Seawater desalination using renewable energy sources

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Abstract

The origin and continuation of mankind is based on water. Water is one of the most abundant resources on earth, covering three-fourths of the planet's surface. However, about 97% of the earth's water is salt water in the oceans, and a tiny 3% is fresh water. This small percentage of the earth's water—which supplies most of human and animal needs—exists in ground water, lakes and rivers. The only nearly inexhaustible sources of water are the oceans, which, however, are of high salinity. It would be feasible to address the water-shortage problem with seawater desalination; however, the separation of salts from seawater requires large amounts of energy which, when produced from fossil fuels, can cause harm to the environment. Therefore, there is a need to employ environmentally-friendly energy sources in order to desalinate seawater.

After a historical introduction into desalination, this paper covers a large variety of systems used to convert seawater into fresh water suitable for human use. It also covers a variety of systems, which can be used to harness renewable energy sources; these include solar collectors, photovoltaics, solar ponds and geothermal energy. Both direct and indirect collection systems are included. The representative example of direct collection systems is the solar still. Indirect collection systems employ two sub-systems; one for the collection of renewable energy and one for desalination. For this purpose, standard renewable energy and desalination systems are most often employed. Only industrially-tested desalination systems are included in this paper and they comprise the phase change processes, which include the multistage flash, multiple effect boiling and vapour compression and membrane processes, which include reverse osmosis and electrodialysis. The paper also includes a review of various systems that use renewable energy sources for desalination. Finally, some general guidelines are given for selection of desalination and renewable energy systems and the parameters that need to be considered.

Introduction

The provision of fresh water 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.

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 salt water in the oceans and 3% (about 36 million km3) is fresh water contained in the poles (in the form of ice), ground water, lakes and rivers, which supply most of human and animal needs. Nearly, 70% from this tiny 3% of the world's fresh water is frozen in glaciers, permanent snow cover, ice and permafrost. Thirty percent of all fresh water 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 fresh water; lakes contain most of it.

Water and energy are two inseparable commodities that govern the lives of humanity and promote civilization. The history of mankind proves that water and civilization are two inseparable entities. This is proved by the fact that all great civilizations were developed and flourished near large sources of water. Rivers, seas, oases, and oceans have attracted mankind to their coasts because water is the source of life. History proves the importance of water in the sustainability of life and the development of civilization. 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 main wheat exporting country in the whole Mediterranean Basin [1]. Due to the richness of the river, various disciplines of science like astronomy, mathematics, law, justice, currency and police were created at a time when no other human society held this knowledge.

Energy is as important as water for the development of good standards of life because it is the force that puts in operation all human activities. Water is also itself a power generating force. The first confirmed attempts to harness waterpower occurred more than 2000 years ago in which time the energy gained was mainly used to grind grain [2].

The Greeks were the first to express philosophical ideas about the nature of water and energy. Thales of Militus (640–546 BC), one of the seven wise men of antiquity wrote about water [3], [4] that it is fertile and moulded (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 BC) developed the theory of the elements [3] describing that the world consists of four primary elements: fire, air, water and earth. These with today's knowledge may be translated to: energy, atmosphere, water and soil, which are the four basic constituents that affect the quality of our lives [5].

Aristotle (384–322), who is 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 wrote for the water cycle in nature [6]:

“Now the sun moving, as it does, sets up processes of change and becoming and decay, and by its agency the finest and sweetest water is every day carried out and is dissolved into vapor and rises to the upper regions, where it is condensed again by the cold and so returns to the earth. This, as we have said before, is the regular cycle of nature.”

Even today no better explanation is given for the water cycle in nature. Really, the water cycle is a huge solar energy open distiller in a perpetual operational cycle. For the seawater Aristotle wrote [7]:

“Salt water when it turns into vapour becomes sweet, and the vapour does not form salt water when it condenses again. This is known by experiment.”

Man has been dependent on rivers, lakes and underground water reservoirs for fresh water requirements in domestic life, agriculture and industry. However, rapid industrial growth and the worldwide population explosion have resulted in a large escalation of demand for fresh water, both for the household needs and for crops to produce adequate quantities of food. Added to this is the problem of pollution of rivers and lakes by industrial wastes and the large amounts of sewage discharged. On a global scale, man-made pollution of natural sources of water is becoming one of the largest causes for fresh water shortage. Added to this is the problem of uneven distribution. For example, Canada has a tenth of the world's surface fresh water, but less than 1% of its population.

Of 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 first. For example drip irrigation, using perforated plastic pipes to deliver the water to crops, uses 30–70% less water than traditional methods and increases crop yield. This system was developed in the early 1960s but until today it is used in less than 1% of the irrigated land. In most places on the earth, governments heavily subsidise irrigation water and farmers have no incentive to invest in drip systems or any other water saving methods.

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 with desalination of this water. Desalinize in general means to remove salt from seawater or generally saline water.

According to 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, while 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 [8]. Excess brackishness causes the problem of 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 within the permissible limit of 500 ppm or less. This is accomplished by several desalination methods that will be analysed in this paper.

Desalination processes require significant quantities of energy to achieve separation of salts from seawater. This is highly significant as it is a recurrent cost, which 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 in and run desalination equipment. 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 year 2000 is about 22 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. It has been estimated that the production of 22 million m3/day requires about 203 million tons of oil per year (about 8.5 EJ/yr or 2.36×1012 kW h/yr of fuel). Given concern about the environmental problems related to the use of fossil fuels, if oil was much more widely available, it is questionable if we could afford to burn it on the scale needed to provide everyone with fresh water. Given current understanding of the greenhouse effect and the importance of CO2 levels, this use of oil is debatable. Thus, 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 burning of substantial quantities of fossil fuels. Given that conventional sources of energy are polluting, sources of energy that are not polluting will have to be developed. Fortunately, there are many parts of the world that are short of water but have exploitable renewable sources of energy that could be used to drive desalination processes.

Solar desalination is used by nature to produce rain, which is the main source of fresh water supply. Solar radiation falling on the surface of the sea is absorbed as heat and causes evaporation of the water. The vapour rises above the surface and is moved by winds. When this vapour cools down to its dew point, condensation occurs and fresh water precipitates as rain. All available man-made distillation systems are small-scale duplications of this natural process.

Desalination of brackish water and seawater is one of the ways of meeting water demand. Renewable energy systems produce energy from sources that are freely available in nature. Their main characteristic is that they are friendly to the environment, i.e. they do not produce harmful effluents. Production of fresh water using desalination technologies driven by renewable energy systems is thought to be a viable solution to the water scarcity at remote areas characterized by lack of potable water and conventional energy sources like 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 fresh water. 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 paper presents a description of the various methods used for seawater desalination. Only methods, which are industrially matured, are reviewed. There are, however, other methods, like freezing and humidification/dehumidification methods, which are not included in this work as they are developed at a laboratory scale and have not been used on a large-scale for desalination. Special attention is given to the use of renewable energy systems in desalination. Among the various renewable energy systems, the ones that have been used, or can be used, for desalination are reviewed. These include solar thermal collectors, solar ponds, photovoltaics, wind turbines and geothermal energy.

Section snippets

History of desalination

As early as in the fourth century BC, Aristotle described a method to evaporate impure water and then condense it to obtain potable water. However, historically probably one of the first applications of seawater desalination by distillation is depicted at the drawing shown in Fig. 1. The need to produce fresh water onboard emerged by the time the long-distance trips were possible. The drawing illustrates an account by Alexander of Aphrodisias in AD 200, who said that sailors at sea boiled

Desalination processes

Desalination can be achieved by using a number of techniques. Industrial desalination technologies use either phase change or involve semi-permeable membranes to separate the solvent or some solutes. Thus, desalination techniques may be classified into the following categories:

  • (i)

    phase-change or thermal processes; and

  • (ii)

    membrane or single-phase processes.

All processes require a chemical pre-treatment of raw seawater to avoid scaling, foaming, corrosion, biological growth, and fouling and also require

Direct collection systems

Among the non-conventional methods to desalinate brackish water or seawater, is solar distillation. Comparatively, this requires a simple technology which can be operated by non-skilled workers. Also due to the low maintenance requirement, it can be used anywhere with lesser number of problems.

A representative example of direct collection systems is the conventional solar still, which uses the greenhouse effect to evaporate salty water. It consists of a basin, in which a constant amount of

Indirect collection systems

The operating principle of these systems involves the implementation of two separate sub-systems, a renewable energy collector (solar collector, PV, wind turbine, etc.) and a plant for transforming the collected energy to fresh water. The renewable energy sub-systems are discussed in Section 6, however, some examples employing renewable energy to power desalination plants are presented in this section. The plant sub-system is based on one of the following two operating principles:

  • (i)

    Phase-change

Renewable energy systems

Renewable energy systems offer alternative solutions to decrease the dependence on fossil fuels. The total worldwide renewable energy desalination installations amount to capacities of less than 1% of that of conventional fossil-fuelled desalination plants [1]. This is mainly due to the high capital and maintenance costs required by renewable energy, making these desalination plants non-competitive with conventional fuel desalination plants.

This section presents a review of the possible systems

Solar thermal energy

Solar thermal energy is one of the most promising applications of renewable energy to seawater desalination. A solar distillation system may consist of two separated devices, the solar collector and the conventional distiller (indirect solar desalination). Indirect solar desalination systems usually consist of a commercial desalination plant that is connected to commercial or special solar thermal collectors. With respect to special solar thermal collectors, Rajvanshi [197] designed such a

Process selection

During the design stage of a renewable energy powered desalination system, the designer will need to select a process suitable for a particular application. The factors that should be considered for such a selection are the following [150]:

  • (i)

    Suitability of the process for renewable energy application.

  • (ii)

    The effectiveness of the process with respect to energy consumption.

  • (iii)

    The amount of fresh water required in a particular application in combination with the range of applicability of the various

Conclusions

In this paper, a review of the various renewable energy desalination systems is presented together with a review of a number of pilot systems erected in various parts of the world. The selection of the appropriate RES desalination technology depends on a number of factors. These include, plant size, feed water salinity, remoteness, availability of grid electricity, technical infrastructure and the type and potential of the local renewable energy resource. Among the several possible combinations

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