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Solar-powered desalination comprises all methods that use the sun’s energy indirectly by means of solar collectors. All solar-driven desalination processes, except solar distillation, can be fired indirectly by solar energy. In this chapter the combinations of solar thermal desalination devices to solar concentrating collectors are explained. It concerns medium to large desalination and solar plants. Initially the various types of solar collectors of large solar concentrating fields are shortly described so that the reader can have a general view of the solar concentrating plants that transform solar radiation into thermal energy. Some specific definitions and nomenclatures are described, to help the reader understand what the solar concentrators and the desalination distillation plants are. Factors that are of importance and affect the selection of the type of the two plants, for the optimum efficiencies of both systems and for the higher water productivity, are given. These determine the suitability of these combinations. Various types of solar collectors are shortly described. They include all types of solar flat-plate collectors up to point focus and heliostats for central receiver systems. The conventional desalination thermal systems, that is, conventional distillation methods, types and configurations of the corresponding evaporators, such as multiple effects distillation, multistage flash, mechanical vapor compression, and thermal vapor compression, and the variations in construction are presented for large capacity installations. For each conventional distillation method, the mathematical equations, concerning heat transfer conditions and heat balance of the system, are briefly depicted. Diagrams of temperature distribution are also given. For economic reasons, the combination of electricity production by the solar plant with hybrid and/or conventional evaporators is of importance to achieve optimum performances, as high-pressure steam from a solar collector field can be used for electricity production and waste, and low-temperature heat for the desalination plant.

Humidification/dehumidification (H/D) is in fact a small to medium capacity desalination method driven by solar energy or waste heat. In this chapter, initially a short historical introduction is given on how this method is derived from solar distillation procedure by using separate devices (champers or towers) for evaporation and condensation processes in contrary to solar distillation where the two operations take place in only one chamber. The basic operating principles of the two configurations, such as closed water–open air and closed air–open water of the single H/D, are presented. The majority of all H/D concepts are driven by solar energy and/or by waste heat. As these H/D systems are based on H/D principles in separate chambers, a short description of Mollier (h–x diagrams) and psychrometric charts, used in drying, with chart examples is presented to facilitate calculations of H/D in the corresponding chambers. The main relationships of mass and heat transfer and mathematical models of the single effect or multiple effect configurations for the humidifier (evaporator) and the condenser (dehumidifier) are given. The corresponding temperature–enthalpy diagrams for the two chambers are presented as well as examples for calculations. The dewvaporation procedure having the two chambers in contact, leaving the vapor to be transferred through a separating wall, is included. Multiple effect H/D offers the possibility for air to be loaded by reheating with as much as possible vapor in the humidification chamber. Mathematical models of this H/D concept including Mollier charts of humidity calculations are also presented. Various other concepts of operation that exist, such as the desiccant absorption/desorption H/D process using zeolite or lithium bromide as vapor absorbent, the membrane drying dehumidification using selective membranes, varied H/D cycle, and the thermocompression method, are shortly described. At the end of this chapter, the thermoeconomics of this method is described. Greenhouses are important in dry regions which have intensive solar radiation and no usable water resources. The combination of solar-driven H/D method and solar stills with greenhouses to supply the necessary humidity to the greenhouse plants gives a good solution to produce simultaneously fresh water and crops in dessert lands. A short description of this combination and figures of installations is presented.

This chapter is the most extended part of this book and is dedicated to solar distillation. Solar distillation was known from the antiquity as a possible way to extract fresh water from the seawater and was used in ships crossing Mediterranean Sea, in a very primitive way. A variety of configurations of solar stills are developed and tested the use of which started almost after Second World War and still today new configurations continue to be under development and improvement. Solar stills are small devices that provide a few liters of fresh water per day and are very popular in small and poor communities in Africa and other remote places. In this chapter, the most important types of solar stills and the trials made to increase their efficiency and decrease their thermal losses are described. Initially, a short description of the specific terms and definitions that govern the construction and the operational variety of the stills is given. The parts from which solar stills are assembled, including construction and operation details are also described. Special care is given on the materials that can be used, as fresh water must not contain harmful elements, which are dangerous for health and must not be expensive. The precautions that must be taken are also described in order to avoid early damage or low efficiencies of the devices. Rain water collection, if is available, can double the production of the solar still during raining days by applying inexpensive canals around the still.

This chapter is dedicated to a promising, relatively new, distillation method using specific membranes. Membrane distillation (MD) is a nonisothermal membrane separation process, in which only water vapor molecules are transferred through a microporous hydrophobic membrane because of the partial pressure difference across the membrane that is created by the temperature difference between the two sides of the membrane. It is an attractive alternative technology for small to medium desalinated water supplies operating at atmospheric pressure and low temperature. The capability of utilizing solar thermal energy or waste heat from other processes makes MD an energy and cost-efficient and environmental friendly process. Initially in the chapter, a short historical introduction to the method is given and the specific terms involved in the MD process are explained. The operating principle of the process is described, as well as its basic configurations, its advantages and disadvantages, and its various applications. An interesting characteristic of the process is the simultaneous mass and heat transfer that occur. The basic relationships of mass and heat transport that govern each of the four different MD configurations are presented. An extensive description of membrane distillation and thermal solar energy is given based on current research and application. Apart from the description of the MD process itself, a comprehensive review of various types of membranes used in the MD process is presented. Membranes are the main tool in hybrid desalination technologies. Initially, an introduction to the membranes related to the MD process and their characteristic properties is given. Then, different types of membranes are analyzed as well as their structure, morphology, and modifications made in order to optimize their performance. Finally, the performance of MD membranes in various operating conditions for different configurations of the process is given.

Seawater or brackish waters are the raw materials used in all desalination processes to obtain fresh usable water. Beginning in 1950, when industrial desalination applications started, many papers concerning sea- and brackish-water chemical, physical, and thermodynamic properties necessary for the proper operation of desalination plants were issued. These properties were available in the ranges of temperatures that the desalination plants operated. At about 2008 the Oceanographic Institutes and Committees, as IASPO, IOC, SCOR, etc., started rearranging all seawater properties in order to create more correct and accurate values. They established new seawater standards, thermodynamic equations, new numerical values, and many more. In this chapter, the new values of properties and the new equations for calculating these water properties are adopted both for seawater and for natural and desalinated water.

Desalination is increasing tremendously every year in fresh water production capacity as more fresh water is needed for communities, industries, and human population especially in dry regions. Many methods exist to transform sea- and brackish water or even polluted water into fresh water suitable for any use.

Relations between contact period, chlorine demand, residual chlorine and chlorine dose were established in the chlorination of seawater samples collected near atomic power plants [1]. Nonpolar, presumably lipophilic organohalogens formed in seawater were studied. The principal component found in all seawater samples was CHBr3. Smaller quantities of CHClBr2 and traces of CHBrCl2 were found. CHCl3 was not present in significant amounts, CHBr3 was found in all unchlorinated sea-water samples analyzed [2]. To prevent biological fouling in cooling systems using seawater, a portion of seawater is electrolyzed in a cell and the chlorinated seawater is injected into the cooling system [3]. In a series of chlorination studies, the initial loss of chlorine reached a saturation level that appeared to be the true organic demand [4]. In case of deoxygenated stagnant seawater, a semilog relation was found between the residual chlorine concentration and corrosion rate. At 95° the corrosion rate was 4 fold that at 40°. The presence of a residual concentration of 0.1 ppm Cl under stagnant conditions increases the extent of corrosion by 5 to 10% compared to the case in which chlorine is absent [5].

The decrease in the temperature Δt in a mulstistage adiabatic water desalination plant is resulting from physicochemical and hydrodynamic parameters. The physicochemical Δt is mainly affected by the concentration of the salt water and slightly by the temperature. The hydrodynamic Δt depends on the hydraulic resistance of the condenser and separator. The Δt resulting from the pressure drop of the secondary steam is relatively high, 1.8°, and must be taken into account in projecting water desalination plants [1]. A review of vertical tube evaporator with corrosion resistant gaskets was presented [2].

Economic comparisons indicate reverse osmosis to be more cost effective than distillation for large operations [1]. Important progress has been made in synthetic semipermeable membranes, leading to a wider scope of reverse osmosis applications, including the purification of used water and saline water for drinking, the supply of ultra-pure water for the pharmaceutical and electronic industries and for feeding boilers [2]. Along with the mass transfer within the membrane surface is important. Reverse osmosis and ultrafiltration processes can be described by heat-transfer equations, but in contrast to the design of a heat exchanger, the equations for calculating flows and concentrations at membranes can only be solved iteratively. For the most important membrane configurations (tube, hollow-fiber, and spiral-wound modules) equations are given for describing their transfer characteristics [3].

Methods of preparing cation- and anion-exchange membranes with low areal resistances were developed for use in lower cost desalination of seawater. Membranes of both types were prepared having the desired resistance of 1 Ω . cm or less. Preparation involved using reinforcements to strengthen polymer compositions. In 1 N sodium choloride the best experimental cation-exchange membranes that could be made consistently had transference numbers of 0.76 to 0.79, compared to commercial membranes with a range of about 0.85 to 0.87, The best experimental anion-exchange membranes had transference numbers of 0.90, the same as commercially available membranes [1].

Textbooks and monographs on water treatment include Theoretical Principles of Thermal Processes [1], Reagentless Methods of Water Treatment in Power Plants [2], Engineering Technological and Chemical Methods of Cooling Water Treatment in Industry and Power Plants: Fresh Water, Seawater, Brackish Water [3], effect of the contamination of outer surfaces of shaped heat exchanger tubes on the operating indexes of distillation units [4] and model boiler studies on deposition and corrosion [5].

A monography on Distillation of water using solar energy was published [1]. A review was presented on the most important and recent studies on solar distillation [2]. Solar water desalination plants of the greenhouse type, plants of high capacity with heat pipe collectors and flat plate collectors in connection with MSF evaporation were reviewed [3]. An assessment of solar distillation plant performance, attainable with presently existing technology, provides the basis for selection of the design for a 5 Mgd solar distillation plant. For plants constructed in the future, the continued escalation of fuel oil cost will make the cost of water from solar driven and oil driven distillers equal within 15 years [4].

Bibliographies on geothermal energy include exploration with 174 abstracts from the NTIS data base [1], corrosion and equipment with 88 abstracts from the NTIS data base [2], technology and general studies with 326 abstracts from the NTIS data base [3] and 323 abstracts from the Engineering Index data base [4]. All bibliographies cover the period May 1976 to July 1978.

A critical review was presented on the application of the heat pump principle to evaporators, operated either by steam jet thermal method or by mechanical recompression using centrifugal or positive displacement pumps. Costs on 1974 basis were reported [1]. In trying to keep up with designed product rates, in spite of decreases in volumetric flow caused by scale or the presence of non-condensible gases, operators usually increase the evaporation temperature, increasing the vapor density and mass flow. However, the decrease in volumetric flow may suddenly shift the operating point of the compressor into the surge region. Predicted and measured performance curves were compared and rules for a safe working range were given [2]. A vapor-compression evaporator, using a seed-slurry process for scale control, was developed. Several features support the system’s broad industrial application [3], An evaporation process, using a liquid heat transfer medium and vapor compression for the concentration of scaling waste waters was presented [4]. The design of a small-capacity seawater distillation plant uses a combination of a wiped-film rotating-disk evaporator unit and a vapor compressor [5].

New editions of Wasser Kalender 1979 [1) and the Manual on Water [2) have been published. A bibliography on water resources in arid and semiarid regions contains 140 abstracts, 31 of which are new entries. The bibliography covers reports on water quality, resource management, potable water, consumption and irrigation, particularly those in the western regions of the U.S. [3). Causes of the shortage of clean water were examined with heavy emphasis placed on the problems of the U.S. Among the solutions that the aurhor suggests are desalination and weather modification [4). The trend of the future and solution to the water crisis will come primarily from increasing emphasis placed on tapping underground reservoirs and desalination of seawater. The solution will also involve judicions water management [5).

A bibliography of citations from the U.S. National Technical Information Service data base with 183 abstracts on membranes for reverse osmosis desalination, electro-dialysis desalination and other osmotic desalting processes was published, covering Federally funded research for the period 1973 to February 1978 [1]. A newer edition contains 198 abstracts, 15 of which are new entries, covers the period up to February 1979 and superseds all previous editions [2]. A similar bibliography of citations from the Engineering Index data base citations covers world wide engineering research and contains 255 abstracts for the period 1974 to February 1978 [3]. A newer edition contains no new entries [4]. A supplement edition contains 51 abstracts, all of which are new entries and cover the period 1978 to February 1979 [5].

Volume 7 of Chemical Oceanography, edited by Riley and Chester, has been published in 2nd edition [1]. A representative group of papers give an insight into the subject of cycling of major elements in seawater, edited by Drever, as Volume 45 of Benchmark Papers in Geology [2], An Introduction to Marine Science with reference to the chemistry of seawater was published [3].

The method of allocation of steam generating costs greatly influences the specific cost of heating steam and desalted seawater. From the point of view of social economics, the cost allocation method, based on the cost of the heating energy generated by a single-purpose heat source, yields sound results. The usual criterion for steam cost evaluation, based on business administration viewpoints, is the steam energy content. The advantages of the cost evaluation, using the geometric mean of heat and exergy, in comparison with the calorific, the exergetic and the arithmetically averaging methods are shown [1].

Ferrous materials are affected by residual chlorine in saline water feed of desalination plants. In stagnant C1 test solution a semilogarithmically increased corrosion rate was observed with increasing dissolved chlorine content [1]. The corrosion resistance of ferritic steels was much greater than that of Cu-Ni alloys or the austenitic stainless steels. No pitting corrosion was observed after 180 days in 3% NaCl solution at 28 to 101°. A low carbon content further increases their corrosion resistance [2]. Nonalloyed or low-alloyed steel corrosion behavior was studied in simulated seawater desalination at ambient temperature to 120° and varying pH and oxygen concentrations [3]. The corrosion rates in artificial seawater were considerably lower for a 6% Cr-steel than for an unalloyed steel C15, a base melt containing C 0.05, Si 0.16, Mn 0.48, Al 0.036, Cu 0.023% and the rest iron, and a 5.5% Ni steel. Transition from uniform corrosion to pitting was observed with increasing oxygen content from 10 to 20 ppb. Sulfide impurities in seawater became corrosion stimulators only at pH

In many areas of the Mediterranean, a residue of a particular sea-grass, Posidonia Oceanica, is often found in large quantities and cause operational problems in desalination plants. An operation oriented study deals with aspects such as concentration in seawater, distribution of sizes in residue samples, sedimentation properties, bulk densities and resistance of sea-grass cakes to seawater flow. Hence, a tailored solution of the problem can be designed [1].

Reverse osmosis is not used only for the preparation of potable water. A great deal of other applications have made reverse osmosis and ultrafiltration a generally accepted unit operation process, especially in the treatment of wastes. With the existing know-how and development of membranes of improved quality, membrane processes might now solve many problems in production, recycling and environmental protection. As these activities are out of the main scope of present literature survey, only a very condensed summary of recent publications will be given in the following.

Experimental data on horizontal tube evaporator-condenser water desalination units agree well with the turbulent model, which covers the whole range of the evaporating film flow rate [1]. The knowledge of the mode of liquid flow between neighboring consecutive tubes in a horizontal bundle is important to the understanding of transport rate characteristics. Drop size, drop detachment frequency and the distance between drop-producing sites at the bottom of the tube were studied as a function of surface tension, tube size and spatial location of the tube in the vertical column in the bundle. Smooth and grooved tubes were examined and the effect of dripping on the mass transfer rates is demonstrated [2].

Various possibilities exist of coupling electricity, heat for district heating and desalination in multipurpose nuclear power plants [1].