The seawater forms 97% of water of the world, and the icecaps account for 2%. The portion of water available for human consumption is barely 0.5% and that also in aquifers. The widening gap between availability and requirement of potable water especially in the Gulf countries has necessitated newer methods and technologies for production of water suitable for human consumption. This situation in the Gulf region has developed due to non- availability of surface water, high growth of population and urbanization, and water depletion. The present paper takes account of the desalinization of seawater plant established in Abu Dhabi in the year 1984. The plant was successfully run for its full term up to 2002. The design of the plant was finalized taking into account the climatic, environmental, and geographical conditions of the area near Abu Dhabi where the plant was planned to be established. The solar energy was used to run the plant.Solar desalination plant was designed to produce on average 80 m3/d of fresh water by using evacuated-tube solar collectors. There had been heated collector system, heat accumulator system, and other systems that were operational with the help of radiation from the sun. The desalinization of seawater was based on vaporization and condensation of the water to make it fit for human consumption. It was a solar energy; that was employed for the desalinization, and hence, the process besides being cost effective, was the environment friendly. The successful run of this plant has opened new vistas for use of renewable energy to protect the environment and to preserve fossil fuels.
The desalination of sea water refers to the processes of making the sea water potable. More than 97.5% of water on earth contains salt, and desalination is commonly used process to make it potable. Almost 16 billion gallons of desalinated water per year is treated only in the countries of the Persian Gulf. In addition, the area has been the hot spot of intense desalinization activities for long, but other areas of the world such as regions of the Red Sea and the Mediterranean, China, California, and Australia are also involved in the desalinization of seawater. The desalination innovations have been presented successfully in last three decades to enhance the water availability in arid areas of the globe. Because of the restriction of higher desalination prices, several countries are not able to pay for these innovations as a fresh water resource. As a matter of fact, the adaptation of salt water desalination modern technologies by some nations have shown that sea water desalination can provide a new water resource independent of variations in rainfall. Although sea water desalination modern technologies is developed enough to be a reliable source of fresh water from the sea, researches are being carried out by many institutions in order to constantly improve the innovations and minimize the expense of desalination (Khawaji, Kutubkhanah & Wie, 2008).
The widening gap between availability and requirement of potable water especially in the Gulf countries has necessitated newer methods and technologies for production of water suitable for human consumption. This situation in the Gulf region has developed due to non- availability of surface water, high growth of population and urbanization, and water depletion. The use of fossil fuels for desalinization of seawater in remote areas is cost prohibitive due to non-availability of electricity, and high cost of transportation of fossil fuels and managing of air pollution. The seawater needs to be desalinized in order to produce enough of potable water for the people of this region. However, this area gets abundant sunshine that could be utilized to produce potable water. That is why; desalinization of seawater has been the major source of clean water in this area. The production of potable water by the process of desalinization has been considered as the most viable alternative for the area. The present paper takes account of the desalinization of seawater plant established in Abu Dhabi in the year 1984 and was successfully run for its full term up to 2002.
A Synopsis on Abu Dhabi Sea Water Desalination Plant
Abu Dhabi, capital of United Arab Emirates (U.A.E.) is virtually an island on the midst of the sea. Being situated in the subtropical zone, the summer of the country is very hot with temperatures ranging from 35 to 48 degrees Celsius. There is no river in the country, and the brackish underground water is limited. That is why; the majority of drinking water is derived there by the desalination of the water of the sea. However, the country gets sunshine in abundance with daily radiation averaging 8 Kwh square meters on the horizontal surface. With so limited resources of fresh water, but solar radiation available in such high magnitude and it become the obvious to explore the possibility of use of solar energy for the treatment of sea water by desalination process. The adoption of solar energy for the treatment of sea water, especially in the remote areas where supply of oil and natural gas is difficult, is logical and warranted (EL-Nasher & Ishii.1985).The use of fossil fuels for desalinization of seawater in remote areas is cost prohibitive due to non-availability of electricity, and high cost of transportation of fossil fuels and managing of air pollution. The seawater needs to be desalinized in order to fulfil the increasing demand for potable water in this region, but conventional processes of desalinization being energy intensive, the high cost of energy especially in remote areas increases operating expenses. Besides the cost of energy complications, none can undermine environmental concerns due to the use of conventional sources of energy. The usage of fossil fuels gives rise to greenhouse gases that pollute the environment leading to ozone depletion and global warming. Hence, renewable sources of energy like solar energy has to be used to forestall, at least reduce, environmental pollution (EL-Nasher, n.d.).
Abu Dhabi that had a solar desalination plant started in September 1984. It has oil and natural gas in abundance, but it experimented with renewable energy, the solar energy, for the production of potable water. The plant ran successfully for its slated life till 2002 and was thereafter dismantled. It was aimed to have a measure of economic and technical feasibility of such plants in providing potable water to the people of far flung areas of the Middle East. The reliability and performance of such plants were aimed to be evaluated.
The plant proved its utility and technical feasibility in operation that required a little medication of the plant to mitigate maintenance problems. A routine regimen was evolved to ensure high performance of the plant. It also proved its cost effectiveness in comparison to a plant run with fossil fuels. (EL-Nasher, n.d.).
The Abu Dhabi solar desalination plant was set up at a place adjacent to the Sas Al Nakhl (Umm Al Nar) power and Desalination station around 20 miles to the east of Abu Dhabi. The project aimed to reduce the adverse effects on the environment by deploying latest technologies, innovative solutions, best management practices, and renewable energy resources ((EL-Nasher, n.d.; Solar desalination, 2014).
The Department of Water and Electricity, Abu Dhabi, reached an agreement with the New Development Organization (NEDO) of Japan to put the idea of setting up an energy efficient plant for desalinization of seawater into practice. Both countries decided to take up a joint program of research and development to testing the operational performance by the system of MES distillation supplied by thermal energy. The solar energy was planned to be generated by solar collectors, while the plant design and construction were the responsibilities of NEDO. The figure1 is presenting the view of Abu Dhabi solar desalination plant.
Figure 1 View of Abu Dhabi Solar Desalination Plant
Source Adopted: El-Nasher, (n.d.).
Functioning of the Plant
The Abu Dhabi solar desalination plant was designed to produce on average 80 m3/d of fresh water by using evacuated-tube solar collectors. These solar collectors were oriented towards the sun to get maximum solar radiation so that collector fluid could be heated to a temperature of around 99 degree Celsius. The bank's collector area was 1862 square meters. The fluid of the hot collector left the collector bank and flowed to the top part of the heat accumulator of 300 m3 capacity. The stratified liquid type dimension of the heat accumulator and the variation of density in the top layer and the bottom layer, the water with higher temperature got located in the uppermost part while the low temperature water accumulated in the lower part. The water with lower temperature of 87 degree Celsius was taken out from the bottom of the tank and pumped by the heat collector pump through collectors. The heating water circulating pump took out the hot collector fluid from the top of the tank, and the fluid was made to flow by force to the heating tubes to the first of 18 effects of the MES evaporator. The evaporator had the capacity of producing 120 m 3/d of distilled water from the sea water with salinity of 55000 ppm. The hot collector fluid is discharged into the accumulator after being cooled down from 99 to 87 degree Celsius (EL-Nasher, n.d.).
The MES evaporator possessed 18 effects each kept on top of the other in a double-stack arrangement with effect one having the highest temperature and the effect 18 having the lowest temperature. All the 18 effects were set in a double-stack arrangement such that the odd number of effects 1,3,517 were kept in one stack while even number effects like 2,4,6.18 were in the second stack. The evaporator also had a final condenser which condensed the vapor accumulating in the last stage, effect 18. The collector fluid passed through different stages evaporating a part of it. The vacuum in the evaporator was created by a displacement pump attached to the ultimate condense. The pressure of condense gradually increases from the first effect to the last one where it maximizes at 50 mm Hg.
The vapor created in the last stage, effect 18, is condensed by the use of sea water while a portion of discharged water of the sea coming out of the final condenser comes back to the sea. The other part of the water was the feedwater of the evaporator. The feedwater that passed through 17 preheaters with one preheater connected at each of the 17 effects barring the last one, had a flow rate of 17.3 m3/ h. The preheaters were so designed as to increase the temperature of the feedwater gradually upward during its flow to the top effect from the bottom one (EL-Nasher, n.d.).
Thus, the process of desalination of seawater in the plant at Abu Dhabi is primarily based on evaporation and condensation that lead to distillation. The seawater passes through the 18 effects or evaporators and condensers in the desalination process. The necessary heat for evaporation comes through solar energy collection system which collects solar energy during sunshine through collector bank, and stores it in the heat accumulator. The heat accumulator makes available the thermal energy to evaporators with little fluctuations in the supplied temperature.
Design Aims and Conditions
The plant was designed keeping in view the following conditions prevailing in the area.
- The Gulf Area's climatic conditions that are dusty, but scorching sunshine.
- The plant should be easily maintained and operated.
- The operation should be automatic.
- The evaporator should be of high quality capable of enhanced performance ratio.
- The heat collectors should be highly efficient.
- The plant should be operated 24 hours a day despite fluctuations in solar heat source.
- The accumulation of dust and dirt on heat collectors. It was predicted that dirt and dust would cause a reduction of solar radiation by 10%. (EL-Nasher & Ishii.1985).
The Heat Collector Subsystem
Thus, solar energy collection system consists of two subsystems, one, the solar heat collection subsystem, and the second, heat accumulator subsystem. Both subsystems are connected to each other through valves, pumps, and piping. The basic unit in the solar heat collector subsystem, the evacuated tube solar collector is a collector flat plate type which deploys coating absorber plates within glass tubes. These glass tubes are kept in high vacuum to the level of 10m4 mm Hg. Each collector incorporates ten glass tubes along with absorber plates. The collector fluid passing through the center pipe absorbs the collected solar energy. There are ten individual tubes in each collector kept in parallel. The collector fluid passes through these tubes so arranged leading it to pass first five in one direction and the rest five in the reverse direction. Each collector's area of absorption measures 1.75 square meters and has a black coating with an absorption factor a = 0.91 and an emissivity e = 0.12. The absorber plate is tilted at an angle of 21’09’ due south. All groups are attached in parallel while each is equipped with two isolation valves at the entrance and the exit. The detailed flow diagram of the process with different sections is presented in Appendix-1(EL-Nasher, n.d.).
The Heat Accumulator Subsystem
Another component of seawater desalination plant of Abu Dhabi is the heat accumulator subsystem which accumulates the heat collected by the heat collector subsystem during daytime when sunshine is available. It provides uninterrupted thermal energy to the evaporators. It has three carbon steel tanks with a capacity of 300 m3 and contains water at a temperature around 74 to 99 degree Celsius. The insulation of tanks is done with a layer of 100 mm fiber glass to prevent loss of heat to the blowing air. The three tanks are cylindrical with the same diameter of 3.8 meters, wall thickness 9 mm, but different heights. While the first tank has a height of 10 meters, the other two have the same height of 7.6 meters. The hot water collected from the bottom of the last tank 3 is passed on to the top of tank 1. The water at the top of tank 1 passes on to the evaporator and then back to the tank 3 bottom. The stratification of the water is managed in such a manner that the uppermost water layers of tank 1 are consistently at the highest temperature while the water layers at the bottom of tank 3 are at the lowest temperature.
The tanks have enough capacity to store thermal energy that can last up to 16 hours after the sunset if the tanks have been charged to the full. It facilitates the operation of the desalination plant even during the night, and thus, 24 hours a day. Only in the case of prolonged overcast sky or hail storm the plant had to be shut down (EL-Nasher, n.d.)..
There are 18 evaporators used in the process of desalination of the seawater in this plant. The heated feedwater is sprinkled into the top part of the first effect, and it comes down the stack of the evaporator, passing as a thin film on the bundle of tube in each evaporator or effect. The feedwater gets cooled by many degrees as it flows down different effects. This feedwater gets collected as cool brine at the bottom end of the plant. The hot water flows down from effect one to the last effect 18, and in the process part of it gets evaporated in each effect. In effect 1, the accumulator hot water is utilized to evaporate partially the thin seawater film on the outside of the tubes. The vapor created flowed to the inner side of the tube in the effect 2 where the condensed part of water formed the product. It further vaporizes, and the process goes on repeating in each of the 18 effects. The input of heat from the accumulator had been used again and again during each heat exchanges in evaporation and condensation in each effect for the production of more new vapor and product. It ultimately desalinizes the seawater. The vapor collected in the last effect gets condensed in a seawater cooled condenser. A part of seawater is utilized as the stack's feedwater while the remaining part of the seawater that carries with it almost all the heat away from the process is discarded.
The desalinization plant of Abu Dhabi had been a test project undertaken in collaboration with Japan which was responsible for its designing, commissioning and production. The agreement was signed b/w the two countries for the setting up of this plant proved a milestone in meeting the need of potable water worldwide. This plant at Abu Dhabi, U.A.E. clearly demonstrated that given the will both the mitigation of miseries of the people and the conservation of the environment are possible. The renewable energy, the solar energy, was used to run the plant, and its successful run teaches a lot to the entire humanity.
The operation of the plant was based on the production of thermal energy by the use of radiation from the sun. The heat collecting and accumulating systems were the essential components of the entire desalinization process. While the heat collecting system collected heat from the sunshine during the day, heat accumulating system stored the energy for use during the night so that the plant is operational 24 hours a day. The basics behind desalinization process had been the evaporation of seawater at different temperatures and its condensation. It was through evaporation and condensation that the saline seawater was treated for human consumption. High quality heat collecting, heat accumulating, and evaporating equipment made the plant a success story. The performance of the plant proved that evacuated tube solar collector, the tanks, and the evaporators or effects functioned properly, and there was no noticeable performance decline over a period. The rube bundles and other devices did not suffer any performance degradation. The process also proved cost effective, and the output had been as per the expectations.
There is a popular perception about the renewable energy about it being cost prohibitive, but given the condition of the Gulf area which though abounds in natural gas and oil, cannot use the fossil fuels due to the lack of infrastructure in far flung areas of the country. Also, the stock of fossil fuels being limited has to be preserved. At the same time; the use of conventional energy resources has environmental complications that threaten the existence of the entire humanity. Hence, it is imperative that the renewable energy sources such as solar energy be extensively used in place of fossil fuels. This desalinization plant of Abu Dhabi amply demonstrated that the solar energy could be used to run such a large plant successfully. It also achieved the expected output. The plant that was commissioned in September 1984 was dismantled in 2002 after its fruitful use for the stipulated period.
EL-Nasher, A.M. (n.d.). Multiple effect distillations of sea water using solar energy-the case of Abu Dhabi solar desalination plant. Solar Energy Conversion and Photoenergy Systems, 2, Retrieved from http://www.eolss.net/sample-chapters/c08/e6-106-30.pdf\
El-Nasher, A.M. & Ishii, K. (1985). Abu Dhabi solar distillation plant, Desalination, 52,217–234.
Khawaji, A. D., –Kutubkhanah, I. K., & Wie, J. (2008). Advances in seawater desalination technologies.Desalination, 221, 47–69.
Solar desalination. (2014). Wikipedia. The free encyclopedia. Retrieved from http://en.wikipedia.org/wiki/Solar_desalination.
Figure 2 Schematic Flow diagram of different processed of solar desalination plant, Source adopted: EL-Nasher & Ishii.(1985)