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A preliminary study on air-lift artificial upwelling systemAonastu

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    A Preliminary Study on Air-Lift Artificial Upwelling System

    1Nai Kuang Liang

    (received 96/4/17, revised 96/5/9 , accepted 96/5/10)

    ABSTRACT

    Due to the characteristics of richness in nutrients and low temperature, deep

    seawater becomes an important renewable ocean resource. Generally, the natural

    upwelling sea area is a fish ground of high production. To bring a huge amount of

    deep seawater to the sea surface by an artificial means can generate a new fishing

    ground. In the past, some proposed artificial upwelling devices are either not

    efficient in pumping deep seawater or not practical. The air-lift process is more

    promising. Air can be compressed and transported to the upwelling pipe or pumped

    by a wave-energy conversion buoy. Following Rautenberg's theory (1972), a

    preliminary estimate is that an air flowrate of 0.03 cms may upwell 4 cms seawater

    from 300 m depth through a pipe of 3 meters in diameter. Certainly there are many

    research needs in ahead of us. Especially a field experiment of artificial upwelling is

    a meaningful proposal, to understand the changes in physical, chemical and

    biological oceanography during upwelling. San-Shian-Tai of Taitung County is an

    ideal site. According to Chern & Wang (1995), South China Sea water which is

    strong stratified flows along east coast of Taiwan in summer. And according to Liu

    et al. (1988), the nutrient concentration of seawater in the vicinity of Taiwan is

    highly negatively correlated to the seawater temperature. Hence, the experiment

    should be executed in summer. Besides, the flexible pipe concept proposed in this

    paper can also be applied to the cold water pipe of a detached OTEC plant.

    (Keyword:deep seawater, nutrients, artificial upwelling, air-lift

    method, flexible pipe)

    INTRODUCTION

    The deep seawater is an important resource of the mankind. Almost all natural upwelling areas are significant fishing grounds. This is owing to the eutrophic nature of deep seawater, which is rich in inorganic nutrient salts, such as nitrate, phosphate and silicate. These nutrient salts are essential for the growth of plants. About 28 years ago, Dr. Oswald A. Roels has headed a research program, in which nutrient-rich deep seawater was pumped to the coast at St. Croix, U.S. Virgin Islands and studied the mariculture of the shell fish without feeding food (Roels et al., 1973). Except the richness of nutrients, the other properties of deep seawater are cleanness, low temperature and stableness (Nakashima et al., 1994). As regards to cleanness the turbidity and the concentration of dissolved organic substances are low, and the quality is bacteriologically stable. It contains few parasites and periphyton and pathological bacteria are also few and doesn't contain harmful artificial pollutants.

    The low temperature is another important resource. The Ocean Thermal Energy 1 Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC.

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    Conversion (OTEC) utilizes the temperature difference between surface and deep seawater. OTEC has been widely and deeply studied between 1975-85. Due to the low price of crude oil and the technological shortage in cold water pipe, a commercial OTEC is not yet built. However, the coldness is still useful in mariculture, because some sorts of fish and plant prefer to live in a cold water environment which is not common locally. Despite the halt of OTEC development the utilization of deep seawater on land is successful. In 1982, the Natural Energy Laboratory of Hawaii (NELH) at Keahole Point, Hawaii commenced to study mariculture using deep seawater (Deniel, 1988). There are several private companies which use the deep seawater. Japan is more successful. Kochi Artificial Upwelling Laboratory(KAUL) was constructed in 1989 (Nakashima, 1995). In KAUL a cascade system for utilization of deep seawater resources is planned and research institutes, local government and private companies work together. A new laboratory was established at Toyama Prefecture in March, 1995 and another one is also planning at Okinawa Prefecture. All these accomplishments prove that deep seawater is useful in food production.

    Another concept of the use of deep seawater is to discharge it directly in the surface of open ocean. According to personal communication with Dr. Nakashima, an upwelling area of 10 km x 10 km can be generated by pumping 300 cms deep seawater with nitrate concentration of 20µM to the surface. The sea area to the northeast of Taiwan is accepted as an upwelling area generated by the interaction between Kuroshio intrusion and shelf break (Yin, 1973; Fan, 1980; Liu & Pai, 1987; Liu et al., 1988; Chern & Wang, 1989; Wong et al., 1989; Chern & Wang, 1990; Gong et al., 1992; Liu et al., 1992; Lin et al., 1992; Lee Chen, 1992; Chen & Lee Chen,1992; Chu et al.,1993; Tang & Tang,1994; Hsu et al.,1995). According to a box model for

    3thermal and mass balance, a total volume transport of 0.2 Sv, i.e. 200×10 cms, is estimated(Liu

    et al., 1992). In the box model, the lower boundary is 60m deep, where the nitrate concentration is 7.9µM. Assuming that the source of upwelling is between 300-400 m and the nitrate concentration is about 20µM, the total volume transport from the source is then reduced to 79 x 310 cms. After a field measurement of chlorophyll in the upwelling zone in spring, 1993, an area

    2of high primary productivity is estimated to be 5000 km (Gong et al., 1995). Then an upwelling

    zone of 10 km x 10 km needs 1580 cms deep seawater, which is about 5 times higher than that suggested by Nakashima. One can believe that a value between 300 and 1500 cms of deep seawater (300-400 m) can make a 10 km×10 km artificial fishing ground. According to a personal communication with Prof. S.Y. Yeh, Institute of Oceanography, National Taiwan University, an annual fishery production due to upwelling in the northeastern sea area of Taiwan

    5is estimated to be about 10 tons. The reason is based on the Annual Report of Taiwan Fishery

    during 1992-94, in which the annual production of purse seine fishery for mackerel & scad is

    3about 4055×10 tons just from this upwelling area. As we know that the world population increases rapidly. The shortage of food threatens us. The artificial upwelling in the open ocean to enhance primary productivity is highly worth to be researched.

    Using horizontal contraction & expansion tube (Venturi-type tube), an artificial upwelling induced by ocean currents was studied (Liang et al., 1978). Instead of contraction & expansion (C & E) tube, a tube-pair (Fig.1), which consists of two C & E tubes was proposed (Liang et al., 1979). The pumping ability has been tested both in laboratory and field. In the field experiment, a mooring system is depicted in Fig.2. The upwelling pipe was of 40 cm in diameter and composed of iron-made ring, strong nylon cloth, protecting steel wires and small buoys which

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    was used to eliminate its weight in water and to make it easily to be retrieved. However, the field experiments were not successful due to the tearing of the nylon cloth close to the main body. By an indirect way, the pumping capacity of a real C & E tube-pair was estimated. An artificial upwelling apparatus of 100 C & E tube-pairs which are 220 cm in length can pump 0.05 cms of subsurface seawater from 140 m to 30 m depth in 1.5 knot current (Liang, 1983). This flowrate is not enough and the depth is rather shallow that the concentration of nutrients is not enough high.

    Fig.1A contraction & expansion tube-pair for artificial upwelling proposed by

    Liang

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    Fig.2The mooring system of an artificial upwelling apparatus proposed by Liang

    A wave-driven artificial upwelling device is designed after Isaacs wave pump (Isaacs et al., 1976; Chen et al., 1994). It consists of a buoy and a vertical long pipe with a flow controlling valve (Fig.3). The valve will open when the downward acceleration of the pipe exceeds a threshold value and vice versa. During the heaving of the buoy and pipe, the valve closes and opens alternatively and deep seawater will be pumped to the surface. After a theoretical calculation, a device, which consists of a buoy 4.0 meter in diameter with a tail pipe of 1.2 m in diameter and 300 m in length, can generate an upwelling fow of 0.45 cms for a regular wave of 1.90 m in height and 12 seconds in period and a value of 0.95 cms for a random wave of the same signifigant wave (Liu and Jin, 1995). However, the author wonders that such a device can survive in a typhoon wave.

    Fig.3Buoy for wave-driven artificial upwelling

    AIR-LIFT PUMP

    The air-lift pump has been well-known since the end of 18th century. It can be used to pump water, dirty water, dangerous fluids and transport solid material. Comparing with other hydraulic transport processes, the air-lift process is a very simple and insensitive and needs little maintenance. Due to these advantages, the air-lift process may be ideal for the marine technology, such as the mining of manganese nodules from deep ocean floor (Weber, 1976). The principle of air-lift pump is simple. As shown in Fig.4, air is compressed into a vertical pipe, which is dipped in water (Rautenberg, 1972). Bubbles ascend and the water level in the pipe will rise due to the density decrease of the air-water mixture. Once the water level reaches the top of the pipe and the water flows out, the water flows from the lower end immediately after the principle of continuity. A main question is what is the relation between the air and water flowrate. Basically it is a problem of energy flux balance (Rautenberg, 1972).

    After Rautenberg (1972), the energy flux balance equation is the following:

    N=N+N+N+N+N(1)LWNRBES

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    N is the energy flux of air flow into the system and equal toL

    ;Pln (P/P)(2)VOEOLO

    ;in which P is the atmospheric pressure, P the pressure at the air entrance of the pipe and VOELOis the air flowrate at pressure P.O

    Fig.4 A description of air-lift pump

    N is the energy flux of water pumped up to H meter above sea level and equal toWNO

    ;Hρg(3)VOWW

    ;in which ρ is the water density and the water flowrate.VWW

    N is the energy flux due to frictional losses. There are two parts, of which N is the part RRUin the lower section of pipe without air bubbles and N is that in the upper section with air RE

    bubbles. N is very simple and equal toRU

    32;Hρ((2DF)(4)VUWW

    in which λ is the coefficient of frictional loss head in pipe flow, H is the length of lower pipe U

    section, D the diameter of the pipe and F the cross-sectional area of pipe. N is more REcomplicated due to air bubbles. Rautenberg employed the following assumptions:

    1)The frictional loss is proportional to the kinetic energy and the coefficient is the same as the

    water flow.

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    2)The speed of water is equal to that of the bubble.3)The pressure head varies linearly along the pipe.4)The state variation of air is isothermal.

    N is then equal toRE

    ;+ρHHVPPP22EOWWOEO;;;;2λ?++VVVV(5)(ln)WWLOLO22DFPPPP?EOOE

    N is the energy flux due to the acceleration of two-phase flow. The acceleration originates B

    in the expansion of bubbles. N is then equal toB

    2;;;VVV?+ρ()WWWLO(6)2F

    N is the entrance energy loss flux at the lower pipe end and equal toE

    3;VρWW(7)ξ??E2F2

    in which ξ is the entrance energy loss coefficient.E

    N is the slip energy loss flux due to the velocity difference between bubble and water. S

    Rautenberg assumed that the average diameter of bubbles is constant along the conveying

    column. N is then equal tos

    kk? ?P(8)12 ges

    where

    HOKF ? ν(PP)1LmEO+HHEO

    FK(PP)2LmEO()EOWgHH

    P is the total pressure loss head except ρ ? g ? H and ν is the average slip speed of GesWOLm.~+PPHHVLooEEobubbles and equal to ?n??FPPPHEooo

    ARTIFICIAL UPWELLING VIA AIR-LIFT METHOD

    For artificial upwelling the deep seawater is pumped only to the surface. Therefore, H in O

    Fig.4 is zero. Then H is not necessarily large and the power of compressed air is much less. If E

    H is zero, the equation(8) can not be employed. Hence, in this study slip energy loss is assumed O

    to be small and can be neglected. The energy flux balance equation is the following:

    NNNNN(9)LRUREBE

    35 2 We assume that λ=0.02, ξ=0.1, ρ=1030 kg/m, P=10kg/mand P= 2P. It is easy to EWOEO

    ;;calculate air flowrate for a given seawater flowrate , total pipe length H+H and VVEULOW

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    pipe diameter D. In Fig.5 the relationships between air flowrate and seawater flowrate of 300 meter deep for different pipe diameters are shown, in which H is set to be 10 meters. It can be E

    found that the pipe diameter plays an important role. In the beginning, the gradient, i.e. the

    ;;change rate of with respect to , is large and decreases as the air folwrate increases. VVWLO

    There must be some limitation on the air flowrate. Hence, an appropriate air flowrate ought to be chosen. Why 300 meter depth is chosen? After Liu (Liu et al., 1988), the surface water in the Kuroshio is very depleted in nutrients, whereas the subsurface water (200-700 m) shows linearly increasing nitrate and phosphate concentrations with decreasing temperature. According to Chern & Wang (Chern & Wang, 1995), in the southern and eastern coast of Taiwan, where deep seawater is close to the land, the South China Sea (SCS) water prevails, which is strong stratified. After the onset of northeast monsoon in late fall, the East Philippine Sea (EPS) water which is less stratified takes the place of SCS water. The temperatures at the 300 meter depth are

    ooabout 12.5C for SCS water and 14.5C for EPS water. These temperatures can guaranty a high

    concentration of nutrients (Pai,1995).

    Fig.5Relationships between air flowrate and seawater flowrate for an artificial upwelling of

    300 meter depth

    PRACTICAL CONSIDERATION

    In the southern and eastern coast of Taiwan, there are many places where the 300 meter deep countour line is close to the shore. Air supply stations can be set up on a shore and through a small pipe the compressed air will be pumped into the upwelling pipe. A diagram as shown in

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    Fig.6 descripts the concept. The upwelling pipe can be similar to that dipicted in Fig.2. However, some modifications may be suggested (Fig.7). Because the pressure of compressed air is not high, i.e. less than one atm., it is suggested to use rotary or dynamic compressor instead of reciprocating one.

    Fig.6Concept for an air-lift artificial upwelling system with a shore-based compressor

    Fig.7A concept design of upwelling pipe

    For the sake of saving energy and extanding the applied area to more remote zones where the air supply station is hard to transport air to the upwelling pipe, wave energy may be

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    employed. The pneumatic wave-energy conversion buoy system has been studied long ago(McCormick, 1976). The concept is quite simple. As shown in Fig.8, the air in the chamber flows through the turbine as the buoy heaves. Instead of one outlet two outlets with one-way valves are employed. One outlet with valve which lets air flow into the chamber and another outlet with valve which lets air flow out of the chamber. The latter is connected to an upwelling pipe by a flexible pipe. Like a piston the water column in the pipe of the buoy may press air into the upwelling pipe in a half cycle of a wave period, when the buoy heaves. A preliminary estimate of air flowrate by the buoy may be based on the energy balance equation as follows:

    12M?vP?V(10)aa

    2

    in which M is the mass of water column, V is the speed of water column with respect to the buoy, P is the pressure of air and V is the volume of air. If the diameter of water column is 0.5 aa

    m, the length of water column 5 m, the relative speed of water column to the buoy is 0.3 m/s, the depth of air inlet on the upwelling pipe is 3 m and the wave period is 8 second, an average air

    -3flowrate is 1.2×10 cms. By employing equations (2,4,5,6,7 and 9), an average flowrate of deep seawater is about 0.3 cms for a upwelling pipe of 2 meter in diameter and 200 m in length. Of course, further detailed study is necessary.

    Fig.8A pneumatic wave-energy conversion buoy

    PROPOSED FIELD EXPERIMENT

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    Although the consequences of a natural upwelling is well known, the detailed time and space variations of an upwelling are still mysterious. The reason is clear, i.e. the upwelling flowrate is not clear and can not be controlled. Due to the 200-mile Exclusive Economic Zone (EEZ), the environmental impact of inland mariculture and the increasing demand of sea food, open ocean mariculture is world-wide important. During 1989-1990, a field experiment was executed in Toyama Bay in the Sea of Japan(Kajikawa, 1991). In the experiment, 0.3 cms deep seawater from 300 m was pumped to the surface, mixed with 0.6 cms surface water and discharged to the sea surface. The phenomena were not clear due to the little flowrate of deep seawater. According to Fig.5, 3 upwelling units of 2 m diameter and 3×0.1 cms flowrate of air can generate totally 10 cms upwelling flowrate.

    Fig.9Bathymetry in the vicinity of San-Shian-Tai,Taitung, Taiwan

    As shown in Fig.9, near the coast at San-Shian-Tai peninsula, Taitung County, the 200 m

    odeep contour is about 1 km offshore. In summer, the temperature at 200 m depth is about 14C.

    at this area (Chiao et al., 1993). Hence, the seawater at 200 m depth contains enough nutrients in summer. The northward Kuroshio current may generate an eddy due to the peninsula. The water

    2may be partially trapped. The area is approximately 3 km. After Nakashima's estimate 10 cms

    upwelling flowrate is enough to create a significant upwelling area. In this experiment, all physical, chemical and biological processes of upwelling can be observed. It is of highly

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