Water Desalination Essay.
The supply of freshwater is a limiting factor in any civilization. Freshwater is defined as waters having less than 1% dissolved solid content. Freshwater is obtained from rivers, lakes, streams and underground sources. Early civilizations were situated near freshwater bodies, enabling the population to bathe and drink, the crops to be regularly watered and livestock to be adequately hydrated. Today, freshwater is utilized in a variety of ways. The most important use of freshwater is still for human consumption (drinking).
Other uses are for irrigation of agricultural crops, for livestock and for lubricating or cooling industrial machineries.
However, freshwater accounts for only a small percentage of the total water content of the earth. Out of this small percentage, only about is accessible to man, the rest is locked up in glacial ice, or stored deep underground. The vast majoriy of the earth’s water is brackish or salty. This means that it has a high dissolved solid content. This gives it a salty taste, and is generally unsuitable for human, livestock and crop utilization.
Water desalination is a broad term for a wide range of different processes which reduces the total amount of solids dissolved in water. It is usually used to generate freshwater from sea or brackish water sources. Water generated through desalination is utilized either for human consumption, industry use or agricultural irrigation. (Smith & Shaw, 1999) Desalination was said to have started from the need of seafaring vessels to have a supply of drinking water. The earliest method of obtaining freshwater from salt water uses the process of distillation.
The process uses heat to evaporate seawater, separating the dissolved solids from the water vapor, and then condensing the vapor to obtain water. Other means of desalination include ion exchange, electrodialysis, reverse osmosis using standard membranes and reverse osmosis using high resistance membranes. Water desalination is highly utilized in coastal, Middle Eastern countries, where the lack of surface fresh water and the proximity and abundance of seawater in the area makes desalination a viable option. Desalination in Abu Dhabi, United Arab Emirates
Abu Dhabi is the capital city of the United Arab Emirates. It is located on a small island on the western coast of the Arabian Gulf. It has a population of 942463 (Census, 2005) the highest among all the Emirates. The United Arab Emirates ranks third in world per capita water consumption (550 liters per head). (Todorova, 2008) Groundwater constitutes 81% of Abu Dhabi’s water supply (805 of which is brackish and 1% fresh), desalinated seawater accounts’ for 15% and 4% is supplied by treated wastewater plants. Abu Dhabi has four main desalination plants; Mirfa, Abu Dhabi, Um al Naar and Taweelah.
(ERWDA, 2002) The largest desalination plant located at the Mirfa power plant uses heat from the fossil fuel powered turbines to desalinize seawater through multi stage flash distillation process. The United Arab Emirates’ power generation and consumption are among the highest in the world. And it is still increasing as the Emirates grow in prosperity and in population. Almost all of the energy consumed in the United Arab Emirates is generated using fossil fuel, about 64% percent from natural gas and the remaining 36% from oil.
(EIA, 2007) Figure 1. 0 Electricity generation of United Arab Emirates form 1994 to 2004 (EIA, 2007) In Abu Dhabi, power generation goes hand in hand with water desalination. Power plants utilize modern and advanced gas turbines which burn natural gas and is capable of burning oil also. Heat generated from the gas turbines is recovered in heat recovery steam generators or boilers before combustion gases are vented to the atmosphere. Boiler water is converted into steam that drives steam turbines to generate electricity. (EAAD, 2007)
Although steam loses energy upon contact with the turbine, it retains enough heat to exchange with seawater in a brine heater. This seawater will have gradually gained heat in Multi-Stage Flash Evaporators before reaching the brine heater, where seawater temperature will rise even more due to steam from the turbines. After exiting the brine heater, the heated seawater will be flashed in the distiller to convert it into pure water vapor that condenses into potable water. (Wagner, 2007) Figure 2. 0 Multi Stage Flash Distillation operation scheme. (Zactruba)
In the multi stage flash distillation process, incoming water (feed water) is heated and is transferred to a series of chambers with increasingly lower pressure. This process utilizes steam from the previous chamber to heat the next so that the latent heat is transferred. The Taweelah plant on the other hand uses both multi stage flash distillation process and multi effect distillation. Multi effect distillation uses tube walls to heat flowing feed water. The vapor produced from this is then utilized to heat the heat exchange tubes in other chambers.
The condensate in each heat exchange tubes is collected and extracted. (Wagner, 2007) Figure 3. 0 Multi effect distillation process (Bartosh, Boerboom, Brzenski, & Checka) Aside from distillation technologies, another method is through the use of membranes, which trap the solutes and allow the pure, free solvent to come out. Solar-powered desalination With the increasing demand for electricity, water and also an international clamor to cut fossil fuel use and resort to renewable sources of energy, Abu Dhabi has a variety of options in terms of water desalination.
Foremost among these options is the use of solar energy to generate electricity as well as to operate a desalination plant. Solar energy is highly viable in desert countries which have an almost consistent yearly day length and high mean temperatures. (Kearney, 2008) Measurement of annual solar radiation in Abu Dhabi indicates that it has a high potential for solar energy generation. (Islam, Kubo, Ohadi, & Alili, 2009) Two general types of technology exist for trapping solar energy. Photovoltaic solar panels (PV) use the sun’s energy to excite electrons and induce a direct flow of current.
Solar thermal energy or concentrating solar power on the other hand uses the sun for heat, and that heat is then utilized to generate electricity. (Kearney, 2008) Figure 4. 0 Solar Desailination (TERRIN) Solar technology requires a high initial budget to obtain the facilities and the technology. Another setback to solar technology is that it is extremely hard to generate electricity consistently for 24 hours since the area is only exposed to the sun for a certain period of time and current battery technology is still inefficient in storing vast amounts of power.
Other forms of renewable energy capable of sustaining desalination systems include geothermal, wind, tidal and reverse osmosis technologies. Geothermal energy utilizes thermal vents from below the Earth’s surface to power steam driven turbines. Reverse osmosis technology uses the difference in osmotic pressure to induce the flow of water, possibly through a turbine which generates electricity. Figure 5. 0 Possible combinations of renewable energy sources with desalination technologies. (Drioli & Curcio, 2001) Conclusion Abu Dhabi is ranked among the highest in terms of current and projected energy and water consumption.
Thus, there is a need to develop technologies which would meet these demands for the future, while shifting away from fossil fuel usage and ground water tapping. The high energy need of Abu Dhabi has lead to cogeneration of water from the sea, a practice that can be continued, albeit power generation will now come from renewable sources of energy. The climate of Abu Dhabi is ideal for solar energy generation and with revenues obtained from tapping into oil and gas reserves in the past, the initial expense of building facilities to tap the sun for energy can be covered.
The future of desalination using solar technology in Abu Dhabi is a promising one, however, further research and development is needed to reduce the initial costs as well as to improve upon existing energy storage technology to further maximize electricity from the sun.
Census, T. -A. D. (2005). . Retrieved from TEDAD Census. Drioli, E. , & Curcio, E. (2001). A study on modern desalination technologies in the Gulf