|M.Sc Student||Gido Ben|
|Subject||Atmospheric Moisture Harvesting|
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor David Broday|
|Professor Eran Friedler|
|Full Thesis text|
The atmosphere contains an enormous amount of water vapor, which can be harvested for drinking purposes. Conventional water-from-air machines use refrigeration cycles for direct-cooling which lower the temperature of the air bulk below its dew point and collect the water that condenses from the over-saturated cool air. Such process is subject to investment of cooling energy required by the condensation process (latent heat), and additionally, a significant cooling energy is invested in the air bulk itself. This additional energy demand can sum up to 40%-90% of the total energy requirement of the process. To demonstrate, quantify, and normalize this phenomenon, a thermodynamic index (moisture harvesting index - MHI) was developed and studied based on climate data from 30 locations around the globe. The index can be used to assess the energy-effectiveness of direct-cooling atmospheric moisture harvesting (AMH) process, and in some cases may indicate time laps in which the operation of direct-cooling system is inefficient. Avoiding operation of the moisture harvesting system in such times can result in significant energy saving, while the water productivity will be marginally affected.
Another approach in order to avoid the excess energy which is invested in cooling the bulk air is suggested, based on vapor separation prior to cooling process (hence, an in-direct cooling system), a Liquid Desiccant Separation (LDS). The system’s performance was examined throughout a range of environmental conditions using a thermodynamic simulation model. The investigation results include the hot fluid working temperature, which should be provided to the desorber unit of the LDS system to facilitate desorption of pure water vapor. The required hot fluid temperature should be between 50-800C, hence, can be obtained by solar heating or any other source of low grade heat. The condensation temperature required for most of the investigated climate conditions is 4-150C, and could be achieved by many existing cooling technologies. The results reveal that the system can produce water in almost any climate conditions but the production rate and energy requirements are climate depended.
Comparison of the energy demands of a direct-cooling atmospheric moisture harvesting (AMH) system to those of a LDS-based AMH system is not straight forward because the systems uses different sources of energy. Therefore, five scenarios were studied for such comparison, and the energy saving by LDS system was examined under the terms of each scenario, with input of eight reference points, representing different climatic conditions of operation. The LDS system was found to be energetically superior over a direct-cooling system in most of the examined scenarios.
The research indicates that LDS-AMH systems have a promising potential for improving the energy requirements of AMH, although the full economic aspects of the implementation of such systems are yet to be examined. Nowadays water production by reverse osmosis remains significantly energetically superior over active AMH processes. AMH may still be attractive as a water source in remote locations, scattered populations, or in areas where the water infrastructure is expensive to facilitate or non-existing (e.g. when the infrastructure has been severely damaged).