Renewable Energy Desalination Systems C. Koroneos, E. Nanaki, K. Moustakas, D. Malamis Unit of Environmental Science and Technology School of Chemical Engineering 9, Iroon Polytechneiou St., Zographou Campus Athens 157 73 Greece
Abstract The Objective of this work is to present an overview of desalination technologies that can be powered using renewable energy and to compare them thermodynamically on their performance. The performance of the systems is directly related to the economic cost and the environmental impact. Currently, the majority of large-scale desalination plants in the world employ Multistage Flash (MSF), Multiple Effect Distillation (ME), and reverse osmosis (RO), because these desalination technologies were the first to mature and because they have large economies of scale. MSF and ME are most appropriate for large-scale systems where heat is available such as geothermal energy, solar energy or waste heat. The RO, vapour compression (VC), and electrodialysis (ED) use only electrical energy. The electricity can be produced from wind energy or photovoltaics. Which system is most appropriate for a given location depends mostly on the quality of the available feedwater, since the feedwater characteristics determine the pretreatment requirements and energy demands, and therefore cost and complexity, of membrane systems.
1. Introduction
Desalination is a treatment process that removes salts from water. Saline solutions other than seawater are typically described as brackish water with a salt concentration from 1000 to 11000 ppm TDS. Normal seawater has a salinity of 35000 ppm TDS or more, mostly sodium chloride. A typical desalination plant consists of water treatment system, the desalination unit and a post treatment system (Figure 1).
Saline water Seawater: >35000 ppm TDS Brackish water: 1000-11000 ppm TDS
Feed Treatment
Thermal Energy Energy High grate Desalination plant Low grate
Reject Brine Post Treatment
Fresh Water
Figure 1. Water and energy flow diagram of a desalination unit
Several desalination processes have been developed but not all of them are reliable and in commercial use. The most important processes are split into two main categories: Thermal (or distillation) processes: the most widely used for seawater desalination are Multistage-Flash Distillation (MSF), the Multi Effect Distillation (MED) and the Vapor Compression (VC) process. Membrane processes: consist of Reverse Osmosis (RO) and Electrodialysis (ED) processes. ED is confined to desalination of brackish water while RO can be used for both, brackish and seawater desalination.
Two critical parameters of the desalination processes are the quality of the produced water and the energy required. Product water quality depends on the desalination process. Distillation processes are producing water around 20 ppm TDS and membrane processes are usually designed to produce water of 100-500 ppm TDS. Potable water for human consumption should comply with the World Health Organization (WHO) limits (Table 1) and should not be totally devoid of salts.
Table 1. WHO standards for potable water Concentration (ppm) Constitutes Limited values Max allowed values Total Dissolved Salts (TDS) 500 1500 Cl 200 600 2+ SO4 200 400 Ca2+ 75 100 Mg2+ 30 150 F- 0.7 1.7 NO3- <50 100 Cu2+ 0.05 1.5 Fe3+ 0.10 1.0 NaCl 250 - pH 7-8 6.5-9
In general, desalination is an energy intensive technology and the energy input may be thermal, (defined in terms of units of water produced per unit of steam or per 2500 kj used) mechanical or electrical, (expressed in kWh/m3)
2. Thermal processes
About 60 percent of the world’s desalted water is produced with heat to distill fresh water from sea water. In the distillation process the saline water is heated, producing water vapor that is in turn condensed to form fresh water. In a laboratory or industrial plant, water is heated to the boiling point to produce the maximum amount of water vapor. In a desalination plant, by adjusting the atmospheric pressure the boiling point of the seawater is controlled. Decreasing the pressure the boiling point is also decreasing. The reduction of the boiling point is important in the desalination process for two major reasons: multiple boiling and scale control. To boil, water needs two important conditions: the proper temperature relative to its ambient pressure and enough energy for vaporization. When water is heated to its boiling point and then the heat is turned off, the water will continue to boil only for a short time because the water needs additional energy (the heat of vaporization) to permit boiling. Once the water stops boiling, boiling can be renewed by either adding more heat or by reducing the ambient pressure above the water. If the ambient pressure is reduced, then the water would then be at a temperature above its boiling point (because of the reduced pressure) and will boil with the extra heat from the higher temperature to supply the heat of vaporization needed. As the heat of vaporization is supplied, the temperature of the water will fall to the new boiling point. To significantly reduce the amount of energy needed for vaporization, the distillation desalting process usually uses multiple boiling in successive vessels, each operating at a lower temperature and pressure. This process of reducing the ambient pressure to promote boiling can continue downward and, if carried to the extreme with the pressure reduced enough, the point at which water would be boiling and freezing at the same time would be reached. For gauging the performance of a thermal desalination plant the performance ratio is used: mass of distillate produced Performance Ratio (1) mass of steam consumed
Aside from multiple boiling, the other important factor is scale control. Although most substances dissolve more readily in warmer water, some dissolve more readily in cooler water. Unfortunately, some of these substances like carbonates and sulfates are found in seawater. One of the most important is gypsum (CaSO4), which begins to leave solution when water approaches about 95 0C (203 0F). This material forms a hard scale that coats any tubes or containers present. Scale creates thermal and mechanical problems and, once formed, is difficult to remove. One way to avoid the formation of this scale is to keep the temperature and boiling point of the water below that of temperature. These two concepts have made various forms of distillation successful in locations around the world. The process, which accounts for the most desalting capacity is multi-stage flash distillation commonly referred to as the MSF process.
2.1. Multi-Stage Flash Distillation (MSF)
The Multi-Stage Flash Distillation (MSF) is the process, which accounts for the most desalting capacity (over 50% of desalination plants capacity worldwide). There are two configurations of the MSF process: The “once through” consists of two sections: 1. The heat rejection section 2. Brine heater
The “brine recirculation” consists of the following three sections: 1. The heat rejection section 2. The heat recovery section 3. Brine heater
The recovery and rejection section consist of a series of stages and each stage consist of a flash chamber and a heat exchanger / condenser. In both configurations, the seawater is heated in the brine heater. The heated seawater then flows into the next stage, where the ambient pressure is such that the water will immediately boil. The sudden introduction of the heated water into the chamber causes it to boil rapidly, almost exploding or flashing into steam. Generally, only a small percentage of this water is converted to steam (water vapor), depending on the pressure maintained in this stage since boiling will continue only until the water cools (furnishing the heat of vaporization) to the boiling point. In the brine recirculation configuration the seawater is taken into the plant and fed through the heat rejection stage (Figure 2). The function of this section is to reject thermal energy from the plant and to allow to the product water and brine to exit the plant at lowest possible temperature. The seawater feed is mixed with a large mass of brine, which is recirculated through the plant (brine recirculation). Then the feed passes through a number of heat exchangers, raising its temperature. The preheated feed then is heated up to its terminal temperature in the brine heater. The flow then passes to the top flash stage where the reduction of pressure causes a small fraction to flash off as vapor. This process continues right down to the bottom stage of the plant in the rejection section, where part is rejected as “blowdown” and the rest is mixed the incoming make up (seawater) and then recycled via the brine recirculation pump.
Steam Ejector Steam from boiler
Seawater Feed
Brine Heater
Product Water
Brine Brine Brine Brine Condensate Brine Discharge
Heat Recovery Section Brine Recirculation
Heat Regection Section Figure 2. Typical flow diagram of Multi-Stage Flash distillation plant
The MSF plants are designed for various performance ratios. A performance ratio of 12 is the practical upper limit for MSF plants. Typically, an MSF plant can contain from 4 to about 40 stages. Increasing the number of stages reduces the heat transfer surface that is required. This has to be off-set against the cost of providing extra stages. Complicated optimization calculations have to be undertaken where the main parameters are capital and operating cost. The MSF plants usually operate at the top feed temperatures (after the brine heater) of 90- 120°C (194-248°F). One of the factors that affect the thermal efficiency of the plant is the difference in temperature from the brine heater to the condenser on the cold end of the plant. Operating a plant at the higher temperature limits of 120°C (248°F) tends to increase the efficiency, but it also increases the potential for detrimental scale formation and accelerated corrosion of metal surfaces. The advantages of “brine recirculation” process over the once through process are: The sea-water pretreated is in the order of only one third of the once through design The tube bundles work with de-aerated brine water with lower corrosion and the in-condensable released are reduced thus achieving higher efficiency of the stages In general MSF plants are relatively easy to operate. Special attention is required in order to avoid scaling and corrosion of materials. The MSF has been developed and adapted for large-scale applications, usually greater than 5000 m3/day. Concerning the manufacturing, Japanese and Korean manufacturers produce 45% of the world production of the MSF plants, European manufacturers produce 43% and the USA 8%. The capital costs and the energy cost of an MSF plant are significant. The main energy requirement is thermal energy. The electricity demand is low and is used for auxiliary equipment such as pumps. A plant that operates at a performance ratio equal to 8 the thermal energy consumption is around 290 kj/kg of produced water and the electricity consumption is 4-6 kWh/m3. 2.2. Multi Effect Distillation The multiple effect distillation (MED) process has been used for industrial distillation for a long time. One popular use for this process is the evaporation of juice from sugar cane in the production of sugar or the production of salt with the evaporative process. Some of the early water distillation plants used the MED process, but the MSF units because of cost factors and their apparent higher efficiency displaced this process. However, in the past decade, interest in the MED process has renewed, and a number of new designs have been built. Most of these new MED units have been built around the concept of operating on lower temperatures. MED like the MSF process, takes place in a series of vessels (effects) and use the principle of reducing the ambient pressure in the various effects. This permit the seawater feed to undergo multiple boiling without supplying additional heat after the first effect (Figure 3). In a MED plant, the seawater enters the first effect and is raised to the boiling point after being preheated in tubes. The seawater is either sprayed or otherwise distributed onto the surface of evaporator tubes in a thin film to promote rapid boiling and evaporation. The tubes are heated by steam from a boiler, or other source, which is condensed on the opposite side of the tubes. The condense steam, from the boiler, is recycled to the boiler for reuse. Only a portion of the seawater applied to the tubes in the first effect is evaporated. The remaining feed water is fed to the second effect, where it is again applied to a tube bundle. These tubes are in turn being heated by the vapors created in the first effect. This vapor is condensed to fresh water-product, while giving up heat to evaporate a portion of the remaining seawater feed in the next effect. This continues for several effects, with 8 or 16 effects being found in a typical large plant. MED plants tent to have smaller number of effects than MSF stages. Usually, the remaining seawater in each effect must be pumped to the next effect so as to apply it to the next tube bundle. Additional condensation takes place in each effect on tubes that bring the feed water from its source through the plan to the first effect. This warms the feed water before it is evaporated in the first effect. MED plants are typically built in units of 2000 to 10000 cum/d (0.5 to 2.5 mgd). Some of the more recent plants have been built to operate with a top temperature (in the first effect) of about 70°C (1580F), which reduces the potential for scaling of seawater within the plant but in turn increases the need for additional heat transfer area in the form of tubes. Although the number of MED plants is still relatively small compared to MSF plants, their numbers have been increasing. There are a number of MED variants. These variations depending on the combinations of heat transfer configurations and flowsheet variations (horizontal/vertical, recirculation/once through, etc), that may be selected independently. Thus, leading to a large number of possible combinations. Some of the heat transfer surface configurations are: Multi-Effect Submerge Tube Multi-Effect Vertical Tube Climbing-Film Multi-Effect Vertical Tube Falling-Film Multi-Effect Horizontal Tube Falling-Film The cost of an MED plant heavily depends on the performance ratio. Capital and energy costs significant factors. The main energy requirement is thermal energy. A plant operating with a performance ratio equal to 8 the thermal energy consumption is around 290 kj/kg of produce water and electrical energy demand is 2.5-3 kWh/m3.
Vacuum Vacuum Vacuum Seawater Feed
1st effect 2st effect 3st effect Final Steam from Condenser boiler
Steam Steam Steam Steam Condensate
Brine Brine Brine Brine
Brine Discharge
Seawater Feed
Product Water
Figure 3. Typical flow diagram of Multi Effect Distillation plant
2.3. Vapor Compression Distillation (VC) The vapor compression (VC) distillation process is generally used for small and medium scale seawater desalting units. The heat for evaporating the water comes from the compression of vapor rather than the direct exchange of heat from steam produced in a boiler. There exist two VC processes: The Mechanical Vapor Compression (MVC) configuration, in which a mechanical compressor is used. The mechanical compressor is usually electrically driven, allowing the sole use of electrical power to produce water by distillation (Figure 4). The compressor creates a vacuum in the vessel and then compresses the vapor taken from the vessel and condenses it inside of a tube bundle also in the same vessel. Seawater is sprayed on the outside of the heated tube bundle where it boils and partially evaporates, producing more vapors. The Thermal Vapor Compression (TVC) configuration, in which a thermo- compressor or ejector is used o increase vapors pressure. With the steam jet-type of VC unit a venturi orifice at the steam jet creates and extracts water vapor from the main vessel, creating a lower ambient pressure in the main vessel. The steam jet compresses the extracted water vapor. This mixture is condensed on the tube walls to provide the thermal energy (heat of condensation) to evaporate the seawater being applied on the other side of the tube walls in the vessel.
The plants, which use this process, are generally designed to take advantage of the principle of reducing the boiling point temperature by reducing the pressure. VC units have been built in a variety of configurations to promote the exchange of heat to evaporate the seawater. Extra care is required with the control of the brine level in the evaporator and the proper maintenance of the compressor. Some manufactures use compressors that rotate at very high speeds. Operation at low temperatures minimizes the formation of scaling and corrosion of materials. VC units are usually built in the 20- to 2000-cum/d (0.005- to 0.5-mgd) range. They are often used for resorts, industries, and drilling sites where fresh water is not readily available. Vapor
Seawater & Brine Vapor Compresor Work in
Seawater Feed Compressed Vapor Product Water
Brine Recirculation Brine
Preheated Feed Preheated Brine
Brine Discharge
Brine Discharge Preheated Feed
Figure 4. Typical flow diagram of Vapor Compression (VC) Distillation plant
Capital and energy costs are significant factors in the determination of the total water production cost. The energy demand mainly required to drive the vapor compressor motor. The operation and maintenance of the vapor compressor motor some times covers half of the total operating cost. The energy requirements of VC plants are between 8 to 12 kWh/m3.
2.4. Solar distillation Solar distillation is a process in which the energy of the sun is directly used to evaporate fresh water from the sea. In solar distillation plants the solar radiation is trapped in a solar still by the greenhouse effect. A solar still consist of a shallow basin of brine, line with black material to increase the radiation absorption, and covered by a vapor-poof, transparent roof designed to act as condenser (Figure 5). The vapor is condensed on the cool surface of the roof. A solar collection area of about one square meter is needed to produce 2.5 liters of water per day with a thermal efficiency of 50%. The main disadvantages of this process are: Large solar collection area requirements Vulnerability to weather-related damage Solar stills are simple in operation and the only maintenance required is the cleaning of the plant. Solar distillation is usually suitable for small-scale application in remote areas, where low land cost is available and solar radiation is high.
Condensation Solar Radiation
Water Vapor
Distillate Distillate Collection Collection
Brine Figure 5. Typical solar distillation plant
3. Membrane Processes Membranes are used in two commercially important desalting processes: Electrodialysis (ED) and Reverse Osmosis (RO). Each process uses the ability of membranes to differentiate and selectively separate salts and water. However, membranes are used differently in each of these processes. Electrodialysis uses an electrical potential to move salts selectively through a membrane, leaving fresh water behind as product water. In the RO process, pressure is used for separation by allowing fresh water to move through a membrane, leaving the salts behind. In both processes the main power supply comes from electricity, however in RO process mechanical energy could be used. Another point is that ED process is used only for the desalination of brackish water, while RO process can be used for both brackish and seawater desalination.
3.1. Reverse Osmosis (RO)
Reverse Osmosis (RO) is a membrane separation process in which the water from a pressurized saline solution is separated from the solutes (the dissolved material) by flowing through a membrane. No heating or phase change is necessary for this separation. The major energy required for desalting is for pressurizing the feed water. In practice, the saline feed water is pumped into a closed vessel where it is pressurized against the membrane. As a portion of the water passes through the membrane, the remaining feed water increases in salt content. At the same time, a portion of this feed water is discharged without passing through the membrane. Without this controlled discharge, the pressurized feed water would continue to increase in salt concentration, creating such problems as precipitation of supersaturated salts and increased osmotic pressure across the membranes. The amount of the feed water discharged to waste in this brine stream varies from 20 to 70 percent of the feed flow, depending on the salt content of the feed water. A typical RO system is made up of the following basic components (Figure 6): Pre-treatment: Feed-water pretreatment is important in RO because the feed water must pass through very narrow passages during the process. Therefore, suspended solids must be removed and the water pre-treated so that salt precipitation or microorganism growth does not occur on the membranes (biofouling). Usually, the pretreatment consists of sterilization, fine filtration and the addition of acid or other chemicals to inhibit precipitation. High-pressure pump: The high-pressure pump supplies the pressure needed to enable the water to pass through the membrane and have the salts rejected. This pressure ranges from 17 to 27 bar for brackish water and from 54 to 80 for sea water. Membrane modules: The membrane assembly consists of a pressure vessel and a membrane that permits the feed water to be pressurized against the membrane. The membrane must be able to withstand the drop of the entire pressure across it. The semi-permeable membranes are fragile and vary in their ability to pass fresh water and reject the passage of salts. No membrane is perfect in its ability to reject salts, so a small amount of salts passes through the membrane and appears in the product water. The concentrated reject stream (brine) emerges from the membrane modules at high pressure. In large plants the reject brine pressure energy is recovered by a turbine, recovering from 20% up to 40% of the consumed energy. There are two types of RO membranes that being used commercially. These are the Spiral Wound (SW) membranes and the Hollow Fiber (HF) membranes, both are used for brackish and seawater desalination. The choice between the two is based in factors such as cost, feed water quality and product water capacity. Post-treatment: It consists of sterilization, stabilization, mineral enrichment and pH adjustment of the product water. Membrane
Water Product Water Pre-Treatment inWork Post-Treatment Brine
High Pressure Pumb Seawater Feed Brine Discharge Figure 6. Typical flow diagram of an RO plant
Due to the operation of the RO process in ambient temperature, corrosion and scaling problems are diminished in comparison with distillation processes. However, effective pre-treatment of the feed-water is required to minimize fouling, scaling and membrane degradation. Generally a seawater RO plant has low capital cost and significant maintenance cost due to the high cost of membrane replacement. The major energy requirement for RO desalination is for pressuring the feed-water. The energy requirements of a seawater SW-RO plant is around 5 kWh/m3 for large units with energy recovery, while for small units they around 15 kW/m3.
3.2. Electrodialysis (ED)
Electrodialysis (ED) is an electrochemical method for the desalination of brackish water. The dependence of energy consumption on the feed water salt concentration makes the ED process not economically attractive for the desalination of seawater. The ED process depends on the following general principles (Figure 7): Most salts dissolved in water are ionic, being positively (cationic) or negatively (anionic) charged. These ions are attracted to electrodes with an opposite electric charge. The electrodialysis membranes are impermeable by water but permit selective passage of either anions or cations. The dissolved ionic constituents in a saline solution are dispersed in water, effectively neutralizing their individual charges. When electrodes connected to an outside source of direct current like a battery are placed in a container of saline water, electrical current is carried through the solution, with the ions tending to migrate to the electrode with the opposite charge. If a pair of membranes that are arranged alternately with an anion-selective membrane followed by a cation-selective membrane are placed between a pair of electrodes, then, a region of low salinity water will be created between the membranes. Between each pair of membranes, a spacer sheet is placed that permits water to flow along the face of the membrane and induce a degree of turbulence. One spacer provides a channel that carries feed (and product) water, while the next carries brine.
As the electrodes are charged and saline feed water flows along the product water spacer at right angles to the electrodes. The anions in the water are attracted and diverted toward the positive electrode and pass through the anion-selective membrane, but cannot pass any farther than the cation-selective membrane, which blocks their path and trap the anions in the brine. Similarly, cations under the influence of the negative electrode move in the opposite direction through the cation- selective membrane to the concentrate channel on the other side. Here, the cations are trapped because the next membrane is anion-selective and prevents further movement towards the electrode. By this arrangement, concentrated and diluted solutions are created in the spaces between alternating membranes. The basic electrodialysis unit consists of several hundred of cell pairs bound together with electrodes on the outside and is referred to as a membrane stack. Feed water passes simultaneously in parallel paths through all of the cells to provide a continuous flow of desalted product water and brine to emerge from the stack. Depending on the design of the system, chemicals may be added to the streams in the stack to reduce the potential for scaling. Brackish Water Feed
A C A C A C + + + + Electrode - - Electrode Anode - - - - - Cathode + + + + - - - - + + + + + + + - - - - Water Brine Water Brine Water
Product Water Brine Discharge Figure 7. Electrodialysis process A: anion permeable membrane C: cation permeable membrane
A ED unit is made up of the following basic components: Pretreatment system Membrane stack Low-pressure circulation pump Power supply for direct current (a rectifier) Post-treatment
In order to inhibit the deposition of inorganic scales and colloidal substances on the membranes without addition of chemicals to the feed water the Reversal Electrodialysis (EDR) process was developed. The EDR process operates on the same principal as standard ED process except that both the product and the brine channels are identical in construction. At intervals of several times an hour, the polarity of the electrodes is reversed, and the flows are simultaneously switched so that the brine channel becomes the product water channel, and the product water channel becomes the brine channel. This development has enhanced the viability of the process considerably, as the process is self-cleaning. The EDR process requires minimum feed water pre-treatment and minimum use of chemicals for membrane cleaning. Generally the ED and EDR processes are economically attractive for low salinity water. EDR requires some extra equipment in comparison with ED, but reduces or almost eliminates the requirement for chemical pre-treatment. The total energy consumption, under ambient temperature conditions and assuming product water of 500 TDS, would be around 1,5 and 4 kWh/m3 for feed water of 1500 to 3500 ppm TDS, respectively.
3.3. Other processes
A number of other processes have been used to desalt saline waters. These processes have not achieved the level of commercial success that distillation, electrodialysis, and RO have, but they may prove valuable under special circumstances or with further development. The most significant of these processes are freezing and membrane distillation. Freezing: During the process of freezing, dissolved salts are naturally excluded during the formation of ice crystals. Cooling the water to form crystals under controlled conditions can desalinate seawater. Before the entire mass of water has been frozen, the mixture is usually washed and rinsed to remove the salts in the remaining water or adhering to the ice crystals. The ice is then melted to produce fresh water. Freezing has some advantages over distillation: lower theoretical energy requirement, minimal potential for corrosion, and little scaling or precipitation. The disadvantage is that it involves handling ice and water mixtures that are mechanically complex to move and process. The process has not been a commercial success in the production of fresh water for municipal purposes. At this stage, freezing desalting technology probably has a better application in the treatment of industrial wastes rather than the production of municipal drinking water. Membrane Distillation (or Pervaporation): The process combines both the uses of distillation and membranes. Pervaporation differs from other membrane processes in that the membrane constitutes a barrier between the feed in the liquid phase and permeate in the in the gas phase. In the process, saline water is warmed to enhance vapor production, and this vapor is exposed to a membrane that can pass vapor but not water. After the vapor passes through the membrane, it is condensed on a cooler surface to produce fresh water. In the liquid form, the fresh water cannot pass back through the membrane, so it is trapped and collected as the output of the plant. The main advantages of membrane distillation lie in its simplicity and the need for only small temperature differentials to operate. Membrane distillation probably has its best application in desalting saline water where inexpensive low- grade thermal energy is available, such as from industries or solar collectors.
4. Thermodynamics of and energy requirements of desalination
There is an absolute theoretical minimum requirement of energy for a completely reversible process, which is independent of the mechanism, or steps of the process but does depend on temperature, concentration and yield. This minimum energy 3 requirement is about 0.8 kWh per m of pure water from a 3.5% NaCl solution at 25 0C based on the assumption that this concentration of pure NaCl is a close approximation to normal seawater. This minimum, also, assumes zero driving forces at every point of the process. The driving forces, resulting in thermodynamic irreversibility, that are inherent in any distillation process are: Pressure difference to overcome fluid friction. Temperature differences in heat exchangers and between the system and its environment and in the fluid mixing. Concentration differences for mass transfer and in mixing fluids
The thermodynamic irreversibilities of the process result in exergy (availability) losses. The MSF and MED processes operate with heat energy so is important to consider the exergy of the heat source, which depends on temperature of the heat source (TH) and the temperature of the heat sink (T0). The relationship between minimum work (Wmin) and minimum heat (Qmin) is:
TH T0 TH Wmin Qmin or Qmin Wmin (2) TH TH T0
For a source temperature 1200C (393 K), sink (environment) temperature 200C (293 3 3 K) and Wmin=0.8 kWh/m the minimum heat required (Qmin) per m of pure water is 3.15 kWh. Steam is more often being used as a heat source. For saturated steam at 1250C the exergy is 640.8 kj/kg or 0.178 kWh/kg so the minimum requirement under these conditions is 4.494 kg of steam per m3 of pure water or 222.5 kg produce water per kg of steam. The performance ratio (mass of distillate per mass of steam) in a MED system can be approximated by the relation: w (1 w n ) P (3) 1 w where: w = the mass of water evaporated per mass of steam in a single effect and assumed to be constant, n = the number of effects.
The work required for VC distillation can be calculated from the equation:
R k 1 k1 k 1 k1 W T (r k 1) 1.283107 T (r k 1) (4) 183.6106 k a in k a in
. -1. -1 where: W = kWh per kg of distillate, R = 8.314 J kgmol K , k = CP/CV specific heat ratio, Tin = inlet temperature (K), a = adiabatic efficiency of the compressor. If CS the cost per kg of steam and CE the cost per kWh of electric energy then from the equations (3) and (4) the number of the effects in a MED system that would have the same energy cost as a single effect VC evaporator can be calculated:
6 7.810 k (1 w) a CS ln1 k1 (k 1) T (r k 1) C n in E (5) ln w
The MSF system requires more stages from the MED system for the same performance ratio but less surface area is needed for the same overall heat transfer coefficient. The thermodynamic irreversibility is greater in the MSF system resulting from the greater driving forces for heat transfer. A simplified relation between performance ratio and the number of stages can be given: n (R Δt) P (6) R n Δt where: P = performance ratio, n = number of stages, R = overall temperature range of brine entering and leaving the MSF system, Δt = terminal temperature difference across the condensers. Although the MSF system needs more stages than MED system for the same performance ratio, it has a design advantage in that the various stages can be combined into a single unit, which gives cheaper construction and also eliminates much of the external piping needed by the MED system. Also, because the evaporation does not occur in contact with the heating surface the MSF system gives less trouble from scale than MED system. For this reasons MSF system is preferred for large capacity units. Reverse Osmosis is a process that uses pressure as driving force to separate the dissolved salts from water. The application of pressure larger than the osmotic pressure of a saline solution against of a semi-permeable membrane has as a result the passage of pure water through the membrane. The basic equation that describes the RO process is:
(ΔP ΔΠ) K W A (ΔP ΔΠ) QW (7) R W d
3. -1 where: Qw = flow rate of water through the membrane (m h ), RW = resistance . . -3 2. -1. -1 (bar h m ), KW = specific permeability of water through the membrane (m h bar ), A = surface of the membrane (m2), d = thickness of membrane (m), ΔΠ = osmotic pressure difference between feed water and produce water (bar), ΔP = pressure difference between feed and product water (bar). The permeability of water through the membrane (KW) depends on the material (polymer) that the membrane is constructed, the temperature and the operational time of the membrane. The temperature affects considerably the water flow rate through the membrane. An increase in temperature of 1 0C results to an increase of about 3%. As it can be seen from equation (7), in order to achieve the desirable flow rate of produce water it is necessary to apply pressure to the feed water above the osmotic pressure of the molecules and ions that are dissolved in the feed water (Table 2).
Table 2. Osmotic pressure of various solutions of salts Salt Concentration (mg/l) Osmotic Pressure (bar) NaCl 35000 27.86 NaCl 1000 0.79 Na2SO4 1000 0.42 MgSO4 1000 0.25 CaCl2 1000 0.58 NaHCO3 1000 0.89 MgCl2 1000 0.67
RO systems work with feed compression 2 to 3 times greater than the osmotic pressure. Seawater with 35000 mg/l TDS has an osmotic pressure of about 350 psi and the applied pressure for RO ranges from 800 to 1000 psi. Due to the fact that no membrane is perfect and the concentration difference between the brine and the product water some amount of salts pass through the membrane. This is a mass transport phenomenon and can be described by the following equation:
K A (C C ) Q S F P (8) W d where: QW = flow rate of salts through the membrane (kg/h), CF ,CP = concentration of salts of the feed and product water (kg/m3), A = the area of the membrane (m3), d = thickness of the membrane (m), KS = mass transfer coefficient of salts through the membrane (m2/h). It is clear that the KW must be as large as possible and KS as small as possible in order to achieve the smallest resistance to water permeation through the membrane and the greater resistance to salts permeation. Two important factors for the membrane are the salt permeation and the salt rejection, which are defined by the following equation: Salt Rejection = 1 – Salt Permeation = CP / CF (9) The Salt Rejection is an important characteristic of the membrane and is different for different ions (Table 3).
Table 3. Rejection of ions from an RO membrane Ion Salt Rejection % Ion Salt Rejection % + NH4 92 Nitrates 85 Na+ 95 Chlorates 95 K+ 95 Fluorides 95 Mg+ 97 Sulfates 97 Sr+ 97 Phosphates 99 Ca2+ 98 Acid carbonates 95
Another important factor is the Recovery Ratio, which is defined as the ratio between the flow rates of product and feed water. RO systems in the case of seawater feed are designed for recovery ratios from 20 to 35 %. The energy consumption, in desalination processes, depends on variety of factors, such as feed water concentration of salts, temperature of operation in membrane processes, performance ratio, heat losses, temperature difference etc, for thermal processes. Thermal processes that rely on a change on water phase (MED, MSF), involve higher energy consumption than processes that do not require a change of phase (Table 4). However thermal processes can utilize exhaust steam from turbines for electrical generation or hot gases from diesel engines or geothermal energy and so are economically attractive and comparable with RO energy cost.
Table 4. Energy consumption of the major desalination processes per m3 of product Process Primary Exergy of Electric energy Electric Energy energy Steam Consumption Equivalent (kWh/m3) (kWh/m3) (kWh/m3) MSF Steam 7.5-11 2.5-3.5 10-14.5 MED Steam 4-7 ~2 6-9 MVC Electricity - 7-15 7-15 SW-RO Electricity - 4-6 with energy 4-6 with energy recovery recovery 7-13 w/o energy 7-13 w/o energy recovery recovery ED Electricity - 0.7-2.5 0.7-2.5
5. Technology comparison and selection of desalination process
Selecting among desalination processes requires considerations of operability, maintenance and complexity as well as capital and operating costs. Also, the choice of one process over the other is very site specific and depends on the conditions of feed water, energy source, demographic distribution, etc. The main characteristics of the major desalination processes are shown in table 5.
Table 5. Characteristics of major desalination processes Feed Water Product Water Quality Max plant Process Energy Source Type (ppm TDS) Capacities (m3/d) MSF Seawater Steam ~10 5000-60000 Steam MED Seawater ~10 5000-20000
VC Seawater Electricity ~10 2400 SW-RO Seawater Electricity ~350-500 128000 ED brackish Electricity ~350-500 45000
The main points that have to be taken into consideration in order to compare and select between the existing desalination processes are: 1. Feed water type: Thermal and RO processes can be used for both brackish and seawater desalination and ED process is more attractive for the desalination of brackish water. The characteristics of the feed water are very important as for each desalination process define the pre-treatment needed in order to prevent scaling and corrosion problems. Pre-treatment such as aeration and acidification is used in most of the processes, but RO often requires more complex pre-treatment such as filtration and chlorination. Generally seawater quality has a little impact on the distillation processes if proper temperatures and concentrations are maintained but affect the amount of required pre-treatment in RO process. 2. Product water quality: Thermal processes produce distillate water with very low TDS (10-20 ppm). The product water from RO and ED processes is usually around 350-500 ppm. The post-treatment required varies according to the use of the produce water. 3. Plant Capacity: MSF and MED processes are available in large unit capacities. The VC is used in small and medium scale applications. MSF units are best suited for large-scale application. MED process is used for small, medium, and large- scale application. Membrane applications can be easily adapted to any plant size and can be found in all capacity ranges. 4. Capital cost: Capacity and performance ratios are the key variables affecting capital cost. Distillation plants have higher investment costs and lower operational than membrane processes. Also, land requirements for thermal processes are higher. 5. Operating and Maintenance Costs: O&M costs include energy requirements, labor and process consumables including chemicals and membrane replacement. The maintenance includes pre-treatment, periodic cleaning of the system, replacement of mechanical equipment and control instruments. The membrane processes require membrane replacement, which constitutes a major cost factor. Chemical requirements for pre-treatment and post-treatment depend on the feed water quality. Distillation processes require fewer chemicals for the feed water pre-treatment than membrane processes. Labor requirements are more or less the same for both distillation and membrane process. Generally O&M costs are less for thermal processes.
Product Water Feed Water Type Capacity Quality
Desalination Technologies
Post-Treatment Capital Cost Energy Source needed
Pre-Treatment Operation & Energy Efficiency needed Maintenance Costs
Chemicals & Complexity Auxiliary Energy
Maintenance
Labour
Selection of Technology
Figure 8.Main points for selection of a desalination technology
Total specific costs of the major desalination technologies are summarized in table 6.
Table 6. Total specific costs of the major desalination processes Total O&M Investment Energy Consumable Labour Maintenance Process costs (Euro/m3day) (Euro/m3) (Euro/m3) (Euro/m3) (Euro/m3) (Euro/m3) MSF 1000-2000 0.6-1.8 0.03-0.09 0.03-0.2 0.02-0.06 0.68-2.15 MED 900-1800 0.38-1.12 0.02-0.15 0.03-0.2 0.02-0.06 0.45-1.53 VC 900-2500 0.56-2.4 0.02-0.15 0.03-0.2 0.02-0.08 0.63-2.83 SW-RO 800-1600 0.32-1.28 0.09-0.25 0.03-0.2 0.02-0.05 0.46-1.78 ED 266-328 0.06-0.4 0.05-0.13 0.03-0.2 0.006-0.009 0.146-0.739 6. Coupling of Desalination with Renewable Energy Sources
Renewable Energy Sources (RES) are available in many areas of the world and their exploitation for desalination purposes especially in isolated areas such as islands where water supplies are limited is essential for local development. A RES driven desalination plant can be designed to operate coupled to the electrical grid or and off-grid (stand-alone or autonomous). The latter case poses the problem of renewable energy variability due to the fact that most RES lack an inherent energy storage mechanism. Most RES are stochastic in power output. The produce energy varies in time for example as wind speed or solar radiance varies so the power has to be consumed directly or else it will be lost. Because water can be stored cheaply in large quantities for long periods, this lack firmness seems not be a problem. On the other hand, the non-steady power inputs force the desalination plant to operate in non-optimal conditions and may cause operational problems. The use of an energy storage system to overcome the problem adds to the total system cost. So the main problem in matching RES to desalination processes is the intermittent of RES and the increased total cost. There are three ways to achieve the matching of RES and desalination processes: Power side management: The power supply plant is designed to provide to the desalination plant a fixed power irrespective of the prevailing energy conditions. This requires the use of a hybrid power package solution having a number of sources of power supply. These might include both renewable and non-renewable unit and also energy storage media and dump loads to dissipate excess power. Load side management: In this design the power plant is based totally on RES and the aim is to vary the characteristic of the load so all of the power produced is absorbed by the load. Integrated management: In this configuration power-matching control is applied both to the desalination plant and to the renewable power supply power plant.
The feasible RES-desalination technology combinations are in the form of a tree in the following diagram.
RES
Solar
Solar- Wind PV system Geothermal Thermal
Shaft Electricity Heat
MVC RO ED TVC MSF MED
Figure 9. Coupling of RES with desalination processes
6.1. Solar Thermal-Distillation Processes
Solar Thermal distillation plants include a field of solar collectors, where a thermal fluid is heated by the solar radiation. The hot fluid is used to warm up the brine circulating through the distillation plant. The collectors must be able to heat the thermal fluid to medium temperatures so that after appropriate heat transfer, the brine fed to the evaporator reaches temperatures between 70 to 120 0C. One major component of the system is the heat accumulator or storage tank, which is used to keep the feed water to, required temperature and compensate for the night hours or the cloudy days. The distillation unit may be either MSF or MED. MSF process requires higher temperatures (120 0C) in order to achieve the required performance ratio while MED process can operate at lower temperatures. Thermal processes require a high-energy input (due to the energy required for change of phase) and also auxiliary electricity is required for pumping. Solar Thermal systems are heavily dependent on solar radiation and weather conditions that the heat accumulator is essential. Generally solar thermal distillation application are “solar assisted” rather than stand-alone. These units are perhaps suitable when low enthalpy energy is also available. The capital cost for a Solar Thermal-MED system of around 80 m3/day can reach the value of 2140000 US$, O&M costs are about 1 ECU/m3.
6.2. Solar PV - RO or ED
The electricity form PV systems can be used for desalination applications such as pumps or in a direct current devise for electrodialysis. The main advantage of PV- desalination systems is the ability to develop small size desalination plants. The energy production unit consists of a number of photovoltaic modules, which convert solar radiation into direct current (DC). RO usually uses alternative current (AC) for the pumps, which means that DC/AC inverters have to be used. ED uses DC and can use the energy supply from the PV panels without major modifications. Energy storage is required and batteries are used for PV output power smoothing or for sustaining system operation when no sufficient solar energy is available. According to publish reports the water costs of a PV-RO system for seawater desalination ranges from 5.5 to 20 ECU/m3 for product water capacity of 120 to 12 m3/day.
6.3. Wind-RO/ED/VC
Wind energy can be coupled with RO, ED and MVC processes for the desalination of water. All three processes require electrical or mechanical energy as primary energy input, which can be provided form a single wind turbine or a wind farm. The selection between the three technologies depends on the feed water quality and the required product water quality. MVC as all distillation processes produce water with very low salinity (below 20 ppm TDS), while membrane processes (RO and ED) produce water with higher salinity (500 ppm TDS). Also ED process is usually used for the desalination of brackish water (up to 6000 ppm TDS). The main characteristic of wind energy is its stochastic nature, which is due to the random nature of wind velocity. Because there is no trend or model like day/night type for solar energy the energy output is difficult to predict. Appropriate power control and conditioning systems are required for the matching of the input power to the desalination load. RO and MVC require AC, while ED requires DC. The power system requires energy dissipation and storage device (flywheels, batteries). Generally, it is difficult to control the usage of wind in a cost-effective way. Coupling of a variable energy supply system to a desalination unit requires either power or demand management. The mean water cost for a wind powered MVC seawater desalination plant that runs stand-alone varies form 3.07 to 3.73 Euro/m3 for capacities 5 to 12.5 m3/h of distillate.
6.4. Geothermal-Distillation processes (MED & MSF)
Geothermal energy is one of the indigenous and environmentally friendly energy resources and has been used successfully for over three decades both for electricity generation and direct utilization in many parts of the world. Geothermal energy has a number of positive features, which make it competitive with conventional energy sources and some renewable sources. These features include: It is a local energy source that can reduce demand for imported fossil fuels thus having a large positive impact on the environment. It is efficient and competitive with conventional sources of energy. The geothermal energy supply is without constraints imposed by weather conditions, unlike other renewable sources. It has an inherent storage capability and is best suited to base-load demand. It is a reliable and safe energy source, which does not require storage or transportation of fuels.
Geothermal resources are suitable for many different types of uses but are commonly divided into two categories, high and low enthalpy and according to their energy content. High enthalpy resources (>150 °C) are suitable for electrical generation with conventional cycles, low enthalpy resources (<150 °C) are employed for direct heat uses and electricity generation using a binary fluids cycle. Geothermal energy is ideal for distillation processes and usually the MED process is preferred due to the lower energy requirements in comparison with MSF process. Generally geothermal energy applications tend to be very site specific and design decisions for one location may not valid for another. Some important considerations are: The drill depth, which affects the temperature, which determines the number of effects in the desalination plant. The distance of the geothermal well from the sea. Whether to use geothermal ground water itself as the feed water for the desalination plant in preference to using its heat to desalinate seawater.
Generally, water costs of less than 1 Euro per m3 of product water are possible, which make the geothermal-MED coupling very attractive.
References Saline water conversion-II, Advances in Chemistry Series 38, American Chemical Society, Washington D.C. , 1963.
Wayne M. Wagner, Donald R. Finnegan, Select a Seawater desalination, Chemical Engineering February 7, 1983
The Desalting ABC’s, International Desalination Association, 1990
Koroneos, C., Dompros, A., Roumbas, G., Renewable energy driven desalination systems modelling, Journal of Cleaner Production, 15 (5), p.449-464, Jan 2007
Fernández-López, C., Viedma, A., Herrero, R., Kaiser, A.S., Seawater integrated desalination plant without brine discharge and powered by renewable energy systems, Desalination, 235 (1-3), p.179-198, Jan 2009
Eltawil Mohamed A., Zhengming Zhao, Liqiang Yuan, A review of renewable energy technologies integrated with desalination systems Renewable and Sustainable Energy Reviews, 13 (9), p.2245-2262, Dec 2009
Michael Papapetrou, Christian Epp, Senem Teksoy, Seval Sözen, Vicente Subiela Ortín, Ulrike Seibert, Gisela Vogt, Co-ordination action for autonomous desalination units based on renewable energy systems — ADU-RESEpp, Desalination, 168, p.89- 93, Aug 2004
Desalination Guide Using Renewable Energy Sources, JOULE-THERMIE, 1998
Mohamed A. Eltawil, Zhao Zhengming, Liqiang Yuan, A review of renewable energy technologies integrated with desalination systems, Renewable and Sustainable Energy Reviews, Volume 13, Issue 9, December 2009, Pages 2245–2262