Optimum thermal design of humidification dehumidification desalination systems

Humidification dehumidification (HDH) process is used for producing fresh water from saline water at sub-boiling temperature. This process uses a low-temperature source such as solar energy or waste heat source. Although these heat sources are available with minimal operating cost, an optimum thermal design is required to maximize the water production rate for a given heat input. In this paper, the main design and performance parameters are investigated for two HDH cycles namely, water-heated and air-heated cycles. First-law based thermal analyses are provided and performance charts are presented by considering assumptions. The design details of both the humidifier and dehumidifier are presented to determine their sizes under different design conditions. It has been demonstrated that optimum mass flow rate ratios exist for each cycle such that the gained-output ratio (GOR) is maximized. In addition, it is demonstrated that higher GOR can be obtained by using large-size humidifiers and dehumidifiers due to increasing their effectiveness. Moreover, increasing the temperature of water entering the humidifier reduces GOR for the water-heated cycle whereas it increases for the air-heated cycle. A comparison is also carried out between the two cycles to provide guidelines for designers in terms of, power requirements and components size.