Artificial Upwelling of Deep Seawater Using the Perpetual Salt Fountain for Cultivation of Ocean Desert

Journal of Oceanography, Vol. 60, pp. 563 to 568, 2004

1 1 2 3 SHIGENAO MARUYAMA *, KOUTARO TSUBAKI , KEISUKE TAIRA and SEIGO SAKAI

1Institute of Fluid Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan 2Japan Society for the Promotion of Science, 6 Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan 3Graduate School of Engineering, Yokohama National University, 79-1 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan

(Received 20 January 2003; in revised form 16 July 2003; accepted 17 July 2003)

Deep seawater in the ocean contains a great deal of nutrients. Stommel et al. have Keywords: proposed the notion of a “perpetual salt fountain” (Stommel et al., 1956). They noted ⋅ Deep seawater, the possibility of a permanent upwelling of deep seawater with no additional external ⋅ upwelling, ⋅ energy source. If we can cause deep seawater to upwell extensively, we can achieve an perpetual salt ocean farm. We have succeeded in measuring the upwelling velocity by an experi- fountain, ⋅ natural convection, ment in the Mariana Trench area using a special measurement system. A 0.3 m diam- ⋅ heat transfer, eter, 280 m long soft pipe made of PVC sheet was used in the experiment. The mea- ⋅ enhancement of sured data, a verification experiment, and numerical simulation results, gave an esti- bio-production, mate of upwelling velocity of 212 m/day. ⋅ flow measurement.

1. Introduction salt fountain” (Stommel et al., 1956). In ocean deserts, The explosion of the earth’s population necessitates the salinity of surface seawater is higher than that of deep a greater food productivity. Primary food productivity seawater. As a result, seawater is stably stratified, and from the ocean is gradually increasing (Berger and Wefer, little convective motion occurs in the vertical direction. 1991), due to aquaculture, but number of captured fish When a pipe is inserted to connect deep seawater and such as salmon is declining because the number of salmon surface seawater, and the pipe is filled with the low-sa- flies released is reaching the limit of the ocean’s produc- linity deep seawater, the salinity of the water inside the tivity. Large areas in the middle ocean have very low pro- pipe is lower than that outside, and hence a buoyant force ductivity of the phytoplankton that forms the beginning occurs in the pipe. The upwelling continues as long as of the food chain (Nozaki, 1998). These areas have suffi- the differences of the temperature and salinity exist. Af- cient sunshine on the surface but little nutrient. The mid- ter Stommel’s proposal, Howard and Stommel tried to dle southern part of the North Pacific is classified as an measure the upwelling velocity using a 1000 m pipe in ocean desert. the ocean, but could not detect any upwelling due to low So-called deep seawater, meaning generally seawater velocity and strong wave motion (Huppert and Turner, deeper than 200 m below the surface (Takahashi and Iseki, 1981). 2000), where sunlight does not penetrate, is cooler and We have proposed “the Laputa project” (Maruyama contains much nutrient compared to surface seawater. To et al., 2001a) which has the potential to solve the global draw up deep seawater, several artificial methods such as food problem without destroying land forests. Figure 1 OTEC (Ocean Thermal Energy Conversion) (Otsuka et shows a diagram of the Laputa project. A number of float- al., 2000) have been proposed. But these methods are ing pipes are deployed with buoys in an ocean desert. expensive to deploy in large areas, and storms might de- The nutrient-rich deep seawater is upwelled by the per- stroy the mechanical structure on the sea during long pe- petual salt fountain to the region where the sunlight riods of operation. Furthermore, pumped-up, cold, deep reaches to cultivate phytoplankton. The drawn up deep seawater may sink down due to its higher density. seawater is heated during its flow up through the pipe Stommel et al. have proposed the idea of a “perpetual and then remains on the surface. The increase in phytoplankton will enhance the food chain, and eventu- ally fisheries and sea plant fields will be formed in the area. Without any mechanical structures except buoys on * Corresponding author. E-mail: [email protected] the ocean surface, the system can be operated for a long Copyright © The Oceanographic Society of Japan. time without maintenance. Since the pressure inside and

563 Sun

Plankton Seaweed Buoy High Temp. Fish & Salinity

Upwelling

Heat 200~1000m

Low Temp. Pipe & Salinity Nutrient Rich Deep Sea Water

Fig. 1. Diagram of the Laputa project: draw up the nutrient- rich deep sea water to the photic region and cultivate phytoplankton using a large number of vertical floating pipes. An independent ecosystem of ocean farm is thus constructed in an ocean desert.

Fig. 3. Diagram of experimental apparatus. Pipe is 280 m long and 0.3 m diameter. Top of the pipe is maintained at 55 m depth, and the velocity measurement system is placed at the 15 m below top of the pipe.

laminar natural convection method in a tube (Aihara et al., 1976, 1986) and obtained an average velocity of 10 to 30 m/day (Takahashi et al., 2002). We also conducted a laboratory experiment in a 0.4 m deep pool, inserting a plastic tube of 5 mm diameter. The velocity in the tube was approximately 2.2Ð3.5 mm/s, which is in good agreement with the numerical simulation (Maruyama et al., 2001b).

Fig. 2. Temperature and salinity distribution measured outside 2. Measurement Method the pipe. Salinity around the bottom of the pipe is lower In order to evaluate the feasibility of the Laputa than most of the upper part. This density difference achieves Project, it is essential to measure the actual velocity of the upwelling. upwelling by the perpetual salt fountain in the ocean. The field experiment was carried out in the Mariana Trench area of the Pacific Ocean from August 2Ð5, 2002, using a outside of the pipe is balanced at each depth, lightweight, research vessel, Hakuho-maru of the University of To- flexible pipes made of plastic can be used. kyo (KH-02-2). The experimental point was located at The above mentioned massive upwelling of deep longitude 142°24′96″ E and latitude 11°25′91″ N in the seawater using floating pipes is called as the Laputa Mariana Trench. The temperature and salinity distribu- Project. Laputa was described by Jonathan Swift (1726) tions measured at the experiment point at noon of 4th in his “Gulliver’s Travels” as a floating island, and a re- August 2002 are shown in Fig. 2. At the same point, the lated animated movie was later released (1986) by H. temperature and salinity distributions were taken on 22nd Miyazaki. If the deployed pipes can upwell deep seawater June 2001, and both records shows similar distributions, in a large area, an ocean farm and consequently a green which shows that these distributions should be almost the floating island can be realized in an ocean desert. same every summer. Fluctuation of the distributions is We have estimated the average velocity by the expected to be small within a period of few days.

564 S. Maruyama et al. The distribution of the salinity near the surface in Fig. 2 is no higher than that at depth. It would seem diffi- cult to induce “the perpetual salt fountain” in this area. A salt fountain occurs due to the difference of integrated density value between the inside and outside of the pipe. At the pipe position shown in Fig. 2, the salinity outside the pipe is mostly higher than the salinity inside it, i.e. salinity at the bottom of the pipe, and the integrated density value outside it is higher than that inside the pipe. Figure 3 shows a diagram of the experimental apparatus. A vertical pipe of 280 m length and 0.3 m diameter was deployed. The pipe was made of nylon-reinforced PVC sheet. The length was chosen to obtain a reasonably high velocity using our numerical simulations (Aihara et Fig. 4. Time variation of temperature and depth at the top and al., 1986) of the experimental conditions. Steel coil rings bottom of the pipe. Measurement was started after deploy- were attached at every 0.25 m to maintain a circular cross ment of the pipe. section. Spherical buoys were attached to decrease the tension of the ropes due to the pipe weight. The buoys and ropes were adjusted so that the pipe exit was approxi- a battery to operate the injection system are installed in- mately 55 m below sea level. side the buoy on the surface. Time variation of the tem- In order to measure the very low velocity, much lower perature and depth at top and bottom of the pipe are de- than that of the undulating motion of the pipe, a special picted in Fig. 4. According to the difference of the depth measurement system was constructed, as shown in Fig. between upper and bottom end, the maximum inclination 3. A tracer injector was sequentially operated, and the two of the pipe is approximately 30 degrees, due to the cur- sensors were placed above and below the injector, which rent in the ocean. This inclination reduces the buoyancy was located 15 m below the pipe exit. The measurement force by up to 15% compared with a vertical pipe. This system was placed at the center of the pipe. The tracer value is not negligible but is not essential to the upwelling. (Rhodamine WT, 5 weight%) was injected into the pipe After the measurements were completed, the pipe was using a motor-driven injector. Observation in a labora- removed and the data were transferred to a computer. The tory experiment showed that the effect of the density dif- temperature, salinity and density distribution in the area, ference between tracer and seawater is negligible. The shown in Fig. 2, were measured during the experiment. container housing the injector and motor assembly was A verification experiment was carried out to investi- filled with silicone oil for lubrication and in order to in- gate the measurement data obtain in the Mariana Trench. sure that it did not collapse under water pressure. Two This experiment was done in Onagawa Bay in Japan. A sensors were placed at 2 m above and below the injector 30 m long pipe was used and sensors were placed 2 m in order to detect the concentration of the tracer and wa- and 5 m above the injector. The seawater was drawn up ter temperature. The concentration of the tracer was de- using a small pump, keeping the pipe exit above sea level. tected by measuring the fluorescence of Rhodamine WT The flow rate of the pump was chosen such that the aver- with a recording frequency of 1 Hz. The upwelling ve- age velocity was equal to that of the velocity measured at locity and apparent mass diffusivity were estimated by the center of the pipe in the Mariana experiment. comparing measured concentration data and a one-dimensional numerical simulation for the advection and diffu- 3. Numerical Simulation Model sion of the tracer. Temperature and pressure at the exit Numerical simulations were performed to investigate and inlet of the pipe were measured by other independent the upwelling phenomena in the pipe. The numerical sensors, as shown in Fig. 4. The RMS fluctuation of depth model is similar to the one we publish previously at the pipe exit was 0.62 m during the experiments. (Takahashi et al., 2002), but the latest measured tempera- After inserting the pipe into the ocean with a 100 kg ture and salinity distributions shown in Fig. 2 were used weight at the bottom, deep seawater was drawn up using as the boundary conditions outside the pipe. A schematic a pump, keeping the pipe exit above the sea level. The of this model is shown in Fig. 6. The computational ge- pipe exit was kept at this position for 12 hours until the ometry is axisymmetric and bounded by solid walls on temperature of the deep seawater in the pipe reached that the right and bottom. The boundary conditions consid- of the outside seawater. The deep seawater was station- ered here are the non-slip conditions on the walls. The ary at this stage. The pipe was then slowly lowered, as top surface is specified as a free surface, which is as- shown in Fig. 3. A Global Positioning System (GPS), and sumed to be adiabatic. The bottom is specified as adi-

Artificial Upwelling of Deep Seawater Using the Perpetual Salt Fountain for Cultivation of Ocean Desert 565 Fig. 5. Comparison of detected concentration distribution with that of numerical simulation. The dots are experimental data, Fig. 6. Diagram of numerical simulation. and curves are simulation results. The red dots and the pur- ple curve are ocean experiment data. Upwelling velocity v at the center of the pipe is estimated to be 2.45 mm/s by one-dimensional simulation. The apparent measured mass diffusivity D was much higher than the molecular diffusiv- ∂()ρT k +∇⋅()ρVT =∇2T ()3 ity. The blue dots were measured in forced convection (av- ∂ t cp erage upwelling velocity is 2.5 mm/s). The upwelling velocity at the center of the pipe is estimated to be 3.0 mm/s. Species equation

∂()ρC abatic. Salinity and temperature boundary conditions on +∇⋅()ρρCDCV =∇⋅() ∇ ()4 the walls have the same distribution as the measured data ∂t shown in Fig. 2. The thermal boundary conditions on the pipe wall are calculated by the heat transfer equation. At where ρ is density, V is velocity vector, t is time, µ is the initial condition, the measured temperature is used viscosity, g is gravitational acceleration, T is tempera- for the whole domain, i.e. the temperature of interior water ture, k and Cp are thermal conductivity and specific heat is the same as that outside the pipe at any horizontal po- at constant pressure, respectively, C is salinity, D is mass sition. Outside the pipe, the salinity distribution is the diffusivity. The density is a function of temperature, pres- same as the data measured in the experiment, and the sa- sure and salinity. linity at the bottom of the pipe is imposed inside the whole Two kinds of flow model were examined. One was pipe. The unsteady state equations are calculated until laminar flow with a molecular diffusion coefficient, while the flow reaches the steady-state condition. The govern- the other was turbulent flow with the measured apparent ing equations are as follows: turbulent diffusivities. The turbulent diffusivity was esti- Continuity equation mated so that the calculation diffusion profile is close to the measured data from the ocean experiment, as shown ∂ρ in Fig. 5. +∇⋅()ρV = 01() ∂t 4. Results and Discussion Momentum equation 4.1 Ocean experiment A special diffusion method was adopted to measure ∂()ρV +∇⋅()ρµVV=∇2 V −∇+ ρρg ()2 the low velocity at 15 m below the pipe exit and at the ∂t center of the pipe. In order to measure the upwelling velocity, the tracer was injected three times, 1, 7 and 13 Energy equation hours after deployment of the pipe. According to the nu-

566 S. Maruyama et al. merical simulation, the data after 13 hours was considered as a steady-state condition. The concentration of the tracer after 13 hours detected by the upper sensor is shown in Fig. 5. A one-dimensional numerical simulation for the advection and diffusion of the tracer was done to esti- mate the velocity needed to match the same result. As shown in Fig. 5, the estimated velocity at the center of the pipe was 2.45 mm/s, i.e. 212 m/day. In order to obtain a similar concentration distribution, the estimated diffusion coefficient of the tracer was estimated to be 1 × 10Ð5 m2/s, which is about five orders of magnitude higher than the molecular diffusion coefficient. The tracer was not detected by the sensor placed below the injector. Fig. 7. Axial velocity and temperature differences profiles at This result shows that we were able to measure the measurement point in experiment as calculated by numeri- upwelling of deep seawater at the center of the pipe. The cal simulation of turbulence model (apparent mass diffusivity is 1 × 10Ð5 m2/s). Temperature difference is the result equivalent mass diffusion is much larger than the mo- of the ambient temperature outside the pipe minus the local lecular diffusion of the tracer, and the flow may be turbu- temperature inside. lent due to the undulating motion of the pipe because wave velocities are much faster than the upwelling velocity.

4.2 Numerical simulation 4.3 Verification experiment The velocity and temperature distribution in the tur- A verification experiment was performed in Onagawa bulent flow model, which corresponds to the velocity Bay in Japan. In this experiment the pump drew up measurement point in the Mariana experiment, is shown seawater inside the pipe at 2.5 mm/s average velocity. in Fig. 7. In this case it is expected that the realistic trans- The Reynolds number of the average velocity is 560, if port is some kind of turbulent flow due to the tidal move- the flow is in the laminar, steady-state condition. ment of the pipe. The velocity profile at the measurement The concentration distributions detected by the sen- point exhibited a double peak (M-shape), and the veloci- sor placed 2 m above the injector are shown in Fig. 5. ties at the pipe center were 0.33 mm/s and 1.6 mm/s in The estimated velocity and apparent mass diffusivity were the laminar and turbulent flow models, respectively. The 3.0 mm/s and 5 × 10Ð4 m2/s, respectively. The velocity at estimated M-shaped velocity distribution was observed the center is a little higher than the average velocity, i.e. in the laboratory experiment (Maruyama et al., 2001b) 2.5 mm/s, but much lower than 5.0 mm/s which is the and the respective numerical simulations. We estimated velocity for fully developed laminar flow. The apparent the Rayleigh number of the natural convection for the turbulent diffusivity was fifty times higher than the ocean experiment. The M-shaped velocity profile can be Mariana experiment. It was inferred that this higher dif- found at the exit (Aihara et al., 1986) and reverse flow fusivity was caused by the difference of the pipe move- can sometimes be found in the relevant flow regime cor- ments induced by the ocean waves between Onagawa Bay responding to the estimated Rayleigh number. and the Mariana Trench area. Further investigation is The measured velocity at the center of the pipe was needed for a better understanding of the mechanism of much higher than predicted by the laminar numerical the fluid flow and the heat transfer in such an upwelling simulation, although the average velocity was similar to flow. the measured velocity at the center. The numerical result for the turbulent model was similar to the measured ve- 5. Summary and Concluding Remarks locity at the center of the pipe. According to the velocity The cultivation of an ocean desert, the Laputa Project, distribution calculated by numerical simulations, the av- has been proposed, in which deep seawater is upwelled erage velocity is much higher than that at the center of by the perpetual salt fountain mechanism. In order to the pipe. According to the numerical simulation, the av- evaluate the feasibility of the project, the actual upwelling erage upwelling velocity was strongly dependent on the velocity by the perpetual salt fountain in the Mariana heat transfer between the pipe wall and the deep seawater Trench area of the Pacific Ocean was measured. in the pipe. The movement of pipe wall due to wave mo- The measured velocity at the center of the pipe was tion may enhance the heat transfer inside the pipe. This 2.45 mm/s, i.e. 212 m/day. This value is much larger than phenomenon is similar to the dream pipe that was pro- the numerical simulation result of laminar flow. A turbu- posed as a heat transfer device for machine elements lent flow model using measured turbulent diffusivity gives (Nishio et al., 1995). a reasonable agreement with the ocean experiment.

Artificial Upwelling of Deep Seawater Using the Perpetual Salt Fountain for Cultivation of Ocean Desert 567 Compared with numerical simulation, it was found References that the velocity measured in the experiment may repre- Aihara, T., I. Tanasawa and I. Michiyoshi (1976): Progress of sent the minimum value of the average upwelling veloc- Heat Transfer 4. Yokendo, Tokyo (in Japanese). ity. The volume of upwelling is 15 m3/day at least, and Aihara, T., S. Maruyama and J. S. Choi (1986): Proc. 8th Inter- the value may be ten times greater than that, according to national Heat Transfer Conf., 4, 1581Ð1586. Berger, W. H. and G. Wefer (1991): Discussion of the iron hy- the numerical simulations. Two-dimensional diffusion pothesis. Limnol. Oceanogr., 36, 1899Ð1918. analysis is required to determine the upwelling velocity Huppert, H. E. and J. S. Turner (1981): Double-diffusive con- more accurately. vection. J. Fluid Mech., 106, 299Ð329. Not only the upwelling mechanism, but also assess- Maruyama, S., M. Ishikawa and K. Taira (2001a): Japanese ments from many different aspects have to be considered patent, #2001-336479. before the Laputa Project can be realized in the ocean Maruyama, S., K. Nakano, N. Takahashi, S. Sakai and K. Taira desert. One of the most important aspects is biological (2001b): Study on upwelling of deep seawater by the per- assessment of the environmental effects. An ocean farm petual salt fountain. Proc. 38th National Heat Transfer achieved in an ocean desert is similar to a forest in a land Symp. Japan, 3, 753Ð754 (in Japanese). desert. Since an ocean farm is relatively isolated from Nishio, S., X.-H. Shi and W.-M. Zhang (1995): Oscillation-in- the surrounding ocean desert, less environmental impact duced heat transport: heat transport characteristics along liquid-columns of oscillation-controlled heat transport is expected compared with upwelling in seashore regions. tubes. Int. J. Heat Mass Transfer, 38, 2457Ð2470. Nozaki, Y. (1998): Global Warming and the Oceans: The Role Acknowledgements of Carbon Cycling. Univ. of Tokyo Press, Tokyo (in Japa- We would like to express our gratitude to Dr. J. Uh, nese). a former researcher of the Ocean Research Institute, Uni- Otsuka, K., A. Bando and H. Inoue (2000): A study on float- versity of Tokyo, Dr. D. Yanagimoto of the Ocean Re- ing-type deep-seawater upwelling system. OTEC, 8, 43Ð48 search Institute, Mr. K. Nakano and Mr. N. Takahashi, (in Japanese). former post graduate students of the Institute of Fluid Stommel, H., A. B. Arons and D. Blanchard (1956): An ocean Science, Tohoku University for their assistance during the curiosity: the perpetual salt fountain. Deep-Sea Res., 3, 152Ð preparation of the experiments. Dr. X. R. Zhang of the 153. Institute of Fluid Science assisted with the numerical Takahashi, M. and K. Iseki (2000): Deep seawater as resource of the 21st century. Kaiyo, 22, 5Ð10 (in Japanese). simulation of the pipe flow. This experiments was con- Takahashi, N., S. Maruyama, S. Sakai and K. Taira (2002): A ducted as a collaborative project between the Institute of numerical analysis of natural convection using temperature Fluid Science, Tohoku University and the Ocean Research and concentration differences. Mem. Inst. Fluid Sci., Tohoku Institute, University of Tokyo. Univ., 13, 21Ð30 (in Japanese).

568 S. Maruyama et al.