Solar Distillation

Background

The Diné people have historically been subject to chronic water pollution and contamination, especially as a result of the mining boom in the 1900s for uranium [1]. The release of heavy metal-laden tailings, combined with the presence of sulfides and iron oxides that promote dissolution of heavy metals, has resulted in widespread contamination of an already scarce water supply [1]. In addition to modern-day contamination, there is a significant reservoir of saline groundwater spanning almost the entire Navajo Nation’s area [4]. Due to the salinity of this water, it is considered unusable for domestic and agricultural use. 

 

One idea to combat water shortages in the Navajo Nation is to treat existing contaminated water in the area. Numerous water treatment methods exist, from reverse osmosis to phytoremediation to ion exchange resins. One possible method that could prove viable in Navajo Nation is the use of solar distillation, a method that draws upon the use of solar energy to separate clean water from contaminants. 

 

What is Solar Distillation?

Solar distillation works by using the differences in physical properties between water and other substances [2]. Water has a boiling point of 100 degrees Celsius (212 degrees Fahrenheit), meaning it is converted into a gaseous form at that temperature [2]. However, heavy metals have a drastically higher boiling point, with uranium having a boiling point of 4131 degrees Celsius (7468 degrees Fahrenheit) and arsenic having a boiling point of 613 degrees Celsius (1135 degrees Fahrenheit) [5]. As a result, solar energy can be directed to heat contaminated water until it boils, leaving behind contaminants [2]. This water vapor can then be cooled and recondensed into liquid, producing clean water [2]. The structure of solar distillers can vary, but generally consist of one container to hold contaminated water, some component to capture and direct water vapor, and a second container to store the clean, recondensed water, all of which is ideally made airtight to prevent loss of water [2]. 

Figure 1: Basin-type still example [7]

Figure 2: Wick-type solar still [7]

 

Evaluation of Solar Distillation

This method is well suited for Navajo Nation’s climate, considering that the area is considered a desert with limited water input and is currently experiencing a megadrought [3]. One similar project, centered on solar desalination, has been piloted in 2018 by the University of Arizona. The group focused on ways to source and purify water from a saline aquifer that encompasses a majority of the Navajo Nation, testing desalination via thermal energy inputs, membrane/nanofiltration, and solar-powered electrochemistry [3].  Their results showed that using sunlight for thermal energy to evaporate and recondense water was most effective at removing contaminants, with almost zero waste produced [3]. 

 

Additionally, there are a wide variety of possible setups. The most basic type simply allows the sun to shine directly into a vessel of contaminated water, and allows the water vapor to condense and collect in a second container [2]. More complex versions utilize wicks and cloths to transport water within the containers more efficiently [2]. The primary maintenance would be cleaning out the components regularly and checking to ensure the containers remain airtight [2][3]. Additionally, most solar still designs have an average operating cost of $0.0135 to $0.23 per liter of clean water, meaning that this is implementable, even in resource limited areas [8]. 

 

Students can make use of the versatility and wide possible range of setups of solar distillation to tackle the engineering challenge of making their solar still. Using materials available to them, students of all ages and grade levels can develop their own designs for a solar still. Then, they can evaluate the efficiency based on the yield of clean water after a set time and the changes in water quality before and after distillation. Based on the feedback, they learn how to improve their designs and can understand how to directly implement this method to source clean water in their own daily lives. 

Figure 3: Basin type solar still example, adapted as classroom project [9]

 

Comparison of common solar distillation setups: [5][6][7]

 

Type	Description	Pros:	Cons:
Single Basin Stills	Basin with an optionally reflective bottom and sloped clear top cover to allow sunlight in, filled with contaminated water. Water vapor condenses on top cover and drains down into a conduit for collection.  Classroom alternative: One large bowl, partially filled with contaminated water. Smaller, clean bowl nested inside the large bowl. Cover with Saran Wrap and weigh down so that the lowest point hangs over the small bowl, allowing the condensed water vapor to drip into the smaller bowl for collection (see figure 3).	Costs of less than $0.01/L of clean water produced Simplest and cheapest to build Easily customizable for size/water volume, sun angle	Less effective at cold temperatures Produces highly concentrated brine/contaminated water that is difficult to evaporate
Wick Stills	Similar to basin stills, but blackened cloth or another wicking material is added within the basin, either covering the bottom and/ or distributed to form banks subdividing the basin.	Higher efficiency  Increases absorption of heat Highly saline/contaminated waters can be treated with less energy Still low cost, approx. $0.05/L of clean water	Wicks will need regular replacement. Wicks will have be discarded safely  if water is contaminated with heavy metals or other toxins
Diffusion Stills	Uses closely spaced vertical partitions in the basin and a set of wicks to maximize evaporation and condensation of water	Highest efficiency  Produces the least brine/highly contaminated water	Requires most materials Wicks will need regular replacement Wicks need to be discarded safely if use with contaminated water Most difficult to clean up