical University, Iraq.

Q1: Reviewers' point:

- "The Al2O3-water nanofluids were prepared…" However, the study investigates ZnO and MgO nanofluids, not Al2O3.

Our Response:

We appreciate this comment. We mean “The ZnO –water and MgO-water nanofluids were”. It is corrected.

………………………………………………………………………………………………………………………..

Q2: Reviewers' point:

- The manuscript claims nanofluid stability based only on visual observation for 48 hours. This is not sufficient for a scientific study.

Our Response:

Thank you for your comment. We conducted a zeta potential to observe the stability of the nanofluids.

The following paragraph has been added before the conclusion section

“In addition, the stability of MgO-water and ZnO-water nanofluids performed by use of Dynamic Light Scattering (DLS) is illustrated in Particle Size Distribution Figures (11-12).

………………………………………………………………………………………………………………………..

Q3: Reviewers' point:

- No validation of calculated properties is provided.

Our Response:

We appreciate your comment. Table 1 has been updated, and the calculated data have been validated against the literature

………………………………………………………………………………………………………………………..

Q4: Reviewers' point:

- No experimental measurement of thermal conductivity.

Our Response:

Thank you for your valuable comment. We apologize for not being able to perform experimental measurements of thermal conductivity in this study, as we did not have access to an accurate device for such measurements. As a result, we relied on equation (3) to calculate the thermal conductivity, which is a widely accepted approach in the literature for nanofluid systems. Additionally, we have included experimental validation of these properties against the literature in Table 1.

………………………………………………………………………………………………………………………..

Q5: Reviewers' point:

- No viscosity estimation is provided (important for pumping power).

Our Response:

………………………………………………………………………………………………………………………..

Q6: Reviewers' point:

- Density and Cp values in Table 1 are confusing and inconsistently formatted.

Our Response:

Thank you for your comment. The density and specific heat capacity (Cp) values in Table 1 were calculated using equations (1-2), based on the density and Cp of water (the base fluid) and the corresponding values for the nanoparticles, as reported in the referenced studies. We acknowledge the formatting issue and will revise the table to ensure consistency and clarity. Additionally, the calculated data have been validated against the literature to ensure accuracy.

………………………………………………………………………………………………………………………..

Q7: Reviewers' point:

- Table 2 lists: Aperture Area = 1.85 × 10-6 mm². This is clearly incorrect (dimensionally and physically unrealistic).

Our Response:

Apologies for this typo error it meant to be 1.85 × 106 mm².

……………………………………………………………………………………………………………………..

Q8: Reviewers' point:

- Clarify temperature units in Equation 8

Our Response:

Thank you for this comment the following sentence is added to clarify the unit of temperature in Equation 8:

“T represents the temperature measured in Kelvin"

………………………………………………………………………………………………………………………..

Q9: Reviewers' point:

Validation against Hamad (1988) is insufficient

Our Response:

We really appreciate your suggested, thank you. Another reference has also been added to support the validation, and this section has been lengthened.

………………………………………………………………………………………………………………………..

Q10: Reviewers' point:

- Language editing required throughout (grammar inconsistencies).

Our Response:

Proofreading is conducted throughout the manuscript.

………………………………………………………………………………………………………………………..