Thermal Performance Analysis of Thermosyphon Solar Water Heating System Using Overlapped and Reverse Flow

2.1 Improvement in the tube geometry

Aldulaimi [1] conducted an experiment to determine the values of Nu and EEC, as well as the temperature and pressure differentials in fluid flow. This study examined the production of reverse and overlapping flows in parabolic and trough collectors by inserting a twisted tube at the center of a circular tube in a variety of configurations. The novel design was validated using five receiver tube (Rt) models, each with two types of overlapping flows. The best model, which used a different mass flow rate, outperformed the regular model by 42.917%-38.528%.

Ayompe and Duffy [2] used information from a field examination that lasted for one year. The thermal efficiency of a solar water heating system with a heat-pipe evacuation tube collector was examined. An automated subsystem for managing hot water draw-offs and electric immersion heaters was designed and installed to mimic the functioning of residential solar water heating systems. The yearly average daily energy production was 20.4 MJd^-1, and the system efficiency was 52.0%.

Yang et al. [3] conducted a series of experiments to enhance the heat transfer performance of two types of molten salt receiver tubes, namely smooth and spiral tubes, under conditions of high temperatures and high heat flux. The experimental results indicated that the Nusselt numbers for the spiral tube with heat transfer improvement ranged from 400 to 1200. These findings suggest that using a spiral tube as part of the heat transfer system significantly improved the performance of the molten salt receiver and reduced radiation and conduction losses.

Rodríguez-Sánchez et al. [4] conducted a year-long field trial to investigate the thermal efficiency of a solar water heating system employing a heat pipe evacuation tube collector. To mimic the typical operation of household solar water-heating systems, an automated subsystem was devised and installed to manage hot water withdrawal and electric immersion heaters. The system exhibited an efficiency of 52.0% and an average daily energy production of 20.4 MJ d-1 over a year.

Ananth and Jaisankar [5] used laminar flow conditions to examine the heat transfer and friction factor properties of solar water heaters fitted with twisted inserts comprising rods and spacers of various sizes. The findings showed that, while the collector performance indicator, the heat loss coefficient, increased, the heat removal factor decreased as the lengths of the rods and spacers increased. Moreover, twisted inserts with rods exhibited a more substantial improvement in heat transfer than those with spacers. Under the same operational conditions, the thermal efficiency of the best-performing model (a completely twisted tube) was 1.15 times higher than that of a standard flat-plate collector (FPC) with a plain tube.

Saravana et al. [6] analyzed the heat transfer and friction factor characteristics of V-trough solar water heaters (SWHs) fitted with helical twist tapes with different twist ratios (Y=3, 4, 5, and 6), including experimental research. The results showed that, in comparison with a flat-plate collector, the V-trough reflector increased the heat transfer efficiency by 13.64%. Moreover, the thermal efficiency of the V-trough collecting tube equipped with the twisted tape, which had a lower twist ratio of 3, was 5.41%, 9.91%, 14.77%, and 18.91%, respectively, greater than the thermal efficiency with twist ratios of 4, 5, 6, and without the twisted tape (PVT collector).

Jaisankar et al. [7] conducted an experimental examination of thermosiphon water heating systems (TSWHS) equipped with helical and left–right twisted inserts with a twist ratio of three. The left–right system provides a two-way swirling flow, which resulted in increased heat transmission and pressure loss compared with the helical system. The helically twisted insert created a one-way swirling flow along the riser tube. The left–right twisted insert collector exhibited a notable enhancement as compared with a simple tube collector; its heat transfer and friction factor were 3.75 times (375%) and 1.42 times (142%) greater, respectively.

Barbosa et al. [8] designed and tested two economical solar heating systems (ESHs) for thermal assessment. Their setups incorporated flat-panel solar collectors and heat storage components, which were constructed from PVC tubing, bottle caps, and durable packaging materials. Three temperature readings were recorded. When compared with the LCSHS, the LCSHP exhibited more outstanding performance. The highest temperatures and efficiency values for LCSHP were 49.7℃ and 40.9%, respectively, and those for LCSHS were 47.8 °C and 37.8%, respectively.

Wenceslas and Ghislain [9] utilized optimization findings to construct a flat-plate solar collector using locally accessible materials. A genetic algorithm was used to write the computer code, which assisted in obtaining the optimal combination of design parameters that optimized the exergy efficiency. The highest energy efficiencies obtained via the experiment and simulation were 68.71 and 66.01%, respectively, whereas the maximum energetic efficiencies were 20.34 and 18.65%, respectively.

Jawad et al. [10] used an experimental setup comprising a pair of copper tubes arranged in a dual-spiral configuration, with the inner surface of the solar collector coated in matte black paint containing a 5% blend of nanomaterials alongside a thermoplastic dye (TiN). The investigation reported a close correspondence between the theoretical predictions and analytical outcomes, indicating a strong agreement between the two sets of results.

Amraoui [11] compared two solar collector designs, with a focus on enhancing Ben Slama Romdhane's collector by introducing two additional airflow passages to boost the heat transfer. They conducted a 3D simulation of a flat air solar collector featuring transverse baffles to induce turbulence and augment the exchange surface area. The ANSYS computational code facilitated swift and cost-effective simulations, yielding prompt results.

2.2 Improved with coating

Sahu et al. [12] developed a novel and affordable combination of Fe and Mg. These coatings were specific to flat-plate absorbers of solar water-heating systems. Various analytical methods were used to characterize the composition of the coating mixtures. Two solar-collector prototypes were built. Fe-modified black paint had better excellent thermal absorption, with FPA2 having a 60% efficiency compared with FPA1's 48%.

Zelzouli et al. [13] found that a horizontally oriented storage unit with low wall conductivity could be merged with a flat-plate collector with a selective black-chrome-coated absorber. These findings indicate that polyurethane is a highly effective material for insulating thermal systems. A 2% enhancement in annual performance was achieved by reducing the collector's heat loss coefficient by 1 W/m² K.

2.3 Improvement in the geometry of solar panels

Lu et al. [14] created dynamic models to predict the thermal efficiency of a residential thermosyphon solar water heating system. A dedicated outdoor experimental setup was devised to confirm the accuracy of the model. The optimal airgap thickness for the thermosyphon solar water heating system was 25 mm, as this was the point at which heat loss through the glass cover plate stopped decreasing. Furthermore, both the energy loss and ultimate water temperature in the tank decreased as the water tank volume increased.

Khalifa et al. [15] developed a thermal storage solar collector composed of six 80-mm-diameter copper pipes linked in series. This collector was used in conjunction with a rear container that held paraffin wax, which served as a phase change material (PCM) to store heat. The ability of the system to heat water is verified through outdoor trials. The temperature of the plate increased at a distance of 2.5 m from the entrance site before stabilizing. The storage system's immediate efficiency varied between 45% and 54%.

2.4 Solar panel tank improvements

Zhang et al. [16] calculated the yearly performance of two-loop thermosyphon (LT) systems using the traditional SWH system investigated in this study. The supply duration, heat gain, and nocturnal heat loss were evaluated for two operational scenarios. They revealed that, based on calculations, the annual average heat loss ratio for the SWH system was 15.07%, whereas that for the LT-SWH system was 6.15%.

Naveen et al. [17] conducted a comprehensive stratified experimental investigation of heat energy-storage tanks in SWHs. Their study encompassed tanks with various configurations, including those without a PCM, those with a PCM but lacking fins, those with a PCM featuring ring-type fins, and those equipped with spiral fins. In each scenario, the efficiencies of hourly charging and discharging were assessed and juxtaposed. The highest charging efficiencies observed in the PCM were 71% for the spiral module, 66% for the cylindrical PCM module without fins, and 58% for the cylindrical PCM module with fins.

4.1 Experimental simulation system

Figures 1 and 2 show a visual illustration of the model and a schematic of the device, respectively. Solar energy simulation relies on simulating the transfer of solar energy to a solar receiver through radiation, which is simulated in a laboratory [3]. The simulated model included a thermal heating element that was used to simulate solar energy to stabilize the thermal energy and change the mass flow rate. It was installed on a structure composed of parabolic steel to reverse the heat flow toward the pipe and prevent significant heat losses. The thickness of the steel sheet was 1 mm and the width was 300 mm with the extension of the device (Table 1). The heater was connected to a variable-voltage device (TDGC-series Contact Voltage Regulator). The tube model was fixed 20 mm from the surface of the heater using the second steel ring connected to the structure of the external device. The four models and normal model were applied to the heat flux. The flow in progress was measured once for the px flow and again for the py flow. A flowmeter was used at the exit point to control the fluid flow velocity. Four different fluid flow rates were recorded for each model. K-type sensors were positioned at the entry, exit, and reflection points along the direction of the HTF, and one sensor was installed on the outer surface of the outer tube to measure the surface temperature. The heat data collected were recorded using twelve-channel, TM 500-model thermal data recorder. The readings recorded by the data logger were stored on an SD card placed inside the recorder and extracted using a computer. Readings were taken after reaching stability at five-minute intervals, and the device was calibrated with another approved meter before use. A differential pressure manometer with 1-mbar precision was used to detect the pressure differential within the Rt.

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Figure 1. Simulated solar collector

Table 1. The characteristics of the simulation system

Feature	Value	Feature	Value
Aperture area, Aa	0.12 m2	Length, l	1 m
Receiver area, Ar	0.11m2	Aperture width, Wa	0.035m
Material of the tubes	copper	Working fluid	Water
Direct heat flux, Idth	1000 W

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1. receiver tube (Rt), 2. electrical flat heater, 3. steel reflector, 4. micro-flowmeter, 5. data logger, 6. differential pressure manometer, 7. p_x flow, 8. p_y flow, 9. contact voltage regulator (TDGC series), 10. electrical source, 11. valve, 12. inlet fluid pressure measurement point, 13. outlet fluid pressure measurement point, 14. thermocouple sensors for surface temperature, 15. temperature of the reverse fluid flow measurement point, 16. inlet fluid temperature measurement point, and 17. outlet fluid temperature measurement point.

Note: The schematic represents the third model (Rt₄) and p_y flow

Figure 2. Schematic of the experimental simulation system

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Figure 3. Photograph of (TSWHS)

4.2 Description of the TSWHS

The system is depicted in Figure 3 and an explanatory diagram is presented in Figure 4. Once the experimental verification was completed and the model that exhibited the best thermal performance was identified, an experimental investigation was initiated for the flat-plate collector system (FPCS). This system was constructed using interconnecting and reverse-oriented tubes along with a standard model for comparative purposes. Copper was chosen as the material for the tubes due to its high thermal conductivity (386 W/m²K) and significant melting point of 1000 °C. Water was used as the operating fluid. The experimental model comprised a frame supporting the SWH models and a storage tank with a capacity of 196 L. The frame was constructed from iron and insulated with two layers of wood and glass fibers to encompass the solar receiver, thereby reducing heat loss. The dimensions of the frame were 1020×1190 mm with a height of 25.5 cm and an insulator width of 5 cm. The tank was installed at the top of the frame and was completely insulated using fiberglass. The external surfaces of the solar collectors were coated with charcoal-black paint to enhance their absorption properties. The K-type thermocouples were attached to the inlet and outlet of the HTF, and six additional K-type thermocouples were positioned on the outer surfaces of the SRT models. Two sensors were inserted in the storage tank to measure the thermal gradient within the tube. Additional details are provided in Table 2.

Table 2. Physical specifications of the solar water heating system

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1. Solar collector, 2. horizontal tank, 3. flow meter, 4. data logger, 5. pump, 6. solar intensity sensor, 7. insulation supported by wooden frame, 8. valve, 9. inlet fluid temperature measurement point, 10. outlet fluid temperature measurement point, 11. two thermocouple sensors for tank temperature, 12. one of the six thermocouple sensors for the surface temperature. Note: Schematic represents the normal model (SRT₁).

Figure 4. Schematic of the experimental system

6.1 Simulation system experimental setup and test procedure

A practical simulation was performed in the laboratories of the Al-Nahrain University College of Engineering. The test was conducted from March 3-15, 2023, with an average maximum atmospheric temperature of 21℃. Readings were taken at 5-min intervals after reaching stability, typically approximately half an hour after activating the heater and systematically logged using a data logger. A minimum of four temperature readings were collected for each flow rate, including measurements of the entry-point temperature, point of reflection of the fluid flow, exit-point temperature, surface temperature of the pipe, and ambient temperature. The experimental simulation started with one flow path (${{p}_{x}}$) and then switched to another (${{p}_{y}}$) with four mass flow rates (100, 150, 200, and 250 mL/min) for each flow path. Measurement points were placed at the entrance and exit to continuously measure the differential pressure, which changed with the change in flow rate.

6.2 Experimental setup and test procedure for the experimental system (TSWHS)

Practical experiments were conducted at Baghdad's Al-Nahrain University within the College of Engineering (located at a latitude of 33.27°N and longitude of 44.38°E in Al-Jadira, Baghdad). These experiments were conducted from May 18-22, 2023, under clear weather conditions. During the measurements, the maximum temperature reached 41℃, with a relative humidity of 22.6% and average wind speed of 2.1 m/s.

Several measurements were conducted, including the inlet and outlet temperatures, as well as the temperatures at six specific points on the external surface of the Rt models. Furthermore, the temperature gradient within the tank was determined using two embedded thermocouples, and the average surface temperature for each Rt model and flow rate were computed. Temperature data from all the thermocouples were recorded every 60 s using data loggers over a 24-h period. The solar radiation intensity was also monitored with data loggers every 30 s, capturing the radiation levels per second over the course of 24 h. The choice of the flow path (py) was determined based on the proposed model because it exhibited the highest thermal efficiency.

The results of the experimental investigation indicated that, in comparison with conventional Rt models, there were enhancements in temperature difference (∆T), Nusselt number (Nu), efficiency (η), and EEC when the mass flow rate (ṁHTF) was in the range of 0.00167–0.00416 kg/s. When expressed as a percentage, these improvements clearly highlighted the differences between the average Rt and the improved models. The practical results showed that, when we compared the improved TSWHS model with the normal one, there was a difference in the thermal gradient inside the tank, which improved the thermal performance of the system. The maximum uncertainties for the five parameters referred to above were approximately ±0.81℃, ±2.23 mbar, ±2.5%, ±2.87%, and ±2.125%, respectively, after calibration of the devices used for measurement.

8.1 Experimental simulation results

8.1.1 Temperature difference

In all the simulated models, the temperature difference and pressure decreased as the mass flow rate increased, as depicted in Figure 8. All the greatest differences in temperature were obtained (15.6-11.8℃) in model Rt₄ compared with the ordinary model Rt₁ (10.1-4.2), with flow path ${{p}_{y}}$ and hydraulic diameter H_d of 1.27, which increased by 54.4-180.8%. To alter the fluid motion and swirl movement of model Rt₄, fins were added to the outside surface of the inner tube (model Rt₅), but the temperature difference increased (13.6-10.7℃) with flow path ${{p}_{y}}$, which was less than that of model Rt₄, with an increase of 34.6-154%. These results were all obtained for the same range of ${{}_{HTF}}~$(0.001667-0.004167 kg/s) and indicate an increase in energy absorption in the selected models.

8.1.2 Pressure difference

Figure 9 indicates a rise in the pressure variation (19-6 mbar) for model Rt₄ at flow path ${{p}_{x}}$, representing a power difference (137-100%) for the range $\text{ }\!\!~\!\!\text{ }{{}_{HTF}}=0.001667-0.004167\text{ }\!\!~\!\!\text{ kg}/\text{s}$, when compared with model Rt₁, which indicated a pressure difference (8-3 mbar) for the same $\text{ }\!\!~\!\!\text{ }{{}_{HTF}}$ range; the largest pressure difference (22-12 mbar) occurred for model Rt₅ with a flow path ${{p}_{x}}$, which increased by 175-300%. This increase occurred because of the increased friction and subsequent elevated pressure losses in the system. Increasing the pressure difference loss led to an increase in the energy expended during the movement of the fluid.

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Figure 8. The relationship between reynolds number and the temperature difference ∆T (℃) for all Rt models

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Figure 9. The relationship between the Reynolds number and the pressure difference (∆P m bar) for all Rt models

8.1.3 Collector efficiency η

For each R_t model and flow rate, calculations were performed using Eq. (2), as shown in Figure 10. There was an increase in efficiency with a decrease in mass flow rate (ṁ_HTF). The maximum η value occurred for model Rt₄ (37.3-70.5%) for a flow path p_y, which increased by 14.7-67.8% compared with the normal model Rt₁, which had an efficiency of 24-42.8%. This amounted to a 42.917-38.528% increase. All of these values were for the same range ($\text{ }\!\!~\!\!\text{ }{{}_{HTF}}=0.00167-0.00167~kg/s)$.

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Figure 10. The mean collector efficiency (η) values for all models of (Rt)

8.1.4 Nusselt number Nu

Figure 11 shows that the Nusselt number (Nu) gradually decreased with an increase in the mass flow rate. This trend can be attributed to a reduction in the heat transfer rate and the relative stability of the surface temperature beyond a certain rate. Nu was estimated for each R_t model. The highest Nu value was obtained for model Rt₄ (17.02-13.7) with the flow path of ${{p}_{y}}$ compared with the normal form Rt₁ (9-7), where an increase of (89–95%) represents the highest increase obtained.

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Figure 11. The relationship between mass flow rate and Nusselt number (Nu) for all Rt models

8.1.5 Efficiency evaluation criterion EEC

The EEC was computed using Eq. (6) to gauge the overall performance of the heat transfer unit, as explained in [1]. The results are presented in Figure 12. The findings indicated that the EEC increased notably as the flow rate increased, primarily because of the decrease in pressure loss at higher flow rates. Model Rt4 with the flow path py achieved the highest EEC value (ranging from 0.53 to 1.4), while model Rt5 with the flow path px recorded the lowest EEC value (ranging from 0.43 to 0.53). This considerable increase in pressure loss had a pronounced impact on the final EEC value.

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Figure 12. The relationship between mass flow rate and efficiency evaluation criterion (EEC) for all models of (Rt)

8.2 Experimental results

8.2.1 Temperature gradient

The experimental findings showed an enhancement in the heat transfer rate within the solar receivers. The normal model (SRT₁) and improved model (SRT₂) were compared, as shown in Figure 13. The thermal gradient inside the horizontal tank was calculated, and the results were recorded for an entire day under similar weather conditions. The highest thermal gradient value (58.4℃) was obtained for the normal model (SRT₁) at (12:16 PM), while the greatest thermal gradient (68.8℃) was recorded in the improved model (SRT₂) at (12:16 PM). A thermal improvement of 17.8% was achieved $(\frac{{{{\bar{T}}}_{Tank~SRT2~}}-{{{\bar{T}}}_{Tank~SRT1~}}}{{{{\bar{T}}}_{Tank~SRT1~}}}\times 100%$). This outcome is primarily contingent on the intensity of incident solar radiation on the TSWHs.

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Figure 13. The relationship between the time of the whole day and the temperature inside the tank

8.2.2 Direct solar radiation

The solar radiation intensity incident on the solar collector was monitored continuously for an entire day, spanning 24 h, and the data logger was used to measure the values and record the data. Values were recorded every 30 s starting at 8:16 AM and ending at 7:16 AM. The highest solar radiation intensity (803 W/m²) was recorded at 12:16 PM, as shown in Figure 14. This value was obtained because of solar orthogonality, the highest value of which occurred at this time.

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Figure 14. The relation between the time of the day and the intensity of the solar radiation