Critical Review Energy Issues in Desalination Processes RAPHAEL SEMIAT* Rabin Desalination Laboratory, Grand Water Research Institute, Wolfson Faculty of Chemical Engineering, TechnionsIsrael Institute of Technology, Technion City, Haifa 32000, Israel
Received May 14, 2008. Revised manuscript received August 29, 2008. Accepted September 2, 2008.
Water, energy, and environmental issues are closely related. Newwatertechniquesconsumeenergy,andinnovativerenewable energy techniques using biofuels and biodiesel consume an incredible amount of water. Different desalination techniques that consume different energy levels from different sources are in use today. Some people, environmentalists, decision makers, and even scientists, mainly in nonscientific publications, consider energy consumption in desalination to be too high and are seeking new ways of reducing it, which often involves increasing capital investment. Efforts should be directed at reducing not only energy consumption but also total water cost. A competent grasp of thermodynamics and heat and mass transfer theory, as well as a proper understanding of current desalination processes, is essential for ensuring beneficial improvements in desalination processes. Thermodynamics sets the absolute minimum limit of the work energy required to separate water from a salt solution. Unavoidable irreversibilities augment the actual energy consumption, yet modern desalination techniques have succeeded in considerably narrowing the gap between actual and limiting energy levels. The implication of this smaller gap is that only marginal energy reductions are possible. The current energy consumption of different desalination processes is reviewed in this paper. A comparison with other common energy-consuming ventures leads to some interesting conclusions.
Introduction Modern life has reduced the percentage of people living on agriculture. Most of the population is concentrated in large cities, places that had served in the past as the basis for settlements around water sources. Over the centuries, the population grew well beyond its ability to supply its water needs. The need for water is therefore essential in many places, while good-quality water flows into the sea in other places. Conveying water from place to place proved to be uneconomical. It is well-known that more than 97% of water on earth has accumulated in the oceans and cannot be used. A quarter of the world population uses water melted from snow and glaciers. Three quarters of the world consumes surface water and aquifer water, altogether less than 0.4% of all of water on earth. Many techniques are available for supplying more water. The most important is wastewater reuse, especially due to the necessity for cleaning water to prevent pollution. Treated wastewater is used all over the world for irrigation and * Corresponding author phone: 972-4-8292009; fax: 972-48230476; e-mail:
[email protected]. 10.1021/es801330u CCC: $40.75
Published on Web 10/22/2008
2008 American Chemical Society
drinking (Europe and United States) and even in very clean industries such as semiconductor manufacturing plants (FABs) in Singapore. Slightly polluted or brackish water is relatively easy to treat, yet the main, unlimited water source comes from the oceans and the seas. Over the years, several desalination techniques have proven to be feasible and are serving as appropriate sources. Reverse osmosis (RO) is currently the fastest growing technique for treating different types of water. Multistage flash distillation is still the most common technique, especially where energy is still not an issue. Multieffect distillation has a greater potential as an evaporation technique, while vapor compression may be suitable for resort areas or small sites. Other techniques have proven to be inefficient, such as the freezing technique, solar stills, and more. Other new technologies are trying to pop up to challenge the existing, already well-established techniques. However, with the continuous energy crisis we are facing, the question of how much energy is needed to make water is constantly emerging. Unfortunately, significant misunderstandings and misinterpretations still prevail, even in the best published papers. This paper will attempt to make some sense of this issue. Prof. Silver, one of the best known desalination developers, said the following about 30 years ago during a different energy crisis (1): “The production of fresh water from seawater or other contaminated water when seen against all the vast and varied industrial activity of the modern world may seem a small thing. But unless it is provided it could prove to be the nail for lack of which the whole battle of civilization might be lost, even if we solve the energy supply situation.”
Water, Energy, and the Environment The future of humankind will be affected by three interrelated major endeavorsswater, energy, and environmental protection. On one hand, energy requires water for cooling purposes, and on the other hand, water is needed for growing biofuels and for biodiesel. Intensified biofuel consumption has already increased food costs, thus straining living conditions of inhabitants in Third World countries. Problematic environmental effects associated with burning oil or biofuel for energy generation are air pollution and global warming. Improper water usage through uncontrolled squandering and neglect of wastewater reclamation are intensifying water pollution and water shortages. In this paper I will concentrate on issues related to energy needs for medium to large water desalination processes. Understanding of real energy consumption in water desalination processes is essential for the scientific community as well as for the decision makers. Minimum Energy for Separation. A thermodynamic analysis of the energy requirement of some desalination VOL. 42, NO. 22, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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processes has been described by many researchers (2-6). Equations for W, the minimum isothermal reversible work of separation, which is applicable to any desalination process regardless of the separation mechanism (2), are presented: -W ) ∆H - T∆S ) ∆F
(1)
where ∆H represents the change in enthalpy between the final and the initial stages, ∆S represents the change in entropy, and ∆F is the change of the free energy. Introducing the free energy relations to the molar concentration of the salt in water results in -W )
∫ ∆F dn ) ∫ RT ln a
w
dn )
∫
n2
n1
RT ln
p dn (2) p0
where n represents the number of water moles in the solution, R is the gas constant, aw is the water activity in the solution, 1 and 2 represent the initial and final stages of water separation from the solution, respectively, and P is the water vapor pressure assumed as an ideal gas. The final expression for the minimal separation energy is given by -W )
0.296T 100 - n2
∫
n2
100
log
p dn p0
(3)
Equation 3 directly gives the theoretical minimum separation energy in (kW h)/m3 of water product. From eq 1 it is possible to calculate that the energy needed per cubic meter produced from an infinite source of 3.5% salt concentration seawater is given by -W ) 0.296T log aw
(4)
as a value of 0.79 (kW h)/m3. However, for 50% recovery based on eq 3, the energy demand is 1.09 (kW h)/m3. On the basis of the chemical potential, µ, the minimum isothermal reversible energy is given by -W ) ∆F ) µc + µp - µf
(5)
where the letters c, p, and f are concentrate, product, and feed, respectively. Utilizing thermodynamic relations F)
∑xµ
µi ) RT ln ai + µi0
i i
(6)
leads to a different equation: -W ) 2.18T(nc(xc log asc + (1 - xc) log awc) + 0.01725mf) (7) where c, s, w, and f represent concentrate, salt, water, and feed, respectively. Introducing values of a into eq 7 yields values similar to those of an integration of eq 3. Similar results have been obtained (4) by direct integration of the third term in eq 3 by using an equation for aw based on the osmotic constant φ:
∑ m /55.51)φ
ln aw ) -(
i
(8)
where mi represents the molality of the different ions. A value of 1.345 (kW h)/t for seawater at 4.5% was suggested (5). The values presented are indifferent with respect to the separation technique. More information on energy consumption in desalination processes may be found (6). Thermal distillation processes suffer from the disadvantage of a high energy penalty associated with irreversibilities in the process. Irreversibilties account for energy losses due to friction, heat losses to the environment, heat losses due to minimal driving forces, losses due to boiling point elevation (BPE), and more. A simple thermodynamic analysis (2, 3) based on ideal Carnot cycles leads to the following equation: 8194
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{
∆Tirrev 1+ Wirrev R ) Wrev η
}
(9)
where Wirrev is the work (energy) required for separating a unit of water in an actual process, Wrev is the minimum separation energy for a reversible process, and η is the Carnot efficiency of the process. The term ∆Tirrev expresses all irreversibilities in a process in terms of the temperature driving force and R is the boiling point rise (BPR). BPR (R) is the temperature difference in a minimum energy reversible separation process. No evaporation can take place when the temperature difference is equal to or smaller than the BPR value. Assuming a 50% recovery process of an initial 3.5% NaCl solution using relations in ref 2, we find that R ) 1 °C and Wrev ) 1.9 (kW h)/m3. Reverse osmosis seawater desalination today consumes about 3.7 (kW he)/m3 of product. Inserting these values into eq 9, we find that, with a Carnot efficiency of 100%, a thermal process aiming to achieve an energy consumption similar to that of an RO process must operate with temperature driving forces less than 1 °C (∆Tirrev expresses all irreversibilities, not only the temperature driving force). If we take a Carnot efficiency of 75%, the temperature driving force should be less than 0.75 °C. Thermal desalination usually works under a much lower Carnot efficiency. All distillation studies consider systems having significantly higher temperature driving forces.
Actual Energy Demand for Different Desalination Systems Evaporation Techniques. The simplest desalination technique, single-stage evaporation of seawater, consumes a tremendous amount of energy, around 650 (kW ht)/m3 of product, depending slightly on the evaporation temperature. The main evaporation techniques, MSF and MED, have overcome this obstacle by reusing energy consumption through multiple stages. About 50% of the world desalination production is based on the MSF technique in the Persian Gulf. The number of times the heat of evaporation is reused is represented by the “gained output ratio”, GOR, defined as the ratio of the number of tons of water produced per ton of steam as energy invested. MSF manufacturers provide a GOR design in the range of 8-12, depending on the steam feed temperature. MED manufacturers claim a GOR of 10-16 in working units and up to 30 in designed prototypes. Thermal energy based on fuel consumption in typical “single-purpose” plants is in the range of 55-80 (kW h)/m3 of product for MSF and 40-65 (kW h)/m3 of product for MED. The work equivalent of these energy consumptions, based on an efficiency of about 45% in a modern power station, is 24-37 (kW he)/m3 for MSF and 18-30 (kW he)/m3 for MED. Pumping energy must be added to this, claimed to be around 1.2-4.5 (kW he)/m3 for both techniques. Table 1 illustrates the confusion in the literature regarding real energy consumption in desalination processes. Estimations are taken from different sources where the real equivalent energy is not always clear. Energy consumption of thermal desalination processes may be reduced considerably in “double-purpose” plants by using heat released by the turbine condensing steam to provide the primary thermal energy required for an MSF or MED desalination process. High efficiency of electrical energy production requires a low steam condensation temperature close to the ambient temperature. Back-pressure of the turbine is therefore increased to provide an adequate heating steam temperature in the range of 70-120 °C. The cost penalty is reduced electricity production per ton of steam, as explained below. The actual work energy consumption of thermal processes in double-purpose plants is around 4-7
TABLE 1. Different Energy Requirements for Industrial Desalination Techniquesa ref 7 8 5 9, 10 11 12
technique RO MSF MED MSF MSF variations MED MED MSF RO RO
heat requirements (thermal) [(kW h)/m3]
electricity requirements (pumping) [(kW he)/m3]
combined energy demand [(kW he)/m3]
40-120 (thermal) 30-120 (thermal) 25-114 (thermal) 34-102 4-5 (electricity)
4-6 2.5-5 2-2.5 4.8 2-2.2 1.5
4-6 21-58 15-58 not clear 17-47 5-6.5 6-12 10-16 3.8 3-4
a
Estimations for thermal units depend on the GOR achieved in the plant and on the steam temperature. The far-right column represents equivalent energies in terms of electrical energy for comparison.
TABLE 2. Energy Consumption for Distillation Techniques Based on Exiting Steam and Heat Losses at Different Temperaturesa energy consumption including pumping [(kW he)/m3] temperature working range of exiting steam (°C) 70-35, MED 100-35, MED 120-35, MSF MED energy losses MSF energy losses a
electrical power potential in the power station (kW he)
boiling point elevation range(13) (°C)
17.0 30.7 38.9
0.98-0.34 1.18-0.34 1.31-0.34
GOR ) 10 4 5.6 8.4 13 17
GOR ) 16 5 6.9
BPE values of seawater are included. Energy values are calculated from steam tables.
(kW he)/m3 of product for steam supply and 1.2-2.5 (kW he)/m3 of product for pumping energy. On a summer day, the seawater cooling system at the end of a multistage evaporation plant may allow the steam to work down to 35 °C. Back-pressure turbine steam at 70 °C can produce up to 17 kW he in the turbine, assuming high efficiency, since most of the energy of this steam was already used or dissipated. The MED process working at this range can produce 16 stages according to claims, with a GOR value of 12, due to heat losses. This means almost 2.2 °C/stage, or an average of 1.5 °C as the net average temperature difference driving force per stage, based on an average of 0.7 °C BPE (Table 2). The total energy consumption of this MED process could theoretically decrease to an equivalent of 1.5 kW he for the evaporation process, or 4 kW he/m3, including the energy for pumping water through the system, control, lighting, etc. Similar calculations may be made for different MED and MSF conditions. Some results are shown in Table 2, where the energy consumption is estimated. The calculations presented here are based on the assumption that the final vapor condensation at the last stage is operated by the electricity plant, that the plant also agrees to review the problems involved together with the customer, that maintenance problems may also interfere with regular production, etc. Hybrid plants apparently work well in the Persian Gulf where power and water supply usually belong to the same management. This method also works in refineries where access energy is available and water is required. More information on desalination processes may be found in refs 14-17. The above estimation appears to be too low since energy losses to the environment may be high. Another way of looking at this is through heat losses from the system walls. Producing an overall heat balance in an evaporation desalination system, the heat supplied by an external source is equal to the overall heat leaving the system. Assuming an MED system with 50% recovery and efficient heat usage, every cubic meter of product leaves the system at a
temperature higher by about 5 °C than the seawater temperature. This is the same for the concentrate leaving the system, resulting in system losses of 10 °C/m3, totaling about 11.6 kW h or 5.2 kW he. These values highly depend on the equipment invested for heat recovery. A similar value may be considered for heat losses through the boundaries of the system, so that total energy consumption is close to 13 kW he including pumping. In MSF with circulation, assuming a higher initial temperature and 40% recovery, the energy consumption may reach around 17 kW he, including the pumping energy. Apparently, the real values lie between those values and the values presented in Table 2. The values estimated above may be reduced on the basis of equipment expenses. Reducing the temperature difference at one or more stages or in the heat recovery between the exit steam and the feedwater involves a larger heat transfer area and reduces energy consumption. The temperature difference across heat transfer surfaces requires larger heat transfer surfaces or a greater investment. An extreme case of this is the use of solar energy. This energy source may be considered as “energy free” since no fuel is used since it is based solely on free sun, yet the cost of this free energy is high due to the large area needed. An optimum exists between the cost of energy used and the cost of the investment. RO Technique. The specific energy for RO desalination has decreased significantly over the past decade and is approaching the theoretical thermodynamic minimum. This was achieved through the development of large pumps having an efficiency of approximating 90% using modern efficient turbines and other energy recovery devices. The newer devices, known as “turbochargers,” “pressure exchangers”, or “work exchangers”snames adopted by different manufacturerssrepresent efficient ways of recovering the energy content of the high-pressure concentrate leaving the membrane module. Turbines convert the concentrate pressure into the velocity of a jet that spins a wheel. This is used either to reduce the power consumption of the motor driving the pump or to boost the pressure of the feed to a second stage. VOL. 42, NO. 22, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Energy Balance, Large-Scale Plant (51) energy (kW he) no. of pumps intake raw water supply feed booster high-pressure aggregate pumps turbine motors auxiliary + lighting total
flow
(m3/h)
diff head (bar)
2200 2200 1042
1.0 2.5 7.7
77 192 281
462 1154 3368
12 12 12
1042 521
69.3 73.0
2381 -980 1444 400
28567 -11763 17323 400
“New” Desalination Processes Recent publications in the scientific literature refer to socalled “new” desalination processes. The physics behind these processes is known. The processes were also mentioned in the old literature, as can be seen below. It is important, however, to analyze the energy needs of these processes. Evaporation-Based Humidification-Dehumidification (52). This is one of the oldest desalination processes. Sailors in ancient times used the technique to collect drinking water during the night after the temperature dropped below the saturation level (dew point); water could be collected underneath the sails. A modern approach based on this “new” technique is illustrated in Figure 1. Hot seawater or salted water is allowed to pass through a stream of relatively dry air. The air is heated, and the water evaporates. The air exits this stage at a higher temperature, depending on the feedwater, and it contains more water. The hot wet air is then allowed to pass through a heat exchanger fed with cooling water. The temperature drops below the dew point so the water is condensed and accumulates as a product. The higher the temperature of the exiting air (the humidification stage) the higher the air humidity. Take, for example, saturated air at 80 °C. The air humidity can easily be found in psychrometric charts; the value is 1.28 kg of water/kg of dry air. Assuming the air is cooled to 35 °C, the humidity of the air exiting the system is 0.09 kg of water/kg of dry air. Assuming all humidity changes are collected (100% saturation at the high temperature and no fog exiting at the 9
total
6 6 12
Additional methods are available for conveying the concentrate stream pressure to the seawater feed steam using simple devices (18-50). The above techniques enable considerable reduction in power consumption used to pressurize the feed. For instance, the pressurizing energy consumption in desalting 3.5% seawater at a recovery rate of 50% using turbines for concentrate energy recovery is as low as 2.7 (kW he)/m3 of product. Pressure exchangers can reduce energy consumption even further to 2.2 (kW he)/m3. Taking into account the additional pumping requirements involved in conveying the feed, concentrate, and permeate steam, the overall energy needs are less than 3.7 (kW he)/m3 of product in seawater desalination. In other desalting applications involving lowsalinity raw waters, such as brackish water streams or slightly polluted resources, energy consumption can be as low as 1 (kW he)/m3 of product or less, depending on the water salinity and energy recovery availability. Working at a lower pressure may save energy, yet greater investment in equipment is required. Table 3 presents the itemized energy consumption for different tasks in a large RO plant. It is important to note that the energy consumption for small RO plants may increase due to the lower efficiency of smaller pumps.
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specific energy [(kW he)/m3 of product) 0.07 0.18 0.54
2.77 0.06 3.63
low temperature), the amount of water collected from 1 m3 of dry air is approximately 1.2 kg. The amount of dry air needed to produce 1 m3 of water under the above conditions is 700 m3, or 1680 m3 of wet air/m3 of water produced. The operation of such a process is possible when hot water is passed once through the air, or the salted water is heated against the condensing vapor from the wet air and a final higher temperature from another hot heat source is reached. The first option presents a hot source of water; hot and salted spring water may be used this way. The second option is when the salted water is heated with another source of waste heat. The first option represents a case where the energy demand for the water production is the evaporation heat, namely, around 650 (kW h)/m3. This does not include the pumping expenses of the hot water, the air flow, and the cooling water. In fact, in terms of energy, this is the highest energy demanding desalination process that may consume above 800 (kW h)/m3. The second option uses another source of energy, yet the GOR value is not high and energy demand is still extremely high. In fact, at high temperatures, as presented in some reports, it is possible to use the heat source to produce electricity and then run an RO system. The energy demand will be much lower, and the production efficiency from the source of waste heat will be much higher. MED operation will produce more water at a much higher efficiency and at a lower cost. The so-called humidification-dehumidification desalination process has other problems. First, the heat transfer coefficient of the condensing vapor from air is much lower than for pure water, and hence, the heat transfer area needed is enormously high, at least 100-fold, depending on the temperature and humidity of the exiting air. All of the pollution in the air used for the process is concentrated in the productsusually dust in remote locations, air pollution
FIGURE 1. Schematic of the humidification-dehumidification process.
FIGURE 2. Schematic view of a membrane distillation design based on MED technology. in cities, and even bacteria and virusessso the water quality produced is poor. Membrane-Based New Techniques. Recent developments have brought significant attention to other types of membrane processes reported mainly in research papers: forward osmosis (53-55) and membrane distillation (56-59). Forward osmosis (FO) or direct osmosis is defined as the water passage from the salt solution or the polluted solution through a membrane to a solution containing dissolved matter of higher osmotic pressure. Loeb (60-62) tried using direct osmosis for energy production from salt and pure water. A possible advantage of such a process is that separating the water from the higher osmotic pressure solution is easier than through the RO process. Such separations were proposed by using magnetic nanoparticles covered with organic matter and separated by a magnetic field (a company called Nanomagnetics published this idea in 2004), distillation of the dissolved material like in cases of ammonium carbonates (53, 63), or possible simpler separation such as crystallization. Removal of the nanomagnetic particles against sheer forces in high flow rate devices involves strong magnetic fields and subsequently high energy consumption. Separating ammonium carbonate from water solution consumes a significant amount of energy, similar to that of water evaporation. It is impossible to separate this salt by evaporation without significant water evaporation, so the energy demand will be higher than conventional high osmotic pressure techniques (RO, NF). Care is needed to avoid osmotic material residuals in product water according to drinking water regulations. Thermodynamics, however, teaches us that the minimum separation energy depends on the concentration. The concentration of the ammonium car-
bonate is much higher than the concentration of the salt in the feed. Therefore, the FO process needs higher theoretical and actual energies. RO separation is closer to the minimal thermodynamic separation energy. Therefore, the aim is to find a process that consumes lower energy than the RO process. The membrane distillation technique is based on open hydrophobic membranes that allow only the passage of water vapor (59, 64). The product quality is expected to be better than that of RO since only water vapor is expected to pass through the membrane, yet like in RO membranes, leakage occurs. Vapor condensation is allowed on colder surfaces adjacent to the membranes or outside the membrane module where vapors are pumped out. Another way is to condense the vapor in direct contact with a cold water stream. The main problem in these techniques is the need to evaporate the water, which involves an energy demand of around 650 (kW h)/m3. This enormous amount of energy may be reduced when energy reuse is possible, similar to the multieffect distillation desalination process. This may cut energy demand to about 60 (kW h)/m3 if energy is reused more than 10 times in the desalination plant. More energy is needed for pumping the water and for the cooling system. Heat transfer flux through the membrane is low; therefore, a large transfer area is needed. Developers of this technique claim that the high energy demand may be supplied by lowgrade, cheap energy, yet this claim also holds for the multieffect distillation process. The only possible advantage of the MED technique is the possibility of lower volume and a low plant footprint. However, the design is complicated, as shown in Figure 2, representing a preliminary membrane distillation design with heat recovery, similar, but not identical, to the MED process. VOL. 42, NO. 22, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Energy demand in the RO process as a function of water recovery. In this design, entering seawater exchanges heat with exiting hot streams of product and concentrate. External heat must be added to the feedwater. The water is then allowed to flow parallel to the membranes where evaporation takes place. Vapor leaving the membrane condenses on the heat transfer wall and transfers the heat condensation to the water in the next stage. In this way, the evaporation energy is reused to heat the concentrating solution. Other designs are possible, yet in terms of energy demand, the technique cannot compete with RO. As previously mentioned, it is similar to MED but suffers from low heat transfer properties and will therefore need more energy. A report on the construction of a membrane distillation process pilot plant similar to MSF operation, called Memstill (65), claimed a possible GOR of 30. If this is achievable, taking into account temperature limits and the BPR of the evaporating water, the energy consumption here will be at least 15 (kW he)/m3 of product. Connecting such a system to a power station may reduce energy consumption, as explained earlier, involving the same and even more problems.
Energy Reduction in RO Processes The contention raised in the literature about a potential energy reduction in the RO process by 50% or even 75% is unrealistic. However, even with the current low energy consumption in the RO process, there is still room for improvement. Savings are possible by using better membranes having a high permeability without losing rejection capabilities. This would enable a reduction in operating pressure and lead to energy savings. In addition, improved pretreatment and fouling control measures would facilitate operation under more optimal conditions. Analyzing the components of RO desalination costs shows that the main ones are energy cost and equipment investment. Itemizing the equipment, membranes, pressure vessels, pumps, tubing, flow devices, and energy recovery units shows that no special item is significantly more expensive than the other. However, membranes play the most important role in possible cost reductions. RO membranes cost about 8% of the overall investment. Membranes may still be improved significantly. Permeation may be increased, maintaining similar rejection properties. Increased flux through the membranes will allow a pressure reduction, and hence less energy, at the same recovery ratio. A reduction in pressure may also reduce the costs of expensive metals used with high-pressure piping in a highly corrosive environment. This will reduce the cost of pumps and flow devices. Larger membrane modules will reduce the plant’s footprint. Figure 3 illustrates an analysis based on the energy consumption of an RO process and pumping cost as a function of the recovery ratio. The analysis was made on the basis of current RO performance with 50% recovery. The line of the RO process was calculated by increasing energy with recovery on the basis of pressure changes. The pumping and pretreatment line was calculated on the basis of increased 8198
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energy demand with increased feedwater to supply the original production. It is possible to see how the energy demand of the RO process increases with the recovery ratio, while the energy required in the “pumping” line decreases with recovery. The two lines are summed into a third line that represents the overall energy required as a function of recovery. This line clearly has a shallow minimum, around 50% recovery. It is obvious, therefore, that, even with major changes, the energy consumption of an RO process cannot be reduced significantly. By improving the RO membranes, it will be possible, yet extremely difficult, to reduce the actual RO energy consumption by 10-30%, which may result in an overall change on the order of 15%. RO will continue to lead in the near future, in the desalination industry, as the lower energy consumption technique.
Energy-Related Issues The energy cost of an optimized desalination plant is approximately 30-44% of the total cost of water produced. Current optimal conditions may move toward higher investments due to the increased cost of oil. As an extreme example, fossil energy may be replaced theoretically by solar collectors of different types. The problem is that since solar energy is available only 25% of the time, the investment fraction increases by a factor of 4, before taking into account the equipment needed for electricity production. The cost of water produced by solar collectors will be much higher compared to that of water produced with the use of a regular energy source. Solar desalination has been investigated over the past 50 years, yet no commercial small- or large-scale plant is currently in operation. More information on renewable energy and desalination may be found in refs 68-75. The optimization is made during the plant design, yet the energy cost may vary significantly during a project’s lifetime. For example, during the writing of this paper, the cost of natural oil was increasing at a significant rate in comparison to its cost at the design stage of the Ashkelon desalination plant. It is difficult to change the optimal design of a plant once it has been built. However, it is possible to minimize losses by designing for more flexible changes in terms of variable energy consumption and equipment costs. A 100 million m3 RO-based seawater desalination plant requires an electrical energy supply of less than 50 MW. A desalination-dedicated power station can work at a much higher efficiency than a regular power station since it operates constantly without the known sine wave, representing day-night and summer-winter changes in consumption. Better efficiency is expected for gas turbines since the high temperature of the gases may also be used. Therefore, the real energy required is lower than for other common uses. Critics among environmentalists are often heard expressing concern about energy consumption for water desalination. Some critics, mainly in nonscientific debates, express concern about the environmental impact of energy consumption involved in water desalination. Water is an essential human need, and energy usage for desalination should take priority over wasteful modern day energy usages such as excessive air-conditioning and high-energy-consuming cars. Table 4 analyzes the energy derived from various fuel sources (natural gas, gas oil, heavy fuel, and coal) in desalination versus other usages. A large-scale power station generates electricity from coal or heavy fuel at an efficiency of around 45%. If operated effectively, gas turbines may reach an efficiency as high as 80%. The magnitude of the energy consumption in desalination can be more easily appreciated by referring to the volume of desalted water produced by RO from 1 kg of fuel or the mass of fuel required to produce 1 ton of water. The table shows that 1-2 tons of desalted water can be produced with 1 kg of fuel, making water the cheapest
TABLE 4. RO Energy Consumption Compared with Other Alternatives natural gas caloric value, kcal/kg of fuel caloric value, (kW he)/kg of fuel electricity production (45% eff), large power station, (kW he)/kg of fuel electricity production (80% eff), high-efficiency gas turbine, (kW he)/kg fuel capacity, seawater desalination (50% recovery), m3/kg of fuel 80% efficiency fuel consumption, kg of fuel /m3 of product water 80% efficiency How many km can I drive in the city with 1 m3 of desalinated water fuel consumption? How many hours can I air-condition a single room (2.5 kW he)? a
9000 10.5 4.7
gas oil
heavy fuel
coal
10750 12.5 5.6
10000 11.6 5.2
7700 9 4
1.6
1.5
1.2
0.6
0.7
0.9
8.4 1.3 2.4 0.7 0.4 2-10a
2-9a
1.4
Depends on the car type, conditions, etc.
commodity manufactured. The fuel consumed to produce 1 m3 of seawater can drive a typical car in a city journey a distance of only 2-10 km. The same amount of energy may be used to operate a small room air-conditioner for 1.4 h. Clearly, the energy cost for desalination is rather small compared to that in other common energy usages. Environmental concern about the CO2 “greenhouse” effect associated with the use of fossil fuel has led to the goal of supplying desalination energy from renewable energy sources. Renewable energy sources may soon be compatible and economical for general electricity production. At that stage, they will also be suitable for desalination purposes. No doubt, greater efforts should be devoted toward exploiting renewable energy sources. However, the real test of any new energy source is its acceptance for electricity production or other common energy uses. Savings on CO2 emissions must be made in terms of other energy forms, and not regarding the very sensitive issue of desalination for freshwater production. The use of nuclear energy, which is currently still more expensive than fossil energy, is dangerous in areas where political instability prevails. It is also problematic where the technology is not accessible, and it is necessary to rely on imported, trained, and sophisticated labor. A possible method of efficient energy use in a sufficiently large desalination plant involves the design of different types of hybrid plants consisting of a membrane unit combined with a vapor compression unit that uses electrical energy and a multieffect evaporation plant that uses heat energy (66, 67). Such an operation is common in the chemical industry. Energy costs could be minimized by coupling a desalination plant with a dedicated power plant generating electricity and waste heat at optimal economic conditions. The advantage of the day-night and summer-winter electricity production cycle is that desalinated water is produced during the night, involving lower power consumption. The main disadvantage is that the desalination equipment is not used for a large percentage of the time. This is a mistake, since, as in any modern plant, production costs are greater if the equipment is not in full use. An efficient desalination plant should therefore be operated 24 h a day, 365 days a year, with exceptions for maintenance only. During this time, a full energy supply is required at the lowest cost. In some cases, it will be possible to combine solar-operated RO during the day with a low tariff grid operated RO during the night, if the energy costs during the day are comparable with that of fossil energy. Equivalent Energy Usage for an RO Desalination Plant. It is interesting to compare energy consumption for desalination with other common energy consumptions in a
household environment. A small family typically consumes water at a rate of about 18 m3/month and uses about 1200 kW he of electricity or other equivalent energy sources per month. If the water is obtained from a seawater desalination plant, the energy consumed for water production (excluding water transportation) is 140 (kW h)/month (fuel value). Moderate driving of a family car amounting to about 1500 km/month consumes 160 L of gas oil/month. Therefore, the energy consumed (expressed in thermal units) for water production is 140 (kW ht)/month, for driving the car it is 1500 (kW ht)/month, and for electricity it is 1200/0.45 ) 2667 (kW ht)/month. These figures show that energy for desalinated water is 9.3% of the transportation energy, 5.2% of the electricity energy, and 3.2% of the total family energy consumption. Clearly, use of desalinated water is affordable for typical households.
Proper Water Usage Desalinated water is considered expensive compared to traditional water sources. The problem starts when these sources are not available. Water savings are possible in households, yet significant efforts are required. This is also true for energy consumption. Some more interesting points are as follows: (1) The cost of water is negligible in regular households compared to other utilities payments. (2) The cost of water is tolerable for most industries. This was achieved in many places by enforcing strict regulations on effluents disposal. (3) The cost of water is significant in agriculture. Better usage of water in agriculture may be accomplished by (i) using greenhouses and retreating and recirculating water and (ii) using drip irrigation, which saves 30-90% of water consumption compared to other irrigation techniques, thus reducing the problem of cost. This paper was written to increase awareness of the research community about the real information regarding energy consumption in salted water desalination. Too much confusion in the literature on this issue exists today, and even the best researchers are making common mistakes in this respect. The main conclusions that may be drawn based on the analysis made here are as follows: (1) Energy consumption for desalination is not very high, especially compared to that utilized in other common household utilities. (2) The current energy consumption for desalination is close to the minimum energy set by thermodynamics; it is impossible to go below this limiting value. (3) Currently, the lowest energy consumption is achieved by reverse osmosis desalination integrated with energy recovery devices. (4) Distillation techniques consume more energy. Combined with electrical power, the energy consumption is higher than VOL. 42, NO. 22, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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in the RO process but is not extremely high. (5) Burning fuels or the use of so-called “low energy sources” consume much more energy than techniques currently available. (6) The so-called “new technologies” for water desalination should be examined, and a real comparison with technologies in use should be made. (7) Examining the real energy consumption of desalination processes, it is important to emphasize that other parameters are involved. The best way is to optimize the designed plant at a given site regarding equipment cost, manpower availability, and other important parameters. The best way is to optimize the designed plant at a given site regarding equipment cost, manpower availability, and other important parameters.
Nomenclature aw BPR GOR F H m MED MSF n P P0 R RO S T W x
water activity in the solution boiling point rise gained output ratio free energy enthalpy molality multieffect distillation multistage flash evaporation molar concentration vapor pressure vapor pressure of pure water gas constant reverse osmosis entropy temperature work (energy) concentration
Subscripts c concentrate f feed e electric i component i irrev irreversible p product rev reversible s salt w water Greek Letters R value of BPE φ osmotic constant η Carnot efficiency µ chemical potential
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