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DESALINATION

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ELSEVIER

Desalination 109 (1997) 277-284

A simulation study to improve the performance of a solar humidification-dehumidification desalination unit constructed in Jordan •

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Naser K. Nawayseh a, Mohammed Mehdi Farld , Abdul Aziz Omar a, Said Mohd. Al-Hallaj b, Abdul Rahman Tamimi b aSchool of Chemical Engineering, University Sains Malaysia (USM), 31750 Tronoh, Pera£ Malaysia Tel. +64 (9) 373-7599 Fax +64 (9) 373-7463 bChemical Engineering Department, Jordan University of Science and Technology, Irbid, Jordan Received 15 October 1996; accepted 28 February 1997

Abstract

Solar desalination with a humidification-dehumidification process has proven to be an efficient method of utilizing solar energy for obtaining fresh water from saline water. The reason behind the success of this method is the use of latent heat of condensed water vapor. With proper use, this process efficiency could be high. A simulation program was written to describe the performance of such units. It was found to predict the performance of a desalination unit constructed in Jordan in 1993. The simulation program was then used to optimize the unit performance by studying the effect of different parameters such as the condenser and humidifier area, as well as the effect of some of the operating conditions such as the feed water flow rate to the unit.

Keywords." Solar; Simulation; Desalination; Humidification

I. I n t r o d u c t i o n

Solar stills have been thoroughly studied and tested for the production of desalinated water using solar energy. It was reported that the *Corresponding author. Present address: Chemical and Material Engineering Department, The University of Auckland, Private Bag 92019, Auckland, New Zealand.

efficiency of the single-basin solar still is in the range of 30-50% [1,2]. The major problem behind this is the loss of latent heat from condensation through the glass cover of the stills. Several workers tried to investigate the effect of the climatic and design parameters on the performance of the basin-type solar still [1,3]. The productivity of these units was improved

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when operated under reduced pressure [3]. However, such an operation was found not practical because of the difficulties associated with reducing the pressure. Some investigators [4] arranged the still in such a way as to make the water flow over the glass cover. Preheating of the feed water by passing it over the glass cover allowed only partial utilization of the latent heat with only a limited increase in the production. However, the flow of water over the glass cover reduced the amount of solar radiation received by the water in the still. The accumulation of salt and the vapor leak were also some of the defects of these units. The unit produced 6 L/m2/d of fresh water. Other investigators [5-7] used a fiat-plate solar collector to supply the heat to the still. The desalination efficiency was slightly improved, but the increase in the cost of the unit was significant. The real improvement in the solar still design was achieved through the multiple use of the latent heat from condensation in the still. In a unit consisting of multi-cells, heat is supplied only to the first cell (effect). Water is evaporated in the second effect as it trickles over a metallic surface heated by the condensation of the vapor from the first effect. This allows the use of the latent heat from condensation at different levels. The development of a rugged design, high-efficiency, multi-stage solar still was studied and analyzed [8]. The unit was a stack of six rectangular cells. In each cell, which is 4 cm thick, a thin film of salty water is partially evaporated as it trickles over a heated vertical wall. The vapor produced is condensed on the opposite wall of the cell. The heat evolved from the condensation is used to evaporate the film trickling in the next cell on the other side of the same plate. The unit produced more than 20 L/m2/d of distilled water. They have used mirrors as solar reflectors to increase the solar energy received by the inclined multi-effect still, allowing the operating temperature to reach 85°C. If the reflector area was included in the calculation, then the unit productivity would not be better than 6 L/m2/d.

Multi-effect solar stills may be used for efficient production of desalinated water but only for small capacities since the condenser and the evaporator are integrated parts of the still. The low heat and mass transfer coefficients in this type of still require operation at relatively high temperatures and the use of large and expensive metallic surfaces for the evaporation and condensation. Extensive research have been carried out by different research institutes in Germany, as reported by Heschl [9], to develop an efficient use of solar energy for water desalination. Multieffect humidification (MEH) plants were constructed and tested at different countries. In these plants, heat was recovered by air circulation between a humidifier and a condenser using natural or forced draft circulation. Some of the MEH units use integrated collector, evaporator and condenser [10]. Air was circulated through the unit by natural draft. A maximum value of 3 was recorded for the gain output ratio (GOR), which is significantly higher than that of a singlebasin still (GOR