There is an increased requirement of fresh water in the Middle East and the North African region collectively known as MENA. The demand can fully be satisfied by appropriate utilization of the already existing renewable sources of water. All the MENA states have a significant potential for energy from the sun. Therefore, it is an obvious approach to apply the technologies of Concentrating Solar Power (CSP) in powering the process of sea water desalination in Qatar. This way, the problem of water scarcity in Qatar and other MENA states can be solved. The German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) has sponsored the AQUA-CSP project which attempts to quantify the efficacy of the CSP technology in Qatar and other MENA countries (Laird, 2001). Among the studies carried out by this project are the environmental impact and the socio-economic effects which may result from the large scale application of the technology. The result from these studies can provide important data necessary in making decisions and policies regarding the national plans in expansions in the water sector.
There is a worsening condition of the deficit of fresh water in Qatar mostly attributed by the fast growth of population, increased urbanization and industrialization with a limited amount of natural resources. There is therefore a need to faster develop modern infrastructure for distributing water in Qatar. An enhanced use efficiency and better management of water need to be established.
It is important to use the desalted sea water in order to maintain a considerable capacity of water supply. An option of using fossil fuels in desalinating seawater is an excellent solution but the method is not only non economical but also not sustainable in the long term making fossil fuel to be over utilized and owing to the fact that fossil fuel provide non renewable energy. The most sustainable way to desalinate seawater is through applying the technology of CSP which can replace the fossil fuels which grow scarce day after day. The technology can also be effective in performing large scale desalination of seawater which may not be done easily with the application of fossil fuel energy (Yelema, Taner, 2001). The challenge which remains in Qatar is the immediate introduction of the technology in the market which will enable mass production of freshwater for the people of Qatar.
The fast rate of population and economic growth of Qatar needs to be approached with sustainable technologies like CSP which will provide freshwater to support the high populations and the busy industrialization process in Qatar. Over the last decade, the exploitation of freshwater in Qatar has greatly surpassed the renewable available surface and the groundwater sources. The needed sustainable approach is to enhance water management and to ensure that there is a more efficient use and distribution of water to the people of Qatar. What is generally affecting Qatar and other MENA countries is the perception that there should be a least cost and rather proven technology which can help provide water in the region. This mentality has dragged the application of the technology by a great magnitude thus making MENA government have hardships in implementing projects which will offer economic sources of energy. The most available of these technologies in Qatar is the CSP because there is plenty of sun radiation in the deserts and the coastal. It is estimated that every square meter in Qatar receives solar radiation comparable to 1.5 million barrels of crude oil (Cheater, 1999). What is required is the technology of converting this great solar energy into meaningful usage. The solar radiation can only be harvested, stored and converted into usable energy by using CSP.
Introduction to CSP (concentrated solar power) Technologies
Concentrated solar power technologies utilize the concept of focusing direct radiations of the sun to a receiver through to optical devices. The solar radiation is then converted into heat energy which can be used in various applications. Examples of such applications include the burning of the Roman war ships in 212 BC using polished bronze shields. The technique was used by Archimedes in an effort to attack Syracuse by setting fire to the wooden war ships although the success of this attempt is not reported. In 1973, the Archimedes experiment was replicated and at the time, a Greek navy determined in science managed to set fire to wooden boats at a distance of 50 meters away. Another supporter of Archimedes concept was Leonardo da Vinci in the 16th century who proposed that it is possible to concentrate sunlight to boil water using concave mirrors (Bahgat, 2008). The historical application of this technology seems to be a violent one or taking the nature of weaponry. However, in todays applications, peaceful uses of the technology have been invented the production of electricity and heat for domestic or industrial applications.
A typical Concentrated Solar Power (CSP) system is comprised of the solar field, solar collector element, the solar receiver and the balance of plant. The solar field consists of the reflective area often represented as square meters (m2). This quoted surface area does not represent the land use area. The solar field collects the suns radiations which can be converted later into heat or electricity. Perhaps the largest component of the CSP system is the solar collector element and it may consist of a single item or assembly of several elements. The solar radiation is then converted into heat by the solar receiver and the balance of plant comprise of all the elements and the necessary installations for the system. However, the balance of plant excludes the elements related to the conversions of solar radiation into heat.
There are five major technologies used to desalinate seawater. To separate water from salts, energy expenditure is inevitable and the process of evaporation needs heat energy which has to be supplied. These desalination methods which rely on evaporation processes are Multi-Stage Flash (MSF), Multi-Effect Distillation (MED), Thermal Vapor Compression (TVC) and Solar Distillation (SD). Other desalination technologies rely on crystallization process and include methods such as Freezing (FR) and Gas Hydrate Processes (GH). Methods relying on filtrations and evaporation processes include Membrane Distillation (MD).
Desalination methods may require to be done in mechanical way and may not require so much of thermal investment. Evaporation and filtration are example of these processes and the technologies behind these processes include the Mechanical Vapor Compression (MVC) and the Reverse Osmosis (RO). In the separation of salts from water, electrical and chemical sources of energy are used with technologies like Electrodialysis (ED) utilized in the selective filtration processes. Ion Exchange (IE) technologies are well applied in the exchange processes with the major source of energy being chemical reactions.
Multi-Effect Desalination (MED) using Concentrating Solar Power The importance of Multi-Effect Desalination (MED) has been highlighted by most writers and scientists working on the sustainable energy projects. MED is a thermal distillation process which involves spraying the feed water. Alternatively the feed water is distributed onto the surface of the evaporator made of tubes of various chamber with different effects. The feed water is often sprayed in thin films in order to encourage evaporation following the preheating in the upper section of every chamber (Baird, 2008).
The steam extracted from the power cycle or sometimes from the boiler is used to heat the evaporator tubes in the first effect. Thereafter, the steam which is produced in the first effect is then condensed in the evaporator tubes of the subsequent effect which again produces vapor. All the entire effects surfaces are heated by the steam which is produced in each of the subsequent effect. It is important that each of the effect has to have a lower pressure than the previous one. The process is then repeated again up to about 16 effects. The last effect produces steam which is then condensed in a different heat exchanger also known as the final condenser. The incoming seawater then cools the steam and then used as the preheated feed water for the process of desalination.
Reverse Osmosis (RO) using Concentrating Solar Power Another important technology for concentrating solar power is the reverse osmosis (RO). Reverse osmosis is a separation process which utilizes membrane technology to recover water from a highly concentrated salt solution. The saline solution is pressurized to some point higher than the solutions osmotic pressure. In practice, the membrane filters will hold back the ions of the salt from the solution which is pressurized and will allow only the water to pass through. The Reverse osmosis membranes are very sensitive to pH, algae, bacteria, oxidizers, fouling, particulate depositions and a wide array of organics (Ghosh, 1993). It is therefore a necessity to treat the feed water prior to any procedure of passing it through the RO. This treatment is necessary to be performed since the agents can impact on the energy consumption of the RO and the overall cost.
Alternatives to chemical treatment of the raw water have been developed. In the recent past, there has been an increased trend of utilizing ultra, micro and nano-filtration technologies so as to prevent the seawater contamination which might be from the additives in the environment of the plants. The ROs can be post-treated to remove the dislodged gases like CO2. Stabilizing the pH is also a necessary procedure through the addition of calcium and sodium salts and then the dangerous substances are removed from brine (Kay, 2001).
Most of the energy expenditure in the RO is accounted for the pressurizing of the sea water which is saline. The salinity of the water implies that the water is high in osmotic pressure and thus more pressure is needed to perform the process of separation since there is a correlation between the pressure required and the salt concentration of the water. In a situation when brackish water has to be used, RO is the dominant method of choice to be used in the separation because here, a range of low to intermediate pressures is needed (Rankin, 1997). The brackish water systems require a range of operating pressure which will have a value from 10 to 15 bars while for seawater systems the bar of 50 to 80 is required.
Comparisons between CSPRO and CSPMED The comparison of the technologies is made possible using a simple Rankine power cycle. By this technique, it is easier to compare a linear Fresnel concentrating solar power system together with the reverse osmosis membrane desalination. Comparisons between the RO desalination and the thermal multi-effect distillation can also be made. To better compare the systems, it is important to design the systems so that they have similar water and electricity demand of 24,000 m3 per day and 20-25MW per day respectively. Establishing the required thermal energy investment and the essential size of the solar collector field will enable the evaluation of the system performance differences (Edens, 1999).
(a) Power generation comparison
The performance difference between CSPRO and the CSPMED has been made clear although not such significant difference can be implicated. MED desalination requires between 2.2-2.4 kWhm3 of electricity consumption which is relatively lower than the requirement of electricity in RO which is between 4.9-5.9 kWhm3 . This can be caused by the difference in salinity of the raw water. The gross output of electricity in MED is relatively low and the cold end temperature of about 700C of the backpressure steam turbine is relatively higher. This negatively impacts the performance of the power generator up to 10 decline of the mechanical work which is delivered (Cordsman, 2004).
(b) Performance difference
A power generator with a condensing steam turbine of a lower cold-end temperature of about 35-450C can have a significantly higher performance when used in RO desalination process (Kay, 2001). This then implies that the thermal energy obtained from the power cycle for MED distillation is equal to the specific loss of electricity of 2.3-2.8 MW with respect to a solely electricity producing system which would be utilized in the process of RO.
There is also a clear difference between MED and RO in terms of internal consumption of electricity of the power block. This way, the parasitic losses are significantly different in both MED and RO. The loss for MED id 0.2 MW and that of RO is 1.8-2.1 MW. The MED plant wholly replaces the conventional power station cooling system. The plant also replaces the power consumption associated with cooling of the fans and the pumping of the water. Another reason to account for the electricity consumption differences is the fact that some of the cooling energy escapes from the plant in the form of distilled water and brine hence saving a lot of electricity which would otherwise be needed to pump cooling water and to cool the evaporation tower fans (Cordsman, 2004). In general, all the effects make CSPMED to have a better technical performance than CSPRO. MED is better applied in areas of high salinity of the seawater like the coastal regions of Qatar.
(c)Economical comparison
There is also economic difference between RO and MED which ultimately affects the implementation of the projects. The environmental conditions will also affect the decision of implementing the projects and a comparison between the two will be necessary to decide on which one will most be preferred. For efficiency and economic gains, the policy makers and government of Qatar may choose to combine the CSPMEDRO technologies. This integration will therefore provide advantages which are gained from individual technologies. Although there is a considerable difference between RO and MED, much of the literature shows that there is too little economical difference between the application of RO and MED (Bahgit, 2008). The two technologies have competing nature in the Qatar market since they have individual advantages and demerits. There are key differences which should be noted between the methods, CSPMED is limited to the coastal regions while CSP can be sited anywhere with the public grid interconnecting the CSP plant with RO. The CSPMED is independent of the quality of the raw water and very high quality of water is produced with this technology. On the other hand, the CSPRO requires brackish raw water and low quality water is produced when the raw water is of low quality. The optimal irradiance of CSPMED is defined by the site at the coast while in CSPRO the CSP can be located at a higher irradiance although a given amount of power is lost by the process of transmission to the RO (Goodbody, Hope, 2002). Dry cooling in CSPRO also leads to a reduced efficiency. Storage options for CSPMED can range from molten salt to concrete and it is possible to have hot water storage and phase change materials. In CSPRO, the options of storage will be molten salt, phase change materials (PCM) and concrete. The general cost of CSPMED is relatively low since it will require the replacement of reverse osmosis membranes in CSPRO. The better technical performance of CSPMED also lowers the cost of the technology as lower fuel consumption is realized of about 10 lower than in CSPRO. The cost of water in CSP.MED is also lower than in CSPRO (Murakami, 1995). The implementation of the projects of CSP will greatly improve the living standard of the people in Qatar owing to the fast economic pace in the region. The fast growing population rate in Qatar requires the most excellent technology which will preserve the integrity of Mother Nature.
There is a worsening condition of the deficit of fresh water in Qatar mostly attributed by the fast growth of population, increased urbanization and industrialization with a limited amount of natural resources. There is therefore a need to faster develop modern infrastructure for distributing water in Qatar. An enhanced use efficiency and better management of water need to be established.
It is important to use the desalted sea water in order to maintain a considerable capacity of water supply. An option of using fossil fuels in desalinating seawater is an excellent solution but the method is not only non economical but also not sustainable in the long term making fossil fuel to be over utilized and owing to the fact that fossil fuel provide non renewable energy. The most sustainable way to desalinate seawater is through applying the technology of CSP which can replace the fossil fuels which grow scarce day after day. The technology can also be effective in performing large scale desalination of seawater which may not be done easily with the application of fossil fuel energy (Yelema, Taner, 2001). The challenge which remains in Qatar is the immediate introduction of the technology in the market which will enable mass production of freshwater for the people of Qatar.
The fast rate of population and economic growth of Qatar needs to be approached with sustainable technologies like CSP which will provide freshwater to support the high populations and the busy industrialization process in Qatar. Over the last decade, the exploitation of freshwater in Qatar has greatly surpassed the renewable available surface and the groundwater sources. The needed sustainable approach is to enhance water management and to ensure that there is a more efficient use and distribution of water to the people of Qatar. What is generally affecting Qatar and other MENA countries is the perception that there should be a least cost and rather proven technology which can help provide water in the region. This mentality has dragged the application of the technology by a great magnitude thus making MENA government have hardships in implementing projects which will offer economic sources of energy. The most available of these technologies in Qatar is the CSP because there is plenty of sun radiation in the deserts and the coastal. It is estimated that every square meter in Qatar receives solar radiation comparable to 1.5 million barrels of crude oil (Cheater, 1999). What is required is the technology of converting this great solar energy into meaningful usage. The solar radiation can only be harvested, stored and converted into usable energy by using CSP.
Introduction to CSP (concentrated solar power) Technologies
Concentrated solar power technologies utilize the concept of focusing direct radiations of the sun to a receiver through to optical devices. The solar radiation is then converted into heat energy which can be used in various applications. Examples of such applications include the burning of the Roman war ships in 212 BC using polished bronze shields. The technique was used by Archimedes in an effort to attack Syracuse by setting fire to the wooden war ships although the success of this attempt is not reported. In 1973, the Archimedes experiment was replicated and at the time, a Greek navy determined in science managed to set fire to wooden boats at a distance of 50 meters away. Another supporter of Archimedes concept was Leonardo da Vinci in the 16th century who proposed that it is possible to concentrate sunlight to boil water using concave mirrors (Bahgat, 2008). The historical application of this technology seems to be a violent one or taking the nature of weaponry. However, in todays applications, peaceful uses of the technology have been invented the production of electricity and heat for domestic or industrial applications.
A typical Concentrated Solar Power (CSP) system is comprised of the solar field, solar collector element, the solar receiver and the balance of plant. The solar field consists of the reflective area often represented as square meters (m2). This quoted surface area does not represent the land use area. The solar field collects the suns radiations which can be converted later into heat or electricity. Perhaps the largest component of the CSP system is the solar collector element and it may consist of a single item or assembly of several elements. The solar radiation is then converted into heat by the solar receiver and the balance of plant comprise of all the elements and the necessary installations for the system. However, the balance of plant excludes the elements related to the conversions of solar radiation into heat.
There are five major technologies used to desalinate seawater. To separate water from salts, energy expenditure is inevitable and the process of evaporation needs heat energy which has to be supplied. These desalination methods which rely on evaporation processes are Multi-Stage Flash (MSF), Multi-Effect Distillation (MED), Thermal Vapor Compression (TVC) and Solar Distillation (SD). Other desalination technologies rely on crystallization process and include methods such as Freezing (FR) and Gas Hydrate Processes (GH). Methods relying on filtrations and evaporation processes include Membrane Distillation (MD).
Desalination methods may require to be done in mechanical way and may not require so much of thermal investment. Evaporation and filtration are example of these processes and the technologies behind these processes include the Mechanical Vapor Compression (MVC) and the Reverse Osmosis (RO). In the separation of salts from water, electrical and chemical sources of energy are used with technologies like Electrodialysis (ED) utilized in the selective filtration processes. Ion Exchange (IE) technologies are well applied in the exchange processes with the major source of energy being chemical reactions.
Multi-Effect Desalination (MED) using Concentrating Solar Power The importance of Multi-Effect Desalination (MED) has been highlighted by most writers and scientists working on the sustainable energy projects. MED is a thermal distillation process which involves spraying the feed water. Alternatively the feed water is distributed onto the surface of the evaporator made of tubes of various chamber with different effects. The feed water is often sprayed in thin films in order to encourage evaporation following the preheating in the upper section of every chamber (Baird, 2008).
The steam extracted from the power cycle or sometimes from the boiler is used to heat the evaporator tubes in the first effect. Thereafter, the steam which is produced in the first effect is then condensed in the evaporator tubes of the subsequent effect which again produces vapor. All the entire effects surfaces are heated by the steam which is produced in each of the subsequent effect. It is important that each of the effect has to have a lower pressure than the previous one. The process is then repeated again up to about 16 effects. The last effect produces steam which is then condensed in a different heat exchanger also known as the final condenser. The incoming seawater then cools the steam and then used as the preheated feed water for the process of desalination.
Reverse Osmosis (RO) using Concentrating Solar Power Another important technology for concentrating solar power is the reverse osmosis (RO). Reverse osmosis is a separation process which utilizes membrane technology to recover water from a highly concentrated salt solution. The saline solution is pressurized to some point higher than the solutions osmotic pressure. In practice, the membrane filters will hold back the ions of the salt from the solution which is pressurized and will allow only the water to pass through. The Reverse osmosis membranes are very sensitive to pH, algae, bacteria, oxidizers, fouling, particulate depositions and a wide array of organics (Ghosh, 1993). It is therefore a necessity to treat the feed water prior to any procedure of passing it through the RO. This treatment is necessary to be performed since the agents can impact on the energy consumption of the RO and the overall cost.
Alternatives to chemical treatment of the raw water have been developed. In the recent past, there has been an increased trend of utilizing ultra, micro and nano-filtration technologies so as to prevent the seawater contamination which might be from the additives in the environment of the plants. The ROs can be post-treated to remove the dislodged gases like CO2. Stabilizing the pH is also a necessary procedure through the addition of calcium and sodium salts and then the dangerous substances are removed from brine (Kay, 2001).
Most of the energy expenditure in the RO is accounted for the pressurizing of the sea water which is saline. The salinity of the water implies that the water is high in osmotic pressure and thus more pressure is needed to perform the process of separation since there is a correlation between the pressure required and the salt concentration of the water. In a situation when brackish water has to be used, RO is the dominant method of choice to be used in the separation because here, a range of low to intermediate pressures is needed (Rankin, 1997). The brackish water systems require a range of operating pressure which will have a value from 10 to 15 bars while for seawater systems the bar of 50 to 80 is required.
Comparisons between CSPRO and CSPMED The comparison of the technologies is made possible using a simple Rankine power cycle. By this technique, it is easier to compare a linear Fresnel concentrating solar power system together with the reverse osmosis membrane desalination. Comparisons between the RO desalination and the thermal multi-effect distillation can also be made. To better compare the systems, it is important to design the systems so that they have similar water and electricity demand of 24,000 m3 per day and 20-25MW per day respectively. Establishing the required thermal energy investment and the essential size of the solar collector field will enable the evaluation of the system performance differences (Edens, 1999).
(a) Power generation comparison
The performance difference between CSPRO and the CSPMED has been made clear although not such significant difference can be implicated. MED desalination requires between 2.2-2.4 kWhm3 of electricity consumption which is relatively lower than the requirement of electricity in RO which is between 4.9-5.9 kWhm3 . This can be caused by the difference in salinity of the raw water. The gross output of electricity in MED is relatively low and the cold end temperature of about 700C of the backpressure steam turbine is relatively higher. This negatively impacts the performance of the power generator up to 10 decline of the mechanical work which is delivered (Cordsman, 2004).
(b) Performance difference
A power generator with a condensing steam turbine of a lower cold-end temperature of about 35-450C can have a significantly higher performance when used in RO desalination process (Kay, 2001). This then implies that the thermal energy obtained from the power cycle for MED distillation is equal to the specific loss of electricity of 2.3-2.8 MW with respect to a solely electricity producing system which would be utilized in the process of RO.
There is also a clear difference between MED and RO in terms of internal consumption of electricity of the power block. This way, the parasitic losses are significantly different in both MED and RO. The loss for MED id 0.2 MW and that of RO is 1.8-2.1 MW. The MED plant wholly replaces the conventional power station cooling system. The plant also replaces the power consumption associated with cooling of the fans and the pumping of the water. Another reason to account for the electricity consumption differences is the fact that some of the cooling energy escapes from the plant in the form of distilled water and brine hence saving a lot of electricity which would otherwise be needed to pump cooling water and to cool the evaporation tower fans (Cordsman, 2004). In general, all the effects make CSPMED to have a better technical performance than CSPRO. MED is better applied in areas of high salinity of the seawater like the coastal regions of Qatar.
(c)Economical comparison
There is also economic difference between RO and MED which ultimately affects the implementation of the projects. The environmental conditions will also affect the decision of implementing the projects and a comparison between the two will be necessary to decide on which one will most be preferred. For efficiency and economic gains, the policy makers and government of Qatar may choose to combine the CSPMEDRO technologies. This integration will therefore provide advantages which are gained from individual technologies. Although there is a considerable difference between RO and MED, much of the literature shows that there is too little economical difference between the application of RO and MED (Bahgit, 2008). The two technologies have competing nature in the Qatar market since they have individual advantages and demerits. There are key differences which should be noted between the methods, CSPMED is limited to the coastal regions while CSP can be sited anywhere with the public grid interconnecting the CSP plant with RO. The CSPMED is independent of the quality of the raw water and very high quality of water is produced with this technology. On the other hand, the CSPRO requires brackish raw water and low quality water is produced when the raw water is of low quality. The optimal irradiance of CSPMED is defined by the site at the coast while in CSPRO the CSP can be located at a higher irradiance although a given amount of power is lost by the process of transmission to the RO (Goodbody, Hope, 2002). Dry cooling in CSPRO also leads to a reduced efficiency. Storage options for CSPMED can range from molten salt to concrete and it is possible to have hot water storage and phase change materials. In CSPRO, the options of storage will be molten salt, phase change materials (PCM) and concrete. The general cost of CSPMED is relatively low since it will require the replacement of reverse osmosis membranes in CSPRO. The better technical performance of CSPMED also lowers the cost of the technology as lower fuel consumption is realized of about 10 lower than in CSPRO. The cost of water in CSP.MED is also lower than in CSPRO (Murakami, 1995). The implementation of the projects of CSP will greatly improve the living standard of the people in Qatar owing to the fast economic pace in the region. The fast growing population rate in Qatar requires the most excellent technology which will preserve the integrity of Mother Nature.
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