Solar water heater (SWH) with serpentine configuration uses a unique parallel flow thermal collector design. Explorations [24] show that the application of serpentine configuration yields approximately 2.62% more usable thermal energy compared to other configurations. In addition, the thermal performance of serpentine-type SWH is significantly influenced by the rate of heat transfer between the absorber surface and the water, as well as the amount of solar radiation incident on the absorber surface [25]. Compared to other SWH designs, serpentine configuration shows superior thermal performance, with a maximum thermal efficiency reaching 82.5% [26]. Eventually, serpentine SWH is also more economical compared to electric water heaters, giving rise to a cost-effective alternative to traditional heating systems [27].

Table 1 presents a comparative analysis of previous research on serpentine collectors and the current research, showing the novelty and differences of the methods. Research by Hasan et al. (2018) [28] using a serpentine thermosyphon flat-plate solar heater collector design showed that the exertion efficiency reached 3.72%. However, the application is limited to the influence of geometric and operational parameters without considering more economical materials. Hossain et al. (2016) [29] reported double-sided serpentine collector efficiency of 70.24% in June and 70.96% in July, but did not explore water flow rate variations that might further influence performance. Wang et al. (2019) [30] used phase change material (PCM) with a high melting point in Serpentine flat plate collectors and different pipelines. The complexity of using phase change material (PCM) showed the potential to increase costs and reduce economic competitiveness. Das et al. (2024) [31] achieved an energy efficiency of 62.28% in serpentine tubes with copper sheets behind photovoltaic (PV) panels. This showed that copper improved efficiency but cheaper pipe materials were not tested. Shahsavar et al. (2021) [32] found that the efficiency of 6 rib rifled serpentine increased by 22.5%, although dependence on nanofluid flow and nano additives can increase the cost and complexity of the system. Abdalla et al. (2023) [33] recorded an energy efficiency of 88.23% in sheet-and-serpentine tube collectors. The research did not sufficiently explore the influence of material variations and simpler designs. Finally, Chopra et al. (2024) [34] reported an efficiency of 82.45% for a serpentine evacuated tube collector (SR-EVTC) with copper annular fin. Meanwhile, the use of expensive materials such as copper can limit the applicability of this technology.

Despite the drawbacks, modifications to serpentine pipe arrangement aimed at increasing the amount of solar energy converted into useful heat had slightly reduced the total system efficiency. The higher complexity of manufacturing collectors with serpentine pipes also posed production challenges and increased material costs. However, the advantages offered by serpentine design served as attractive options for SWH applications. To further improve the thermal performance and efficiency of serpentine SWH, several investigations and developments are being conducted. These include optimizing serpentine flow design, exploring the use of new materials with higher thermal conductivity, and combining reflective methods to maximize solar radiation absorption.

This research focuses on a serpentine solar water heater (SWH) using low-cost copper pipe material and varying water flow rates. The results obtained 81.93% efficiency of heat absorbed by water, with the highest value achieved at a flow rate of 3 lpm. This is an important novelty, showing that efficiency can be achieved with more affordable materials, as well as identifying the potential of higher flow rates to reduce efficiency due to increased inlet temperatures. The results contribute to the development of more efficient and economical serpentine collectors, which are relevant in the application of more affordable renewable energy technologies. The current research position and collectors' research on serpentine can be seen in Table 1.

Solar water heater (SWH) featuring serpentine pipe configuration was used in the research. The system used copper pipes with a diameter of 12.7 mm and a length of 1850 mm, bent at a 90-degree angle. The collector, measuring 1500 × 840 mm, was painted black to improve heat absorption. The pipes were installed with a 50 mm spacing to ensure even heat distribution. The collector was tilted at 30° facing north to maximize exposure. The specifications of SWH with a serpentine design are presented in Table 2.

Before the experiment was conducted, all equipment and materials were prepared. These materials included serpentine SWH, a water pump with a capacity of 3600 L/h, water hoses, a 75-liter container box, and measuring instruments such as flow meter, solar power meter, anemometer, and arduino thermocouple in Table 3 as:

Figure 1 shows SWH research instrument used to obtain data, including the collector design and configuration. The collector plays a role in absorbing heat from solar radiation. In this research, the collector uses a serpentine configuration with materials namely copper pipes.

Figure 1.

Collector design and configuration

The research was conducted in Solo, Central Java, Indonesia, at an elevation of 105 meters above sea level with latitude and longitude coordinates of -7.560180 and 110.774247, respectively. Data collection was performed from 9:00 AM to 3:00 PM (Western Indonesian Time), with readings taken every 10 minutes. To ensure consistency and reliability of results, the experiment was conducted on three different days for each water flow rate (60 L/h, 120 L/h, 180 L/h, and 240 L/h) under similar conditions.

The testing was conducted three times for each variation of water flow rate, totaling nine days of testing. The obtained data were analyzed to determine the thermal efficiency of the SWH with serpentine configuration. Efficiency was calculated based on the temperature difference between the inlet and outlet as well as the radiation intensity received by collector. Additionally, control testing was performed before data collection to ensure uniform experimental conditions, specifically regarding the heat contribution from the pump. Water was circulated through identical pipes for all models in terms of dimensions, material, and length to normalize the thermal input from the pump before entering collector. Furthermore, statistical analysis was conducted on the inlet temperature data, ensuring that the heat contribution from the pump was neglected and did not alter the results statistically. Consistent inlet temperatures across all models ensured that efficiency measurements reflected collector's performance independently, and any recorded temperature increase was solely due to the solar heating effect from collector.

Energy efficiency analysis is a significant method for evaluating energy usage [35]. Increasing energy efficiency will enhance high levels of resource use and reduce emissions [36]. This is an effort carried out to reduce the amount of energy required to operate energy-related equipment or systems and maximize output energy. In SWH, energy efficiency can be calculated using eq. (1), eq. (2), and eq. (3) as follows:

Solar radiation data were obtained from NASA [https://power.larc.nasa.gov/data-access-viewer] using the All-Sky Surface Shortwave Downward Irradiance or Global horizontal irradiance (GHI) variable to measure the total solar radiation received on a horizontal surface. Following the context, the radiation reflected real-world conditions experienced by the Earth's surface throughout the day under various weather conditions. Additionally, Figure 2 shows the changes in GHI and air temperature to understand the thermal environmental conditions at a height of 2 meters above ground level at the research location during the year 2023. The data were processed into daily averages for one year for simulation purposes, allowing observations of seasonal trends and anomalies more accurately.

Figure 3 shows the variation of collector, absorber, and total efficiencies with different water flow rates. First, the consistent collector configuration during testing minimized variations in collector efficiency. However, environmental factors such as solar radiation intensity and ambient temperature affected collector's performance in absorbing heat. In this context, the heat transfer rate was directly proportional to the surface area and the temperature difference between the two media. This shows that small changes in environmental surroundings such as ambient temperature as presented in Figure 2 influenced collector efficiency.

Figure 2.

Global horizontal irradiance (GHI) radiation and ambient temperature

Figure 3.

Efficiency comparison of collector, absorber, and total on the different flow rate

Second, the absorbed heat efficiency of water was affected by the mass flow rate and temperature difference between the inlet and outlet water. At a higher flow rate, a larger mass of water allowed for more heat absorption, with a smaller temperature change. At a flow rate of 240 L/h, efficiency decreased because the high inlet temperature led to heat loss to collector.

Third, the total thermal efficiency of the SWH system was influenced by both collector and absorber efficiencies. However, at a flow rate of 240 L/h, the total thermal efficiency decreased due to the high inlet temperature causing heat loss. Thermal efficiency was optimized by adjusting flow rate of water, ensuring that the temperature difference between inlet and outlet water was at an optimal condition for maximum heat transfer. This condition supported the fundamental principle of convective heat transfer, which increased with the rise in fluid flow rate to a certain point.

Based on the test results, flow rate of 180 L/h was identified as optimal, providing the highest thermal efficiency. The result higher than the previous research Hossain et al. [37] which the efficiency results are 70.24% and 70.96% which use two-side serpentine tube collector design. In addition, it shows that low water inlet temperature can reduce the absorber surface temperature significantly. This process showed the importance of adjusting the water flow rate appropriately to achieve maximum thermal performance in a solar water heating system with serpentine configuration. Additionally, the higher efficiency observed at this flow rate showed that the optimal contact time between water and collector as well as the optimal temperature difference can significantly improve heat transfer.

The influence of water flow rate on the efficiency and performance of the SWH with serpentine configuration was analyzed through variations in flow rate of 60 L/h, 120 L/h, 180 L/h, and 240 L/h respectively, as shown in Figure 4. At flow rate of 60 L/h, the outlet water temperature gradually increased, reaching a peak of around 42.75 °C at 12:00. Furthermore, collector efficiency at this flow rate was recorded at 69.15%, while the absorbed heat efficiency of water was 66.72%. A lower flow rate allowed for a longer contact time between water and collector surface, improving heat transfer, and increasing heat loss to environmental surrounding.

Figure 4.

Comparison of average output temperatures of various flow rate

At flow rate of 120 L/h, the outlet water temperature increased more significantly, reaching a peak of around 43.83 °C at 12:50. Collector efficiency slightly decreased to 68.94%, but the absorbed heat efficiency of water increased to 76.62%. However, this process showed that a higher flow rate helped to reduce heat loss to environmental surroundings due to a shorter contact time, allowing more heat to be absorbed by the water. Fundamental thermodynamic principles supported the observation, showing that there was an optimal point where heat transfer from collector to the working fluid was most effective.

The outlet water temperature reached the highest peak of around 44.53 °C at 13:50 at flow rate of 180 L/h. Collector efficiency was 69.60%, and the absorbed heat efficiency of water was 81.93%. Furthermore, this flow rate proved optimal, providing maximum heat transfer with minimal heat loss. According to heat transfer theory, there was an optimal flow rate for fluid in heat transfer systems. A study by Hossain et al. [38] also found similar results, where there was an optimal flow rate that maximized heat transfer efficiency​.

At flow rate of 240 L/h, the outlet water temperature peaked at around 45.67 °C at 13:50. Despite this high outlet temperature, collector’s efficiency was 70.39%, but the efficiency of the absorbed heat by the water dropped to 61.34%. Furthermore, the very high flow rate led to too short of a contact time between water and collector surface, reducing heat transfer efficiency. The high inlet temperature caused greater heat loss to collector, as the temperature difference between the water and environmental surrounding decreased, reducing the drive for convective heat transfer. Moreover, a study [39] also found that an excessively high flow rate led to increased heat loss, reducing the general system efficiency.

Life cycle assessment analysis (LCA) was crucial for understanding environmental impact of the SWH with serpentine configuration throughout its life cycle. Furthermore, LCA helped evaluate environmental contribution of each stage from production to disposal, providing an understanding of how this technology could be optimized for better sustainability. This analysis enabled the identification of areas with the greatest environmental impact and finding ways to mitigate, thereby supporting the aims of reducing carbon emissions and improving energy efficiency.

LCA analysis was conducted using ANSYS software [40] to assess environmental impact of the materials and production processes used in the manufacturing of serpentine SWH in this study. Figure 5 shows the results of the LCA analysis of the SWH with serpentine configuration. Furthermore, in Figure 5a, the use phase dominated energy consumption with significantly higher values compared to other phases. This dominion showed that although serpentine SWH relied on solar energy, there was a significant energy requirement during its operation, such as for water circulation and system control. The energy contribution from materials and manufacturing was relatively low, showing efficiency in production processes and material selection. Additionally, Figure 5b showed that the largest cost came from the material phase, followed by the manufacturing phase. The high material costs were due to the use of high-quality materials such as copper and soda-lime glass essential for the optimal performance of the system. Moreover, transportation and usage costs were relatively low, showing cost efficiency in distribution and operations.

Figure 5.

Life cycle assessment analysis for SWH with serpentine configuration in: Energy (a); Cost (b); Climate change (c); and Life phase (d)

Carbon dioxide equivalent (CO_2eq) emissions showed that the material phase contributed the most to climate change, followed by manufacturing and transportation in Figure 5c. The use phase showed a significant reduction in CO₂ emissions, reflecting that during operation, serpentine SWH was capable of reducing CO₂ emissions compared to conventional fossil fuel-based water heating systems. Additionally, the End of Life (EoL) potential showed a decrease in CO₂ emissions, indicating that materials could be recycled and reused, thereby reducing general environmental impact. Figure 5d presented the relative contributions of each life cycle phase to energy, climate change, and cost. The graph showed that the material and use phases had the highest impact on energy consumption and cost. Meanwhile, the use phase also showed a significant contribution to climate change, although with a positive impact in the form of CO₂ emission reduction due to the use of renewable energy. To improve sustainability, the focus was placed on improving energy efficiency during the use phase and optimizing production processes to reduce material costs and emissions. Generally, this serpentine SWH showed great potential as a low-cost, environmentally friendly, and efficient water heating solution.