MED versus MSF and Reverse Osmosis - Research Paper Example

?Harris Kamran Renewable Energy Research Paper 16 July MED versus MSF and Reverse Osmosis In the incessantly developing social and technologicalconditions, the uninterrupted and adequate availability of clean and cheap energy is the staple need, and the prerequisite for the economic stability and growth globally (Eltawil, Zhengming, and Yuan 2009: 2245). However, the use of conventional fuel types is not sufficient anymore to meet the increasing needs of the population, and with the growing concern for the environment, the use of these fossil fuels is frequently frowned upon, owing to their voluminous carbon foot prints, that is, the capacity to release green house gases (Eltawil, Zhengming, and Yuan 2009: 2246). Therefore, there has been a considerable interest in the use of renewable or alternative energy sources to meet the demands of the industry (Eltawil, Zhengming, and Yuan 2009: 2245), since the supply of these renewable forms is potentially unlimited and the generation of energy is almost completely clean. That, and given their low cost of maintenance (Eltawil, Zhengming, and Yuan 2009: 2245), such energy sources, especially the harvesting of solar energy, has been getting much attention lately. The ultimate use of solar energy is for the desalination of brackish water in order to produce and supply WHO approved drinkable water to areas that are almost deprived of fresh water resources (Eltawil, Zhengming, and Yuan 2009: 2246). Water is the fundamental need of humanity, and a fundamental requirement for the functioning of industry and society. However, the overwhelming ratio of water is in the form of seawater or other forms of brackish water; it is estimated that only 3% if water is available as fresh water, and out of that, less than 1% is accessible (Eltawil, Zhengming, and Yuan 2009: 2246). These statistics are alarming even without the population spurt that is estimated to occur over the coming years; it is predicted that this population bloom will effect the developing countries the most, with Africa experiencing a 50% increase in its population, and Asia 25% (Eltawil, Zhengming, and Yuan 2009: 2246). It coincides with the fact that it is mostly in these very developing countries that there is the most dire shortage of fresh water; the Middle East has almost completely run out of all fresh water supplies (Eltawil, Zhengming, and Yuan 2009: 2246). Already, over a billion of the world’s population has no access to fresh water, and the demand is likely to increase by 40% of the present amount (Eltawil, Zhengming, and Yuan 2009: 2246). In light of these statistics, it has become imperative that new avenues be explored that should provide with an easy, cheap and clean method of fresh water production. To that end, the utilisation of solar energy is the most effective. This paper analyses three methods of harbouring this technology, namely Multi-effect distillation or MED, Multistage Flash Distillation or MSF, and Reverse Osmosis, the distribution for which is given below. Fig. 1: “Distribution of renewable energy powered desalination technologies” (Eltawil, Zhengming, and Yuan 2009: 2248). It explores which of these methods is the most efficient and cost-effective. It will start with a discussion of each of these methods, as follows, followed by a study of advantages of MED over the other methods. Multi-effect distillation, or MED, is the most common and the oldest of all the techniques used for freshwater desalination (Bruggen 2003: 7). It harnesses the concept of evaporation-condensation, in that steam obtained from brackish water by evaporation is allowed to condense at a low temperature and pressure, and the heat thus given off is utilised to evaporate more water from the brine (Bruggen 2003: 7). The water produced upon condensation of steam is collected as non-saline fresh water; the ultimate goal of the set up. It is obvious that initially some energy from an external source would be needed in order to produce steam and start the cycle; the rest of the steps in the cycle are known as effects (Bruggen 2003: 7). This initial energy can come from two sources: electricity or solar energy, but mostly solar energy is utilised for this purpose (Agha, Wahab, and Mansouri n.d.: 5). The solar energy is collected through the use of flat-plate collectors, solar panels and towers, and solar ponds (Agha, Wahab, and Mansouri n.d.: 5). A comparison between the energy needed in the form of electricity and thermal energy reveals that it requires “0.75-1.75 kW/h/m3” of electrical energy, and “30-120 kW/h/m3” of thermal energy (Agha, Wahab, and Mansouri n.d.: 5). It might seem like electrical energy would prove more cost-effective, however, the production and maintenance costs of electrical energy come to be much more than the direct use of thermal energy. The downside of MED is the corrosion of the metallic surfaces that come into contact with the brine, like the condensers and heat exchangers (Bruggen 2003: 7). Fig. 2: “Schematic presentation of horizontal tubes multi-effect distillation (MED) plant” (Eltawil, Zhengming, and Yuan 2009: 2251). The other method that is frequently used is that of Multistage flash desalination technique, or MSF. This technique was developed more recently as compared to MED, and was designed to overcome some of the problems caused by MED, such as erosion (Bruggen 2003: 7). It also involves a different approach to desalination than evaporation-condensation: the brine or seawater is eveaporated under low pressure to obtain steam, which is then condensed to form fresh water (Bruggen 2003: 7). The chambers for this low pressure evaporation are arranged in series and are called flash chambers; hence, the name multistage flash (Bruggen 2003: 7). The heat released during condensation of steam is used to preheat the seawater for evaporation (Bruggen 2003: 7). The solar energy is collected, again, through the use of flat-plate collectors, solar panels and towers, and solar ponds (Agha, Wahab, and Mansouri n.d.: 5). A comparison between the energy needed in the form of electricity and thermal energy reveals that it requires “2.64-3.96 kW/h/m3” of electrical energy, and “55-120 kW/h/m3” of thermal energy (Agha, Wahab, and Mansouri n.d.: 5). Fig. 3: “Schematic diagram of a basic multi-stage flash (MSF) desalination process” (Eltawil, Zhengming, and Yuan 2009: 2250). The third most common technique and relatively latest in developed as compared to the other two techniques is that of reverse osmosis. This method depends on a principle entirely different from that of MED or MSF; there is no evaporation or condensation for the desalination (Bruggen 2003: 8). Instead, the method relies on the principle of the difference in diffusivity of the particles, which in turn depends upon their size and the ionized state (Bruggen 2003: 8). The separators are polymer membranes or sheets which act as filters to separate brine from fresh water (Bruggen 2003: 8). The salts in the brackish water do not cross over mostly due to their ionized state (Bruggen 2003: 8). This process requires low pressures (Bruggen 2003: 8). This process depends solely on electrical energy, and if thermal energy needs to be used, then photovoltaic cells have to be employed (Agha, Wahab, and Mansouri n.d.: 5). The electrical energy required for this process is “5-7 kW/h/m3” (Agha, Wahab, and Mansouri n.d.: 5). It is quite effective due to its high efficiency, especially when used in conjunction with MSF (Bruggen 2003: 8). Fig. 4: “Schematic of a simple reverse osmosis (RO) system” (Eltawil, Zhengming, and Yuan 2009: 2252). All the three methods of desalinating brackish water using thermal energy are beneficial for the production of fresh water and for meeting the ever-increasing needs of the growing population. Each of the three techniques has its pros and cons as regards to the industrial set up, maintenance, and the overall yield of production. A study of the three systems in depth reveals that the best possible technique for desalination might be the multi-effect, MED. The reasons for this inference are as follows. Let us compare the figures discussed earlier with the discussion of each technology about the energy consumption for each m3 of fresh water obtained. Since the reverse osmosis method does not employ thermal energy directly, but needs photovoltaic cells for its conversion into electrical energy (Agha, Wahab, and Mansouri n.d.: 5), we will consider the values in terms of electrical energy only. It becomes evident that reverse osmosis requires the highest amount of electrical energy per cubic meters of fresh water obtained, followed by MSF and MED, the latter requiring the least energy for operation. This is a huge plus point in terms of the costs incurred for the production and generation of energy required to run the processes; if the energy need is less, the operational costs will be less. MED not only has the least operational cost, but also the start-up cost and the costs for maintenance are also the least of the three (Agha, Wahab, and Mansouri n.d.: 9), the reasons for which will be discussed shortly. Being the cheapest gives this technology the advantage of being adequate for the economically weak areas of the world, such as the developing countries, which have the greatest need for desalination, as already discussed (Eltawil, Zhengming, and Yuan 2009: 2246). The very fact that reverse osmosis cannot utilise thermal energy directly puts it at odds with the other methods of desalination under discussion. The cost of converting solar energy first into electrical energy by photovoltaic cells and then using those cells for the desalination process are higher than the costs incurred for the direct utilisation of the thermal energy, as with MED and MSF. This means that it is the most expensive of the three techniques to run (Eltawil, Zhengming, and Yuan 2009: 2256). In terms of efficiency as well, this mechanism reduces the yield of the process. The advantage of MED in this regard is that it relies upon fairly raw energy input, with almost no preprocessing of the initial energy required to start the cycle of effects (Agha, Wahab, and Mansouri n.d.: 9). This means that it can be set up almost anywhere with minimum hassle of pre-production. Again, this is helpful in the rural areas. Another advantage of the MED over the other two systems is the minimal pretreatment of its raw material (Agha, Wahab, and Mansouri n.d.: 9), the brackish water, as compared to the others. MSF requires some form of preheating of the water (Bruggen 2003: 7), as does reverse osmosis, in which the water needs to be cleaned of some impurities to prevent fouling (Bruggen 2003: 8). This ensures a speedy start-up for the MED, with less set up costs. This points towards a special factor that needs to be taken care of while operating reverse osmosis especially, and even MSF: the poisoning of the equipment as a result of either constant exposure to brine (Bruggen 2003: 7) or due to reactions with other chemical agents in the seawater. The polymer membranes of the reverse osmosis are especially vulnerable to poisoning, a process known as fouling (Bruggen 2003: 8) of the membranes. These, then, have to be changed almost every 5-7 years (Eltawil, Zhengming, and Yuan 2009: 2256). MSF, although as discussed, reduces the occurrence of erosion often encountered in MED plants, does undergo poisoning especially if the material used for the manufacturing of its flash chambers is of low quality or durability (Eltawil, Zhengming, and Yuan 2009: 2256), as might be the case with plants operating in areas that are already deficient economically and need to save on their start-up costs of the plants. MED, comparatively, has a higher tolerance to poisoning and erosion (Eltawil, Zhengming, and Yuan 2009: 2256), and can function in even highly contaminated waters. The product quality, on the other hand, is greatest of the MED (Eltawil, Zhengming, and Yuan 2009: 2256) as compared to the other three even though it can stand the dirtiest waters. In reverse osmosis, especially, the salt content of the fresh water, though still within the recommended limits by the WHO, is, nevertheless, higher than that from MED or MSF (Bruggen 2003: 8). MED has the advantage of having the capability of combining with power generation systems to produce electricity (Eltawil, Zhengming, and Yuan 2009: 2256); a secondary product of the plant that is, by all means, very important and useful. This electricity, for instance, can be coupled with reverse osmosis plants for their operation, or can be utilised elsewhere in homes and industries. This saves up on the cost of power generation in separate plants. Another advantage of MED is the non-specificity of its operation; that is, it does not require workers to possess special technical information and expertise in order to operate the plants (Eltawil, Zhengming, and Yuan 2009: 2256). On the other hand, reverse osmosis requires the most technical expertise of its workers out of the three methods (Eltawil, Zhengming, and Yuan 2009: 2256). This means that workers would have to be trained beforehand, which requires funds, and would have to use sophisticated machinery, which, again, requires extra funding. None of these issues are encountered for the operation of MED plants, so it reduces the expenditure. The comparison among the three methods is summarized below in a tabulated form for easy reference: MED MSF RO Energy required (Agha, Wahab, and Mansouri n.d.: 9) “0.75-1.75 kW/h/m3” “2.64-3.96 kW/h/m3” “5-7 kW/h/m3” Costs (Agha, Wahab, and Mansouri n.d.: 9) Least Relatively more Most Direct Utilisation of thermal energy (Eltawil, Zhengming, and Yuan 2009: 2256) Yes Yes No Pretreatment of raw material (Agha, Wahab, and Mansouri n.d.: 9) and (Bruggen 2003: 8) Minimum More (“Pre-heating”) More (“Cleaning and pressurized”) Resistance to poisoning (Eltawil, Zhengming, and Yuan 2009: 2256) Most Fair (“depends upon quality of material used”) Least (“membranes have to be changed every 5-7 years”) Product quality (Bruggen 2003: 8) and (Eltawil, Zhengming, and Yuan 2009: 2256) Greatest Good Fair (“highest salt content” out of the three) Capability of integration with other systems (Eltawil, Zhengming, and Yuan 2009: 2256) Yes Yes Yes Training required for operation (technical knowledge required) (Eltawil, Zhengming, and Yuan 2009: 2256) Least Less Most (Agha, Wahab, and Mansouri n.d.: 9) , (Bruggen 2003: 8), and (Eltawil, Zhengming, and Yuan 2009: 2256) From the preceding discussion, it can be clearly deduced that MED comes to be the most efficient and cost-effective method of desalinating brackish water out of the three most common methods discussed (Agha, Wahab, and Mansouri n.d.: 10). 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