The provision of freshwater is becoming an increasingly important issue in many areas of the world. In arid areas, potable water is very scarce and the establishment of a human habitat in these areas strongly depends on how such water can be made available. A brief historical introduction to solar desalination is given in Chapter 1, Section 1.5.2.
Water is essential to life. The importance of supplying potable water can hardly be overstressed. Water is one of the most abundant resources on earth, covering three-fourths of the planet’s surface. About 97% of the earth’s water is saltwater in the oceans and 3% (about 36 million km3) is freshwater contained in the poles (in the form of ice), ground water, lakes, and rivers, which supply most human and animal needs. Nearly 70% from this tiny 3% of the world’s freshwater is frozen in glaciers, permanent snow cover, ice, and permafrost. Thirty percent of all freshwater is underground, most of it in deep hard-to-reach aquifers. Lakes and rivers together contain just a little more than 0.25% of all freshwater; lakes contain most of it.
8.1.1 Water and energy
Water and energy are inseparable commodities that govern the lives of humanity and promote civilization. The history of humankind proves that water and civilization are inseparable entities. All great civilizations developed and flourished near large sources of water. Rivers, seas, oases, and oceans have attracted humankind to their coasts because water is the source of life. Maybe the most significant example of this influence is the Nile River in Egypt. The river provided water for irrigation and mud full of nutrients. Ancient Egyptian engineers were able to master the river water, and Egypt, as an agricultural nation, became the most important wheat-exporting country in the whole Mediterranean Basin (Delyannis, 2003). Due to the richness of the river, various disciplines of science, such as astronomy and mathematics, as well as law, justice, currency, and police protection, were created in the region at a time when no other human society held this knowledge or sophistication.
Energy is as important as water for the development of a good standard of life because it is the force that puts in operation all human activities. Water by itself is also a power-generating force. The first confirmed attempts to harness water power occurred more than 2000 years ago, at which time the energy gained was mainly used to grind grain (Major, 1990).
The Greeks were the first to express philosophical ideas about the nature of water and energy. Thales of Miletus (640–546 B.C.), one of the seven wise men of antiquity, wrote about water (Delyannis, 1960) that it is fertile and molded (can take the shape of its container). The same philosopher said that seawater is the immense sea that surrounds the earth, which is the primary mother of all life. Later on, Embedokles (495–435 B.C.) developed the theory of the elements (Delyannis, 1960), describing that the world consists of four primary elements: fire, air, water, and earth. With today’s knowledge, these elements may be translated to energy, atmosphere, water, and soil, which are the four basic constituents that affect the quality of our lives (Delyannis and Belessiotis, 2000).
Aristotle (384–322 B.C.), one of the greatest philosophers and scientists of antiquity, described in a surprisingly correct way the origin and properties of natural, brackish, and seawater. He also described accurately the water cycle in nature—a description that is still valid. In fact, the water cycle is a huge solar energy open distiller in a perpetual operational cycle.
Aristotle wrote that seawater becomes sweet when it turns into vapor, and the vapor does not form saltwater when it condenses again. In fact, Aristotle proved this experimentally.
8.1.2 Water demand and consumption
Humanity is dependent on rivers, lakes, and underground water reservoirs for freshwater requirements in domestic life, agriculture, and industry. However, rapid industrial growth and a worldwide population explosion have resulted in a large escalation of demand for freshwater, both for household needs and for crops to produce adequate quantities of food. Added to this is the problem of the pollution of rivers and lakes by industrial wastes and the large amounts of sewage discharge. On a global scale, human-made pollution of natural sources of water is becoming one of the greatest causes of freshwater shortage. Added to this is the problem of uneven distribution. For example, Canada has a 10th of the world’s surface freshwater but less than 1% of the world’s population.
Of the total water consumption, about 70% is used by agriculture, 20% is used by the industry, and only 10% of the water consumed worldwide is used for household needs. It should be noted that, before considering the application of any desalination method, water conservation measures should be considered. For example, drip irrigation, using perforated plastic pipes to deliver water to crops, uses 30–70% less water than traditional methods and increases crop yield. This system was developed in the early 1960s, but today it is used in less than 1% of the irrigated land. In most countries, governments heavily subsidize irrigation water and farmers have no incentive to invest in drip systems or any other water-saving methods.
8.1.3 Desalination and energy
The only nearly inexhaustible sources of water are the oceans. Their main drawback, however, is their high salinity. Therefore, it would be attractive to tackle the water shortage problem by desalinizing this water. Desalinize, in general, means to remove salt from seawater or generally saline water.
According to the World Health Organization (WHO), the permissible limit of salinity in water is 500 parts per million (ppm) and for special cases up to 1000 ppm. Most of the water available on earth has salinity up to 10,000 ppm, and seawater normally has salinity in the range of 35,000–45,000 ppm in the form of total dissolved salts. Excess brackishness causes the problem of bad taste, stomach problems, and laxative effects. The purpose of a desalination system is to clean or purify brackish water or seawater and supply water with total dissolved solids (TDS) within the permissible limit of 500 ppm or less. This is accomplished by several desalination methods that are analyzed in this chapter.
Desalination processes require significant quantities of energy to achieve separation of salts from seawater. This is highly significant because it is a recurrent cost that few of the water-short areas of the world can afford. Many countries in the Middle East, because of oil income, have enough money to invest and run desalination equipment. However, people in many other areas of the world have neither the cash nor the oil resources to allow them to develop in a similar manner. The installed capacity of desalinated water systems in the year 2012 was about 75 million m3/day, which is expected to increase drastically in the next decades. The dramatic increase of desalinated water supply will create a series of problems, the most significant of which are those related to energy consumption and environmental pollution caused by the use of fossil fuels. The production of 75 million m3/day requires considerable amounts of energy. Given the current concern about the environmental problems related to the use of fossil fuels, if oil were much more widely available, it is questionable whether we could afford to burn it on the scale needed to provide everyone with freshwater. Given the current understanding of the greenhouse effect and the importance of CO2 levels, this use of oil is debatable. Therefore, apart from satisfying the additional energy demand, environmental pollution would be a major concern. If desalination is accomplished by conventional technology, then it will require the burning of substantial quantities of fossil fuels. Given that conventional sources of energy are polluting, sources of energy that are not polluting must be developed. Fortunately, many parts of the world that are short of water have exploitable renewable sources of energy that could be used to drive desalination processes (Kalogirou, 2005).
Solar desalination is used by nature to produce rain, which is the main source of the freshwater supply. Solar radiation falling on the surface of the sea is absorbed as heat and causes evaporation of the water. The vapor rises above the surface and is moved by winds. When this vapor cools down to its dew point, condensation occurs and freshwater precipitates as rain. All available manmade distillation systems are small-scale duplications of this natural process.
Desalination of brackish water and seawater is one way to meet the water demand. Renewable energy systems (RESs) produce energy from sources that are freely available in nature. Their main characteristic is that they are friendly to the environment, that is, they do not produce harmful effluents. Production of freshwater using desalination technologies driven by RESs is thought to be a viable solution to the water scarcity at remote areas characterized by lack of potable water and conventional energy sources such as a heat and electricity grid. Worldwide, several renewable energy desalination pilot plants have been installed and the majority have been successfully operated for a number of years. Virtually all of them are custom designed for specific locations and utilize solar, wind, or geothermal energy to produce freshwater. Operational data and experience from these plants can be utilized to achieve higher reliability and cost minimization. Although renewable energy-powered desalination systems cannot compete with conventional systems in terms of the cost of water produced, they are applicable in certain areas and are likely to become more widely feasible solutions in the near future.
This chapter presents a description of the various methods used for seawater desalination. Only methods that are industrially mature are included. Other methods, such as freezing and humidification–dehumidification methods, are not included in this chapter, since they were developed at a laboratory scale and have not been used on a large scale for desalination. Special attention is given to the use of RESs in desalination. Among the various RESs, the ones that have been used, or can be used, for desalination are described. These include solar thermal collectors, solar ponds, photovoltaics, wind turbines, and geothermal energy.
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