Desalination

0011-9164/04/$– See front matter © 2004 Elsevier Science B.V. All rights reserved

Desalination 160 (2004) 167–186

Solar desalination with a humidification–dehumidificationtechnique — a comprehensive technical review

Sandeep Parekha, M.M. Faridb, J.R. Selmana, Said Al-Hallaja*

aCenter for Electrochemical Science and Engineering (CESE), Department of Chemical and Environmental Engineering,Illinois Institute of Technology, 10 W 33rd Street, Chicago, IL 60616, USATel. +1 (312) 567-5118; Fax: +1 (312) 567-6914; email: [email protected]

bDepartment of Chemical and Materials Engineering, The University of Auckland, Auckland, New Zealand

Received 10 February 2003; accepted 27 May 2003

Abstract

Major desalination processes consume a large amount of energy derived from oil and natural gas as heat andelectricity. Solar desalination, although researched for over two decades, has only recently emerged as a promisingrenewable energy-powered technology for producing fresh water. Solar desalination based on the humidification–dehumidification cycle presents the best method of solar desalination due to overall high-energy efficiency. This paperprovides a comprehensive technical review of solar desalination with a multi-effect cycle providing a better under-standing of the process. Discussion on methods to improve system performance and efficiency paves the way towardspossible commercialisation of such units in the future.

Keywords: Solar desalination; Humidification; Dehumidification; Multi-effect humidification

1. Introduction

Water is available in abundance on the earth;however, there is a shortage of potable water inmany countries in the world. In the Gulf countriesand elsewhere, non-renewable energy from oiland natural gas is used to desalinate water fromsea water in multi-effect evaporators. It is alsocommon in some places to use electric

*Corresponding author.

power to run reverse osmosis units for waterdesalination. In the first method, a large quantityof heat is required to vaporize the water, whilethe second method requires electric power togenerate high pressure to force the water compo-nent of seawater through a membrane. Bothmethods consume large amounts of energy andrequire high skill operation. Nevertheless, thesetwo methods, until recently, were considered asthe most practical way of desalinating seawaterbecause the Gulf countries, known for their

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shortage in drinking water, are also known for theavailability of oil as a cheap source of energy.Due to the fossil-fuel-based energy consumptionin both methods, CO2 emission will always be anissue of environmental concern. Also there aremany places where energy is too expensive to runsuch desalination processes. Sometimes freshwater is required at locations far from the energygrid-lines, requiring a local source of energy.Hence, even countries with rich resources ofenergy, such as the Gulf countries, have shown astrong interest in the desalination processes thatoften utilize renewable energy sources.

Water shortages occurs most at places of highsolar radiation, which usually peaks during thehot summer months of maximum solar radiation.Hence, solar desalination could be one of themost successful applications of solar energy inmost of the hot climate countries having limitedresources of fresh water.

2. History and background: solar stills

Interest in seawater desalination goes back tothe fourth century BC, when Greek sailors usedto obtain drinking water from seawater. The firstwork published on solar desalination was that ofArab alchemists in 1551. However, the first solarstill was designed and constructed in Chile by theSwedish engineer, Carlos Wilson, in 1872, asdescribed by Malik et al. [1]. Following that, nowork was conducted on solar desalination till theend of the First World War. During WorldWar II, Telkes [2] developed a plastic stillinflated with air, which was used by the US Navyand Air Force in emergency life rafts. Shepublished a series of more than 20 papers onthese types of solar stills, up to 1964.

It is not the objective of this paper to com-prehensively review the work done on solar stills;however, certain features of their design led tonew solar desalination processes such as themulti-effect humidification–dehumidification(MEH), which will be discussed in this paper.

Even with very good insulation, the single-basin still was found to distill water with lowefficiency (usually below 45%), depending on theoperating conditions, as reported by Malik et al.[1], Cooper [3], Kudish et al. [4] and Farid andHamad [5]. The low efficiency of the still ismainly due to the high heat loss from its glasscover. A double glass cover reduces heat losses,but it also reduces the transmitted portion of thesolar radiation and increases the cost signifi-cantly. The productivity increases when the solarstill is operated under reduced pressure (Yeh et al.[6]); however, this was found impractical becauseof the difficulties associated with re-ducedpressure operation.

Solar radiation received by a horizontalsurface is not at its maximum except near theequator. Many investigators have modified thehorizontal single-basin still, usually fixed on ahorizontal surface, to an inclined type to receivemaximum solar radiation. Later, the tilted trayand the wick-type solar still were developed.However the construction cost of these com-plicated stills added significant cost penalties,while the increase in the production of the stillswas very limited.

The loss of energy in the form of latent heat ofcondensation of water at the glass cover is themajor problem of the single-basin still. Someinvestigators (Tiwari et al. [7]) arranged the stillin such a way as to have the water flow over theglass cover. Preheating of the feed water bypassing it over the glass cover allowed onlypartial recovery of the latent heat, with anincrease in the still production up to 6 L/m2d. Theflow of water over the glass cover reduced theamount of solar radiation received by the water inthe still. Accumulation of salts and vapor leaksalso frequently caused defects in these units.

Further work in improving the efficiency ofsolar stills was carried out by El-Bahi and Inan[8]. The effect of adding an outside passivecondenser to a single-basin-type solar still withminimum inclination (4°) was investigated

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experimentally. This solar still yielded a dailyoutput of up to 7 L/m2. They have reported stillefficiency of 75% during the summer months.When the solar still was operated without acondenser, the yield decreased to 70% of thatwith a condenser.

A major improvement in solar still design ispossible through the multiple use of the latentheat of condensation in the still. In such a unitconsisting of multi-cells, heat is supplied only tothe first cell from a flat-plate solar collector.Water is evaporated in the second effect as ittrickles over a metallic surface heated by thecondensation of the vapor from the first effect.This allows the utilization of the latent heat atdifferent levels. A comprehensive review andtechnical assessment of various single- and multi-effect solar stills was presented by Fath [9], whohighlighted the impact of utilizing latent heat ofcondensation via multi-effect solar stills. Mink etal. [10] conducted an in-depth study on heatrecycling using a laboratory-scale solar still of1 m2 area, designed to recycle the condensationheat of the distillate. The exposed wick surfacearea was 1.0 m2, with thermal incident energy of650 W/m2 being supplied by a solar simulator ata tilt angle of 20°. The forced circulation ofambient air was achieved using a low-pressurevariable speed ventilator. Preliminary resultsshowed an increase in productivity per unit areaby a factor of two to three, compared with tilted-wick or basin-type solar stills, respectively.

One of the most recent designs of such a stillis that described by Grater et al. [11] andRheinlander and Grater [12] of a four-effect still,as shown in Fig. 1. The evaporation process in afour-effect still for the desalination of sea andbrackish water was experimentally investigated ina test facility under different modes andconfigurations of heat recovery. The unit wasoperated under both natural and forced convec-tion in the four distillation chambers (“effects”).The experimental unit consisted of a base moduleof the four-effect distillation unit with an active

Fig. 1. Multi-effect still: technology for the desalinationof 10 m3/d of water (Rheinlander and Grater [12]).

evaporation cross-section of 1.7 m2 and theheating and cooling cycles.

The four-effect distillation unit consisted offour stages comprised of the heating and coolingplates and three intermediate plates. The multi-effect still unit was tilted by a very small angle ofaround 3–5° from the vertical line. A heatexchanger was used to provide the necessaryheat, simulating a flat-plate solar collector. Theenergy of the last condensation surface wasabsorbed by the cooling cycle, connected to a wetcooling tower via a heat exchanger. All possibleoperating conditions were examined by using amathematical model. It was observed that themain advantage of the heat recovery system is thesubstantial improvement of the gain output ratio(GOR) (defined as the ratio of the energyconsumed in the production of the condensate tothe energy input) by decreasing the demand forprimary heating of the first effect. The GORincreases by up to 80% due to heat recovery fromthe distillate latent heat. Intermediate screens andforced convection in the colder effects improvedthe distillate output which lead to improved GORof the investigated four-effect still. The modelpredicted a distillate output in the first effect thatwas 50% higher than the measured values whenthe feed temperature was raised to 90°C.

It is to be noted that the unit of Grater et al.[11] and Rheinlander and Grater [12] was


operated with hot water at a constant temperatureof 90°C. Under such high operating conditions,the evaporation and condensation will be veryefficient, but long-term operation is not practicaldue to scale formation. Furthermore, if a solarcollector is used to drive the desalination unit,then its collection efficiency will drop to a verylow value at such a high temperature.

Thus, multi-effect solar stills may carry outmore efficient desalination of seawater comparedto a single-basin still, but for only small capa-cities since the condenser and the evaporator areintegral parts of the still. The low heat and masstransfer coefficients in this type of still requireoperation at a relatively high temperature and theuse of large and expensive metallic surfaces forthe evaporation and condensation.

In the following sections, a new class of solardesalination system is discussed, whose design isbased on a more efficient utilization of the latentheat of condensation.

3. Humidification–dehumidification technique

3.1. Principle of the humidification–dehumidi-fication process

The most promising recent development insolar desalination is the use of the humidifi-cation–dehumidification (HD) process. The prin-ciple of functioning of the HD process has beenreviewed by Bourouni et al. [13]. The HD processis based on the fact that air can be mixed withlarge quantities of water vapor. The vaporcarrying capability of air increases with tempera-ture: 1 kg of dry air can carry 0.5 kg of vapor andabout 670 kcal when its temperature increasesfrom 30°C to 80°C. When flowing air is incontact with salt water, a certain quantity ofvapor is extracted by air, which provokes cooling.Distilled water, on the other hand, may berecovered by bringing the humid air in contactwith a cooled surface, which causes the conden-sation of part of the vapor in the air. Generally,

the condensation occurs in another exchanger inwhich salt water is preheated by the latent heat ofcondensation. An external heat contribution istherefore necessary to compensate for the sensi-tive heat loss.

The HD technique is especially suited forseawater desalination when the demand for wateris decentralized. Several advantages of thistechnique can be presented which includeflexibility in capacity, moderate installation andoperating costs, simplicity, and possibility ofusing low-grade thermal energy (solar, geo-thermal, recovered energy or cogeneration). Inthis process, air is heated and humidified by thehot water received from a solar collector. It isthen dehumidified in a large surface condenserusing relatively cold saline feed. Most of thelatent heat of condensation is used for preheatingthe feed.

3.2. Non-solar methods of extracting water fromhumid air under atmospheric conditions

Prior to focusing on the details of the differenttypes of solar HD processes and their analysis, webriefly review work aimed at using the humidityin the atmosphere as a source of fresh water andmethods for extracting the water from humid air.Methods for water extraction from humid airinclude mechanical, refrigeration (absorption andvapor compression), adsorption and absorption.

3.2.1. Atmospheric water vapor processingRecently, Wahlgren [14] conducted a com-

prehensive review of atmospheric water vaporprocessing (AWVP). Three classes of “processormachines” for potable water production wereidentified. The machine design types mentionedare based on:C surface cooling by heat pumps or radiative

coolingC concentrating water vapor through use of solid

or liquid desiccants, andC inducing and controlling convection in a tower


structureNo costs or capacities for these machines arementioned, but energy requirements stated for thethree classes of machines mentioned are rela-tively higher than those of solar-based HDprocess.

3.2.2. Dew collectionIn 1981, Rajvanshi [15] reported a scheme for

large-scale dew collection as a source of freshwater supply. In the desert environment, dewcollection takes place due to night sky radiationcooling. This, however, results in an insufficientquantity of water production. A more efficientmethod proposed would be to pass deep-sea coldwater through suitable heat exchangers for dewcondensation. A heat exchanger field of129,000 m2 can condense 643 m3 of dew over aperiod of 24 h. Cold water for dew condensationmay be obtained from a depth of 600 m. Three200 kW wind machines power the pumping of8,320,000 kg/h of this cold water. This methodhas been studied by Wahlgren [14] too in hisdesign of AWVP machines (mentioned above).

3.2.3. Adsorption methodAlayli et al. [16] in 1987 reported a study to

extract water from wet air based on the adsorp-tion principle. A two-phase cycle comprised of anocturnal phase and a diurnal phase wasproposed. In the nocturnal phase, an adsorbentcomposite material, type “A”, is exposed to thesurrounding atmosphere in which the temperatureand relative humidity rate can vary. Material “A”is humidified by physicochemical adsorption. Inthe diurnal phase, solar radiation heats up the wetcomposite material “A” to about 100°C. Thewater contained in the material is drawn up and itcondenses on a cold plate. Experimental inves-tigations with a certain type of composite material(“A”) under the conditions of 20°C temperatureand relative humidity of 50% yielded 1 L/m2/d ofdrinking water. The quantity of water increased to

2–4 L per m2 of material surface if the relativehumidity is increased to 80% and if anothercomposite material (type “B”) is used.

3.2.4. Absorption–refrigeration methodAmong the absorption–refrigeration methods

to extract fresh water from humid air, Aly [17]presented a non-conventional method suitable forcollecting the humidity from air in hot regions. Inthis process, water is a by-product of a cycle,originally used in air-conditioning. A solar drivenLiBr-H2O absorption-cooling machine is usedwith an open absorber where the ventilation air isdehumidified by direct contact with a concen-trated LiBr-H2O solution. The diluted solution isregenerated in a generator (concentrator) wherethe collected water is recovered to allow the con-centrated solution to be recycled. The recoveredvapor is condensed (fresh water by-product) andthe condensation heat is re-used to promote therequired cooling effect for the air-conditioningevaporator. Besides its efficient air-conditioningfunction, the process contributes to decentralizedfresh water production in hot regions. The by-product water production amounts to 3.1 L/m2/dof collector area, which is higher than that ofbasin solar stills.

3.2.5. Vapor compression–refrigerationmethod

Vlachogiannis et al. [18] proposed a noveldesalination concept combining the principles ofHD and mechanical vapor compression refrige-ration. They constructed a laboratory prototypeunit to analyze and study the concept. Theirprocess combines the principles of intensiveevaporation and vapor compression refrigerationwith a heat pump (mechanically intensifiedevaporation MIE). This process re-uses latentheat of condensation of water in successiveevaporation chambers, and experiments haveindicated that the prototype was successful.


A study to investigate the combination ofdesalination with cooling and dehumidificationair-conditioning was conducted by Khalil [19] forthe the climatic conditions of the United ArabEmirates coastal regions. The quantity of freshwater obtained depends on different parameterssuch as properties of the humid air, air velocity,cooling coils, surface area, and heat exchangearrangement. To achieve the maximum conden-sate yield, the heat and mass transfer mechanismswere analyzed and coil conditions optimized.

3.2.6. Absorption methodAmong the absorption-based techniques,

Abualhamayel and Gandhidasan [20] proposedthe use of a suitable liquid desiccant to extractfresh water from humid air. The night-timemoisture absorption and the daytime moisturedesorption take place in the same unit. The per-formance of the unit was predicted analyticallyfor typical summer climatic data in Dhahran,Saudi Arabia, by solving the energy balanceequations. For given operating conditions it wasshown that it is possible to obtain about 1.92 kg/m2 of the unit. The influence of absorbent con-centration and flow rate on the performance ofthe system was analyzed, and it was found thatthe increase in the absorbent solution flow rateincreases the rate of absorption of water from theatmosphere but decreases the desorption rate ofwater during daytime operation. Further study isrequired to determine the economic feasibility ofthe system.

3.2.7. HD using hydrophobic capillary con-tactors

Novel methods of desalination based on theHD principle have been proposed and studied byother investigators. Korngold et al. [21] andBergero and Chiari [22] studied a new desali-nation process consisting of air HD usinghydrophilic or microporous hydrophobic hollowfibers. Hot saline water is passed through hollow

fibers in a recycled air-sweep prevaporationprocess, the saline water being heated by wasteheat, solar energy or any other sources of energy.The flux of water through the hollow fibers is inthe range of 1.5–3.0 L/m2 h, with water tempera-ture being 45–65°C. The calculated energyrequirement for pumping air and water in a pilotplant unit of capacity 6.3 L/h with 4 m2 of anion-exchange hollow fibers was about 2 kWh/m3

when hot water temperature was 60°C [21]. Ex-periments indicated high mass transfer efficiencyfor both humidification and dehumidification.

3.3. Solar HDIn 1968, Garg et al. [23] reported a study on

developing the HD technique for water desali-nation in arid zones of India. A solar stilldeveloped in the first stage in the Central Salt andMarine Chemicals Research Institute, Gujarat,India, had a productivity of 2.94– 3.91 L/m2 ofstill area, depending upon the varia-tions in theintensity of solar radiation. Certain drawbacks ofthe solar still technique were overcome in thesolar-powered HD technique. In the first stage ofdevelopment of the HD tech-nique, a 3 L/d (24 h)capacity experimental unit was manufacturedhaving a packed tower with a packing height of30 cm, using Raschig rings as packing material.The total height of the humidifier was 60 cm,with 15 cm top and 15 cm bottom heads. Thehumidification unit was coupled with a surfacecondenser (dehumidifier). The distillate collectedfrom the unit had a concentration of less than 50ppm of salt. An electric heater was used to heatthe brine. From experimental runs, it wasdetermined that for lower temperatures of around55°C, a liquid–gas ratio (L/G) of the order of 3was suitable. This unit had a production of 3.4L/d for a brine temperature of 60°C. A secondunit was con-structed with a capacity of 136 L/d(24 h). Measurements showed that the rate ofproduction of fresh water increased with theincrease in temperature of brine, if other


conditions such as liquid and gas rates were keptconstant. The Reynolds number for air flowcalculated for the 3.4 L capacity unit was 285 andfor the 136 L capacity was 300. From theexperiments on the 136 L/d capacity unit, the rateof production of fresh water obtained was about61.2 L/d for a brine temperature of 59°C and aL/G ratio of 2.0. The production was lower thanthe designed capacity as the heat transfer areaprovided in the condenser unit was not sufficient,even though the heat transfer coefficient wasrelatively high, so that complete condensation ofwater vapor was not achieved. Results showedthat an L/G of 3 was favorable, resulting inproduction of 72 L/d. A pilot plant with acapacity of 4540 L/d of fresh water was designedfor further study.

Due to the scarcity of water and fossil fuel inthe Canary Islands, solar-assisted desalinationwas studied on the Islands. A forced convectionHD process was analysed by Veza and Ruiz [24].The process of forced convection solar distilla-tion differs from the conventional still in thatvapor from salt water is absorbed by flowing airand dragged out to an external cooler where it iscollected as condensate. The authors studied theeffect of convection during water evaporation andvapor condensation at an external condenser(Fig. 2).

The authors developed two simulation modelsfor the proposed process and predicted its outputin terms of temperature and humidity. A simpli-fied model using mass and energy balance

Fig. 2. Flow diagram of force convection solar distillation(Veza and Ruiz [24]).

relationships is presented as well as a generalmodel that predicts the system behaviour. Amethod for determining energy and exergy effi-ciency has been included, which is also appli-cable to other solar collection and conversionprocesses. An experimental design was adapted tovalidate the model by simulating variousexperimental pilot plant conditions. The designconsisted of two stills with similar character-istics, except for the lapilli (walnut-sized volcanicdebris) on the bottom for one case, to assessdifferences in performance. Still dimensions were10 m length and 1 m width for each still, with acover of standard glass. At the exit, a seawater-cooled condenser was present and fans inducedthe forced flow of air. Feed water and air flowrates as well as water temperature (at threedifferent points along the still) and air tempera-ture and humidity were measured. Ambienttemperature and humidity, wind velocity andsolar irradiance were also measured at theexperimental site. The model results comparefavorably with experimental data obtained at thepilot plant. The relationship between differentparameters was determined, and optimum opera-ting values were selected in the study.

3.4. Multi-effect humidity (or HD) process (MEH)

3.4.1. Principle of MEHThe principle of MEH plants is the distillation

under atmospheric conditions by an air loopsaturated with water vapor. The air is circulatedby natural or forced convection (fans). Theevaporator-condenser combination is termed a“humidification cycle” because the air flow ishumidified in the evaporator and dehumidified inthe condenser. The term “multiple effect” usedhere is not in reference to the number of con-structed stages, but to the ratio of heat input toheat utilized for distillate production (GOR>1).

As noted earlier (in studies by Grater et al.[11]), efficient evaporation and condensation can


be achieved at high temperature (close to 100°C);however, the thermal efficiency even for thehighest quality flat-plate collector drops signi-ficantly at such elevated temperatures. On theother hand, at moderate operating temperatures,intensive heat and mass transfer must bemaintained in the evaporator and condenser. Thisnecessitates the development of a new generationof solar desalination units.

In recognition of this fact, extensive researchhas been carried out at different research insti-tutes in Germany, as reported by Heschl [25], todevelop a more efficient utilization of solarenergy for water desalination. Grune [26] intro-duced the MEH (or HD) process in the early1960s, and his 1970 study gave interesting insightinto the process. The multiple-effect processpromises higher specific productivity due toseparation of the three basic processes of energycollection, evaporation and condensation.

The MEH process further extends the conceptof the forced convection solar still by separationof the heat collection and evaporation units. TheUniversity of Arizona, based on pilot plant workperformed from 1956 to 1963, initiated con-struction of a pilot solar energy MEH plant in1963. The plant was constructed to test thefeasibility of using solar energy as a heat sourcein a humidification system. Further work wasinitiated in 1964 by the University of Arizona incooperation with the University of Sonora,Mexico, whereby a larger pilot-scale solar desalt-ing plant at Puerto Penasco, Sonora, Mexico, wasconstructed. The MEH process was developedover the years and a few units constructed andtested in different countries. They are of twotypes: the open-water/closed-air cycle and open-air/closed-water cycle, as described below.

3.4.2. MEH units based on the open-water/closed-air cycle

In these plants, heat is recovered by air

Fig. 3. Schematic diagram of an experimental MEHdesalination unit operated with forced or natural aircirculation (Farid [27]).

circulation between a humidifier and a condenserusing natural or forced draft circulation. Asshown in Fig. 3 [27], the saline water feed fed tothe condenser is preheated by the evolved latentheat of condensation of water. This heat isusually lost in the single-basin still. The salinewater leaving the condenser is further heated in aflat-plate solar collector and then sprayed over thepacking in the humidifier. Some of the MEHunits used an integrated collector, evaporator, andcondenser [28]. The reported efficiency of thesedesalination units was significantly higher thanthat of a single-basin still. These types ofdesalination units are very suitable for small pro-duction capacities in remote areas. The process iseasy to operate and maintain and it does notrequire skilled operators.

Chaibi [29] carried out a performance study ofa solar MEH unit installed in the south of Tunisiafor potable water production and irrigation.Several tests on storage, evaporation, and con-densation were carried out, and an estimation ofthe cost of fresh water production was also given.The study showed that the plant, which wasintended to produce 12 L/m2 d of fresh water, didnot reach its goal. The highest production wasabout 6 L/m2 d, which is only marginally higherthan that of the efficient single-basin still.


Heschl [25] constructed an MEH plant, whichuses natural-draft air circulation. Textile heatexchangers were used for efficient evaporationand condensation with minimum pressure drop. AGOR value higher than 3 was reported.

Khedr [30] performed a techno-economicinvestigation of an air HD desalination process.The results showed that 76% of the energyconsumed in the humidifier is recovered bycondensation. Their cost calculations showed thatthe HD process has significant potential as analternative for small-capacity desalination plantsand allows operation of systems with an output aslow as 10 m3/d.

An increase in desalination productivity wasachieved by increasing the water temperature atthe inlet to the humidifier of the MEH unit. Also,air circulation was found essential for raising thesystem performance. Madani and Zaki [31]constructed a test rig of an MEH solar desali-nation plant working on the humidificationprinciple. The unit yielded 0.63–1.25 L/m2 h offresh water on a typical summer day at noontime(2.5–5 L/m2 d for a 4-h/d peak time operation),which is as low as some efficient single-basinstills.

During the period 1990 to 1996, Farid and co-workers built three MEH desalination units inIraq, Jordan, and Malaysia. The unit constructedin Iraq was operated with forced air circulation[32] as shown in Fig. 3, while the unit con-structed in Jordan was operated with both forcedand natural draft air circulation [33–37]. Based onthe experience of operating these units, a thirdunit operated with natural draft air circulation wasconstructed in Malaysia [36,37]. These units werebuilt with a single stage for the purpose ofgenerating sufficient information to construct arigorous mathematical model that can be used inthe design and simulation of such units and alsoto optimize the performance of existing MEHunits. The design and performance simulation ofthese units is discussed in detail in a previouspublication [38].

The Bavarian Center of Applied EnergyResearch and T.A.S. GmbH & Co. KG atMunich, Germany, has addressed the perfor-mance optimization of a MEH unit built in theCanary Islands in Spain. The unit was based on apatent design developed at the University ofMunich, as described earlier. The design of theunit was similar to that used by Farid et al. [27]except that the humidifier and condenser werekept in the same unit and the unit was designedfor higher capacity. The design of these two unitswas based on natural convection and not forcedconvection. After installation, their long-termperformance was measured from 1992 to 1997.These distillation units illustrate the energysaving procedure of MEH. Water is evaporated atambient pressure and condensed where more than70% of the heat of evaporation can be recovered.The performance of the units has been improvedover the years, and an average daily production of100 L from an 8.5 m2 collector area (11.8 L/ m2 d)was obtained without thermal storage.

Muller-Holst et al. [39] studied an MEH unitas shown in Fig. 4. The desalination plantconsists of a solar collector, which provides thethermal energy, and a desalination module thatuses multi-effect distillation to treat the water.Seawater fed to the unit evaporates under ambientpressure, and the saturated air is trans-ported byfree convection to the condenser area where itcondenses on the surface of the plastic heatexchanger. The evaporator consists of verticallysuspended tissues or fleece made of poly-propylene over which the hot seawater isnormally distributed. In the evaporator, partialevaporation cools the brine, which leaves theevaporation unit concentrated at a temperature ofabout 45°C. The condenser is a polypropylenebridged double-plate heat exchanger throughwhich the cool brine is pumped upwards. Thecondensate runs down the plates and trickles intoa collecting basin. The heat of condensation ismainly transferred to the cold brine, as it flowsupwards inside the heat exchanger. The


Fig. 4. Illustration of a multi-effect distillation unit with-out storage implementation (Muller-Holst et al. [39]).

temperature of the brine rises from 40°C to about75°C. The brine is then heated to the evaporatorinlet temperature, which is between 80 and 90°Cby a heat source such as the highly efficient solarcollectors, by heat from the thermal storage tank,or by waste heat. Salt content of the brine as wellas condenser inlet temperature can be increasedby a partial reflux from evaporator outlet to brinestorage tank. If re-circulated, brine needs to becooled, e.g., by sending the feedwater through acooler before it reaches the condenser.

Based on this concept, a pilot plant with directflow through the collectors has been workingalmost without any maintenance or repair for aperiod of more than 7 years on the island ofFuerteventura [39]. Results from Fuerteventurafor a distillation unit without thermal storageshowed that the daily averaged heat recoveryfactor (GOR) was between 3 and 4.5. A similardistillation unit in the laboratory at ZAE Bayernyielded a GOR of more than 8 at steady-stateconditions [39]. The optimized module produced40 L/h of fresh water, but it was shown that aproduction of 1000 L/d is possible when the unitwas operated continuously for 24 h. Based on a

collector area of 38 m2, the daily productivity ofthe optimized module works out to be about26 L/m2 of collector area for a 24-h run and withthermal storage under optimized laboratory con-ditions. It was realized that an improvement ofthe overall system efficiency could be reached byadding a thermal storage as alternate heat sourceto enable 24-h operation of the distillationmodule. This was achieved by using extra col-lectors and hot water storage tanks. In a relatedstudy, Ulber et al. [40] investigated the concept ofa solar thermal desalination plant with a heatstorage tank installed.

A unit was constructed in 1997 in Tunisia,with the financial support of the German Min-istry of Economic Cooperation and Development(BMZ). In addition, a unit for drip-irrigation wasimplemented to reduce the water consumption. Anew concept was developed and implemented inSfax, Tunisia, which includes the use of aconventional heat storage tank and heat exchangebetween the collector circuit (desalted water) andthe distillation circuit. This enabled continuous(24 h/d) distillate production. A major factorprompting a 24-h/d operation of these units wasthe realization that the major capital cost of theseunits is due to the condenser and humidifier. Itwas suggested to include a 2 m3 storage tank inthe MEH unit constructed in Tunisia, which uses38 m2 collectors, to improve its performance. Asimilar suggestion was made to extend theoperation of the unit constructed in Jordan andMalaysia to be operated in a 24-h/d mode [34,37].

3.4.3. MEH units based on the open-air/closed-water cycle

In the study of multi-effect processes byBöhner [41], a description of a closed-watercirculation system was presented, as shown inFig. 5. The closed-water circulation is in contactwith a continuous flow of cold outside air in theevaporation chamber. The air is heated andloaded with moisture as it passes upwards


Fig. 5. MEH unit with open-air/closed-water cycle(Bohner [41]).

through the falling hot water in the evaporationchamber. After passing through a condensercooled with cold seawater, the partially dehu-midified air leaves the unit, while the condensate(distillate) is collected. Water is recycled or re-circulated. Incoming cold air provides a coolingsource for the circulating water before it re-entersthe condenser. This system with a closed saltwater cycle ensures a high utilization of the saltwater for fresh water production. In the closedwater cycle, the salt water is continuouslyevaporated in the evaporation chamber. Forexample, 1 m3 saltwater with 1% salt results in330 L distillate water and a brine concentration ashigh as approximately 15%.

In further research, some investigators [42]applied an open-air cycle for obtaining goodproductivity. The air is vented to the atmosphereafter its partial dehumidification in the con-

denser, while the water is circulated in a closedcycle. The productivity of the units working onthis principle was high, but the power requiredfor air circulation was also very high. The systemconsisted of a humidifier, a solar still in the formof a flow channel, a condenser, and a pond. Thesolar still was a long glass-covered channel about200 m long. Sensitivity studies carried out on thechannel still explored parameters such as windvelocity, air flow rate and inlet water temperatureand flow rate. The channel selected had a lengthof 177 m and a width of 1.69 m. Vertical dimen-sions chosen were 0.31 m × 0.76 m. The perfor-mance increased with increasing air flow rate butpractically leveled off at about 2500 kg/h.

Recently, an MEH unit based on this principlewas built at Kuwait University [43]. It receivedenergy from a salt gradient solar pond of 1700 m2

in area, used to load the air with humidity. Freshwater is then collected by cooling the air in a de-humidifying column, producing 9.8 m3/d of dis-tillate. In a similar study, Younis et al. [44]described a solar-operated HD desalination sys-tem which consisted of a solar pond, a humidi-fying column, a dehumidifying stack andnecessary fans and pumps. For an average21,000 kJ/m2 solar intensity and 22% efficiency,a heat rate of 90.9 kW thermal energy can beobtained for the 1700 m2 solar pond builtat Kuwait. A performance ratio of 3 (or 800 kJ/kgdistillate) is obtainable as mentioned in the study,with an output of 9.8 m3/d.

The process described by Khalil [19], asdiscussed earlier, where the moist air is passedover a cooling coil of an air conditioner, fallsunder the open-air/closed-water cycle MEHcategory. He noted that the method might beeconomical only if the produced fresh water wasconsidered as an air-conditioning by-product.

Dai and Zhang [45] also built an MEH unitoperated in an open-air, closed water cycle. Theunit was 1 m×1 m×1.5 m in dimension and wascapable of producing up to 100 L/h of freshwater. They replaced the collector by a boiler to


provide the hot water, which was sprayed at thesurface of honeycomb packing of the humidifier.A fan was used to force the process air to flowthrough the humidifier in a cross flow arrange-ment. The hot humid air was then passed througha condenser, cooled by cold seawater prior tofeed to the boiler. The seawater captures some ofthe latent heat of condensation thereby improvingthe efficiency of the unit. The water from thehumidifier was recycled to the storage tank sinceit is warm and its salinity is not very high.However, some bleeding of this water wasrequired to prevent the accumulation of salt in theunit. An efficiency of 80% was obtained usinghot water feed from a boiler. This corresponds toabout 68% only when a solar collector is used.The unit production did not exceed 6.2 L/m2 d.The authors showed a strong effect of thehumidifier feed water temperature, which hasbeen reported previously in all types of MEHunits. The effect of air flow rate on the pro-duction efficiency showed a maximum value.Increasing air flow rate first increases the heatand mass transfer coefficients in the humidifierand condenser but eventually lowers theoperating temperature. This is the reason for themaximum efficiency observed.

Another MEH unit based on an open-air cycle— and referred to as “dewvaporation” — wasbuilt at Arizona State University for the pro-duction of 45.4 kg/d of condensate, with GORvalues in excess of 7.5. The evaporator unit wasconstructed out of strips of thin, water-wettableplastics and was operated at a low-pressure drop.This system, studied and experimentally operatedby Beckman [46], could emerge as economicallyfeasible for small-capacity plant applications. Asmentioned in his study, RO technology facescompetition from other seawater desalinationtechniques such as MVC, MSF, and MED withand without thermal vapour compression. Theelectrically driven MVC plants consume moreelectricity than RO plants. The thermally drivenplants attempt to recycle the applied heat

continually to minimize the operating costs. Theenergy reuse factor economically varies from 6 to12. The optimum GOR value depends on factorssuch as plant capacity, cost of energy, cost ofmaterials, interest and tax rates.

4. Other processes based on humidification–dehumidification

Other studies carried out on desalinationsystems based on the HD principle are describedin the following sections. Although all theseprocesses are based on the HD principle, therespective researchers have presented them underdifferent process titles and descriptions.

4.1. Solar multiple condensation evaporationprocess

In 1991, Graef [47] studied a desalinationprocess based on a solar multiple condensationevaporation (SME) cycle, and in further relatedstudies, Ben Bacha et al. [48] presented a study ofa water desalination installation using thesolar multiple condensation evaporation cycle(SMCEC). In the study by Graef [47], it ismentioned that the number of heat recoverycycles depends on the condenser surface area andtemperature of the cooling water. A collectorefficiency of 58% and a water temperature of65–75°C can produce 6 L/m2d of condensatebased on 1 m2 collector and condenser surfaceareas. Tests in Sfax, Tunisia, produced con-densate of 4 L/m2d with a collector efficiency of46% (theoretical 14.3 L). Two types of desali-nation units — namely SME 3.6 and SME 200 —were manufactured by Aquasolar GmbH & Co.,and Aquasolar (Tunisia) in the presentation byGraef. The SME 3.6 is most suitable for a singlefamily, producing up to 50 L/d, and has been inseries production since 1991 in Tunisia.

The SMCEC-based desalination unit pre-sented by Ben Bacha et al. [48] belongs to a newgeneration of decentralized installations for water


desalination using solar energy with heatrecuperation. Similar to the solar HD and MEHunits, the SMCEC-based units are well suited fordeveloping countries with extended rural areasbecause of their simplified design, low main-tenance, extended life-time (over 20 years),almost zero energy consumption and low capitalcost. A detailed modelling, simulation andexperimental validation for this type of instal-lation permits the optimisation of size of the solarcollectors, evaporation tower and condensationtower (similar to the modelling and simulation foran MEH unit studied and presented in the studyby Farid et al. [38]). The SMCEC-baseddesalination unit consists of three main parts:solar collector, condensation tower and evapo-ration tower. The flat-plate collector is equippedwith an absorber made of polypropylene materialcovered by a Hostoflan membrane or glass. Theabsorber is made up of very thin and tightlyspaced capillary tubes where the salty watercirculates. The evaporation tower produces thewater vapour. Thorn trees are utilized to increasethe water spray and improve evaporation. At thebeginning, the brackish or seawater is heated bythe solar collector. Then, hot water is injectedinto the top of the evaporation tower. Anatomizer with a special shape is used to insure auniform pulverization of the hot water in all thesections of the tower. Air circulation in theevaporation is possible either by natural or forcedconvection.

To examine the validity of the model pro-posed by Ben Bacha et al. [48], experimentalmeasurements were taken using the pilot desali-nation unit located at the National School ofEngineering of Sfax, Tunisia. The specificationsof the pilot unit are: solar collector with an areaof 7.2 m2 (effective transmission absorption of0.83 and loss co-efficient 3.73 W/m2K), evapo-ration tower size of 1.2 m × 0.5 m × 2.55 m, withsolid packing of thorn trees, and a condensationtower of size 1.2 m × 0.36 m × 3 m. Based onmodel simulation and experimental validation,

the optimum operation and production for theSMCEC unit require a perfect insulation of theunit, a high water temperature and flow rate at theentrance of the evaporation tower, a low watertemperature at the entrance of the con-denser, hotwater recycling by injection at the top of theevaporation chamber, and a storage tank to storethe hot water excess that would extend waterdesalination beyond sunset.

4.2. Aero-evapo-condensation process

Bourouni et al. [49] conducted an experi-mental investigation with a desalination plantusing the “aero-evapo-condensation” process.The unit consists of a falling film evaporator andcondenser made of polypropylene, and isdesigned to work at low temperatures (70–90°C),specifically using geothermal energy. The proto-type was patented by the firm Caldor-Marseille(France) in 1994. This prototype includes twocross-flow heat exchangers, a horizontal fallingfilm evaporator, and horizontal falling filmcondenser. The two exchangers were made ofpolypropylene and affect the humidification–dehumidification of air. The influence of thevarious thermal and hydrodynamic parameters onthe unit performance was investigated. Resultsshowed that the performance of the unit increasedwith the increase of inlet hot water and airtemperatures. On the other hand, it was observedthat the performance of the unit decreased whenthe air velocity and hot liquid flow rate increased.A critical film flow rate corresponding to the filmbreakdown was determined. At this value, amaximum amount of evaporated water wasobtained. Horizontal-tube falling film evaporatorshave an advantage over vertical-tube evaporatorsin dealing with problems such as liquiddistribution, levelling, non-condensable gases onthe tube side, fouling and liquid entertainment.Another parameter affecting the heat transfercoefficients is the water distribution system at thetop of the horizontal tube. Instead of the common


“perforated-plate” water distribution system, themore accurately controlled “thin-slot” water dis-tribution system was shown to be preferable.

4.3. Carrier gas process

Larson et al. [50] and Hamieh et al. [51] havepresented results of the “carrier-gas process”(CGP) of EvCon Corp., which demonstrated apotential for desalination of seawater andbrackish water and for the concentration ofvarious process streams and industrial waste-waters. It operates at temperatures below thenormal boiling point and at ambient pressure.This process is similar to the HD process withtwo chambers (one for evaporation and the otherfor condensation) being physically separated bya common heat-transfer wall. The CGP processprovides results over a wide range of perfor-mance ratios and production densities simply byvarying the temperatures and airflows. Thesystem can also be operated using renewable heatsupplies including solar and ambient air. Thus,this system too is a convenient choice for remoteand arid regions of the world where conventionaltechnology is too expensive.

ECOTERM [52], an organization formed byengineers and architects based in Barcelona,claim to be developing a pilot plant for desali-nation using the CGP. This principle is similar tothe HD technique and has been studied separatelyas an alternative desalination process. Expectedresults from the proposed pilot unit include aproduct water flowrate of 1 m3/h for an air flow-rate of 7.55 kg/h and a heat exchange surface areaof 500 m2. The possible plant sizes sug-gested arefor productivity between 100 L/h and 1000 m3/hor higher. Capability to work with lowtemperatures of approximately 40°C and usingforms of energy such as residual heat of cogene-ration, solar energy, and geothermal energy arethe possibilities for this proposed unit.

5. Summary of studies conducted on the HDdesalination process

A summary of main technical results reportedby various researchers is shown in Tables 1 and2. Almost all the investigators state that the effectof water flowrate on the performance of the unitis important. The effect of air flowrate onproductivity is termed insignificant by all authorsexcept Younis et al. [44].

Also, all researchers express a preference fornatural convection since air flowrate has aninsignificant effect on unit productivity. How-ever, forced circulation could be feasible withanother cost-effective source of energy such aswind energy. The effect of air flowrate is onlynoticeable at temperatures around 50°C, asreported by Al-Hallaj et al. [34].

Another variable tested was the packingmaterial in the humidifier. Packing materialshould generally be of such a size and shape as toprovide a high contact surface and a low pressuredrop. The choice of packing material tends tohave an effect on the thermal efficiency andproductivity of the unit. Examples includeRaschig rings, Berl saddles, Pall rings, Lessingrings, Prym rings, meshed curtains, wooden slats,wooden shavings, and fleece made of polypropy-lene or honey-comb paper as used by someresearchers.

6. Conclusions

A comprehensive review of the solar HDtechnique and the various desalination unitsbased on this principle has been presented in thisstudy. Solar desalination plants based on thistechnique have yet to be commercially imple-mented, and a detailed understanding and com-pilation of all relevant information of the processis another step in that direction. A detailed re-view of other desalination processes based on theHD principle would also help in improving


Table 1Studies on closed-air/open-water cycle systems

Reference Unit features Parameters varied

Younis et al.[44]

C Solar pond HDDC Forced air circulationC Packing material-meshed curtains

C Air flowrate has a significant effect on collected amountof fresh water.

C Study of different packing materials is required forimproving performance.

Abdel-Salamet al. [53]

C Two closed loops: air and waterC Forced air circulation

C Inlet water and air dry-bulb temperature to the coolingtower was varied.

C Water flowrate through cooling tower and air cooler wasvaried.

C An increase of inlet water and air dry-bulb temperature tothe humidifier and increasing humidifier size increasessystem productivity and decreases energy required inpumping water and air.

C Air recirculation is essential for raising systemperformance.

Farid et al.[54]

C Closed-air cycleC Natural and forced circulationC Humidifier packing-wooden slats

C The effect of water flowrate on the heat and mass transfercoefficients in the condenser and humidifier issignificant.

C The effect of air flowrate is small and natural circulationis preferred over forced convection.

C A simulation program is required to conduct a detailedstudy.

Farid andAl-Hajaj [32]

C Closed-air cycleC Forced circulationC Humidifier packing-wooden

shavings

C Forced circulation is not suitable due to highconsumption of electrical power and is practical utilizingwind energy.

C Decreasing the water flowrate causes more efficientevaporation and condensation.

C Decreasing the water flowrate below 70 kg/h reduces theperformance factor due to an expected decrease inefficiency of collector at elevated temperatures.

C Production of 12 L/m2/d of desalinated water wasachieved.

Nawaysehet al. [35]

Unit in Jordan was testedC Closed-air cycleC Forced and natural circulationC Humidifier packing- wooden

packing

C A simulation program was developed to describeperformance of units.

C A significant effect of water flowrate on unit productionwith two opposite effects was observed: a lowerevaporation and condensation efficiency and highercollector efficiency.

C The effect of air flowrate is negligible. Naturalconvection is preferred over forced convection.

C The surface area of condenser was varied; it wasobserved that a significant improvement in productionwas possible if the condenser area was doubled.

C A humidifier surface area >5.6 m2 can increaseproduction (although insignificantly).

C An increase in evaporator and condenser surface areaincreases productivity significantly


Table 1, continued


Al-Hallajet al. [34]

Two units (bench scale and pilot) were built and tested in JordanC Closed-air cycleC Forced and natural circulationC Humidifier packing-wooden slats

C The condenser area was varied from 0.6 m2 in bench unitwith single condenser and 8.0 m2 in pilot unit withdouble condenser.

C Humidifier inlet temperature was between 60 and 63°C.C No significant improvement in performance with forced

circulation was noticed. A significant effect was observedonly at higher temperatures above 50°C.

C Production of the pilot MEH unit increased with nightoperation using rejected water from humidifier. Producti-vity around 8 L/m2/d was estimated.

C Large mass of the outdoor unit (approximately 300 kg)was a negative factor.

C Use of a lighter material of construction for the unit isproposed.

C Night-time operation of the unit is recommended.Nawaysehet al. [36]

Two units in Jordan and one inMalaysia were studiedC Closed-air cycleC Natural circulation

C Due to a larger contact area of humidifier and condenser,productivity improved significantly in the unitconstructed in Malaysia compared with that in Jordan.

C The increase in humidifier area was 136%, while theincrease in the single condenser area was 122.5% anddouble condenser area was 11.25%.

C The humidifier cross-sectional area of the unit con-structed in Malaysia was reduced by 39% to that of theJordan unit; but complete wetting was not achieved.

C A computer simulation for design of the unit componentsis required to achieve optimisation of unit.

C The effect of water is significant on heat and masstransfer coefficients in the humidifier and condenser.

C The effect of air flowrate on performance is small.Nawaysehet al. [36]

Units in Jordan and Malaysia were studied

C A simulation program was developed to correlate withexperimental results.

C The effect of air flowrate is found to be insignificant.C Increasing water flowrate decreases production due to

lower evaporation. However, reducing water flowrate toextreme values lowers production due to drop in collectorefficiency.

C Simulation shows fast convergence of temperatures andproduction rate to final values.

Müller-Holstet al. [39]

Pilot solar MEH unit at CanaryIslands and desalination unit atSfax, Tunisia, were studiedC Closed-air cycleC Natural circulationC Evaporator and condenser were

located in same unitC Evaporator packing material-

fleeces made of polypropylene

C A simulation tool for optimising collector field andstorage was developed at ZAE Bayern.

C The cost of water was determined using laboratory-basedsimulation criteria.

C Laboratory unit yielded reduced specific energy demanddue to better evaporation surfaces and thinner flat-plateheat exchangers on the condensation side of the unit.

C The simulation results for optimisation of 24-h operationare to be verified in field tests.


Table 1, continued


Müller-Holstet al. [55]

Simulation of solar thermal seawaterdesalination system using TRNSYS

C Simulation results were compared with data from pilotplant in Fuerteventura, which were in good agreement.

C Increasing evaporator inlet temperature causes risingdistillate volume flow.

C A higher distillate volume flow was achieved byincreasing brine volume flow (load flow>600 L/h).

C The optimum condenser inlet temperature was found tobe 40°C.

C A simulation tool for the desalination unit would be usedin a unit configuration with storage tank.

Table 2Studies on open-air/closed-water cycle system

Reference Unit features Parametric analysis

Khedr [30] Open-air, closed-water system C Studies covered: water to air mass ratio ranging from 60 to 80and concentration ratio between 2 and 5 at temperaturedifference between 10 to 16°C, along the liquid fordehumidification.

C 76% of the energy consumed in humidifier was recovered bycondensation.

C An increase of concentration ratio to 5 can reduce make-upwater and rejected brine by approximately 58% and 24%,respectively.

C The HD process has a significant potential for small-capacitydesalination plants as small as 10 m3/d.

Dai andZhang [45]

Closed-water, open-air systemC Forced circulationC Packing material-honey comb

paper

C The thermal efficiency and water production increases withincrease of mass flow rate of water to the humidifier.

C The effect of rotation speed of fan on thermal efficiency wasstudied. It was observed that the lower the temperatures ofinlet water to humidifier, the smaller the optimum rotationspeed required.

C An optimum mass flow rate of air exists. A higher or lowermass flow rate of air is not recommended for increasing bothwater production and thermal efficiency.

C The thermal efficiency and water production increasesignificantly with temperature of inlet water into humidifier.

C Productivity achieved was around 6.2 kg/m2/d.

the design of current solar-based HD units.Modification and study of the effect ofintroduction of horizontal falling film evaporatorand horizontal falling film condenser andmodification in the water distribution system at

the top of the condenser unit in current solarMEH units could lead to improvement in systemefficiency. Also, as already observed in thisstudy, thermal storage and a 24-h operation ofthe units is required to improve productivity.


From work carried out, it was observed that anincrease in evaporator and condenser surfaceareas significantly improves system productivity.But prior to implementing any techniques indesign improvement, it is necessary to optimizethe MEH unit by optimizing component size tounderstand the effect of feed water and air flowrates. Although a fair amount of simulationstudies have been conducted in the past, furtherdesign simulation is required to fully understandthe complicated effects of air and water flowrates, the optimum size of individual componentsor modules of the unit and to generate a com-prehensive model for the system — bothtechnical and economical. Construction ofadditional bench-scale and pilot units is alsonecessary to validate simulation results as prac-tical tests show the actual drawbacks, quite oftennot explained by simulation. Along with thethrust towards unit optimisation, the economicviability of these units is also important tounderstand the status of solar-based HD units inthe current desalination market scenario.

7. OutlookAlthough solar desalination units based on the

HD principle have been in existence for a while,more work needs to be performed to increase theproductivity and decrease the cost of waterproduced from these units. Work has beeninitiated to add thermal storage modules to theMEH units. Alternate sources of energy could beutilized to heat water stored for nocturnal use ina 24-h operation. An interesting but relativelynovel technology, i.e., fuel cells, could emerge asa successful integration for a fuel cell/solar MEHhybrid system in the future. For systems at remotelocations and far from the grid, with substantialpresence of solar energy, low-temperature fuelcells would provide power as a replacement/alternative to grid power and at the same timeprovide power to the auxiliary components (such

as fans, pumps) of the MEH unit. Waste heat —a form of “free energy” from the fuel cell —could be used to preheat the feed to the MEHunit, which could help increase pro-ductivity ofthe MEH units at no additional expenditureexcept for the thermal storage units. Furtherstudies, simulation and process improve-mentdesign of MEH units with a 24-h operation andthermal storage modules should be considered inthe future as a critical step for the com-mercialization of these units. Also, an economicmodel and analysis for these types of solar-basedunits is required to understand the marketpositioning and potential end-users of these units.

Acknowledgements

This study was supported financially by TheMiddle East Desalination Research Center,Sultanate of Oman under Contract No. 98-BS-032b.

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