Design of plants for solar desalination using the multi-stag heating/humidifying technique Original Research Article

Results are presented of a completed R&D project to develop a new process for solar seawater desalination. The central idea was to use the sun for heating air flow and to humidify that hot air by contact with seawater. After loading the air flow by a large amount of vapour, the water content can be easily recovered by cooling the humid air in dehumidification equipment. A main feature of this desalination process is to conduct the heating/humidifying procedure in several stages in order to load the air with a high amount of humidity and to reduce the volume of circulating air. Several pieces of equipment had to be developed in order to use this desalination process. Suitable and economic collectors, humidifiers and dehumidifiers were designed, manufactured and tested. Design guidelines for the new equipment and calculation methods to predict its performance are provided. This enables future construction of desalination plants working according to the new process. Furthermore, methods for pre-calculation of the optimum operating conditions of the complete process were developed. This research was based on a modern computation code as a tool. A further procedure could be derived to optimise the operation of the plant not only from a thermodynamic point of view but also from an economic one. Here the effect of multiple-stage operation on the economy of the plant could be investigated. The new equipment as well as the process were tested in two different pilot plants. The results obtained from several tests verify the applicability of the process and the reliability of the developed prediction, calculation and optimisation methods. Based on these results, an optimised design of a complete desalination plant for a small community to produce 10 m3/d of desalted water is presented. A summary of the engineering data of this plant, a process flow sheet and a plan view are given. A preliminary study on further R&D actions and possible measures to enhance plant economy are also discussed.