Solar Distillation for Potable Water Provision in Remote Regions: Implementation of Inclined-Angle Solar Stills on the Galápagos Islands

The Galápagos Islands

Water scarcity has been a persistent challenge in numerous regions of the globe, including arid and semi-arid climate zones and islands in particular (Reyes et al., 2017). As the global population continues to grow and economies expand, the water demand is consistently rising, while the availability and security of clean water are gradually declining, especially in regions highly affected by climate change. These developments are especially relevant to the species-rich and fragile ecosystem of the Galápagos archipelago (Figure 1), which is isolated approximately 1000 km off the coast of Ecuador in the Pacific Ocean. The population of the four inhabited main islands is estimated to have increased to over 30,000 residents over the last decades. Concurrently, the number of tourists has risen to over 329,000 during 2023, with this figure still rising steadily (Barrionuevo, 2024). In the past, the small resident population on these islands relied on collecting rainwater and shallow brackish water, as there are very few natural water sources. Currently, approximately 97% of the islands’ surface area is subject to strict nature conservation measures, effectively precluding any use for the development of energy or water infrastructure (Grube, 2020). In the present era, the most populous island, Santa Cruz, has three principal sources of fresh water: municipal supply, bottled water, and private extraction. While the water from the public supply is brackish and not suitable for human consumption, the use of plastic bottles requires transportation over thousands of kilometers and contributes to an increase in waste. The extraction of water from a few crevices on private properties on Santa Cruz is unregulated, and in some cases, high levels of contamination have been detected (Reyes, 2015).

Figure 1.

Solar radiation map for Ecuador (Ordóñez, 2019)

One low-tech and low-cost method of extracting potable clean water out of brackish or salty sources, is solar distillation (Practical Action, 2012). Imitating the natural cycle of evaporation and condensation of water, leading to rain, distillation processes are quite energy intensive. Evaporating one kilogram of water, which equals one liter, requires 2.26 Megajoules (MJ). This means, that gaining one liter of potable water by distilling dirty water is not possible without providing a minimum heat input of 2.26 MJ. Because the efficiency of distillation systems is less than 100%, even greater amounts of energy are needed (Practical Action, 2012). One beneficial aspect of the Galápagos Islands’ location below the equator is the availability of large quantities of energy in the form of so-lar radiation, as illustrated in Figure 1. The red areas indicate a particularly high potential for the utilization of solar energy. The Galápagos Islands receive an average of 5.7 kWh/m²/day of Global Horizontal solar Irradiation (GHI), which could theoretically yield up to nine liters of potable water through distillation per square meter of surface area daily. However, it is evident that the practical yield is significantly less due to energy losses. Nevertheless, the potential of solar radiation to contribute to the provision of drinkable water on the Galápagos is considerable (Eras-Almeida et al., 2020).

Solar Distillation Systems

Solar distillation systems, also referred to as solar stills (SS), employ the process of evaporation and condensation of water to remove salts and other impurities using the energy of the sun. It is recommended that drinking water contains a minimal amount of minerals, such as salt, to maintain electrolyte levels. This may necessitate the addition of saline water to the distilled output. A wide range of potential variations exists with regard to the design and technology of solar stills, as illustrated in Figure 2. In remote regions, passive solar stills with more accessible construction, size, operation, and lower cost are more suitable than active solar stills that use solar cells or waste heat from the industry as external heat sources (Aljubouri, 2017). Passive solar stills can be further classified into several subcategories, including basin-type stills, wick-type stills, stepped-basin stills, and multi-effect solar stills, among others (Hasan, 2020).

Figure 2.

Classification of solar stills with schematic diagrams of a selection of relevant basin-type designs

Table 1 provides a concise overview of the most prevalent designs and experimentally determined freshwater yields, with a particular focus on basin-type solar stills. Basin-type stills typically exhibit the highest output at the lowest cost due to their simple design. Furthermore, this technology is considered here on the basis of its robustness and comparative ease of construction and utilization. Findings from the literature show that the possible yield rises with the complexity and cost of the solar still technology.

Table 1.

Review of a selection of solar still technologies

Economic Considerations – Small Is Beautiful

The greatest potential of the low-tech solution of solar distillation of drinking water lies in its capacity for self-construction using inexpensive, available materials by the local population. From literature review, commercially available basin-type inclined-angle solar stills claimed to generate outputs of up to 5-7 L/day. While featuring surface areas for evaporation of around 1 m², these systems were available for approximately $500 and could not establish themselves on the market (Taslim et al., 2007). It can thus be concluded that the promotion of a self-creatable SS designed for local conditions is a feasible solution that will contribute to a more sustainable future. Schuhmacher (1970) suggests that small, local and decentralised economic structures are often more effective and sustainable than large, centralised systems. Local self-sufficiency and autonomy are key principles here. Self-containable solar stills represent a suite of technologies adapted to people’s needs, with the overarching objective being the promotion of social and environmental goals, as opposed to a narrow focus on increasing productivity (Schumacher, 1973).

In 2013, an experimental study was conducted in Rajasthan, India, to assess the feasibility of constructing a small household inclined-angle solar still using locally available materials. The still’s material costs of $42 were still deemed excessive for low-income populations in remote regions. However, economies of scale may make larger production feasible (Gray et al., 2013). Mehta et al. constructed a rudimentary inclined-angle still, utilizing low-cost materials that were locally sourced in Gujarat, India. The device had a surface area of 0.252 m², achieving yields of approximately 0.5 L per day. Based on historical exchange rates from the time of the study in 2011, the material costs equaled approximately $9 per unit, resulting in a cost of roughly $35 per square meter surface area. Given the low-cost construction, the still was expected to have a lifespan of four years (Biswasa et al., 2017). In a separate project in India, a 15 m² solar still was constructed at a cost of $575, which equates to $38.30 per square meter. If the system can be expected to last for over a decade, this may represent a viable option for villages in remote regions, depending on the operating costs (Practical Action, 2012). Taslim et al. (2007) designed a more technologically advanced inclined-angle solar still, utilizing durable materials and incorporating advanced features such as a water level regulation system through a float valve. It thus seems probable that this still will achieve higher yields and may have a lifespan of several decades. The total cost of materials purchased was $528.39 (Taslim et al., 2007). Even if the cost per unit is expected to decrease substantially with mass production, the price for stills of this nature would remain prohibitively expensive for the majority of potential applications.

According to economic analysis of Rufus et al. (2016) the cost of advanced designs continues to increase significantly in comparison to the basic basin-type inclined-angle still. Experiments conducted with various pyramid-shaped stills, comprising four glass panes converging at the apex in a pyramidal configuration, have resulted in operating costs exceeding $27 per year. Hemispherical stills can require complex manufacturing processes for the domed glass resulting in capital costs of several hundreds of dollars per square meter surface area. While a cover made of plastic could be a more cost-effective solution, the majority of potential materials degrade over time due to solar radiation. Furthermore, it is more problematic for water to condense onto them. The cohesive properties of water result in a greater tendency to form beads on plastic surfaces than on glass. Rather than flowing off the plastic, the water becomes aggregated into beads that obstruct the incoming solar radiation, preventing heat transfer to the bottom of the still. Meanwhile, parabolic forms utilizing evacuated tube collectors or distinct advanced single- and multi-effect still designs cause costs in the range of several thousand dollars (Rufus et al., 2016).

Single-Basin Inclined-Angle Solar Still

In general, distillation units with glass surfaces facing in different directions, such as triangular or pyramid stills, allow more energy to reach the evaporation surface. This is particularly true on sunny days due to the reduced shading effects compared to simple inclined-angle stills. Conversely, the number of exposed glass surfaces is directly correlated with radiation losses, and the basin of inclined-angle stills may provide superior insulation. Based on this characteristic, Rufus et al. further conclude that, although triangular solar stills may enhance yield on certain days, they offer no advantage when considering yearly performance due to radiation losses (Rufus, et al., 2016). Moreover, as indicated in a publication by the Schumacher Centre, single-basin stills in general are the only design that has been proven effective in the field (Practical Action, 2012). Given these considerations and the aforementioned characteristics, this research will focus on the design of a single-basin inclined-angle solar still.

According to a publication by Practical Action, the optimal water depth in the basin of an inclined-angle still is 20 mm. However, the installation of a water level control system, for instance through the use of a float valve, was not carried out due to the unavailability of the necessary components on the islands, the relatively high costs involved, and the probable moderate yield increases. A transparent cover made of glass should have a thickness of at least 4 mm to reduce the likelihood of breakage. Alternatively, plastic foils can be employed for short-term use. To optimize the solar radiation reaching the evaporation area in the basin, the angle of tilt of the transparent cover of the distillation system should be equal to the latitude of the area in which it is installed. Given that the Galápagos Islands are situated beneath the equator, this would equate to a zero-degree tilt, which would prevent the condensed water from flowing off and, thus, prevent the basic function of a solar still. In that case, Practical Action proposes an inclination angle of 10-20°. There is otherwise no consensus in the literature regarding the optimal angle. For instance, Junior et al. (2023) propose an angle of 15-25°, whereas Mohamed et al. (2021) stipulate 15° as an absolute minimum for functionality and select 32° for their model. An investigation of various inclinations in Iraq revealed a maximum output at an inclination of 20° (Aljubouri, 2017). Taslim et al. (2007) recommend 23° for regions with high irradiation near the equator. With a suitable inclination angle, the typical efficiency of single-basin SS can be estimated to be around 35%.

The objective of this project was to construct and evaluate a low-tech and cost-effective solar still as illustrated in Figure 3. The system was constructed with minimal expenditure using materials and tools that are readily available in remote regions, such as the village of Puerto Ayora on Santa Cruz Island in the Galápagos.

Figure 3.

Schematic diagram of a simple basin-type inclined-angle still

The construction of one prototype, designated as proto#1, was completed at the lowest possible cost, with the majority of resources obtained free of charge from locations such as landfills or natural sources. The second prototype, referred to as proto#2, was built with higher-quality materials, resulting in a slightly higher cost. This enabled the determination of the extent to which the water yield can be increased through the use of superior materials. However, care was also taken to ensure that the second model was constructed as cost-effectively as possible in accordance with the improved design. Both stills were tested over several weeks regarding the daily output of distilled freshwater, the construction costs, strengths and weaknesses, and further details of the models were analyzed.

Construction Process

For the construction of inclined-angle solar stills in Puerto Ayora, comparatively few equipment and tools were used. This included a circular saw and a handsaw, a knife, a cartridge gun, and abrasive paper in different grit sizes. The base of the minimum-cost inclined-angle still #proto1 was built from old pieces of furniture from the trash. The 2-centimeter-thick boards of cedar wood, as illustrated in Figure 4, were sawn and screwed together to form a basin with an evaporation area measuring 60 × 42 cm and thereby 0.25 m² (b, c). On one side, two holes with different diameters were drilled to later accommodate the discharge pipe for the distilled water, and (the bigger hole on the right) for filling and refilling dirty water and as a drain for cleaning purposes. The selected angle of the condensing surface is 18 degrees.

Figure 4.

Wooden frame of #proto1: Cedar wood boards (a) and assembled frame with holes for pipes (b, c)

The basin was lined with a black polyethylene foil. The tubes were installed with the assistance of small amounts of 2K epoxy resin adhesive. The runoff for distilled water at the lower end of the slope consists of a plastic tube, of which the internal segment within the still was excised and opened to the top in order to facilitate the collection of condensed water. At the point where the pipe goes through the hole in the outer wall, it remains closed all around. A second layer of foil was then inserted into the selectively glued lining of black foil around the lower area of the basin. This additional layer of foil ensures tightness and can be removed for cleaning purposes or replaced if necessary. The edge was covered with silicone strips to which a 0.3-millimeter-thick transparent polyethylene film of around 65 × 55 cm, serving as a condensation surface, was applied. The foil is affixed to the upper and lower long sides of the still with wooden slats through the application of pressing force. In the present design, the aforementioned pressing force is maintained through the use of simple ropes around the still (see Figure 5).

Figure 5.

Low-cost inclined-angle solar still prototype #proto1

The frame of the optimized #proto2 was constructed from wood in the same way as #proto1, but with a higher quality wood, which was a surplus from a construction project. A refill hole was unnecessary, as #proto2 has a lid for removing and refilling the basin. The lid was affixed with conventional hinges, can be closed with a simple slider, and seals with the assistance of single-sided foam tape. The basin itself was constructed from a flat, almost square container made of thickly coated wood, sourced from the local garbage dump. The base and walls of the container were coated with a black food-grade silicone sealant. This guarantees a tight seal and optimizes the absorption of solar radiation due to its color. The front and rear walls were shaped to fit by sawing and filing on the upper side, where the glass would later be positioned. To facilitate the collection of condensed water and its subsequent transfer to the runoff, a plastic pipe, which had previously demonstrated efficacy in #proto1, was cut in half to the length of the interior of the front wall of the still. The section of the pipe that traversed the frame was left intact. These details are visualized in Figure 6.

Figure 6.

Construction details of #proto2: The removable basin (a), the associated lid (b) and the runoff tube (c, d)

The frame was painted white on the sides and coated with remnants of the black silicone sealant on the top to protect the wood in the event of subsequent contact with water. The halved pipe was affixed at the front inner wall, where water from the glass pane arrives, with 2K epoxy resin adhesive. The angle of the drain on the side where the distilled water should flow is slightly inclined. Foam tape, covered with polyethylene foil, was applied to the silicone on the rim of the frame to seal the glass. The rectangular glass pane was purchased from a local shop in Puerto Ayora. It measures 58 × 59 centimeters with a thickness of 4 millimeters to prevent breakage. Once the glass was positioned on the frame, the runoff pipe was connected to a canister via a hose. The final system of #proto2 is illustrated in Figure 7.

Figure 7.

Improved inclined-angle solar still prototype #proto2

It was decided that additional insulation was not a cost-effective measure due to both the associated costs and the space requirement, weight increase, and likely small impact. The same can be said of the regulation of water levels with a float valve. Moreover, this item was not available in the appropriate size on the Galápagos Islands.

Testing

The test series had the objective of investigating the performance of the two prototypes. Figure 8 provides an overview of the Galápagos archipelago as the location of the experiment and the solar potential of the specific location. The map presented here is available for consultation on the territorial information platform of the Universidad del Azuay (UDA), Cuenca. The differently colored dots, visible in select areas on the map, represent the locations of installed solar thermal systems (Orcatec, 2024). In addition, the location of the meteorological station of the Charles Darwin Foundation is displayed, from which meteorological data was obtained. Both systems were tested on their shadow-free rooftop in Puerto Ayora, located near the Laguna de las Ninfas.

Figure 8.

Overview of the Galápagos Islands with focus on the experimental location including an indication of the average GHI and solar thermal systems installed by Orcatec (IERSE, 2024)

The key variable under consideration in the experiment was the yield of distilled water per day. Furthermore, the yield per square meter of evaporation area and potential annual yields were analyzed. The evaporation area is equivalent to the floor area of the water basin in the distillery, which is 0.25 m² for the two prototypes constructed here. The freshwater yield represents the volume of water evaporating through the supply of thermal energy from the sun and subsequently condensing on the comparatively colder interior of the inclined transparent cover. The water flows out of the still through the bisected pipe described above and is collected in a bottle of 2 Liters. The stills were typically examined in the early morning hours to quantify the output of the previous day using a 500 ml measuring cup with a 10 ml scaling. This output included the initial hours of the previous night, during which the water temperature was sufficient to maintain the evaporation and condensation process. Concurrently, the stills were refilled when necessary to accommodate the optimal water depth of approximately 20 millimeters. In some instances, the stills were permitted to operate independently for several days to ascertain their long-term operational efficacy. The #proto1 model was tested continuously for a period of six weeks. Due to the later date of completion of #proto2, taking into account the experience gained with the first prototype, it was tested over a shorter period of three weeks. During the test phase, only minor alterations were made to the design of the individual distilleries. The water utilized was tap water from the municipality. Several studies found out that the water quality of the tap water does not comply with the standards for drinkable water. Tap water in Puerto Ayora, is sourced from a brackish basal aquifer that is highly vulnerable to contamination due to urban settlement directly above the water source and inadequate sanitation systems. Numerous studies, including Liu and d’Ozouville (2013), have consistently detected elevated levels of Escherichia coli in both the aquifer and household water samples, indicating significant fecal contamination. The widespread use of poorly maintained septic tanks and the absence of sealed drainage systems have contributed to bacteriological pollution of the groundwater. Moreover, the piped distribution network and common household storage practices facilitate recontamination, resulting in water that does not meet national drinking water standards (Liu & d’Ozouville, 2013).

Construction Cost

The following sections present a summary of the costs associated with the materials utilized in the construction of the stills, as presented in Table 2 and Table 3. The indicated real cost was the direct cost of constructing the still in Puerto Ayora. It would not be feasible to construct additional stills at these costs. This analysis did not include the use of materials sourced from the garbage dump or the cost of operating materials. It is possible, though not guaranteed, that a single system could be constructed under comparable circumstances at a similar cost. The theoretical cost is the estimated expense that would be incurred if all materials were to be procured from a prominent do-it-yourself (DIY) retailer in Ecuador. These costs are representative of those that would be anticipated when constructing a significant number of distilleries. It was assumed that the necessary tools would be readily accessible at no cost. Additionally, labor costs, depreciation, and other expenses related to wear and tear were not incorporated into the analysis.

Table 2.

Materials used for #proto1 and their cost

Table 3.

Materials used for #proto2 and their cost

The low-cost prototype, #proto1, was constructed at a significantly reduced cost in comparison to the enhanced prototype, #proto2. The use of more economical and readily available materials, sourced from the local area, was a primary factor in the reduced cost. The theoretical cost per square meter of evaporation area for the #proto1 is $42.20, while the cost for the #proto2 is $118.56. It seems reasonable to posit that economies of scale in the actual realization of larger distilleries will result in significantly lower costs than those extrapolated here. It can be concluded that larger distilleries would still be more expensive overall, but would result in a reduction in costs per unit of water distilled. If we consider a five-year lifespan for the #proto2, before the necessity of cost-intensive repairs becomes apparent, the daily freshwater output would have a theoretical cost of $0.016 per liter.

Freshwater Output

Figure 9 illustrates the daily distilled water output of #proto1 over the initial 21-day testing period. The lowest output of 110 ml was recorded on the first day of testing. This may be attributed to the low temperature of the water and the system at the outset. On the first day of the test period, the entire system was required to reach its operating temperature before the process could commence. Subsequently, the system did not undergo a complete cooling process during the nighttime hours, which resulted in an increased output during the following mornings compared to the first day. The second-lowest output recorded after the launch on July 30, 2024 was observed on August 17. On this day, 160 milliliters of water were distilled. On both days, the weather was characterized by high cloud cover and intermittent drizzle, with temperatures reaching 22 °C. This demonstrates that direct radiation is not a prerequisite for the evaporation and condensation process to occur. The highest yield achieved during the initial three-week testing period was recorded on August 2nd, with a total of 430 ml. As with the other days exhibiting high performance, the weather was sunny and warm, though there were significant clouds in the early morning and late evening hours. The mean daily yield over the three-week period was 301 milliliters, which equates to 1,204 milliliters per square meter of evaporation area per day.

Figure 9.

Daily distilled water output of #proto1 during test phase 1

The two-month period in Puerto Ayora was characterized by a lack of completely clear and sunny days. This can be attributed to the local microclimate at this time of year. From sunrise until approximately 9 a.m., thick clouds were consistently present, which limited the duration of sunshine and thus the amount of incoming solar energy. A clear correlation between hours of sunshine and the drinking water yield can be observed when examining the weather data from the climatology database of the Charles Darwin Foundation in Puerto Ayora. The second of August, which was the day of the maximum yield during test phase 1, also exhibited the highest Solar Distillation for Potable Water Provision in Remote Regions: Implementation of Inclined-Angle Solar Stills on the Galápagos Islands duration of sunshine, with a total of 9.6 hours. In contrast, days such as August 17th, which exhibited minimum yields, also exhibited the least amount of sunshine. The mean value for this three-week period was 2.73 hours of sunshine per day (Charles Darwin Foundation, 2024).

However, daily sunshine duration is a suboptimal proxy for solar still performance, as solar irradiance (MJ/m²) near noon has a disproportionately greater impact on evaporation rates than low‑angle morning light. Unfortunately, more accurate irradiance data for Puerto Ayora are not available, and it is unlikely that the Charles Darwin Foundation’s station provides global horizontal irradiance measurements - its records only include sunshine duration via heliograph, air temperatures, humidity, and precipitation. Zander et al. (2023) analyzed MODIS cloud-mask data across the archipelago and documented substantial intra-island variability in cloud frequency related to elevation, exposure, and time of day - especially pronounced during the June - December Garúa season (Zander et al., 2023). Given this highly localized nature of cloud occurrence and insolation patterns on Galapagos - where conditions can differ within just a few kilometers - interpolated satellite-based irradiance data would carry substantial uncertainty and thus were not feasible for this study.

After the improved prototype with glass cover was finished, it was placed next to #proto1 on the same rooftop in Puerto Ayora. Over the next three weeks, the output of both stills was observed on a regular basis. In two cases, the stills were left for more than one day without measuring the output. After these periods, the yield was measured and divided by the number of days to obtain the mean value per day. This can be observed between the dates of the 21^st to 25^th of August and the 31^st of August to the 5^th of September, as indicated by the constant averages over these periods in Figure 11.

Figure 10.

Daily hours of sunshine in Puerto Ayora during test phase 1 (Charles Darwin Foundation, 2024)

Figure 11.

Daily distilled water output of both prototypes during test phase 2

An investigation of the duration of sunshine during the second test phase revealed that the level of solar radiation was generally low. Consequently, the average yield was constrained by the limited duration of sunshine, which averaged only 1.45 hours per day. Nevertheless, the greatest quantities of distilled water were once again produced on days with the highest levels of solar radiation, while the lowest quantities were obtained on days with no sun. The mean temperature exerts minimal influence on the daily yield variation, as it remains relatively constant in Puerto Ayora. The mean daily air temperature for the entire month of August ranged from a minimum of 20°C to a maximum of 22°C, according to data from the Charles Darwin Foundation. The lowest recorded temperature of 15°C was observed during the nighttime hours of the final days of August, while the highest temperature of 24.8°C was recorded on the 19^th of August (Charles Darwin Foundation, 2024).

The enhanced #proto2 generated a considerably superior output compared to #proto1, as evidenced by the summary of measurement values in Table 4 and its graphical representation in Figure 13. The yields observed on the initial testing day were not considered in the determination of the minimum and maximum values, as the potential for distortion introduced by the aforementioned initial heating process may have affected the results. The substantially higher freshwater output of #proto2 in comparison to #proto1 serves to demonstrate the effectiveness of the measures implemented in the improved model. A detailed account of these measures can be found in the subsequent section dedicated to “Construction Findings”. The substitution of glass for polyethylene foil may have exerted the most significant influence on the enhanced output.

Table 4.

Measurement results: Freshwater yield of #proto1 and #proto2

Figure 12.

Daily hours of sunshine in Puerto Ayora during test phase 2 (Charles Darwin Foundation, 2024)

Figure 13.

Average and extreme freshwater yields per square meter evaporation area of #proto1 and #proto2 during the test phase and a forecast over the course of a year

The mean output of #proto1 was found to be higher during the initial test phase in comparison to the subsequent one. The decline was from 301 to 237 milliliters per day, resulting in the overall mean value of 269 milliliters per day illustrated in Table 4. This decrease can be attributed to the 47% reduction in the number of hours of sunshine in the second test phase. Although the effects of wear and tear cannot be entirely discounted, it is unlikely to have a substantial influence, particularly given the relatively low 21% reduction in yield during the relatively high reduction in solar irradiance.

The mean number of daily hours of sunshine over the past 20 years, from January 2004 to September 2024, as recorded by the meteorological station of the Charles Darwin Foundation, was 4.28 hours per day. During the test period investigated here, this value was found to be 2.09 hours. The assumption that the yield of solar stills increases proportionally with the duration of sunshine allows the extrapolation of a possible annual yield of 201 liters for #proto1 and 378 liters for #proto2. These values, which are also illustrated per day in Figure 13, are only valid under the assumption of a linear correlation with a factor of 2.05 relating to the difference in hours of sunlight. This would result in a 2.05 times difference in yield. However, this approximation is not exact, as yields are generated even with zero hours of sun per day. To conduct a more detailed statistical analysis, additional data would be necessary.

Economic Implications

The cost per liter of distilled water was calculated to be $0.021 for #proto1 and $0.032 for #proto2. This calculation is based on a service life of five years of daily operation with the average yields measured during the previously discussed less sunny period of the year in Puerto Ayora. The analysis presented does not account for the full range of cost components and accounting details that can be associated with this project. For instance, the rental costs associated with the space required for the stills, the labor costs associated with the construction of the system, and the interest costs associated with the capital investment were not included in the analysis.

In consideration of the projected yields over the course of a year, including the high-yield months during the dry season, the cost decreases to $0.010/L for #proto1 and $0.016/L for #proto2. The five-year operational lifespan, prior to the necessity of costly repairs, may prove to be a significant challenge for the low-cost #proto1. With a more realistic maintenance-free operational lifespan of one year, the projected costs of #proto1 at $0.052 per liter of freshwater would be considerably higher than those of #proto2. From a purely economic point of view, the use of superior materials such as glass as a cover is therefore recommended despite the higher acquisition costs. In this calculation, the theoretical cost mentioned in Table 2 and 3 was used. By making use of readily available materials and resources free of charge, the expenditures could be substantially reduced, as indicated by the provided “real cost” of this project.

Construction Findings

During the construction process, especially of #proto1, many valuable insights were gained that could be implemented in the construction of the second prototype. The low-cost solar still exhibited deficiencies including the following:

• A slight taste of plastic in distilled water

• Durability issues

• Heavy droplet formation

• Evaporation of distilled water from the collection bottle

• Difficulty of cleaning

The majority of these deficiencies could be addressed through the use of glass in place of polyethylene foil as a transparent covering. Glass is tasteless, can be used for many years without any loss of properties, is simple to clean, and has cohesive properties that result in fewer beads. As illustrated in Figure 14, the glass cover of the second prototype (b) allows the water to flow away quickly, preventing the formation of droplets and increasing the radiation that reaches the interior of the still. In contrast, the polyethylene foil of #proto1 (a) blocks a significant portion of the sun’s rays, reducing the solar energy that reaches the evaporation area. In addition to the enhanced ease of cleaning glass in comparison to foil, the entire #proto2 is more suitable for maintenance purposes due to the lid and the removable basin.

Figure 14.

Comparison of droplet formation on the polyethylene cover of #proto1 (a) and the glass cover of #proto2 (b)

In addition to the previously mentioned construction improvements, several further factors may have contributed to the performance differences between the two prototypes. Firstly, the solar transmittance of the transparent cover is influenced not only by the material (glass vs. polyethylene film) but also by light scattering due to dust and droplet formation, which were significantly more prominent in #proto1, as shown in Figure 14. Secondly, the sealing quality of the transparent cover likely played a crucial role: while the glass panel in #proto2 was firmly and immovably fixed, the flexible PE foil of #proto1 may have allowed minor vapor leakage, particularly along the diagonals where mechanical reinforcement was limited (cf. Figure 5). Lastly, thermal conductivity differences between PE film and glass could have impacted the heat insulation performance of the cover, thus influencing the internal temperature and evaporation efficiency.

At the beginning of testing #proto1, the bottle for distilled water was just placed under the end of the runoff tube. After a short time, transparent polyethylene film, that remained following the cutting of the transparent cover, was employed as a protective covering as depicted on the left side of Figure 15(a). This was to prevent falsified results due to the ingress of rainwater and contamination and to reduce evaporation from the bottle. However, an unknown amount still evaporated, especially on sunny days. The runoff of #proto2 was therefore implemented airtight, with a fitting hose that connects to the bottle for storing the distilled water via a sealed connector (cf. Figure 15b, c).

Figure 15.

Distilled water runoff design comparison of #proto1 (a) and #proto2 (b, c)

Climatic and Meteorological Context

As previously discussed, the meteorological conditions and climate during the experimental phase of this study are not representative for the entire year in Puerto Ayora, nor for other locations within the Galápagos archipelago. The highest average daily GHI of 7.40 kWh/m² is observed during the dry season, typically in March. Meanwhile, the minimum daily GHI of 4.40 kWh/m² occurs from July until September, which coincides with the testing phase of this work (Eras-Almeida et al., 2020). In addition to this circumstance, which affects the entirety of the islands, Puerto Ayora exhibits a distinct microclimate that further reduces solar radiation. The transportation of humid air from the southern ocean to the north during the wet season results in the formation of clouds south of Santa Cruz’s mountains. Puerto Ayora is located in this precise geographical position. It is reasonable to assume that higher yields could be expected if the stills had been tested in the northern region of the island at the same time. The weather conditions in this region are sunnier as a result of the clouds being trapped in front of the island’s geographical barrier. It is important to note that the average of 2.09 hours of sunshine per day recorded in Puerto Ayora during the testing period remains a relatively low figure, even when considering the aforementioned circumstances. The average yields determined in this study therefore represent a conservative minimum value. When viewed at other locations or during the sunnier dry season, it is highly probable that these values can be exceeded. Nevertheless, the considerable yields of #proto1 (1076 ml/m² evaporation area/day) and #proto2 (2020 ml/m² evaporation area/day) can be attributed to the location of the experiment in close proximity to the equator. In this region, solar radiation typically exhibits a high energy density, which can have a corresponding effect on yields even when it is cloudy. Consequently, at other latitudes, lower yields could be anticipated for the same number of hours of sunshine. The predicted daily yields of 2.2 L/m² for #proto1 and 4.1 L/m² for #proto2 over the year are intended to compensate for these meteorological distortions and represent a reasonable value, although their experimental verification has not yet been achieved.

Water Quality

Due to limitations in the available facilities on the Galápagos Islands, it was not feasible to conduct experimental analyses of the distilled water. Consequently, no reliable quality parameters, such as pH value, salinity in terms of chloride concentration, hardness levels or total dissolved solids are known. Nevertheless, the water was consumed on a regular basis by the author, who did not experience any adverse health effects as a result of its consumption. No discernible impurities were observed on visual inspection. On occasion, the water from #proto1 exhibited a slight plastic-like flavor, which was likely attributable to the transparent polyethylene film utilized as a condensation cover. This effect might also be influenced by volatile compounds from the black plastic liner used in the basin. Although no health issues were reported during short-term consumption, regular intake of water with plastic-like odor is not advisable without proper chemical analysis or use of certified food-grade materials. Other studies in the literature have provided clear evidence that solar stills can reliably produce clean distilled water if they are constructed and maintained in accordance with appropriate design and maintenance protocols. As demonstrated by Biswas et al. in experimental studies that consider the WHO standards, this also applies to models with polyethylene foils as a transparent cover (Biswasa et al., 2017). The objective of this experiment was to assess the feasibility and performance of solar stills rather than to provide specific insights into the quality of the distilled water. To ensure the hygienic and pure quality of the water, it is crucial to implement comprehensive cleaning procedures and water assessments prior to utilizing the distilled water for human consumption.

Figure 16.

Determined freshwater yields compared to other experimental studies on similar single-basin inclined-angle solar stills in the literature

Comparative Analysis of Freshwater Outputs

The technology of solar distillation for potable water had not previously been applied on the Galápagos Islands. The majority of preceding experimental studies on inclined-angle solar stills were conducted in the Middle East region, situated between 10 and 35 degrees north of the equator. In contrast, Puerto Ayora is situated at a latitude of approximately 0.7 degrees south of the equator. Given that position, the region would be likely to receive more solar energy than regions situated farther from the equator. However, the presence of clouds, which are promoted by the region’s climatic conditions, results in higher GHI values in the Middle East. The distilled water outputs of single-basin inclined-angle solar stills comparable in design and construction from the literature in comparison to the here determined yields are presented in the following graph.

Economic Viability

The cost of freshwater was determined to be $0.016 per liter for #proto2. Thus, the cost of 20 L of potable water would be $0.32. In Puerto Ayora, a corresponding gallon of drinking water containing 20 L can be purchased for $2.00, including delivery to the consumer’s doorstep and collection of an empty gallon for reuse. While the environmental impact of the still may be lower when it is in operation for an extended period, the economic competitiveness of this approach is undermined by the associated effort. The operator is required to attend to the system at least every few days in order to refill and collect the water, unless autonomous operation is feasible. The implementation of complete automation would demand the incorporation of additional components, including a floating valve, a more sophisticated and larger water supply and storage system, and other requisite elements. The elevated costs would result in an even lower economic viability. A frugal one-person household could potentially be sustained with a weekly supply of 20 liters of drinking water. The production of this quantity of water by means of a solar still can be accomplished with an estimated workload of 10 minutes every two days, allocated to the refilling of the still and the collection and transportation of the distilled water. A savings of $1.68 per gallon could be achieved with a weekly investment of 35 minutes of labor, based on the cost of #proto2. This value would correspond to an hypothetical hourly wage of $2.88. It is unlikely that this compensation would provide sufficient incentive for the population of the Galápagos Islands to operate their own solar stills. It can be argued that the small-scale distillery technique demonstrated here would only be viable in remote regions with severely limited resources. In the context of the Galápagos Islands, which have a relatively high standard of living due to their thriving tourism industry, personal solar stills would therefore rather serve as a means of maintaining a minimum level of hygienic drinking water in the event of natural disasters or other emergency situations. It is reasonable to posit that the costs per liter of fresh water would decrease with the implementation of larger distillery systems, due to the advantages of economies of scale. To assess the economic viability of solar stills on a larger scale, further research would be required.

Alternatives to Solar Distillation

Some publications posit that solar stills are typically only suitable for the purpose of removing dissolved salts from water. If brackish groundwater or polluted surface water is present, the use of a slow sand filter or other pretreatment devices may be a more cost-effective solution. In the absence of fresh water, rainwater collection is an even less complex technique than solar distillation. It may be preferable in areas with more than 400 mm of rainfall annually, but it requires a greater area and usually a larger storage tank (Practical Action, 2012). Given an average annual precipitation of 380 mm in Puerto Ayora, this may prove to be a challenging aspect of a sustainable solution to water scarcity. The village of Bellavista, situated in the highlands approximately five kilometers north of Puerto Ayora, experiences precipitation levels of approximately 1100 mm per year (Reyes, et al., 2017). This is attributable to its geographical position and the aforementioned microclimate of Santa Cruz. Given these precipitation levels, rainwater collection shows considerable potential there. For outputs of 1 m³/day or more, reverse osmosis or electrodialysis should be considered as an alternative to solar stills, depending on the availability and price of electrical power. For outputs of 200 m³/day or more, vapor compression or flash evaporation are typically regarded as the preferred option in terms of economic efficiency (Practical Action, 2012). A project undertaken by the Faculty of Science and Technology at the Universidad del Azuay is investigating the implementation of desalination systems on Floreana, the least populated of the inhabited Galápagos Islands. To obtain clean water from seawater, the research employs the principles of evaporation, similar to the solar stills evaluated in this study. The system’s design aims to achieve high outputs, ensuring sufficient water for the island’s total demand. To meet this energy demand, the project utilizes external solar panels and electric resistances to heat thermal oil as heat transfer medium. These alternatives to solar distillation should be mentioned briefly here, but are otherwise a different subject area and not within the scope of this study.