Water scarcity presents one of the most major problems facing many countries today. According to the United Nations, more than two billion people are living now with the risk of encountering water shortage. Each day, around one thousand children die due to polluted water and sanitation related diseases. It is expected that by 2050, at least one in four people is likely to live in a country affected by chronic diseases resulting of reduced access to fresh water resources. Fortunately, desalination can be one of the solutions of the water shortage problem. It can offer unlimited access to pure water by removing salt from vast oceans surrounding us. Many advantages of desalination are reported in in the work of Cooley, Gleick, & Wolff (2006). Some of these advantages are high water quality, system reliability, unlimited water access and the ability to preserve current freshwater supplies. However, Cooley et al. and Einav, Harussi, & Perry (2003) also pointed out many disadvantages of using desalination as the high cost of building and operating desalination plants, the large land needed to build desalination plants, the possibility of hurting the marine environment, the potential of affecting human health, the noise pollution resulting from the high pressure pumps, and especially the intensified use of energy.
Food & Water Watch, (2009) stated that the cost of energy is the most expensive factor for seawater desalination. A lot of energy is consumed to pump water across membranes to remove salt. The reverse osmosis (RO) plant for example requires large amount of energy to intake water, to use high pressure pumps, and to build transport and waste disposal systems. In addition, as the amount of the salt in water increase, the consumed energy, and thus the cost increase too. Although there is an improvement in the equipment used in desalination, the increase price of oil around the globe prevents the cost of desalination technologies to decrease.
The authors, Rao, Morrow III, Aghajanzadeh, Sheaffer, Dollinger, Brueske, & Cresko (2018), reported that the amount of energy used by desalination facilities can be over twenty five times larger than the energy used by freshwater systems. They also reported that the average use of water per person in the U.S is 0.37 m3/day and that the energy required to produce 1m3 of pure water is equivalent to the energy consumed by a typical U.S. home in 2.6 h. Rao et al. reviewed in their paper the existing energy requirements for membrane and thermal-based seawater desalination systems to produce clean water. They evaluated two categories of energy intensity:
- State-of-the-Art (SOA) which describes the lowest energy intensity based on best commercially available technology.
- Thermodynamic Minimum (TM) which describes the minimum energy intensity under ideal conditions.
They divided the desalination system into five unit operations: intake, pre-treatment, the desalination process, post-treatment of the product water, and disposal of the concentrate (concentrate management) and reviewed the energy requirement of each operation. The main energy requirement for the intake operation is to pump seawater from the ocean to the plant location. The required energy is directly proportion with the density of seawater and the gravitational acceleration, and inversely proportion with the pump efficiency and the motor efficiency. For the post-treatment operation, the energy consumption is calculated as the energy of the combination of flocculation, coagulation, sand filtration, and cartridge filtration. To estimate the energy usage of the desalination process, the intensity values for various desalination technologies are evaluated in the paper. It was shown that RO has the lowest reported energy intensity of any of the technologies reviewed. The energy required by the post-treatment operation is the sum of the energies used by the remineralization, the disinfection, and the fluoridation technologies. As the energy requirement of the concentrate management is primarily due to pumping, the energy intensity was assumed to be identical to the intake energy intensity. The energy intensities reported in this paper are shown in Table 1.
Where kWhe is the kilowatt-hour of electrical energy. This is used when the energy source is entirely from electricity. kWhT,equiv is the kilowatt-hour of total electrical equivalent energy. This is used when a system component (or combination of system components) employs both electrical and thermal energy. So, the intent of this paper was to better understand the energy implications of seawater desalination.
The paper presented by Semiat (2008) mentioned that “the energy cost of an optimized desalination plant is approximately 30-44% of the total cost of water produced.” This paper reviewed the actual energy demand for different desalination systems. The evaporation technique is the simplest desalination technique with single-stage evaporation of seawater, but consumes a large amount of energy, depending on the evaporation temperature. Some evaporation techniques as multistage flash evaporation (MSF) and multieffect distillation (MED) were able to mitigate this issue by reusing energy consumption through multiple stages. Mainly the energy can be reduced by reusing the heat released by the turbine condensing steam to provide the primary thermal energy required for an MSF or MED desalination processes. On the other hand, the development of large pumps with a 90% efficiency leads to the decrease of the energy consumed by the RO desalination technique. This technique uses turbines to convert the concentrate pressure into the velocity of a jet that spins a wheel. This is used either to reduce the power consumption of the pump motor or to rise the pressure of the feed to a second stage. However, the low efficiency of smaller pumps may increase the energy consumption of small RO plants. In the forward osmosis (FO) desalination process, the water is separated from the higher pressure osmosis solution using magnetic nanoparticles. The removal of theses nanomagnetic particles against sheer forces in high flow rate devices involves strong magnetic fields and thus high energy consumption. The main problem of the membrane desalination techniques in general is the need to water evaporation which requires a large energy usage. The energy reuse may reduce this enormous amount of energy. If energy is reused more than ten times in the desalination plant, the energy demand may be cut to the tenth.
Al-Karaghouli and Kazmerski (2013) pointed out that the energy cost represents 50% of the total water cost. They stated that the MSF distillation is an energy-intensive process that requires both thermal and electrical energy. The thermal energy is in the form of low-pressure bleed steam and the electrical energy is required for driving the pumps. The energy consumption of the MSF depends on various factors: maximum temperature of the heat source, temperature of the heat sink, salt concentration in the flashing brine solution, number of stages, construction materials, geometrical configuration of the flashing stage, and design configuration of heat exchange devices. Regarding the mechanical vapor compression distillation, it depends on the heat generated by the compression of water vapor to evaporate salt water. The electrodialysis (ED) uses electrical current to move salt ions to obtain clean water. The RO plant uses electrical energy to drive the pumps to generate the pressure needed to force the water to pass through the membrane. Energy consumption of the RO unit depends mainly on the salinity of the feed water and the recovery rate. Therefore as the salinity of water increases, it requires a higher amount of energy due to higher osmotic pressure. The energy consumption of the RO plant is lower than other desalination processes due to the high energy needed by other processes for water vaporization, and to the continuous improvement in the technology of the RO process membrane. However, the electrical energy consumed by the RO still costs around 44% of the total water cost.
Due to the enormous amount of energy used by desalination processes, renewable energy sources are an alternative way to derive desalination technologies. Different sources of renewable energy are solar energy, wind power, nanotechnology and nuclear energy. These sources are recognized as effective and feasible solutions towards energy‐efficient desalination.