Introduction

Access to fresh, clean, and safe drinking water is still an ongoing global issue. The issue is further exacerbated in off-grid communities, where modern water purification technologies are often unaffordable or entirely absent, leaving populations vulnerable to water scarcity and health risks1. Solar distillation systems are one of the suggested solutions to overcome these problems as they offer a simple, low cost and sustainable method to address these issues2,3,4. Long term studies have also demonstrated the feasibility of solar stills in harsh climates confirming its potential beyond areas with high solar radiations5,6. A challenge lies in the low productivity of solar stills especially for portable designs where cost, weight, and performance must be balanced7. A number of performance enhancement strategies exist to increase the productivity of solar stills, such as wicks, coatings, reflectors, nano-materials, and phase change materials (PCMs)8,9,10,11. Several techniques have improved the performance of many solar desalination systems, identifying solutions that are both effective and affordable for remote communities still remains a huge challenge12. Computational Fluid Dynamics (CFD) models have also been employed for fin simulations indicating enhanced internal heat distribution and vapor circulation inside the solar stills13,14. Another challenge lies in the field deployment, i.e. minimizing maintenance and reducing operating expenses15.

Enhancing the performance of solar stills can also be achieved through geometrical modifications of the still basin. A significant portion of enhancement studies focus on hemispherical or simple basin designs, with relatively little attention paid to stepped still configuration optimization, which cost relatively less16. Recent research has shown that optimizing both the shape and insulation of a solar still leads to a significant gains in performance17. Even though spherical and other geometries have been given priority in innovations18. Despite their potential, there has been limited experimental research on square-stepped stills incorporating innovative fin designs. Fundamentally, the core challenge in many solar desalination systems is achieving efficient thermal management i.e. controlling thermal gradients and heat localization to boost performance19. The efficacy of stepped and cascading designs lies in their ability to expand the evaporation surface area and lower thermal inertia, with some studies reporting productivity gains as high as 96%20. For performance improvements, conical and pyramid geometries have also been assessed21. Reviews usually list both active and passive methods as efficient ways to increase evaporation area and decrease thermal inertia22,23. Pyramid geometries have also been assessed for potential performance enhancements24. Nevertheless, long-term performance is still constrained by issues like inadequate thermal management, wall heat losses, desalination issues and condensation inefficiencies25,26.

Additional enhancements such as inclusion of PCMs, external condensers etc. often produce significant yields however they also increase complexity and costs27,28,29. According to the reviews, PCMs and nano-enhanced PCMs significantly improve thermal retention during non-sunshine hours30,31. Also, recent reviews have showed that PCMs improve thermal storage during night time whereas fins and modified absorber geometries enhance day time evaporations32,33. A study used highly conductive hollow cylindrical copper fins embedded with PCMs to improve the performance of conical stills. These fins greatly improved evaporation stability and internal heat transfer34. For instance, using nano-PCM into pyramidal configurations improved the performance but also increased the system complexity as well as its cost35. It has also been observed, that low cost solutions such as porous materials and surface coatings have shown notable improvements36,37. Simple and cheap solutions remain more feasible for remote deployment as shown with the successful performance of tubular design solar stills38. Stepped and cascading units improve upon single-stage designs by maximizing evaporation and minimizing thermal inertia39. Experimental results indicate that freshwater productivity can be increased by adding stages and increasing the heat input in multi-effect diffusion stills40. Similarly, In hemispherical systems, incorporating nano-PCMs with structural alterations speeds up evaporation and increases productivity41,42 especially in areas where there is abundant sunlight. Studies on the solar stills also indicate absorber material selection involves trade-offs between cost and performance43. One of the most useful and efficient improvements is still geometric modification, especially stepped and cascading configurations44,45.

Improving performance without sacrificing cost or manufacturing viability remains a major challenge. Modest improvements such as the addition of reflectors/condensing covers can reduce losses and increase yield. On the other hand, structural defects like tray cracks reduce thermal efficiency46. Studies have investigated adsorption properties and effectiveness in raising distillate yield to improve sustainability47. The overall system performance can be enhanced, provided the techniques are merged correctly. A good example is when capillary wicks and condensers are used together48. Parameters that improve distillate yield include fin geometry, water depth, and basin water temperature49. Notable yield increases have also been demonstrated by straightforward passive augmentations, such as bags filled with sand50. Copper fins when tested in a variety of geometries have been shown to significantly increase productivity by improving the rate of evaporation and heat transfer51 with performance strongly influenced by various parameters such as temperature, water depth and fin geometry52. Convective and evaporative heat transfer can be significantly increased with altered absorber shapes and inclined fin configurations53. Significant improvements can also be achieved without adding unnecessary complexity by combining passive and active techniques, such as coupling reflectors with thermal storage54. In a similar vein, research has looked into the adsorption capabilities and efficacy of chemical dyes in increasing distillate yield in order to enhance solar absorption55.

Fin shape has been studied extensively from simple to very complex shapes56,57. Al Adel et al. optimized fin distance using prismatic fins. Their findings showed fins having height range 30–50 mm increase basin temperature by 2.6%, boosting freshwater productivity by 0.03 L/m2.h, and improving solar still efficiency by 1.74%. Widening fin width raises water basin temperature by 10.4%, resulting in 4% efficiency enhancement58. Mohammed et al. using conventional pyramidal solar stills (CPSS) compared the hollow cylindrical perforated fins (HCPF) with inclined perforated rectangular fins (IPRF) for the enhancement of solar still performance. The HCPF and IPRF improved the productivity by 31.3% and 55.9%, respectively, compared to that of the finless CPSS59. Dhaoui et al. studied cylindrical fins with largest fin diameter (80 mm) achieving a 14.07% increase60. Vembu et al. changed the material by adding coal made cylindrical fins for economic reasons, which led to an increase from 22.04% to 32.46%61. Falah et al. also studied the same cylindrical fin but under a hemispherical dome, providing a peak efficiency improvement exceeding 64.7% over the finless model62. Kaviti et al. compared the parabolic fin solar still (PFS) with magnets with the truncated fin solar still (TCFS). PFS produced 20%, 15%, and 16% more distillate at water depths of 1 cm, 2 cm, and 3 cm, respectively, than the TCFS, owing to the combined effects of magnetism and the larger heat absorption area63. Mehta et al. investigated the solar still fin efficiency improvement using hemispherical and trapezoidal fins. They concluded that the hemispherical fins provided superior thermal performance, having higher radiation heat loss of 0.709 W compared to 0.683 W for trapezoidal fins with fillets but at reduced surface area64. Nagori et al. studied the hemispherical solar fins as well, but with sparkling water. The Hemispherical Solar Still (HSS) with fins and water sprinkling provides 24% more distillate output65. Qu et al. proposed a three-level fractal fin design for enhanced performance which improved efficiency compared to straight fins by 13.53%31.

Leaf structure applications in other thermal systems have been studied for its efficient thermal distribution. For instance, in thermal energy storage and metal hydride reactor systems, leaf-vein inspired fins have demonstrated superior performance by optimizing heat conduction pathways and promoting more uniform temperature distribution, leading to ≈ 70% faster reaction times compared to traditional radial or longitudinal fins66. This principle extends to passive cooling as well. Novel leaf-vein-like heat sink designs have proved to have good conductive properties, thus significantly improving convective heat dissipation and reduced thermal resistance by 39.84%. In addition, inspired by natural branching designs of fractal and hierarchical fins have shown improvements in convective heat transfer (13.53% improved efficiency) by disrupting thermal boundary layers and increasing the effective surface area for interaction with the surrounding fluid31,67.

Key operational parameters including water depth, glass inclination, aperture area, and feed-water temperature also play a significant role in optimizing the efficiency of the solar still systems68. Although hybrid nanofluids show great promise for improving thermal performance, problems with long-term stability and operational dependability make their practical use difficult69. Combining passive and active elements, such as reflectors with thermal storage, provides a way to achieve notable improvements without adding undue complexity70. Similarly noticeable gains in output were observed by applying top cover cooling strategies in hemi-spherical solar stills71.

Fig. 1
figure 1

Enhancement of efficiency using bio-inspired fins.

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Although literature identifies several performance enhancement strategies that can increase the productivity of solar stills, including wicks, coatings, glass cooling, reflectors, nanofluids, and thermal storage. Even though fins are frequently used to improve heat transfer, most studies use traditional shapes (flat/rectangular) for desalination. Still, there has been no reported experimental assessment of bio-inspired leaf-shaped fins in square-stepped geometries, especially for solar stills, which represent an untapped potential for performance improvements. These fins can encourage better internal conduction, and more efficient thermal distribution by imitating the natural leaf profiles of plants like bananas, willows, and maples (Fig. 1). The main novelty of this work is filling this gap by introducing and experimentally assessing the performance of a square-stepped solar still with bio-inspired copper fins in an outdoor environment.

Methods

This section outlines the detailed design, construction and testing procedures employed for evaluating the performance of the Novel Solar Still integrated with three distinct leaf shaped configurations. The experimentation was carried out under real environment conditions in Islamabad Pakistan in the month of July, August and September. This specific summer period was selected to ensure high and consistent solar irradiance minimizing the performance instability that that may arise from variation in solar geometry across different seasons72.

Design overview of the solar still

The solar still used in this study was a square stepped basin type solar still, designed to enhance heat absorption and increase the rate of evaporation (Fig. 2). It consisted of a 6 mm thick double-slope glass cover, selected as per literature existing design thickness73. The basin was divided into 5 square mild steel trays, arranged in a stepped manner with each inner tray raised approximately 2 inches above the previous. The outermost tray (\(\:20x20i{n}^{2}\)) was utilized for freshwater collection and was painted white to reduce re-evaporation losses. This design ensured effective solar energy utilization by exposing multiple water layers exposed to direct sunlight.

Fig. 2
figure 2

Experimental setup of a square Stepped Sized Solar Still with reflector used for freshwater production testing.

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In the modified design presented in this study, the square stepped solar still was fitted with three copper leaf-shaped inspired fins (maple, banana, and willow). The system retained the primary 20 × 20 \(\:i{n}^{2}\) basin, over which stepped trays (16 × 16, 12 × 12, 8 × 8, and 4 × 4 \(\:i{n}^{2}\)) were concentrically arranged. Leaf fins (maple, banana, and willow shapes) were distributed across trays and immersed in the water to enhance internal conduction and vapor flow. A total of 25 vertically attached leaf-shaped plates (of shapes as shown in Fig. 3a-c) each were introduced on the basin and arranged in accordance with the allowable space between each step. The schematics of leaf-shaped fin arrangement is shown in Fig. 3d. The double-slope transparent glass cover allowed solar radiation to enter while retaining heat and vapor within the system (Fig. 3).

Fig. 3
figure 3

Manufactured and schematic representation of the (a) Maple, (b) Willow, (c) Banana inspired leaf fins, and (d) Schematic of the square MS steel plates with fin arrangement.

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The plates used in the solar still were made up of MS Steel material. The double slope glass was made up of high-strength soda lime glass shaped to minimize heat losses and to facilitate the process of condensation. The reflector was also made up of MS steel which was then covered with the help of aluminum foil and aluminum tape to make its surface highly reflective. This strategy is in line with effective uses in pyramid stills, where it has been demonstrated that reflective surfaces improve productivity and absorption of solar radiation74. All materials used in this study were chosen because of their durability, cost effectiveness, and local availability. Transparent tapes were used for attaching the sensors onto the plates of the system (Fig. 4).

Fig. 4
figure 4

Fabrication and assembly stages of the square stepped solar still equipped with leaf-shaped copper fins: (a) maple fins installed in stepped metal trays without sensors or glass cover; (b) willow fins with temperature sensors installed, without glass cover; and (c) banana leaf fins with sensors, glass cover, and external reflector.

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Fin-Configurations

To improve heat transfer and promote internal convection within the system, we have used three unique leaf-shaped fins made up of copper material. These fins inspired by natural leaf forms, due to their large surface area and their potential for improved thermal distribution. The selected leaf-shaped fin types were:

Fin-Type 1: maple leaf shaped fins

These fins have a multi-lobed design with sharp edges like the design of an actual maple leaf. Its edges and corner help in creating larger surface area allowing it to transfer and absorb more heat from the water. This shape causes small disturbances in the water which helps distribute the heat inside the solar still.

Fin-Type 2: banana leaf shaped fins

These fins are long and wide just like the design of an actual banana leaf. It has a huge surface area that allows it for more better and consistent heat transfer. With its gentle curve, it promotes smooth water movement inside the solar still, thereby improving heat exchange between the fins and the water.

Fin-Type 3: Willow leaf shaped fins

These fins are quite slim and slightly curved resembling the shape of an actual willow tree leaf. Their shape is quite simple and efficient which does not disturb the water present inside the solar still. They are known for their effective heat transfer and for their capability to fit inside tight spacings.

Each fin was fabricated from 1.6 mm thick copper sheet and were cut with the help of CNC Machine. The fins were then painted Matte black to maximize solar radiation absorption, a technique that is widely documented to improve thermal input and evaporation efficiency in solar distillation systems75. The fins were then placed inside the water trays and were distributed evenly to ensure proper heat transfer during the testing phase. A total of 24 fins of each type were tested and compared. Their placement was designed to stand-upright, and adjustments were made when required.

Experimentation and performance factors

The solar still system was installed in Islamabad H-13, Pakistan. The system was tested between 9:30 AM to 5:30 PM. A fixed amount of saline water (2.25 L) was added daily at the start of each test. This amount of saline water was added to maintain an ideal water depth, with around 50% of the leaf fin area under the water, which is a critical parameter. This critical parameter has shown significant impact on productivity in single slope solar stills76 and has also been highlighted in other broader literature reviews77. This specific mass was selected on the basis of previous experimental research studies which demonstrated that fresh-water yield output is adversely affected beyond a certain range of water mass where optimal productivity is achieved78. This is in line with research showing that water depth has a significant effect on system performance even when employing cutting-edge heat transfer fluids like nanofluids79. In addition, continuous operation of the solar still can lead to salt fouling, which, if not addressed, may adversely affect the yield80. To prevent this, the solar still was washed with ambient-temperature fresh water and wiped clean with rags prior to each day of testing. The seven temperature sensors were placed at locations presented as follows:

  • Temperature Sensor 1 at Plate 1

  • Temperature Sensor 2 at Plate 2

  • Temperature Sensor 3 at Plate 3

  • Temperature Sensor 4 at Plate 4

  • Temperature Sensor 5 at Plate 5

  • Temperature Sensor 6 at ambient air temperature near the setup.

  • Temperature Sensor 7 on the outer surface of glass

A digital data logging sensor was used to measure solar intensity at regular time intervals. Fresh-water yield was measured at the end of the experiment using TDS meter (Fig. 5). To maintain consistency, environmental data (Temperature, solar irradiance) were recorded to compare productivity under comparable settings.

Fig. 5
figure 5

(a) Digital temperature sensors installed at multiple points of the solar still to monitor and record temperature distribution; (b) digital lux meter used to measure total solar irradiance; and (c) graduated beaker used to measure distilled water yield after each experiment.

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Solar still efficiency was evaluated by calculating the ratio of the thermal energy consumed in water vaporization to the incident solar energy on the system, as represented by the following Eq. 

$$\:\eta\:=\frac{\sum\:{M}_{w}\:\times\:L}{\sum\:I\:\times\:{A}_{a}\times\:3600}$$
(1)

where, \(\:{M}_{w}\) is the total fresh-water yield collected, \(\:L\) is the latent heat of vaporisation, \(\:I\) represents the solar irradiance (\(\:W/{m}^{2}\)) and \(\:{A}_{a}\) is the glass aperture area. \(\:{\prime\:}L{\prime\:}\) was calculated using the empirical relation shown as follows:

$$\:L\:=\:\left(2.4935\:\right(1\:-\:(9.4779\times\:{10}^{-4}\times\:\:{T}_{v}))\:+\:(1.3132\times\:{10}^{-7}\times\:{{T}_{v}}^{2}\:\:\:)\:-\:(4.7974\times\:{10}^{-9}\times\:\:{T}_{v}^{\:3}\left)\right)$$
(2)

where \(\:{T}_{v}\) is the vapor temperature in Celsius. The collected data were tabulated for each time interval. Productivity was calculated in L/\(\:{m}^{2}\)/day by dividing the total daily output by the aperture area. The daily productivity \(\:{M}_{\text{d}}\) is then computed as

$$\:{M}_{\text{d}}=\frac{{Q}_{\text{u}}}{{h}_{\text{fg}}}$$
(3)

Results and discussions

Experimentation was conducted in the months from July to Sep 2025 at the location of 33.6364° N, 72.9895° E in a subtropical region. We began testing at approximately 0930 h and continued till sunset, which occurred near 1730 h. The diurnal temperature profiles for each fin configuration are detailed in Fig. 6a, b and c. Peak solar radiation occurred slightly before the temperature peaks, around 1200 h (Fig. 7). All 3 designs demonstrated the characteristic trend of rising temperatures in response to increasing solar irradiance, reaching peak values between 12:00 and 13:00 h, followed by a decline till sunset. Differences in thermal behavior were observed. The Banana leaf fin reached highest peak plate temperatures (exceeding 62 °C), indicating significant heat accumulation. The Willow leaf fin operated at markedly lower temperatures of ~ 61 °C. The 3rd profile (Maple leaf fin) showed an intermediate trend. The lower operating temperature of the Willow leaf indicates that its geometry enables more efficient heat transfer to the water, thereby promoting evaporation, rather than retaining heat within the fin structure itself.

Fig. 6
figure 6figure 6

Diurnal Profile of average plate temperatures for the Solar Still (a) Willow leaf (b) Banana leaf, and (c) Maple leaf.

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Fig. 7
figure 7

Average daily solar irradiance trends observed during the testing period.

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All fin designs were tested for water desalination quality as well as production rate. The baseline we tested both with and without the solar reflector showed average efficiencies of 0.1900 and 0.2327, respectively (Fig. 8). This provided a benchmark to test the 3 designs. The overall efficiency was computed using Eq. 1, which caters for area and solar irradiance normalization. Still, under real world conditions, variations in efficiency are commonly observed. The average efficiencies for each fin design were as follows: maple leaf: 0.2807 ± 0.0653 (mean ± standard deviation), banana leaf: 0.2997 ± 0.0257, willow leaf: 0.3013 ± 0.0530. During the testing, the Maple leaf fins showed relatively lower efficiency. However, their performance, when they peaked, reached the highest (0.3605). The Banana leaf fins demonstrated consistently intermediate performance. Although the Willow leaf fins showed behavior which was less consistent than the Banana leaf design but were more stable than the Maple leaf fins.

Fig. 8
figure 8

Comparison of daily efficiency with and without performance-enhancing fins.

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The input water had a TDS of approximately 650 PPM, which was reduced to an average of ≈ 77 PPM after purification. An average 565 ml of distilled water was collected, where the testing for the initial samples showed an average impurity of 140 ± 2 PPM. However, as the day(s) progressed, TDS showed a trend of gradual decrease, indicating improvement in purification efficiency over time (Fig. 9). Our individual result sets may differ (Fig. 10), but the overall water quality never plunged below 150 PPM. As the day(s) progressed, the impurity levels dropping from an initial 141.5 PPM reached a minimum of 45 PPM.

Fig. 9
figure 9

Simultaneous measurement of production, purity, and temperature during desalination.

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Fig. 10
figure 10

Time-profile of cumulative distillate production and associated total dissolved solids (PPM).

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During the period of the day when we had maximum solar irradiance and thermal energy at around 11:30 am, water collection started showing some significant output. As evaporation rates increased, and thermal rates remained stable, water quality kept improving. When the basin temperature dropped after 4:00 PM, the impurity levels stopped falling and even crept back up. This decline in performance makes sense; with less thermal energy, the evaporation rate drops, making the separation of pure water from contaminants less efficient, allowing some contaminants to carry over into the final distillate.

The data also confirms that by adding fins, we get a reduced absorber plate’s operating temperature, with the baseline (no fins) recording the highest average temperature of 59.10 °C (Fig. 11). Among the finned designs, the Willow leaf fin achieved the lowest temperature (51.43 °C), while also having the smallest total area (355,354 mm2). The Maple leaf fin, with an intermediate area of 368,361 mm2, also reached a similarly low temperature of 52.81 °C. This temperature reduction correlates directly with improved thermal efficiency (Fig. 12). The Willow leaf fin’s combination of the lowest temperature and the highest efficiency (30.13%) demonstrates that its geometry is most effective at transferring heat for evaporation, rather than simply storing it, outperforming the Banana leaf fin which has a larger area (375,019 mm2) but higher temperature (54.91 °C) and lower efficiency (29.97%).

Fig. 11
figure 11

Temperature drop and surface area of bio-inspired fins in solar still.

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Fig. 12
figure 12

(a) Efficiency Performance and Surface Area of Bio-Inspired leaf Shaped Fins in Solar Still, (b) Bar chart comparing the efficiency per unit area (m²) for the baseline configuration (no fins) and three bio-inspired leaf-shaped fin designs. The Willow leaf Shape Fin shows the highest performance in this metric.

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Thermal performance can be dependent upon various factors which may lead to an artificial increase in the efficiency of any given design. It is therefore pertinent to eliminate as many such factors as possible before concluding on the efficacy of any design. The efficiency equation takes care of the irradiation effect due varying days of data acquisition, however the presence, as well as the size of fins could have a direct correlation with the efficiency. Therefore, in Fig. 12b, we report the efficiency of each design normalized with the total surface area available for heat and mass transfer process. Doing so helps us much clearly analyze how the specific geometry of each bio-inspired fin influences the underlying heat and mass transfer processes. The baseline (no-fin) configuration establishes a benchmark efficiency of 75.14% per m² of absorber area. The integration of fins enhances the normalized efficiency, but to varying degrees as dictated by their shapes.

The Willow leaf fin achieved the highest normalized efficiency (84.79%/m²) despite having the smallest total surface area. Its slim, tapered, and slightly curved profile minimizes the conductive resistance along its length, allowing heat from the absorber plate to be quickly distributed to its entire surface recording lowest average plate temperature (Fig. 12b) and thusly, the highest conversion efficiency of absorbed heat into evaporation. The Banana leaf fin, although having the largest surface area, yielded a relatively lower normalized efficiency (79.91%/m²). Although its broad, flat geometry provides extensive surface area for convection, its geometry may also present a longer, less optimal conductive path from the base to the edges of the fin. The Maple leaf fin exhibited least normalized efficiency (76.20%/m²) amongst the bio-inspired designs. This same complexity, with multiple acute angles and a less continuous conductive path from the base to each lobe tip, may introduce slightly higher thermal resistance for conduction compared to the more streamlined Willow design. This trade-off between convection enhancement and conductive efficiency is reflected in its performance metric.

Conclusion

Plants have existed for hundreds of millions of years and have undergone continuous evolutionary refinement. The primary function of their leaves being intercepting sunlight for photosynthesis, makes the leaf morphologies an excellent natural model for solar-energy-capture applications. The current study integrated bio-inspired fins into a double slope stepped solar still to enhance its performance. Testing under real meteorological conditions, all three fin designs (Maple, Banana, and Willow leaf) proved effective in increasing daily efficiency beyond the baseline established by stills with and without a reflector. Compared to the finless baseline efficiency of 0.2327, the Maple, Banana, and Willow leaf fins increased performance to 0.2807, 0.2997, and 0.3013, respectively. An indicator of the enhanced performance of a fin was the absorber plate temperature, which was the lowest for the willow leaf geometry of 51.43 °C, despite having a 14.7% smaller total area than the Banana leaf fin.

Results indicated that the reflectors’ bio-inspired fins addition improved efficiency for each of the 3 fin cases. The Maple leaf Fin achieved the highest single-day efficiency (0.3605), indicating a strong potential for maximum yield. The Willow leaf Fin recorded the highest average efficiency (0.3013), a result of its superior heat dissipation. In contrast, the Banana leaf fin, while slightly lower in average efficiency, delivered the most consistent and reliable performance throughout the entire testing period. An in-depth analysis of efficiency per unit area (normalized efficiency, %/m²) revealed the Willow leaf fin achieved the highest normalized efficiency (84.79%/m²), despite having the smallest total surface area. Its slim, tapered profile minimized conductive resistance, facilitating rapid heat distribution from the absorber plate to its entire surface. In addition, maple leaf did not perform as per expectations. Further experimentation in temperate climate regions, where such leaves are abundant, may further dictate the structure-property relationships. Future works could also look further into more intrinsic bio-inspired designs as well as improvement in the existing geometries and exploring hybrid designs that merge their consistency with the high-peak potential of the other fins.