Enhancing Parabolic Trough Collector Performance Through Surface Treatment: A Comparative Experimental Analysis ^†

Abstract

Parabolic trough collectors (PTCs) are effective solar thermal systems, but their performance can be significantly enhanced through surface treatments. This research investigates the enhancement of thermal performance in parabolic trough collectors (PTCs) by experimentally evaluating the results of surface coating on the absorber tube surface. To achieve this objective, a closed-loop PTC system was fabricated to conduct an experimental comparison between a conventional simple copper tube and a black-painted copper tube. The experimental setup was placed in Islamabad, Pakistan, operated under both laminar and turbulent flow conditions to measure key performance metrics, of temperature difference (ΔT) between the inlet and outlet. The results demonstrate a significant performance advantage for the black-painted tube. In laminar flow, the black-painted tube achieved an average ΔT of 3.54 °C, compared to 2.11 °C for the simple copper tube. Similarly, in turbulent flow, the black-painted tube’s ΔT was 2.1 °C, surpassing the simple copper tube’s 1.57 °C. This superior performance is primarily attributed to the black surface’s high solar absorptivity, which more effectively captures and converts solar radiation into thermal energy. The findings highlight the critical role of surface treatment in optimizing PTC efficiency and provide a practical method for improving solar thermal energy systems.

1. Introduction

The world energy demand is increasing exponentially, and most nations have begun to implement renewable energy technologies due to continuously deteriorating non-renewable energy sources [1]. Fossil fuel-based water heating systems contribute to greenhouse gas emissions, such as carbon dioxide (CO₂), which cause climate change [2]. Warming spaces and heating domestic water are among the vital needs of these systems and in the majority of cases, they utilize electric energy which is rather expensive.

In this respect, parabolic trough collectors (PTC) could be used directly for water heating purposes [3]. The PTCs are line-focus concentrators to drive solar radiation onto the collector axis. These collectors have massive applications in concentrated power technology which is used to create power and even in generating heat that may be utilized by several processes and other industries as well [4]. The technology of PTC is evidently adopted by many industries such as desalination, water purification and food processing due to the efficient transfer of heat and possibility to meet the heating needs [5]. Alghamdi et al. [6] studied the effect of selective coatings on efficiency and conducted a detailed performance analysis of PTSCs using 1D analytical, 2D, and 3D numerical models. The study examined the impact of two types of selective coatings—cermet and black chrome—on the absorber tube. The results of black chrome were superior. Research by Suppan et al. [7] conducted on absorber surface material variation experimentally analyzed PTC with different absorber surface materials including aluminum sheets, aluminum foils, and mirror stickers. Their results indicated that the aluminum sheet absorber achieved the highest outlet temperatures and heat gain, suggesting superior performance in terms of thermal energy conversion. A broader review by Alamr et al. [8] on advanced coatings and nanofluids emphasized the role of surface coatings and nanofluids in enhancing PTC performance. Selective coatings such as black nickel and black chrome were found effective in minimizing radiative heat losses. The authors also discussed the use of nanofluids like Al₂O₃–water and CuO–water mixtures as working fluids. These nanofluids conducted heat more efficiently than traditional heat transfer fluids, and when combined with coated absorbers, provided an efficiency enhancement of up to 4.3%. The integration of hybrid nanofluids and selective coatings presents a promising direction for future improvements in PTSC performance. Research by Olson and Talghader et al. [9] focused on optimizing solar selective coatings for direct steam generation systems by creating a non-ideal spectral, environmental, and material property model. Their goal was to move beyond idealized models to more accurately predict the performance of real-world systems. The study concluded that while an ideal system’s optimum transition wavelength is 1.4 μm, a more realistic model shifts this to 3.4 μm, which is a critical insight for designing cost-effective and efficient systems. Yang et al. [10] developed a comprehensive spectral optimization model to analyze the effect of coating properties on the performance of parabolic trough receivers. They specifically investigated the cutoff wavelength of solar selective absorbing coatings and its relationship with operational parameters. The key finding was that the optimal cutoff wavelength increases with higher solar irradiation flux but decreases as the absorber temperature rises, with thermal efficiency being sensitive to the coating’s width.

Kumaresan et al. [11] provided a detailed review of various experimental and numerical studies aimed at enhancing thermal performance in the receiver part of PTC. The paper outlined a strategy of exploring different physical modifications and material properties to improve heat transfer, such as that the use of black chrome coating has high absorbance when compared to other selective coatings.

The literature reports several studies aiming to enhance the thermal performance of the receiver tube using different types of coating. In this study we have proposed a simple and effective way of enhancing the thermal performance by painting the receiver tube using black paint which is a good absorbent. The primary objective of the present study is to investigate the performance enhancement of a solar parabolic trough collector (PTC) by coating the receiver surface with black paint. The performance of the treated tube is then experimentally compared against a baseline configuration, which utilizes a simple, uncoated copper tube. The core performance metric targeted in this investigation is the maximization of ΔT, defined as the temperature difference between the fluid inlet and outlet of the absorber tube. This comprehensive analysis aims to provide a robust understanding of the effectiveness of surface modifications in optimizing the thermal efficiency and overall output of SPTCs.

2. Problem Formulation and Experimental Procedure

This study aims to enhance the thermal performance of the receiver of the PTC using coating of black paint on the receiver tube. The unpainted copper tube is considered as the reference base case. The objective of the PTC is to heat the water; therefore, the working fluid is water with the thermophysical properties mentioned in

Table 1. Thermophysical properties of water.

Name	Property	Value	Unit
Density	P	997	Kg/m3
Specific Heat Capacity	Cp	4.18	KJ/Kg·k
Dynamic Viscosity	µ	0.890	mPa·s
Boiling Point	Tb	100	°C

. The geometric dimensions of the PTC are taken from the reference studies of Tabassum et al. [12] and Byiringiro et al. [13]. The solar energy which is received by the collector is transferred to the water in the absorber tube where it circulates in the tube.

The experimental system comprises a parabolic trough collector which is composed of a chromium sheet that plays the role of a mirror with an aperture area of 1.30 m² where the incoming solar radiation is focused on the absorber tube. The connection of the absorber tube to the tank, flowrate meter and the pump requires a plastic flexible tube that is insulated. The dimensional details of the parabolic trough collector setup are mentioned in

Table 2. Different parameters and their values for the fabricated PTC.

Parameter	Notation and Unit	Value
Length of Parabolic Collector	Lp (m)	1.91
Length of Copper Tube	Lt (m)	1.91
Rim Angle	Ψ (degrees)	45–90
Collector Tube Outer Diameter	do (m)	0.026
Collector Tube Internal Diameter	di (m)	0.025
Aperture Area	Ap (m2)	1.30
Concentration Ratio	CR	8.33
Thermal Conductivity of Copper	K (W/m·K)	401
Absorptivity of Black-Painted Copper Tube	α	0.95
Chromium Reflectance	ρ	0.90

.

Various devices and their values for the fabricated PTC are shown in

Table 3. Different devices and their values for the fabricated PTC.

Variable	Device	Value	Accuracy	Rangle
Temperature, °C	Digital Thermometer	1.91	±0.1	−50–110
Solar Radiation W/m2	Light Dependent Resistor	1.91	±10	0–2000
Water Flowrate L/m	Digital Turbine Water Flow Sensor	45–90	±0.5	1–30
Dimensions (m)	Measuring Tape	0.026	±1	0–100

.

The schematic diagram and experimental setup of PTC are shown in Figure 1.

The solar parabolic trough collector (PTC) is operated in a closed-loop configuration, where water was pumped from the storage tank through the receiver tube and back into the storage tank. A DC pump maintained the circulation, moving the fluid through connecting pipes between the tank and the collector’s absorber tube. To monitor thermal performance, temperature sensors were strategically placed at the inlet and outlet of the absorber tube, with an additional sensor located inside the storage tank. A digital turbine flow sensor measured the volumetric flow rate of the water as it passed through the absorber tube. To account for environmental conditions, an ambient temperature sensor was positioned next to the experimental setup. All sensor readings were recorded and logged into a Microsoft Excel spreadsheet for data analysis. This experiment was carried out at the Capital University of Science and Technology, Islamabad (longitude 33.6996°, latitude 73.0362°).

2.1. Tilt Angle Calculations

The Sun’s position in the sky is constantly changing, both throughout the day and from one season to the next. The position of the Sun can easily be predicted at any given time because the Earth and Sun move in a very systematic and predictable way. This predictability is especially important for solar energy systems like parabolic solar troughs, which need to be positioned precisely to capture the most sunlight. For this the formulas that calculate the Sun’s location at a particular time and on a specific day are as follows:

2.1.1. Declination Angle

The solar declination is the angular measure of the Sun’s rays to the north (or south) of the equator with north declination assigned the positive and south declination the negative value, where N represents the number of day of the year [13].

$$\delta =23.45\mathit{sin}\left[{\displaystyle \frac{360}{365}}\right]\left(284+N\right)$$

(1)

2.1.2. Solar Altitude Angle

The formula for the solar angle of altitude is a mathematical equation that changes based on the local latitude, which is the angle between a line from the Earth’s center through your location and the equatorial plane; latitudes north of the equator are positive, and those south of it are negative [13].

$$\alpha =\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{s}\mathrm{i}\mathrm{n}(\mathit{sin}L\mathit{sin}\delta +\mathit{cos}L\mathit{cos}\delta \mathit{cos}h)$$

(2)

2.1.3. Solar Azimuth Angle

The solar azimuth angle is the horizontal angle of the Sun’s rays, measured from true south in the Northern Hemisphere or north in the Southern Hemisphere, with westward angles considered positive [13].

$$z=\arcsin \left({\displaystyle \frac{cos \delta \mathit{sin}h}{\mathit{cos}\alpha }}\right)$$

(3)

2.1.4. Angle of Incidence

In the case of solar-concentrating collectors, which is being rotated in east-west axis continuously to obtain the maximum beam radiations. The angle of incidence for such a solar-concentrator collector is found as [13].

$$\theta =\arccos \sqrt{1-{cos}^2\left(\delta \right) {sin}^2\left(h\right)}$$

(4)

The optimal angle of incidence throughout the day is given in

Table 4. Optimal angles throughout the day.

Time	$\mathbf{Tilt} \mathbf{Angle} \mathbf{in} \mathbf{Degrees} \left(\mathit{\theta }\right)$
9:00	41.41
9:30	35.23
10:00	29.17
10:30	23.37
10:45	20.64
11:00	18.09
11:45	15.83
12:00	13.99
12:15	12.76
12:30	12.33
12:45	12.76
13:00	13.99
13:15	15.83
13:30	18.09
13:45	20.64
14:00	23.37
14:30	26.23
15:00	29.17

. At these calculated angles, the parabolic solar trough is at the optimum position that maximum solar radiations are being struck on to the parabolic collector. Graphical representation of optimal tilt angle throughout the day is shown in Figure 2.

3. Results and Discussions

This study compared the performance of a simple copper absorber tube with a black-painted copper absorber tube integrated into a parabolic solar trough system, delving into their underlying physical explanations. The primary metrics for comparison included the temperature difference across the absorber tube (ΔT), the maximum temperature achieved in the storage tank, and the influence of solar irradiance.

The experiments for both laminar and turbulent regimes were performed and compared for both the simple copper tube and black-painted copper tube. The experiment were performed in a closed loop, and a DC pump circulated fluid from a tank through the absorber tube of a collector via connecting pipes. To track the system’s thermal performance, temperature sensors were installed at the inlet and outlet of the absorber tube, as well as inside the storage tank. The volumetric flow rate of the water was measured by a digital turbine flow sensor. An additional sensor was placed near the setup to record the ambient temperature. All data from these sensors were recorded and logged into a Microsoft Excel spreadsheet for data analysis.

3.1. Laminar Flow

In laminar settings, the average Reynold number was 715 and the average irradiance was 850 W/m². Average ambient temperature was at 30 °C and the duration of the experiment was 4–6 h. Inlet, outlet and ambient temperature readings in laminar flow for simple copper tube are shown in Figure 3.

The simple, unpainted copper tube exhibited an average ΔT temperature difference between the inlet and outlet of 2.11 °C. Inlet, outlet and ambient temperature readings for the case of laminar flow using black copper tube are shown in Figure 4.

The black-painted copper tube exhibited an average ΔT temperature difference between inlet and outlet of 3.54 °C.

3.2. Turbulent Flow

In turbulent settings, the flowrate was kept high, the average Reynold number was 4900 and the average irradiance was 850 W/m². Average ambient temperature was 34.5 °C and duration of the experiment was 4–5 h. Inlet, outlet and ambient temperature readings for the case of turbulent flow using the simple copper tube are shown in Figure 5.

The simple, unpainted copper tube exhibited an average ΔT temperature difference between inlet and outlet of 1.57 °C. Inlet, outlet and ambient temperature readings for the case of turbulent flow using the black copper tube are shown in Figure 6.

The black-painted copper tube exhibited an average ΔT temperature difference between inlet and outlet of 2.1 °C.

Regarding the temperature difference (ΔT) between the inlet and the outlet, the black-painted copper tube consistently demonstrated superior performance indicating a more efficient heat transfer to the working fluid. In contrast, the simple, unpainted copper tube exhibited low ΔT values. This significant temperature difference was because of the materials differing in solar absorptivity values. A simple copper surface, with its inherent metallic luster, reflects a considerable portion of incident solar radiation, leading to lower absorption. Conversely, a black-painted surface was used for high solar absorptivity, typically ranging from 90% to 95%. This enabled it to absorb a much larger fraction of the incoming solar energy, converting it more effectively into heat which was transferred to the fluid circulating within the tube.

Results show that the black-painted copper tube consistently achieved a higher temperature difference as shown in the Figure 7.

Enhanced heat absorption of the black-painted tube directly resulted in higher maximum temperatures achieved in the storage tank. The black-painted copper tube consistently elevated the tank’s temperature. This observation is a direct consequence of the higher ΔT experienced in the black-painted tube. Since it absorbed solar energy more efficiently and raised the fluid’s temperature as it flows through the absorber, it delivered hot fluid to the storage tank, leading to higher overall system temperatures.

This difference was particularly noticeable during laminar flow tests. Lower flow rates in this regime could allow for more extended heat absorption if heat losses were effectively managed. Conversely, turbulent flow involves chaotic, disordered fluid motion, which significantly enhanced the convective heat transfer due to increased mixing. For a given heat input, higher flow rates (often associated with turbulent flow) typically lead to a lower ΔT, but a higher total heat collected per unit time, as a larger mass of fluid was being heated, despite the black-painted tube consistently achieving higher ΔT values. This indicated the enhancement in solar energy absorption was more because of black paint than the changes being made in water flow (laminar and turbulent) for both tube types.

4. Conclusions

This research experimentally evaluated the impact of absorber tube surface treatments on the thermal performance of parabolic solar trough collectors (PTCs). The primary objective was to enhance PTC efficiency for advancing renewable energy applications. A closed-loop PTC system was fabricated to compare the performances of a simple copper tube and a black-painted copper tube.

The results demonstrated a significant performance advantage for the black-painted tube. In laminar flow, the black-painted tube achieved an average ΔT of 3.54 °C, compared to 2.11 °C for the unpainted tube. Similarly, in turbulent flow, the black-painted tube’s ΔT was 2.1 °C, surpassing the unpainted tube’s 1.57 °C. This superior performance was primarily attributed to the black surface’s high solar absorptivity, which more effectively captures and converts solar radiation into thermal energy.

This improved heat absorption leads to a higher storage tank temperature. A significant difference in performance was because the black-painted surface absorbed more sunlight and turned it into useful heat more effectively. These findings underscore the critical role of surface treatment in optimizing solar thermal collector efficiency.