Methods

Methods:

Attempt 1:

Set Up:

The overarching goal of this study was to desalinate water using a solar still. In Attempt 1, three solar stills were made from Medium Density Fiberboard (2726N83) based on Edoja et al.’s study. These stills were fashioned to replicate the single basin solar still design used in Karthick et al.’s study (view Figure 1.0). While constructing the stills, protective eyewear and gloves were worn. The cover sheet was set to a 12º incline to match the slope used in Edoja et al. study. However, many papers suggest that an inclination of 20º to 25º optimizes solar still performance (Azooz,1970). It may be smart to look into altering the inclination of the cover sheet to 20º to 25º in future iterations of this study.

In Edoja et al.’s study, they used a trough to collect the distillate and funnel it into the collection valve. In this study, a similar trough was made, using Plumb Pak 200WK P Trap Pipes with 1 ¼ inch diameters. They were cut in half using a Ridgid R4513 15 Amp 10 in. Heavy-Duty Portable Table Saw with Stand (R4513). The table saw was used under professional supervision and goggles and lab coats were worn. A Clear Scratch- and UV-Resistant Cast Acrylic Sheet 24" x 24" x 1/8" (8560K259) was used as the cover sheet for the still instead of glass due to its durability (Pawar, 2016). Additionally, plexiglass is a cheaper material than glass; Clear Scratch- and UV-Resistant Cast Acrylic Sheet (8560K259) (the plexiglass) from Amazon cost $26.08 per sheet, while 24 Inch Sq. - Handrolled Clear Sheet Glass (1GBS-24-60-00-F) (glass sheets) cost $77.28 each from Slumpy’s. It is also important to note that delivery from Slumpy’s would have taken significantly longer as Slumpy’s is not a Berkeley Carroll pre-approved vendor.

According to Kaviti et al., maximum yield can be achieved when the solar still is equipped with a black lining because the color black is capable of absorbing all frequencies of light, thus increasing heat absorption, so the still was lined with a black tarp (2015). The distillate dripped from the trough into a plastic Quantum Storage Systems Dividers For Shelf Bins, 6 5/8" Width, 50/Ct (Dsb102/104/106). The plastic bin was covered with a layer of saran wrap to prevent the effluent from evaporating.

Each still was fabricated using the exact same measurements (seen in figures 1.1, 1.2, 1.3, 1.4, and 1.5) and materials to ensure zero variability across designs; the only difference across the stills was the thickness of the plexiglass: Still 1 was made with one layer of plexiglass (4 mm), Still 2 was made with two layers of plexiglass (8 mm), and Still 3 was made with three (12 mm). The thickness of the cover sheets (the independent variable) were varied to observe how it affects the amount of effluent produced. This was done because, according to Edoja et al., the temperature difference between the cover sheet and water in the still affects the yield. Without a temperature difference, the salt water will not evaporate rapidly because the cover sheet will be hotter than the salt water in the still. Additionally, it will not condense on the cover sheet because water molecules cannot condense on a surface that has an increased temperature (Khalil, 2015).

Still 1 served as the experimental group because a 4 mm cover sheet is proven to augment total effluent (Gnanaraj, 2021). There is no “true” positive control for this experiment because there is no standard solar still blueprint nor a commercially available still. However, based on Edoja et al.’s study, it is known that Still 2 (8 mm cover sheet) is expected to produce less effluent than Still 1. Regardless, Still 2 is expected to desalinate salt water, so it served as a known control condition. Still 3 (12 mm cover sheet) served as the negative control because it was treated under the same conditions as Stills 1 and 2, but was expected to produce no effluent due to the increased thickness of the plexiglass cover sheet (Edoja, 2015).

Testing:

All three stills were tested under the same testing conditions. First, each still was filled with 4 liters of saltwater with a uniform concentration measured using a SPER Combined Conductivity, Salinity, TDS, Temperature Meter (AP10068). The ocean water in each still was made in one large batch to ensure consistent salt concentration. If the salt concentration had been inconsistent, the amount of salt water desalinated by each still would have been disproportionate and incomparable. Synthetic ocean water was made using 456 g of Instant Ocean Sea Salt for Marine Aquariums, Nitrate & Phosphate-Free (B000255NKA) dissolved in 12 L of water. The instant ocean mix was measured using an FlinnScientific Electronic Balance, 410 x 0.01-g (OB2142).

The average concentration of ocean water is 35 ppt (Webb, n.d.). Water with a higher salt concentration has a higher boiling point, and requires more heat to evaporate (Sharqawy, 2011). Since this experiment was conducted in a classroom with synthetic sunlight created by 150 W Halogen Bulbs (SL-150), purchased from Amazon, screwed into Simple Deluxe 2-Pack Clamp Lamp Light with 8.5 Inch Aluminum Reflector up to 150 Watt E26 (B08X4C6ZXY), the salt concentration was lowered to 25 ppt. By starting at 25 ppt, the potential for more steady evaporation earlier in the experiment was maximized. These lamp shades were clipped onto Neewer 75"/6 Feet/190CM Photography Light Stands for Reflectors, Softboxes, Lights, Umbrellas, Backgrounds (10000117@@4). Secondly, the heat lamps were all positioned 1 meter above each still, so that they would be equidistant from one another. This allowed for equal exposure to heat for all three of the stills. Halogen bulbs were used to simulate sunlight because, according to Souza et al., halogen light bulbs emit all wavelengths of visible light in addition to ultraviolet and infrared (non-visible light). Of all artificial lighting sources, halogen bulbs most accurately reproduce natural sunlight, which is why they were used (Souza, 2019). Thirdly, the stills were set up in the same room, side by side. As a result, each still experienced the same temperature and lighting conditions. If left unmitigated, both of those factors could have negatively altered the data. If one still is run in a warmer area, condensation may occur more rapidly as a result. A warmer climate would pose a confounding variable. Confounding variables need to be mitigated within studies because, if they are not, these external variables may suggest a non-existent correlation within the data (Skelly, 2012). However, since all three stills were exposed to those same conditions, heating and lighting could not have disproportionately affected the data. Finally, all stills ran for the same amount of time to ensure that none of them had disproportionate time to desalinate.

Cause of Failure:

The solar stills were not adequately sealed leading to two issues: heat escaping and water leaking. In Attempt 1, fiberboard was used for the construction of the still. The pieces of wood were cut individually, and then held together by 20 screws. Though the screws did hold each piece of the still together, they did not seal the edges of the box. Additionally, the wood was not a uniform height, so the plexiglass cover sheet was unable to sit flatly on the surface, thus creating space in the seam between the plexiglass cover and the wood basin. Solar stills require high internal humidity to function (Hammadi, 2019) and these gaps in the basin seams allowed the humidity to escape. The first indication of this issue arose when one of the solar stills leaked two liters of water. Prior to this, there had been some evidence that there were holes in the seams as condensation was not occurring at a rapid rate within the solar still. Heat escape also posed a major problem. The water in the still cannot evaporate if the temperature in the still does not increase. Thus, with consistent heat escaping, condensation did not take place and the basin solar stills were nonfunctional.

An additional factor that contributed to the loss of heat was the black tarp stapled inside of the still. All solar stills must have black basins. The color black increases still efficiency as it increases heat absorption more than any other color (Canlas, 2016). The pieces of plywood were light brown, not black, so a means of blackening the base was needed. A black tarp was utilized to address this issue. Using a staple gun, the stills were lined with black tarp cutouts. At the time, this seemed like the smartest decision as tarps are waterproof, could be folded to match the shape of the basin, and are black. In order to secure the tarp to the basin, the tarp had to be folded over one of the four edges of the basin as seen in Figure 1.6. This exacerbated the unevenness of the basin's rim as the tarp further raised one of the four sides and, as a result, the plexiglass could not sit evenly on the basin’s rim. Unlike the tarp, painting the base black would not have caused space between the plexiglass and base of the still. The reason paint was not used was because high temperatures within the still can cause certain paints to peel and even melt.

Attempt 2:

Set-up:

Current methods are based on the causes of failures from the first attempt. Attempt 2 was built similarly to Attempt 1, except for two stark differences. Attempt 2, uses a circular basin (made from a Plastic Pail - 5 Gallon, Black (S-7914BLK)) rather than a square basin and the trough was built differently (view Figure 2.6).

Using the 1 ¾ inch piece from the Craftsman 7-Piece Set Bi-metal Non-arbored Hole Saw Set (CMAH1SET7), a hole was cut at 10 inches above the base of the bucket (view Figures 2.0, 2.1, and 2,2). In order to drill this hole, the 1 ¾ non-arbored hole saw was screwed into a Ryobi P1832 18V One+ Handheld Drill/Driver and Impact Driver Kit (6 Piece Bundle, 1x P277 Drill / Driver, 1x P235 Impact Driver, 1x P118 Dual Chemistry Charger, 2x P102 18V Batteries, 1x Tool Bag) (B01M2CGHKM).

Using an Eclipse 70-CP1R Wood Handle and Steel Frame Coping Saw, 1" Thickness, 12-3/8" Length x 5-1/8" Width (70-CP1R), one of the pipes from the set of Plumb Pak 200WK P Trap Pipes with 1 ¼ inch diameters had a piece cut out of it (view Figure 2.3). This piece was cut out of the trough so that when the water drips off of the cover sheet, it can fall into the opening in the trough (view Figure 2.4). Without that opening, there would be no way for the effluent to drip into the trough and exit the still. Sashco 5oz Sealants Clear Lexel Adhesive Caulk, 5-Ounce (13013-2) was applied around the trough to ensure that it was adequately sealed (view Figure 2.5). Safety goggles were worn at all times and the construction of this still was done under the supervision of a professional. The effluent dripped from the trough into a container with a small nozzle (view Figure 2.6). This was done to avoid the evaporation of the effluent. Since the collection container must be positioned under the trough, it is hit with direct light from the halogen bulbs. The small nozzle ensures that if any effluent were to evaporate, it would not exit the collection container.

Otherwise, both still designs have singular slopes, the cover sheets for both stills are set to a 12º incline, both utilize the same plexiglass, and Attempts 1 and 2 both used the same halogen bulbs to simulate heat from the sun.

The following alterations were made to address three crucial issues with Attempt 1:

1. Leaking Wood:

As stated previously, the wood was sawed/cut by hand leaving room for systematic errors. Also, gaps in the seams of the box led to leaking humidity escaping. Using a bucket as the basin eliminated these issues as the bucket was already built and had no seams.

2. Non-Uniform Rim:

The plexiglass in Attempt 1 did not sit flatly on the rim. Similar to the spaces in the seams, these gaps allowed heat to escape. The bucket shape addresses this issue (view Figure 2.7). The bucket does not have a uniform circumference. This means, more specifically, that the circumference at the top of the bucket is larger than the circumference at the bottom. This allowed the plexiglass to sit inside of the bucket on an incline without any additional support (see Figure 2.8). In order to calculate the size of the plexiglass, guessing and checking was used because the nonuniform circumference made it impossible to measure and calculate the ideal size for the cover sheet. Using an Epilog Fusion M2 Laser Series and the “Measure” app (an app that measures the incline of a surface), a larger piece of plexiglass was slowly cut down until it sat at a 12º angle and had minimal gaps. The diameter of this circle was 285 mm. In order to seal the still, Sashco 5oz Sealants Clear Lexel Adhesive Caulk, 5-Ounce (13013-2) was applied to the circumference of the plexiglass (view Figure 2.9). The glue dried for 24 hours before any salt water was put into the still to ensure that it had completely set.

3. Paint Contamination:

Though paint would have avoided the non-uniform rim issue caused by the black tarp in Attempt 1, paint contamination, melting, and peeling prevented its use. The buckets used in Attempt 2 are innately black, which makes black paint or a tarp unnecessary.

Testing:

The first testing set-up with the single slope, circular basin still was exactly the same as the testing set up for Attempt 1. The same halogen light bulbs, light fixtures, batch of Instant Ocean mix, and salinity meter were used and the stills were put in the same location. Additionally, the halogen lights were raised to the same height (1 meter above ground level) as in Attempt 1. View the “Testing” section in “Attempt 1” for further details regarding the testing set up.

Cause of Failure 1:

The still sat under the halogen bulbs for two hours, and no condensation occurred. The sides of the buckets were not yet hot, so the light was moved from one meter above ground level to 18 inches in order to increase the amount of heat infiltrating the still.

Cause of Failure 2:

The basin of the stills are made of plastic, so, when the light was moved closer, the bucket's rim melted. The melted area was sealed with ceramic clay to avoid any further heat from escaping. While sealing up the melted area, it was noted that there were still very minor gaps between the plexiglass and basin.

Cause of Failure 3:

Clay was added to seal all of these spaces and avoid any further escape of heat (view Figure 2.91). Sand is a strong insulator due to its high thermal conductivity, so, the bottom third of the still was additionally submerged into sand (Munirah, 2011).

Cause of Failure 4:

Now that the still was well sealed, it should have functioned. It was then realized that the plexiglass created an entirely new obstacle. Plexiglass has a lambda value that is 0.19 W/mK higher than that of glass. A higher lambda value indicates that a material absorbs more heat and, as a result, insulates less (“What Are U-Values, R-Values and Lambda Values?”, 2016). This means that the plexiglass cover sheet was absorbing a majority of the heat and thus minimal heat reached the salt water in the basin. One of solar still’s driving forces is the temperature difference between the cover sheet and the water in the basin (Edoja, 2015). The water in the basin must be hotter than the inner surface of the cover sheet in order for it to condense on the plexiglass (Edoja, 2015). The high lambda value of the plexiglass cover sheet caused it to become hotter than the water in the basin. As a result, the salt water could not condense on the cover sheet. To test this theory, an ice pack was placed on the cover sheet in order to lower its temperature. Five minutes later, once the plexiglass cooled down, condensation was visible on the inside of the still, proving the theory to be right (view Figures 2.92 and 2.93). This indicates that, once the temperature of the plexiglass is lowered, condensation does take place. Since glass has a lower lambda value, it should not pose this same issue.

Attempt 3:

Set-Up:

The set-up for Attempt 3 is identical to that of Attempt 2 except that glass was used as the cover sheet instead of plexiglass. The exact same materials (bucket, trough, and adhesive caulk glue) were used in both Attempts. View the “Set-Up” section in “Attempt 2” for further details.

The glass cover sheet was cut from Borosilicate Glass Sheet 12" x 12" (8476K23) purchased from McMaster. Using a Glass Cutter Tool Set 2mm-20mm Pencil Style Oil Feed Carbide Tip with 2 Bonus Blades and Screwdriver (B07Y1D243H), the square sheets were cut into circles. In order to ensure that the circle was cut to the accurate size, a compass was used to draw a circle with a 285 mm diameter onto the glass sheets prior to cutting it. These glass sheets were cut by continuously scraping the surface of the glass with the carbide tip pencil until the piece broke off. Safety goggles were worn at all times and all cutting was supervised by a professional engineer. Once the glass was cut, it was sealed into the still using the Sashco 5oz Sealants Clear Lexel Adhesive Caulk, 5-Ounce (13013-2), just like in Attempt 2.

Testing:

The testing set-up for Attempt 3 is almost identical to that of Attempt 2. The same halogen light bulbs, light fixtures, batch of Instant Ocean mix,, and salinity meter were used and the stills were placed in the same location (view the “Testing” section in “Attempt 2” for further details). Based on the insulation issues in Attempt 2, clay was added around the rim and sand was added around the base to increase insulation (view Figure 3.0 and “Cause of Failure 3” for further details).

Cause of Failure:

As stated previously, the optimal angle of inclination for the coversheet of a solar still is between 20° and 25° (Azooz,1970). As a result, this solar still failed to produce effluent.

Attempt 4:

Attempt 4’s setup is a simplification of Attempt 3’s design. Unlike the designs for Attempts 1, 2, and 3, this model does not require any sort of fabrication. This was helpful because it eliminated many of the obstacles posed by the solar still-fabrication process.

Set-Up:

Methods for this design are based on the design of a DIY emergency solar still (Marinoff, 2020). This solar still model is considered an “emergency still” because it requires few materials. Set up for this solar still includes a 5 gallon bucket (FG296300GRAY), saran wrap, three Four Brothers 1/2" Replacement Chromium Steel Balls for Spacerail Game (Pack of 10) (1004), Heathrow Scientific Cube Rack (HS29050A), and 4 Inch Small Glass Bowls 12 PCS,WERTIOO Mini Food Prep Bowl Tiny Glass Ramekins for Kitchen Dessert, Dips, and Candy Dishes or Nut Bowls (B07TYP1JHW).

In order to catch the distillate dripping from the cover sheet, a plastic cube with a glass bowl sitting on top of it was placed into the bucket after the salt water was poured in (view image 4.0). Next, a 20 inch piece of saran wrap was cut using scissors. This served as the cover sheet. Afterwards, the piece of saran wrap was laid on top of the bucket's opening. Since saran wrap folds and is light, tape was used to seal it to the bucket. It was important to make sure that the tape sealed all the gaps between the saran wrap and bucket because heat escapage would lead to no effluent production. While sealing the cover sheet is important, there must be some slack left in the saran wrap so that when the metal marble is placed onto the cover sheet, it sags down about 0.5 inches (view image 4.1). This is to ensure that after the distillate condenses into water droplets on the cover sheet, it drips down into the distillate collector (the glass bowl).

Testing:

The testing set-up for Attempt 4 is almost identical to that of Attempts 2 and 3. The same halogen light bulbs, light fixtures, batch of Instant Ocean mix, location of the stills, and salinity meter were used (view the “Testing” section in “Attempt 2” for further details).