An Experimental Comparison of the Performance of Various Evacuated Tube Solar Collector Designs

Abstract

An experimental study was carried out to assess the thermal performance of a few evacuated tube solar collectors (ETSCs) for water heating. The thermal performance of two kinds of ETSC (heat pipe ETSC and direct-flow ETSC) was investigated using an indoor experimental apparatus in lab testing conditions with a solar simulator. Several experimental tests were carried out for the heat pipe ETSC system under different operating conditions, such as the solar intensity (300, 500, and 1000 W/m²) and the tilt angle (0°, 30° and 90°) of the ETSC and the water flow rate (0.6, 1.2, and 2.4 LPM). Moreover, four configurations of direct-flow ETSC (U tube, double U tubes, coaxial tubes, and helical tube) were examined and compared to the conventional heat pipe ETSC. The results of the experiment proved that the ETSC system presents a great performance at higher solar irradiance and lower water flow rates, and the experiments indicated that with a 30° tilt angle, the ETSC reaches the maximum thermal efficiency of 36%. Furthermore, compared to the conventional heat pipe ETSC and the other proposed configurations of direct-flow ETSCs, the helical tube-based ETSC has a better thermal efficiency, 69%, and can be considered a greater potential heat exchanger that can be integrated in ETSCs. To the best of our knowledge, it is the first time this helical tube type been integrated into the ETSC and tested under these conditions.

1. Introduction

Solar energy is considered to be the most likely renewable energy source due to its accessibility around the globe. [1]. The use of solar thermal collectors is one of the most effective methods of harvesting solar energy in order to fulfill rising energy needs and reduce greenhouse gas emissions [2]. In general, there are two kinds of solar thermal collectors: a flat-plate collector and an evacuated tube solar collector (ETSC). Several temperature ranges may be attained by using these collectors. The flat-plate collectors can operate in the temperature range of 20 to 80 °C, while ETSCs can achieve a temperature range of 50 to 200 °C [3,4]. ETSCs are the most effective solar collectors because they outperform flat-plate collectors in terms of efficiency and operating temperature [4]. An ETSC is a solar collector made of two extremely sturdy glass tubes. A very high transmissivity and very little reflection characterize the outer tube, which allows for the passage of radiation. A selective coating on the internal surface of the glass tube helps to lock in heat by maximizing solar energy absorption and reducing reflection. The two tubes are fused together in order to create the vacuum that serves to decrease the losses by convection, and the upper tube causes the greenhouse effect. There are two main categories of ETSCs [5,6]: (i) Direct-flow ETSC; and (ii) Heat-pipe ETSC. Different researchers have focused their attention on several elements, including construction, design, heat transfer fluid type, radiation capture techniques, and tilt angle, which have a substantial impact on the ETSC’s performance. Several innovative research efforts on enhancing ETSCs’ thermal efficiency exist in the literature. To examine the thermal performance of a single ETSC with coaxial tubes, Badar et al. [7] developed an analytical model. A solar system’s thermal efficiency created to meet the needs of a normal home’s space heating, cooling, and domestic hot water was numerically examined by Ghoneim et al. [8]. Experimental and numerical research was carried out by Hazami et al. [9]. According to their findings, the ETSC produced about 9% more energy than the flat-plate collector. As a way of creating more affordable and effective solar water heating systems that would suit Turkish conditions, Comakl et al. [10] adjusted the size of a solar collector system. A comparison between a flat-plate collector and an ETSC for water heating was conducted. Liangdong et al. [11] investigated analytically the thermal performance of a U-tube ETSC. They proved that the surface temperature of the absorbing coating is an essential parameter to consider when assessing ETSC performance.

Even though ETSCs outperformed other solar collectors in terms of efficiency, scientists are still looking for methods to make them even better. Several design advances have made it feasible to develop new ways of boosting efficiency and to make collectors capture the most solar radiation. Jowzi et al. [12] conducted an experimental and numerical analysis into the effectiveness of a modified ETSC. The results reveal that the modification in structure enhanced the collector’s efficiency by up to 11%. In an experimental investigation, Xu Jia et al. [13] compared the thermal efficiency of two semi-concentric parabolic concentrator designs with that of a standard ETSC. According to the results, systems with combined parabolic concentrators increase water temperature by around 20% over typical solar collectors. Bracamonte et al. [14] studied the impact of inclination angle (10°, 27°, and 45°) on the thermal efficiency of ETSCs in quasi-tropical regions. Higher inclination angles, according to their findings, cause the hot and cold streams to completely mix, lowering temperatures, whereas low tilt angles cause lower velocities. An experimental study on the efficiency of the ETSC with and without a solar reflector and at various tilt angles was undertaken by Dabra et al. [15]. The findings demonstrate that the tilt angle significantly impacts the thermal performance of the ETSC. Recent studies have shown that the working fluid significantly affects the solar collector’s efficiency. The most prevalent working fluids in solar energy systems are air, water, and oil; however, their thermal conductivity is low [3]. In order to improve the collector’s effectiveness, researchers are now focusing their studies on different working fluids, such as nanofluids. In an experimental study of a U-tube ETSC utilizing Al₂O₃ nanofluid, Hyeongmin et al. [16] concluded that it was beneficial in enhancing the ETSC’s efficiency. By employing a copper oxide/distilled water as a nanofluid, an experimental evaluation of the thermal performance of ETSC was conducted by Sadeghi et al. [17]. The findings indicated that raising the nanofluid’s flow rate and concentration improved the thermal efficiency of the ETSC. Because of their excellent capacity to store energy, phase change materials (PCMs) are often employed by researchers to improve the performance of solar collectors. In this technique, the PCM is either inserted within the manifold or the heat pipe or absorber pipe, or both are submerged in the PCM, or else it is inside a storage tank connected directly or indirectly with the collector. Papadimitratos et al. [18] suggested an experimental evaluation of a heat pipe ETSC integrating two PCMs. According to their findings, efficiency rose by 26% compared to the situation without PCM. A heat pipe with ETSC combined with PCM was theoretically studied by Naghavi et al. [19]. The findings show that an ETSC combined with a PCM has better thermal performance than a standard ETSC. A study on the heat transfer between an air heating system based on ETSCs and a latent heat storage tank was carried out by Arkar et al. [20], using both experimental and computational methods. According to the findings, between 54 and 67% of the heat produced by the solar air heating system during the day might be used to heat homes at night. An ETSC combined with PCM was the subject of experimental research by Sheng [21]. The author found that the efficiency of the ETSC with PCM is higher than that of an ETSC without PCM. A study on a heat pipe ETSC combined with a latent heat storage system was conducted by Flinski et al. [22]. According to their findings, the charging efficiency of the PCM varies between 33 and 66% depending on the sun’s intensity and the melting temperature of the PCM. Additionally, the system shows an improvement of about 20.5% compared to a case without PCM. Almari et al. [23] developed experimentally an ETSC connected to a heat storage tank integrated with a helical coil heat exchanger and PCM. According to their findings, the efficiency improved from 28.4% to 40.5% when the water flow rate was raised from 5 to 13 LPM. Because the PCM has poor conductivity, researchers have tried to enhance it in order to ameliorate the ETSCs’ efficiency. Either by adding fins to the collector or by integrating nanoparticles instead of PCM, or else by using metal foam. Elbrashy et al. [24] used an ETSC equipped with a nano-enhanced PCM (copper oxide nanoparticles with paraffin wax) at different mass flow rates to experimentally evaluate a solar air heater. The experimental findings showed that at a mass flow rate of 0.05 kg/s, the suggested ETSC attained maximum thermal efficiencies of 62.66% with PCM and 37.34% without PCM. Dhaou et al. [25] studied numerically an ETSC incorporated with PCM and metal foams and fins. Two distinct kinds of metal foams were tested. The result revealed that adding metal foam and fins significantly enhanced the thermal performance of the ETSC. The total processing time was reduced by about 9% when compared to pure PCM. Furthermore, it was revealed that the pore size of the metal foams had a negligible impact on the dynamic process of heat storage and release in the ETSC/PCM system. Elarem et al. [26] conducted numerical studies on a direct-flow ETSC integrated with a nano-PCM equipped with copper fins. Results show that the PCM melts more quickly with a decreased fin thickness. An U-tube ETSC with a nano-enhanced PCM (a combination of paraffin wax and copper nanoparticles) was experimentally explored by Algarni et al. [27]. Aluminum fins were added to enhance the heat transfer in the system. As a consequence, the collector efficiency was improved by integrating the PCM, and the system’s overall performance was improved by 32% in the case of paraffin wax that had been nano-enhanced. The presence of an aluminum fin within a U-tube ETSC coupled with PCM was examined by Abokersh M. et al. [28]. According to their experimental findings, the finned U-tube ETSC had a daily system efficiency that was approximately 14% greater than the unfinned U-tube ETSC. In an experimental investigation, Kumar et al. [29] used an ETSC with PCM and nanocomposite PCM (nanoparticles of SiO₂ with paraffin wax). Compared to the situation without PCM, the solar collector’s energy efficiency increased by 16.05%. The literature review shows that despite the several studies [30,31,32,33] that have been conducted on ETSC systems, none of the studies have compared different configurations of a direct-flow ETSC and a conventional heat pipe ETSC. The impact of the heat exchanger design on the performance of ETSC still needs to be investigated. The main goal of this paper is to study the thermal performance of four direct-flow ETSC configurations and compare them to the conventional heat pipe ETSC. The optimal ETSC design with the maximum thermal efficiency may be found by researching the impact of various operating situations on the heat pipe ETSC’s thermal performance.

2. Description of the ETSC Systems

The main purpose of this work is to conduct a comparative experimental test on the performance of the two most common categories of ETSC, namely, the direct-flow ETSC and the heat-pipe ETSC. The study was performed on the thermal performance of the ETSC according to the ETSC design and its operating conditions.

2.1. Heat Pipe ETSC System

Ten evacuated glass tubes connected to a manifold form the heat pipe ETSC system, as seen in Figure 1. The evacuated tubes are made from low-emissivity borosilicate glass with 700 mm of length and 58 mm of outer diameter. Inside each evacuated tube is inserted a heat pipe of 650 mm in length and a 21 mm diameter. A copper pipe with a larger diameter bulb at one end makes up the heat pipe. Within the heat pipe is a small amount of high-purity liquid water that is heated and evaporated at a high temperature in the base of the pipe (the evaporator), and the heated vapor rises to the top of the pipe (the condenser). Heat is transferred from the vapor that has reached the condenser to the solar thermal collector manifold. The heat then travels via the manifold to the heat medium. The fluid in the heat pipe condenser then cools and returns to the heat pipe evaporator at the bottom. As long as thermal energy is available, this heating and condensation cycle will continue. The outside of the inner evacuated tube is covered by a solar selective coating, which is in contact with a circular aluminum fin. The selective covering absorbs the radiation, which is collected as thermal energy and transferred to the heat pipe via the aluminum fins. The heat pipes are connected to a manifold that contains flowing water. To greatly reduce heat losses, the manifold is completely insulated using foam insulation materials.

Table 1. The geometric parameters of the evacuated tubes.

Parameters	Heat Pipe ETSC	Direct-Flow ETSC
	Heat Pipe	U-Tube	Double U-Tubes	Coaxial Tubes	Helical Tube
Glass tube diameter (mm)	58/54	58/54	58/54	58/54	58/54
Glass tube length (mm)	700	700	700	700	700
Pipe diameter (mm)	15.88/25.4	7.94	7.94	7.94/34	7.94
Length (mm)	680	1360	2720	680	5730

is a list of the geometric parameters of the ETSC system.

2.2. Direct-Flow ETSC Configurations

Two tubes of glass—one thinner and one thicker—combine to form the ETSC. The glass tubes are both coated with a selective material and connected to curved aluminum fins that are part of a copper tube heat exchanger. Water runs through the tubes of the heat exchanger as the heat transfer fluid. Direct-flow ETSCs come in several varieties, distinguished by the arrangement and design of the tubes in the heat exchanger. Four heat exchanger configurations were proposed and tested experimentally in this study: U-tube, double U-tube, coaxial tubes, and helical tube. Table 1 presents the geometric parameters of these heat exchanger configurations.

(i)

U-tube configuration

The U-tube shape is one of the most prevalent heat exchanger designs in the direct-flow ETSC, and the reason for that is simple geometry. The U-tube is made up of copper material with a length of 1360 mm, an outer diameter of 7.94 mm, and 36 mm distance between its two legs. The U-shaped tube is wrapped with an aluminum fin with a 0.8 mm thickness to absorb the heat received from the evacuated tube. A schematic of the U-tube ETSC type is shown in Figure 2a.

(ii)

Double U-tube configuration

The double U-tube configuration consists of two U-shaped cooper tubes, each of which has an outer diameter of 7.94 mm and a length of 2720 mm. The twin U-tubes are attached to an absorber fin. It should be possible to firmly fix the fin within the inner glass tube, and it is broad enough. The fin is made of aluminum and attached over the full height of the double U-tube. Figure 2b presents the double U-tubes ETSC type.

(iii)

Coaxial tube configuration

The direct-flow ETSC is integrated with two coaxial pipes, which are defined by a single pipe-in-pipe structural penetration inside the tube envelope. The coaxial tubes are made of twin coaxial pipes welded at the bottom and connected to an absorber fin. The water enters the inner pipe and leaves through the outer pipe, as shown in Figure 2c. The coaxial tubes were 650 mm in length, and the diameters of the outer tube and the inner tube are 7.94 mm and 34 mm, respectively.

(iv)

Helical tube configuration

Helical tubes make efficient heat exchangers because they increase the surface area in contact with the fluid to be heated. The helical tube has 59 turns and a total pipe length of 5730 mm with an outside diameter of 54 mm. The helical tube was fitted on the inner glass tube (Figure 2d).

Figure 2. Configurations of the direct-flow ETSCs: (a) U-tube configuration; (b) Double U-tube configuration; (c) Coaxial tubes; and (d) Helical tube.

Figure 2. Configurations of the direct-flow ETSCs: (a) U-tube configuration; (b) Double U-tube configuration; (c) Coaxial tubes; and (d) Helical tube.

3. Experimental Methodology and Set Up

3.1. The Experimental Setup

Figure 3 depicts the experimental setup, which consists of an ETSC system (with 10 tubes) covered by a solar light simulator. It has an open-loop circuit and the necessary measuring equipment.

Experiments were carried out by placing the collector facing the solar light simulator (Figure 3). For evaluating and testing the ETSCs in a lab setting, the solar simulator has been employed as a controllable indoor test device. The sunlight simulator was built from an array of 15 iron base lamps, each with a maximum electrical power usage of 400 W and covering a 1 m² area. K-type thermocouples with an accuracy of 0.5% were installed in the test setup to measure the ambient temperature under the solar simulator, the inlet and outlet water temperatures in the ETSC, and finally, the condenser temperature and the fin absorber temperature. Using a data recorder, all these experimental parameters were monitored and recorded. The recordings are all averaged across intervals of 2 s.

The indoor experimental apparatus was created and manufactured to examine the performance and dynamic behavior of the two categories of ETSC (heat pipe ETSC and direct-flow ETSC) under identical testing settings and without the interference of the outside environment. Several experimental tests were carried out for the heat pipe ETSC system under different operating conditions, such as the solar intensities and the tilt angle of the ETSC, as well as the water flow rate. Moreover, a number of experimental tests were conducted especially for different heat exchanger designs for the direct-flow ETSC.

In general, at the beginning of each experimental test, the test rig was implemented, where a flow meter was used to adjust a constant water flow and a pyranometer was used to adjust the constant irradiance that was delivered by the solar simulator. The inlet and outlet water temperatures are measured for calculating the difference in temperature and the useful heat. Then, the ETSC’s thermal efficiency was calculated.

3.2. Performance Analysis

The thermal efficiency of the ETSC, which is the ratio of output energy to input energy, may be used to measure thermal performance. The heat gained by water flowing through the manifold header in this case is the output energy, while the solar radiation hitting evacuated tubes is the input energy.

$$\eta =\frac{Q_{gain}}{A·I}$$

(1)

where A is the area of the evacuated tubes, I is the solar irradiance and $Q_{gain}$ is the amount of heat gained by the water; it can be written as

$$Q_{gain}=\dot{m}{C}_p\left(T_{out}-T_{in}\right)$$

(2)

where $\dot{m}$ is the mass flow rate, C_p is the specific heat of water, T_in and T_out are the inlet and outlet water temperatures.

4. Results and Discussions

To determine the uncertainty of the experimental results, the method of propagation errors was employed. The following expression was used to determine the findings’ uncertainty

$$U_R\hspace{0.17em}=\sqrt{{\displaystyle {\sum }_{i=1}^n{\left(\frac{\partial R}{\partial x_i}{U}_i\right)}^2}}$$

(3)

where R is a function of the measured quantities x_i (e.g., R = R(x₁, x₂,… x_n)) and it is the total uncertainty (i.e., systematic and random errors)

$$U_i\hspace{0.17em}=\sqrt{{\displaystyle {\sum }_{i=1}^n{\epsilon }_{s,i}^2}+{\displaystyle {\sum }_{i=1}^n{\epsilon }_{r,i}^2}}$$

(4)

The uncertainties of measured parameters are shown in

Table 2. The uncertainty of the instruments.

Instruments	Measurement	Accuracy (±%)
Thermocouple: type K	Temperature (°C)	0.5
Pyranometer	Radiation (W/m2)	10
Flowmeter	Flow rate (LPM)	2.6

. Thermal efficiency uncertainty was assessed using the degree of measurement uncertainty. An overall uncertainty of 3.6% was revealed for the thermal efficiency.

To conduct comparative studies, two experimental set-ups of two ETSC water heater systems were installed and tested. In the first one, a heat pipe ETSC system was installed, allowing the study of the effect of the solar irradiation, the tilt angle and the water flow rate on the outlet water temperature, and the thermal efficiency of the heat pipe ETSC. The second experimental set-up was used to test different configurations of a direct-flow ETSC and to identify the optimal design for the greatest thermal performance. Indeed, four configurations of direct-flow ETSC (U-tube, double U-tubes, coaxial tubes, and helical tube) were tested and compared to the conventional heat pipe ETSC. The outcomes of the various experiments are shown in the following sections.

4.1. Study of a Conventional Heat Pipe ETSC System

The dynamic characteristics and thermal performance of the typical heat pipe ETSC system were studied for each of the three operating parameters (solar irradiation, tilt angle, and water flow rate). The solar radiation that penetrates the ETSC system is first absorbed by the inner tube; then, it is transmitted to the aluminum fin in the ETSC, the heat pipe’s evaporator, and subsequently, the heat pipe’s condenser, through an evaporation and condensation process. The heat pipes are connected to a manifold containing flowing water. The impact of the most important operating parameters on the ETSC was comprehensively discussed and analyzed to identify the most effective system.

4.1.1. Effect of the Solar Simulator Intensity

The developed sunlight simulator was used as controllable indoor test equipment under laboratory conditions for varying the solar irradiance to test the proposed evacuated solar collectors. To study the impact of solar irradiation on the heat pipe ETSC performance, three intensities were chosen: 300, 500, and 1000 W/m². The flow rate and the inlet water temperature in this experiment, for all cases, were fixed at 0.6 LPM and 36 °C, respectively. The inlet water was heated through the manifold connected to the heat pipes in the ETSC system. The experiment-related data sets were acquired and examined.

Figure 4 illustrates the temperature profiles over time for the top and bottom glass tube surfaces, the fin aluminum absorber surface, and the condenser surface at various solar irradiances (300, 500, and 1000 W/m²). These temperature variations of the heat pipe ETSC rise progressively until they reach the maximums of 145 °C, 62 °C, 130 °C, and 105 °C, respectively. After 2500 s, the temperature changes start to fall quicker in order to reach the initial temperature since the solar intensity has significantly decreased. Figure 4a,b shows that the top glass tube temperature and the bottom glass tube temperature range between 28–145 °C and 28–62 °C, respectively, for solar irradiance values of 300 to 1000 W/m². The temperature changes are directly related to the rise in solar irradiance.

Figure 4c,d presents the temperature changes of the aluminum fin absorber and the condenser surface of the ETSC heat pipe. It can be seen that the temperature changes range between 28–130 °C and 28–105 °C, respectively. It was noticed that higher solar irradiance leads to larger fins and condenser temperatures. This has a significant influence on the heat pipe ETSC performance efficiency. The degree of these temperature changes is directly related to the rise in solar intensity.

The profile of the outlet water temperature at different solar irradiance fluxes (300, 500, and 1000 W/m²) is shown in Figure 5a. In every situation, 36 °C of inlet water temperature is provided. According to the figure, a higher amount of solar radiation causes an increase in the water’s temperature as it passes through the manifold. The findings further highlight the temperature profile’s non-linear feature. Additionally, as seen in Figure 5a, the outlet water temperature rises as the solar intensity rises.

The thermal efficiency of the ETSC heat pipe system has been calculated for different solar irradiances, as illustrated in Figure 5b. It can be seen from this figure that the thermal efficiency of the ETSC decreases as solar irradiance increases. Similar experimental results were found by Mahmoud Sh. et al. [34]. This is an understandable finding given that at higher solar radiation values, the ambient temperature just underneath the simulator and the surface temperatures of the ESC tubes increases. Hence, its operating effectiveness is significantly impacted by this. This can be clarified by the fact that the solar collector’s heat output had surpassed its saturation limit.

4.1.2. Effect of the Tilt Angle

Experiments were carried out for various tilt angles of the collector (with constant solar irradiance of 1000 W/m²), and in every situation, an inlet water temperature of 22 °C was provided. The heat pipe ETSC system was exposed to the solar simulator with different tilt angles of 0°, 30°, and 90° from the horizon in order to investigate the impact of the inclination on the thermal efficiency of the ETSC system. Figure 6 illustrates the impact of the tilt angle on the condenser temperature, the outlet temperature, and the heat pipe ETSC system’s efficiency. In Figure 6a,b, it is evident that the wall condenser temperature fluctuates; this is due to the condensation phenomenon in the condenser, where the vapor releases its heat to the header. Then, the vapor condenses and returns to the evaporator, where it is heated again. As long as thermal energy is available, this heating (i.e., vaporization) and cooling (i.e., condensation) cycle will continue; hence, a fluctuation of the condenser wall temperature happens, as seen in Figure 6b.

Additionally, experiments revealed that for the ETSC-30, the heat pipe end received the circulation of the condensed water, which then evaporated and returned as steam to the condenser with a clear water circulation loop; therefore, the condenser reached its highest temperature, as shown in Figure 6a,b. Whereas for the ETSC-0 and ETSC-90, the situation was the same as the ETSC-30; however, without a clean water circulation loop, the condensed water on its route to the heat pipe end was partially or completely mixed with the hot steam returning to the condenser; furthermore, such mixing became more intense with a tilt angle of 0° and 90°, i.e., horizontal and vertical situations of solar tubes. To allow the internal liquid water of the heat pipe to flow back down to the evaporator at the bottom of the tube, the evacuated tube collector and heat pipe must be positioned with a minimum tilt angle (of around 30°). This indicated that there is an optimum tilt angle for the heat pipes, increasing or lowering the heat pipes’ tilt angle relative to the optimum angle had no beneficial influence on the water’s thermosiphon circulation inside the heat tubes.

To compare the heat pipe ETSC’s thermal efficiency even further, another comparison at a different tilt angle for the outlet water temperature and the ETSC’s thermal efficiency is presented in Figure 6c,d. In the figure, it is clear that the outlet water temperature and the thermal efficiency of the ETSC-30 were still higher than those of the ETSC-0 and ETSC-90. According to the experimental data, the thermal efficiency of the heat pipe ETSC was significantly impacted by the tilt angle. The results indicated that with a 30° tilt angle, the ETSC reached a maximum outlet water temperature of 36.5 °C and a thermal efficiency of 36%. The experimental results are similar to those found by Madhuri et al. [35]. It was observed that the behavior of heat pipes is very poor in horizontal (0°) and vertical (90°) situations (especially for high power input). The tilted situation is preferred for obtaining high thermal performance from the heat pipe [35]. Due to gravity’s impact on inclination, when the heat pipe is tilted (at around 45°), its thermal resistance is lower compared to that in vertical and horizontal situations [35].

4.1.3. Effect of the Mass Flow Rate

The ETSC system was tested using various mass flow rates. Figure 7 depicts the trend of output water temperature for three different flow rates (0.6, 1.2, and 2.4 LPM) with a constant solar heat flux (1000 W/m²) and inlet water temperature of 22 °C. Higher flow rates result in lower outlet water temperatures and a nearly non-linear temperature profile, as seen in the figure.

The outlet water temperature profile over 80 min is shown in Figure 7, using different flow rates of 0.6, 1.2, and 2.4 LPM. Results reveal that at 0.6 LPM, the outlet water temperature climbs to a maximum of 34.1 °C, while at 1.2 LPM and 2.4 LPM flow rates, it rises to 29.5 °C and 24.1 °C, respectively. Furthermore, at the 2400 s, the outlet water temperature starts decreasing as solar irradiation remains at 0 W/m², and it continues to decrease until it becomes constant with the inlet water temperature. It was shown that for the ETSC system, the maximum outlet temperature seems to be much smaller at a 2.4 LPM flow rate than at a 0.6 LPM flow rate. This is caused by a decrease in the amount of time that water has to contact the condensers as a result of an increase in water flow rate. The experimental results proved that at lower flow rates, the heat pipe ETSC system performs effectively.

4.2. Effect of the Direct-Flow ETSC Configuration

Four single ETSC configurations were tested under identical experimental conditions in order to assess the ETSC’s effectiveness in accordance with the design of the direct-flow ETSC type. Experiments were carried out for different ETSC configurations with constant solar irradiance (1000 W/m²), and in every situation, an inlet water temperature of 22 °C was provided. The ETSC’s performance was analyzed based on the inlet and outlet temperature differential’s fluctuation. A comparison between two types of ETSC is studied in this part of the article; the heat pipe ETSC and the direct-flow ETSC, as presented in Figure 8. This depicts the change in the outflow water’s temperature over time for five types of heat exchangers: (1) Heat pipe; (2) U-tube; (3) Double U-tube; (4) Coaxial tube; and (5) Helical tube. By using a single evacuated tube for each case and during the 2400 s results show (Figure 8a) that for the helical tube, the outlet temperature rises to a maximum of 27.5 °C, while for the coaxial tube, double U-pipe, and U-tube ETSC heat exchangers, the outlet temperatures reach 26.1 °C, 26 °C, and 25.4 °C, respectively. Furthermore, the ETSC heat pipes attained a maximum temperature of 24.8 °C. Temperatures then begin to drop as soon as the collectors stop receiving solar irradiation, reaching 23.5 °C, 23.4 °C, 23.3 °C, 23 °C, and 22.8 °C for heat pipe, double U-tubes, coaxial tubes, helical tube, and U-tube, respectively. It is concluded that an ETSC with a helical tube can provide hot water at a higher temperature than a typical heat pipe ETSC.

The ETSC system with a helical tube has the highest outlet water temperature compared to the other ETSC configurations (Figure 8a). This can be explained by the fact that the helical tube is connected directly to the inner area of the glass tube (coated surface) without aluminum fins. However, for the other configurations, the cooper tubes are wrapped with aluminum fins to absorb the heat received from the evacuated tube. Despite the fact that copper has a greater conductivity than aluminum, this outcome is caused by the different surface areas and forms of copper tubes and aluminum fins.

Measurements of efficiency were used to assess the performance of the direct-flow ETSCs (Figure 8b). The highest efficiency was 69%, which was obtained by the direct-flow ETSC system with the helical tube. It was also observed that the greatest water temperature at the smallest flow rate (0.6 LPM) is seen for the helical tube ETSC. Compared to the reference case (i.e., ETSC with heat pipe), the thermal efficiency of the ETSC is improved by integrating the different configurations. According to Figure 8b, the ETSC with a heat pipe has the lowest efficiency, i.e., the heat transfer rate is low, due to the lower surface area and the deficient heat transfer between the absorber and the heat pipe due the losses heat, consequently affecting the difference’s temperature water and the efficiency. Finally, it was concluded that the design parameters of the integrated heat exchanger should be optimized to avoid the delay of the heat transfer rate in the ETSC.

Additionally, the results demonstrate that the water flow rate is the most important factor in determining the ETSC’s thermal efficiency and the resultant temperature difference. In addition to having high thermal performance, the proposed design is also straightforward, robust, dependable, and reasonably priced. This design can be used for water heating. By including fins in the heat transmission (heat exchanger) parts, the study’s findings may be used as a tool to improve ETSC performance. Future research will examine the effects of adding bigger aluminum fins with a simpler parabolic concentrator to the system to further improve collector efficiency. The heat transmission between the water and the copper tube will be improved by the increased fin regions.

4.3. Comparison with Previous Studies

The findings of several ETSCs’ thermal performance assessments are summarized in

Table 3. Summary of previous studies on performance assessment of ETSC water heaters.

Type of ETSC	Solar Irradiation (W/m2)	Flow Rate (LPM)	Temperature Difference (°C)	Efficiency (%)	Reference
Heat pipe ETSC
Helical finned heat pipe (pitch 40 mm) (with PCM)	230–751	0.165	10.5	47.39	[36]
	50–850	0.335	8.5	50.29
	180–780	0.5	5.1	63.25
	50–750	0.665	3	62.85
Conventional heat pipe (with PCM)	230–751	0.165	9.2	47.30	[36]
	50–850	0.335	7.5	52.20
	180–780	0.5	3.1	54.97
	50–750	0.665	1.8	55.32
Helical finned heat pipe (pitch 20 mm) (with PCM)	740	0.165	18.2	63.5	[36]
	749	0.335	8.8	60.78
	730	0.5	5	66
	798	0.665	8.5	68.66
Helical finned heat pipe (pitch 40 mm)	740	0.165	18	60.56	[37]
	749	0.335	8.5	57
	730	0.5	4.9	61.27
	798	0.665	8.3	39
10 evacuated tubes (with PCM)	992	8	43	79.98	[38]
		12	31	81
		16	28	82.5
		20	23.5	87.8
		24	21	84.2
10 evacuated tubes (without PCM)	1000	8	74	40.92	[39]
		16	61	53.22
		24	53.7	54.1
Direct-flow ETSC
U-tube (with PCM)	550–93	-	26	22.96	[40]
U-shaped spiral tube with 2-lobe (with PCM)	550–930	-	29	25.79	[40]
U-shaped spiral tube with 3-lobe (with PCM)	550–930	-	30	26.5	[40]
U-shaped spiral tube with 4-lobe (with PCM)	550–930	-	31	27.91	[40]
U-pipe (with nano-PCM)	950	0.08	10	29	[27]

. Some of these studies employed a parabolic concentrator to raise the water’s outlet temperature. Various numbers of evacuated tubes were utilized in the investigations, depending on the particular subject. The flow rate and number of tubes utilized have a considerable effect on the efficiency of ETSC. Table 3 makes it evident that utilizing a PCM raises the thermal performance of the ETSC system. The table shows that, similar to this investigation, the biggest temperature differences between intake and output were observed when taking into account the lower flow rates. Based on the analysis of the review table and the findings of the experiment, the flow rate has the greatest impact on both the measured outlet water temperature and the ETSC’s thermal efficiency.

According to our knowledge, it is the first time that this helical tube type has been integrated into the ETSC and tested under these conditions. Along with being thermally efficient, the proposed design is also uncomplicated, robust, dependable, and reasonably priced.

5. Conclusions

An indoor experimental test rig was created to investigate the thermal performance of two categories of evacuated tube solar water heater systems, namely, the direct-flow ETSC and the heat-pipe ETSC. Several experimental tests were carried out under lab testing settings using a solar simulator. The following two features constitute this work’s novelty: (i) The heat pipe ETSC system was tested under different operating conditions, such as solar intensity, tilt angle, and water flow rate; (ii) Four configurations of direct-flow ETSC (U-tube, double U-tube, coaxial tubes, and helical tube) were tested and compared to the conventional heat pipe ETSC. The following conclusions were obtained from the experimental studies.

For higher solar irradiation (1000 W/m²), the ETSC heat pipe collector has given higher outlet water temperatures of about 40 °C.

To allow the internal liquid water of the heat pipe to flow back down to the evaporator at the bottom of the tube, the evacuated tube collector and heat pipe must be positioned with a minimum tilt angle (of around 30°). This indicated that there is an optimum tilt angle for the heat pipes. The thermosiphon circulation of the water inside the heat tubes was unaffected by changing the tilt angle of the heat pipes compared to the optimum tilt angle.

The experimental results proved that ETSC presents a great performance at lower flow rates.

The ETSC with helical tubes provided the highest thermal performance (69%), followed by the ETSC with coaxial tubes (54%), then the double U-tubes (52%), then the U-tube (41%), and then the heat pipe (31%). When compared to other configurations, the ETSC with the helical tube showed a higher outlet water temperature.

To the best of our knowledge, this is the first helical tube type to have been integrated into the ETSC and tested in this manner. Additionally, the ETSC’s efficiency and the water temperature were much higher for the four other ETSC designs (U-tube, coaxial tube, and heat pipe).