Opportunities for solar water desalination worldwide: Review

Opportunities for solar water desalination worldwide: Review

Sustainable Cities and Society 9 (2013) 67–80 Contents lists available at SciVerse ScienceDirect Sustainable Cities and Society journal homepage: ww...

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Sustainable Cities and Society 9 (2013) 67–80

Contents lists available at SciVerse ScienceDirect

Sustainable Cities and Society journal homepage: www.elsevier.com/locate/scs

Opportunities for solar water desalination worldwide: Review Mahmoud Shatat ∗ , Mark Worall, Saffa Riffat Institute of Sustainable Energy Technology, University of Nottingham, Nottingham NG7 2RD, UK

a r t i c l e Keywords: Water Desalination Solar energy

i n f o

a b s t r a c t Water desalination is increasingly becoming a competitive solution for providing drinking-water in many countries around the world. The desalination of saline water has been recognized as one of the most sustainable and new water resource alternative. It plays a crucial role in the socio-economic development for many communities and industrial sectors. Currently there are more than 14,000 desalination plants in operation worldwide producing several billion gallons of water per day. Fifty-seven percent are in the Middle East and Gulf region where large scale conventional heat and power plants are installed. However, since they are operated using fossil fuels, they are becoming expensive to operate and the pollution and greenhouse gas emissions they produce are increasingly recognized as harmful to the environment. Moreover, such plants are not economically viable in remote areas, even in coastal regions where seawater is abundant. Many areas often experience a shortage of fossil fuels and inadequate and unreliable electricity supply. The integration of renewable energy resources in desalination and water purification is becoming more viable as costs of conventional systems increase, commitments to reducing greenhouse gas emissions are implemented and targets for exploiting renewable energy are set. Thus, solar energy could provide a sustainable alternative to drive the desalination plants, especially in countries which lie on the solar belt such as Africa, the Middle East, India, and China. This paper explores the challenges and opportunities of solar water desalination worldwide. It presents an extensive review of water desalination and solar desalination technologies that have been developed in recent years and the state-of-the-art for most important efforts in the field of desalination by using solar energy, including the economic and environmental aspects. © 2013 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Challenges to international water desalination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nexus between renewable energy, conventional and water desalination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water desalination technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Thermal desalination processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Membrane desalination processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Spiral wound membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Hollow fine fibre (HFF) membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Potential integration of desalination with renewable energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solar water desalination technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Direct solar desalination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Indirect solar desalination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1. Solar still coupled with solar collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2. Solar humidification and dehumidification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3. Water desalination powered by solar photovoltaics (PV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abbreviations: DC, an electrical direct current; ED, electro dialysis; MD, membrane distillation; MED, multiple effect evaporation; MSF, multi-stage flash distillation; VC, vapor compression evaporation; PV, photovoltaic cells; RO, reverse osmosis; ppm, part per million; UN, United Nations; UNEP, United Nations Environment Programme; TDS, total dissolved solids (mg/L); WHO, World Health Organization; MENA, Middle East and North Africa. ∗ Corresponding author. Tel.: +44 0115 9513158; fax: +44 0115 9513159. E-mail addresses: [email protected], [email protected] (M. Shatat), [email protected] (M. Worall), [email protected] (S. Riffat). 2210-6707/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scs.2013.03.004

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Economics and performance of desalination processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Economics of thermal desalination processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Economics of water desalination using membrane processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3. Cost analysis of renewable and solar water desalination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Water and energy are necessary for life on Earth and sustain the modern world. In many parts of the developed world, the control and exploitation of water and energy has driven economic development and progress. In the developing world, many regions suffer from shortages of fresh water and energy supplies. The United Nations Environment Programme (UNEP) stated that one third of the world’s population live in countries with insufficient freshwater to support the population. Consequently by 2025, two thirds of the world population will face water scarcity (UNEP, 2012). Drinking water of acceptable quality has become a scarce commodity. The World Health Organization estimates that over a billion people lack access to purified drinking water and the vast majority of these people are living in rural areas where the low population density and remote locations make it difficult to install traditional clean water solutions (Qiblawey & Banat, 2008). Unfortunately, in addition to being scarce, freshwater resources are also unevenly distributed geographically worldwide (Mohammed, 2011). Desalination of seawater is known to be one of mankind’s earliest forms of water treatment, and it has become one of the most sustainable alternative solutions to provide fresh water for many communities and industrial sectors. This plays a crucial role in socio-economic development in a number of developing countries, especially in water stressed regions such as Africa, Pacific Asia and countries in the Middle East. Hence, the increase in population together with the industrial and agricultural development in emerging countries will accelerate rapidly the deterioration and depletion of the available freshwater resources. Desalination is a process in which saline water is separated into two parts using different forms of energy, one that has a low concentration of dissolved salts (fresh water), and the other which has a much higher concentration of dissolved salts than the original feed water (brine concentrate) (Buros, 2000). Saline water is classified as either brackish water or seawater depending on the salinity and water source. Large commercial desalination plants that use fossil fuels are in use in most of the countries suffering from water shortages. For instance, a number of oil-rich countries use fossil fuel to supplement the energy for water desalination supply. In contrast people in many other areas of the world have neither the financial nor oil resources to allow them to develop in a similar manner. The production of 1000 m3 per day of freshwater requires 10,000 tonnes of oil per year (Kalogirou, 2005), which can be considered a highly significant energy consumption, as it involves a recurrent energy expense which few of the water-short areas of the world can afford. Recently, the utilization of renewable sources (e.g., solar, biomass, wind, and geothermal) to drive desalination plants has emerged as a promising sustainable solution for fresh water supply in regions lacking energy supply. This may be especially significant in regions where water is needed and renewable resources are available such as Africa and the Middle East region. The conversion of solar radiation into direct utilization has been investigated for many years (Kalogirou, 2005). Recently, attention has been directed towards improving the conversion efficiency of solar energy systems, desalination technologies and their optimal coupling to make them economically viable for small and medium scale applications

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(Quteishat & Abu-Arabi, 2012). Extensive research and development activities have been conducted to explore the opportunities for sustainable and feasible methods of producing drinking water using solar energy (Buros, 1999). Conventional desalination systems are operated using fossil fuels, resulting in increase in costs because of the rise in world energy prices, increase in environmental pollution and the emission of greenhouse gases. In spite of these problems, it is likely that seawater desalination in large scale heat and power plants will continue to play a substantial role in providing fresh water for domestic and industrial use in areas of high population density, especially in oil rich Gulf countries and parts of India and China. However, such plants are not economically viable in remote areas where electricity infrastructure is poor and supplies inadequate and unreliable. The development of alternative, compact, small-scale water desalination systems is imperative if the populations in such areas are to gain access to fresh and safe water supplies (Buzás, Farkas, Biró, & Németh, 1998; Gleick, 1998). Semi-arid regions generally have a large solar energy utilization potential, as solar desalination concepts and methods are specifically suited to supplying dry regions with fresh water. The key point is that efficient and environment-friendly solar energy coupled with desalination technologies would be an appropriate alternative to producing fresh water on both small and medium scales. This solution is suitable for supplying up to a half of the rural population living in arid regions that lack conventional fossil fuels, whilst reaching an average of 15% in oil-rich countries (Chaibi, 2000). Solar thermal water desalination is proving to be a viable method of producing fresh water from saline sources (Al-Kharabsheh, 2003) in remote locations; humidification and dehumidification solar water desalinations units and conventional basin solar stills with a relatively large footprint are an example of such simple technologies, but the main problems with the use of solar thermal energy in large-scale desalination plants are the relatively low productivity, the low thermal efficiency and the considerable land area required. However, since solar energy utilized is “free” the operating costs are significantly reduced compared with conventional plant (AlKharabsheh & Yogi, 2003; Naim, Mervat, & Abd El-Kawi, 2003; Qiblawey & Banat, 2008). This paper aims to explore the opportunities for adopting solar water desalination technologies worldwide to minimize water shortage. It also presents an extensive review of the published literature on the various desalination technologies using solar energy and including both the implementation challenges as well as economic and environmental impacts. 2. Challenges to international water desalination In the next few decades, access to water for drinking, agricultural and industrial use will become an increasingly crucial challenge for many countries around the world. Currently, freshwater is becoming a scare commodity and is used unsustainably in the majority of the world’s regions. Over 70% of the Earth’s surface is covered by water but most of it is unsuitable for human consumption. With total global water reserves of about 1.4 billion km3 , around 97.5% of it is in the oceans and the remaining 2.5% is fresh water present in the atmosphere, ice mountains, freshwater lakes, rivers and ground water, as shown in Fig. 1. Only about 0.014% is directly available

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Fig. 1. Distribution of world water resources (UN Water, 2012; World resource, 2012).

for human beings and other organisms (World resource, 2012). In spite of water scarcity, freshwater resources are also very unevenly distributed across the world. The world’s water consumption rate is doubling every 20 years, and is outpacing population growth by two times. Accordingly, water scarcity is expected to affect one in three people on every continent of the planet. Almost one fifth of the world’s population live in areas where water is scarce. It is projected that by the year 2025 water demand will exceed supply by 56% due to persistent regional droughts, and shifting of the population to urban coastal cities (Water and Process Technology, 2012). As of today, about three billion people have no access to a potable source of water and about 1.76 billion people live in areas already facing a high degree of water shortage (Ma & Lu, 2011). The mismatch between the need for fresh water and its availability will intensify as competing needs for water grow due to population growth, urbanization, the impact of greenhouse gases on the environment and increases in household and industrial demand for water (Safe Drinking-water, 2012). However most projections estimate that the world population will stabilize at between 8 and 9.50 billion by around 2050 and that most of this growth will take place in the developing world, population growth will not only increase domestic water consumption, but also impacts on the consumption of agricultural, industrial and other products, and energy use. The population of the world has tripled in the last century, with a six fold increase of the global water use (Fresh water shortage, 2012). Recently, the rapid economic growth of many countries, particularly in China and India, has led to higher incomes stimulating greater consumption of goods and services. The production of goods and services requires huge quantities of water. Decoupling income growth from water consumption is one of the major challenges to water management. Urbanization is another demographic trend resulting in greater water use. Such rapid growth would add tremendous stress on the region’s water resources. Even today, the major cities rely on deep wells to abstract water from aquifers. The discharge of inadequately treated wastewater in many developing countries contributes negatively in the pollution and degradation of its limited water resources. Lack of fresh water reduces economic development and lowers living standards. Clearly, there is a critical worldwide need to improve the management of this increasingly valuable resource (Miller, 2003; Water Desalination, 2012). Consequently desalination of saline water can be used to augment the increasing demand for fresh water supplies. However, desalination is a very energy intensive process, often using energy supplied from fossil fuel sources which are vulnerable to volatile global market prices. In the light of water scarcity and limited available fresh water resources, the growth of the desalination market in the world is rapidly developing to meet the increasing water demand

utilizing seawater, brackish water, river water, and brine. The installed capacity was 60 Mm3 /day in 2010 and is expected to be doubled by 2015. 38 Mm3 /day of these plants are planned to be installed in the Gulf region and 59 Mm3 /day in the rest of the world as shown in Fig. 2 (Lattemann, Kennedy, Schippers, & Amy, 2010). Seawater desalination technology, available for decades, made great strides in many arid areas of the world, such as the Middle East, the Mediterranean, and the Caribbean. Fig. 3 shows the location of the existing desalination plants worldwide. The vast majority of high production capacity plants are installed in the Middle East. Seawater desalination in the Gulf region represents 65% of global water desalination capacity due to abundance of the world’s largest oil reserves, with an acute shortage of potable water resources as shown in Fig. 3 (Lattemann et al., 2010). Saudi Arabia, UAE, U.S., Spain and China have the highest desalination capacity, but India and Israel have seen a significant growth since 2002, when most of their capacities were installed (Global water, 2012). 3. Nexus between renewable energy, conventional and water desalination At present, the majority of desalination plants have been located in regions with high availability and low costs of conventional energy. Current statistics on desalination shows that only 1% of total desalinated water is based on energy from renewable sources (Water Desalination, 2012). Renewables are becoming increasingly reliable and mainstream with costs decreasing year-on-year, thus making renewable energy a viable option in many regions. With increasing demand for desalinated water in energy-importing countries such as India, China and small islands, there is a large

Fig. 2. Current and projected growth of desalination market worldwide. Adapted from Lattemann et al. (2010).

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Fig. 3. Global distribution of desalination capacities worlwide (Lattemann et al., 2010).

market potential for renewable energy-powered desalination systems worldwide. Renewable resources are plentiful but still remain largely untapped. The reliance on fossil fuels is set to continue since it is still considered by many to be the most cost effective and reliable energy form. However, mass deployment of desalination has presented a number of challenges, resulting in very high energy demand and CO2 emissions from fossil fuelled sources (Desalination Technologies, 2011). There is significant potential in developing integrated desalination and renewable energy technologies as a medium and long-term strategy, by encouraging technically feasible renewable energy systems with funding and investment (Renewable Desalination, 2010). The extent to which sustainable desalination schemes could be technically feasible for providing significant amounts of water remains to be seen. The use of sustainable energy sources such as hydropower or biofuels may limit their viability as a solution, through negative impacts on the water–energy–food nexus (Global water security, 2012; Water issues, 2012). Recently, the current water stress challenges have led to extensive research and development in sustainable water resources. The sustainable alternative could be attained through adopting an integrated water management strategy utilizing the renewable energy sources for water treatment technologies. This strategy aims to achieve sustainable use of the nation’s water resources by protecting and enhancing their quality while maintaining economic and social development (Water Resources and Use in Australia, 2012). These alternatives should involve water desalination, waste-water reuse and rain water harvesting systems. Consequently, recent developments in desalination technologies have achieved a breakthrough in terms of an affordable, low cost water supply and high energy efficiency. As per unit of desalinated water, the energy consumed by the desalination processes has been reduced significantly in recent years meaning that, if solar technologies are to be used, fewer PV modules would be required and the solar collector areas that are required will be reduced (Quteishat & Abu-Arabi, 2012). These advancements can make abundant fresh water both from seawater and brackish water with a very low environmental impact when it is integrated with renewables and solar energy in

particular, where most of MENA’s region, Gulf area, India, China and countries in Africa lie on the Sunbelt. Therefore, to alleviate water shortages and tackle the real threat to resource sustainability, the GCC countries are making continuous efforts to produce water by constructing new desalination plants. For instance Saudi Arabia and UAE are currently trying to manage the available water resources and to meet the water demand growth in order to stop rapid deterioration and depletion of its brackish water aquifers. Hence Saudi Arabia looks towards solar energy to sustain the growth of water and energy demand. An ambitious initiative has been recently announced, to convert the Saudi Arabia desalination plants to solar driven ones in 10 years. The first large scale solar energy driven (PV-SWRO) desalination plant is being developed. The plant will produce 30,000 m3 /day of drinking water to more than 100,000 inhabitants. In addition to that Abu Dhabi in the UAE has just committed to invest $2 billion in two thin film photovoltaic solar cells (PV) factories in Germany and Abu Dhabi and it has also started the construction of 2 × 1400 MW(e) nuclear power plant (Mezher, Fath, Abbas, & Khaled, 2011). 4. Water desalination technologies The majority of water desalination processes can be divided into two types: phase change thermal processes and membrane processes, as shown in Fig. 4, both encompass a number of different processes. In addition, other alternative technologies of freezing and ion exchange, but they are not widely used. All are operated by either a conventional or renewable energy sources to produce fresh water. 4.1. Thermal desalination processes Thermal desalination is based on the principles of evaporation and condensation. Water is increased in temperature until it reaches its saturation temperature, beyond which evaporation occurs. The salt is left behind whilst vapour is taken away and condensed in another heat exchanger to produce fresh water (Winter, Pannell, & McCann, 2005). The thermal energy is produced in steam

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Fig. 4. Water desalination technologies (Shatat & Riffat, 2012).

generators, waste heat boilers or by the extraction of back-pressure steam from turbines in power stations (Raluy, Serra, Uche, & Valero, 2004). The most common thermal desalination processes are:

4.2. Membrane desalination processes

concentrations to produce fresh water. A membrane is a thin film of porous material that allows water molecules to pass through it, but simultaneously prevents the passage of larger and undesirable molecules such as viruses, bacteria, metals, and salts. Membranes are made from a wide variety of materials such as polymeric materials that include cellulose, acetate, and nylon, and non-polymeric materials such as ceramics, metals and composites. Two of the most successful membranes are spiral wound and hollow fine fibre (HFF) and both of these are used to desalt brackish water and seawater (Bou-Hamad, Abdel-Jawad, & Al-Tabtabaei, 1998).

Membrane technology was originally limited to municipal water treatment such as micro-filtration and desalination, but with the development of new membrane types, uses have expanded to cover not only the water industry, but also high return processes such as chemical separations, enzyme concentration and beverage purification. This technology uses a relatively permeable membrane to move either water or salt to induce two zones of differing

4.2.1. Spiral wound membrane This type of membrane element is most commonly manufactured as a flat sheet of either a cellulose diacetate and triacetate blend or a thin film composite usually made from polyamide, polysulphone, or polyurea polymers as illustrated in Fig. 5 (El-Dessouky & Ettouny, 2001).

• • • •

Multi-stage flash distillation (MSF); Multiple-effect distillation (MED); Vapour-compression evaporation (VC); Solar water desalination.

Fig. 5. Cutaway view of a spiral wound membrane element (El-Dessouky & Ettouny, 2001).

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Fig. 6. Hollow fine fibre membrane module (El-Dessouky & Ettouny, 2001).

4.2.2. Hollow fine fibre (HFF) membrane HFF is a U-shaped fibre bundle housed in a pressure vessel. The membrane materials are based on cellulose triacetate and polyamide and its arrangement allows the highest specific surface area of all the module configurations, resulting in compact plants. Fig. 6 illustrates the HFF formation (El-Dessouky & Ettouny, 2001). Membrane processes are also useful in municipal water treatment; reverse osmosis (RO) and electrodialysis (ED) are replacing phase-change desalting technologies for supplying water to coastal and island communities all over the world. RO in particular, is becoming an economical alternative to the traditional water softening processes (DWTMM, 2012). It includes several processes, but the principal difference between them lies in the size of the entities, ions, molecules and suspended particles that are retained or allowed to pass through the membranes. Typical separation processes are nano-filtration, ultra-filtration, micro-filtration and filtration used in the pre-treatment stages of desalination to remove large particles, bacteria, ions, and for water softening (Water Desalination Technologies in the ESCWA Member Countries, 2001). Fig. 7 shows the effective range of membrane processes and applications.

5. Potential integration of desalination with renewable energy The worldwide installed desalination capacity is increasing rapidly. Currently there are more than 14,451 desalination plants in operation worldwide producing more than sixty million cubic metres per day, according to the International Desalination Association (IDA) (Henthorne, 2009) and it is expected to reach one hundred and twenty million cubic metres per day by 2020 of which forty million cubic metres is planned for the Middle East (Middle East, 2012). Since they are operated with fossil fuel, they are becoming very expensive to run and the environmental pollution they produce is increasingly recognized as very harmful to the globe (Fischetti, 2007). Moreover, such plants are not economically viable in remote areas, even near a coast where seawater is abundant. Many areas often experience a shortage of fossil fuels and inadequate electricity supply. The International Energy Agency (IEA) statistical data (IEA, 2012) show that modern renewable technologies are growing rapidly and would overtake gas soon to become the second-largest source of electricity behind coal and account for 40% of global power generation by 2030. In 2006 about

Fig. 7. Effective range of membrane processes and applications (Bou-Hamad et al., 1998).

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Fig. 8. Combinations of renewable energy resources with water desalination technologies (Shatat & Riffat, 2012).

12.3% of world consumption of energy generated by renewables with the largest fraction of 10.1% comes from traditional biomass sources as well accounting for 16% of global electricity production (Isabel & Andrea, 2010a). The potential use of renewable energy in small-scale desalination in remote communities has received increasing attention in recent years (Werner & Schäfer, 2007). The integration of renewable energy sources such as solar, wind and geothermal energy with desalination systems holds great promise for tackling water shortage and is a potential for viable solution of climate change problems and water scarcity (Renewable Desalination, 2010). Fig. 8 shows the integration of renewable energy resources with desalination technologies. In the meantime, the cost of desalination and renewable energy systems are steadily decreasing, while fossil fuel prices are increasing, reserves are being depleted, and concerns about energy security increase. The desalination units powered by renewable energy systems are uniquely suited to provide water and electricity in remote areas where water and electricity infrastructures are currently lacking (Mahmoudi, Abdellah, & Ghaffour, 2009). In 2008, 10% of the generated electricity worldwide was produced by renewable energy sources, such as (hydropower, biomass, biofuels, wind, geothermal, and solar). Current statistics on desalination shows that only 1% of total desalinated water is based on energy from renewable sources (Water Desalination Using Renewable Energy, 2012). Recent assessment conducted by the US Energy information administration forecasts that by 2035, consumption of renewable energy will be about 14% of total world energy consumption which shows strongest growth in global electric generating capacity (US Energy, 2010) motivating the use of water desalination technologies with renewable energy sources. The desalination systems that use renewable energy sources can be divided into three categories: wind, solar (photovoltaics or solar collectors) and geothermal energy. These renewable energy sources can be coupled with thermal distillation or membrane desalination systems to produce water (Ali, Fath, & Armstrong, 2011; Mathioulakis, Belessiotis, & Delyannis, 2007). The decision on which energy source is used should be made on the basis of economic, environmental and safety considerations. Due to its desirable environmental and safety advantages, it is widely believed that where possible, solar energy should be utilized instead of energy derived from fossil fuels, even when the costs involved are slightly higher (Kalogirou, 2004).

Currently, solar energy is the most widely used among the renewable sources, as shown in Fig. 9 (Quteishat & Abu-Arabi, 2012). So that by the utilization of solar energy for fresh water production, three main problems can be addressed: fresh water scarcity, fossil energy depletion and environmental degradation due to greenhouse gas emissions and hydrocarbon pollution (Bouchekima, 2003; Desalination Water Purification Technologies, 2010). A comprehensive literature revealed that solar energy can be identified as a viable energy source to be utilized in producing fresh water from saline water (Al-Kharabsheh, 2003), especially in many African and Asian countries, and MENA’s region which are located in semi-arid and sunny climates with average global solar irradiation of 6–7 kWh/m2 /day (Africa Direct Noranl Solar, 2012). A further advantage of solar water desalination technology is that peak solar energy harnessed in summer seasons coincides with high water demand in semi-arid areas. Consequently, the development of affordable, inexhaustible and clean solar energy technologies will have a significant long term benefits. It will increase energy security through the use of a localized indigenous, inexhaustible and mostly import-independent resource, enhance sustainability, reduce pollution, lower the costs of mitigating climate change, and keep fossil fuel prices lower (International Energy Agency, 2011). 6. Solar water desalination technologies Solar water desalination has a long history. The first documented use of solar stills was in the sixteenth century and, in 1872, the Swedish engineer, Carlos Wilson, built a large-scale solar still to supply a mining community in Chile with drinking

Fig. 9. The use of renewable energy sources in water desalination (Quteishat & AbuArabi, 2012).

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Fig. 10. Possible configurations of solar energy resources with water desalination technologies.

water (Intermediate Technology Development Group, 2007). The solar energy can be captured for use either by photovoltaic (PV) devices and direct absorption using solar collectors or solar ponds as thermal energy (Quteishat & Abu-Arabi, 2012). Solar powered desalination processes are generally divided into two categories, direct and indirect systems as shown in Fig. 10. 6.1. Direct solar desalination The direct systems are those where the thermal desalination processes take place in the same device and it is mainly suited to small production systems, such as solar stills, in regions where the freshwater demand is less than 200 m3 /day (Ma & Lu, 2011). Solar still distillation represents a natural hydrologic cycle on a small scale. The simple solar still is shown in Fig. 11. The solar still is working as a trap for solar radiation that passes through a transparent cover it consists of a basin containing salt water, a pair of glass or plastic panels sloping at an angle above the basin and meeting at the apex, creating a structure much like a greenhouse. The basin is generally painted black to maximize the absorption of long wave radiation falling on the surface. Solar radiation falls on the sloping panels and the greenhouse effect that is produced in the

inside raises the temperature of the salt water held in a basin. Water at the surface is evaporated, the water vapour rises in the still and reaches the sloping panels, where it condenses to liquid water and runs down the sides of the panels. The water is collected and drawn off to provide fresh water. Solar stills can produce 3–4 L of fresh water per day per square metre. Because of low production rates, it is important to minimize capital costs by using very inexpensive construction materials. Efforts have been made by various researchers to increase the efficiency of solar stills by changing the design, by using additional effects such as multi-stage evacuated stills and by adding wicking material, and these modifications have increased production per unit area (Buros, 2000). In the simple solar still shown in Fig. 11, the latent heat of condensation is dissipated to the environment. However, the latent heat of condensation can be used to pre-heat the feed-water, and this leads to an improvement in the efficiency. Solar still technology requires a large area for solar collection so it is not viable for large-scale production, especially near cities where land is scarce and expensive. The comparative installation costs tend to be considerably higher than those of other systems. Solar stills are also vulnerable to damage by the weather. Labour costs are likely to be high due to the need for routine maintenance to prevent scale formation and to repair vapour leaks and damage to the glazing panels (Buros, 2000; Miller, 2003). However, they can be economically viable for small-scale production for households and small communities, especially where solar energy and low cost labour are abundant (Ali Samee, Mirza, Majeed, & Ahmad, 2007; Buros, 2000). 6.2. Indirect solar desalination

Fig. 11. Solar still desalination unit (Ali et al., 2011).

In these systems, the plant is separated into two subsystems, a solar collector and a desalination unit. The solar collector can be a flat plate, evacuated tube or solar concentrator and it can be coupled with any of the thermal desalination processes types which use the evaporation and condensation principle, such as multistage flash distillation (MSF), vapour compression (VOC), multiple effect evaporation (MED), and membrane distillation (MD) for possible combinations of thermal desalination with solar energy. Systems that use photovoltaic (PV) devices tend to generate electricity to operate reverse osmoses (RO) and electro dialysis (ED) desalination

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Fig. 12. Water desalination technologies coupled with solar power sources installed worldwide (Ali et al., 2011).

processes (Miller, 2003; Ali et al., 2011). Fig. 12 shows the worldwide use of the various desalination technologies using solar power sources (Kaushal & Varun, 2010). 6.2.1. Solar still coupled with solar collectors In order to increase still productivity many small-scale system configurations have been examined such as coupling single stills or multi-effect stills with solar collectors, as shown in Fig. 13. Coupling more than one still with such solar collectors produces an increase in efficiency by utilizing the latent heat of condensation in each effect, which is then delivered to the next stage (Shatat & Mahkamov, 2010). 6.2.2. Solar humidification and dehumidification In this process, saline water is evaporated by thermal energy and the subsequent condensation of the humid air that is generated (normally at atmospheric pressure) produces freshwater (DESWARE, 2007). Air has the capability to hold large quantities of water vapour and its vapour carrying capability increases with temperature (Parekh, Farid, Selman, & Al-hallaj, 2004). Many studies on desalination using humidification–dehumidification have been conducted with a variety of fabricated devices (Orfi, Galanis, & Laplante, 2007). The principle of this process is based on the evaporation of water and the condensation of steam from humid air. The humid air flows in a clockwise circuit driven by natural convection between the condenser and the evaporator, as shown in Fig. 14. In this example, the evaporator and condenser are located in the same thermally insulated box. Seawater is heated in the evaporator and distributed slowly as it trickles downwards. The air moves in a counter-current flow to the brine through the evaporator and the air reaches saturation. Partial evaporation cools the brine that

Fig. 14. Humidifcation and dehumidification desalination unit coupled with solar collector (Müller-Holst et al., 1999).

is left in the evaporation unit, leaving it at a higher concentration, while the saturated air condenses on a flat plate heat exchanger. The distillate runs down the plates and trickles into a collecting basin. The heat of condensation is mainly transferred to the cold

Fig. 13. Schematic diagram of single solar and multi effect solar still coupled with a solar collector (Al-Kharabsheh, 2003).

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Fig. 15. RO desalination unit coupled with a PV generator (Al-Karaghouli and Kazmerski, 2011).

seawater flowing upwards inside the flat plate heat exchanger. Thus the temperature of the brine in the condenser rises from 40 ◦ C to approximately 75 ◦ C. In the next step, the brine is heated to the evaporator inlet temperature, which is between 80 and 90 ◦ C. The salt content of the brine as well as the condenser inlet temperature can be increased by a partial reflux from the evaporator outlet to the brine storage tank (Müller-Holst, Engelhardt, & Schölkopf, 1999). Then distillate can be collected in a vessel and the brine goes also to saline water tank to recover a portion of the heat. 6.2.3. Water desalination powered by solar photovoltaics (PV) Solar photovoltaic (PV) systems directly convert sunlight into electricity by using solar cells made from silicon or other semiconductor materials and are connected together to form a PV module, which can then supply power to the desalination unit. The PV generator can be connected either with RO or ED water desalination technology, as described previously (Mahmoud & Ibrik, 2006). Fig. 15 shows the assembly of a RO desalination plant coupled with a photovoltaic generator. This configuration includes a set of battery blocks to stabilize the energy input to the RO unit and to

compensate for solar radiation variations, a charge controller to protect the battery block from deep discharge and overcharge, and a RO unit to desalinate the water. 7. Economics and performance of desalination processes Geographically, the largest market of desalination will continue to be the Middle East and Gulf region, where the combination of rapidly growing populations, depleted groundwater resources and the retirement of capacity built during the oil boom years will require upgrading and doubling of the total capacity of production (The outlook, 2012). However Asia-Pacific is expected to experience a higher growth rate aided by its rapidly developing economy, urbanization and population growth while America and Europe continue to be a steady market in which desalination plants are currently used for preserving rapidly depleting groundwater as an alternative water source (Water Pump, 2012). The desalination industry has achieved significant technological advancements in order to meet the growing demand for high corrosion resistance materials, energy efficiency and reliability.

Fig. 16. Cost of thermal and RO desalination processes (Lattemann et al., 2010; SWRO, 2011).

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Table 1 Cost of desalinated water in thermal processes. Desalination process

Capacity of desalination plant (m3 /day)

Desalination cost per m3 (US$)

Multi effect distillation MED

Less than 100 12,000–55,000 Greater than 91,000

2.5–10 0.95–1.95 0.52–1.01

Multi stage flash MSF Vapour compression

23,000–528,000 1000–1200

0.52–1.75 2.01–2.66

Fig. 17. Development of achievable energy consumption and cost in RO desalination processes.

However, the desalination processes require large amounts of energy which is expensive and costly in environmental pollution where large commercial desalination plants using fossil fuel are in use in most of the world. Many economic studies of water desalination costs appear regularly in water desalination and renewable energy related publications. Cost estimates of desalinated water per cubic metre can be significantly influenced by several factors. These factors include the capacity and type of desalination facility, location, feed water either (seawater or brackish water), and labour, type of energy (conventional or renewable energy) (Karagiannis & Soldatos, 2008). For instance, the phase change water desalination processes, which use conventional sources of fuel and energy and which usually have large production capacities are more expensive than membrane plants because of the large quantities of fuel required to vaporize salt water. Membrane methods are also more economical for brackish water desalination. Fig. 16 shows the cost analysis for large scale commercial plants for thermal and RO desalination processes.

Table 2 Cost of desalinated water in membrane (RO) plants (Al-Wazzan, Safar, & Ebrahim, 2002; IDA, 2006–2007; Jaber & Ahmed, 2004; Sambrailo, Ivic, & Krstulovic, 2005; Voivontas, Misirlis, Manoli, Arampatzis, & Assimacopoulos, 2001; Wilf & Bartels, 2005; Zejli, Benchrifa, Bennouna, & Zazi, 2004). Desalination cost per m3 (US$)

Type of feed water

Capacity of desalination plant (m3 /day)

Brackish water

Less than 20 20–1200 40,000–46,000

5.63–12.9 0.78–1.33 0.26–0.54

Seawater

Less than 100 250–1000 15,000–60,000 100,000–320,000

1.5–18.75 1.25–3.93 0.48–1.62 0.45–0.66

Thermal process desalination plants are mostly driven by fossil fuel sources, which add a high cost of desalination per unit cost. In the case of multi-effect distillation (MED), the cost for large systems with a daily production from 91,000 m3 to 320,000 m3 ranges between 0.52 $/m3 and 1.01 $/m3 . These costs refer to examples of plants built between 1999 and 2009 with the most recent having lower costs and the older installations being the most expensive (Karagiannis & Soldatos, 2008). Table 1 shows the cost of desalinated water using thermal processes.

and composition of the feed water. Large scale RO plants can use brackish water containing total dissolved solids (TDS) of from 2000 ppm to 10,000 ppm (Delyianni & Belessiotis, 1995) but, as TDS concentrations increase, the unit cost of the desalinated water also increases, as shown in Table 2 (Liu, Rainwater, & Song, 2011). The cost of brackish water desalination in Middle East was 0.26 $/m3 with a TDS concentration of 2300 ppm, whilst the cost for brackish water desalination in Florida was 0.27 $/m3 with a concentration of 5000 ppm (Avlonitis, 2002). Whilst for seawater desalination the estimated water production cost for the world’s largest RO seawater desalination plant in Ashkelon, Israel, was 0.52 US$/m3 for a capacity of 320,000 m3 /day, (SWRO, 2011) and the cost of an RO plant in Florida was $0.56/m3 for a capacity of 94,600 m3 /day (Wilf & Bartels, 2005). Table 2 shows typical costs for RO desalination of brackish water and seawater (Karagiannis & Soldatos, 2008).

7.2. Economics of water desalination using membrane processes

7.3. Cost analysis of renewable and solar water desalination

Recent developments in membrane materials, pumping and energy recovery systems have dramatically reduced the energy consumption in RO desalination processes, as shown in Fig. 17 (Karagiannis & Soldatos, 2008; Stover, 2006). The cost of water desalination in membrane processes varies according to the type

From the literature reviewed, it has been revealed that the cost of water produced from desalination systems using a conventional source of energy, was much lower than those powered by renewable energy sources. Generally water desalination prices have fallen over the recent years due to technical improvements and research

7.1. Economics of thermal desalination processes

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Table 3 Type of energy supplied and water production cost (Karagiannis & Soldatos, 2008). Type of feed water

Type of energy

Water cost (Euro/m3 )

(US$/m3 )

Brackish water

Conventional fuel Photovoltaic cells Geothermal

0.21–1.06 4.5–10.32 2

0.27–1.38 5.85–13.42 2.6

Sea water

Conventional fuel Wind energy Photovoltaic cells Solar collectors

0.35–2.70 1.0–5.0 3.14–9.0 3.5–8.0

0.46–3.5 1.3–6.5 4.08–11.7 4.55–10.40

advancements in technologies. When renewable energy sources are used, the costs are much higher, and in some cases exceed 10 Euro/m3 , due to the use of the most expensive energy supply systems. However, this cost is counterbalanced by the environmental benefits. Table 3 summarizes the cost of fresh water when the desalination systems are powered by conventional and renewable energy sources. 8. Conclusions and outlook Water and energy are the most essential pillars for a sustainable life. The scarcity of water limits the socio-economic development of many countries in the world especially in developing countries. The desalination of saline water is proving to be a sustainable resource of fresh water and is contributing to tackling the world’s water scarcity. The majority of desalination plants installed in most of the world’s countries suffering from water shortages are operated by fossil fuel, however, since the desalination processes are energy intensive, they are becoming expensive to run and the environmental pollution they produce is increasingly recognized as very harmful to the environment. The thermal desalination process is exclusively used for desalination of seawater in the most of oil rich countries and the RO process becomes the second important technology on a global scale. Currently RO is considered the first choice in many industrialized and developing countries where conventional energy resources are scarce. However, the MSF thermal distillation will continue to be the largest desalination market for the foreseeable future because thermal cogeneration facilities predominate in oil-rich countries of the Middle East (Isabel & Andrea, 2010b). Recent developments and technical improvements in desalination technologies have significantly reduced the cost of desalination in recent years. The desalination of brackish and seawater will increase rapidly as technologies develop and demand for fresh water grows. The use of renewable energy for desalination is becoming a reliable and technically mature alternative to the conventional systems, especially in regions with poor infrastructure, lack of access to conventional forms of energy, and abundant renewable resources available. Currently, coupling desalination plants with renewable energy resources is becoming more attractive due to the rise in fossil fuel prices and the harmful impact of burning fossil fuels. The recent developments and improvements in both solar and desalination technologies have made them mature technologies. Thus the use of solar energy for water desalination in countries in the Middle East region and Africa which lie on solar energy belt could be a promising solution and would contribute both towards solving water scarcity problems and reducing carbon dioxide emissions by means of an environmentally friendly process. However the solar distillation may be advantageous for seawater desalination, as solar-MED and MSF are recommended for large-scale solar desalination and other solar applications have to be investigated such as photovoltaics and solar-thermal

collectors, which could be suitable for different desalination processes at a competitive cost and which are well suited to providing fresh water in remote areas where water and electricity infrastructure are currently lacking. The connection of RO membrane technology is considered the most cost-competitive solar desalination technology and approaches that of conventional desalination costs. Hence, the integration between solar photovoltaic cells and membrane water desalination processes could be considered as the second most competitive alternative for brackish water desalination (El-Nashar, 2001). The on-going and future research should focus on the development of low cost, affordable, inexhaustible and clean solar energy technologies which will have a significant long term benefits and can be integrated with various desalination processes. Although they are a noticeable developments and improvements on the desalination processes, further R & D should be carried out to find out a new desalination membranes in terms of forward osmosis, and to reduce the energy consumption through an innovative pumping systems, heat and pressure recovery systems. At the meantime parallel research on the development of low cost solar energy technologies should be undertaken to look for a low cost materials (chemicals, membrane), inexpensive solar collectors, low and durable PV’s materials. The decision about the adoption of desalination alternative revolves around a complicated evaluation of local circumstances such as water demand, available water resources, renewable energy resources, economics, environmental and social impacts. Furthermore, there is a need for on-going research and demonstration projects to gain experience, knowledge and trust in new environmentally friendly technologies as well as a state incentives through the introduction of new polices and financial support. These measures would most likely result in lower energy consumption and production cost of desalinated water. Meanwhile some advantages of wastewater desalination over seawater desalination should also be highlighted and taken into account in any future research because of cheaper cost and lower energy consumption in addition to that most of collected wastewater is near the urban areas which save a transportation cost through utilizing it for landscaping activities and industrial use. Collective and comprehensive research and development programmes involving all the stakeholders – consumers, local communities, governments, industries, universities and research institutions are required to develop sustainable water desalination polices and strategies to tackle the water scarcity and protect the rapid deterioration in available water resources worldwide. These programmes should include a holistic, system wide design processes of water reuse and desalination technologies with an optimal processes optimization to minimize waste recover and energy consumption.

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