Using TRNSYS Simulation to Optimize the Design of a Solar Water Distillation System

Abstract

We present the modeling, TRNSYS simulation, and parametric analysis of a solar water distillation system based on a humidification-dehumidification cycle. The thermal processes that constitute the cycle are carried out in devices designed for maximum individual efficiency. In order to achieve this, it is necessary for the evaporative process (in which an air current is humidified) to happen at the highest temperature attainable without boiling. This maximizes the amount of vapor that can be carried by the air current. This process is performed in a vertical packed column, where a stream of hot water falls as a current of hot air ascends in countercurrent and directly contacts the water. The air at the exit of the tower is saturated with vapor at the same temperature as the entering water, thus maximizing the amount of moisture carried. The hot and humid air then passes through a condenser that releases heat to the atmosphere, bringing air to nearly ambient temperature and maximizing distillate condensation. In order to achieve the previous, it is necessary for the thermal capture and storage system to work with thermal oil, an insulated storage tank, and evacuated-tube solar collectors. The system must maintain a steady oil temperature of 110°C. We propose a condenser based on heat pipes and with excess heat transfer area for dumping heat to the atmosphere, in order for the condensation temperature to be as close as possible to atmospheric. The efficiency of the distiller is substantially increased by forcing the process to occur between the described temperature limits, not unlike what happens in the power cycles of heat engines. The thermal oil transfers energy to the air and water currents through two heat exchangers, and the insulated storage tank makes it possible to operate the system at night for as long as sufficiently hot oil is still available. A flow control system regulates the temperature reached by the water that flows to the evaporation tower.We used the simulation platform TRNSYS to model this system, including the evaporation tower and condenser. Using the climatic conditions of the city of Chihuahua, Mexico, we performed a parametric study of the system and determined the effect of varying the number of solar collectors, volume of the thermal tank, and flow rate of water. We simulated the behavior of the system over a year of continued operation, measuring the amount of condensate produced during that period. The objective of this analysis was to determine the variation of distillate production, in kilograms of water distilled per year per square meter of solar collector, and per cubic meter of thermal tank. This was used to determine the optimal characteristics of the proposed distillation system.