Modeling, sensitivity analysis, and multi-objective optimization of a humidification-dehumidification-desalination system

Humidification-dehumidification desalination (HDH) is a promising technique that offers a valuable solution to the problem of fresh water shortage. In this paper, an accurate and a complete model that displays all needed expressions for all pressure drops, heat and mass transfer coefficients in the humidifier and dehumidifier and takes into consideration the effects of fouling and salinity of seawater was presented, a local sensitivity analysis was performed to determine the most influential variables for the objective functions of the total cost and gain output ratio (GOR), an innovative design for the dehumidifier that significantly enhances the overall heat transfer coefficient and reduces the pressure drop for water flow was introduced, and optimization problems were formulated through which the total cost of producing fresh water and GOR were optimized separately and simultaneously relative to 10 optimization variables using a genetic algorithm and the direct search method. The main optimization results showed that the HDH system runs at higher temperatures at the minimum cost than that at the optimum GOR and that the average overall heat transfer coefficient in the dehumidifier at the minimum cost is about 5.3 times that at the optimum GOR and that minimizing the total cost reduced the cost at the optimum GOR by 89% and increased its production rate by 121%. Fouling reduced the average overall heat transfer coefficient by 10 to 21% and increased the annual cost. Replacing the copper tubes of the dehumidifier with stainless steel tubes reduced the cost further by 39% at the same production rate. The distributions of the Pareto-optimal point solutions of multi-objective optimization results for the total cost and GOR as a conflict-objective functions were plotted and showed that all points are non-dominated. The consideration of the salinity of seawater shifted the Pareto-optimal point solutions to higher costs and GOR.