Goal
The primary objective of this challenge was to design a full-scale wind powered water treatment system, specifically applicable for treating brackish water in rural, third world areas. Before beginning any project design, it was necessary to separate the broad objective of using wind power to treat brackish water into smaller, more specific challenges. The team determined that appropriate groupings for these challenges would be wind energy capture, power transmission and energy storage, and finally the water treatment itself. Each of the three teams was composed of 4-6 students, and all three teams worked closely with each other due to the highly related nature and necessary compatibility of the three challenges.
Team
Wind Energy Capture Subteam: Matthew Ball, Alison Ernst, William Liew, Lyndsey Morgan and Deshira Wallace. Power Transmission Subteam: Alexander Brehm, John Peter Dolphin, Trisha Lowe and Nicholas Millar. Water Filtration Subteam: Samantha Beardsley, Pim Dangkulwanich, Aaron Lee and Natalia Rossiter-Thornton.
Project Considerations
The wind energy capture team chose to design a vertical axis Savonius rotor for this application; Savonius rotors are particularly well suited to low wind speeds, near-ground construction, and can accept wind from nearly any direction. They are also easy and inexpensive to construct; the compromise made is their low efficiency in comparison to horizontal axis turbines. The primary design consideration for the turbine is sizing its swept area to provide enough torque to run the water treatment process. This is dependent on both the pressure required by the water filtration system and the power transmission capabilities, so the team worked backwards using these considerations. In addition, to prevent stalling which can occur if the wind is blowing directly perpendicular to the concave section of the rotor, the team decided to stack multiple turbines vertically and perpendicular to each other. This increases swept area and percentage of the day the turbine spins. In addition, the team chose aluminum sheet metal to construct the rotor, which is light, strong, and easy to work with. The shaft material is steel, as it will be subject to nearly constant cyclic loading and must transmit a large torque.
The power transmission team’s objective was to transform the torque into a pressure that could be used to push the water through the filter. In order to do this, they first designed a gear system which allows a much greater torque and slower rotational speed to be obtained from the turbine, which is ideal for this design scenario. This gearing system is attached via a rope and pulley system to one end of a long lever arm, which rotates around a pivot at its opposite end. This lever arm is situated with several pegs that allow weight to be added, which provides a method of storing potential energy. Near the pivot, two small area plunger pumps are connected to the lever arm: a low pressure pump to run water through the pre-treatment process, and a high pressure pump, with a smaller area but longer stroke length and thus an equal volume per stroke, for the main filtration. As the lever arm rises, the plungers rise as well and pull water into the pumping chamber. Then, as the lever arm falls, the pumps complete their downstroke and push pressurized water through the system. These pumps were chosen for their simplistic design and ability to generate high pressure, given the compromise of low flow rate.
The water filtration team had numerous considerations to make given the quality of brackish water. They determined first to implement a pre-treatment system to filter out the organic material in the water; an activated carbon filter is best suited to this task, and requires a pressure of near 30 psi. The main filtration system chosen was reverse osmosis, which does not require electricity or large amounts of heat energy, as many filtration methods do. Reverse osmosis filters do, however, require fairly high pressures, which is the primary challenge of this design. Reverse osmosis requires applying a pressure to a solution that overcomes the natural osmotic pressure of the system, pushing to solvent through the filter while rejecting the solute. The team chose a spiral-wound thin-film composite membrane for this task. A storage tank is provided at the end of the system to hold the filtered water, and a separate tank collects the brine waste. In order to regulate the flow of the water through the system, our team decided that implementing an optional control system would be ideal. Because this system would be only a monitoring and feedback device that would not actually carry out the main function of filtration, we decided to include an option for a solar-powered electrical control system in our design. This system, using PID controllers, would monitor the flow rate and provide a feedback loop to provide nominal adjustments to improve performance and safeguard against inconsistent flow.
Solution
The team chose a sample location of rural Namibia, a highly arid region in southwest Africa, which meets the WERC ideal location criteria. We have designed this system for a village of approximately 100 people, requiring a total of 1000 liters per day of water for drinking and cooking uses. The system was designed conservatively with the assumptions that wind blows for 12.5% of the day, far lower than the 65%-90% average for most wind farms (WNCREI), at a wind speed of 8.5 meters per second, the highest distribution wind speed in our location. This allows for over 1300 liters per day to be pumped through the system. Based on the percent recovery of the membrane, approximately 80% to use a very conservative estimate, this allows for 1050 liters per day of clean water. Even if the wind blows more slowly, less frequently, or both, there is still a sufficient error margin to provide enough clean water to the residents.