Technical Analysis
The technical analysis of the Water Harvesting System can be broken into different parts, comprising of the heat transfer analysis, condensation analysis, electrical analysis, and water treatment.
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Prior to making any technical analysis calculations, there were several assumptions that needed to be made. The basic assumptions made were that the atmospheric pressure was at 1 atm and the saturation temperature of water was at 100 °C. Since the target environment temperature for the system would be in hot and humid locations, after a brief research on the average humidity levels and temperatures at places near the equator, it was assumed that the system would be held at a constant humidity level of 60% and the outside air temperature was at 80 °F. As the peltier plates would be able to reach below freezing points, it was assumed that the peltier plate surface temperatures would be at 40 °F. In order to be able to do heat transfer calculations on simpler geometry, the pipe and coils were considered to be one solid cylinder of 2.2 inch diameter. Specific assumptions made for each area of the technical analysis will be written out in the following pages.
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Heat Transfer Analysis
Our system uses peltier coolers to drop the temperature of the pipes and coils in order to induce condensation and form droplets of water. Peltier plates are generally used for a refrigeration system and since they can go below freezing temperatures, this uses a lot of energy. For our system, the peltier plates will be used to drop the temperatures of the pipes to around 40 degrees Fahrenheit. After calculating the conduction of the system at this temperature, pipes made from copper resulted in the highest conduction heat transfer of around 148.33 Btu/h on one pipe. Heat loss to the air while the peltier plates are running varies on the atmospheric temperatures but the strength of the peltier plates will maintain the pipes at the lowest temperature possible. Our team is still in the process of calculating the exact time it will take the peltier plates to cool down our design to 40 degrees Fahrenheit. However, the peltier coolers give off enough cold energy to maintain the necessary dew point temperature differences for water to condense and collect on our pipes and coils.
The chosen pipe and coil material had to be a type of metal that was able to conduct the most heat while not contaminating the water in the process of condensation and being economically cheap and available. In order to analyze this, the conduction heat transfer equation from the textbook was used. After the area, two end temperatures, and the length was calculated for, conduction coefficients for three different metals, copper, aluminum, and zinc, were found. After testing all three coefficients, copper had the highest heat conduction rate of 25.09 W. This material was what the team decided on for the pipe and coil as it was also cheap and easily accessible.
The above two equations for Q for very long fin and specified temperature at fin tip is calculated for how much heat is absorbed from the air. About 40W of heat energy is absorbed from the ambient temperature of 80 °F. Analytically, this Q value should thus be how much heat the peltier plates need to absorb from the pipe in order to prevent the pipe from heating up from the ambient temperature heat.
Condensation Analysis
Using the average temperature and humidity of Mexico as an example, we were able to calculate the approximate volume of water the Water Harvesting System can extract from the air in Mexico's specific location. First, we calculated what temperature the surface of the coils had to be in order to cause condensation. Then, using the heat transfer of condensation from the vapor in air to the coil, we were able to determine how much water per hour our system could collect under ideal conditions.
Electrical Analysis
The economic feasibility of the device was determined by calculating its daily operating cost. Shown below in Table 5, by summing the total power needed to power the device, 405.04W, the amount of energy in kWh per day could be calculated as 3.24, assuming the device would be in use 8 hours per day. The price of electricity is different all over the world, even city by city, so it is impossible to calculate a universal operating cost. As an example, the group determined what the operating cost per day would be using the cost per kWh in Mexico, which, when converted to USD, was only 45 cents per day. This cost is relatively cheap, but it is important to remember that the scale of this device only allows for relatively small water generation. Considering the theoretical amount of water that the device could generate over a period of 8 hours, the cost of water would come to about 5 cents per liter of water. This allowed the group to come to the conclusion that given a relatively hot and humid environment, the device would be an economical water source for the end user(s).
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We want to ensure that our product produces enough water to make the amount of energy spent to create it worth it, so that our customers can enjoy water that is affordable. By calculating the amount of electrical energy our design will need, we come to the end price of an average of $0.45 in North America. This price will be different around the world, and our team will use the calculated energy requirement to ensure that the cost is feasible in areas that it would theoretically be useful.
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Water Treatment Analysis
Some calculations were necessary for choosing design parameters for the two-step treatment process of filtration and disinfection. The dimensions of the sand filter, including the depth of each component, as well as the amount of activated carbon, were calculated from AWWA's Manual for Water Treatment.
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