SER l/TP-252-2149 UC Categories: 62e, 64 DE84000086 An Artificial Upwelling Driven by Salinity Differences in the Ocean D.H.Johnson J. Decicco December 1983 To be presented at the .ASME 7th Annual Energy Sources Technology Conference and Exhibition New Orleans, Louisiana 12-16 February 1984 Prepared under Task No. 1390.21 FTP No. 415 Solar Energy Research Institute A Division of Midwest Research Institute 1617 Cole Boulevard Golden, Colorado 80401 Prepared for the U.S. Department of Energy Contract No. DE-AC02-83CH10093 Printed in the United States of America Available from: National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 Price: Microfiche A01 Printed Copy A02 NOTICE This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. SERI/TP-252-2149 AN AlrrIFICIAL Ul'WELLING DRIVEN BY SALINITY DIFFERENCES IN THE OCEAN D. H. Johnson J. Decicco Solar Energy Research Institute 1617 Cole Boulevard Golden, Colorado 80401 ABSTRACT the ocean in such a manner that its bottom was exposed to cold, relatively fresh water and its top to warm saline water, a continuous flow up the pipe could be A concept for an artificial upwelling driven by salin­ maintained after priming the fountain. His explana­ ity differences in the ocean to supply nutrients to a tion was that the ascending water in a pipe suitable mariculture farm is described and analyzed. A long for heat exchange would exchange heat but not salinity shell-and-tube counterflow heat exchanger built of in­ with the ambient ocean and would be accelerated due to expensive plastic and concrete is suspended vertically its deficiency in salt (and thus density) relative to in ::he ocean. Cold, nutrient rich, but relatively fluid at the same level outside the pipe. Stammel fresh water from deep in the ocean flows up the shell also attempted an experiment in the ocean to demon­ side of the heat exchanger, and warm but relatively strate this effect, but the results were inconclusive. saline water from the surface flows down the tu be The small boat he used to suspend his pipe from was side. The two flows exchange heat across the thin strongly affected by waves, so he could not separate plastic walls of the tubes, maintaining a constant the effects of wave pumping from a flow caused by temperature difference along the heat exchanger. The salinity differences. plastic tubes are protected by the concrete outer shell of the heat exchanger. The flow is maintained Since Stommel's 1956 article, most of the effort by the difference in density between the deep and sur­ related to this phenomenon has been to understand face water due to their difference in salinity. This naturally occurring "salt fingers'" in the ocean as phenomenon was first recognized by the oceanographer described in Stern (2). This work has recently been Stommel, who termed it "The Perpetual Salt Fountain." summarized by Huppert and Turner (3). Our search of The heat transfer and flow rate as a function of tube the literature since Stommel's original paper in 1956 number and diameter is analyzed and the size of the found only one discussion of the engineering aspects heat exchanger optimized for cost is determined fer a of using the salinity difference in the ocean to drive given flow of nutrients for various locations. Rea­ an artificial upwelling (4). Groves estimated flow sonable sizes (outer diameter on the order of 5 m) are rates and diameters for a single pipe exchanging heat obtained. The incremental capital cost of the with the ambient ocean as originally described by salinity-driven artificial upwelling is compared to Stommel. It was found that a 600-m- long, 20-cm­ the incremental capital cost and present value of the diameter copper pipe installed at an angle so its operating cost of an artificial upwell fueled by bottom is 300 m below the surface might produce a flow liquid hydrocarbons. The salinity-driven upwelling is rate of 5.5 L/s. This upwelling was assumed to supply generally cheaper. phosphorus for fish. It was estimated that it would produce 54 kg/yr of edible nutrients for fish. The 1. 0 BACKGROUND same amount of phosphorus could be added by dumping in 290 kg/yr of Peruvian guano, whose phosphorus content One of the major factors inhibiting implementation of is 4.6%. Thus, Groves (4) concluded that the artifi­ mariculture farms in the ocean is a cost-effective way cial upwelling is not likely to be worth the effort. of fertilizing the mariculture. The deep ocean con­ However, there are several reasons to reexamine the tains inorganic material that could fertilize the practicability of an artificial upwelling driven by mariculture if brought to the surface. Present !'leth­ salinity differences. ods of doing this are too expensive to make maricul­ ture farms commercially feasible. The deep water in Commercially viable mariculture is likely to be on a the ocean is usually less saline and colder than the large scale (much greater than 10 acres). If an surface water. In 1956, the oceanographer Stommel (1) effective upwelling system can be constructed that noted that if a long vertical pipe were lowered into does not run on fuel shipped to the site, then at some 1 SERI/TP-252-2149 size the upwelling may become cheaper than shipping maintained by Sto1lllllel 's salt fountain effect. Warm fertilizer. There are also other mariculture products salty surface water is drawn from just above the besides fish that may be commercially viahle. For bottom of the surface mixed layer (typically at a example, kelp for use as a feedstock to produce syn­ depth of SO m) and flows down the tubes inside the thetic natural gas has been proposed as a mariculture shell. These tubes are supported at stations along product. Nitrogen is not present in sufficient quan­ the length of the shell so they do not bear a signifi­ tities in surface water to provide a commercially cant load. They are constructed of very thin (a few viable kelp yield. A recent study (5) concluded that tenths of milHmeters thick) extruded plastic. Cold, commercial nitrogen fertilizer would be too expensive relatively fresh, nutrient rich water from 300 m to to use as a nitrogen source even when considering only 500 m below the surface is qrawn into the shell side the energy required to produce it. Deep ocean water of the heat exchanger and flows towards the surface. is typically rich.in nitrogen. The commercial viabil­ The deep water and surface water flow counter to each ity of a kelp farm depends on finding an economical other and ma.lntain an approximately equal temperature way of getting this deep water to the surface. The difference throughout the length of the heat concept presented in this paper holds potential for exchanger. This is the most efficient arrangement for building such a cost-effective artificial upwelling. heat transfer and is the major difference between our concept and Stommels' original idea. 2.0 THE SERI ARTIFICIAL UPWELLING CONCEPT When the deep water reaches the top of the heat exchanger it has been warmed to a temperature slightly The SERI concept is essentially a large shell-and-tube less than the temperature of the surface water but is counterflow heat exchanger (Fig. 1). Structural sup­ more bouyant than the ambient surface water because of port is provided by the outer shell of diameter D . s lower salinity. The warmed, nutrient-rich deep water This shell would be constructed in a manner similar to is directed towards the surf ace by the upper manifold. an Ocean Thermal Energy Conversion (OTEC) cold water It rises and floats on the surface due to its buoy­ pipe (6). .It might be made of concrete, plastic, or ancy. The streams of water entering and exiting the some other inexpensive material. No significant heat top of the heat exchanger are kept separate by the transfer occurs across the shell. stratification set up by their density differences. When the surface water reaches the bottom of the heat exchanger, it has been cooled to a temperature Nutrient-Rich, � slightly higher than the deep water but is heavier \\l!� · - - --- - · - � �ar ·- -- ·· me than the ambient deep water because of higher salin­ :i:::!::!l!l:� ������=Zli'i:l �=::fJ 5 ity. It is discharged directly downward through the lower manifold and sinks below the depth from which +-Salty deep water is being withdrawn until it reaches a depth Surface where the surrounding water has the same density. The Water two streams of water are kept separate by the natural stratification that exists in the ocean at these depths. Once a flow has been established through the heat ex­ changer it will be maintained by the buoyancy differ­ ence between the warmed deep water and the cooled sur­ face water due to their difference in salinity. However, since this natural pump must first be primed, mechanical pumps will be required to initiate the flow. I 300-SOOm L Any artificial upwelling operating far from land will have to provide a method of keeping the n�trient-rich deep water on the surface once it is brought there from the depths.