Conceptual Designs
Two environmental factors that are important when designing an atmospheric water harvesting system are the dew point and the humidity. The dew point is the temperature at which the water vapor in the air condenses into liquid water at the same rate that it evaporates. When the air comes into contact with surfaces below the dew point temperature, the water vapor will leave the air and condense on the surface where it can be collected. Humidity is the quantitative amount of water vapor in the air. Both high temperatures and high humidity levels are ideal conditions for a large accumulation of atmospheric water.
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The general idea of using condensation to harvest water from the atmosphere can be summed up by creating a device that forces a phase change. Using energy to cool a copper coil with a large surface area and placing the coil in a hot, humid environment would cause water vapor to condense as droplets on the coil's surface. The accumulated droplets could then be stored and treated for potable use.
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The team brainstormed two different designs to compare and contrast, shown below.
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Design 1 - Peltier Plate System
Design 1 uses peltier plates to cool the surface of coils below the dew point in order to cause condensation. In this design, copper coils are wrapped around pipes that are in contact with peltier plates. By conduction, heat is removed along the entirety of the pipe, which removes heat from the coils, therefore dropping the surface temperature of the coils. The peltier plates would be powered by a renewable source such as a solar panel or solar powered rechargeable batteries.
Design 2 - Coolant System
Design 2 uses a liquid coolant to cool the surface of the coils below the dew point, mimicking Freon's refrigeration system. A pump would transport the coolant throughout a pipe, which is surrounded by a copper coil. The chemical properties of the coolant, the velocity that the coolant is traveling through the pipe, and the pressure drop all contribute to the conduction process. Heat is removed along the entirety of the pipe, which removes heat from the coils, therefore dropping the surface temperature of the coils. The pump would be powered by a renewable source such as a solar panel or solar powered rechargeable batteries.
Concept Selection Process
To decide between the two water harvesting designs, the team created a concept selection matrix. The most important criteria considered were low power consumption, low cost, and high efficiency. The team also considered the timeliness, feasibility of testing, maintenance, and ability to withstand weather conditions.
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The Concept Selection Matrix comparing the peltier plate and coolant designs is shown below. It was a close score, but the peltier plate design had the higher rating and won.
The main downside of the peltier plate design is that it uses more power to run the system than the coolant design. Four peltier plates would use an approximate total of about 450 W. A pump would only require about 100 W of power. However, using the peltier plate design would be less expensive, since the price of a pump is very high, and more efficient, because it cools the pipes faster than the coolant would. Overall, the peliter plate design is the better choice for the team's senior design project.
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The figures above show more detailed conceptual designs, which are summarized below.
1) Balsa Wood (18" x 36" x 8")
2) Copper (D=1", L=34")
3) Peltier Plates (40mm x 40mm)
4) Water Collector/Filter
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Water Treatment is an important subsystem to this design, and it will be connected to the main system shown as (4) in the above designs. Atmospheric water is already relatively clean, so this subsystem will consist of a two-step treatment. Filtration, the first step, will be a simple sand filter that is layered by diameter of aggregate. This will help strain out any floc, solids, and particles in the water. The second step, disinfection, will be performed by granular activated carbon (GAC). A sketch of the water treatment subsystem is provided below.
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