Chapter 8 Salinity Energy

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Chapter 8

SALINITY ENERGY As many fresh streams meet in one salt sea...

King Henry V, Shakespeare

Ocean energy is mostly bound in thermal and chemical forms, yet where rivers flow into the ocean another, practically untapped source of energy is available. A Greek term could be forged to designate it: “halothalassic energy” ;it is represented by a large osmotic pressure difference between freshwater and salt water. In recent years increased thought has been given to tapping, at reasonable economic cost, these salinity gradients for electricity generation. SALINITY

Salinity is defined as the total amount of solid material dissolved in seawater when all the bromine and iodine have been replaced by the equivalent amount of chlorine, all the carbonate has been converted to oxide, and all the organic matter has been oxidized completely. Salinity is expressed in grams per kilogram of seawater, thus in parts per thousand. Salinity is distributed both horizontally and vertically. Extreme salinities occur in enclosed seas and near certain coasts. While the normal seawater salinity varies from 33 to 37%0, in some Red Sea locations it reaches 42%0 and in the Gulf of Bothnia it falls to 5%0. In some areas of the world powerful rivers debouch in the ocean bringing into contact large masses of freshwater and the saline seawater. The energy thus released equals powerful heads, and. if captured, could provide considerable electrical power; the basic principle of operation is osmosis. The mixing of 1 m3/s (35.3 ft3/s) of freshwater with seawater would release 2.24 MW of energy. Loeb and Bloch thus calculated that 30 million MW could be captured yearly! The Amazon River would generate 470 x lo3 MW, the Congo River, 130 x lo3 MW Even all the wastewater from the continental United States would generate theoretically 11,000 MW. A 213 m (700 ft) head is thus obtained in some sites. When a high salinity solution is separated from a solution of low salinity by a semipermeable membrane that allows the passage of water but not of the salts dissolved therein, osmosis takes place. Water flows from the low-salt-concentration solution into the high-concentration solution, so that both solutions will be at the same concentration (osmotic equilibrium is then reached). However, in the

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process the amount of liquid increases on the originally “high-concentration” side and solution level will be higher on one side of the membrane than on the other, creating a pressure difference (osmotic pressure). Osmos comes from the Greek word wapo5 meaning a push or impulse: the salinity gradient (difference) energy converters use this osmotic pressure by converting it into mechanical energy and then electricity, although in cells or batteries (e.g., dialytic battery) electricity can be directly generated. Among the energy conversion methods proposed are equalization of chemical potentials; electrochemical properties of solutions with different salinities separated by anion- and cation-exchange membranes; cyclical expansion and contraction of polymers immersed in solutions of different salinities at the same temperature; and utilization of heat generated when freshwater and salt water irreversibly mix. Research is currently carried out in the United States, Sweden, Israel, and elsewhere. Preferred sites for plant locations, in addition to river mouths, include hypersaline sinks, salt marshes, evaporation ponds; plants could also be linked with solar energy systems and electromechanical concentration cells. Francis Richards calculated that the osmotic pressure will be raised from 12.87 to 29.33 atm or 1305 to 2974 kPa (189.14 to 431 psi) when chlorinity is increased from 1.0% to 2.2% at a temperature of 25°C (77°F). Chlorinity (Cl) is a measure of the chloride content, by mass, of seawater: if S is the salinity expressed in parts per thousand, C1 = S/1.80655 Hence, a semipermeable membrane stretched across the mouth of an estuary debouching into a sea with a 35%0 salinity would be subject to a pressure of 24 atm (352.7 psi), or the equivalent of a dam providing a 228 m (748 ft) head to turbines. For salt ponds, such as the Dead Sea where salinity reaches 270%0, the head equivalent would be over 1,890 m (1 mi.), since pressure equals 189 atm (2,776 psi). With A T being the temperature, C1 being the chlorinity, P being the pressure, and ATr: being the lowering of the freezing temperature of water due to salinity, P = Po-

+

273 T 273

with Po = -12.08 ( A T F )

A T = -9.66 x lo-’ C1- 5.2 x

C13

Levenspiel and de Nevers have proposed to drive a vertical tube, equipped with a semipermeable membrane at its lower end, into salt water. Eventually osmotic pressure would be reached and freshwater would rise in the tube and pass through a turbogenerator.

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Their scheme has many advantages since it provides additional usable freshwater, a head for electricity generation, and diffusion of freshwater at considerable depth, limiting environmental impact. MAGNITUDE OF THE RESOURCES

Salinity gradient energy has the highest density and where large rivers debouch in the ocean the large osmotic pressure difference could be harnessed to produce electricity. Salt domes, often sites of oil and gas deposits, could also provide electrical power : dissolved salt or brine pumped to the surface could be brought into contact with less saline sea water. Salinity power is currently uneconomic because of the cost of required membranes. Currently much attention is paid to the concept of reverse electro-dialysis which uses alternately anion-permeable and cation-permeable membranes in a stack, thus adding up voltage. Another proposed method is pressure-retardedosmosis using directly osmotic pressure. Bromley calculated that the equivalent pressure head between 0.5 M seawater and freshwater is about 238 m (781 ft; 24 atm). Because of this osmotic pressure difference, a 240 m (787 ft) waterfall theoretically exists at the mouth of every river and stream in the world. Few dams are this high. At present, river water irreversibly mixes with ocean water with no social gain. However, if one-half of the flow of the Columbia River (U.S.A.) could be converted into electricity at only 30% efficiency, 2300 MW would be produced. This is the size of two gigantic power plants, and compares with power recoverable from ocean thermal gradients handling comparable volumes of water. Where the Jordan River empties into the Dead Sea, the energy density is even more spectacular. The nearly saturated brines of the Dead Sea have an osmotic pressure of about 500 atm, corresponding to a dam more than 5000 m (16,400 ft) high! Every cubic meter of water flowing into the Dead Sea per second could theoretically generate more than 27 MW of power. The potential power available from runoff of major rivers in the world and from other sources, including drainage into some hypersaline lakes, has been estimated at 2.6 x lo9 kW. Global runoff is 1.1 m3/s (38.8 ft3/s). This number represents the total renewable resource of salinity gradient energy resulting from evaporation from the oceans, precipitation on land, and runoff back into the ocean. There are even larger sources of salinity gradient energy, but some of them are non renewable. Nonetheless, the renewable salinity gradient energy can make a dent in our energy budget (Table 8.1). How then, in principle, can we extract this energy that exists in salinity gradients? Consider this example: if a solution of freshwater and a solution of salt (or sugar) water are separated by a semipermeable membrane (a membrane that allows only water to pass, but not salt, or sugar), the water would flow through the membrane from the freshwater to the salt water side (Fig. 8.1). This is not a new discovery.

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Chapter 8

TABLE 8.1 Potential power resulting from salinity gradients (from Wick, 1978) Source

Flow rate (m3/s)

Osmotic pressure difference (atm)

Power (watts)

Global runoff Amazon River (Brazil) La Plata-Parana River (Argentina) Congo River (Congo/Angola) Yangtze River (China) Ganges River (Bangladesh) Mississippi River (U.S.A.) Salt Lake (U.S.A.) Dead Sea (Israel/Jordan) U.S.A. waste water to ocean

1 . 1 x 106 2 x 105 8 x lo4 5.7 104 2.2 104 104 2 1.8 104 125 38 500

24 24 24 24 24 24 24 500 500 22.5

2.6 x 4.7 x 1.9 x 1.3 x 5.2 x 4.1 x 4.2 x 5.6 1.8 x 1.1 x

lo'* 10" 10"

10" 10"' 1Olo

10"' 109 10' 109

It was observed in ancient times when wine was stored in bladders of sheep or pigs, and cooled in vats of water. The bladder, being a semipermeable membrane, allowed the water to pass into it and dilute the wine; sometimes the bladders swelled until they burst (Fig. 8.1). In our example, the freshwater would pass through the membrane and elevate the salt water until the pressure resulting from the height of the salt water is equal to the osmotic pressure difference. In the case of freshwater and seawater, the osmotic pressure difference is equivalent to 24 atm., or the pressure at the bottom of a column of water 240 m (787 ft) high. In September 1974 a group of scientists met in San Francisco to consider possible entrees into the energy represented by the salinity concentration gradients between freshwater and seawater or between any two bodies of water. Numerous possibilities were presented including direct utilization of the osmotic pressure

BEFORE

Fig. 8.1. How osmosis works.

AFTER

351

Salini(y energy

difference, of the electrochemical potential, of the vapor pressure difference, and of the mechanochemical phenomena. Other schemes were suggested as well. Salinity gradient power was not then economically feasible with the current state of membrane technology; research conducted to develop new techniques and/ or materials could completely change the situation. Schemes that do not use membranes may be developed. More immediate application may be found for salinity power from brine that is more concentrated than seawater, where the density is much larger. Salinity-gradient energy has the highest density, especially for brine, and ranks with thermal gradients as having the greatest power available. If all the salinity gradient power from rivers were converted, it would supply about 10% of the present global power demands (Fig. 8.2). By coincidence, the theoretical potential of hydroelectric energy from dams is approximately equal to that of salinity power from the global runoff of freshwater into the oceans. Therefore, in principle, there is the possibility of extracting from river and stream flow at !east as much energy as is extracted from hydroelectric dams. At present, hydroelectric energy provides a bit more than 10% of the electrical power utilized in the United States. There is little chance that this percentage will increase. Therefore, we cannot expect salinity gradient power from rivers to provide a much greater percentage of the U.S. total. Nonetheless, it is not a trivial amount. Comparing the renewable energy and the energy density in terms of pressure

lo4 loz

E

(a)

tn

n

n

Hydro 10-4LI I electric Geo thermal

n

n

Tidal

IITiiI n

iquivalent water head 10-z

Tides

currents

Waves

salinity Thermal gradients gradients

(AT: 12°C)

Fig. 8.2. a. Concentration of energy in different ocean sources (meters of head). b. Power of energy flux of various sources of ocean energy. (Source: Wick and Schrnitt, 1977.)

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Chaprer 8

head, it appears that if 1 kg (2.2 Ib) of water was acted on by the source, the number of joules generated would equal 10 times the pressure head. (The acceleration of gravity is 10 m/s2 (32.8 ft/s2).) ENERGY EXTRACTION

It takes energy to separate salt from water. Thus, we might expect that the mixing of salt and water would release energy. Numerous methods have been developed to desalinate water. If they could be operated in reverse, many would yield energy.

Direct mechanical osmotic effect utilization Whether or not more sophisticated means exist for the direct conversion of the osmotic potential to electric power, it appears that a “brute force” approach is not impossibly far from technical feasibility and perhaps, ultimately, economic feasibility. This is a two-step process. In the first step, the river water is allowed to flow through hydroelectric turbines into a reservoir at a level that is some hundreds of meters below sea level. The difference in heights is maintained by the osmotic pressure difference. Ideally it would be as much as 238 m (780 ft), but it should be enough less than the osmotic equilibrium level 238 m (780 ft) to provide the driving force for the second step which follows. This second step is “waste disposal”, which constitutes the discharge of the freshwater directly into the salt water through “semipermeable membranes” - membranes permeable to water but not to dissolved solids. These membranes must be washed continuously by large volumes of ocean water in order to carry away the freshwater and prevent “concentration polarization” due to dilution of the salt water adjacent to the membrane surfaces (Fig. 8.3). Such a scheme appears to be fairly straightforward, but, when one calculates the amount and cost of the membranes, it becomes prohibitive at the present time.

Fig. 8.3. Mechanical osmotic effect scheme.

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Hollow fibers were suggested as an appropriate form of the membranes. A material that represents a starting point is cellulose acetate, such as was invented at the University of California at Los Angeles by Loeb and Sourirajan and was developed by the Dow Chemical Company for desalination and related purposes [337]. A major difficulty here is that the river water contains a finite concentration of dissolved salts that would accumulate on the freshwater side of the membranes. Since hollow fibers are assembled in dead-ended units, they may be ruled out. A better configuration might be a spiral-wrapped unit from which brackish water at some optimized concentration could be removed. Two major inefficiencies for this process would be the diminution of the osmotic potential and the power necessary for the flushing pumps. Crude calculations indicate that possibly the largest single operating expense of producing power with semipermeable membranes - more or less in their present state of development - would be replacement of the fibers (made necessary by compaction under pressure) at about $0.20 per kWh. Capital cost (in 1974 dollars), may be as much as $36,000 per kW compared with $200-$500 for conventional power plants. While these figures need two orders of magnitude improvement, the numbers suggest that the scheme is not completely unlikely. In addition to the problem of generation of appropriate materials and configurations, a great many other technical problems of constructing a huge plant much of which is hundreds of feet below sea level have to be solved. Other problems include desilting of the river water, prevention or accommodation of biological fouling, and minimization of concentration polarization. But since stainless-steel membranes of the same thickness as the membranes considered here can hardly withstand the corrosion and biological fouling of the oceans, it is hard to see how these cellulose acetate membranes will survive in an ocean environment; effects on aquatic life and other ecological and aesthetic factors should also be considered. One could conceive alternative schemes that elevate the seawater rather than depress the freshwater, but similar problems occur here. Massive earthworks either 315 m (700 ft) above or below sea level can be avoided by maintaining the pressure head with pumps or turbines and standard pressure vessels. Such equipment is commonplace in chemical and mechanical engineering processes. It would be simpler and more practical than excavating the ocean. Such a pressure-retarded osmosis has been investigated in Israel using Dead Sea brines. As it involves defects also common to electrodialysis, it is examined with such schemes.

Electrodialysis When two solutions of different salt concentrations are separated by a “charged membrane”, an electrical voltage is created between them. In the case of freshwater and seawater, this voltage is about 80 mV (0.08 V) across one membrane. It is possible to stack 1000 such cells in a series and generate 80 V. The voltage varies as the logarithm of the ratio of the salt concentration in the adjacent cells. Although

354

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there are geographical salinity variations it reaches, for instance, 42 ppm in the Red Sea - seawater salinity is relatively constant, around an average of 35 ppm. The biggest drawback of the series arrangement is that if electrodes are included in each cell - so that ionic current be converted to electric current - a 1000 MW power plant would dissolve millions of tons of metal present in the electrodes. This excludes use of silver/silver chloride electrodes, because they are too expensive and there is not that much silver available in the world! The idea is thus to avoid using electrodes in each cell: that is the principle of reverse electrodialysis. For a reverse electrodialysis stack, two types of charged membranes are used, called anion- and cation-permeable membranes. The cation-permeable membranes allow the positive ions (in this case mainly sodium ions, Na+) to pass through, and the anion-permeable membranes allow the negative ions (mainly chloride ions, C1-) to pass through. If one alternately stacks anion-permeable and cation-permeable membranes, filling the alternate cells with freshwater and salt water, respectively, the voltages add (Fig. 8.4). Electrodes are only needed at the ends of the stack. Under operating conditions, an anode of platinum-plated titanium and a cathode of steel waste 2-3 V. With 1000 membranes, the inefficiency caused by the electrodes is almost negligible. In conventional electrodialysis, a voltage is applied across a stack, similar to the one shown in Fig. 8.4. In this mode, all of the cells would be filled with brackish water, and the end product would be freshwater and salt water in alternate cells. Electrodialysis is used for many commercial processes, such as sweetening orange juice by removing some of the citric acid. Some initial studies by J. Weinstein and F. Leitz at Ionics Corporation indicated that it would cost about $50,000 (1975 dollars) per kilowatt to build a reverse electrodialysis power plant [338].

Fig. 8.4. Electrodialysis stack.

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355

Fig. 8.5.Toroidal stack of dialytic cells

This figure is about 50-100 times greater than the capital cost of a conventional power plant. With thinner mass-produced membranes, it may be possibly to reduce the capital cost to about $600 per kilowatt, becoming more competitive with other sources. Operating costs of about 2-4 cents per kilowatt-hour were estimated for reverse electrodialysis. This compares favorably with 2-4 cents per kilowatt-hour for power delivered to the local meter. Research groups in the United States and in Sweden are exploring further the reverse electrodialysis concept. The biggest problem is the membranes - they are costly, subject to degradation, and require pretreatment of the solutions. By stacking dialytic cells in a toroidal arrangement the ionic current would flow in a loop continuously in a single direction (Fig. 8.5). The electrode problems would be eliminated. Energy could be extracted from such a system by periodically interrupting the current flow to generate a modulated

356

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Fig. 8.6. Saltwater battery (source: Sea Frontiers).

C

C

A

I

I

I I I

1 1

I

I I

I I

R E P E A T I N G UNIT O R C E L L PAIR

Fig. 8.7. Reverse electrodialysis cell pair; concentration gradients at membrane surface. Existing concentrations within solutions shown by lines 1-4 and 5-8. (Source: redrawn from Lacey, 1980).

Salinity energy

357

current that can be coupled as in a transformer to a secondary alternating-current circuit. Where brackish water occurs near seawater, the two water types could alternate in successive cells of a torus and anion-exchange membranes could alternate with cation-exchange membranes. If brackish water were substituted in some seawater compartments, the predominant electrical current would drive the dissolved ions into adjacent compartments leaving fresher water behind, thus desalinating brackish water (Fig. 8.6). The idea of converting to useful energy the difference between chemical potentials of concentrated and dilute solutions by reverse electrodialysis was studied by Pattle [339]. Murphy [340], used reverse electrodialysis as a direct source of energy to desalt brackish water by the osmionic process. His theoretical expressions, revised by Lacey [341], led to new equations used to predict performance for the osmionic process. Weinstein and Leitz [342], simplified equations predict the power output from reverse electrodialysis units and show experimental data obtained with a small reverse-electrodialysis unit agree with their predictions of gross power outputs which do not, however,include the power required to pump solutions through the solution compartments; their equations involve simplifying assumptions, and do not include factors such as concentration polarization, water transfer by osmosis and electroosmosis, and the power required to pump solutions. Lacey, Belfort, and Guter developed equations to predict the performance of electrodialysis units, which confirmed experimental results. Forgacs, somewhat simplifying the calculations and assuming that membranes would be permeable selective, obtained, for reverse electrodialysis, results quite close to those actually obtained with a sodium chloride brine simulating the Dead Sea brine electrolytic concentration and a more dilute solution simulating seawater. Swedish researchers, Emren and Bergstrom, studying river and seawater contact also obtained similar results. They developed extra thin membranes, and cells and stacks; when equilibrated in seawater the membranes have resistances of about 2 Q/cm2. Resulting costs of 1 kWh would run between 3.5 and 4 cents, and capital investment would amount to $725 per kW. Other estimates place the cost of 1 kW at slightly different amounts. The equations for predicting gross power output are expressed in terms of the voltage and resistances in a cell pair. A cell pair, which is the basic repeating unit in reverse electrodialysis, comprises one anion-exchange membrane, one cationexchange membrane, one concentrated solution, and one dilute solution (Fig. 8.7). When current flows at steady state, there are concentration gradients in the nearly static solutions in the boundary layers adjacent to the membrane surfaces. For the development of the equations, the Nernst idealization of completely static boundary layers and completely mixed bulk solutions was used. Since this idealization has been used in other research on electrodialysis, the boundary-layer thicknesses at various solution velocities can be taken from those references. The voltage output from a cell pair is given by eq. 8.1. On the right-hand side of eq. 8.1, the first term in brackets is the voltage component, which is composed of four diffusion potentials and two membrane potentials. The second term in brackets is the voltage drop through the resistive elements of a cell pair, i.e. the product of

358

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the current density, i, and the sum of the resistances of each boundary layer, each bulk solution, and each membrane [343]. With eqs. 8.1-8.5, the voltage per cell pair, Eo can be calculated for any selected value of current density i. The product of E and i gives the gross power output in W/cm2.

where:

Eo = voltage output from one cell pair (V); i = current density (mA/cm2); R = gas constant (S.314 JPK); f d t , fd- = transference number of cation and anion in dilute solution, respectively; rct, tc- = transference number of cation and anion in the concentrated solution, respectively; LAM+, CAM- = transference number of cation and anion in the cation-exchange membrane, respectively; fCMt, ~ C M - = transference number of cation and anion in the anion-exchange membrane, respectively; N1, N4, Ns, NU = interfacial concentrations at points 1, 4, 5 , and 8 in Fig. 8.7 (eq/cm3) calculated by eqs. 8.2-8.5; Nd = concentration of the bulk of the dilute solution (eq/cm3); N, = concentration of the bulk of the concentrated solution (eq/cm3); yl, y4, yd, ys, yh, y8 = activity coefficients for the concentrations designated by the subscripts; Ad, A c = equivalent conductances, of the dilute and concentrated solutions, respectively (cm2/R) (eq); A = spacing between membranes (cm); 6 = thickness of boundary layers (cm); R A M ,RCM = resistances of the anion- and cation-exchange membranes (R-cm2).

Salinity energy

359

The equations to calculate the interfacial concentrations N1, N4, Ns and N8, are:

where:

F = the Faraday (96,500 C/eq); D = the diffusion coefficient for the salt in solution (cm2/s); and the other symbols have been defined previously. With this procedure, one can show the sensitivity of Eo and gross power output to current density. By selecting a range of values of i, one can determine the optimum value of current density and power output along with values of Eo and the voltage and resistive components of Eo for each value of i. From examination of the voltage and resistive components of Eo for a range of current densities, the effects of boundary-layer thickness, 6, and current density i on the interfacial concentrations N1 through N Xand thus on the voltage component of Eo and power output can be seen, as well as the effects of spacing A between membranes and membrane resistances on the resistive component of Eo. These data provide a basis for evaluating the factors that will effect the largest decreases in the cost of power. The gross power output is:

Pg = Eoi To achieve a given thickness of boundary layers, 6, each solution must be pumped through each cell. The power required to pump the solutions, P, must be furnished by reverse electrodialysis device. Therefore, the net power output is:

P, = Pg - Pp The pumping power required to achieve given thicknesses of the boundary layers is obtained from graphs such as shown in Fig. 8.8. The net power output - the difference between gross power and the pumping power - can also be predicted (Fig. 8.8). The voltage component of Eo is proportional to the logarithm of the ratio of salt concentrations in the dilute and concentrated solutions. There is maximum brine concentration that is desirable (eq. 8.1); the more dilute solutions provide the highest Eo values, but such solutions have very high resistances. Lacey showed that some design factors are very important to net power output, while others have far less influence. The thinner the dilute compartment and the

360

Chapter 8 0.10.

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I

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cm’rec

Fig. 8.8. Solution velocity on boundary thickness layers in Aquachem 1575 forced-flow electrodesalination stack. 70°F = 21°C. (Source: R.E. Lacey, 1980.)

lower the solution velocities, the better; the thickness of the brine compartment plays an increasingly important role when brine concentrations decrease. Membranes with the lowest possible resistance are preferred; the highest possible ratio of brine to dilute solution concentration should be aimed for, although, as stated before, there is an optimum concentration. The Swedish Group for Energy Production Research experimented with a 100 MW system that involved two types of polymers, one with a 3 s extension-contraction cycle, the second with such a 30 s cycle. The short-cycle polymer trapped in a cylinder linked to the crank shaft of a generator produced 0.5 W per gram of polymer (1.25 kW per cylinder), but the long-cycle polymer, also using a 390 m3/s (13,773 ft3/s) flow produced only 0.05 W To generate 100 MW, 80,000 and 80,000 cylinders, respectively, would be necessary [344]. To produce 1 kWh using a short-cycle polymer would cost 1.9 U.S. cents and for the long-cycle type, 5.5 cents; capital costs were $423/kW and $1,47O/kW, 3.2 cents at approximately 20%, for a

Salinity energy

361

capital cost of $630/kW. Total investment is estimated at $133 million with 68% required by the battery modules. These Swedish researchers made a special arrangement of membranes allowing salinity) and 1500 m3/s a cross-flow of 390 m3/s (13,773 ft3/s) of freshwater (0.6%~~ (52,978 ft’/s) of salt water ( 3 0 % ~salinity). ~ A floating structure of 1,000 by 250 m (3,280 by 820 ft) made up of 20 kW battery modules would have a 200 MW output capacity. Another method of direct energy conversion, which is subject to some of the same defects as reverse electrodialysis, is known as pressure-retarded osmosis (PRO). In 1975, Israeli researchers led by Sidney Loeb invented a device that directly utilizes osmotic pressure for power [345]. Their method uses pumps, pressure chambers, and turbines to achieve the same effect as the 240 m (787 ft) column of water cited at the beginning of this chapter. High-salinity water is pumped to a hydraulic pressure equal to about one-half the osmotic pressure difference between the high-salinity water and the lowsalinity water used in the device. The two fluids are separated by semipermeable membranes, allowing the fresher water to flow into the more saline water. In order to permeate the salt water, the freshwater must flow against the pressure on the saltwater side of the membrane. Essentially, the osmotic pressure drives the freshwater into the pressurized salt water (as long as this imposed pressure is not greater than the osmotic pressure difference). The power is generated when this permeated freshwater is released through a turbine (Fig. 8.9). As the mixture of seawater and freshwater leaves the PRO device, new supplies of freshwater and seawater must enter the converter. The seawater, however, must first be pressurized by passage through a pump in order that the pressure in the chamber can be maintained. This added pressure is called the hydraulic pressure. According to Loeb: “In PRO, the hydraulic pressure is less than the osmotic pressure so that water flux is against the hydraulic pressure gradient, this fact being the basis for energy production.”

Fig. 8.9. Diagram of pressure-related osmosis energy-conversion device. (Seawater is pumped to pressure P ... osmotic pressure difference ...)

362

Chapter 8

Loeb maintains that PRO will be the most economical osmotic power production process. He estimates that the total cost of producing 1 kWh to be about 40 U.S. cents. An order of magnitude reduction in this cost would be needed for commercial use of this converter. Less expensive and better membranes may result in the necessary cost reductions. The adverse environmental impacts of the PRO system are likely to be less severe than those resulting from the estuarine salinity gradient converter, because massive excavation of the ocean and construction of huge dams are avoided. Loeb’s latest research, conducted in the United States, has allowed an estimate of the concept’s economics. The calculations indicate that it would cost $10,000 per kilowatt to construct a plant, and 30-40 cents per kilowatt to deliver it to the user. Improvements in membranes could reduce the cost to 10-14 cents per kWh, making it economically feasible. However, there are still some basic problems to overcome and more research to be done.

Vaporpressure difference utilization Both of the two preceding energy conversion ideas depend on membranes semipermeable for pressure-retarded osmosis and ion-selective for reverse electrodialysis. There are numerous technical difficulties with membranes, in addition to the high cost, deterioration, and solution pretreatment requirements mentioned previously. However, there is a promising method that requires no membranes. Power can be extracted utilizing the vapor pressure difference between fresh(or low-salinity) and salt (or high-salinity) water. At the same temperature, water evaporates more readily from freshwater than it does from salt water. As a result of the lower vapor pressure on the salt water side, water vapor rapidly transfers from freshwater to salt water in the evacuated chamber. If a turbine is placed between the two solutions, power can be extracted. The corresponding desalination method is called vapor compression desalination. In reverse vapor compression, the vapor would expand through the turbines. The surfaces of the water act as membranes. Because of the low vapor density and low pressure differentials, large turbines would be required to extract power. Similar ideas have been proposed for ocean thermal energy conversion schemes. There is a comparable vapor pressure difference between cold deep ocean water and warm surface water. Modern designs incorporate 24 m (79 ft) diameter turbine blades. The proposed ocean current turbines are even larger. For a 12°C (22°F) temperature difference between surface and deep water, the vapor pressure difference is about 7 mm Hg (0.93 kPa; 0.28 in. Hg). Taken as a function of temperature, vapor pressure curves for freshwater, seawater, and brine can be compared. The operating temperature of the saltwater system would have to be elevated in order to be competitive with the ocean thermal vapor pressure difference. At 70°C (158°F) the freshwaterheawater vapor pressure difference is 6 mm Hg (0.8 kPa;

Salinity energy

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CLEAR T H I N

P L E X I G L A S

C O P P E R

T U R B l N E

Fig. 8.10. Salinity gradient conversion double spiral pump (source: Science, 1979).

0.24 in. Hg) and the seawater/brine difference is 60 mm Hg (8 kPa; 2.4 in. Hg) (Figs. 8.10 and 8.11). To raise the temperature some generated power must be fed back into the system as heat, with an ensuing loss of efficiency. When the vapor transfers between the two solutions, it carries energy in the form of latent heat of vaporization. This is the heat that is released by the vapor to its surroundings when it condenses and that is absorbed from its surroundings when it evaporates. More energy is transferred by the latent heat of vaporization than is present in the vapor motion. This heat transfer would tend to slow down the process and would eventually stop it unless the heat were returned to the freshwater reservoir or the system were flushed before much of the energy had been extracted. To overcome this problem evaporation and condensation can take place on opposite sides of an efficient heat exchanger plate, as happens in vapor compression desalination. Figure 8.10 shows a model that Mark Olsson built at the University of California. It consisted of a spiral heat exchanger, doubling as a mixing pump when the unit was enclosed in a slowly rotating cylinder. In tests, Olsson, Isaacs, and Wick obtained power densities of as much as 10 W/m2 (0.93 W/ft2) of heat exchanger surface [346]. This value is more than 10 times higher than for reverse electrodialysis. Furthermore, heat-exchanger surfaces such as copper are much cheaper and longer-lived than membranes. Since water pretreatment is not necessary and biological fouling and corrosion are not so important, “reverse

364

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LIQUID

/kr,

4.54 4.4’0

10

20

30

40

50

60

70

80

90

100

TEMPFRATLJRE W )

Fig. 8.1 1. Vapor pressure as function of temperature and water type.

vapor compression” appears to be the most cost-attractive. Very high efficiencies, approaching loo%, are possible for low vapor transfer rates. Since the vapor pressure difference increases sharply with temperature, it would be advantageous to place a power unit near a low-grade source of heat, such as geothermal heat or waste heat from extant power plants. However, above 80°C (176°F)scale deposits may occur in some brines because of precipitation of gypsum. Another problem is maintaining a vacuum of rapid vapor transfer. Gases dissolved in the water need to be evacuated continually. This may not pose a serious problem, but it needs to be considered in the overall operation. Wick remarked in 1975 that to extract energy from salinity gradients some sort of membrane is necessary and that water surfaces are by far the cheapest. Using spray nozzles in both the salt- and freshwater reservoirs will increase the surface, speeding up the process. Air dissolved in the water will need to be evacuated regularly from

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365

the chambers. In a 1975 paper he discussed the possibility of reviving Claude’s salinity gradient system [347]. Earlier in the century, the French engineer Claude built a power generator designed to operate between the heat reservoirs of the deep cold ocean water and the water surface water [348]. One scheme was to use the difference in vapor pressure between water at these two temperatures to drive a turbine connecting the reservoirs. Such a device has greater potential using freshwater or seawater as one reservoir and brine in salt pans as the other. The pressure can be employed to drive turbine blades. The vapor pressure curves as a function of temperature are shown in Fig. 8.11, for freshwater, seawater, and brine. The power available from a generator operating between these two reservoirs will be proportional to the product of the pressure difference times the efficiency of the process. In general the efficiency of salinity power devices is not limited by the Carnot cycle, since it applies only to heat engines. But in this case, the modified Claude system behaves as a heat engine. The Carnot efficiencies for the ocean temperature gradient of 12°C (22°F) would be 12/273 = 4.3%. The efficiency for the salinity gradient at 70°C would be about 2.196, giving an overall improvement of the modified Claude system of 10 x (21/32) N 5 . A 12°C (22°F) temperature difference gives an energy of 12 cal/g. Freshwater flowing into the Dead Sea can support an osmotic pressure of 500 atm (7350 psi), yielding an energy of about 12 cal/g (50.3 J; 0.05 Btu). With comparable efficiencies the salinity gradient Claude process has no advantage over the thermal gradient Claude process. However,in processes where Carnot efficiency does not apply, the salinity gradient is favored. Claude failed to persuade potential sponsors that his system worked economically because he could not model the ocean intake. But the advantages of the salinity gradient over the temperature gradient could lead to redirected revival of Claude’s machine. However, latent heat transfer and the consequent temperature changes will require good heat exchange between the two reservoirs. Nonetheless, the Claude salinity gradient system is worth pursuing.

Expansion and contraction Another energy scheme is to employ a substance that will expand in contact with freshwater and contract in contact with salt water or vice versa. Indeed, there are substances, such as collagen, which is one of the active ingredients in muscles and tendons that have these properties. This behavior is typical for any unreinforced, not highly cross-linked ion-exchange resin. Such a device was proposed and built several years ago by Katchalsky and his co-workers [349]. It was not very efficient since it took a long time for the collagen to expand and contract. One can imagine a 19th century device with pistons moving up and down as the various fibers contract and expand driving a big flywheel. This suggestion, although not taken very seriously, may nevertheless merit some attention.

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Concentration of the free energy Clark showed that when freshwater and salt water mix irreversibly, the amount of heat generated is almost negligible [350]. The solubility of NaCl in water is practically temperature-independent and, concomitantly, the heat of dilution is close to zero. The total change in free energy (AG) from mixing river water and seawater is equivalent to the potential energy of a 240 m (785 ft) dam. The temperature rise of water falling over such a dam is about 0.5"C (1°F) and is insignificant compared with the free energy of the elevated water. The situation is worse in the present case because almost all of the free energy (AG) is expressed as a change in entropy ( T A S ) rather than as a change in enthalpy ( A H ) , or available heat. However, it may be possible to concentrate more of this free energy as heat through the use of an intermediate reaction. As an example of this scheme, consider the following reactions, presented not as practical example but as an illustration of the principle. If dilute sulfuric acid (HzSO4) can be encapsulated in semipermeable membranes, it will become more concentrated when the capsules are surrounded by salt water. The salt water would extract water from the H 2 S 0 4 solution owing to the osmotic pressure difference. Then if the dehydrated, encapsulated sulfuric acid were surrounded by freshwater, it would absorb water through the membranes and generate a significant amount of heat. This heat could raise the water temperature by an amount suitable to drive a generator. Although it is clear that sulfuric acid is not the ideal chemical for such a scheme, there are other reactions that may be suitable. A simple calculation with the inorganic salt magnesium sulfate (MgS04) demonstrates the efficacy of this approach. If MgS04 encapsulated in semipermeable membranes is placed in contact with a 5 molar (M) solution of saturated brine, it will reach a 3.3 M solution if one can extrapolate from dilute solutions. However, it is more reasonable to assume that the MgS04 would have a higher molarity than 3.3 as it does not dissociate at high concentrations as much as does sodium chloride (NaCI). This point is academic because the solubility limit of MgSO4 is about 3 M at 30°C (86°F). Thus it will become crystallized when separated from saturated NaCl by a semipermeable membrane. If this encapsulated, crystallized MgS04 is 20 kcal/mole (83.6 kJ). If three-quarters of this heat is released when the salt is dissolved, 45 kcal (189 kJ) will go into 1,000 ml (0.26 gal.) of water raising its temperature by 45°C (81°F). This temperature rise is considerably larger than that obtained by irreversible mixing of brine and freshwater -0.01"C (-0.018"F). Other simple compounds reduce the temperature of solutions, thus allowing a twofold increase of the temperature gradient.

Osmotic pump Levenspiel and de Nevers have investigated the osmotic pump power plant (Fig. 8.12). This device depends on the fact that the flow of water through a semipermeable membrane is affected by pressure differences on opposite sides of

Salinity energy

:

367

A J

SEMI-PERMEABLE M E M B

Fig. 8.12. Osmotic pump. (Source: M. McCormick. Energy from the Oceans: Fact or Fantasy? Conference Proceedings, Raleigh, NC, January 27-28, 1976, p. 39.)

the membranes. It is possible to reverse the normal direction of water flow that occurs because of osmosis by applying sufficiently high pressure on the side of the membrane with the greater concentration of salt. Thus, instead of water flowing from the solution with the low salt concentration through the membrane into the solution with the high salt concentration, as one would expect because of osmosis, water may be forced to flow in the opposite direction [351]. In the case of the osmotic pump, a large vertical tube with a semi-permeable membrane on its lower end is immersed into salt water. The tube remains empty until the membrane reaches a depth at which the osmotic pressure is reached, which for seawater would be about 240 m (785 ft). As the tube is lowered beyond this depth, the pressure becomes so great on the ocean water, that it exceeds the osmotic pressure and water flows through the membrane in the opposite direction to normal osmotic flow (the pressure in any liquid increases with depth). Now since seawater is more dense than freshwater, by about 3% the level of freshwater would rise

368

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above the level at which the osmotic pressure is reached, i.e. 240 m (785 ft) level in seawater. If the freshwater is allowed to pass through a turbogenerator and empty into a tube that displaces or draws less than 240 m (785 ft) then the movement of freshwater through the turbogenerator would produce electrical energy. It appears that any practical use of the osmotic pump must await advances in membrane technology. Levenspiel and de Nevers maintain that if membranes far superior to those now available are developed and if the ratio of the cost of power to charges on fixed capital greatly increases, then osmotic pumps might merit further consideration for practical application. TECHNICAL PROBLEMS

There remain several technical problems to be resolved before any large-scale utilization of the impressive halothalassic potential. They include the electrodes and membranes, corrosion and fouling, electrical and hydrodynamical resistance, and efficiencies. Electrodes One of the largest expenses in these systems could be the electrodes, as was mentioned earlier. The maximum efficiency is obtained with reversible reactions. Using reversible electrodes requires an unreasonable amount of material. They dissolve in the solution and some metal is lost. Electrodes figure prominently in most of the electrochemical schemes. But by stacking several cells together the electrodes do not play such a large part. In the terminal cells, rather than employing reversible electrodes, one might simply use irreversible electrodes making the stacks so large that the electrode effect is small. An anode of platinum-plated titanium and a cathode of stainless steel will require 2-3 V at the current densities of interest. With 1,000 membranes generating 80 V, the inefficiency due to electrodes is almost negligible. Taking a loss in power, one might also drive some useful chemical reaction with the ionic current. In this scheme useful by-products as well as electrical power would be produced. The chemicals might be produced directly from the electrochemical potential or a metabolic process might be driven in reverse such as generating ATE which is a highly concentrated source of energy. It would also be possible to generate hydrogen from electrolysis of water and employ it in reduction reactions to produce a liquid fuel, or to use the hydrogen directly as an energy source. This latter process is, in principle, somewhat less efficient. Also, a toroidal arrangement of cells completely eliminates the electrodes.

Membranes Another overwhelming cost is that of the membranes. With the present technology it is quite clear that membranes will be a considerable expense even if

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369

the electrode problem is avoided. However, technology does seem continually to improve. A cost reduction of a factor of 100 and an improvement in membrane performance may be sufficient for the economical realization of salinity power. Further research is necessary, especially for establishing a meaningful “figure of merit” for the membranes if their performance is to be improved by a factor of 100. Owing to its thinner cross section, the Loeb-type membrane has about 10 times the flow rate of the more-common hollow fiber membrane, but the cost per unit flow is comparable. With a better fabrication process for hollow Loeb membranes, these schemes may become competitive with other sources of power. Since there is no depletion layer in reverse electrodialysis caused by polarization in a micro-layer adjacent to the membranes, it is impossible for the membranes to be overdriven in contrast to conventional electrodialysis. In electrodialysis the most damage due to polarization occurs on the surface of the anion-permeable membrane. On the concentrate stream (anode) side of the membrane, a layer of high alkalinity is formed due to “water splitting” (acid-base generation) on the dilute stream side caused by polarization and subsequent transfer of hydroxide (OH-) ions. The alkalinity can in turn cause the precipitation and deposition of calcium carbonate (CaC03) and magnesium hydroxide (Mg(0H)Z) in the form of scale on the membrane surface. Higher electrical resistance results. Also, scale deposits lead to embrittlement and rapid deterioration of the membranes. During desalination by electrodialysis, if the solution were thoroughly mixed, the ions migrating near the membrane on the dilute side would inhibit “water splitting”. Israeli investigators reported the successful reduction of polarization by reversing periodically the current for a fraction of a second after several seconds of normal operation. Crudely speaking, this current reversal “bounces” the polarization layer off the membrane.

Corrosion and fouling Corrosion, biological fouling, and sedimentary fouling may be very serious problems if the membranes are in seawater. There will need to be some sort of filtration on both seawater and river water. Perhaps even pretreatment of the water will be necessary in order to prevent fouling and corrosion and also in order to increase the efficiency of the membranes or to increase the voltage in some way. There is some pretreatment in electrodialysis in order to minimize the deleterious effects. In the example of hypersaline sinks, the lack of organisms will eliminate one of these problems.

Resistance - electrical and hydrodynamical If the interstitial spaces in the cells are small, there will be a tremendous amount of energy expended in pumping the water. Additional calculations will have to be done to determine if that pumping energy is greater or less than the amount that would be retrieved through the salinity gradient. If it is greater, the whole approach would be self-defeating, of course.

310

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Also, one will have to consider internal electrical resistance. The internal electrical resistance due to the water reduces the power that is available at the terminals. This is especially acute for freshwater. There is a lesser problem of internal resistance in the concentrated cells because the dissolved ions effectively increase the conductivity. But in the dilute cells there is a large internal resistance. In order to limit internal ohmic losses in the freshwater cells, they should be made narrow. This construction unfortunately increases the flow resistance and boosts the pumping power necessary to maintain a reasonable flow rate. These parameters need to be optimized. Recent calculations have been obtained by Weinstein and Leitz [352]. For the example of salt pans along desert coasts, we would be generating an electromotive force between seawater and more concentrated brine. The internal electrical resistance would not be as prominent as for the freshwater case. Thus the hydrodynamical resistance might not be so inhibiting either. However, membrane deterioration may be more serious in brine.

Eflciencies One point that needs considerable investigation is the thermodynamic efficiency and its comparison with the economic efficiency. It seems quite likely that by operating at very low thermodynamic efficiency we will be able to overcome some of the problems that require expensive components. For example, we may not need to use the most efficient, most expensive membranes as we can have an abundance of water and potential power, allowing us to waste much of the energy through inefficient processes in early stages of practical development. It has been pointed out that during the use of the nonrenewable source of energy, fossil fuel, early development, capitalized on abundance and low efficiency. With osmotic power, a renewable resource, such an early approach is more justifiable. ENVIRONMENTAL EFFECTS

The environmental impact of the development of salinity power at the mouths of rivers probably would be minimal except for the structures - some form of aqueduct - necessary to bring the two bodies of water together in a relatively small space. In the mixing process, the amount of heat that is generated is trivial, raising the temperature less than 0.5"C (0.9"F) actually less than would result from natural mixing. The by-products would be discharged in much the same way as under natural circumstances. Thus it appears likely that deleterious environmental effects can be minimized. Estuaries - among the most productive areas in the marine environment - are found at the mouths of many rivers. Any development concepts should be designed so that these vanishing areas are not put under further stress. If dilution of the saline solution is the method used, and huge reservoirs are placed in estuaries, estuarial zones would likely be suppressed and sedimentary processes may be disturbed, though not necessarily more than with dams. If estuaries

Salinity energy

37 1

were severely affected, it would mean possible loss of primary breeding grounds and concomitantly severe reduction in production. Other important problems that need to be solved are the management of sediments carried by the rivers and the protection of marine animals that might be sucked into inlet pipes from the ocean. Corrosion, biological fouling, and silting may be very serious problems for the concepts that employ membranes. Some sort of filtration system will have to be developed for both the seawater and the river water. Pretreatment of the water may even be necessary to prevent fouling and corrosion and, also, to increase the membrane’s efficiency. There is some pretreatment now used in electrodialysis to minimize harmful effects. In the example of hypersaline sinks, the absence of organisms eliminates one of these problems. In concepts using vapor-pressure differences, fouling and corrosion do not appear to be serious problems. Fouling may not even occur in the evacuated chambers required for these methods. The environmental effects from brines interfaced with seawater or freshwater depend on the location. Using lagoons along desert coasts, the end-product could be safely discharged into the ocean since it originated there. Brines derived from salt deposits are somewhat different. They are not renewable and would represent an additional salt burden wherever they are discharged. In regions with continuous ocean currents, the resulting dilution of brine discharge would hardly be felt. However, other products, such as oil remnants, may need to be removed. Injection or reinjection of waste products into the earth has been suggested. The geological structure would need to be examined to ensure isolation. Also, the expense might prohibit this form of disposal.

ENERGY FROM GEOTHERMAL BRINES

Because of the high concentration of ions, geothermal brines also merit consideration as sources of energy. According to Klei and Maslan [353] the initial capital cost for a thermal power plant based on flashed steam from geothermal hot brines is from $180 to $200 per kW, of which the wells and piping amount to about $65 per kW. Fixed charges based on the capital investment (e.g., depreciation, taxes, insurance, and return) account for 80-90% of the electrical production costs (Table 8.2). A typical brine contains sodium, calcium, potassium and magnesium, and traces of strontium, zinc, and iron. The major anions are C1 and HCO3, some SiO3 and BO3, and occasionally ammonium and hydrogen sulfide. The normality of the salts is approximately 3.5. Some brines would require pretreatment prior to use in reverse electrodialysis. These brines are currently examined as heat sources. Besides the use of flashed steam, additional power could be generated by reverse electrodialysis using the brine after the flashing and the condensed steam from the turbine: the warmth of the two streams is advantageous to the process. Brines from oil and gas wells could still be another source of energy.

372

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TABLE 8.2 Comparison of energy available from the salts and the oil in selectcd salt domes Dome

Salt volume (mi.3)

Oil production (lo3 barrels)

Salt energy (MW-years)

Oil energy (MW-years)

High yield Thompson (Ft. Bend, Texas) Hull (Liberty, Texas) Humble (Harris, Texas)

0.4 2.6 9.8

259,623 156,830 138,639

14,000 93,000 350,000

44,000 27,000 24,000

Medium yield Avery Island (Iberia, Louisiana) Bayou Blue (Ihervillc, Louisiana) Belle Isle (St. Mary, Louisiana)

4.0 4.6 1.9

53,054 20,806 10,316

140,000 161,000 68,000

9,000 3,500 1,700

Low yield Lake Hermitage (Plaquemines, Louisiana) Bethel (Anderson, Texas) East Tyler (Smith, Texas)

0.9 8.0 4.3

2,415 1,017 55

32,000 280,000 150,000

420 172 9

From Wick, G.L. and Isaacs, J.D., 1978. Salt domes: is there more energy available from their salt than from their oil? Scicnce, 199: 1436-1437.

ENERGY FROM SALT DOMES

There is another source of salinity-gradient energy: salt domes. The salt in salt domes that originated on the floors of ancient, shallow seas and that is often involved with the formation of oil reservoirs when in contact with freshwater may often have more energy potential than the petroleum entrapped by them. The Hull field (Liberty, Texas) has a salt volume of 10.4 km3 (2.5 mi.3), and could yield approximately the energy containing 450 million barrels of oil. Wick and Isaacs estimate that an average Gulf of Mexico salt dome could provide the energy needed to produce 1,000 MW during 30 years. These subterranean formations of brine or solid salt, located adjacent to or under the sea, contain a large amount of energy, perhaps even more significant than river runoff. The brine or salt dissolved from the domes could be pumped to the surface and interfaced with the seawater (or nearby groundwater similarly pumped). Salt domes are of interest because they are likely sites for oil and natural gas deposits. Numerous formations have been monitored and drilled, particularly along the coastal zone of the Gulf of Mexico. These domes have yielded some of the largest oil strikes in the United States. Thus it is surprising that we may be able to convert greater amounts of energy from this salt supply than from the oil and gas. A barrel of oil is equivalent to 5 x lo6 Btu or about 5 x lo9 J. This is 170 W-years. Assuming that a salt dome oil field yields 6 x 10 barrels, a dome provides 1.7 x 10 MW-years of energy. However, there are more low-yield wells than other types, and, furthermore, there have been more dry holes than strikes in the hundreds of salt domes that have been drilled. In fact, the majority contains no oil. Thus, even if it could be converted

Salinity energV

373

at 5% efficiency, this salt supply would be a large untapped energy resource. And recent research indicates that much higher efficiencies are possible. Salt domes energy is a nonrenewable source.

ENERGY FROM SALT PANS

Another likely source of salinity-gradient energy is the dried lagoons or salt pans along arid and semiarid coasts. This has been discussed in part in a preceding chapter. By controlling the influx of seawater into these lagoons, a concentrated brine can be maintained through solar evaporation. Then this brine could be interfaced with seawater, which would serve as the dilute solution. Salt flats can also be used as ponds for solar energy utilization. Hence, the use of high concentrations of salt in the generation of electric power is not limited to the development of osmotic pressure, or electric potentials, when in contact with freshwater. Lucien Bronicki showed that in countries where steady high temperatures prevail, a salt pond can be used as a solar heat intensifier; the layer of less saline water above the brine allows heat rays to enter and raise the temperature of the salt water to near boiling point. The fresher layer acts as a heat insulating blanket to the salt layer, preventing loss of heat back to the atmosphere by evaporation or reradiation. As the lighter freshwater does not mix with the denser salt water, heat is not lost by convection, and floating plastic windbreaks prevent the formation of wind waves that would induce mixing of the layers. The hot brine is used to boil a low-boiling-point fluid whose vapor passes through a turbine and is condensed by relatively cooler water drawn from the top layer of the pool.

CURRENT FUNDING AND PROBLEMS FOR RESEARCH

The Stone and Webster Corporation made estimates of plant costs for specific geographical locations for reverse electrodialysis schemes. Lacey used them to assess the costs of a 20 MW plant (the figures provided for a 100 million gallon a day station); he updated the 1970 estimate and conclude that the total investment required would come close to $60 million, or $2995 per kW. An estimate for a 100 mgd plant at Indian Point, New York, foresaw direct construction costs of $4,045,100 of which $1,197,300 was for materials and $847,800 was for labor, which rose to $2,238,100 after addition of distributable costs, and to $2,510,300 with indirect costs included (Fig. 8.12). When owner’s costs, involving real estate acquisition, interest charges, and miscellaneous expenses were added, the total investment reached $2787,800 for the raw water intake portion of the plant. Distributable costs ($1,245,900 for materials and $144,400 for labor) include office at the site, construction equipment and supplies, taxes, etc. Direct costs are those resulting from site development,

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structures, foundations, pumps, motors, piping, electrical needs, etc...; indirect costs are engineering, design fees, and miscellaneous charges. The pretreatment portion of the plant is the portion from raw water intake distribution flume to and including treated clear water well. Direct costs amount to $14,385,200, indirect costs $1,652,900, distributable costs $ 1,218,700, totaling $ 17,274,800 of which $9,652,900 was €or materials. With added owners costs of $ 1,304,100 the total investment is $18,578,900 (Table 8.3) for the pretreatment portion. Comparing four brine sources and assuming a cost of membranes of $ 10.33/m2 ($ l/ft2) with a resistance varying from 0.3 R-cm3 to 1.0 R-cm3 (5-16.4 R - i d ) , salt domes would provide energy at 6 cents per kWh for an installed power cost of $2,889 per kW to 6.9 cents per kWh and $3,060 per kW, seawater energy would cost from 12 to 12.8 cents per kWh and $5,610 to $5,890 per kW; lake water (i.e. Great Salt Lake) 5.8 to 6 cents per kWh and $2,756 to $3,020 per kW; and with a natural gas well the price of energy would run from 9 to 10 cents per kWh and installed power from $4,560 to $4,820 per kW. With membranes costing 50% more, resulting increases of energy and installed power are in the range of 10 to 50%. The cost of membranes varies considerably; some membranes may cost as little as $ 3-4/m2 (30-40 cents/ft2), others command prices running as high as $ 216/m2 ($20/ft2). Not only is the cost of the original membrane - e.g. cellulose acetate or polyamide hydrazide - occasionally high, it is subject to considerable stress and biofouling once placed in a marine environment, possibly affected also by sedimentary transport. Configuration of the membrane also plays an important role: while fouling poses no problem with tubular types, whose tubes are easily replaceable, there types are very expensive. Like the low-cost spiral type, they have low parasitic pressure losses. The spiral type is subject to fouling, but is somewhat cheaper $40/m2 ($3.7/€t2).The very inexpensive hollow fiber type has a higher parasitic pressure loss, and is very susceptible to fouling, and the membrane flux is quite lower than that of the other types [354]. Wick calculated some years ago membrane cost and size for salinity gradient power plants. For a 1,000-MW plant utilizing and equivalent water head of 500 ft (152 m) (300 ft = 91 m less than theoretical maximum) at 100% efficiency, the water flow would be: PVRh Power = P = t

where: = water density = 1 g/cm3; V / t = volume flow/unit time; ,q = acceleration of gravity; h = height p

P = 1, 000 MW x 1 0 ~ u . sgaI./min, . or I, 000 MW x 37.9 x lo7 Vmin.

375

Salinity energy TABLE 8.3 Cost estimate for a plant at Indian Point, New York (condensed from Stone et al.) Material costs

I. RAW WATER INTAKE PORTION OF PLANT

Construction and indirect costs Owner’s costs Total investment costs

2. PRETREATMENT PORTION OF PLANT Construction costs: Direct COSIS Site development Structures and foundations Clari tiers Chemical storage and feed equipment Misc. piping and instruments Electrical Activated carbon filtration Structures and foundations Activated carbon filters Misc. piping and instruments Electrical Activated carbon Carbon regeneration equipment Sludge disposal Building service Total direct costs Disrributoble cosr.s Office at works Construction equipment, temporary construction, construction supplies Federal and state taxes and other distributables Total distributables Total construction costs Indirect costs Engineering and design, fee and miscellaneous headquarters office Allowance lor indctcrminatcs Total indirect costs Total construction and indirect costs (not including an allowance for labor productivity or change in base wage rate Allowance lor labor productivity Allowance for differences in base wage rates Total construction and indirect costs including labor productivity and wage rates for site Owner’s costs: l a d - 27.6 acres at $25,000 per acre Interest during construction Spare parts inventory Start-up labor and materials Miscellaneous, including legal, travel and public relations Total owner’s costs Total investment costs

($1

Labor costs ($)

1,405,000

1,104,600

1,405,000

1,104,600

18,300 1,046,700 1,900,000 85,000 192,100 62,100

46,700 1,658,500 570,000 76,400 141,700

65,000 2,705,200 2,470,000 86,800 268,500 203,800

1,265,600 1,600,000 465,100 2,400 1,267,200 200,000 360,500 49,700 8,514,700

1,545,200 480,000 174,700 5,300 21,100 15,000 52,500 x 1,600 5,870,500

3,810,800 2,080,000 639,800 7,700 i,2xx.3on 215.000 4 13,000 I 3 I ,300 14,385,200

14,400

292,200

306,600

215,000 3,200 232,600 8,747,300

I Y I ,700 502,200 936.1 00 6,856,600

406,700 505,400 1,218,700 15,603,900

677,200 228,400 905,600 9,652,900

423,700 341,600 765,300 7,621,900

1,100,900 570,000 I ,670,900 17,274,800

9,652,YnO

none none 7,62 1,900

none none I7,274,800

1,800

Total costs ($)

2,509,600 277,500 2,787,500

690,000 476,800 65,300 54,400 17,600 1,304,100 18,578,900

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Chapter 8

If the flux in the membranes is 0.2 gal./day/ft2 (0.07 l/m2) (commensurate with flows in semipermeable membranes used in reverse osmosis), the required surface area would be: 7 gal. min h 1 x 24x 10 - x 60 = 7.2 x 10'" ft2 min h day 0.2 gal. day-' ft-2 ~

= 6.7 x lo9 m2

If the fibers were packed 3.3 m2/m3 (lo4 ft2/ft3), the entire volume would be: 7.2

lo6 ft3(= 0.2

1 0 ~ ~ ~ )

or a cube -200 ft (= 58.5 m) on a side. This is large but not necessarily impossible. Assuming a cost of $5.38/m2 (50 cents/ft2) for the fibers, capital costs for the fibers only would be $3.6 x lo1" or $36,000 per kW compared with $200-500 for fossil fuel or nuclear power plants. Another calculation of membrane cost was made using the Mississippi -Missouri River System as an example. The drainage of the Mississippi-Missouri River system is approximately 6.4 x 10' ft3/s (1.8 x lo4 m'/s). To completely convert this energy would require concentrating it to seawater. The necessary electrical current for 0.5 N seawater could be calculated using F , Faraday's constant (10' coulombs). The current would be I = 1 0 ' ~amperes. Assuming a dialytic battery that would run reversible at 64.5 x lo3 A h 2 (comparable to electrodialysis units), the required membrane area for a single cell would be: 15.5 x 10l2 in2 (1014 cm2) At a cost of 0.05 cents/cm2 (50 cents/ft2)the total capital expense for membranes would be $10 billion to produce 46,000 MW, which is 3.5% of the U.S. consumption. This does not include other material, such as 1014 cm2 of electrodes, if they are used. While Norman, and Wicks and Isaacs, calculated the cost per kilowatt hour at no more than 20 cents for a membrane, capital costs are more than double those required for traditional power plants. While Georges Claude's approach dispenses with membranes and electrodes, and reduces the fouling factor, by driving a turbine run by a vapor pressure differential of approximately 18 kPa (2.6 psi) for a temperature differential of 12°C (22°F) many engineering problems must be resolved and current research places less emphasis on that system. One area that needs considerable investigation is a comparison of thermodynamic efficiency with economic efficiency. Also, the scale of the project must be considered. Small conversion plants could serve the nation's purpose better than large plants. The location of the plant, of course, would bear on its size. In remote regions without electricity, the

Salinity energy

377

salinity power of streams of salt pans could provide electrical power. Immediate applications also could be found for salinity power from brine, where the power density is much larger than for seawater. Potential candidates for additional research and development support are research to reduce membrane costs to improve their structure, and to develop new fabrication methods; environmental impact analysis; determination of geographical sites including magnitude of the resource, legal implications, political consequences, and socioeconomic impact; research directed at improving converter’s efficiency, their design, prototype scaling (for laboratory use), demonstration models; and examining the parameters and new configurations of such schemes, and the study of the interaction of water systems with membranes. Both N:O.A.A. and the U.S. Department of Energy supported research; however, with current emphasis on frugality in regards to the spending of U.S. federal funds, one may have reservations as to the dispensing of major support at this time. COSTING - ELECTRODIALYSIS PLANT

Lacey [355] updated a cost analysis made by the Stone and Webster Engineering Corporation in 1970 [356] for electrodialysis plants of 100 to 200 mgd. He took into consideration costs for membranes, filters, feeds pumps, instruments, controls, buildings, services, RED piping, and heat exchangers, using the ratio of the Chemical Engineering plant cost indices. This led to an estimate, in 1978 prices, of $14,282,000 for the membranes, $5,551,400 for all other costs, for a total of $19,833,400. A detailed analysis of the electrodialysis portion of a plant, according to Lacey is provided in Table 8.4. With a well needed to pump fresh brine and another to reinject used brine, an additional $2,920,000 are required; if a salt dome is used, raw water intake and activated-carbon sections would add another $36,665,000 to the capital investment. The grand total would reach close to $60,000,000 for a 20,000 kW plant, or $2,995 per kW. The carbon-activated section ensures removal of organic contaminants; if water quality would permit the reduction of the costs by one-third $20,000,000 would be lopped off. A LOOK INTO THE FUTURE

Halothalassic power has been put to use: batteries using seawater are in rather common use (Figs. 8.6, 8.12). They are dry-charge primary batteries. Because they are immediately activated upon immersion in water, they are useful in emergency situations and for marine safety devices. They have been used aboard the Tneste at depths of over 600 m (1969 ft) and could probably function at still greater depths. Salt resources are clearly abundant worldwide, but certain conditions must be met in order to utilize salt for energy. The most important condition is the proximity

378

Chapter 8

TABLE 8.4 Cost estimate for electrodialysis portion of Indian Point

Construction costs: Direcf costs Site development Structures and foundations Electrodialysis stacks Elcctrodialysis filters E.D. feed pumps E D . stage pumps Hrine recirculation pumps Brine waste pumps Chemical storage and feed equipment Waste gas equipment l<,ll.piping and valves for tixed staging Additional E.D. piping and valves for mixes staging Head tank, pump and filter E.D. structures Instruments and controls Electrical for cells Electrical other than cells Huilding services Total direct costs DlSlrlbUfUbk C0SI.Y Ofice at works Construction equipment, temporary construction, construction supplies Federal and state taxes and other distributables Total distributables Total construction costs Indirect cosfs Engineering and design, fee and miscellaneous headquarters office Allowance for indeterminates Total indirect costs Total construction and indirect costs (not including an allowance for labor productivity of changc in labor hosc wagc rates) Allowance for labor productivity Allowance for differences in base wage rates ‘lbtal construction and indirect costs including labor productivity and wage rates for site

Owner’s costs: Land - 20.6 acres at $25,000 per acre Interest during construction Spare parts inventory Start-up labor and materials Miscellaneous including legal, travel and public relations ‘liital owner’s costs Iota1 investment costs

Material costs

Labor costs

lbtal costs

($)

($)

($)

24,300 877,700

x,ino,wo 128,000 880,000 450,000 I ,200,nw 45,oon 332,500 7,300 1,170,300 424,800 5,600 360,000

360,000

62,200 1 ,556,20O 648,000 I2,OOO I 76,nno 90,000 240,000 9,000 26,900

6no 2X1.300 59,xoo

600

I 26,000 97,200

X6,500 2,433,9011 X,74X,(lO(l l40,00(1

I,056,000

540,000 I,440,000 54,000 359,40(1 7,y00 1.45 1,6011 4X4,600 6,200 486,000 457,200 3.04X.400

2,350,300 I23,XOO 70,700 16,910,300

69X, I00 36,700 sx,700 4, I79,300

l2Y,4O(l 21,OXY,h00

35,oW

2 I 4,200

249,200

520,500 6,200 561,700 17,472,oUO

140,500 357,hOO 712,300 439 l,600

66 I,OOO 363,XOO 1,274,000 22,363,600

1,500,300 456,100 1,956,400 19,428,400

3 1 O,hOO 243,700 554,3011 5,445,900

I,XIO,lJOO 699,800 2,s 10,700 24,x74.300

none none

none none

5,445,YOO

24,x74,300

IY,42X,400

l6o,soo

5 15,000 hX6,50(1 108,500 ‘JO.000

S4.20(1

I ,454,hOO 26.328.‘100

Note: Thc elcctrodialysis portion of the plant includes that portion of the plant from and including the E l ) . feed pumps to and including the product water sump.

Salinity energy

319

of a large body of freshwater: this requirement is quite restrictive. Sunshine is needed to renew the salt resource, and precipitation is required for the freshwater. Generally speaking, these two conditions do not occur in the same region. If membranes could be developed to use saline water at the “freshwater” and brine as the concentrated solution, many regions would open up for salinity gradient energy. Reverse vapor compression might be a more obvious approach. Significant portions of the United States have saline groundwater. The salt domes in the Gulf of Mexico also can be used with seawater as the dilute solution. Similar situations exist in other countries. Possibly, membranes suitable for use with brines already exist. The Japanese have a problem of limited salt resources rather than limited freshwater resources. Thus, in their electrodialysis units, they highly concentrate the brine and discharge the freshwater. Membranes have been developed to tolerate highly concentrated salt solutions. It may be advantageous for salinity gradient researchers to examine these membranes. Some preliminary work in this area has been done by Kurt Spiegler at the University of California. By all indications, it is certainly possible to produce power from salinity gradients. We have seen that cost could be the most critical factor. We need an improvement of at least a factor of 100 in the cost of membranes before these concepts, such as reverse electrodialysis, would be worth pursuing. With current concern for the shortage of freshwater and equally for disposal and potential use of “used” waters, the salinity schemes offers an additional “dividend” since it is not even necessary to have potable freshwater. Even polluted waste water, or seawater, would suffice to tap the great osmotic pressure potentials of nearby saltwater brines. As in all systems for converting the potential energy flow of the oceans, waves, tides, currents, temperature gradient (OTEC), or salinity gradient, the problems are not a lack of energy but its dilution and the high capital costs of conversion. The cost of building a reverse dialysis power plant was estimated by Ionics Corporation at about $50,000 per kilowatt, between 50 and 100 times that of a conventional steam plant. On the other hand, operating costs have been estimated at about the same. Nevertheless, the rate at which energy is developed by rivers meeting the salt ocean is as great as the power that theoretically could be developed from ocean temperature gradients using the OTEC systems, and greater than the power from ocean currents by the Coriolis scheme, or from waves, or from tides. It is not necessary to put all the nation’s energy into one or even two baskets. If an alternative energy source can provide even a few percent of the country’s energy demands, then it is worth pursuing. Initially, ocean energy may make the best sense in select locations and in small-scale applications. It is improbable that ocean sources will singlehandedly solve the massive energy appetites of the globe. But as we gain experience, it would not be surprising if ocean sources make their mark around the turn of the century.