- Email: [email protected]
Salinity-gradient vapor-pressure power conversion
Energ) Vol. 7, No. 3. pp. 237.246, 1982 Printed in Great Britain
036&5442/82/030237-10103 W/O Pergamon Press Lrd
SALINITY-GRADIENT VAPOR-PRESSURE POWER CONVERSION MARK S. OLSSON Scripps Institution of Oceanography, Institute of Marine Resources, Mail Code A-008, La Jolla, CA 92093, U.S.A. (Receioed 12 Nooember 1981)
Abstract-The interface between water bodies of different salinities represents a large unexploited source of energy. An energy conversion approach that does not require the use of membranes but uses the differences in vapor pressure between solutions is examined. The resource potential, source solutions, system components, and operating characteristics are evaluated and, where similar, compared to research and development on open-cycle OTEC (Ocean Thermal Energy Conversion). It is shown that salinitygradient, vapor-pressure power generation is within reach of current technology.
Due to its salt content, seawater has a lower vapor pressure than freshwater, and it therefore requires more energy to evaporate water from the sea surface than from a freshwater lake. This energy difference is approx. 0.5 Cal/gof evaporate or about 0.1% of the latent heat of evaporation. This is stored solar energy and is, in fact, more than is available per unit mass from the gravitational potential energy of the water stored behind the 220 m high Hoover Dam on the Colorado River. The worldwide energy flux for evaporation from the oceans has been estimated to be 2.4 x lOI W.’ Since only about 10% of the 1.3 m of water evaporated annually from the surface of the oceans returns as freshwater runoff from the continents where it is accessible, the upper limit on the exploitable resource has been placed at 2.6 x 10” W, which is approximately equal to world hydroelectric potential.* Salinity-gradient energy is liberated where freshwater flows into and mixes with seawater. This free energy of mixing is converted into entropy or increasing disorder. As a result, heat is not released in natural mixing since NaCl, the principal constituent of sea salt, has a very small heat of dilution. The osmotic pressure head, which exists at the marine termini of rivers, is about 24 atm and is equivalent to a 240 m water head. Thus, an untapped 240 m high conceptual waterfall exists at the mouths of all rivers that flow into the ocean. In theory, this salinity-gradient power amounts to 2.24 MW for a freshwater flow of one m3/s into the sea.3 The energy density can be an order of magnitude greater for rivers flowing into hypersaline lakes: each m3/s flow of the Jordan River into the Dead Sea would yield 27 MW (a potential energy comparable to a dam over 5000 m high)! Among the renewable sources of energy in the ocean, salinity gradients rank with temperature gradients as having the greatest energy density and potential for conversion. Relic solar energy in the form of salt deposits could also be utilized. The problem of extracting power from salinity gradients is essentially the reverse of water desalination. Efficiency considerations aside, the energy required to desalinate water is the same as can be extracted during the process of salination. Various techniques for the extraction of this energy have been proposed.‘-9 Two general categories of techniques exist, viz. those which utilize membranes (osmosis, reverse electrodialysis or electrochemical processes) and those which exploit vapor-pressure differences. In addition, a technique using membrane-based fibers, which expand and contract when exposed to varying salinity, has been suggested.” In some sense, the vapor pressure approach uses the water surface as a membrane, and so might also be considered a membrane technique. Wick and Isaacs state:’ “In order to extract energy from salinity gradients some sort of membrane is necessary. Water surfaces are by far the cheapest”. However, Isaacs and Schmitt” later note another approach suggested by Emre’n that might be feasible in arctic regions and which apparently does not require any sort of membrane. In this procedure, seawater or brine at a temperature below the freezing point of 237
238
MARKS. OLSSON
freshwater can be used to freeze freshwater in pressure vessels, and the resulting high pressure from expansion is used to produce power. Osmotic pressure between solutions, as well as the vapor pressure differences of solutions, are essentially different manifestations of the same physical process (see Fig. 1). For isothermal conditions, the same amount of energy is available and the same limiting principles apply. The relationship between vapor pressure and temperature for freshwater, seawater and brine is shown in Fig. 2. Vapor pressures are a strongly increasing function of temperature, while osmotic pressure is not; however, the amount of energy available is independent of temperature. Extraction rates will increase with temperature. That is, for a given size extraction device, the temperature will determine the rate at which it can consume its salinity fuel but will not change the amount of fuel required to convert a given amount of energy. Production of commercial quantities of power using freshwater and seawater will require processing volumes of water comparable to those passing through the turbines of a hydroelectric installation. For seawater to brine gradients, an order of magnitude less volume is required for the same power production. Currently, it is not technically or economically feasible to process such volumes through osmotic membranes on ti production basis. Utilizing osmotic membranes to produce appreciable quantities of power would require enormous membrane areas. In addition, pressure support, flux, salt rejection, fouling, life expectancy, and cost are formidable obstacles to overcome.” Ion specific membranes have similar problems. Even in the unlikely event of a major breakthrough in membrane technology, extensive water pretreatment seems unavoidable. The vapor-pressure approach to salinity-gradient power exploits the small difference in vapor pressure between waters of different salinities. Fluid streams to be mixed during power extraction would be degassed before entering separate halves of a heat exchanger or would be continually deaerated inside the heat exchanger. Solutions inside the heat exchanger would be in equilibrium with water vapor at their respective vapor pressures. The more saline side of the heat exchanger would be at a lower pressure; therefore, a vapor flow could be established, with condensation on the saltier side and evaporation from the fresher side. A turbine placed in this vapor flow would produce usable power. The vapor pressure approach is technologically conventional and bypasses many of the problems associated with the membrane approach. The power cycle utilizing vapor pressure
EQUIVALENCE OF OSMOTIC PRESSURE AND VAPOR PRESSURE DIFFERENCES
FRESd’ATER h=240m
SALTiVATER
FOR SEMATER/FRESHWATER
Fig. 1. A closed tubular loop from which all noncondensable gases have been removed graphically demonstrates the equivalence of osmotic pressure and vapor pressure differences. Freshwater will flow through the semipermeable membrane until the pressure at the base of the fluid column h is equal to the osmotic pressure of the solutions. The same equilibrium can also be reached by vapor transport above the solutions due to the differences in vapor pressure. If these two processes were not exactly equal in effect, a net perpetual flow around the loop would occur, in violation of thermodynamic principles. Raoult’s law states that the vapor pressure of a substance in solution is proportional to its mole fraction.
Salinity-gradient vapor-pressure power conversion
239
8OC
60C
PRESSURE (mm Hg) 4oc
3oc
2OC
IO<
4.5, 4.4’ TEMPERATURE
(“C)
Fig. 2. Vapor pressure vs. temperature curves for freshwater, seawater and brine. There is a rapid rise with temperature and a small difference between freshwater and seawater compared to brine.
differences is also similar to the proposed open-cycle OTEC (Ocean Thermal Energy Conversion) process. An open-cycle ocean thermal energy converter was actually constructed and tested over 50 years ago.13 Research and development on OTEC has resulted in a number of studies on the open-cycle concept. These studies address problems which are almost identical to those faced by vapor-pressure salinity-gradient power conversion. Many parallels can be drawn between the results of this work and the technology of salinity-gradient power. The major engineering differences between open-cycle OTEC and vapor-pressure salinity-gradient power conversion lie in the accessability of source waters, turbine pressure drop and heat exchanger design. Heat engines operating between small temperature differences, such as OTEC, are inherently inefficient. For the same flow rates, the salinity-gradient source (freshwater and seawater) is equivalent to a Carnot cycle operating between source temperature differences of about 13C. However, unlike the thermal cycle, the salinity-gradient cycle is not limited to Carnot efficiencies. Hydroelectric power is about 90% efficient,14 but fossil fuel power plants operate at much lower efficiencies and nuclear plants, because of safety considerations, operate at still lower efficiencies. Optimum power production for the salinity-gradient cycle occurs when half of the total available pressure head is dropped across the turbine and the other half is used to drive the vapor flow. This process results in a cycle efficiency of 50%. Turbine and generator losses, coupled with pumping costs, will reduce the overall plant efficiency below 50% and the range of 3040% might be expected. THE CYCLE
An operational power production cycle would consist of the following: (1) conveyance of the source fluids to the plant site; (2) removal of floating debris, possible removal of suspended
240
MARKS. OLSON
solids and exclusion of larger organisms; (3) nearly complete degassing of all fluids inside or entering the heat exchanger; (4) evaporation from the low salinity side of the heat exchanger; (5) energy extraction from the vapor stream by a turbine; (6) condensation on the high salinity side of the heat exchanger; and (7) discharge and reaeration of the mixture. Since the vapor pressure is temperature dependent, the transfer of latent heat tends to retard the process. In order to overcome this problem, a heat exchanger is needed to transfer the latent heat of condensation on the saline side of the turbine back to the fresh side. If the saline side is allowed to warm up and the fresh side to cool down, the vapor pressures on both sides of the turbine will equilibrate and the process will stop. An adverse temperature difference of only 0.5 C will halt evaporation and condensation between freshwater and seawater and a difference of 5 C will stop the process between seawater and brine. Because of the importance of efficiently returning this latent heat, a uniquely designed heat exchanger is required. It has been previously suggested that it would be necessary to operate the salinity vapor cycle at elevated temperatures to increase the pressure drop across the turbine.5 Experiments with scale models have suggested that higher vapor transport rates for a given sized system can be achieved at higher temperatures, but that reasonable transport rates are possible at 27 C.6 SOURCES
Initial engineering tests should be done with high salinity gradients against saturated brine, but the major world and U.S. resource potential lies in the mixing of fresh and ocean waters. Power plants would be located at or near the mouths of rivers. Riverwater could be diverted nearly at the mouth and the seawater could be brought to the plant site by way of a sea level canal. Local mixing, estuarine flow and prevailing ocean currents would have to be considered in locating such canals, to ensure an.undiluted source of seawater and low salinity riverwater. In certain cases, it might be desirable or necessary to construct a large pipe running offshore, similar in size and length to existing sewage outfall and power-plant coolant discharge pipes. Extracting seawater at depth could have several advantages: (1) deep water is less likely to be diluted with river water; (2) deep water is generally cooler (except at high latitudes) than surface water and some thermal energy might also be extracted (the relative effects of a favorable thermal gradient and reduced cycle temperatures would have to be determined); (3) deep water contains more nutrients and the productivity of the discharge water might be improved; (4) deep water, in general, contains fewer organisms that might be injured or killed by the cycle. Waste water is discharged to the ocean in significant quantities (500m3/s in the U.S.). Because of dissolved solids, its energy potential relative to seawater is about 6% less than that of freshwater. Still, 1100MW of power is theoretically available. If properly harnessed, perhaps many coastal sewage treatment facilities could be energy independent. Several places in the workd are particularly suited to the developement of salinity power. In the U.S., the Great Salt Lake has an annual average energy potential of 4200 MW’ and might be an ideal location for a proof of concept engineering test facility. The Dead Sea offers similar possibilities. The Israel Electric Corp., Ltd., has been doing a feasibility study on a hydroelectric power plant linking the Mediterranean and the Dead Sea.” For equal flow rates, more than 10 times as much power is available from the salinity gradients than from the elevation differences. The Congo River has a calculated potential of 10’ MW," a submarine canyon associated with the mouth of this river might make diversion and disposal of working fluids easier than in many areas. The evaporation of seawater in shallow ponds could be an additional resource. The Leslie Salt Company produces annually 2.3 x lo6 m3 of brine at 27% salt by weight from 50 km* in South San Francisco Bay.14 This is a sustained flow rate of 0.073 m3/s which, if mixed with seawater, might produce about 1.5 MW. In arid areas, evaporation rates would be higher, with a corresponding increase in energy production per pond area. In the U.S. Gulf Coast area, numerous salt domes have been mined for their hydrocarbon deposits. Wick and Isaacs16 have shown that the energy potential of the salt in salt domes is generally greater than that of the oil they might contain. Using water injection to produce brine, these salt domes might be mined for their salinity energy. The resulting brine could be mixed
Salinity-gradient vapor-pressure power conversion
241
with freshwater, which is in abundant supply in the region. The mixture could be reinjected into the salt dome to produce more brine, or discharged directly to the Gulf. In the case of naturally occurring brines in this region, geopressured methane gas and heat might be extracted during degassing as an additional bonus.” A number of Gulf Coast salt domes are presently being solution-mined to form storage caverns for the National Strategic Petroleum Reserve.18 During leaching and filling, an average of 267 x lo6 e/day of concentrated brine will be discharged to the gulf through two 0.9 m diameter pipelines with diffusers, for a period of 6-7 years. Almost 100 MW of salinity power is available as this brine mixes with seawater. Geothermal brines constitute an additional resource. These brines could be utilized down stream of a conventional thermal energy-extraction process. The waste heat inherent in thermal power conversion can be used to raise the operating temperature of the salinity cycle and increase its power production for a given heat exchanger size. With a pilot geothermal plant (thermal equivalent loop, no turbine) tapping the Nyland geothermal reservoir in the California Imperial Valley, the feasibility of producing approx. 500 MW at a flow rate of 4.7 m3/s has been demonstrated. It has been estimated that an additional 40 MW could be produced by mixing these geothermal brines, which have a salinity of 20%, with Salton sea water.14
WATER TREATMENT
Freshwater runoff is often heavily laden with suspended solids; concentrations vary seasonally and from one river to another. Ocean water generally contains less suspended material, though in some locations floating debris and seaweeds would have to be excluded. Because of the flow-through nature of at least some fraction of the liquid streams on both sides of the heat exchanger and the flushing action of the process of rotation and surface wetting, some silt and other suspended matter should be tolerable. Since the heat exchanger rotates, a certain amount of fine sand or silt might act as an abrasive and help keep the heat exchanger surface clean. Of course, too much abrasive action would be undesirable. Chemical treatment to prevent biofouling of heat exchanger surfaces will probably not be necessary because of the low oxygen levels and pressures maintained inside the heat exchanger. Existing techniques should be adequate to deal with all aspects of necessary water pretreatment. Two strategies are available to minimize pumping losses required to lower the pressure within the heat exchanger, i.e. a barometric arrangement in which the heat exchanger is elevated approximately one atmosphere above the incoming solution streams (tides or varying river levels might complicate this) or an energy recovery scheme using a turbine on the incoming streams directly coupled to a pump on the outgoing streams.”
DEGASSING
The level of noncondensable gas inside the heat exchanger must be kept very low so that internal pressures are nearly equal to the vapor pressures of the solutions. When the pressure is lowered as the fluid streams enter the heat exchanger, any gas in these fluids can come out of solution and create an undesirable increase in pressure which will inhibit vapor transfer. Even though the percentage by volume of air produced by flashing seawater is less that 0.3%,*’ nearly complete removal of noncondensable gases is required to ensure good vapor transfer rates because of the low operating pressures and small pressure differences. Two different strategies are available: pre-deaeration of fluids entereng the heat exchanger or deaeration in the condenser downstream of the power turbine. Deaeration technology is presently used in several industrial processes. Feedwater for steam boilers in conventional power plants is routinely deaerated (to 10 ppb oxygen) to prevent boiler corrosion?1 and the flash distillation technique of water desalination requires the degassing of substantial volumes of water. However, the open-cycle approach to OTEC necessitates the degassing of much larger volumes of water. OTEC pioneer Georges Claude solved the degassing problem on a small scale and found that parasitic power losses were not unreasonable. In a recent OTEC study, it was estimated that parasitic power losses due to degassing would be about 6% of gross turbine output?
242
MARKS. OLSON
Open-cycle OTEC studies have shown that degassing would be best accomplished with staged high efficiency axial compressors and that removal of noncondensable gases at the condenser might be more cost effective than pre-deaeration.22-24In the sanity-gradient case, it may also be possible to deaereate within the heat exchanger itself. The operating conditions inside the heat exchanger are similar to those expected inside the condenser of proposed open-cycle OTEC systems and desalination plant deaerators. If the saline side of the heat exchanger is forced to the vapor pressure of the fluid by active pumping, the energy cycle will be maintained and some of the energy required for pumping can be recovered in the turbine. Noncondensables could be removed using staged axial compressors with condensers between stages to remove condensables. Degassing in stages is preferred from an efficiency standpoint because most condensables can be removed well before atmospheric pressure is reached so that only noncondensables have to be pumped to full atmospheric pressure.24The condenser side of the heat exchanger and the degassing system intake would be designed to minimize the volume of water vapor entrained during degassing. Rejected heat from this condensation could be transferred to the evaporative side of the turbine.
HEAT EXCHANGER
One possible heat-exchanger design is shown in Fig. 3. This spiral pump design allows for the fabrication of very large surface areas that are necessary for evaporation, and condensation as well as for return heat transfer to maintain isothermal conditions6 Because of the rolled nature of this design, it should be possible to develop fabrication techniques which are not unduly expensive. Condensation and evaporation would occur primarily on the thin heattransfer membrane and the latent heat would be directly returned. Salinity differences are maintained by passing appropriate volumes of saltwater and freshwater in and out of both sides
CLEAR THIN
PLEXIGLAS
i
COPPER
Fig. 3. Diagrammatic views of the “double spiral” pump for converting salinity gradient energy. A cross section (top) and an end view (bottom) are shown. Rotation of the cylinder causes the copper surfaces to be wetted by the adjacent solutions and maintains fluid circulation. Since most of the evaporation and condensation occurs on the copper surfaces, latent heat is efficiently transferred. The turbine is driven by vapor transfering from the freshwater side to the brine side. The dimensions of the laboratory model are 20 cm in diameter by 40 cm long.
Salinity-gradient vapor-pressure power conversion
243
of the heat exchanger, as vapor is transferred from one side to the other. The double spiral pump design has the additional advantage of providing a mechanism for wetting surfaces while circulating the mixing the fluids. Since the available pressure drop is small, pressure drop losses inside the heat exchanger must be minimized and any design should reflect this, allowing for the unimpeded flow of vapor. The pressure gradients the heat exchanger is required to support internally are very small, as long as the water levels are kept equal. Since the system operates in a near vacuum, the container vessel must support about one atmosphere of external pressure. The heat exchanger might thus be housed inside a steel or reinforced concrete tube that should easily handle one atmosphere of external pressure. Plastics might be suitable for heat-exchanger materials because of the low pressures and temperatures involved. The use of plastics would allow the consideration of novel fabrication techniques. Thin plastic films coud be laminated onto a supporting and separating matrix and rolled into large diameter double spirals. Tremendous surface areas and corrosion-free long life might be achieved with thin plastic sheeting at a reasonable cost. The fact that thermal contact resistance may prove more limiting than conduction may make thin plastic cross sections thermally feasible, even though most plastics are poor thermal conductors. Various fillers can improve the thermal conduction properties of plastics considerably and composite, filled plastic heat exchangers have been considered for OTEC. Saran films have been evaluated as a candidate heat exchanger material. Plastic laminates covered with 0.013 mm (0.5 mil) Saran have an estimated minimum 12 year working life?’ Saran has a low diffusivity and minimal creep at the expected low temperatures and pressures. Small leaks in the heat exchanging film would probably have a minimal impact on the operating characteristics of the system. TURBINES
For freshwater and seawater, the pressure drop across the turbine will be quite small, less than 0.1 of that of an open-cycle OTEC. The energy available per unit volume of liquid water processed is approximately equal to OTEC because of increased vapor flow. For seawater and brine, the pressure drop is comparable to OTEC but the available energy is ten times greater. About one third of one mmHg of pressure drop would be maximally available across a turbine operating between freshwater and seawater at 20C. In comparison, the dynamic pressure available from a 17 knot wind blowing at sea level is also only one third of one mmHg. This pressure difference is small, but the absolute working pressure is also small (only 17 mmHg). If wind power were considered from the standpoint of pressure drop across a turbine, the problem might intuitively appear more difficult than it is. Similarly, the small pressure drops associated with the vapor-pressure cycle should not, u priori, be considered an insurmountable obstacle to power extraction. Given that the absolute pressure is so low, this problem might be more properly considered from the standpoint of momentum transfer than from pressure drop. Relatively high velocities and large volume flow rates should make power extraction feasible. Several large-diameter, low-speed turbines or many small, high-speed turbines could be used?6 Advances in the technologies of helicopter blades, cooling tower fans, and wind turbine construction provide a firm base upon which the design and construction of such turbines can be built. Even if smaller diameter turbines are somewhat less efficient than the very large designs proposed for open cycle OTEC, the use of smaller units operating at reduced efficiencies may be preferred because of the more conventional design and the possibility of constructing many smaller, decentralized units rather than one large facility. While generally more expensive, decentralization has enviromnental and strategic advantages. In an open cycle OTEC report, we find the following statement: “The steam turbine has been the major concern and a previously stated objection to the open cycle systems . ...The adoption of helicopter-blade technology, coupled with extensive steam turbine experience, essentially eliminated all the feasibility and cost concerns of the turbine.“” In this study, a turbine efficiency of 81% is estimated and a gross output of 42 MW is found for six 600 rpm, 16 m diameter (2.7 m blades, 10.7 m rotor) turbines. ENVIRONMENTAL
IMPACT
Adverse environmental impacts could arise from the large scale development of salinity
244
MARKS. OLSSON
power. 27In many areas, freshwater is in short supply and any further demands on this resource would be deleterious; however, in many areas, freshwater is in abundant supply and even the large volumes required for salinity power would not tax the sources. Where brines are available, seawater (if available) can be used in place of freshwater with only a slight loss in efficiency. Objections to filtering the suspended load from great volumes of river water have been raised, based on the assumed use of membranes. The vapor-pressure approach should not require the removal of the suspended load. Therefore, this objection should not apply. Particles larger than the gaps in the matrix which supports the heat-exchanging film would have to be removed to prevent clogging, but these should consist mostly of floating debris, and their exclusion should not have a major adverse impact. With careful engineering and site selection, the environmental impact of salinity-gradient power could be minimal in many areas. The mixing of fresh and saline waters is a natural process which the energy extraction process would not modify significantly. After power extraction, in the case of mixing freshwater and seawater, the mixture would be cooled, but less so than from natural mixing. The temperature difference between the two cases would be less than 0.5 C.“’ Large quantities of undiluted brine are presently being discharged through diffusers into the Gulf of Mexico without apparent adverse environmental impact.18 Diluted brine produced near hypersaline sinks could be discharged back into the sink with minimal impact. Brine production near an inland geothermal field or salt dome would present a disposal problem if reinjection was not feasible. Low pressure and oxygen tension encountered during the power cycle may adversely affect many aquatic organisms entrained. Screens could be used to exclude larger organisms, but smaller and microscopic organisms would pass through the power cycle. Some small organisms such as phytoplankton may be able to survive a limited exposure to these conditions. Many freshwater organisms cannot tolerate oceanic salinities and many marine organisms cannot tolerate reduced salinities, hence mortalities occur as a natural process where rivers discharge to the sea. Reaeration of the mixture might be required to minimize the biological impact of discharge waters. In most instances involving larger rivers, only a portion of the flow would be utilized so that fish migration, shipping and sediment fluxes would be largely unimpeded. Estuarine areas involving substantial tidal fluxes would probably prove difficult to develop from environmental and engineering considerations. Wide seasonal variations in freshwater flow and the related fluctuations in salt water intrusion might make source water diversion difficult and potentially environmentally damaging. River discharge areas such as the Columbia or the Congo would be preferred sites for development since they do not involve extensive tidal flats, coastal lagoons and delta type environments. It has been estimated that if half of the mean flow of the Columbia River were mixed with Pacific Ocean water at 30% conversion efficiency, 2300 MW of electricity could be generated.’ LABORATORYEXPERIMENTS
Preliminary investigations of the vapor pressure approach have proved encouraging.6 A small model (Fig. 3), operating between freshwater and saturated NaCl brine, transferred 12.3e/s of vapor during one experiment. Ten watts of salinity energy were dissipated and almost 700 W of latent heat were transferred across the 1 m2 heat exchanger surface of this simple device. The calculated vapor velocity in this experiment was over 22 m/s (80 kph). Scaling this model up to a cylindrical heat exchanger 30 m in diameter by 30 m long with a 10 m hollow core for the turbine, and using a heat exchanger sheet separation thickness of 5 mm, the heat exchanger surface area would be approx. 7.5 x lo6 m2. At an efficiency of 40% this exchanger could generate 30 MW. By mixing seawater and freshwater, 3 MW could be expected. Higher rates per unit area heat exchanger surface may be possible in a large flow through system, compared to the model which utilized pre-deaerated solutions in a batch process. Waste heat from a conventional power plant or an industrial source could be used in combination with counter current heat exchangers to raise the cycle operating temperature and increase the power output. SCALING
From an engineering standpoint, salinity-gradient power extraction does not have the size
Salinity-gradient vapor-pressure power conversion
245
scaling problem that is associated with OTEC. Large volumes of continuously moving water are required by OTEC to maintain the thermal gradient. A large pipe and relatively direct access to cold water is needed to minimize pipe conduction losses, which makes plant location in or very near deep water preferred. Scaling restrictions should be more relaxed for salinity power, making a 1 MW or smaller plant feasible and perhaps economically reasonable in certain locations. Parasitic power losses due to pumping and degassing as well as turbine and heat exchanger efficiency will vary with plant size. These engineering considerations balanced against the current cost of energy in combination with the economics of source water procurement and subsequent disposal will place a lower limit on feasible plant size.
SUMMARY
Because of the subtlety of the resource and its past association with membranes and their attendant problems, salinity-gradient power has been largely overlooked in the development of renewable energy resources. The resource is large, the energy density high, and the engineering and environmental obstacles do not appear to be insurmountable. The difficulties are seemingly less severe than in several technologies currently being investigated. An effort needs to be made to prove or disprove salinity-gradient power as a partial solution to our energy needs. Acknowledgements-This work would not have been possible without the inspiration and guidance of the late Prof. J. D. Isaacs. Support was provided by the Institute of Marine Resources, Scripps Institution of Oceanography and by the Foundation for Ocean Research, San Diego CA. REFERENCES 1. G. L. Wick, Energy 3,95 (1978). 2. Report on “Energy From the Ocean”, prepared for the Subcommittee on Advanced Energy Technologies and Energy Conservation, Research, Development and Demonstration, of the Committee on Science and Technology, U.S. House of Representatives, 95th Congress, 2nd session, by the Science Policy Research Division, Congressional Res. Service Library of Congress Serial cc. (April 1978). 3. R. Norman, Science 186,350 (1974). 4. G. S. Wick, “Salt Power: Is Neptune’s Ole Salt a Tiger in the Tank?“, Oceanus 22(4)(Winter 1979/80). 5. Cl. L. Wick and J. D. Isaacs, Utilization of the Energy from Salinity Gradients, Institute of Marine Resources, University of California, La Jolla; CA 92093,IMR Ref. No. 78-2, 26pp (1978). 6. M. Olsson, G. L. Wick, and J. D. Isaacs, Science 206,452 (1979). 7. A. T. Emre’n, “Concentration Cell for Salinity Power Production, Economic Potential of the Concentration Cell”, presented at Miami Int. Conf. on Alternative Energy Sources, Univ. Miami, Florida (15 Dec. 1980). 8. S. Loeb, F. Van Hessen, and D. Shahaf, J. Membrane Sci. 1, 249 (1976). 9. M. Reali, Energy 6,227 (1981). 10. A. Emre’n, Energy 4,439 (1979). 11. J. D. Isaacs and W. R. Schmitt, Science 207,265 (1980). 12. G. Wick and J. D. Isaacs, “Utilization of the Energy from Salinity Gradients”, in Wooe and Salinity Gradient Energy Conversion WorkshopProceedings. University of Delaware, Newark, Delaware, ERDA report no. COO-2946-l (24-26May 1976). 13. G. Claude, Mech. Eng. 52, 1039(1930). 14. N. T. Monney, “Ocean Energy from Salinity Gradients”, in Ocean Energy Resources, The Energy Technology Conference, Vol. 4, Ocean Energy Division ASME, 345 E. 47th St. New York (18-23 Sept. 1977). 15. D. Weiner, project manager, private communication. 16. G. L. Wick and J. D. Isaacs, Science 199, 1436(1978). 17. R. A. Kerr, Science 207, 1455(1980). 18. R. M. Davis, Science 213,618 (1981). 19. C. E. Brown and L. Wechsler, “Engineering an Open Cycle Power Plant for Extracting Solar Energy from the Sea”, OTC No. 2254,in 7th Ann. OTC Conf., Houston, Texas, pp. 111-126(6 May 1975). 20. E. D. Howe, A. D. K. Laird, B. Beorse, and B. W. Tleimat, “Ocean Thermal Power and Water Production Research by the Sea Water Conversion Laboratory” in Proc. 5th Ocean Thermal Energy Cowersion Conf. (Edited by A. Lavi, T. N. Veziroglu) Vol. 3. U.S. Government Printing Office, Washington, DC 20402(2&22 Feb. 1978). 21. U.S. Dept. of Interior, Office of Saline Water, Deaerators for_Desalination Plants Research and Development Progress Report No. 314, U.S. Government Printing Office, Washington, DC 20402(Dec. 1976). 22. W. H. Coleman, Energy 5,493 (1980). 23. T. J. Rabas and J. M. Wittig, “OTEC 100-MWe Alternate Power Systems Study”, in Ocean Thermal Energy Conversion, 6th OTEC Conf. Ocean Thermal Energy for the 80’s, Washington, DC (Edited by G. L. Dugger). The Johns Hopkins University, Applied Physics Laboratory, Johns Hopkins Rd., Laurel, MD 20810(19-22 June 1979). 24. A. D. Watt, F. S. Mathews, and R. E. Hathaway, “Open Cycle Thermal Energy Conversion: A Preliminary Engineering Evaluation” in Proceedings of the Fifth Ocean Themtal Energy Conversion Conference, (Edited by A. Lavi and T. N. Veziroght), Vol. 3. U.S. Government Printing Office, Washington, DC 20402(2&22 Feb. 1978). 25. W. B. Suratt, G. K. Hart, and E. N. Sieder, “Plastic Heat Exchanger for Ocean Thermal Energy Conversion”, in Pmt. 3rd Workshop on Ocean Thermal Energy Conversion (OTEC) Houston, Texas (Edited by G. L. Dugger). The Johns Hopkins University, Applied Physics Laboratory, Johns Hopkins Rd., Laurel, MD 20810(8-10 May 1975).
246
MARKS. OLSSON
26. L. M. Bartone Jr., “Alternate Power System for Extracting Energy from the Ocean: A Comparison of Three Concepts”, in Proc. 5th Ocean Thermal Energy Conuersion Con/. (Edited by A. Lavi and T. N. Veziroglu), Vol. 3. U.S. Government Printing Office, Washington DC 20402(20-22 Feb. 1978). 27. J. M. Zeigler, P. V. Hyer, and M. L. Wass, “Environmental Effects Arising from Salinity Gradient and Ocean Wave Power Generating Plants”, in Wave and Salinity Gradient Energy Conversion Workshop Proceedhas. University of Delaware, Newark, Delaware, ERDA report no, COO-2946-l(24% May 1976). 28. G. L. Wick, “Salinity Energy”, in Harvesting Ocean Energy (Edited by G. L. Wick and W. R. Schmitt), Chap. 5. Unesco Press, 7 place de Fontenoy, 75700Paris, France (1981).
036&5442/82/030237-10103 W/O Pergamon Press Lrd
SALINITY-GRADIENT VAPOR-PRESSURE POWER CONVERSION MARK S. OLSSON Scripps Institution of Oceanography, Institute of Marine Resources, Mail Code A-008, La Jolla, CA 92093, U.S.A. (Receioed 12 Nooember 1981)
Abstract-The interface between water bodies of different salinities represents a large unexploited source of energy. An energy conversion approach that does not require the use of membranes but uses the differences in vapor pressure between solutions is examined. The resource potential, source solutions, system components, and operating characteristics are evaluated and, where similar, compared to research and development on open-cycle OTEC (Ocean Thermal Energy Conversion). It is shown that salinitygradient, vapor-pressure power generation is within reach of current technology.
Due to its salt content, seawater has a lower vapor pressure than freshwater, and it therefore requires more energy to evaporate water from the sea surface than from a freshwater lake. This energy difference is approx. 0.5 Cal/gof evaporate or about 0.1% of the latent heat of evaporation. This is stored solar energy and is, in fact, more than is available per unit mass from the gravitational potential energy of the water stored behind the 220 m high Hoover Dam on the Colorado River. The worldwide energy flux for evaporation from the oceans has been estimated to be 2.4 x lOI W.’ Since only about 10% of the 1.3 m of water evaporated annually from the surface of the oceans returns as freshwater runoff from the continents where it is accessible, the upper limit on the exploitable resource has been placed at 2.6 x 10” W, which is approximately equal to world hydroelectric potential.* Salinity-gradient energy is liberated where freshwater flows into and mixes with seawater. This free energy of mixing is converted into entropy or increasing disorder. As a result, heat is not released in natural mixing since NaCl, the principal constituent of sea salt, has a very small heat of dilution. The osmotic pressure head, which exists at the marine termini of rivers, is about 24 atm and is equivalent to a 240 m water head. Thus, an untapped 240 m high conceptual waterfall exists at the mouths of all rivers that flow into the ocean. In theory, this salinity-gradient power amounts to 2.24 MW for a freshwater flow of one m3/s into the sea.3 The energy density can be an order of magnitude greater for rivers flowing into hypersaline lakes: each m3/s flow of the Jordan River into the Dead Sea would yield 27 MW (a potential energy comparable to a dam over 5000 m high)! Among the renewable sources of energy in the ocean, salinity gradients rank with temperature gradients as having the greatest energy density and potential for conversion. Relic solar energy in the form of salt deposits could also be utilized. The problem of extracting power from salinity gradients is essentially the reverse of water desalination. Efficiency considerations aside, the energy required to desalinate water is the same as can be extracted during the process of salination. Various techniques for the extraction of this energy have been proposed.‘-9 Two general categories of techniques exist, viz. those which utilize membranes (osmosis, reverse electrodialysis or electrochemical processes) and those which exploit vapor-pressure differences. In addition, a technique using membrane-based fibers, which expand and contract when exposed to varying salinity, has been suggested.” In some sense, the vapor pressure approach uses the water surface as a membrane, and so might also be considered a membrane technique. Wick and Isaacs state:’ “In order to extract energy from salinity gradients some sort of membrane is necessary. Water surfaces are by far the cheapest”. However, Isaacs and Schmitt” later note another approach suggested by Emre’n that might be feasible in arctic regions and which apparently does not require any sort of membrane. In this procedure, seawater or brine at a temperature below the freezing point of 237
238
MARKS. OLSSON
freshwater can be used to freeze freshwater in pressure vessels, and the resulting high pressure from expansion is used to produce power. Osmotic pressure between solutions, as well as the vapor pressure differences of solutions, are essentially different manifestations of the same physical process (see Fig. 1). For isothermal conditions, the same amount of energy is available and the same limiting principles apply. The relationship between vapor pressure and temperature for freshwater, seawater and brine is shown in Fig. 2. Vapor pressures are a strongly increasing function of temperature, while osmotic pressure is not; however, the amount of energy available is independent of temperature. Extraction rates will increase with temperature. That is, for a given size extraction device, the temperature will determine the rate at which it can consume its salinity fuel but will not change the amount of fuel required to convert a given amount of energy. Production of commercial quantities of power using freshwater and seawater will require processing volumes of water comparable to those passing through the turbines of a hydroelectric installation. For seawater to brine gradients, an order of magnitude less volume is required for the same power production. Currently, it is not technically or economically feasible to process such volumes through osmotic membranes on ti production basis. Utilizing osmotic membranes to produce appreciable quantities of power would require enormous membrane areas. In addition, pressure support, flux, salt rejection, fouling, life expectancy, and cost are formidable obstacles to overcome.” Ion specific membranes have similar problems. Even in the unlikely event of a major breakthrough in membrane technology, extensive water pretreatment seems unavoidable. The vapor-pressure approach to salinity-gradient power exploits the small difference in vapor pressure between waters of different salinities. Fluid streams to be mixed during power extraction would be degassed before entering separate halves of a heat exchanger or would be continually deaerated inside the heat exchanger. Solutions inside the heat exchanger would be in equilibrium with water vapor at their respective vapor pressures. The more saline side of the heat exchanger would be at a lower pressure; therefore, a vapor flow could be established, with condensation on the saltier side and evaporation from the fresher side. A turbine placed in this vapor flow would produce usable power. The vapor pressure approach is technologically conventional and bypasses many of the problems associated with the membrane approach. The power cycle utilizing vapor pressure
EQUIVALENCE OF OSMOTIC PRESSURE AND VAPOR PRESSURE DIFFERENCES
FRESd’ATER h=240m
SALTiVATER
FOR SEMATER/FRESHWATER
Fig. 1. A closed tubular loop from which all noncondensable gases have been removed graphically demonstrates the equivalence of osmotic pressure and vapor pressure differences. Freshwater will flow through the semipermeable membrane until the pressure at the base of the fluid column h is equal to the osmotic pressure of the solutions. The same equilibrium can also be reached by vapor transport above the solutions due to the differences in vapor pressure. If these two processes were not exactly equal in effect, a net perpetual flow around the loop would occur, in violation of thermodynamic principles. Raoult’s law states that the vapor pressure of a substance in solution is proportional to its mole fraction.
Salinity-gradient vapor-pressure power conversion
239
8OC
60C
PRESSURE (mm Hg) 4oc
3oc
2OC
IO<
4.5, 4.4’ TEMPERATURE
(“C)
Fig. 2. Vapor pressure vs. temperature curves for freshwater, seawater and brine. There is a rapid rise with temperature and a small difference between freshwater and seawater compared to brine.
differences is also similar to the proposed open-cycle OTEC (Ocean Thermal Energy Conversion) process. An open-cycle ocean thermal energy converter was actually constructed and tested over 50 years ago.13 Research and development on OTEC has resulted in a number of studies on the open-cycle concept. These studies address problems which are almost identical to those faced by vapor-pressure salinity-gradient power conversion. Many parallels can be drawn between the results of this work and the technology of salinity-gradient power. The major engineering differences between open-cycle OTEC and vapor-pressure salinity-gradient power conversion lie in the accessability of source waters, turbine pressure drop and heat exchanger design. Heat engines operating between small temperature differences, such as OTEC, are inherently inefficient. For the same flow rates, the salinity-gradient source (freshwater and seawater) is equivalent to a Carnot cycle operating between source temperature differences of about 13C. However, unlike the thermal cycle, the salinity-gradient cycle is not limited to Carnot efficiencies. Hydroelectric power is about 90% efficient,14 but fossil fuel power plants operate at much lower efficiencies and nuclear plants, because of safety considerations, operate at still lower efficiencies. Optimum power production for the salinity-gradient cycle occurs when half of the total available pressure head is dropped across the turbine and the other half is used to drive the vapor flow. This process results in a cycle efficiency of 50%. Turbine and generator losses, coupled with pumping costs, will reduce the overall plant efficiency below 50% and the range of 3040% might be expected. THE CYCLE
An operational power production cycle would consist of the following: (1) conveyance of the source fluids to the plant site; (2) removal of floating debris, possible removal of suspended
240
MARKS. OLSON
solids and exclusion of larger organisms; (3) nearly complete degassing of all fluids inside or entering the heat exchanger; (4) evaporation from the low salinity side of the heat exchanger; (5) energy extraction from the vapor stream by a turbine; (6) condensation on the high salinity side of the heat exchanger; and (7) discharge and reaeration of the mixture. Since the vapor pressure is temperature dependent, the transfer of latent heat tends to retard the process. In order to overcome this problem, a heat exchanger is needed to transfer the latent heat of condensation on the saline side of the turbine back to the fresh side. If the saline side is allowed to warm up and the fresh side to cool down, the vapor pressures on both sides of the turbine will equilibrate and the process will stop. An adverse temperature difference of only 0.5 C will halt evaporation and condensation between freshwater and seawater and a difference of 5 C will stop the process between seawater and brine. Because of the importance of efficiently returning this latent heat, a uniquely designed heat exchanger is required. It has been previously suggested that it would be necessary to operate the salinity vapor cycle at elevated temperatures to increase the pressure drop across the turbine.5 Experiments with scale models have suggested that higher vapor transport rates for a given sized system can be achieved at higher temperatures, but that reasonable transport rates are possible at 27 C.6 SOURCES
Initial engineering tests should be done with high salinity gradients against saturated brine, but the major world and U.S. resource potential lies in the mixing of fresh and ocean waters. Power plants would be located at or near the mouths of rivers. Riverwater could be diverted nearly at the mouth and the seawater could be brought to the plant site by way of a sea level canal. Local mixing, estuarine flow and prevailing ocean currents would have to be considered in locating such canals, to ensure an.undiluted source of seawater and low salinity riverwater. In certain cases, it might be desirable or necessary to construct a large pipe running offshore, similar in size and length to existing sewage outfall and power-plant coolant discharge pipes. Extracting seawater at depth could have several advantages: (1) deep water is less likely to be diluted with river water; (2) deep water is generally cooler (except at high latitudes) than surface water and some thermal energy might also be extracted (the relative effects of a favorable thermal gradient and reduced cycle temperatures would have to be determined); (3) deep water contains more nutrients and the productivity of the discharge water might be improved; (4) deep water, in general, contains fewer organisms that might be injured or killed by the cycle. Waste water is discharged to the ocean in significant quantities (500m3/s in the U.S.). Because of dissolved solids, its energy potential relative to seawater is about 6% less than that of freshwater. Still, 1100MW of power is theoretically available. If properly harnessed, perhaps many coastal sewage treatment facilities could be energy independent. Several places in the workd are particularly suited to the developement of salinity power. In the U.S., the Great Salt Lake has an annual average energy potential of 4200 MW’ and might be an ideal location for a proof of concept engineering test facility. The Dead Sea offers similar possibilities. The Israel Electric Corp., Ltd., has been doing a feasibility study on a hydroelectric power plant linking the Mediterranean and the Dead Sea.” For equal flow rates, more than 10 times as much power is available from the salinity gradients than from the elevation differences. The Congo River has a calculated potential of 10’ MW," a submarine canyon associated with the mouth of this river might make diversion and disposal of working fluids easier than in many areas. The evaporation of seawater in shallow ponds could be an additional resource. The Leslie Salt Company produces annually 2.3 x lo6 m3 of brine at 27% salt by weight from 50 km* in South San Francisco Bay.14 This is a sustained flow rate of 0.073 m3/s which, if mixed with seawater, might produce about 1.5 MW. In arid areas, evaporation rates would be higher, with a corresponding increase in energy production per pond area. In the U.S. Gulf Coast area, numerous salt domes have been mined for their hydrocarbon deposits. Wick and Isaacs16 have shown that the energy potential of the salt in salt domes is generally greater than that of the oil they might contain. Using water injection to produce brine, these salt domes might be mined for their salinity energy. The resulting brine could be mixed
Salinity-gradient vapor-pressure power conversion
241
with freshwater, which is in abundant supply in the region. The mixture could be reinjected into the salt dome to produce more brine, or discharged directly to the Gulf. In the case of naturally occurring brines in this region, geopressured methane gas and heat might be extracted during degassing as an additional bonus.” A number of Gulf Coast salt domes are presently being solution-mined to form storage caverns for the National Strategic Petroleum Reserve.18 During leaching and filling, an average of 267 x lo6 e/day of concentrated brine will be discharged to the gulf through two 0.9 m diameter pipelines with diffusers, for a period of 6-7 years. Almost 100 MW of salinity power is available as this brine mixes with seawater. Geothermal brines constitute an additional resource. These brines could be utilized down stream of a conventional thermal energy-extraction process. The waste heat inherent in thermal power conversion can be used to raise the operating temperature of the salinity cycle and increase its power production for a given heat exchanger size. With a pilot geothermal plant (thermal equivalent loop, no turbine) tapping the Nyland geothermal reservoir in the California Imperial Valley, the feasibility of producing approx. 500 MW at a flow rate of 4.7 m3/s has been demonstrated. It has been estimated that an additional 40 MW could be produced by mixing these geothermal brines, which have a salinity of 20%, with Salton sea water.14
WATER TREATMENT
Freshwater runoff is often heavily laden with suspended solids; concentrations vary seasonally and from one river to another. Ocean water generally contains less suspended material, though in some locations floating debris and seaweeds would have to be excluded. Because of the flow-through nature of at least some fraction of the liquid streams on both sides of the heat exchanger and the flushing action of the process of rotation and surface wetting, some silt and other suspended matter should be tolerable. Since the heat exchanger rotates, a certain amount of fine sand or silt might act as an abrasive and help keep the heat exchanger surface clean. Of course, too much abrasive action would be undesirable. Chemical treatment to prevent biofouling of heat exchanger surfaces will probably not be necessary because of the low oxygen levels and pressures maintained inside the heat exchanger. Existing techniques should be adequate to deal with all aspects of necessary water pretreatment. Two strategies are available to minimize pumping losses required to lower the pressure within the heat exchanger, i.e. a barometric arrangement in which the heat exchanger is elevated approximately one atmosphere above the incoming solution streams (tides or varying river levels might complicate this) or an energy recovery scheme using a turbine on the incoming streams directly coupled to a pump on the outgoing streams.”
DEGASSING
The level of noncondensable gas inside the heat exchanger must be kept very low so that internal pressures are nearly equal to the vapor pressures of the solutions. When the pressure is lowered as the fluid streams enter the heat exchanger, any gas in these fluids can come out of solution and create an undesirable increase in pressure which will inhibit vapor transfer. Even though the percentage by volume of air produced by flashing seawater is less that 0.3%,*’ nearly complete removal of noncondensable gases is required to ensure good vapor transfer rates because of the low operating pressures and small pressure differences. Two different strategies are available: pre-deaeration of fluids entereng the heat exchanger or deaeration in the condenser downstream of the power turbine. Deaeration technology is presently used in several industrial processes. Feedwater for steam boilers in conventional power plants is routinely deaerated (to 10 ppb oxygen) to prevent boiler corrosion?1 and the flash distillation technique of water desalination requires the degassing of substantial volumes of water. However, the open-cycle approach to OTEC necessitates the degassing of much larger volumes of water. OTEC pioneer Georges Claude solved the degassing problem on a small scale and found that parasitic power losses were not unreasonable. In a recent OTEC study, it was estimated that parasitic power losses due to degassing would be about 6% of gross turbine output?
242
MARKS. OLSON
Open-cycle OTEC studies have shown that degassing would be best accomplished with staged high efficiency axial compressors and that removal of noncondensable gases at the condenser might be more cost effective than pre-deaeration.22-24In the sanity-gradient case, it may also be possible to deaereate within the heat exchanger itself. The operating conditions inside the heat exchanger are similar to those expected inside the condenser of proposed open-cycle OTEC systems and desalination plant deaerators. If the saline side of the heat exchanger is forced to the vapor pressure of the fluid by active pumping, the energy cycle will be maintained and some of the energy required for pumping can be recovered in the turbine. Noncondensables could be removed using staged axial compressors with condensers between stages to remove condensables. Degassing in stages is preferred from an efficiency standpoint because most condensables can be removed well before atmospheric pressure is reached so that only noncondensables have to be pumped to full atmospheric pressure.24The condenser side of the heat exchanger and the degassing system intake would be designed to minimize the volume of water vapor entrained during degassing. Rejected heat from this condensation could be transferred to the evaporative side of the turbine.
HEAT EXCHANGER
One possible heat-exchanger design is shown in Fig. 3. This spiral pump design allows for the fabrication of very large surface areas that are necessary for evaporation, and condensation as well as for return heat transfer to maintain isothermal conditions6 Because of the rolled nature of this design, it should be possible to develop fabrication techniques which are not unduly expensive. Condensation and evaporation would occur primarily on the thin heattransfer membrane and the latent heat would be directly returned. Salinity differences are maintained by passing appropriate volumes of saltwater and freshwater in and out of both sides
CLEAR THIN
PLEXIGLAS
i
COPPER
Fig. 3. Diagrammatic views of the “double spiral” pump for converting salinity gradient energy. A cross section (top) and an end view (bottom) are shown. Rotation of the cylinder causes the copper surfaces to be wetted by the adjacent solutions and maintains fluid circulation. Since most of the evaporation and condensation occurs on the copper surfaces, latent heat is efficiently transferred. The turbine is driven by vapor transfering from the freshwater side to the brine side. The dimensions of the laboratory model are 20 cm in diameter by 40 cm long.
Salinity-gradient vapor-pressure power conversion
243
of the heat exchanger, as vapor is transferred from one side to the other. The double spiral pump design has the additional advantage of providing a mechanism for wetting surfaces while circulating the mixing the fluids. Since the available pressure drop is small, pressure drop losses inside the heat exchanger must be minimized and any design should reflect this, allowing for the unimpeded flow of vapor. The pressure gradients the heat exchanger is required to support internally are very small, as long as the water levels are kept equal. Since the system operates in a near vacuum, the container vessel must support about one atmosphere of external pressure. The heat exchanger might thus be housed inside a steel or reinforced concrete tube that should easily handle one atmosphere of external pressure. Plastics might be suitable for heat-exchanger materials because of the low pressures and temperatures involved. The use of plastics would allow the consideration of novel fabrication techniques. Thin plastic films coud be laminated onto a supporting and separating matrix and rolled into large diameter double spirals. Tremendous surface areas and corrosion-free long life might be achieved with thin plastic sheeting at a reasonable cost. The fact that thermal contact resistance may prove more limiting than conduction may make thin plastic cross sections thermally feasible, even though most plastics are poor thermal conductors. Various fillers can improve the thermal conduction properties of plastics considerably and composite, filled plastic heat exchangers have been considered for OTEC. Saran films have been evaluated as a candidate heat exchanger material. Plastic laminates covered with 0.013 mm (0.5 mil) Saran have an estimated minimum 12 year working life?’ Saran has a low diffusivity and minimal creep at the expected low temperatures and pressures. Small leaks in the heat exchanging film would probably have a minimal impact on the operating characteristics of the system. TURBINES
For freshwater and seawater, the pressure drop across the turbine will be quite small, less than 0.1 of that of an open-cycle OTEC. The energy available per unit volume of liquid water processed is approximately equal to OTEC because of increased vapor flow. For seawater and brine, the pressure drop is comparable to OTEC but the available energy is ten times greater. About one third of one mmHg of pressure drop would be maximally available across a turbine operating between freshwater and seawater at 20C. In comparison, the dynamic pressure available from a 17 knot wind blowing at sea level is also only one third of one mmHg. This pressure difference is small, but the absolute working pressure is also small (only 17 mmHg). If wind power were considered from the standpoint of pressure drop across a turbine, the problem might intuitively appear more difficult than it is. Similarly, the small pressure drops associated with the vapor-pressure cycle should not, u priori, be considered an insurmountable obstacle to power extraction. Given that the absolute pressure is so low, this problem might be more properly considered from the standpoint of momentum transfer than from pressure drop. Relatively high velocities and large volume flow rates should make power extraction feasible. Several large-diameter, low-speed turbines or many small, high-speed turbines could be used?6 Advances in the technologies of helicopter blades, cooling tower fans, and wind turbine construction provide a firm base upon which the design and construction of such turbines can be built. Even if smaller diameter turbines are somewhat less efficient than the very large designs proposed for open cycle OTEC, the use of smaller units operating at reduced efficiencies may be preferred because of the more conventional design and the possibility of constructing many smaller, decentralized units rather than one large facility. While generally more expensive, decentralization has enviromnental and strategic advantages. In an open cycle OTEC report, we find the following statement: “The steam turbine has been the major concern and a previously stated objection to the open cycle systems . ...The adoption of helicopter-blade technology, coupled with extensive steam turbine experience, essentially eliminated all the feasibility and cost concerns of the turbine.“” In this study, a turbine efficiency of 81% is estimated and a gross output of 42 MW is found for six 600 rpm, 16 m diameter (2.7 m blades, 10.7 m rotor) turbines. ENVIRONMENTAL
IMPACT
Adverse environmental impacts could arise from the large scale development of salinity
244
MARKS. OLSSON
power. 27In many areas, freshwater is in short supply and any further demands on this resource would be deleterious; however, in many areas, freshwater is in abundant supply and even the large volumes required for salinity power would not tax the sources. Where brines are available, seawater (if available) can be used in place of freshwater with only a slight loss in efficiency. Objections to filtering the suspended load from great volumes of river water have been raised, based on the assumed use of membranes. The vapor-pressure approach should not require the removal of the suspended load. Therefore, this objection should not apply. Particles larger than the gaps in the matrix which supports the heat-exchanging film would have to be removed to prevent clogging, but these should consist mostly of floating debris, and their exclusion should not have a major adverse impact. With careful engineering and site selection, the environmental impact of salinity-gradient power could be minimal in many areas. The mixing of fresh and saline waters is a natural process which the energy extraction process would not modify significantly. After power extraction, in the case of mixing freshwater and seawater, the mixture would be cooled, but less so than from natural mixing. The temperature difference between the two cases would be less than 0.5 C.“’ Large quantities of undiluted brine are presently being discharged through diffusers into the Gulf of Mexico without apparent adverse environmental impact.18 Diluted brine produced near hypersaline sinks could be discharged back into the sink with minimal impact. Brine production near an inland geothermal field or salt dome would present a disposal problem if reinjection was not feasible. Low pressure and oxygen tension encountered during the power cycle may adversely affect many aquatic organisms entrained. Screens could be used to exclude larger organisms, but smaller and microscopic organisms would pass through the power cycle. Some small organisms such as phytoplankton may be able to survive a limited exposure to these conditions. Many freshwater organisms cannot tolerate oceanic salinities and many marine organisms cannot tolerate reduced salinities, hence mortalities occur as a natural process where rivers discharge to the sea. Reaeration of the mixture might be required to minimize the biological impact of discharge waters. In most instances involving larger rivers, only a portion of the flow would be utilized so that fish migration, shipping and sediment fluxes would be largely unimpeded. Estuarine areas involving substantial tidal fluxes would probably prove difficult to develop from environmental and engineering considerations. Wide seasonal variations in freshwater flow and the related fluctuations in salt water intrusion might make source water diversion difficult and potentially environmentally damaging. River discharge areas such as the Columbia or the Congo would be preferred sites for development since they do not involve extensive tidal flats, coastal lagoons and delta type environments. It has been estimated that if half of the mean flow of the Columbia River were mixed with Pacific Ocean water at 30% conversion efficiency, 2300 MW of electricity could be generated.’ LABORATORYEXPERIMENTS
Preliminary investigations of the vapor pressure approach have proved encouraging.6 A small model (Fig. 3), operating between freshwater and saturated NaCl brine, transferred 12.3e/s of vapor during one experiment. Ten watts of salinity energy were dissipated and almost 700 W of latent heat were transferred across the 1 m2 heat exchanger surface of this simple device. The calculated vapor velocity in this experiment was over 22 m/s (80 kph). Scaling this model up to a cylindrical heat exchanger 30 m in diameter by 30 m long with a 10 m hollow core for the turbine, and using a heat exchanger sheet separation thickness of 5 mm, the heat exchanger surface area would be approx. 7.5 x lo6 m2. At an efficiency of 40% this exchanger could generate 30 MW. By mixing seawater and freshwater, 3 MW could be expected. Higher rates per unit area heat exchanger surface may be possible in a large flow through system, compared to the model which utilized pre-deaerated solutions in a batch process. Waste heat from a conventional power plant or an industrial source could be used in combination with counter current heat exchangers to raise the cycle operating temperature and increase the power output. SCALING
From an engineering standpoint, salinity-gradient power extraction does not have the size
Salinity-gradient vapor-pressure power conversion
245
scaling problem that is associated with OTEC. Large volumes of continuously moving water are required by OTEC to maintain the thermal gradient. A large pipe and relatively direct access to cold water is needed to minimize pipe conduction losses, which makes plant location in or very near deep water preferred. Scaling restrictions should be more relaxed for salinity power, making a 1 MW or smaller plant feasible and perhaps economically reasonable in certain locations. Parasitic power losses due to pumping and degassing as well as turbine and heat exchanger efficiency will vary with plant size. These engineering considerations balanced against the current cost of energy in combination with the economics of source water procurement and subsequent disposal will place a lower limit on feasible plant size.
SUMMARY
Because of the subtlety of the resource and its past association with membranes and their attendant problems, salinity-gradient power has been largely overlooked in the development of renewable energy resources. The resource is large, the energy density high, and the engineering and environmental obstacles do not appear to be insurmountable. The difficulties are seemingly less severe than in several technologies currently being investigated. An effort needs to be made to prove or disprove salinity-gradient power as a partial solution to our energy needs. Acknowledgements-This work would not have been possible without the inspiration and guidance of the late Prof. J. D. Isaacs. Support was provided by the Institute of Marine Resources, Scripps Institution of Oceanography and by the Foundation for Ocean Research, San Diego CA. REFERENCES 1. G. L. Wick, Energy 3,95 (1978). 2. Report on “Energy From the Ocean”, prepared for the Subcommittee on Advanced Energy Technologies and Energy Conservation, Research, Development and Demonstration, of the Committee on Science and Technology, U.S. House of Representatives, 95th Congress, 2nd session, by the Science Policy Research Division, Congressional Res. Service Library of Congress Serial cc. (April 1978). 3. R. Norman, Science 186,350 (1974). 4. G. S. Wick, “Salt Power: Is Neptune’s Ole Salt a Tiger in the Tank?“, Oceanus 22(4)(Winter 1979/80). 5. Cl. L. Wick and J. D. Isaacs, Utilization of the Energy from Salinity Gradients, Institute of Marine Resources, University of California, La Jolla; CA 92093,IMR Ref. No. 78-2, 26pp (1978). 6. M. Olsson, G. L. Wick, and J. D. Isaacs, Science 206,452 (1979). 7. A. T. Emre’n, “Concentration Cell for Salinity Power Production, Economic Potential of the Concentration Cell”, presented at Miami Int. Conf. on Alternative Energy Sources, Univ. Miami, Florida (15 Dec. 1980). 8. S. Loeb, F. Van Hessen, and D. Shahaf, J. Membrane Sci. 1, 249 (1976). 9. M. Reali, Energy 6,227 (1981). 10. A. Emre’n, Energy 4,439 (1979). 11. J. D. Isaacs and W. R. Schmitt, Science 207,265 (1980). 12. G. Wick and J. D. Isaacs, “Utilization of the Energy from Salinity Gradients”, in Wooe and Salinity Gradient Energy Conversion WorkshopProceedings. University of Delaware, Newark, Delaware, ERDA report no. COO-2946-l (24-26May 1976). 13. G. Claude, Mech. Eng. 52, 1039(1930). 14. N. T. Monney, “Ocean Energy from Salinity Gradients”, in Ocean Energy Resources, The Energy Technology Conference, Vol. 4, Ocean Energy Division ASME, 345 E. 47th St. New York (18-23 Sept. 1977). 15. D. Weiner, project manager, private communication. 16. G. L. Wick and J. D. Isaacs, Science 199, 1436(1978). 17. R. A. Kerr, Science 207, 1455(1980). 18. R. M. Davis, Science 213,618 (1981). 19. C. E. Brown and L. Wechsler, “Engineering an Open Cycle Power Plant for Extracting Solar Energy from the Sea”, OTC No. 2254,in 7th Ann. OTC Conf., Houston, Texas, pp. 111-126(6 May 1975). 20. E. D. Howe, A. D. K. Laird, B. Beorse, and B. W. Tleimat, “Ocean Thermal Power and Water Production Research by the Sea Water Conversion Laboratory” in Proc. 5th Ocean Thermal Energy Cowersion Conf. (Edited by A. Lavi, T. N. Veziroglu) Vol. 3. U.S. Government Printing Office, Washington, DC 20402(2&22 Feb. 1978). 21. U.S. Dept. of Interior, Office of Saline Water, Deaerators for_Desalination Plants Research and Development Progress Report No. 314, U.S. Government Printing Office, Washington, DC 20402(Dec. 1976). 22. W. H. Coleman, Energy 5,493 (1980). 23. T. J. Rabas and J. M. Wittig, “OTEC 100-MWe Alternate Power Systems Study”, in Ocean Thermal Energy Conversion, 6th OTEC Conf. Ocean Thermal Energy for the 80’s, Washington, DC (Edited by G. L. Dugger). The Johns Hopkins University, Applied Physics Laboratory, Johns Hopkins Rd., Laurel, MD 20810(19-22 June 1979). 24. A. D. Watt, F. S. Mathews, and R. E. Hathaway, “Open Cycle Thermal Energy Conversion: A Preliminary Engineering Evaluation” in Proceedings of the Fifth Ocean Themtal Energy Conversion Conference, (Edited by A. Lavi and T. N. Veziroght), Vol. 3. U.S. Government Printing Office, Washington, DC 20402(2&22 Feb. 1978). 25. W. B. Suratt, G. K. Hart, and E. N. Sieder, “Plastic Heat Exchanger for Ocean Thermal Energy Conversion”, in Pmt. 3rd Workshop on Ocean Thermal Energy Conversion (OTEC) Houston, Texas (Edited by G. L. Dugger). The Johns Hopkins University, Applied Physics Laboratory, Johns Hopkins Rd., Laurel, MD 20810(8-10 May 1975).
246
MARKS. OLSSON
26. L. M. Bartone Jr., “Alternate Power System for Extracting Energy from the Ocean: A Comparison of Three Concepts”, in Proc. 5th Ocean Thermal Energy Conuersion Con/. (Edited by A. Lavi and T. N. Veziroglu), Vol. 3. U.S. Government Printing Office, Washington DC 20402(20-22 Feb. 1978). 27. J. M. Zeigler, P. V. Hyer, and M. L. Wass, “Environmental Effects Arising from Salinity Gradient and Ocean Wave Power Generating Plants”, in Wave and Salinity Gradient Energy Conversion Workshop Proceedhas. University of Delaware, Newark, Delaware, ERDA report no, COO-2946-l(24% May 1976). 28. G. L. Wick, “Salinity Energy”, in Harvesting Ocean Energy (Edited by G. L. Wick and W. R. Schmitt), Chap. 5. Unesco Press, 7 place de Fontenoy, 75700Paris, France (1981).