An experimental analysis of the performance of direct absorption parabolic trough collectors with laser-processed copper surfaces

Abstract

The growing demand for renewable energy sources has made solar energy an ideal choice due to its unlimited and readily available nature. Direct and indirect absorption parabolic solar collectors are widely used to harness solar energy for water heating in laboratory and industrial applications. In this research, modifications were made to the absorber tube to enhance the efficiency of a Direct Absorption Parabolic Trough Collector (DAPTC). Fiber laser processing is applied to copper surfaces placed at the center of the absorber tube. The spectroscopy technique was used to analyze the main laser parameters, including frequency, speed, and power. It was determined that applying a laser with a frequency of 20 kHz, a speed of 20 mm/s, and 80% of the power of a 30 W laser machine significantly reduces surface reflection. Various patterns, including horizontal lines, vertical lines, and circle patterns, were engraved on the surfaces to evaluate their impact on heat transfer and thermal efficiency improvement. The findings are presented based on an experimental approach, wherein a system was constructed to investigate these effects, and the relevant considerations are discussed. The results indicate that using an unprocessed copper surface increases the thermal efficiency of the collector by up to 32.83%. When laser-processed surfaces with different patterns are used, this increase reaches up to 38.99%. The maximum pressure drop across these surfaces is reported to be 247 Pa, and the performance index ranges between 0.84 and 1.06, indicating the effective operation of the collector using these surfaces.

Introduction

The solar collector, a fundamental component of solar energy and water heating systems, functions as an environmentally friendly heat exchanger¹. The primary function of a solar collector is to absorb solar energy and transfer it to the heat transfer fluid within the collector². Among various types of solar collectors, concentrating collectors, which focus solar radiation onto smaller absorber surfaces, are particularly suitable for high-temperature applications. Parabolic trough and dish collectors are the most effective examples, achieving significantly higher temperatures than flat-plate or non-concentrating collectors³. In this study, a direct absorption parabolic trough collector (DAPTC) is employed, where the heat transfer fluid directly absorbs solar energy⁴. Unlike indirect absorption collectors, a copper tube is not used for fluid transfer. Instead, the transparency of the glass tube allows solar radiation to penetrate and interact with the fluid throughout its volume⁵.

Despite these advantages, the thermal efficiency of a DAPTC with water as the heat transfer fluid and high-transparency glass as the absorber tube is relatively low. Mashhadian et al.⁶ and Heyhat et al.⁷ reported maximum thermal efficiencies of 21.85% and 25.09%, respectively, under similar conditions. Enhancing the thermal efficiency of these collectors requires modifications to the absorber tube, such as combining surface and volumetric absorption. Qin et al.⁸ investigated the application of a semi-cylindrical absorber coating on the external surface of the tube through a numerical analysis. Their results showed that this configuration significantly improved thermal efficiency over a wide range of inlet temperatures compared to a reflective surface. Furthermore, when the absorber coating was applied to the internal surface of the tube, the efficiency increased by 4.5% at an inlet temperature of 350 K.

This study utilizes a novel approach to combined absorption by placing metal surfaces inside the glass absorber tube. The laser surface modification process is a single-step, environmentally friendly, and integrated technique that transforms the metal surface into an effective solar absorber without adding significant complexity, weight, or associated risks. This process creates grooves on the metal surface that enhance its heat absorption properties⁹. Alternative methods for improving thermal efficiency, such as using nanofluids¹⁰ or incorporating porous materials¹¹, have shown promise but come with challenges. Nanofluids face issues related to long-term stability¹², while porous materials often result in relatively high pressure drops within the system¹³. The subsequent section provides an overview of research studies focusing on laser processing for fabricating absorber surfaces.

Hwang et al.¹⁴ investigated the effects of femtosecond (fs) laser pulse irradiation on nickel (Ni) surfaces, producing two-dimensional quasi-periodic arrays of nanostructure-covered conical microstructures (NC-CMs). By controlling the laser fluence, the reflectance of fs laser-treated Ni surfaces could be dramatically changed in the infrared while maintaining high absorptance in UV-visible wavelengths. At a laser fluence of 1.4 J/cm², the treated Ni surface achieved high spectral selectivity, with UV-visible absorptance remaining above 0.9 and infrared reflectance reaching up to 0.45. These findings demonstrate the potential of fs laser-treated Ni surfaces as effective solar absorbers for high-performance energy conversion devices​​.

Garcell et al.¹⁵ investigated the effects of femtosecond laser ablation on copper surfaces submerged in a water container, demonstrating that varying the laser scan speed from 0.5 to 2 mm/s produces different colors, ranging from deep purple to light orange. The laser-induced formation of nanoparticle structures influenced the color and reflectance properties of the surface. Optimal conditions for creating uniform colors with high reflectance were identified. This research provides new insights into producing high-quality structural colors on metals, showing potential applications in enhancing the optical properties of materials for various applications.

Zhao et al.¹⁶ presented a method for enhancing laser-induced periodic surface structures (LIPSSs) on molybdenum using two-color femtosecond laser pulses (400 and 800 nm) with identical linear polarizations. The technique improves structure quality by preventing disordered ridge-splitting, achieving a periodicity down to 140 nm and feature sizes as small as 70 nm. They reported that by adjusting the time delay and fluences of the laser pulses, controlling the generation of uniform LIPSSs is possible. This process enables precise nanostructuring with applications in concentrated solar energy devices and other fields, such as optoelectronics, tribology, and biomedicine.

Wang et al.¹⁷ used a femtosecond laser raster-type in situ repetitive direct writing technique to fabricate anti-reflective microhole structures on germanium (Ge) surfaces in the visible to near-infrared range (300–1800 nm). These structures reduced the average reflectance of Ge surfaces from 41.5 to 2.25%, with the lowest reflectance reaching 1.6%. The Ge microhole structures also exhibit a water contact angle of 133°, providing strong self-cleaning capabilities. This simple and precise method for producing these structures has diverse applications in solar panels, photovoltaic sensors, and other optoelectronic devices.

Mastellone et al.¹⁸ applied circularly polarized femtosecond laser pulses to create two-dimensional laser-induced periodic surface structures (2D-LIPSS) with sub-wavelength periodicity of 150 ± 20 nm on semi-insulating 6 H-SiC samples. Micro-Raman spectroscopy verified that the characteristic peaks of 6 H-SiC remained unchanged across all treated samples, indicating that the laser technique did not cause significant alternations in the material composition. The modified samples demonstrated notably improved optical absorption and solar selectivity, achieving solar absorptance values ranging from 10 to 75% and selectivity ratios as high as 2.0 at temperatures exceeding 1000 K. These findings underscore the capability of fs-laser textured 6 H-SiC surfaces as efficient solar absorbers, suitable for high-temperature energy conversion systems.

The studies mentioned above demonstrate that laser processing can significantly enhance the solar radiation absorption capacity of surfaces. While this enhancement has typically been validated through spectroscopic technique, the present research takes a novel approach by employing laser-processed metal surfaces in the environmental testing of a DAPTC for the first time. Various patterned surfaces will be placed inside the absorber tube of the constructed collector. Temperature differences across the tube at different flow rates will be measured to determine the thermal efficiency of the collector. These results will be compared with scenarios where no surface is placed inside the tube and where an unprocessed metal surface is used.

Laser processing of surfaces

For laser processing, a fiber laser machine is employed, capable of engraving a wide range of materials, including metals like gold, silver, stainless steel, iron, brass, copper, and aluminum, as well as non-metals such as opaque plexiglass, paper, leather, plastic, coated MDF, and more¹⁹. In this study, a 30 W fiber laser machine is used to create micro-scale grooves on copper and aluminum surfaces. This device provides high precision in implementing various patterns, enabling the application of almost any design onto the target surface.

A laser processing machine consists of several key components, including a controller, power supply, beam expander, mirror, and lens²⁰. A schematic diagram of these components is presented in Fig. 1. Figure 2 illustrates the different configurations of copper surfaces investigated in this study.

Fig. 1

Components of a laser processing machine.

Fig. 2

Different configurations of copper surfaces for placement inside the absorber tube.

Several factors influence the outcomes of laser surface processing, including laser beam diameter, beam angle, power, beam movement speed, the thermal properties of the material, and its capacity to harden under laser exposure. The hardening capacity and heating rate of the material also depend on the initial state of the workpiece. The primary factors affecting laser heat treatment are frequency, power, and operational speed⁹. To analyze these parameters more precisely, six different laser intensities were applied to both copper and aluminum surfaces. Variations in light reflectivity were then observed using the spectroscopic technique. Higher movement speed generally produces shallower engraving, while lower speed produces deeper engraving. Similarly, increased power leads to deeper engraving. Frequency has an inverse relationship with power; thus, lower frequencies produce darker and deeper engravings^19,21.

Figure 3 presents two samples of copper and aluminum surfaces, each with a thickness of 1.5 mm, that were processed using six different laser intensities, as specified in Table 1. The parameters presented in the table are the only adjustable values provided by the laser processing equipment. The selected values were determined based on the research objective of creating optimal absorber surfaces and the technical limitations of the laser system. Considering these factors, the frequency was set to its lowest possible level, 20 kHz, to ensure stable and precise operation. Maximum achievable power was utilized to effectively observe the impact of laser processing on the surface. Different operational speeds were employed to account for their varying effects: lower speeds allow for greater laser energy absorption, while higher speeds enable faster processing.

Two lens options were available for the laser processing: 10 cm and 30 cm. The 10 cm lens, which provides higher power and produces deeper engravings, was selected for this study.

Fig. 3

Laser processing with six different lasers on (a) Copper surface and (b) Aluminum surface.

Table 1 Specifications of various lasers applied to surfaces.

Given that the laser-processed surfaces are intended for use in a direct absorption mechanism, it is essential to evaluate their reflectivity using a spectroscopic technique. For this purpose, the Noora 200 spectrometer, manufactured in Iran, was utilized. This device measures light reflectance from the surface across a wavelength range of 400 to 1000 nm. As anticipated, lower reflectance corresponds to higher absorption. The spectroscopic results are presented in Fig. 4.

Fig. 4

Reflectance results of different lasers on two Surfaces: (a) Copper and (b) Aluminum.

The spectroscopic results for both materials indicate that the engravings become darker as the laser processing speed decreases, leading to reduced surface reflectance. Consequently, utilizing surfaces with lower reflectance enhances the absorption capability of the solar collector. Based on these findings, copper surfaces processed with a laser speed of 20 mm/s, a frequency of 20 kHz, and 80% of the working power of the device were selected for integration into the absorber tube for testing in the DAPTC. The length of the surfaces and their thickness were 1.5 m and 1.5 mm, respectively.

The surface thickness was chosen to ensure sufficient rigidity while maintaining approximately 1 cm of clearance at the top and bottom for fluid passage. Three laser-processed patterns—horizontal lines aligned with the direction of fluid flow, vertical lines perpendicular to the fluid flow, and circular patterns—were selected to examine the effects of different laser patterns, as illustrated in Fig. 5. These patterns were designed to ensure that an equal area of the surfaces was laser-processed. The width of the laser-engraved lines and the spacing between adjacent lines were set to approximately 1 mm. Both sides of the surfaces were laser-processed to allow solar radiation to interact with the surfaces from both directions.

Fig. 5

Laser-processed surfaces for installation in the absorber tube.

Experimental setup

Details of the constructed parabolic solar collector

Constructing a solar collector is a complex process requiring meticulous attention to detail to minimize structural errors. Any deviations in focal distance, unevenness of the reflector, or misalignment between the reflector and the absorber tube can significantly affect the performance of the collector, leading to a substantial reduction in efficiency²². Furthermore, careful attention must be given to properly positioning the absorber tube and ensuring that the angle of the collector can be easily adjusted for optimal operation. Figure 6 presents a schematic of the collector, which includes a copper surface within the absorber tube. Solar rays strike this surface both directly and by reflection.

Fig. 6

Schematic of DAPTC with the copper surface in absorber tube.

The research conducted by Kalogirou²³ indicates that the highest efficiency is achieved when the collector dimensions are 1.5 m by 0.7 m, with an edge angle of 90°. Therefore, these geometric specifications are used in the construction of the collector. The absorber tube is aligned longitudinally with the collector (1.5 m). The inner and outer diameters of the absorber tube are specified as 22 mm and 26 mm, respectively.

To minimize environmental losses, a protective tube with inner and outer diameters of 50 mm and 60 mm, respectively, is utilized²⁴. The space between the two tubes is evacuated to 300 mmHg to mitigate environmental influences²⁵. A mirror-finished stainless-steel sheet, grade 316 L, is used for the reflective panel due to its ability to reflect up to 93% of the incident solar radiation. This high reflective capability makes it an optimal choice for the reflective surface, which is a critical component of the parabolic collector. The thickness of the reflector is 1 mm, ensuring both the requisite flexibility for bending and adequate strength to prevent breakage. Moreover, a mechanism is integrated into the collector to ensure consistent perpendicular alignment of the sunlight relative to the collector surface. The geometric specifications of the collector are illustrated in Fig. 7.

Fig. 7

Geometric specifications of the DAPTC.

Measuring equipment and test procedure

In this experiment, the performance of the solar collector was evaluated under steady-state conditions in an outdoor environment using a hydraulic cycle. The test was conducted in accordance with ASHRAE Standard 93-2010, with distilled water serving as the heat transfer fluid²⁶. The collector was aligned along the north-south axis to enable optimal east-west tracking. A mechanism was designed to continuously adjust the orientation of the collector to maximize efficiency during the test period (from 11 AM to 3 PM).

The Vito VNC 25 − 8/180 circulation pump, manufactured in Iran, was used for water circulation. This pump can provide a flow rate of 20 to 120 L/h. The system was equipped with two valves for flow rate adjustment. An ultrasonic flow meter model, SONIC-PRO NX-SERIES from Akam-Madar, Iran, was used to measure the flow rate. Two PT100 thermocouples were placed at the inlet and outlet of the absorber tube to record temperature, while an additional sensor was used to monitor ambient temperature.

The pressure difference across the tube was measured by the Endress Hauser differential pressure transmitter. Solar radiation was recorded with a CEM DT-1307 device from China, and wind speed was measured using a Benetech GM8908 anemometer, also from China. Table 2 provides the measurement ranges and uncertainties for all instruments utilized in the experimental setup.

An appropriate cooling system was required to maintain the inlet temperature of the system. for this research, a compression refrigeration system was designed and constructed to ensure that the tests complied with the required standards. This system consisted of a compressor, condenser, capillary tube, evaporator, ice bank, and thermostat. The temperature of the ice bank was adjusted to achieve the specified inlet temperature for the tests. The inlet temperatures of 20 °C, 30 °C, and 40 °C were used for the experiments. The tests were conducted in June and July in Tehran Province. Each test has been repeated to ensure accurate results. Figure 8 shows the test circuit schematic, and Fig. 9 displays an image of the experimental setup.

Table 2 Devices and relevant considerations.

Fig. 8

Schematic of the DAPTC testing circuit.

Fig. 9

Solar collector experimental setup and measurement equipment.

Performance of the DAPTC calculations

The objective parameter of this study is the thermal efficiency of the DAPTC, which is defined as the ratio of the thermal energy generated by the collector (Q ) to the solar radiation energy received. The thermal efficiency for all configurations is calculated using Eq. 1. In this equation, $\:{T}_{in}$ ​and $\:{T}_{out}$ ​are the temperatures indicated by the inlet and outlet thermocouples, respectively. Additionally, $\:\dot{m}$ represents the fluid flow rate, $\:{C}_{P}$ ​denotes the specific heat capacity of the heat transfer fluid, $\:{G}_{T}$ ​is the solar radiation intensity measured by the solar meter, and $\:{A}_{c}$ ​is the collector area²⁷.

$$\:{\eta\:}_{th}=\frac{Q}{{G}_{T}{A}_{C}}=\frac{\dot{m}{C}_{P}({T}_{out}-{T}_{in})}{{G}_{T}{A}_{C}}$$

(1)

The performance index (PI) evaluates the effectiveness of each copper surface configuration in enhancing efficiency relative to the pressure drop it induces. Equation 2 demonstrates the calculation of this parameter²⁸.

$$\:PI=\frac{\raisebox{1ex}{${Q}_{copper\:plates}$}\!\left/\:\!\raisebox{-1ex}{${Q}_{simple\:tube}$}\right.}{{\left(\raisebox{1ex}{${f}_{copper\:plates}$}\!\left/\:\!\raisebox{-1ex}{${f}_{simple\:tube}$}\right.\right)}^{\frac{1}{3}}}$$

(2)

The friction factor ($\:f$) is calculated using the formula derived from Eq. 3²⁸. In this equation, $\:\varDelta\:P$ is experimentally determined using a pressure difference measurement device.

$$\:f=\frac{\varDelta\:P}{\frac{1}{2}\rho\:{u}^{2}\left(\frac{L}{d}\right)}$$

(3)

The time required for data acquisition is determined using the time constant, which represents the time taken for the temperature difference to reach 63.2% of its final value (Fig. 10). This time was found to be 7 min and 40 s in the experiment. The time constant is calculated using Eq. 4²⁹, where $\:{T}_{o,i}$ ​represents the outlet temperature at any given time, $\:{T}_{i}$ denotes the inlet temperature at any moment and $\:{T}_{o,ss}$​ is the steady-state outlet temperature.

$$T_{{o,i}} - T_{i} = 0.632(T_{{o,ss}} - T_{i} )$$

(4)

Fig. 10

The time constant of the collector.

Uncertainty in experimental measurements reflects the accuracy and precision of the obtained results, providing a range within which the true values are expected to lie. The uncertainty analysis follows the Moffat method, which accounts for error propagation from independent parameters influencing the target parameter. The uncertainties in efficiency and the friction factor (related to pressure drop) are derived from Eqs. 5 and 6^30,31. The uncertainty in efficiency is influenced by flow rate, temperature, solar irradiance, and collector area, while the uncertainty in the friction factor depends on flow rate, pressure drop, tube length, and tube diameter. The uncertainties related to measurement equipment are listed in Table 2. Specifically, the uncertainties for the collector area, tube length, and tube diameter are 0.0015 m, 0.001 m, and 0.00001 m, respectively. Based on these values and the measured data, the uncertainties in efficiency and friction factor are determined to be 2.65% and 2.07%, respectively. The greatest uncertainty in efficiency is attributed to temperature difference and irradiance, whereas the highest uncertainty in the friction factor is due to pressure drop.

$$\:{U}_{{\eta\:}_{th}}=\sqrt{{\left(\frac{\partial\:{\eta\:}_{th}}{\partial\:\dot{m}}{U}_{\dot{m}}\right)}^{2}+{\left(\frac{\partial\:{\eta\:}_{th}}{\partial\:\varDelta\:T}{U}_{\varDelta\:T}\right)}^{2}+{\left(\frac{\partial\:{\eta\:}_{th}}{\partial\:{A}_{C}}{U}_{{A}_{C}}\right)}^{2}+{\left(\frac{\partial\:{\eta\:}_{th}}{\partial\:{G}_{T}}{U}_{{G}_{T}}\right)}^{2}}$$

(5)

$$\:{U}_{f}=\sqrt{{\left(\frac{\partial\:f}{\partial\:\dot{m}}{U}_{\dot{m}}\right)}^{2}{+\left(\frac{\partial\:f}{\partial\:(\varDelta\:P)}{U}_{\varDelta\:P}\right)}^{2}+{\left(\frac{\partial\:f}{\partial\:L}{U}_{L}\right)}^{2}+{\left(\frac{\partial\:f}{\partial\:d}{U}_{d}\right)}^{2}}$$

(6)

The exergy balance for the system under investigation is presented in Eq. 7. In this equation, $\:{\dot{Ex}}_{sun}\:$ represents the exergy of the solar radiation received, $\:{\dot{Ex}}_{in}$ is the inlet flow exergy, $\:{\dot{Ex}}_{out}$ is the outlet flow exergy, $\:{\dot{Ex}}_{loss}$ denotes the exergy losses, and $\:{\dot{Ex}}_{d}$ represents the exergy destruction³².

$$\:{\dot{Ex}}_{sun}+{\dot{Ex}}_{in}={\dot{Ex}}_{out}+{\dot{Ex}}_{loss}+{\dot{Ex}}_{d}$$

(7)

The exergy of solar radiation is calculated according to Eq. 8³².

$$\:{\dot{Ex}}_{sun}={G}_{T}{A}_{C}\left[1-\frac{4}{3}\left(\frac{{T}_{amb}}{{T}_{sun}}\right)+\frac{1}{3}{\left(\frac{{T}_{amb}}{{T}_{sun}}\right)}^{4}\right]$$

(8)

The difference between the inlet and outlet flow exergy represents the useful exergy, as obtained through Eq. 9³³.

$$\:{\dot{Ex}}_{useful}={\dot{Ex}}_{out}-{\dot{Ex}}_{in}=\dot{m}{C}_{P}\left({T}_{out}-{T}_{in}\right)-\dot{m}{C}_{P}{T}_{amb}\text{l}\text{n}\left(\frac{{T}_{out}}{{T}_{in}}\right)-\dot{m}{T}_{amb}\left(\frac{\varDelta\:P}{{\rho\:T}_{m}}\right)$$

(9)

According to Eq. 10, the ratio of useful exergy to the exergy of the received solar radiation equals the exergy efficiency³³.

$$\:{\eta\:}_{Ex}=\frac{{\dot{Ex}}_{useful}}{{\dot{Ex}}_{sun}}$$

(10)

Results

Temperature rise across the absorber tube of DAPTC

Figure 11 illustrates the temperature increase for various configurations of the solar collector, including a simple tube (without a copper surface), a tube with an unprocessed copper surface, and three configurations with laser-processed copper surfaces. The figure presents the results for an inlet temperature of 20 °C, which exhibits the greatest temperature differential among the three tested inlet temperatures.

The temperature differences were measured for five different configurations. Figure 11-a depicts the temperature difference for the configuration without a copper surface inside the glass absorber tube. This configuration shows the lowest temperature increase among the five cases. At a flow rate of 20 L/h, the water temperature reaches 25.6 °C.

When an unprocessed copper surface is inserted into the tube, a higher temperature difference is achieved due to effective radiation absorption and thermal conductivity properties of copper, as shown in Fig. 11-b. In this configuration, at a flow rate of 20 L/h, the water temperature rises to 27.9 °C.

Even higher temperature differences are achieved with the three laser-processed copper surface configurations, attributed to their enhanced radiation absorption properties. The highest temperature difference is observed with the vertical line pattern, where the water temperature reaches 29.1 °C at a flow rate of 20 L/h. For the circle and horizontal line patterns, the water temperatures are 28.6 °C and 28.3 °C, respectively.

As the flow rate increases, the outlet water temperature decreases for all configurations. The slightest temperature increase is observed at a flow rate of 120 L/h across all tested cases.

Fig. 11

Temperature difference at the inlet and outlet of the absorber tube as a function of flow rate and solar irradiance rate in five configurations: (a) Without Copper Surface, (b) Copper Surface Without Laser Processing, (c) Laser-Processed Copper Surface with Horizontal Line Pattern, (d) Laser-Processed Copper Surface with Vertical Line Pattern, (e) Laser-Processed Copper Surface with Circle Pattern.

Thermal efficiency of the DAPTC without copper surface

To compare and validate the findings with previous research, Fig. 12 presents the test results from two studies^6,7 conducted on a collector with similar dimensions and an inlet temperature of 20 °C. Validation was performed for the configuration in which water flows through a simple glass tube without any internal copper surface. Both researchers tested comparable systems, with a discrepancy of up to 2.08% observed between their results. This variation can be attributed to factors such as differences in the calibration of measurement instruments, adherence to the time constant during data acquisition, and the accuracy of the inlet temperature stabilization system.

Fig. 12

Comparison of the DAPTC thermal efficiency for a simple tube with results from other studies.

Thermal efficiency of the DAPTC using laser-processed surfaces

Figure 13a–c present the thermal efficiency for inlet temperatures of 20 °C, 30 °C, and 40 °C, respectively, across all experimental conditions. The tests were conducted at six different flow rates: 20, 40, 60, 80, 100, and 120 L/h. The results indicate that laser-processed surfaces significantly improve efficiency. The highest efficiency of 38.99% was achieved with the vertical line pattern at an inlet temperature of 20 °C and a flow rate of 120 L/h. In comparison, the maximum efficiency with an unprocessed copper surface inside the collector tube reached 32.83%. For the simple tube configuration, lacking any internal copper surface, the maximum efficiency was recorded at 21.95%.

The vertical line pattern enhances thermal efficiency due to its ability to increase surface roughness, reduce reflection, and improve heat distribution. The orientation of the grooves in this pattern optimizes the interaction between the incident solar rays and the copper surface, enabling more efficient absorption by the heat transfer fluid. Additionally, the vertical grooves disrupt the thermal boundary layer along the fluid flow, promoting better fluid mixing and enhancing heat transfer. This pattern also increases the effective contact area between the fluid and the copper surface, facilitating more efficient energy transfer. Furthermore, the vertical grooves ensure uniform heat distribution along the absorber surface, preventing localized overheating and enhancing overall performance. The increased surface roughness minimizes solar radiation reflection, maximizing energy absorption. Considering the laminar flow conditions, the vertical line pattern optimizes heat exchange near the absorber surface by disturbing the velocity profile of the fluid, further improving heat transfer. These combined effects result in superior thermal performance compared to other patterns.

Due to its geometric configuration, the circle pattern promotes a more uniform temperature distribution across the absorber tube, resulting in an efficiency of 36.32% at an inlet temperature of 20 °C. In contrast, the horizontal line pattern demonstrates lower efficiency, as the alignment of grooves along the flow direction reduces the surface area available for direct heat transfer to the fluid, limiting overall absorption capacity.

Thermal efficiency decreases with reduced flow rates, a trend consistent across all conditions. At lower flow rates, the prolonged contact time between the fluid and the absorber tube increases thermal losses, thereby reducing system efficiency. The lowest thermal efficiency is observed at a flow rate of 20 L/h for all inlet temperatures. The highest efficiency is observed at 120 L/h flow rate. As the flow rate drops to 20 L/h, efficiency decreases by 12.73%. Among the inlet temperatures, the lowest efficiency is recorded at 40 °C due to a temperature difference of approximately 5 °C with the ambient. As the inlet temperature increases, efficiency decreases due to a larger temperature difference with the ambient. The average reduction in thermal efficiency between inlet temperatures of 20 °C and 40 °C is 5.07%. However, the difference in efficiency between inlet temperatures of 20 °C and 30 °C is only 1.38%, as the temperature difference with the ambient is minimal in these cases.

Fig. 13

Thermal efficiency of the DAPTC using laser processed copper surfaces (a) inlet temperature of 20 °C, (b) inlet temperature of 30 °C (c) inlet temperature of 40 °C.

Pressure drop analysis

The pressure drop in the absorber tube was experimentally measured using a differential pressure transmitter device, and the results show an increase in pressure drop with higher flow rates. The collected data were then used to calculate the friction factor under different conditions, as shown in Fig. 14. The results indicate that the friction factor decreases as the flow rate rises for all configurations, with the lowest value observed at a flow rate of 120 L/h. The highest pressure drop, measured at 247 Pa, was recorded for the configuration containing a copper surface with a vertical line laser pattern. This pattern increases the surface area in contact with the fluid, leading to greater flow resistance.

Similarly, the surface with a circle laser pattern demonstrates higher flow resistance compared to the surface with horizontal lines. On average, the pressure drop for laser-processed surfaces is 12.73% higher than that for untreated surfaces and 369.17% higher than that for the simple tube configuration.

Fig. 14

Comparison of friction factors of all configurations.

Analysis of performance index for laser-processed surfaces

Figure 15 illustrates the performance index (PI) for each condition. A higher PI value indicates that the heat absorbed by the system significantly outweighs the pressure drop under the respective condition. The results reveal that the surface processed with the vertical line pattern exhibits the highest efficiency compared to other patterns despite its higher pressure drop. For this configuration, the maximum PI value is 1.06 at a flow rate of 20 L/h, as the pressure drop decreases significantly at lower flow rates. The PI values are higher for all laser-processed surfaces compared to the unprocessed condition. The lowest PI value of 0.84 is recorded at a flow rate of 60 L/h for the non-patterned configuration.

Fig. 15

PI of each copper surface at various flow rates.

Exergy analysis

Equation 7 through 10 are used to calculate the exergy of solar radiation, useful exergy, and exergy efficiency for all conditions. Figure 16 illustrates the exergy efficiency as a function of flow rate for all configurations, with data corresponding to the highest inlet temperature of 40 °C. An increase in inlet temperature is associated with higher exergy efficiency, with the maximum value observed at this temperature. Among the tested configurations, the highest exergy efficiency of 4.88% is achieved with the laser-processed surface featuring a vertical line pattern. In contrast, the lowest exergy efficiency of 3.39% is observed for the untreated surface. The exergy efficiency of the simple tube is 2.64%. Lower inlet temperatures, closer to ambient conditions, result in reduced exergy efficiency. This decline is due to the increased influence of the second term in Eq. 9, which negatively impacts the useful exergy.

Fig. 16

Exergy efficiency of all configurations at various flow rates.

Economic analysis

The Levelized Cost of Energy (LCOE) is a key performance metric for evaluating the economic feasibility of energy systems. The LCOE calculation in this study is performed using Eq. 11, where the investment cost ($\:{Z}_{investment}$), operation and maintenance cost ($\:{Z}_{O\&M}$), and annual operational hours $\:({t}_{annual}$) were integrated^33,34. Table 3 summarizes the key parameters utilized for the LCOE estimation. The investment cost includes the expenses associated with the main components of the DAPTC, as well as the copper surfaces and laser processing required to create the patterns on the surfaces. The implementation cost of different patterns on the copper surfaces is assumed to be the same.

$$\:\text{L}\text{C}\text{O}\text{E}=\frac{CRF\times\:{Z}_{investment}+{Z}_{O\&M}}{\left(\dot{m}{C}_{P}\left({T}_{out}-{T}_{in}\right)\right)\times\:{t}_{annual}}$$

(11)

Capital Recovery Factor ($\:CRF$) is calculated according to Eq. 12 to account for the periodic recovery of the investment over the system lifetime, which is assumed to be 20 years in this study, considering an interest rate of 8%³⁵.

$$\:CRF=\frac{i*{(i+1)}^{n}}{{(i+1)}^{n}-1}$$

(12)

Table 3 Key parameters for LCOE Estimation in the present study.

Figure 17 illustrates the variations in the LCOE across different flow rates for various configurations at an inlet temperature of 20 °C. As observed in this figure, the incorporation of copper surfaces inside the absorber tube significantly reduces the LCOE. For the configuration without any patterns, the LCOE decreases to 0.037 $/kWh, compared to 0.052 $/kWh for the simple tube. However, the additional cost associated with laser processing results in a higher LCOE for the horizontal line pattern compared to the unpatterned configuration, indicating that this pattern does not provide an economic advantage. Among all configurations, the laser-processed surface with the vertical line pattern achieves the lowest LCOE. This is because it incurs the same laser processing cost as the other patterns while offering superior thermal efficiency. The LCOE is reduced to 0.033 $/kWh with this configuration.

Fig. 17

LCOE of all configurations at various flow rates.

Conclusion

This study experimentally investigated the impact of laser processing on copper surfaces for application in a Direct Absorption Parabolic Trough Collector (DAPTC). The key findings are summarized as follows:

• Analysis of critical laser processing parameters, including speed, power, and frequency, revealed that a laser speed of 20 m/s, a frequency of 20 kHz, and 80% of the device power significantly reduced surface reflectance. For copper surfaces, reflectance decreased to less than 4% across a wide range of wavelengths, while for aluminum surfaces, it dropped below 10%, demonstrating the high absorption capacity of laser-processed surfaces.

• The largest temperature difference in the DAPTC tests was observed at the lowest flow rate (20 L/h) and the lowest inlet temperature (20 °C). For the simple tube, temperature differences ranged from 1.3 °C to 5.6 °C across flow rates of 20 to 120 L/h. Among the four main configurations—no pattern, vertical line pattern, circle pattern, and horizontal line pattern—the temperature differences were 7.6 °C, 8.3 °C, 8.6 °C, and 9.1 °C, respectively.

• An increase in flow rate and a decrease in inlet temperature enhanced the thermal efficiency of the collector. However, the difference between 20 °C and 30 °C inlet temperatures was minimal. The highest thermal efficiency of 38.99% was achieved with the vertical line patterned laser-processed surface at an inlet temperature of 20 °C (with a 1 °C temperature difference from ambient) and a flow rate of 120 L/h. This efficiency is 1.78 times greater than the simple tube, representing a substantial improvement.

• Due to the favorable thermal efficiency and relatively low pressure drop of the copper surfaces, the performance index ranged from 0.84 to 1.06, confirming the effective performance of the collector with these surfaces.

• The highest exergy efficiency was observed at an inlet temperature of 40 °C and a flow rate of 120 L/h. Under these conditions, the exergy efficiency was 2.64% for the simple tube, 4.09% for the tube with an untreated surface, and 4.87%, 4.62%, and 4.35% for the tubes with vertical line, circle, and horizontal line patterns, respectively.

• Laser processing demonstrated its potential to effectively reduce the LCOE. With the vertical line pattern, the LCOE reached 0.033 $/kWh at a flow rate of 120 L/h and an inlet temperature of 20 °C.