Mechanical Engineering Department
BS in Mechanical Engineering
The active fog catching senior project team has been asked to develop a device that will harvest water from the atmosphere, namely fog, using active means. The project will be centered on maximizing water collection rate, with efficiency as a secondary concern. The final design utilizes a refrigeration system in which fans pull air across the cold evaporators inside of a duct. Water condenses on the evaporators and drain into a collection container. It was decided to focus on maximizing water collection at the cost of efficiency because the feasibility of actively harvesting fog must be proved before trying to make the system efficient. A Copeland condensing unit that provides 2,490 Btu/min of heat transfer from the evaporator to the incoming fog is used in tandem with three expansion valves and four evaporators. Two Vernier expansion valves will allow the system to be finely adjusted while a third ball valve is used to coarsely adjust the flowrate. In addition, temperature and pressure gauges will be used to monitor the performance of the system and add additional control over the system. Speed controllers are connected in series to the fans in order to allow for variable flowrate. For testing, the system was placed in a high humidity environment and the fans were set to run at a predetermined flowrate. The Vernier expansion valves were adjusted until the first couple coils were frosting and the kWh consumption of the system was monitored using a watt meter. The results showed that the optimal flowrate for this system was between 140-150 cfm. In addition, it was discovered that pulsing the fans (toggling between the set point flowrate and the maximum flowrate for a brief period of time) would promote condensate to form droplets and drip out of the evaporators. The results showed that at the optimum flowrate, with pulsing, and a relatively high humidity the system could capture an experimental maximum steady state condensation rate of .307 gal/kWh. This amounted to approximately one gallon of water collected in six hours, at an energy cost of approximately forty cents.