Low-cost Solar Desalinator Design Team Lauren Fennelly, Steven Roy, John Shea Design Advisor Mohammad Taslim Abstract The goal of this project is to improve upon the solar desalination device built as a previous Capstone project. The original design consists of a pyramid shaped aluminum frame encompassed by a plastic tent. A tray of saltwater sits within the frame, which evaporates when heated by solar radiation. Water vapor condenses on the plastic tent and drips down into a collection basin as drinkable water. The second phase is working to make improvements to this design, mainly by implementing an automatic water feeding system. This system will control the flow of saltwater in the trays by setting and keeping the water level at a shallow height. In addition to the introduction of a water feeding system, four smaller trays subsitute the single large tray, and a PVC frame replaces the aluminum frame. Automatic feeding of saltwater into the evaporation tray eliminates the need to disassemble the device to pour more water into the tray throughout the day. Cost, size, portability, and ease of use also factor into the redesign of the device. By switching to a PVC frame, the cost decreases and the portability increases. Replacing the single tray with four trays minimizes the space used during packing and shipping.
The Need for Project An inexpensive, easy to use device is needed to generate clean drinking water in impoverished regions. This low-cost solar desalinator is an invaluable resource to approximately 780 million people worldwide that do not have access to clean drinking water. The device is high-output, low-cost, easy to use, and portable to ensure a life-changing impact on all of its users. Solar desalination is the process of removing salt from water by using the sun s energy. Once saltwater is heated by solar radiation, the water separates from the salt and becomes pure water vapor. This water then condenses on a clean surface and collects in large amounts and can be gathered as drinking water. The Design Project Objectives and Requirements The desalinator must be lowcost, high-output, portable, and The objective of this project is to develop a solar desalination Design Objectives easy to use. device that is low-cost, high-output, portable and easy to use by people of all skillsets. Since the device will mainly be used in impoverished regions, minimizing the price is of utmost importance. The device must also produce enough drinking water for a small family, and must be portable so it can be shipped across countries and easily assembled upon arrival. Finally, the device must be easy to use so all people can operate it and benefit from it. Design Requirements The desalinator prototype should be priced under $100 and must yield at least 5 liters of drinking water per day in optimal conditions. A water-feeding device must maintain a constant height of one-quarter inch in the evaporation trays at all times. Minimizing cost and maximizing output ensures the user will get the most value from the product. When broken down, it must fit inside a duffel bag so it can be transported by airplane. The device must be user-friendly so an inexperienced user can easily figure out how to operate it. Design Concepts Considered Main design concepts included a hanging feeder, a soft bag feeder, and a frame feeder. In order to increase output of the desalinator, a water-feeding system needed to be developed to control the level of water present in the system at a given time. Limiting this water level allows the water to
heat faster from the sun s radiation, therefore increasing the overall yield of clean drinking water. Hanging Feeder One concept for this water feeder involves creating a single solid container that hangs from inside the frame of the device. This container holds seawater. Branching off from this container are four legs that each lead to an evaporation tray. Mimicking the simple technology used in automatic pet feeders, the legs fill each tray until the water level covers the mouth of the feeder leg, upon which the water stops flowing. The height of the water is dictated by the height from the bottom of the tray to the mouth of the feeder leg. If this height is fixed to a quarterinch, then the water will fill only that high. Soft Bag Feeder Another concept involves using a large, soft plastic bag in lieu of a single solid container. Instead of rigid PVC legs, it features four flexible vinyl tubes of a small diameter. These tubes each lead to an evaporation tray. Saltwater is stored in the large plastic bag and is delivered to the tray by the vinyl tubes. This feeder design would operate based on the same principles as the hanging feeder; the tubes fill the tray with water until the water level reaches the mouth of the tube and prevents it from filling any further. Switching from a large rigid container to soft, flexible plastic means this component can be rolled or folded during shipping, therefore making packaging lighter and more compact. Frame Feeder The final concept makes use of the hollow PVC frame of the device as a storage area for the saltwater. Instead of attaching a separate container for saltwater, this concept stores the water directly in the frame itself, thus minimizing the amount of parts. A thicker diameter PVC pipe is used on the four slanted sides of the pyramid. From each larger pipe extends a thin, flexible vinyl tube. The thicker pipe will have a removable cap from which it can be filled with saltwater. When this cap is replaced, water flows out from the smaller tube and fills the tray based on the same principles as the previous concepts. The water will fill the tray only until the water level reaches the mouth of the vinyl tube.
Recommended Design Concept The frame feeder concept most effectively The final design of the solar desalinator was developed to most minimizes the water level while also minimizing effectively minimize cost, maximize the output, and ensure the design cost and improving portability and ease of use. is portable and easy to use. Design Description The final design of the solar desalinator utilizes the frame feeder concept. The frame of the device is constructed mostly from half-inch Schedule 40 PVC to keep costs as low as possible and to allow the device to be broken down, shipped, and reassembled. The slanted side of the pyramid is constructed from larger 2-inch diameter Schedule 40 PVC. Located within the device are four black, plastic, square trays. Saltwater is poured into the large 2-inch diameter pipes from a removable cap and the small vinyl tubes fill the trays to one-quarter inch. The sun s radiation will heat and evaporate this water. As the water evaporates from the trays and the water level slowly drops, the small vinyl tubes refill the tray to one-quarter inch automatically. As an added benefit, the water in the 2-inch pipe preheats before it even enters the tray, thus improving the rate of evaporation. The upper half of the device is a pyramid shape with a onequarter inch thick piece of plastic holding the four feeder legs together at the top. A polyethylene film covers the pyramid to allow solar radiation to enter the device, while providing a surface for the condensation of water vapor. Beneath the trays is a large collection basin made from the same polyethylene film as the upper tent. As the evaporated water condenses on the upper tent film, it will drip down past the evaporation trays and into this basin. This basin features a spigot so it can be emptied at the end of the day or as desired. Experimental Investigations The one aspect of the desalinator that needed to be tested. extensively was the performance of the automatic water feeder. Testing showed that we could only rely on the pet feeder principle if the container was airtight. The two initial concepts proposing the large rigid container and the soft bag each featured the four feeding legs branching off from the same water source, meaning the system was not airtight unless all four mouths of the feeder were covered by the water
level. It was found in many tests that one tray would overflow before the other trays were at the desired level. This is why switching to the built-in frame feeder greatly improved the effectiveness of the design. Each water storage compartment is filled and sealed separately and has only one feeding tube to each tray. Tests show that this maintains the water level at onequarter inch consistently, since it operates independently from the other three tray and feeder systems. Key Advantages of Recommended Concept Using the frame feeder concept is the most advantageous design option as it limits the water level in the tray without requiring any extra effort from the user. A smaller height of water heats and evaporates faster than a greater height, so minimizing the water height in the evaporation trays increases the output of drinkable water. Additionally, this concept does not require more parts to be added to the design. Since the saltwater storage compartments are already part of the frame, no extra storage area needs to be added. This means an improvement in cost by using less material, and an improvement in portability and ease of use by minimizing the number of different parts. The portability is also greatly improved since the PVC frame can be taken apart, unlike the welded aluminum frame of the original Phase I design. Replacing the single large tray from the Phase I design with four small trays also greatly improves portability since the small trays are stacked during shipping. Financial Issues The solar desalinator prototype The device was design for use in impoverished countries, so cost $90.12 developing an inexpensive solution will maximize the positive effect of the desalinator. The goal pricing for this project was to develop a product priced under $100. The prototype desalinator costs $90.12, but quantity pricing will further reduce this cost. Recommended Improvements Molded parts will make the device even more affordable. Molded parts, when buying/selling at large quantities, would reduce cost and decrease amount of parts. Different size options could be available to accommodate the various requirements of the customer.