Experimental investigation of a low-cost evacuated tube in a parabolic trough collector with and wit

Abstract

Keywords: Circular ring wire mesh insert; Evacuated tube; Nusselt number; Parabolic trough collector; Thermal efficiency

Introduction

In parabolic trough collector (PTC), the receiver tube is considered to be the crucial component of the system.

In general, the sun rays from the sun are concentrated on the receiver tube with the help of the reflector, which is placed at the focal distance from it [1]. The working heat

*Corresponding author. *E-mail address: pmghodasara@gmail.com This paper was recommended for publication in revised form by Editor-in-Chief Ahmet Selim Dalkılıç Published by Yıldız Technical University Press, İstanbul, Turkey Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

transfer fluid (HTF) is passed from the heating element or tube, which collects the thermal energy from the peripheral surface of the tube [2]. The thermal energy collected by the working fluid can be helpful for industrial and commercial applications. The importance of the receiver tube in PTC becomes significant due to its cost and efficiency. The cost of the receiver tube can be nearly 30 to 40% as compared to the overall cost of the system [3]. This cost consists of metal and glass tubes, coating, vacuum insulation, sealing, manufacturing and assembly. Out of this, vacuum insulation, sealing and coating are major components that make the receiver tube more expensive. On the other hand, including these three components on the receiver tube is a very vital part from the efficiency point of view. Selective coating on the receiver tube, which has high absorptivity and low emittance, is selected [4]. Absorptivity helps to absorb more concentrated radiation from the sun, which ultimately maximizes the heat gain of the receiver tube and causes a rise in the temperature of the receiver surface and HTF. The low emittance property of the coating helps to decrease the heat losses due to an increase in the temperature of the tube [5]. This coating includes complex chemical depositions, which require advanced technologies, which makes this coating expensive [6-7]. Furthermore, vacuum insulation and sealing between metal and glass tubes reduce the conduction and convection losses from the receiver tube at higher temperatures. Sustaining a vacuum for a long time and managing the thermal stress of the receiver tube with heating and cooling requires precise manufacturing, which adds complexity and high cost [8]. Evacuated receiver tube with double side opens are very expensive compared to the evacuated tube used in the non-concentrating collector. The reason for this is the lower demand in the market, less production, and very limited manufacturers available globally. Evacuated tube used in non-concentrating collectors is designed for low-temperature applications up to 150°C. In contrast, the evacuated tube used in PTC is designed for high-temperature applications up to 400°C [9]. It has been noticed that due to the limited manufacture of such evacuated tube in the world, the additional cost of importing the receiver tube of specific configurations from different parts of the country will make it more expensive. Further, there is also the possibility of breaking glass tube due to fragile material and the loss of vacuum of the evacuated tube during adverse working industrial conditions. Figure 1 indicates the evacuated tube used in PTC. Therefore, the authors have attempted to develop an in-house double-end open evacuated receiver tube for small PTC in the present experimental work. The evacuated tube is described in detail in the next section. The above discussion states the importance of an evacuated tube for improving the efficiency of the PTC by restricting heat losses from the external part of the tube. The next focus of the authors was to emphasize the

performance by internal means of the receiver tube. The authors have recognized two types of methods from various literature reviews, such as the use of nanofluid [10] and inserts [11]. The use of nanofluid can enhance the working fluid conductivity, which can boost the thermal gain and overall efficiency of the solar collector. Hussein et al. [12] evaluated the thermal performance of the flat plate collector with nanofluid as CuO/H2O over three months in Iraq. Results indicate that 1% CuO/H2O nanofluid improves the collector efficiency by 32%, and 11.3% performance surpasses was observed while using pure H2O. Ajbar et al. [13] carried out a simulation analysis on PTC with eight hybrid nanofluids. The thermal efficiency of PTC in all hybrid nanofluids was found to be better compared to the base fluid as Syltherm 800. Challenges such as nanofluid stability, chances of corrosion and erosion of inside tube material, and thermal instability at high temperatures may affect the performance of the PTC for a long time [14]. Next, a literature review was focused and conducted on inserting an insert in the receiver tube to improve the internal heat transfer coefficient of the fluid, which can lead to internal heat gain of the fluid and the overall performance of the PTC. Generally, two types of inserts are categorized: active and passive [15]. From the literature, authors have come across that active types of inserts are not more favorable due to their complexity and high cost [16]. Meanwhile, passive types of insert enhance thermal energy transfer and improve the system performance without requiring any external energy. Various researchers have proven that inserts inside the receiver significantly improve the heat transfer enhancement in solar thermal systems compared to the plain tube. Twisted tapes [17], [18], wire coils [19], fins [20], [21], wire mesh [22], [23], perforated plates [24], and porous inserts [25] are found to be passive inserts, which various authors have researched. Most of the inserts have been researched in the field of heat exchangers and solar thermal systems. Several experimental and numerical studies have been conducted on PTC using inserts such as twisted tapes and fins; however, most investigations have been limited to numerical analysis. Farhad et al. [26] carried out a numerical study involving

dual twisted tape inserts within a Reynolds number range of 10000 to 20000. The Nusselt number was found to be 19.58% higher compared to a plain tube, demonstrating enhanced heat transfer performance [26]. Piyush et al. [27] performed an experimental study on a solar-based air heater utilizing dual-arc artificial roughness, which revealed a significant enhancement in heat transfer performance. The Nusselt number was found to increase by a factor of 2.27 compared to a conventional solar air heater. However, this improvement was accompanied by a substantial rise in pressure drop, which was 3.10 times higher than the plain solar air heater. Karunaraja et al. [28] carried out an experimental investigation on solar drying using a solar tunnel dryer under the meteorological conditions of Negamam. The solar tunnel dryer using thermal storage materials exhibited 2–3% higher thermal efficiency than no storage material. Sand as a thermal storage material was found to be the most effective in this study. Prem et al. [29] conducted a review study, presenting various techniques for enhancing the efficiency of solar stills. Among these, passive augmentation techniques were highlighted as an effective approach for converting polluted water into potable water. Furthermore, these inserts perform differently, whether they are solid or porous. Twisted tapes, conical displacers, and fins are considered to be part of the solid inserts. Such types of inserts have high durability and low-pressure drop, but they can cause high fouling resistance and are heavier in weight. Meanwhile, porous inserts have their own advantages, such as being light in weight and generating turbulence with moderate pressure drop with chances of low to moderate fouling resistance. Pooja et al. [23] experimentally studied the behaviour of the wire mesh inside the receiver tube and compared its results with that of the plain tube. Two types of inserts were studied under the laboratory setup: wire mesh twisted tape

(WMTT) and circular ring wire mesh (CRWM). Parameters such as friction factor, Nusselt number, and performance evaluation criteria (PEC) were determined to compare the various inserts. Figure 2 (a) and (b) illustrate WMTT and CRWM inserts. Experimental results show that wire mesh performs better as compared to plain tube. Meanwhile, the efficacy of CRWM inserts has been found to be more promising than that of WMTT. Although CRWM demonstrates a higher Nusselt number, it also results in a greater pressure drop. In contrast, WMTT shows a slightly improved Nusselt number, while maintaining a relatively lower pressure drop. The PEC for both inserts was evaluated to assess overall performance, revealing that CRWM exhibits a superior PEC compared to WMTT. Therefore, in the current research paper, the authors have attempted to investigate the other configuration of CRWM for optimizing the thermal performance of the PTC. This research paper consists of two broader aims: first, develop a notch-based CRWM inserts for improving the thermal retention of the fluid and second, fabricate low-cost in-house evacuated receiver tube for optimizing the cost of the PTC. An experimental investigation of in-house developed tube with and without insert is carried out to evaluate the performance of the PTC. The following subsections of this paper discuss the development of an in-house evacuated tube, CRWM insert, design and experimental part of PTC, data analysis and validation, and results and discussions.

Development Of In-House Evacuated TUBE For PTC

Evacuated tube consists of four major parts, i.e. metal tube, glass tube, coating and vacuuming between metal and glass tube. Metal tube must be selected based on higher thermal conductivity, corrosion resistance, temperature

(b) CRWM Figure 2. (a) and (b) Illustrates WMTT and CRWM inserts.

Figure 3. Illustration of in-house developed evacuated receiver tube.

range and mechanical strength. The most common materials used in solar thermal collectors are copper and stainless steel [30]. It has been observed that stainless steel as a material is more suitable for higher temperature applications above 250°C due to its strong mechanical properties. At the same time, copper is the best option for low to medium-temperature applications [31]. Secondly, a glass tube made of borosilicate is generally used to cover the metal tube, which has high transmittance and is capable enough to withstand high temperatures. The vacuum is created in this concentric metal and glass tube, which maintains thermal insulation to prevent energy losses at high temperatures. It also protects the metal tube and its coating from the surrounding environment. Next, coating with thickness in a range of 0.2 to 2 microns is used on the external peripheral surface of the receiver tube, which helps to enhance absorption of the sun radiation and simultaneously reduces thermal energy emission losses with an increase in temperature. Black chrome and matte black paint coating can be used on the external peripherals surface of the tube, which is locally available, less expensive, and easy to apply [5]. Coatings, such as ceramic-metal composites, carbon or graphene-based, titanium nitride oxide, etc., are also examined in PTC [32]. Lastly, an enclosure of metal and glass tubes with coating is vacuumed and sealed for thermal insulation. Different types of sealing, such as glass to metal, metal bellows, elastomer, epoxy resin, brazed sealing, and compression sealing options, are available to make a perfect vacuum sealing. Selection of sealing is based on temperature range, thermal stability and vacuum retention [33]. From the literature review, it was noted that sealing such as epoxy resin, compression sealing, and elastomer can withstand up to a temperature range of 150 - 200°C. While glass-to-metal, brazed

sealing and metal bellows are more suitable for higher temperature ranges up to 500°C. In the present study, a metal tube made of copper material was selected, with inner and outer diameters of 22 mm and 25 mm and an effective tube length of 1000 mm. Illustration of an in-house developed evacuated receiver tube is shown in Figure 3. A locally available matt black coating was applied on the external surface of the copper tube. Borosilicate glass tube having inner and outer diameter of 47 mm and 50 mm were selected to form an enclosure on the metal tube. Finally, the rubber seal was cut and fitted according to the space between the copper and glass tube. A heat-resistant adhesive was applied to enhance the bonding between the contact surface of the seal and copper. The Glass tube was slid and rotated over the copper tube to maintain a proper gap, disturbing the adhesive uniformly and preventing air pockets from forming. A vacuum pump was used to evacuate air and obtain the desired pressure level between the copper and glass tube to integrate the vacuum seal. The vacuum pressure of -25 cm of Hg, equivalent to -0.3333 bars, was kept between enclosures. Pressure monitoring was done for two continuous days to ensure the vacuum and seal between glass and metal were tightened, ensuring leak detection. Figure 4 highlights the vacuum process conducted in the laboratory. Circular Ring Wire Mesh Insert Wire mesh is a porous type of insert used in the present study to examine its impact on the heat transfer performance of the PTC. Wire mesh of stainless steel with 18 pores per inch (PPI) (70% porosity) was selected due to being readily available in the local market. It was cut into a circular ring with the help of tin spins and fitted into the circular rod of 3 mm diameter and 1000 mm length. Twenty numbers of CRWM were located on the circular rod with a

Figure 4. Highlights the vacuum process conducted in the laboratory.

Figure 5. (a) and (b) Represents real and CAD model of circular ring wire mesh-18 PPI.

pitch distance of 50 mm. Furthermore, a V-notch cut was made in the circular ring, generating more turbulence and a larger surface area to increase heat transfer and help minimize the pressure drop. Real and CAD models of circular ring wire mesh-18 PPI are shown in Figure 5 (a) and (b). Design of Parabolic Trough Collector The concept design of the parabola is demonstrated in Figure 6. The PTC contains of a parabolic reflector with an eccentricity equal to one, designed to concentrate all sun radiation on the focal point. The receiver tube is located on the focal point, which absorbs the concentrated radiation. Generally, glass or aluminum sheets with high reflectivity are selected for a parabolic reflector. Parameters such as focal length, aperture size, rim angle, receiver tube diameter, and concentration ratio can be analyzed while designing PTC. The following equations (1 to 4) were used to design PTC [34]. The specifications for the design PTC are shown in Table 1. Aperture area of the collector can be calculated using equation (1),

(1) Surface area of receiver tube can be determined using equation (2), (2) Where, dgo = diameter of the glass tube and L = glass tube length Concentration ratio can be calculated using equation (3) [33], (3) Focal point of the parabola can be calculated as mentioned in below equation (4), (4) Where, Ф = rim angle Table 1. Specification of PTC

Experimental Setup and Procedure The experimental setup was fabricated and installed at Marwadi University, Rajkot, which is at 22o18’N latitude and 70o47’E longitude. The setup consists of a parabolic-shaped mirror reflector, evacuated receiver tube, inlet and outlet fluid thermocouple, pyranometer, anemometer, temperature gun, data logger, and miscellaneous accessories for fitting.

Figure 7. Experimental setup of PTC. The PTC was installed on the ground and oriented southward with manual tracking (east to west) to ensure uninterrupted solar radiation. The copper receiver tube, with a diameter of 25 mm, was coated with matte black and mounted coaxially within a glass envelope that was evacuated to minimize heat loss, as depicted in Figure 3. Ro water was used as the HTF and properties are calculated at the bulk mean temperature. K-type thermocouples were connected at both ends of the tube to measure the fluid inlet and outlet temperature. The ends of the receiver tube were insulated with fiber wool to avoid thermal losses. A pyranometer (Class C) was employed to monitor solar radiation throughout the experiment. To measure the wind velocity and ambient temperature of the surrounding atmosphere, an anemometer was used. The average surface of the receiver tube was measured with the help of an infrared thermometer, which is also known as a

temperature gun. Lastly, a data logger was used to record the fluid temperature reading and solar radiation for an interval of every 1 minute. The experimental setup of PTC is shown in Figure 7. Experiments were performed on the PTC in January 2025 from 9.30 am to 4.30 pm. Two types of cases are examined in PTC: plain tube (without insert) and tube having notched CRWM insert. Data were collected under consistent ambient conditions and solar flux to facilitate a fair performance assessment of both cases. The study was conducted under three different discharge rates to judge the performance of the evacuated receiver tube with and without the insert. The discharge rate was measured using a calibrated beaker, and a flow control valve was used to adjust the flow rate. The experiment begins by allowing an inlet of water from one side with a known mass discharge rate to pass through the other end of the evacuated receiver

Table 2. Parameters and equations for data reduction Parameters

tube. Manual tracking of PTC in the east-to-west direction was continuously carried out to concentrate radiation on the receiver tube. The performance parameters, such as the Nusselt number, outlet fluid temperature, heat gain and thermal efficiency, were assessed to identify the significance of the insert in PTC.

of ±2.6%. Furthermore, the results also indicate with an increase in Reynolds number, the Nusselt number shows a corresponding amplifies, suggesting an enhancement in convective heat transfer. These observations validate the accuracy and reliability of the experimental setup, confirming its suitability for the study.

Data Analysis and Validation For the calculation and analysis of the data, reading from all instruments was considered on hourly basis. The equations and parameters applied during data processing are presented in Table 2. Figure 8 represents a comparison of the experimental and theoretical calculated Nusselt number. The DittusBoelter correlation for the Nusselt number mentioned in equation (9) was used to validate the turbulent flow behavior within the tube. These equations are relevant for fully developed turbulent flow conditions. The data used for the experimental Nusselt number was obtained across two mass discharge rates (0.06 and 0.08 kg/s) under nearly identical solar heat flux conditions (760 W/m2). The mass discharge rate of 0.04 kg/s is not considered here in the validation due to the Reynolds number coming to less than 4000, which does not apply to the Dittus-Boelter equation. The data presented in Fig. 8 exhibits a strong correlation with the theoretical models, with deviations

Measurement of Uncertainty This section outlines the inherent uncertainties associated with the experiment. Various instruments, including the thermocouples, solar pyranometer, flow meter, and data logger are employed to gather data during the test. The statistical uncertainty X (z) of the variables used in the data analysis can be calculated from the equation (13), where Y (z) represents the accuracy of the components. Additionally, the uncertainty for F (b) can be determined using equation (14) [36]. Table 3 provides the accuracy and uncertainty of devices. The cumulative uncertainty of the experiment is 1.34%. (13)

(14) Where A, B, C and D are uncertainty in solar pyranometer, thermocouple, flow rate and data logger acquisition respectively.

Results And Discussion

Figure 8. Comparison of experimental and theoretical Nusselt number.

Solar Flux with Time The data presented in Figure 9 demonstrates the variation in solar flux over several days, measured between 9:30 am and 4:30 pm under no cloud conditions at Marwadi University, Rajkot (22o18’N latitude and 70o47’E longitude) in January 2025. The data selected for processing corresponds to the six different days with the least cloud cover. It is important to note that, on all selected days, the overall solar energy received by the collector was nearly identical. The maximum solar flux was available between 12.30 to

Conclusion

This paper focuses on optimizing the cost and performance of the evacuated tube used in parabolic solar trough collector. In-house development of the tube with locally available materials in the market approach was used to fabricate the low-cost evacuated receiver tube. A novel notch-based porous insert was fabricated and experimentally examined to improve the thermal retention of the fluid inside the tube. An experimental setup with an aperture area of 2.36 m2 with a focal length of 0.50 m was designed and constructed to examine the in-house developed tube and porous insert. Water is used as the working fluid, and three mass discharge rates (0.04, 0.06 and 0.08 kg/s) were selected for this study. In six days with different mass discharge rates, experimental readings from the time duration of 9.30 am to 4.30 pm were conducted in the month of January (2025). Parameters such as Nusselt number, outlet fluid temperature, heat gain and thermal efficiency were evaluated to compare performance with and without porous inserts in the tube. The key outcomes of this experimental study are as follows:

1. The solar radiation reading for six different days in the

month of January (2025) for the 22o18’N latitude and 70o47’E longitude was measured with the help of a pyranometer. The average solar flux of 863 W/m2 was noted during the peak solar noon between 12.30 to 1.30 pm. 2. Nusselt number was examined at different mass discharge rates between a plain tube and a tube with an insert. The result reveals that the tube with an insert has a higher Nusselt number than the plain tube at all three mass discharge rates. The rise in Nusselt numbers is 10.6%, 17.64%, and 20.46% for mass discharge rates of 0.04, 0.06, and 0.08 kg/s, respectively.

3. In a similar way, the outlet fluid temperature of the

water was studied with and without an insert in the tube at three different mass discharge rates. The experimental results indicate that the tube with the insert has a higher outlet temperature rise of 8.1%, 4.42% and 3.41% for mass discharge rates of 0.04, 0.06 and 0.08 kg/s, respectively.

4. Finally, heat gain and thermal efficiency of the collector

were determined for both cases at different mass discharge rates. It was noted that tube with inserts show a superior performance and thermal efficiency rise compared to the plain tube. Additionally, both parameters in tube with inserts show excellent results at higher mass discharge rates. This study also signifies that the tube with an insert improves the thermal retention of the fluid, which ultimately enhances heat gain and thermal efficiency.

5. The cost of the in-house developed evacuated receiver

tube was evaluated and compared with existing literature, which shows a positive impact. This type of evacuated tube is more suitable for the small size of parabolic trough collectors, which has high potential for low and medium-temperature industrial applications. The findings of this study demonstrate a significant enhancement in the efficiency of the collector. This improved efficiency makes the collector highly suitable for a wide range of industrial applications. Its enhanced

performance ensures reliable and cost-effective thermal energy solutions for diverse sectors. In the future scope, the developed evacuated receiver tube can be further examined for higher temperatures using oil as the working fluid along with the notched circular ring wire mesh insert.

Nomenclature

There are no ethical issues with the publication of this manuscript.

Aperture area (m2) Receiver area (m2) Fluid Specific heat (KJ/kg.K) Tube internal diameter (m) Glass outer diameter (m) Focal length (m) Heat transfer coefficient (W/m2.K) Thermal conductivity (W/m.K) Mass discharge rate (Kg/s) Prandtl number Useful heat gain (W) Reynolds numbers Bulk fluid temperature (K) Inlet and outlet fluid temperature (K) Receiver surface temperature (K)

Abbreviations

CR Concentration Ratio CRWM Circular ring wire mesh HTF Heat transfer fluid LPM Liter per minute PEC Performance evaluation criteria PPI Pores per inch PTC Parabolic trough collector WMTT Wire mesh twisted tape Greek symbols μ Fluid dynamic viscosity (kg/m.s) ρ Fluid density (kg/m3) v Velocity of fluid (m/s) Ф Rim angle (°)

Acknowledgment

The authors sincerely thank Marwadi University, Rajkot, for the financial support (Grant No. MU/R&D/21-22/ MRP/FT05) to conduct this research project under the Minor Research Project scheme.

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Statement On The Use Of Artificial Intelligence

Artificial intelligence was not used in the preparation of the article.

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