Providing clean water for human consumption has become a
major challenge at local, regional, national and global levels [1].
This is mainly due to increasing demand prompted by population
growth and urbanization [2]. Over the last century, global population
has tripled while water demand per capita has doubled,
resulting in a six-fold increase in water withdrawals. This suggests
that not only has the number of water users increased globally, but
individual consumption rate has also increased due to high living
standards. For example, Energy Information Administration (EIA)
reports a population increase of 70 million in USA alone by 2030.
The direct domestic water demand and indirect industrial, agricultural,
and environmental water demand needed to sustain this
growth is expected to place serious strains on currently available
water resources. At the same time, this growth in population is
expected to increase the electricity demand by approximately 50%
[3], which will place additional demands on available water
sources in USA. For example, thermoelectric power plants
accounted for 48% of the total water withdrawal in the US in the
year 2000. The consumptive use of water for electricity production
could more than double from 3.3 billion gallons per day in 1995 to
7.3 billion gallons per day in 2030 [4]. Although this consumptive
use is not high compared to the total US consumption of 100 billion
gallons/day, large volumes of water are to be dedicated to
thermoelectric power plant operation.
Although 71% of the earth’s surface is covered with water, the
oceans hold over 95% of this water, all of which is salt water not
suitable for drinking purpose, while the remainder (about 2.5%) is
fresh water in rivers, lakes, and underground, and polar ice caps,
which is expected to supply most needs for human and related
consumption. On the other hand, freshwater demand is expected
to rise sharply at global level. About 3 million people, i.e., 40% of
the current world population do not have access to clean and safe
drinking water [5]. In addition, 90% of infectious diseases are
caused by consumption of unsafe water. Moreover, the World
Resources Institute predicts that by 2025, at least 3.5 billion people
will experience water shortages [6]. Global agencies (including
WHO, UNDP, UNICEF, etc.) expect that 24 of the least developed
countries, many of them along coastal areas without access to
freshwater and electricity, need to more than double their efforts
to reach the Millennium Development Goals (MDGs) for basic
health, sanitation, and welfare. Seawater can serve as an excellent
water source in many of these countries which also indicates the
need for development of sustainable technologies for water
production.
The relation between water and energy source production and
utilization is inseparable (Fig. 1). Provision of clean water inevitably
requires energy, which is currently being provided essentially
by nonrenewable fossil fuels. It has been estimated that
production of 1 m3 of potable water by desalination requires an
equivalent of about 0.03 tons of oil [7]. Extraction and refining of
fossil fuels and production of energy not only places additional
demands on water, but also results in pollution of water sources
and air (greenhouse gas emissions). Thus, the projected global
demand for clean water supplies for the future will significantly
accelerate not only depletion of fossil fuel reserves but also concomitant
environmental damage and emission of greenhouse
gases [8]. This situation provides the basis for renewable energy