Studies of the Heat/Mass Transfer in a Desalination System Due to the Integration of a Solar Collecting Chamber and a Heat Dissipating Chimney

Abstract

Given the growth of the world's population and climate change, a lack of clean water has become a major issue that has been widely discussed for many years. Earth is mostly covered by water, but only 3% of this water is clean and usable. Data from research and investigation have shown that the salinity of most of the water on earth is more than 10,000 ppm. Sea water’s salinity ranges between 35,000-45,000 ppm. Based on a World Health Organization report, the highest level of salinity the human body can safely tolerate is normally 500 ppm. If people drink water with salinity higher than 1,000 ppm, this can cause harm to the human body. To solve this problem, various desalination technologies have been designed implemented, and improved to obtain purified water throughout the course of the human history. Because conventional desalination technologies have all required high energy consumption and had resulting pollution issues, more and more researchers have examined combining renewable technologies with desalination systems. The common sources of renewable energy are wind, marine, solar thermal, geothermal, etc. Based on research, solar energy is the most appropriate source for desalination because solar energy can be converted into both thermal and electrical energy that can be applied to all desalination technologies. Solar energy driven desalination systems could be categorized as either direct systems or indirect systems. Indirect systems encompass all conventional desalination systems, including humidification-dehumidification (HDH), multi-effect desalination (MED), multi-stage flash desalination (MSF), membrane distillation (MD), electrodialysis (ED), and reverse osmosis (RO). Direct systems include solar stills and solar chimney desalination. All these desalination technologies have their unique advantages and disadvantages; however, the universal disadvantages they have when they are combined with solar energy are the high capital cost and low thermal efficiency (except RO). To address these shortcomings, a solar thermal desalination system to achieve the goal of energy efficient, low maintenance, and low cost water and salts production, while leaving no waste discharge and having a minimal impact on the environment, is proposed in this paper. This system has two major integrated components: a solar collecting chamber that is also utilized as a water basin, and a heat dissipating chimney. The solar collecting chamber has a layer of seawater or brackish water that is evaporated with the collected heat. The heat dissipating chimney is used to draw airflow from the ambient environment to the solar collecting chamber, which carries the water vapor from the chamber to the heat dissipating chimney. While the moist air is ventilated by the chimney, the water vapor condenses at the heat dissipating chimney's inner surface. Relying on solar energy, the system can produce distilled water and collectible salt while leaving no waste discharge to the environment. Chapter 3 of this paper presents a preliminary experimental study, which was used to demonstrate the concept of desalination technology with freshwater produced under the solar insolation conditions of Tucson, Arizona, USA. This on-sun experimental study also indicated the necessary direction of investigation and optimization for subsequent work. Chapter 4 of this dissertation focuses on in-laboratory experimental tests. These in laboratory experimental tests provided controlled heating of the water in the chamber and a controlled ambient air temperature; studies of how water harvest performance was affected by the temperatures of water in the chamber and the ambient air could then be accomplished. The investigation of the effect of water temperature, the correlated dimensions of the system (including dimensions of the solar collecting chamber and the chimney) that affects the amount of airflow entering the system, and the resulting clean water collection rate in the heat dissipating chimney was also completed in this part. The evaporation of water could utilize as much as 84% of the energy supplied, which demonstrated that the chimney-effect-induced airflow takes the moisture away effectively and thus enhances evaporation. Given the limited chimney height in the lab test, the energy efficiency—the latent heat from harvested water against supplied energy—was limited to 24%. However, with a higher chimney and larger cooling capacity being available in a future outdoor test, much higher efficiency is anticipated. Nevertheless, the results from the current stage of studies have proven that the technology is promising; it is anticipated that this work can help in the design optimization and operation of large-scale systems for further studies. Chapter 5 of this paper presents a 1D modeling and computational analysis for this solar desalination system. A mathematical model was formulated for the in-drawing of airflow due to the chimney effect, the heat/mass transfer of water evaporation in the solar collecting chamber, and the condensation in the heat dissipating chimney. Computational results provide an essential understanding of the amount of airflow drawn in the system and the amount of evaporation and condensation of water, as well as the optimization of the diameter of the solar collecting chamber correlated to the diameter and height of the chimney. This study provides a tool and guidance for optimizing the design of such a system at a large scale and for operating outdoors under actual sunlight conditions in the future.