Abstract Throughout the world, there are regions of vast extent that have many favorable features, but whose development is principally limited by the lack of fresh water. In arid areas where large-scale development has already occurred, e.g. parts of the Middle East and North Africa, the extraction of fresh water via desalination plants requires very large energy consumption. This motivates the development of solar-desalination systems, which are desalination systems that are powered by solar energy. With the goal of identifying key technical challenges and potential opportunities solar-desalination, we review a variety of solar energy technologies used for capturing and concentrating heat energy, and also review various technologies for desalination systems including advanced techniques for energy-recovery. Existing solar-powered desalination plants have generally been indirect solar-desalination systems that first (i) transform solar energy into electrical energy and then (ii) employ the resulting electrical energy to drive desalination systems. Other, potentially more efficient direct solar-desalination systems directly convert the solar energy to pressure and/or heat, and use these to directly power the desalination process. We compare the cost-effectiveness, energy-efficiency, and other relevant quantities of these potential technologies for solar-desalination systems. We conclude that the direct solar-desalination systems using solar-thermal collectors appear to be most attractive for optimization of the energy-efficiency of solar-desalination systems. Further, we consider the economics and other practical issues associated with employing solar-desalination systems to provide for economic water sources for urban and agricultural areas. We consider factors that have significant impact to the use of solar-desalination systems: including location, climate, the type of water source (ocean water or brackish water sources), as well as land-use and ecological issues. We observe that the most favorable locations are those with high solar irradiance, lack of fresh water, but access to large brackish water sources and/or proximate seawater. We review the known locations of global brackish water reserves and areas with proximate seawater. Finally, we determine what appear to be the most favorable candidate locations for solar-desalination systems, which include considerable sections of North and East Africa, the Middle East, Southern Europe, Western South America, Australia, Northern Mexico, and South-West USA. We conclude that the development of cost-effective and energy-efficient solar-desalination systems may in the immediate future the key to a future “terraforming” of otherwise desert and near-desert regions of the world, providing a “greening” of these regions.
Keywords –solar energy; desalination, solar-desalination, brackish water, solar concentrators, alternative energy
1 Introduction 1.1 A Historical Prospective: Prior Greening of the World at the End of the Pleistocene
Interestingly, many areas such as the Middle East and North Africa were not always arid. At the end of the Pleistocene, roughly 12,000 years ago, the melting of glacier ice allowed many such areas have considerable fresh water. These conditions persisted to a degree even up to the Classic period 2000 years ago, and in those times for example certain areas that are now deserts in North Africa were a significant source of grains for Rome.
1.2 Current Freshwater Reserves
Figure 1: World map of freshwater (in green) reserves [Gleeson, 2012, Figure 1]
We will use the term fresh water to denote water with no more than approx. 500 to 100 ppm salinity; fresh water constitutes only 3%-5% of the world’s water [ADA, 2002]. To determine the areas where desalination is of use, see the above Figure 1, which provides a world map of current freshwater water reserves [Gleeson, 2012].
1.2Rapidly Diminishing Accessible Freshwater Reserves The high rate of population growth and climate change presents increased need for freshwater, and in the next decades many further areas of the world are expected also to require substantial use of desalination. Agriculture currently uses approximately 70% of fresh water, and overall agricultural water use will increase substantially with population growth, perhaps by 50% within 15 to 20 years. Agriculture use of fresh water competes with the industrial (approx. 20%) and household (approx. 10%) use of freshwater. A number of arid areas (e.g., much of the Middle East) already completely utilize all available sources of fresh water, and need to rely on desalination. In the future, with demand for fresh water approx. doubling every twenty years, many more regions will need to rely on desalination for a growing proportion of their fresh water needs.
We use the term “Green Terraforming” to describe the goal of transforming now arid areas of the world (e.g., sections of North and East Africa, the Middle East, Southern Europe, Western South America, Australia’s interior, and South-West USA) to areas with considerable available fresh water. We will be discussing technology that with further improvement and the overcoming of some considerable technical challenges may lead to such as “Green Terraforming” of arid regions.
1.4 Classification of Waters A key issue for desalination is the source volume, salinity, and other dissolved solids of the feed water used for desalination.
Table 1: Classification of waters by total dissolved solids[Rhoades, 1992, Table 1]
The above Table 1 gives Total Dissolved Solids (TDS) in grams per liter (g/l), as well as electrical conductivity (EC), expressed in units of deciSiemen per meter (dS/m). (Note that for TDS consisting only of Na Cl salts, the TDS of 1 gram per liter (g/l) is the same as 1,000 ppm.) Observe from the Table 1 that irrigation water can, depending on the crop, be up to approximately three times the TDS of drinking water. Also, ground water has a wide variation of TDS, depending on the drainage and topsoil.
Classification of waters by salinity: Brackish water is water with salinity between that of fresh water and seawater (in the range of approx. 5,000-35,0000 ppm, but typically approx. 10,000-15,000 ppm), and constitutes approximately 23% of the world’s water [ADA, 2002]. The salinity of seawater ranges between 35,000 to 45,000 ppm, and constitutes approximately 58% of the world’s water [ADA, 2002]. Other water consists of wastewater (approximately 5%), and river water (approximately 7%), and other sources [ADA, 2002]. Unfortunately, a large proportion of wastewater of developing nations is released directly into rivers, thus further limiting sources of fresh water.
Saline and Brackish Water Reserves
The salinity and composition of the input feed to any desalination system is critical, and so it is essential to know the accessible sources, saline concentration of nearby saline and brackish water.
The first issue is the situation of the saline and brackish water. The Figure 2 below gives a world map of situations of saline water reserves, with Basin (red), Sedimentary-Basin (yellow), Mountain (green), volcanic (blue) [Weert, 2009]. Note that brackish water reserves of Figure 3 often result from freshwater sources that are in contact with saline sediments or seawater seepage. Furthermore, brackish water can be found often near salt domes, and so collocated near deposits of oil or natural gas.
The next issue is the geographic locations of the saline and brackish water. Figure 3 below gives a world map of brackish water reserves [Weert, 2012] and in Figures 4 and 5 maps are also given for brackish water reserves in the Middle East and North Africa, respectively. Observe the extent of brackish water with partial marine origin (in blue), e.g., those ringing much of Africa, and particularly evident in North Africa. Also, observe the large brackish water reserves of natural terrestrial origin (in red) in eastern Saudi Arabia, which may be associated with salt domes.
An extensive study of seawater and brackish water desalination in the Middle East, North Africa and Central Asia is given in [Schenkeveld , 2004].
Figure 2: World map of situation of saline water reserves [van Weert, 2009]